Atc88 november 2011

BGAS-CSWIP Painting Inspection - Grade 2

ATC88

Training & Examination Services Granta Park, Great Abington Cambridge CB21 6AL, UK

Copyright © TWI Ltd

Rev 2 June 2011 Contents

Copyright TWI Ltd 2011

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BGAS-CSWIP Painting Inspection - Grade 2

Contents Section Subject.

1 Corrosion 1.1 Electrical circuit 1.2 The chemical reaction

2 Surface Preparation Methods and Standards 2.1 Dry abrasive blast cleaning 2.2 Abrasives 2.3 Sizing of abrasives 2.4 Adhesion and profile 2.5 Profile 2.6 Shot blasted profile 2.7 Profile measurement 2.8 Assessing a profile to BS 7079 Pt C ISO 8503.1 2.9 Use of the comparators 2.10 Using the comparators 2.11 Preparation of steel substrate before application of paints and related

products 2.12 Abrasive blasting grades 2.13 Equipment 2.14 Water blasting 2.15 Flame cleaning 2.16 Method 2.17 Pickling 2.18 Vapour degreasing 2.19 Weathering

3 Surface Contaminants and Tests for Detection

4 Paint Constituents and Basic Technology 4.1 Liquid paints containing solvent 4.2 Solvent free 4.3 Powders 4.4 Binder 4.5 Binder – solvent groups and compatibility 4.6 Polyurethanes use ketones and esters with aromatic diluents. 4.7 Polymers 4.8 Linear polymers 4.9 Branched polymers 4.10 Cross linked polymers 4.11 Oils 4.12 Pigments 4.13 PVC



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4.14 Solvents 4.15 Other additives

5 Solutions and Dispersions

6 Drying and Curing of Paint Films 6.1 Solvent evaporation 6.2 Oxidation 6.3 Chemical curing 6.4 Coalescence

7 Paint Systems 7.1 Primer

8 Water Borne Coatings

9 Paint Manufacture 9.1 Direct charge dispersing mills

10 Testing of Paints for Properties and Performance 10.1 Tests on paint 10.2 Paint density 10.3 Hegman grind gauge 10.4 Viscosity

11 Film Thicknesses 11.1 Comb gauges 11.2 Tests done on dry paint films 11.3 Destructive test gauges 11.4 Non destructive test gauges 11.5 Tests for mechanical properties on paint films 11.6 Accelerated testing 11.7 Drying and curing tests 11.8 BK drying recorders 11.9 Other tests 11.10 Trough type 11.11 Black and white fused plates 11.12 Hiding power charts and micrometer adjustable film applicator 11.13 Degree of gloss 11.14 Adhesion

12 Specified Coating Conditions 12.1 The whirling hygrometer, aspirated hygrometer or psychrometer 12.2 Steel temperature measurement

13 Cathodic Protection 13.1 Sacrificial anode systems 13.2 Impressed current system 13.3 Interference 13.4 Monitoring CP 13.5 Cathodic disbondment

14 Holiday/Pinhole Detection 14.1 Voltage setting



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15 Paint Application 15.1 Brush application 15.2 Roller application 15.3 Spray application (Conventional, Airless, Elecrostatic) 15.4 Other paint application methods

16 Metal Coatings 16.1 Galvanising 16.2 Sheradising 16.3 Calorising 16.4 Anodising 16.5 Electro-plating 16.6 Hot metal spraying 16.7 Powder system 16.8 Electric arc system 16.9 Wire and pistol system

17 Coating Faults

18 Colour

19 Health and Safety 19.1 Hazard warning symbols 19.2 Responsibilities 19.3 Maximum exposure limit (MEL) 19.4 Occupations exposure standard (OES)

20 Duties of An Inspector

21 List of Specification and British Standards (BS) Numbers

22 Quality

23 Revision Questions

Appendix 1 – Insulation

Appendix 2 – Data Sheet Examples

Section 1

Corrosion

Rev 2 June 2011 Corrosion


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1 Corrosion Corrosion can be generally defined as degradation of a metal by chemical or electro-chemical means. From this definition it is obvious that two mechanisms are involved, firstly an electrical circuit and secondly a chemical reaction.

1.1 Electrical circuit

In a corrosion circuit the current is always direct current (DC). It is conventionally thought that a current passes from positive + to negative -, ie from anode to cathode. In fact electrons are flowing in exactly the opposite direction, from cathode to anode. For corrosion circuit to exist three things are needed: Anode An anode is a positively charged area. It becomes positively charged because the atoms release two electrons each, thus causing an imbalance between protons and electrons, positive and negatively charged units. In its passive state, the iron atom has 26 of each, protons and electrons, when the two electrons are released the atom still has 26 protons, but now only 24 electrons. In this state the atom is now an ion, overall positively charged by two units and written as Fe++. (An ion is a charged particle and can be positive or negative, a single atom or a group of atoms, known as a molecule.) This losing of electrons can be shown as: -Fe Fe++ +2e. The Fe++ is called a positive iron ion. An ion can be positive or negative and is a charged particle, an atom or a group of atoms. A passive iron atom Fe 26 protons and 26 electrons.

An iron ion Fe++, 26 protons and only 24 electrons

Figure 1.1 Iron atoms. Cathode A cathode is a negatively charged area where there are more electrons than needed in its passive state. These are electrons released from the anode. At the cathode the electrons enter into the electrolyte to pass back to the anode.

Nucleus



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Electrolyte An electrolyte is a substance, which will conduct a current and be broken down by it, (dissociate into ions). Water is the most abundant electrolyte and also very efficient. Acids, alkalis and salts in solution are also very efficient electrolytes. As the electrons pass into the electrolyte it is dissociated into positive and negative ions, as shown by the formula: -2H2O2H+ + 2OH-. Simultaneously the electrons couple back with the hydrogen ions to form two full Hydrogen atoms, which join together diatomically to form hydrogen gas. This is termed as being evolved, or given off from the cathode. The hydroxyl ions return to the anode through the electrolyte carrying the electrons. The corrosion triangle, as shown below, can illustrate the electrical circuit. The electron circuit can be seen to be from anode A, to cathode C, through the electrolyte E, back to A. Figure 1.2 The corrosion triangle.

1.2 The chemical reaction

From the above we can see that no chemical reaction, (combination of elements) has occurred at the cathode, or in the electrolyte. The chemical reaction, the formation of corrosion products, only occurs at the anode. The positive iron ions, Fe++, receive the returning hydroxyl ions and ionically bond together to form iron hydroxide, which is hydrous iron oxide, rust and is shown by the formula: Fe++ + 2OH- Fe (OH)2 It is now apparent that corrosion only occurs at the anode, never at the cathode, hence the term cathodic protection. If a structure can be made to be the cathode in a circuit, it will not corrode. The corrosion triangle shows the three elements needed for corrosion to occur, anode, cathode and electrolyte. If any one of these three is removed from the triangle, corrosion cannot occur. The one most commonly eliminated is the electrolyte. Placing a barrier between the electrolyte and the anodic and cathodic areas, in the form of a coating or paint system does this. If electrolyte is not in direct contact with anode and cathode, there can be no circuit and so no corrosion. The basic corrosion reaction, as explained above, occurs fairly slowly at ambient temperatures. In common with all chemical reactions certain factors can increase the reaction rate, listed below are some of these.

E

A C



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1 Temperature. Steel, in common with most metals, is thermodynamically unstable. The hotter the steel is the faster the corrosion will occur.

2 Hygroscopic salts. Will attract water and dissolve in it. When salts are

present on a substrate and a coating is applied over them, water will be drawn through the film and the resulting solution builds up a pressure under the film. Eventually the film is forced up to form blisters. These blisters are called osmotic or hygroscopic blisters and are defined as pinhead sized water filled blisters. Sulphates and chlorides are the two most common salts, chlorides predominant in marine environments and sulphates in industrial areas and sometimes agricultural.

3 Aerobic conditions, (presence of oxygen): By introducing oxygen into the

cathodic reaction the number of Hydroxyl ions doubles. This means that double the number of iron ions will be passivated and therefore double the corrosion rate. Shown by :-2H2O + O2 + 4e 4OH-

4 Presence of some types of bacteria on the metal surface, for example

sulphur reducing bacteria, better known as SRBs, or, metal eating microbes (MEMs).

5 Acids and alkalis. 6 Bi-metallic contact. Also known as bi-metallic corrosion. Metals can be listed in order of nobility. A noble metal is one, which will not corrode. In descending order, the further down the list the metal is, the more reactive it is and so, the more anodic it is, the metal loses its electrons to become reactive ions. The degree of activity can be expressed as potential, in volts. The list can be called a galvanic list, but when the free potentials of the metals are known it can also be called the electro motive forces series or the electro-chemical series. On the following page is a table of some metals in order of nobility with potentials as measured using a copper/copper sulphate half-cell reference electrode, in seawater at 25°C.



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Material Known potential av. values

Graphite + 0.25 v

Titanium 0.0 v

Silver - 0.1 v

Nickel 200 - 0.15 v

Lead - 0.2 v

Admiralty brass - 0.3 v

Copper - 0.35 v

Tin - 0.35 v

Mill scale - 0.4 v

Low alloy steel - 0.7 v

Mild steel - 0.7 v

Aluminium alloys - 0.9 v

Zinc - 1.0 v

Magnesium - 1.6 v

From the list above it can be seen that millscale is immediately above steel on the galvanic list. This means that millscale is cathodic to steel and if left on the surface of steel will accelerate the corrosion of the steel substrate. Mill scale is formed during the rolling operation of steel sections eg RSC, RSA, RSJ. The oxides of iron form very quickly at temperatures in excess of 580°C. The first oxide formed is FeO, iron oxide, the next is Fe3O4 and last of all Fe2O3. Common names in order are wustite, magnetite and haematite. These oxides are compressed during the rolling operation to produce blue millscale. The thickness of millscale varies from 25-100m. Because millscale is only produced during rolling, when it has been removed by any surface preparation method, it can never recur.

Section 2

Surface Preparation Methods and Standards

Rev 2 June 2011 Surface Preparation Methods and Standards


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2 Surface Preparation Methods and Standards If the products of the corrosion reactions and other contaminants, were left on a substrate and paint applied over them, the adhesion of the coating and thus the coatings life would be far from satisfactory. Surface preparation involves removing these contaminants and in some instances increasing the area available for adhesion by roughening up the substrate. A good surface preparation grade (degree of cleanliness) along with a suitable surface profile can give 10 years life from a typical four coat paint system. The same system applied over a substrate with little or no profile and contaminant remaining might give four to six years, or even less. Therefore two factors need to be considered when inspecting a surface preparation. 1 Degree of cleanliness. 2 Surface profile (degree of roughness). If a specification gives criteria for both of these factors, then quality is not achieved until both criteria are satisfied. Surfaces can be prepared for paint application in several different ways, each one varies in cost, efficiency, ease and suitability. a Dry abrasive blast cleaning. b Water blasting. c Hand and power tool cleaning. d Flame cleaning. e Pickling. f Vapour degreasing. g Weathering.

2.1 Dry abrasive blast cleaning

Dry abrasive blast cleaning involves compressing air and forcing it along a hose and out of a small aperture called a nozzle. A pressure of 100psi results in the air exiting the nozzle at approximately 450mph. If abrasive particles are mixed in with the air and travel at the same speed, they will carry a lot of work energy. This energy is used in chipping away millscale and other detritus from the substrate. With some abrasives part of the energy is used in shattering into small pieces and with others all the energy is used in impinging into the steel surface, roughening the surface and increasing the surface area to increase adhesion properties. Because all standards refer to the amount of contamination remaining on the surface, the longer the time spent on this operation, the higher the degree of cleanliness.



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2.2 Abrasives

Abrasives come in many forms and can be classified in several different ways, as shown below. None metallic (mineral) expendable

Metallic (recyclable) Agricultural by-product

Copper slag ACI (angular chilled iron) Walnut shell

Nickel slag Steel grit Coconut shell

Boiler slag Steel shot Eggshell

Glass bead Grit and shot mix Corn cob husk

Aquamarine (olivine) Garnet Peach husk

It can be seen that the recyclable abrasives are the more costly and therefore justify a cleansing operation before re-use. In the context of this course we are considering the following: Sand It is not permitted to use sand. SI 1657 states that any mineral used as an abrasive must release less than 1% free silica on impact. (Silica causes pneumonicosis or silicosis.) COSHH regulations do not allow the use of sand containing silica for dry blasting. Sand itself is perfectly safe, but shattering on impact releases silica which can be inhaled. Copper slag Although the name implies metallic content the amount of copper in the structure is extremely minute. Minerals smelted with the copper, liquefy and form a protective cover over the molten copper to prevent reaction with the atmosphere like slag on a weld. When the copper metal is run off the slag is rapidly cooled in cold running water, which causes it to shatter. The material is supplied in grit form (random, sharp edges, amorphous) and is very brittle, shatters into smaller pieces on impact and should be used only once and then discarded and so it is classed as expendable. Garnet A natural mineral classed as being of diamond type hardness, can be either expendable or recyclable. If the situation justifies, cleansing units are available to extract contamination so that the material can be reused, usually up to three times. Doesn’t shatter on impact but does suffer some wear, supplied in grit form. Metallic grit In this context, steel and iron are both metallic. Cast steel grit being the softer of the two tends to round off on impact and loses its sharp edges. Angular chilled iron chips off small slivers on impact to produce sharp cutting surfaces on its next cycle. The finings so produced are extremely abrasive and cause extreme wear on moving parts of the recovery systems.



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Metallic abrasives are recyclable because the particles reduce in size slowly. Hence it can be re-used many times and still perform a useful function in a working mix. A working mix is an accepted ratio of large and small particles, where the large particles cut the profile and the smaller particles clean out the troughs. Metallic shot Shot is spherical and doesn’t shatter (otherwise it would form grit). When supplied the particles are virtually uniform in size and shape, (not a working mix) but like the grit they wear down slowly in size. Regular addition of new abrasive as, with grit, will then maintain a working mix. The particles are worn down eventually to finings and are drawn out of the system during cleansing. Metallic shot and grit mixed A mix of shot and grit results in a more uniform profile. The grit cuts the profile and the shot, being unable to enter the troughs produced, controls the peak height and so greatly reduces the number of rogue peaks. A rogue peak is one, which is well proud of the acceptable profile range and if painted over due to contraction of the paint, will leave bare metal in contact with the atmosphere, thus allowing corrosion to occur. When rogue peaks are in concentrated area the effect is of a rash, hence rust rashing or rust spotting. A typical mix ratio of shot to grit as used in a pipe coating mill would be 70-80% shot to 20-30% grit. Other properties of an abrasive have an effect on the resulting substrate also, these being. Size of the particles. Hardness of the material. Density of the material. Shape of the particle. For example steel has a density of approximately 7.6gm/cc and copper slag, depending on composition, approximately 4.2gm/cc. If one particle of each material, of identical size, hit a steel substrate, then it would be logical to say that the steel would impinge further into the substrate, resulting in a deeper trough. A spherical particle would not impinge as deeply because the large smooth surface area would use its energy up in peening or work hardening the surface rather than cutting into it. So a shot blasted surface is different in appearance and texture to that of a grit blasted surface.

2.3 Sizing of abrasives

G prefix = grit amorphous, points and cutting edges, irregular profile. S prefix = shot spherical, smoother profile.



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The G or S notation is followed by a number, which denotes the particle size. eg G24 or S330. From system to system the number can represent vastly different values, eg with the now defunct BS 2451 the 24 means nominally 24 thousandths of an inch where as in the SAE system it represents 1/24 inch = approximately 40 thou. The new BS 7079 pt E uses a different method again, in metric units. G140 would mean a nominal particle size of 1.4mm

2.4 Adhesion and profile

A commonly used definition of adhesion is the force required to separate two surfaces in touch. A newly rolled plate, perfectly smooth, 1m x 1m has an apparent surface area of 1m2 and an actual area of 1m2. Abrasive blasting roughens the surface and increases the actual area, (the apparent area is still 1m2), thus increasing the adhesion. Two theories of adhesion are: 1 Molecular interference

Because the surface is rough and uneven the paint wets and locks into the profile. Analogy – velcro. Physical.

2 Molecular attraction

Negatively charged particles attracted to positive areas and vice versa. Analogy - magnet (sometimes called ionic bonding). Chemical.

2.5 Profile

Surface profile, anchor pattern, key, peak to trough height and amplitude are all expression meaning the cross section of a blasted area, as measured from the top of the peaks to the bottom of the troughs. The surface profile requirements are given on the specification for the job, eg for BGAS 30-75 microns.

Actual sample size: 0.5mm x 0.5mm. Profile height 28.5 microns. Blasted with grit.



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2.6 Shot blasted profile

Also amplitude, key, anchor pattern, surface profile. Figure 2.1 Terms relating to preparing surfaces. Other terms relating to preparing surfaces are illustrated below. Figure 2.2 Grit blasted profile. Hackle: A small surface lamination, which stands upright like a needle after blasting. Approximately ≤ 13mm. Easily removed. Lamination: Appears to be a longitudinal crack, one lip curling back, any laminations (slivers) found must be referred to engineer for ultrasonic check.

2.7 Profile measurement

If a profile requirement is specified, it is the inspector’s duty to ensure that the specification requirements are met. This can be done in two ways. a Measuring using gauges with and without replica tape. b Assessing using surface comparators. Digital gauges are common nowadays, but refineries, gas plant, etc. have stringent safety requirements and batteries can produce sparks, so the dial gauges are still very often used. The dial gauges fall into two categories, surface profile needle gauge and dial micrometers and replica tape.

Peak to trough

Hackle Rogue peak

Lamination or sliver



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Surface profile needle gauge The gauge is applied to the blasted substrate and the needle can be felt to locate a trough. Then by applying a slight pressure to allow the flat ‘foot’ of the gauge to sit firmly on the peaks of the blasted substrate, the needle will pass into the trough as far as it can. To measure the difference from peak to trough we need to zero the gauge when the point of the needle is on the same plane as the flat foot, ie on a smooth piece of glass. This is done by applying slight pressure to the foot to ensure that it is perfectly flat on the glass. By loosening the locking screw, the bezel can now be moved easily in any direction. Still applying the slight pressure, the bezel should be moved so that the zero on the gauge is immediately behind the needle, then tighten the locking screw and the gauge is ready for use. When using this type of gauge it is normal to work to an average figure. Several readings are taken, usually more than ten, in random positions over the substrate and the average calculated. This type of gauge is not ideally suited for curved areas such as pipes.

Figure 2.3 Surface profile needle gauge. Dial micrometer and replica tape Replica tape, more often referred to by its trade name Testex, is also sometimes called corn plaster method. Although more costly than the needle gauge this method provides a permanent record and the traceability required from quality systems. The tapes are supplied in two grades: Course and extra coarse grade, to cover two different ranges of blasted profiles. Coarse grade for measuring profiles 0.8-2 thou inch. 20-50m Extra coarse grade for measuring profiles 1.5-4.5 thou inch 37-115m



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The correct tape should be selected otherwise the readings will not be accurate. Figure 2.4 Cross section of a replica tape. The procedure for using replica tape is: 1 Zero the dial micrometer. Clean the anvils (paper or fingers) and allow

the contacts to come together, release the locking screws and adjust the bezel so that the zero is immediately behind the large needle.

2 Remove the backing paper from the replica tape, ensuring that the small

white disc with the black ring is detached also. Stick the replica tape to the area to be measured.

3 Using a pen or pencil end, or the specially provided plastic stick, rub

firmly and evenly all over the area of the mylar. This causes the testex paste to pass into the troughs and the peaks of the blast will butt up to the transparent mylar.

4 Remove the replica tape and check. The mylar area should no longer be

white (now grey) and pinpricks of light should be visible through the mylar when held up to the light.

5 Place the testex paste area between the anvils of the micrometer and

allow them to gently close together. From the final reading on the gauge deduct two thou if using an imperial gauge or 50m if using a metric gauge. The balance figure is the peak to trough height of the profile.

Paper

Mylar tough transparent polyester plastic

Testex paste



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Figure 2.5 Metric micrometer for testex measurement in microns. 1mm = 1000m 25.4m = 0.001 inch (1 thou.) 40 thou inch = 1mm 25.4mm = 1 inch

Replica tape (50m of plastic backing)

10m

2m



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Example: Micrometer is reading 93m, subtract 50m for testex plastic backing. The surface amplitude is therefore 43m. Figure 2.6 Metric micrometer for testex measurement in microns.

Testex (allow 50 microns 0.05mm for plastic backing)

1 100mm

10 microns

100 microns

0.10mm

Micrometer is reading 80 microns (0.080mm) subtract 50 microns (0.050mm) for testex plastic backing, the surface amplitude is therefore 30 microns.



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Figure 2.7 Imperial micrometer for testex measurement in 1000 of an inch. Reading the gauges There are four common scales for dial micrometers, one of which, the 2μm scale is also used on the needle gauge. The common scales are: 0.01mm = 10 microns/small division 0.002mm = 2 microns/small division 0.001 inch = 1 thou/small division 0.0001 inch = 1/10 thou/small division With all four scales the value given represents the smallest increment on the periphery of the large scale. The small dial at 11 or 1 o-clock position gives the number of complete revolutions of the needle on the main scale. Typically the 2μm scale is 200μm per full revolution. Most profiles are around 75-100μm. Therefore the small dial can be virtually ignored for normal use.

2.8 Assessing a profile to BS 7079 Pt. C ISO 8503.1

Grit and shot abrasives produce different surface profiles, therefore two comparators are specified. One for grit blasted profiles, G. and one for shot blasted profiles, S. When a mix has been used then the reference comparator should be G. In all instances the entire area should be blasted to SA21/2 or SA3 grade (discussed later).

Micrometer is reading 4.6 thou (0.0046 inch), subtract 2 thou (0.002 inch) for testex plastic backing, the surface amplitude is therefore 2.6 thou (0.0026 inch)

1 10 thou

0.0001 inch

1 thou

0.001 inch

Testex (allow 2 thou (0.002 inch) for plastic backing



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2.9 Use of the comparators

Three methods can be employed to assess the roughness characteristics of blast cleaned steel. 1 Naked eye. 2 Visual aid, not exceeding 7x magnification. 3 Tactile. (Note: The comparators are not for assessing cleanliness.) The comparators to BS 7079 are approximately 8cm square with a 2cm diameter hole in the middle and are divided into four segments, by smooth strips. On each strip is an arrow indicating the segment number. Segment one is the smoothest and the degree of roughness progressively increases up to segment four.

2.10 Using the comparators

With all three methods it is important to remember that the prepared surface should not be touched (contamination). For the tactile method the fingernail or a clean wooden stylus may be used. The principle is to compare the surface profile of the blasted steel with the segments on the ISO/BS comparator, looking for two segments between whose profile the test surface lies. The grading used is: Fine -Profiles equal to segment one and up to, but excluding

segment two. Medium -Profiles equal to segment two and up to, but excluding

segment three. Coarse -Profiles equal to segment three and up to, but excluding

segment four. Any profile below the lower limit for fine grading is referred to as finer than fine. Any profile above the upper limit for coarse grading is referred to as coarser than coarse. Because the blasted surface is considered to be a secondary profile, the primary profile is the surface of the steel prior to abrasive blasting. The primary profile is therefore going to have an effect on the secondary profile. It is customary to report on the condition of the substrate before preparation in the following manner.



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2.11 Preparation of steel substrate before application of paints and related products

Rust grades. BS 7079 Pt. A, ISO 8501, SS 05 59 00 The numbers given all refer to the same book, which gives high quality pictorial standards for condition and cleanliness before and after surface preparation, by abrasive blasting, hand and power tool cleaning and flame cleaning. The steel can then be graded. eg B. SA 3, from the definitions below. Rust grade A: Steel surface largely covered with adherent mill scale

with little if any rust. Rust grade B: Steel surface, which has begun to rust and from which

the mill scale has begun to flake. Rust grade C: Steel surface on which the millscale has rusted away or

from which it can be scraped, but with slight pitting visible under normal vision.

Rust grade D: Steel surface on which the millscale has rusted away and on which general pitting is visible under normal vision

The original rust grade is then given a degree of cleanliness, ie a grading relating to how much contaminant is left on the surface after preparation. The degree of cleanliness is mainly dependent on the time spent on the area and the velocity of the particles.

2.12 Abrasive blasting grades

Before surface preparation commences any oil or grease should be removed (by specified solvent or proprietary degreaser) and heavy rust and scale removed by chipping. After preparation the surface should be free from dust and debris. Sa 1 Light blast cleaning. When viewed without magnification, the

surface shall be free from visible oil grease and dirt and from poorly adhering mill scale, rust, paint coatings and foreign matter.



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Sa 2: Thorough blast cleaning. When viewed without magnification, the surface shall be free from visible oil grease and dirt and most of the millscale, rust, paint coatings and foreign matter. Any residual contamination shall be firmly adhering.

Sa 21/2: Very thorough blast cleaning. When viewed without

magnification, the surface shall be free from visible oil grease and dirt and from mill scale, rust, paint coatings and foreign matter. Any remaining traces of contamination shall show only as slight stains in the form of spots or stripes.

Sa 3: Blast cleaning to visually clean steel. When viewed without

magnification the surface shall be free from visible oil grease and dirt and shall be free from mill scale, rust, paint coatings and foreign matter. It shall have a uniform metallic colour.

From the above definitions it can be seen that Sa 1 and Sa 2 are not achievable on rust grade A and consequently there are no photographs for the grades. The American SSPC and NACE (Steel Structures Painting Council and National Association of Corrosion Engineers) have their own systems and compare as below. BS 7079 Pt. A SSPC NACE

Sa 3 White metal SP5 Grade 1

Sa 21/2 Near white metal SP10 Grade 2

Sa 2 Commercial finish SA6 Grade 3

Sa 1 Light blast and brush of SP7 Grade 4

2.13 Equipment

Wheelabrators These are sometimes known as centrifugal blast units are a mechanised way of preparing components for coating. They are ideal for long production runs on similar section components such as pipes in a pipe coating mill, or bridge steelwork. They are usually referred to by the number of ‘wheels’ which they operate eg six wheel. Special machines are designed for special circumstances eg flat steel plated for fabrication yards or ship yards, pneumatically driven operator controlled machines for blasting decks or internal tanks, magnetic crawlers for tank externals. The operators of these machines prefer shot as an abrasive, grit cuts the impellers and entails large amounts of downtime, but when the specification demands, it must be used. The abrasive is gravity fed into the centre of the wheel. Centrifugal forces carry it to the end of the impeller where it is impelled at the component to be cleaned at a speed of approximately 220mph in a fan pattern. The fast



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moving metallic abrasive shatters mill scale, cuts a profile etc ricochets and eventually, its kinetic energy spent, drops. The floor of the unit is open grating over a V shaped pit, in the bottom of which is a rotating screw which carries the spent abrasive plus detritus into a hopper. A conveyer system then carries the abrasives to the top of the machine, dispenses it, to start a gravity fed path back to be re-used. As an integral part of the system the abrasive passes aver a tilted plated, known as a weir plate. As the abrasive and detritus cascades over the edge of the weir plate, a current of air is drawn through it. This draws out low density materials such as rust, mill scale, flakes of paint etc. and finings, abrasive worn so small that it is no longer useful. This is known as an air wash separator, the same principle is used in enclosed grit blasting pens. Meanwhile the cleansed abrasive is fed back into a common hopper with feed lined to all the wheels, to be re-used. As mentioned previously new abrasives need to be added periodically to maintain an adequate working mix. Considerations The quality can be controlled by adjusting the feed roller speeds and therefore is more consistent. Because the system is totally enclosed there is efficient use of abrasives. More operator safety because the operator is not involved. The systems can be far more productive (dependent on supply of components) than open blasting. One major problem is access to bolt pockets, gussets and stiffeners, etc. Because the wheels are fixed, there is no manoeuvrability and thus shadow areas arise. One way to avoid this is manually blast difficult areas prior to machine blasting. Air blasting Site blasting is normally carried out using expendable abrasives and open blasting systems. Open blasting systems operate using: a Compressor. b Pot containing the abrasives. c Vapour traps for oil and water (knock out pots). d Hose, usually carbon impregnated. e Nozzle f Deadmans handle for operator safety. Compressor Compressors are rated by two factors. Air pressure - measured in pounds per square inch (psi). Capacity, the amount of air it can deliver at the pressure required, in

cubic feet per min (cfm), or litres/min.



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It is normal in the UK for portable compressors to be set at 100psi, which is considered to be the ultimate pressure for open blasting. Air abrasive mix and stand-off being constant it is considered that blasting at 100psi gives 100% efficiency. Using pressures over the 100psi uses more abrasives, more fuel, more effort from the operator, more work by the compressor, without a proportionate increase in area blasted, whereas every 1psi drop in pressure results in an efficiency drop of 1½%. 80psi blasting pressure results in 70% efficiency. Although this is not a responsibility of the inspector it is required information. It is far better to have a large capacity compressor working below its capacity than to have a smaller rated compressor working to full capacity. Blast pot For site work the most common is the pressurised blasting pot. These are supplied in various sizes and are selected according to purpose for example it would not be economical to recharge the pot every 5 minutes when blasting a large crude oil tank. The pots are charged with abrasives and when pressurised, seal, rubber to rubber, by means of a mushroom shaped cap. The abrasive is blown by air pressure into the air stream feeding the nozzle. The abrasive flow can be adjusted by means of a metering valve on the conical base of the pot. This is sometimes called a ‘miser’ valve. Vapour traps Air contains water vapour and when air is compressed the water vapour in the air is compressed. Compression produces heat and as the air heats up its capacity to hold water increases, every 11°C rise in temperature the air’s capacity to hold water doubles. Conversely when the air cools rapidly on expansion, exiting the nozzle, water droplets are formed. Should this water contact the substrate, corrosion would result. Also atomised oil (from the cylinder lubricants) needs to be extracted, otherwise low surface energy material, oil, on the substrate will adversely affect adhesion. The knockout pots are on the main airline and are inverted transparent glass domes. A small cock on the bottom allows them to be emptied and usually are kept slightly open. In the UK climate it is not unusual to blow downstream 20 gallons of water in an eight-hour working day. Carbon impregnated hose Because pressure drops along the length of the hose, line lengths are better restricted to around seven to eight metres. Internal couplings reduce the hose diameter and act as pressure reducers, cause turbulence and wear, so external couplings should be used. Hose diameter is related to nozzle size and should have an internal diameter at least three to four times the nozzle diameter. Any specified blasting pressure refers to pressure as taken at the nozzle. This can be measured using a hypodermic needle gauge. The needle is placed through the hose near the nozzle with the needle facing towards the nozzle.



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Nozzles The air consumption and air speed are directly related to the nozzle aperture size. The larger the nozzle size the more air will be needed to maintain pressure. Typically a ¼ inch nozzle will need 103cfm to maintain 100psi, whereas a ½ inch nozzle needs 413cfm. Therefore big nozzle, large bore hose, needs high capacity compressor. Sometimes the nozzles are lined with tungsten carbide or ceramics to reduce wear. Various types of nozzles exist including angled nozzles, straight bore and venturi type. The venturi nozzle provides a larger blast pattern with a more even spread of abrasives and higher velocity of the particles (approximately 450mph). The straight bore nozzle produces a small concentrated area of abrasive contact, with a fringe area of lower concentration and particle speed of around 200mph. The stand-off distance for both types varies according to hose size and nozzle aperture size, but an average figure is around 450mm. Safety to IGE SR 21 (Institute of Gas Engineers) With enclosed systems like wheelabrators, personnel passing the equipment are far safer than in site situations, abrasives are confined in a small area. When abrasive blasting is taking place on a construction site or pipeline, access is not restricted and vehicles and personnel can be within close proximity of the equipment. It is therefore necessary to have warning signs advising that abrasive blasting is in progress, along with warning buntings segregating the area. Other safety considerations are: a The hose should be carbon impregnated to reduce the chance of the

operator getting electric shock from static. b A dead man’s handle should be under direct operator control for his/her

own safety. c Hoses should be kept as straight and as short as possible, to avoid kinks

and blowouts and to maintain pressure at the nozzle. d Use reinforced hoses if possible. e Use external bayonet type couplings, continually bonded. f Maintain operating pressure at 100psi. g Correct protective clothing should be worn by the operator, including

direct air fed helmet, with adequate visors, leather aprons and gloves, boots and ear protectors.

2.14 Water blasting

Surface preparation methods using water are more environmentally friendly than open blasting and also, from the safety aspect, spark free. They are ideal for removal of soluble salts, sulphates and chlorides, (the hygroscopics) although complete removal needs high pressure ranges. Wet blasting methods are also ideal for removing layers of toxic materials, eg red lead, calcium plumbate and zinc chromate primers. These materials are safe during application but removal by abrasion results in fine particulate



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matter passing into the air, which can then be inhaled and passed into the bloodstream. There are certain disadvantages related to wet blasting eg supply of large amounts of water and disposal of the resulting slurry (water and detritus as an entity) and also mixing substrate inhibitors if the specification demands it. (Substrate inhibitors are substances usually sodium compounds, added to the water, to retard the formation of corrosion products) Some organisations, including BG do not allow the use of inhibitors, in which case wet blasting is followed by dry blasting, to remove light oxidation. High pressure water blasting up to 30,000psi (water jetting) Using pure water, usually out of a rotating head, giving alternating pencil and fan jets. Water usage is about 60 litres per minute. To work efficiently the head must be near to the surface, within 25-35mm and as the distance increases the efficiency reduces, until at approximately 250mm only loose and flaking material will be removed. The principle of operation is simple and flexible, but operator fatigue is a problem. This system will remove soluble contamination and millscale at the higher pressure ranges but will not cut a profile. It will only clean up the original profile on rework areas. High pressure water plus abrasive injection This system operates at about 20,000psi and uses abrasives, either gravity fed into the system, suction fed or mixed as a slurry. Marine growths eg barnacles, are easily removed with this system and it us often used in dry-docks on ship hulls. Because of the abrasives a profile is cut using this method. Low pressure water plus abrasive injection Uses normal blasting pressures of 100psi. but with water as a propellant rather than air. The abrasive content is semi-soluble eg sodium bicarbonate crystals, talc, chalk and ideal for use on non-ferrous metals and GRP. Sodium bicarbonate is excellent for acidic or greasy situations. This method is very slow and controllable and can if needed, remove one coat of paint. The abrasives have a very gentle action but leave masses of problematic slurry. Steam cleaning Ideal for oily and greasy situations, but steam production requires a heat source, which is not conducive with the oil and gas industry. Air blasting with water injection Water is injected, with or without an inhibitor into the air/abrasive stream, either immediately after it exits the nozzle or immediately before it enters the nozzle. Water usage with this method is approximately one to one and a half litres per minute, which is sufficient to control dust.



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Hand and power tool cleaning. BS 7079 Pt. A, ISO 8501, SS 05 59 00 Any hand operated or power tools, including needle guns, wire brushes, emery cloth and grinders can be used to achieve these standards. Hand and power tool cleaning methods are tried and tested over many years, but are now considered to be far less efficient than other modern methods. Limited access or environmental considerations may be factors which influence the choice of methods. Hand and power tool cleaning is often specified for short term maintenance programmes. One major disadvantage of this method is the lack of surface profile. Wire brushing will not produce a profile and in most cases will actually reduce an existing profile, sometimes resulting in burnishing, which is polishing and a smooth shiny area does not provide good adhesion. Burnishing needs to be treated by abrading with coarse emery. As with abrasive blasting heavy rust, oil and grease need to be removed prior to preparation of the substrate. St2: Thorough hand and power tool cleaning. When viewed without magnification the surface shall be free from visible oil, grease and dirt and from poorly adhering millscale rust, paint coating and foreign matter. St3: Very thorough hand and power tool cleaning. As for St2 but the surface shall be treated much more thoroughly to give a metallic sheen arising from the metallic substrate. There are no wire brushing grades for Rust grade A as the mill scale is much harder than the bristles on the brushes, which are of non-sparking alloys such as phosphor bronze and beryllium bronze. If needle guns, Jasons hammers, are used they tend to leave a very coarse profile which invariably needs to be reduced by abrading with emery, or grinding.

2.15 Flame cleaning

Not likely to be used on oil and gas plants, but it is an approved method of surface preparation, with photographic standards. The BS 7079, ISO 8501 (SS 05 5900) contains four photographs showing flame cleaning standards from the original rust grades A-D. The designation given is AFl, BFl, CFl and DFl. There is only one flame cleaning standard for each rust grade. Three factors contribute to how flame cleaning works. 1 Expansion

All materials have different co-efficient of expansion. ie. all expand and contract at different rates per degree centigrade rise or fall in temperature. Millscale is chemically bonded to the steel and applied heat causes the materials to expand at different rates, thus breaking the chemical bond.



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2 Dehydration Water in the corrosion products and in the fissures etc. is evaporated away, facilitating the removal of the corrosion products.

3 Heat penetration

The heat is conducted efficiently into the substrate aiding the drying of the steel and removal of penetrated oil or grease. It is not wise to use this method of surface preparation on any fasteners relying on tension, eg rivets, screws, nuts and bolts.

2.16 Method

The operator slowly passes an oxygen/HC gas flame (butane, propane, acetylene) over the area to be cleaned, (weld preheat torches or specially adapted lances) to burn and de oxidise the corrosion products and other contaminants. This leaves a grey coloured ash deposit. A second operator follows on with a power brush to remove the now loose, ash deposits. The primer can now be applied over the warm steel, reducing the need for addition of thinners. Other benefits are that the heat reduces the viscosity of the paint and gives better flow properties. The paint can then 'wet out' better and pass into tiny cavities and irregularities on the surface. The heat also accelerates the drying process and keeps the steel above dew point temperature.

2.17 Pickling

Pickling is a general term relating to the chemical removal of oxides (rust), from a metal substrate. The metals can be either dipped (totally immersed) in the pickling fluid or sprayed with it. Usually aqueous solutions of acids are used for steel, they convert the oxides into soluble salts, eg sulphuric acid produces iron sulphate salts. Sulphuric is the most common acid used for economic and safety reasons. Footners duplex system involves the pickling process followed by a passivation process using phosphoric or chromic acid along with a small percentage of iron filings, which produces iron chromate or iron phosphate salts, which are not soluble. These form a rust inhibitive layer, which passivates the surface and increases the adhesion properties. They are also extremely resistant to cathodic disbondment. A typical process would be: 1 Any oil or grease needs to be removed by using a suitable solvent eg

xylene or as specified. Oil and grease show up as fluorescent yellow/green under an ultra violet light.



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2 Totally immerse in a bath of sulphuric acid, 5-10% concentration at a temperature of 65-70°C. Time can vary from 5-25 minutes depending on degree of contamination but is invariably at the lower end.

3 Rinse using clean warm water to remove the layer of soluble salts

formed. If required the component could be coated after pickling. Likewise components can be blast cleaned and sent on for phosphating/chromating, but the patented process is only called Footners when pickled then phosphated/chromated.

4 Immerse in a bath of phosphoric/chromic acid, 2% solution at 80°C for

approximately one to two minutes with iron filing (0.5%) (and an inhibitor to prevent embrittlement). This leaves a very thin layer of iron phosphate/chromate, which acts as a rust preventative for a limited time.

5 Rinse in clean water and check for pH values.

pH is a measure of acidity or alkalinity of a substance and is measured using pH indicator strips. An indicator such as litmus will only tell if a substance is an acid or an alkali. Indicator strips give a measure of acidity or alkalinity, based upon the scale below.

Figure 2.8 pH scale. This is a logarithmic scale and seven is neutral, the pH value of distilled water. From 7-0 the acidity increases and from 7-14 the alkalinity increases. A typical requirement after rinsing will be in the region of pH 4.5-7.0, slightly less acidic than household vinegar.

2.18 Vapour degreasing

Fumes from a solvent bath condense on a component suspended over the bath and dissolve any oil or grease, which then drips back into the bath. Very rarely used because of modern regulations regarding strong hydrocarbon solvents.

2.19 Weathering

Weathering relies on co-efficient of expansion properties as mentioned in flame cleaning. When left in a stockyard, open to temperature changes, day and night, the millscale sheds. This can now leave the steel open to atmospheric corrosion, which produces such as sulphate salts.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Alkaline Acid

Section 3

Surface Contaminants and Tests for Detection

Rev 2 June 2011 Surface Contaminants and Tests for Detection


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3 Surface Contaminants and Tests for Detection Any contaminants left on a prepared substrate will affect the adhesion of a coating to that substrate and therefore specifications often request that certain tests are done to ensure that contamination is within set criteria. Some tests are qualitative and some are quantitative. A qualitative test is one, which give a result as accept/reject, pass/fail, go/no go, whereas a quantitative test is one, which gives a result in known units eg milligrams/m2. Test for soluble iron salts This is a qualitative test, it will not even differentiate between the salts. It will detect the presence of either sulphates or chlorides. This test is known as the potassium ferricyanide test, although it is now under a new universal naming system, known as potassium hexa-cyanoferrate, a name more descriptive of its formula. Test papers, usually Whatman No.3 laboratory filter papers are soaked in a 5-10% solution of potassium ferricyanide and distilled water and left to dry. The result is a lime green paper, fringed with an orange brim. The area of blast to be tested is sprayed with a fine mist of distilled water, (any other water is likely to contain dissolved salts) and left a few seconds to allow the salts, if present, to dissolve and form a solution. A potassium ferricyanide test paper is then applied to the area and by capillary action draws up the solution like blotting paper. If there are any dissolved salts they react with the potassium ferricyanide to form potassium ferrocyanide. The ferrocyanide is Prussian blue and shows as blue spots on a lime green background. Test to detect soluble chlorides The test for detecting chloride salts is known as the silver nitrate test. As with the previous test a solution of silver nitrate, 2% with distilled water is made and the Whatman papers cut into strips. The strips are then soaked in the solution and pressed onto the area under test for about 20 seconds, then washed in distilled water. The reaction between silver nitrate and any chloride salts present produces silver chloride, which remains on the strip after washing. If the strip is then dipped into photographic developer the chlorides show up as black/brown.



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Other tests for salts 1 Merkoquant

A salts/distilled water solution is made and by swabbing an area of 150mm x 150mm, using approximately 22.5ml. Merkoquant strips, which consist of a small disposable plastic strip and a chemically impregnated cotton pad, are then dipped into the solution and the resulting colour change is compared to a master chart on the container. The concentration level of the salt contamination is read directly from the chart.

2 Bresle sample patch

A test with a reported accuracy of approximately 95%. An adhesive patch with a rubber diaphragm is firmly affixed to the substrate and distilled water is injected by hypodermic needle through the side of the patch. The distilled water is extracted and injected several times to produce a solution of any salts present. By measuring the electrical conductivity change in the solution, the level of salt contamination can be determined. It is a quantitative test.

3 Salt contamination meters

Salt contamination meters measure the resistivity or conductivity of a given sample and convert this value into a concentration (mg/m2).

With any of the above tests, if the amount of salts present is greater than specified, the area should be washed down with copious amounts of clean water, reblasted and retested. Test to detect the presence of mill scale Mill scale being cathodic in relation to steel can cause corrosion cells under a paint film and subsequent early disbondment. Millscale in small quantities is permitted on a SA2½ blast standard, but not on an SA3. Therefore the test needs to be carried out only if the specification requires an SA3. Blasted steel is dark grey in colour and mill scale is dark blue, so by naked eye the contrast is difficult. However, if the surface is sprayed with a fine mist of slightly acidic copper sulphate solution, the solution ionises and tints the steel copper colour and blackens the mill scale, if present, thus providing a better contrast. If this test indicates mill scale presence then it should be reblasted and then retested. Test to detect the presence of dust on a substrate Any dust on a blasted substrate will adversely affect the adhesion of a paint film. In conditions of low relative humidity, dust and finings passing down a blast hose become electrostatically charged and stick onto the substrate. Brushing or air blowing the surface will not remove them, self-adhesive tape however, will.



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If a piece of self-adhesive tape is stuck onto the surface and snatched off, the dust/finings sticks to the tape. By then sticking the tape onto white paper the dust can easily be seen. Test to detect the presence of moisture on a substrate Presence of moisture, even in the teeniest amount, can affect the choice of paints and if work can be done or otherwise. A very simple test for the presence of moisture is to sprinkle with talc or powdered chalk and then lightly blow away. The powder will stick to areas where moisture is. Test to detect the presence of oil or grease Other than ultra violet light, oil and grease can be detected by dropping solvent onto the suspect area and absorbing the solution on Whatman or blotting paper. The solvent will evaporate and oil or grease will give a darker appearance.

Section 4

Paint Constituents and Basic Technology

Rev 2 June 2011 Paint Constituents and Basic Technology


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4 Paint Constituents and Basic Technology Paint is a material which will change the texture colour or appearance of a surface and give some form of protection to the underlying surface. Paint has been classified in many ways eg by principle involved. 1 Barrier

The material forms a thick impermeable layer of a high electrical resistance eg urethane.

2 Passivation

Causing a chemical reaction between the paint constituents and the substrate eg rust inhibitive primers.

3 Cathodic protection

Employs the bi-metallic principles by using a less noble metal as pigmentation eg zinc in zinc rich primers.

By function eg Anti-fouling: Inhibits marine growth on ship hulls. Road marking: Gives white or yellow lines on roads. Fire proofing: Provides resistance to fire. Heat resistant: For surfaces working at high temperatures. Anti-corrosive and many more. Paints can be classified by binder type (the main constituent) or by colour and in some cases even by the pigment type. No matter which identification system is used, all the paints contain the same basic ingredients. 1 Binder. 2 Pigments and other additives. 3 Solvent (where applicable). It is the chemical structure and composition of these constituents, which gives the paints their own individual properties. Paints are supplied as either liquids or solids in powder form and can be subdivided into groups.

4.1 Liquid paints containing solvent

This group is still the largest in terms of sales. It is important to realise that solvent does not relate solely to Hydrocarbon solvents, but also includes water. Due to the modern environmental protection act (EPA) requirements, manufacturers are researching into new paint technology involving vastly reduced amounts of volatile organic compounds. Some are using water based technology, some are concentrating on the solvent free materials.



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4.2 Solvent free

As the name implies these materials contain no (or in some cases a minute amount of) solvent. These are generally chemical curing materials which require the mixing of two or more components and usually go under the name of MCLs (multi component liquids). Some MCLs are made using solvent borne materials.

4.3 Powders

Virtually solvent free MCLs, which are solid at room temperatures. The base resin and the chemical activator, along with the other constituents required to complete the formulation are heated up to the resins melting point, mixed into an homogeneous liquid, cooled and ground into powder form. In theory every particle contains all necessary ingredients to effect a cure into a protective film. The powder can be applied onto a preheated substrate (in the case of substantial steel thicknesses) at about 240°C or onto thin plate electro statically and post heated. In either case the powder melts undergoes a chemical reaction and in approximately three minutes the reaction is complete. The three subdivisions are all made up from the basic ingredients mentioned earlier, binder, solvent, pigment and other additives.

4.4 Binder

The binder is the main constituent of a paint and is often referred to as a film former. Other terms are vehicle and non-volatile. A paint binder is selected according to the function the paint has to perform within vastly different environments. Some major considerations of a binder are: 1 Ease of application (flow properties or viscosity). 2 Adhesion to the substrate for the expected life of coating. 3 Resistance to abrasion. 4 Resistance to chemical attack according to environment. 5 Cohesive strength, its ability to hold together as a film. 6 Dialectric strength. 7 Ability to resist the passage of water. 8 Ability to change from a liquid as applied, into a solid to provide the

above properties and others, for the expected life of the coating. Several materials satisfy the criteria above for different environmental conditions, among them are: a Acrylic

Synthetic resins can be used in HC solvents or water. Good colour retention, good film properties can be hybridised with other binders eg urethane modified acrylic as used by BG.



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b Alkyd A term derived from alcohol – acid reaction, usually associated more with the domestic market; they have a low resistance to alkalis. In common with most resins they are brittle and need modification with oils.

c Asphaltic bitumens

Petroleum based materials, thermoplastic, relatively inexpensive, known for water proof properties, poor resistance to sunlight, very low resistance to HC solvents.

d Cellulose resins

Synthetic material, not extensively used for industrial coating but a good example of reversible materials.

e Chlorinated rubber

Organic resins derived from reaction of rubber with chlorine. Widely used years ago until strict VOC regulations came into force. Especially resistant to alkalis and acids and were used on chemical plants, water treatment plants etc. Very poor resistance to hydrocarbon solvents, but can still be found in many locations and structures.

f Emulsions

Obviously not used for anti-corrosive systems but are included for other factors eg drying mechanisms.

g Epoxies

Synthetic organic resins generally provide good chemical, solvent and water resistance. Good exterior durability but are prone to chalking (discussed later). Epoxies come as two pack, single pack, solvent free and solvent borne.

h Ethyl and methyl silicates

Inorganic materials with excellent weathering, solvent and heat resistance. When cured the binder is a silicate, which contains a high percentage of zinc dust, thus protecting by galvanic action.

i Natural oils

Many natural oils can be used in the paints industry but because of their slow drying properties, cannot be used on their own as binders. They are mixed with resins to modify the film properties. Some natural oils used in the paint industry are linseed oil, tung oil (also known as China wood oil), soya oil, tall oil and safflower oil.

j Natural resins

Natural resins are brittle by nature and fast drying. As mentioned above they need to be mixed with oils to modify some properties. A mixture of oil and resin is known as oleoresinous. Examples of natural resins are copals, dammars and coumarones. Natural resins are not soluble in water.



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k Phenolic resins Made from phenol and formaldehyde, coal derivatives, characterised by excellent adhesion properties and resistance to heat and chemicals. Were used in temperature ranges where Chlorinated rubber couldn’t be used eg greater than 65°C. Commonly called hot drying oils.

l Polyurethanes

Can come in several forms, moisture curing, two pack polyurethanes, chemically curing and single pack. Industrial coatings are mainly the first two. They produce an excellent synthetic coating, with outstanding abrasion resistance, chemical resistance and good exterior gloss and colour retention with a minimum of chalking.

m Silicones

Designed as high temperature service materials for temperatures ideally above 150°C service temperature. Usually carbon or aluminium pigmented, they are used to seal inorganic zinc silicates or metal sprayed surfaces.

n Styrene

Sometimes referred to as a binder and is used to modify other properties. Styrene is referred to as a vinyl type monomer and is used to cross-link the film.

o Vinyl

The actual composition of the vinyl depends upon the designed end use, but in general has slightly better properties than a similar material, chlorinated rubber. However vinyls use a different solvent group and water. CR is limited to one solvent group.

4.5 Binder – solvent groups and compatibility

A solvent free binder, or a binder using a very weak solvent, will cause very few problems when over coating another product. Usually in this situation the problem would be limited to different expansion and contraction ratios. Providing a key by abrading can mostly rectify or at least minimise this. However, a very strong chemically curing binder like epoxy, needs a strong solvent and can cause problems over coating other materials, even when they are fully cured. Guide to binder solvent combinations. Emboldened name is the main one used.



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Solvent strength in descending order

Common names Binders

Water Emulsions PVC/PVA Vinyls Acrylics – other materials eg epoxy Bitumins, polyurethanes, Alkyds, acrylated rubbers

Aliphatic hydrocarbons White spirit Turpentine Turpentine substitute Solvent naphthas Hexanes upwards

Natural oils Natural resins Alkyds Phenolics

Aromatic hydrocarbons Xylene Toluene Benzene

Chlorinated rubber

Ketones Acetone Methyl ethyl ketone Methyl ISO butyl ketone

Epoxy

4.6 Polyurethanes use ketones and esters with aromatic

diluents.

In descending order down the table the solvent groups increase in strength. It is not advisable to use a binder with a strong solvent over an existing coating, which uses a weak solvent. For example chlorinated rubber coated over an alkyd would result in lifting and wrinkling, but alkyd over chlorinated rubber would have no ill effect. Because an epoxy is chemically cured, there is no problem over coating with polyurethane two pack, chemically cured, but a hydrocarbon solvent borne epoxy coating applied over chlorinated rubber would not be advisable. Ethyl and methyl silicates do not appear on the list because they are high (or low) temperature performance coatings, the criteria for compatibility with these materials for over coating is working temperatures. ie will the over coating material withstand the operating temperature? Usually the only material suitable is silicone. Ethyl and methyl silicates will not adhere over any substrate other than bare, clean steel. Any binder which can be converted into a polymeric salt can be modified to be water based and many of the binders mentioned above fall into that category. Chlorinated rubber doesn’t, neither can it be made using reduced amounts of solvents and therefore to comply with modern EPA requirements its usage now is limited, although many structures, both on and off shore are still coated in this material. It used to be the main material for ambient temperature usage for BG. The advantages of using this material were:



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1 Because of the chlorine content, high resistance to mould growth. 2 Again because of the chlorine, non-flammable after solvent release. 3 Very resistant to chemical attack eg acids and alkalis. 4 Very high resistance to water vapour transmission. 5 Material is non-toxic and provides a very durable film. 6 Very easily maintained, no abrasion required, clean surface only. Disadvantages were: 1 Its position on solvent compatibility list shows low resistance to solvents

ie only resistant to aliphatics and water. 2 Low temperature tolerance, 65°C maximum. 3 Spray application often results in cobwebs.

4.7 Polymers

One of the properties expected of a binder is to change from a liquid into a solid to form a film. To perform this function all binders form polymers or use polymers already partially formed. The word polymer means literally many parts, poly = many, mer = single unit or part. Mer (meras GK) can be a single atom, or a molecule, (a group of atoms) and polymerisation can be described as being a string or structure of repeated units and polymerisation is the joining together of a string or structure of repeated units. In the case of most paints the main constituents of the polymers are: H - hydrogen. C - carbon. N - nitrogen. O - oxygen. Cl - chlorine. Although there are variations, the main three polymer types are linear, branched and cross-linked.

4.8 Linear polymers

As the name implies the atoms or molecules which form the polymer, join on at the end of the structure and in so doing saturate the structure. This type of polymer is also referred to as a solution polymer. Figure 4.1 Linear polymer. Each shape represents a molecule (mer) joined to the next by a single line, an ionic bond, an electron joining to the next molecule. The process depends upon the properties of carbon, which forms the backbone of the structure. Carbon can give away electrons, take in electrons, share electrons, or join with itself in many ways.



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H | H – C – H | H

H H | | H – C – C – H | | H H

H H | | C = C | | H H

Methane saturated Ethane saturated Ethylene or ethyne unsaturated

The ethylene or ethyne molecule is defined as being unsaturated, the two carbons are sharing electrons, hence leaving potential for the spare electrons to combine with another molecule or radical. Figure 4.2 Ethylene molecules close together. The above figure represents ethylene molecules close together. The dotted line being the weaker bond (the secondary valency bond). This being the one that joins to the next molecule giving: Figure 4.3 Ethylene molecules polymerise. This gives saturation throughout the polymer, no activity points (polyethylene polymers, depending upon density vary from 25-35,000 molecules) and both ends are closed off with a hydrogen atom. It can be seen that linear polymers, once formed, cannot react with anything to chemically produce another compound and until destruction will maintain the same structure and properties. A linear polymer is a non-convertible or reversible material and also thermoplastic. From the binder types the linear polymers are acrylics, vinyls, chlorinated rubber, asphalt and coal tars and cellulosic resins.

H H | | C ..... C | | H H

H H | | C ..... C | | H H

H H | | C ..... C | | H H

H H | | C ..... C | | H H

H H | | C C | | H H

H H | | C C | | H H

H H | | C C | | H H

H H | | C C | | H H



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4.9 Branched polymers

Branched polymers are formed by combining oxygen with the double bonds available. Oxygen, from the atmosphere, a very reactive element, combines with a constituent of natural oils and resins called fatty acid esters. The double bonds in these fatty acid chains are not at the end of the structure, but in the middle. So any combination doesn’t occur lengthways to elongate the chain, but forms a branch from the main carbon backbone. Because of the abundance of reactive oxygen in the atmosphere, the branching carries on and on over several years until eventually the matrix becomes cross linked and very brittle and cracks and flakes off. Binders, which fall under this category, are natural oils and natural resins and isomers such as alkyds and phenolics. By combining with another element and chemically reacting to form another compound, these materials become non-reversible or convertible coatings, thermosetting. Figure 4.4 Branched polymer.

4.10 Cross linked polymers

Cross linking, or chemical curing is a three dimensional polymerisation process which occurs fairly rapidly using only components provided in the cans. Because the components are in calculated amounts the cross linking stops when all the available bonds are occupied. Some urethanes fully cure in 16 hours, some epoxies in three days and others in seven days, dependant on temperature.

| C H | H C H | H C H H

| C = C - C = C - C | | | | H H H H

OH

O

Oxygen Another chain



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Figure 4.5 Cross linked polymer.

4.11 Oils

Natural oils (vegetable oils) are produced from seeds of a plant, well known examples being linseed, castor, olive, coconut, soya and tung oil. In order to be usable as a paint binder the oil must be of a type that will combine with oxygen, ie it must be unsaturated. A saturated oil cannot be used as a binder because it will not solidify by polymerisation to form a film. Therefore, oils can be divided into three groups. Drying oils. Semi-drying oils. Non-drying oils. Drying oils Drying oils are oils which have three sets of double bonds along the carbon backbone and react with oxygen readily at ambient temperature. Semi drying oils Semi drying oils have one or two sets of double bonds and may need heat addition, or some other catalyst to promote oxidation. Non-drying oils Non-drying oils will not oxidise and therefore cannot be used as binders. Instead these are used as plasticisers in paint formulation, to modify properties of a resin. Although linseed oil and tung oil used to be referred to as rapid drying oils, the term rapid was compared to some other oils and in fact it could be many weeks before a reasonably resilient film was formed. Treated natural resins have the exact opposite properties, ie fast drying and very brittle. Oils and resins are mixed to give a binder with modified properties.



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Long oil paint - more than 60% oil to resin, elastic, slower drying properties suitable for domestic applications, decorative materials. Medium oil paint - between 45-60% oil to resin. Short oil paints - less than 45% oil to resin, faster drying material, suitable for steelwork. More brittle with shorter over coating time.

4.12 Pigments

Pigments have many properties and characteristics. They are derived from many sources, animal, vegetable, mineral and synthetically produced and can be in a wide variety of particle sizes and shapes. Pigments used in paints must remain as solid particles within the vehicle (the binder plus the solvent if a solvent is used) and not dissolve. If it dissolves it is known as a dye, not a pigment. Pigment particles contribute to the paint films strength cohesively, its abrasion resistance, durability, opacity, in some cases impermeability and resistance to ultra violet rays. Some pigment particles are as small as 1/10th micron. Pigments can be subdivided into groups according to the main function they perform in paint. Rust inhibitive pigments. Anti-corrosive Rust inhibitive pigments are added into primers to protect the steel substrate by passivation. Typical materials in the category are: h Red lead. i Calcium plumbate. j Coal tar. k Zinc chromate. l Zinc phosphate. m Barium metaborate. n Zinc phosphosilicate. Zinc phosphate is the most commonly used material from the list. The four marked with an asterisk are toxic and restricted in use. Red lead is a basic inhibitor and works in the presence of fatty acid esters in natural oils and resins only. These systems provide lead soaps, which give the actual inhibition. Metallic pigments Metallic pigments are also used on a steel substrate to protect the steel, but this time by cathodic protection.



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If a metal which is less noble than steel, (more electronegative) is included in the film and an electrolyte eg water, passes through the film, contacting substrate and pigment particles, then a circuit can be engaged whereby the pigment particles will receive the hydroxyl ions and thus suffer corrosion in preference to the steel substrate. In order to satisfy this requirement the metal pigment must be below the position of steel on the galvanic list. The two most amenable metals to satisfy this are: Zinc. Aluminium. Zinc is the better of the two for galvanic protection but aluminium is excellent for solar protection, reflecting the ultra violet A and B. For every other layer of paint other than the metallic primers, colouring pigments are used, usually know as opaque pigments. Opaque pigments Opaque pigments are inert particles with excellent light scattering properties in order to give covering power, (opacity) and colour.

1 Carbon Black

2 Compound of cobalt Blue

3 Compound of chromium Greens, yellows and oranges

4 Compound of iron Browns, reds and yellows

5 Compound of calcium Reds and yellows

6 Titanium dioxide White

Extender pigments Sometimes known simply as extenders or fillers, these materials provide some of the main properties expected of the film, such as adhesion, cohesion, film strength and durability. They also have a role in application and flow, levelling and other mechanical properties of the film and are an aid to inter coat adhesion and can reduce gloss. Materials used as extenders are usually low priced readily available materials such as: Clays eg Kaolin, China clay Chalk Calcium carbonate Talcum Magnesium silicate Slate flour Aluminium silicate Laminar pigments Plate like pigments such as MIO (Micaceous Iron Oxide), aluminium flake, glass flake, mica and graphite, provide excellent barriers. These pigments have a leafing effect and in theory overlap when the coating dries. MIO sometimes known as specular haematite is widely specified and to be



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regarded as pigment quality material quite often has to meet quite stringent requirements eg 85% of the total mineral compound has to be Fe2 O3, haematite, of this 85% less than 1% should be permeable to moisture, thus giving a paint film with high resistance to water permeation.

Figure 4.6 MIO in wet film and dry film. In theory when moisture passes into the film, on contact with the MIO platelet, it has to pass around it, thus almost doubling the distance to reach the substrate. Glass flake as a laminar pigment is usually for abrasion resistance, but in common with the others, improves the tensile strength of the film. Aluminium flake and MIO have good ultra violet A and ultra violet B reflectance properties, protecting the underlying binder from attack and subsequent degradation.

4.13 PVC

The pigment to binder ratio is a very important factor in the design and manufacture of paint and is known as the pigment volume concentration. There is an ideal pigment binder ratio, which varies from paint to paint, pigment to pigment and this is known as critical pigment volume concentration (CPVC). CPVC is defined in BS 2015 as the particular value of the pigment volume concentration at which the voids between the solid particles that are nominally touching are just filled with binder and in the region of which certain properties are changed markedly.



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Figure 4.7 Below CPVC Figure 4.8 Near CPVC Figure 4.9 Above CPVC Figure 4.7 Too much binder to solids ratio, would give a film of good gloss properties, but poor covering power (opacity) and with a tendency to blister (low cohesive strength). Figure 4.8 A film with lower gloss properties but greater cohesive strength and just enough resin to encapsulate each particle, giving good resistance to water permeation. Figure 4.9 The CPVC is exceeded and all particles are not wetted, the film would be porous, low in cohesive strength and adhesion.

4.14 Solvents

Solvents are added to paints to reduce the viscosity and ease application properties. The solvents used in paints have to fulfil various other requirements, for example if a solvent evaporates away too quickly the film will not dry evenly, if it evaporates too slowly drying will be protracted and on vertical surfaces the paint is likely to sag. The four important properties of a solvent are: 1 Solvent strength

Low molecular weight solvents are stronger than high molecular weight solvents and, strong binders such as epoxies and polyurethanes, need strong solvents to cut or separate the molecules. Hence Ketones and aromatics are used for these materials. Natural resins don’t have the same attraction between the molecules and therefore need weaker solvents, higher molecular weight, such as aliphatics.

4 Evaporation rate

The evaporation rate governs at what point the polymerisation starts. For decorative materials a long wet edge time is needed, so a long slow evaporation rate is needed, otherwise dragging and ropiness would occur when joining area to area. Industrial coatings need to dry quickly for protection and so that further coat can be applied.



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5 Flash point The flash point of a solvent is a safety consideration. Roughly defined as ‘the minimum temperature of the solvent at which the vapours given off are flammable if a source of ignition is introduced’. The higher the flash point, the safer the solvent.

6 Toxicity

Solvents, especially modern solvents, are substances hazardous to health and therefore have predetermined concentrations to which humans can be safely exposed. These limits are expressed in parts per million, ppm.

4.15 Other additives

Other than the main constituents of a paint viz, binder, solvent, pigment and extenders, there are approximately fifty other materials which can be added to give other, or alter existing properties. These can be grouped into aids to manufacture, aids to storage, aids to application, aids to film formation, aids to film curing and others. Some are used more than others, among them being. Anti-settling agents An anti-settling agent is an aid to shelf life. It is a thixotrope, a thickener, which also allows a higher film thickness. Thixotropic paints are jelly paints, non-drip and if stirred change to normal liquid consistency. When left they slowly revert to thixotropic consistency. Thixotropic agents are bentones and waxes and help keep solid particulate constituents in dispersion within the paint ie. stop settlement. Plasticisers A plasticiser basically gives paint flexibility and reduces brittleness and therefore needs to be compatible with the binder and have a very low volatility in order to stay in the film for a long time. Alkyd resin was used extensively in chlorinated rubber binders, but for natural resins and their isomers non-drying oils are used, saturated oils, which will not polymerise. Castor oil, coconut oil and some palm oils fall into this category. Driers Also known as oxidants, used in oxidising oils and resins. These are heavy metal salts, rich in oxygen, which are added to the paint during manufacture. Instead of relying on atmospheric Oxygen penetrating the paint layer, the oxygen is already there, to allow even through drying of the film. Common salts are octoates or naphthanates of cobalt, manganese and zirconium eg cobalt naphthanate. (The acids producing the salts from the heavy metals are octoic acid and naphthanic acid.)



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Anti-skinning Anti-skinning agents are also known as anti-oxidants. These are added to oxidising paints to retard the formation of a skin on the surface of the paint. If a skin forms it cannot be stirred back into a solution and must be removed. Because the anti-oxidant works against the oxidant they are added in very small controlled amounts and are liquids usually eg methyl ethyl ketoxime.

Section 5

Solutions and Dispersions

Rev 2 June 2011 Solutions and Dispersions Copyright TWI Ltd 2011

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5 Solutions and Dispersions Solutions A solvent is a liquid, which will dissolve another material, liquid or solid. A solute is the material dissolved by the solvent. A solution is the resulting liquid. Salt and water, sugar and water are solutions, a binder and solvent are also a solution. Dispersions A paint consists of solid particles suspended in the vehicle, where there is no solubility, so a paint is a dispersion. A dispersion can be either a solid or liquid dispersed within another liquid, where there is no solubility. A suspension A suspension is when fine particulate solids, eg pigment and extenders are dispersed within a liquid, the vehicle. Ideally after the manufacturing process, each particle should be completely wetted by the vehicle. However because the pigment particles are so small, they cluster together to form agglomerates or aggregates. In some paints, especially gloss, the size of these aggregates is a very important factor and so has to be checked. The aggregate size is known as degree of dispersion of fineness of grind. An emulsion An emulsion is a liquid dispersed in another liquid when there is no solubility. In vinyl or acrylic emulsion, very tiny droplets of resin are suspended within water, which can now be seen to be a non-solvent. In an emulsion water is a carrier, not a solvent. Water is called the continuous phase and oil/resin is called the dispersed phase.

Section 6

Drying and Curing of Paint Films

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6 Drying and Curing of Paint Films During the drying/curing process a paint changes from a liquid into a solid. It does this by various mechanisms and combinations of mechanisms. The time it takes to undergo this physical change is governed by several factors including temperature. Generally three terms are used to refer to drying/curing temperatures. a Air drying

This refers to normal ambient temperatures.

b Forced drying When heat is needed to effect a cure or accelerate the reaction it is called forced drying, but the temperature range for forced drying is ambient to 65°C.

c Stoving

When temperatures above 65°C are used, using ovens or infra-red, the term used is stoving.

Industrial paints, with a few exceptions eg intumescents, are generally in the air drying category and the liquid to solid transition is dependent on one of the following four drying mechanisms.

6.1 Solvent evaporation

Paints employing this drying mechanism are linear polymer materials, sometimes referred to as solution polymers. Solution polymers dissolve in the solvent, when the paint is applied the solvent evaporates away allowing the fully formed linear polymers, saturated, with no activity points, to come out of solution and form a film on the substrate. The polymers lie in a random interlocking pattern, similar to cooked spaghetti or noodles and loosely bond together by secondary hydrogen bonds. The solvents used by these materials are strong solvents and, when reapplied onto the paints, easily penetrate between the polymers and split the secondary bond, allowing the polymer to go back into solution. Materials, which can do this are, called reversible or non-convertible. Chlorinated rubber, vinyls, acrylics, cellulosic materials and lacquers fall into this category.

6.2 Oxidation

Paints using this mechanism form a film by oxidative cross linking (polymerisation) using atmospheric oxygen and in some cases, the oxygen contained in the driers. First of all if a solvent is present, the solvent evaporates away, allowing the oxidation to begin. Oxygen then combines with the unsaturated bonds on the fatty acid esters, progressively linking them together, to form the film. Once the oxygen has reacted with the binder, it has changed the chemical structure of the binder and cannot be removed. These materials are therefore convertible or non-reversible. Because oxygen is in abundance in the atmosphere the reactions continue,



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ad infinitum, until the materials crack and peel, having formed a very complex cross-linked matrix. Alkyds, phenolics, natural oils and resins are materials from this category.

6.3 Chemical curing

Chemical curing paints need addition of a second material, (in some cases as in moisture curing, water from the atmosphere) but generally the second material, the activator, is supplied in a can, hence the term two-pack or multi component liquid. In order to obtain the desired film the whole of the contents of both cans should be thoroughly mixed together and instructions on the materials data sheet should be strictly observed. Some materials will require an induction period and most data sheets will state the pot life. An induction period is the length of time after mixing which the paint should stand before use. Induction time is also called stand time or lead time and is recommended to allow thorough wetting of the solids. During the induction period the chemical reaction will commence and will be either: Exothermic reaction

Giving off heat, the container will warm up. Endothermic reaction

Taking in heat, the container will cool forming condensation. A typical induction period is 20-30 minutes. Pot life is the period of time after mixing in which the paint must be used and with industrial paints, dependant on temperature is usually 6-8 hours. After the recommended pot life the material becomes very user unfriendly and if in bulk, is quite often subject to spontaneous combustion. Two-pack materials curing agents Amides Epoxy curing agents, usually quote seven days to full cross linking at 20°C. Amines Epoxy curing agents, three days to full cross linking at 20°C. Isocyanates Mainly used for urethanes but also for some epoxies where low temperature application is unavoidable, -10°C being typical. Ambient temperature urethanes, especially for pipeline use quote 16 hours to full cure. Note: Isocyanates are very toxic and need great care during use. Chemically curing materials are convertible or non-reversible.



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6.4 Coalescence

Coalescence means to physically join together. In an emulsion the resin droplets are dispersed in the continuous phase, water. Upon application the water evaporates away allowing the resin droplets to come close together until they are touching. At this stage small amounts of high boiling point solvents are concentrated in the voids between the spheres, from where they migrate into the spheres, plasticise them and allow them to fuse together. In doing so they also increase the Tg of the material (Tg = gloss transition and is the temperature at which the material changes from a rubbery to a glossy solid and vice versa). If the Tg wasn’t changed, the resulting film would stay as a liquid and be easily wiped away. These materials eg acrylics and vinyls are reversible. It is important to remember in this case that water is not a solvent, but if the true hydrocarbon solvent was used the material would form a solution.

Section 7

Paint Systems

Rev 2 June 2011 Paint Systems


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7 Paint Systems A paint system is one or more layers of paint, which will give corrosion protection by one of, or a combination of corrosion protection methods. For example, a single layer of fusion bonded epoxy or urethane would give excellent protection employing the barrier principle. A zinc phosphate pigmented primer would be a passivation system but would need further protection in the form of a barrier system to protect it. An organic zinc rich epoxy would provide galvanic protection through bimetallic principles but would last longer with a barrier system to protect the zinc. Each layer within a system has a function to perform as follows.

7.1 Primer

A primer will not work as designed anywhere else other than in contact with the substrate. A primer, normally low volume solid materials, wets out the substrate and provides excellent adhesion and also provides a key for any subsequent layer. The binders usually have a relatively low resistance to vapour transmission and allow water into the film to carry tiny amounts of the rust inhibitive pigmentation onto the substrate to form a passivating layer. Older versions of BG specifications required that all primers should be brush applied. This was to ensure that any dust or detritus left on a substrate was worked into the film and not left lying where air could be entrapped, forming pinholes. Other primers exist for non-ferrous substrates such as Wash or Mordant primers and PVB etch primers. Mordant means of a corrosive nature, or will bite into and as suggested contains an acid, phosphoric acid. Their use is limited nowadays, mainly through the EPA requirements to reduce VOC emissions. These materials contain approximately 96% VOCs in the form of ketones and approximately 4% phosphoric acid, tinted with copper phosphate (blue). Their primary use was for etching new galvanising. The reaction turns the surface black (zinc phosphate salts). Some specifications allowed painting as soon as dry, but others required a water wash. Etchants do not leave a measurable thickness. PVB etch primers, Polyvinyl butyrol are principally used on Aluminium, but were used on virtually every non-ferrous metal. PVBs are two pack materials, low volume solids with a dry film thickness of 15 to 25μm. This material also contains phosphoric acid. The acid etches the aluminium (aluminium phosphate) provided a key for the vinyl binder. The general appearance when dry is a matt yellow translucent film, with an underlying black or darkened substrate. Some specifications require coating before 16 hours. Because of the acid content if is not wise to spray apply these materials without extraction facilities.



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Mid-coats Mid-coats are mainly barrier coats. They are applied over the primers to prevent further water passing into the film and leaching out the inhibitive pigmentation, without which there would be no passivation. Mid-coats also build up the film thickness and even out any irregularities. They also provide a key for any subsequent layer to adhere to. This is done by aggregates and extenders. Some extender materials have particle sizes of 40μm, if there is a high concentration of extenders in the coating then many of these large particles will protrude through the surface, increasing the area available for adhesion. Finishing coats Finishing coats of a system are mainly aesthetic, but also need certain properties. Colour and appearance are important eg gloss. To have a gloss finish the surface must be perfectly smooth and this also helps in the removal of dust and dirt and natural drainage or shedding of water. The storage facilities of volatile materials need to have solar reflective properties to reduce boil off and materials needing distillation require heat input and are very often black, to absorb heat. Moisture tolerant systems Pipelines transport many different products at different temperatures and pressures. Gas is transported in non-insulated pipes, over huge distances subsea and subterranean. Therefore the gas is cool. Where a pipeline comes above ground (an AGI, Above Ground Installation) the gas in the pipes is much cooler than the ambient temperature and condensation forms on the pipes, posing a problem for repainting and maintenance. Either the gas stream can be diverted along another route, or special materials can be used, tolerant of the situation. The BG National Grid specifications include a clause permitting the latter alternative, the use of moisture curing polyurethane or a high solid epoxy. (Section SPA4 in paragraph 10). Three definitions apply when referring to quantity of water present. Damp, moist and wet (paragraph 10). Damp and moist conditions will allow the use of the materials specified, but wet conditions require excess water to be removed. Single pack moisture curing polyurethanes are materials which use moisture from the atmosphere to cure, not standing water on the substrate. Surface preparation as per the specification, then any excess water should be swabbed off, before brush application of the material. Because the material cures by using air borne moisture, as soon as the lid is removed from the can the cure reaction starts. The more moisture there is present in the atmosphere, the faster the cure. The criteria with this type of material is not high RH, 100% is no problem, but low humidity. Some manufacturers state 35% as minimum RH criteria.



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Powder coating materials As mentioned earlier, powder coatings are solvent free materials, which are solid at room temperature. Thermosetting Thermosetting means the material will cure with the application of heat and therefore are convertible or non-reversible materials like epoxy and urethane. With thick steel sections like underground pipes the powders are electrostatically sprayed onto a preheated substrate, approximately 245°C, as soon as the powder hits the heated steel, it melts, undergoes a chemical cure and is fully cross linked in approximately three minutes. This group of materials is used extensively on subsea and subterranean pipes, office furniture and kitchen white goods. Thinner plate sections are post heated, after electrostatic application of powder. Thermoplastic Thermoplastic materials soften with the application of heat, are linear polymer and therefore reversible or non-convertible, polyethylene and polypropylene being examples. Usually flame sprayed as repair systems on existing thermoplastic coatings. Sacrificial coatings As the name implies, this classification of materials sacrifices itself to protect the underlying substrate. In order to work the sacrificial component must be less noble (more electronegative) than the substrate which it is protecting. Zinc and Aluminium are the most common materials used to protect ferrous substrates. Zinc and Aluminium have relatively low melting points and so are commonly used in the form of metal spray, applied by flame onto structural steel eg bridges, as an added form of protection which purportedly can extend the major maintenance free life of steel work by as much as 20 years. Zinc is used in hot dip galvanising of steel, to totally encapsulate a section. In this situation the zinc works as a barrier coat initially and undergoes atmospheric corrosion itself forming corrosion products such as zinc sulphates and zinc carbonates. To stop this natural process on the zinc it is usual to paint over the galvanising. However, if the galvanising is damaged, exposing the steel underneath so that both metals are in contact with electrolyte, the zinc then starts working sacrificially, corroding in preference to the steel, producing zinc oxides on the damages faces until the damage is filled to exclude electrolyte contact. The zinc then works as a barrier again. If the galvanised coating suffers damage of more than a scratch or gouge, a repair might be a better option. In this instance a zinc rich epoxy might be used. These materials contain a very high percentage content of zinc pigment. Specifications vary but 90% by weight of the dry film is a typical requirement. If moisture, an electrolyte, passes into a film of this nature,



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each particle of zinc needs to be in contact with at least one other, in order to form the metallic circuit through to the steel for the electrons. These electrons, in the form of hydroxyl ions will then return through the electrolyte to the zinc and the zinc will corrode, sacrificially. In order to hold the high concentration of zinc particles together, a very strong binder is required. This is usually an organic epoxy. Inorganic binders such as ethyl or methyl silicates are zinc pigmented but are primarily designed for high temperature service and need sealers such as aluminium or carbon pigmented silicones.

Section 8

Water Borne Coatings

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8 Water Borne Coatings Compliant is a term often used nowadays and refers to a material, which complies with COSHH Regulations and EPA requirements. Progressively, year by year, stricter regulations are brought into force regarding solvent emissions into the atmosphere. For example a 60%VS (volume solids) paint using a hydrocarbon solvent will release 400cc of solvent into the atmosphere for every one litre of paint applied irrespective of thinners added and cleaning solvents used. Hydrocarbon compounds are known to be harmful to the environment, the ozone layer and human life. Paint manufacturers have therefore taken steps to comply with these requirements by using alternatives, in the form of solvent free, high volume solids and water borne. Many binder types can now be modified to use water among them being. d Alkyds. e Epoxies. f Polyesters. g Polyurethane. h Vinyls. i Acrylics. j Silicones. Every material has advantages and disadvantages. Water as a solvent, poses no problems with compatibility over any other material but may prove problematic for adhesion. Abrasion will almost certainly be required, but generally the following will appertain. Advantages Disadvantages

Water is of a suitably low viscosity for any application method, brush, roller or spray

Water usually needs a small amount of a co-solvent for modification

Water is recyclable cheap, abundant, non-toxic and non-flammable

In periods of high humidity drying will be retarded

Water is not harmful to environment, the ozone layer or to mankind

Needs controlled storage conditions, in low temperatures certain components may come out of solution

Water can be applied over any existing binder type with impunity

Not as versatile as HCs for application windows

In good conditions several coats can be applied in one working day

Section 9

Paint Manufacture

Rev 2 June 2011 Paint Manufacture


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9 Paint Manufacture Paint manufacturers buy ingredients from many sources, mix them and process them into their own formulations, which they then sell on as their paint. Part of this manufacturing process is grinding aggregates and agglomerates down to a suitable size for the paint type being processed. For example a gloss paint with a dry film thickness of 30μm would need an aggregate size of far less than 30m, typically 20m or in some instances 10m, because an aggregate of larger size than the nominal film thickness would protrude and deflect light. Whereas an undercoat or mid coat would require a larger degree of grind (some extender have 40m particle size to aid with cohesion and inter coat adhesion). Paint manufacture basically involves three main stages, once all constituents are available: 1 Premixing

Pigment/binder/solvent are mixed in proportions suitable to give a consistency of premix or mill base, suitable for the machinery to be used in the next part of the operation.

2 Dispersion or grinding or milling

The actual dispersion or grinding or milling of the paste from the above. 3 The letdown process

Where the remaining amounts of binder/solvent and any other additives are finally and thoroughly mixed prior to quality checks and canning.

9.1 Direct charge dispersing mills

Ball mill A ball mill is a horizontal steel drum, typical dimension 1m diameter x 1½m long, which is approximately half-filled with various types of balls. Steel balls for darker colours and porcelain or selected flint for lighter colours. The balls are approximately 1-1½ inch diameter. Mill base is added to the drum until the balls are covered, about 50% capacity of the drum. The hatch is then sealed off and the drum started rotating at such a speed so that the balls cascade down and do not stick on the drum due to centrifugal forces. Shear forces are applied to the mill base as the balls cascade both between the balls and balls and vessel walls. A typical dispersion time would be overnight and not the preferred system for paint making due to the time taken. Attritor mill The attritor mill is a vertical version of the ball mill, but more efficient and also static. The balls are driven by paddles. The mill base is continually circulated by pump from bottom to top and gives adequate dispersion in less time. Used to be regarded as a fixed charge M/C but largely modified now for continuous use.



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Figure 9.1 Ball mill. High speed disperser Sometimes called a high speed dissolver, this piece of equipment can be used for mill base production or complete batch production, but mainly for the former. It is analogous to a large food mixer with a flat toothed impeller blade at the end of a shaft. Dispersion is achieved because of the extreme turbulence that occurs at very high shaft rotation speeds near the impeller blade. The mill base produced then undergoes a further process in a bead mill (sand mill or pearl mill are alternate names). Figure 9.2 High speed disperser. Kady and Silverson mills Both the Kady and Silverson mills are suitable for rapid dispersion of aggregates in aqueous emulsions and other water borne material.



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Sand mill Also known as a bead or pearl mill, the sand mill is particularly suited to long production runs on popular paint colours and is still a common method of paint manufacture. The mill base is pumped under pressure up through the vessel which is partially filled with sand or other grinding mediums. Through the centre of the vessel runs a shaft with fixed discs, which causes the abrasives to be moving constantly. As the mill base passes through this moving abrasive, it is subject to shear dispersion. As the paint exits at the top it passes through a fine screen, which retains the abrasive in the vessel. A cold water cooling jacket is needed because of the heat generated by friction.

Figure 9.3 Sand mills.



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Colloid mill Also known as high speed stone mills, usually fairly small, using stone grinding discs containing carborundum, approximately 10 inch in diameter. The top stone is stationary and the lower stone is rotating fast at speeds up to 3600 revs per minute. Gravity fed low viscosity slurry enters the centre of the static top stone and is passed between the two stones by centrifugal force, where it is subjected to extreme turbulence and shear forces to affect the dispersion.

Triple roll mills Three rollers made from chilled steel or granite, run parallel to each other and each one rotates at a different speed and each contact face passes in the opposite direction to the adjacent roller. The gap between them, the nip, can be adjusted. These machines need a thick paste like mill base to operate efficiently. The mill base is fed into the nip between rollers one and two and the final product is taken from roller three by means of a scraper bar.



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Figure 9.4 Triple roll mill. Single roll mills This system utilises a single chilled steel roller. Mill base is gravity fed from a hopper into a small gap between a longitudinal bar and the rotating oscillating roller. The material is thus subjected to shear and dispersion. The bar can be adjusted to control the gap by screws or hydraulic pressure along the length of the bar. There are two types of bars which can be operated, a single roll refining bar and a recessed bar. The final product is removed by scraper bar.

Figure 9.5 Triple roll refiner.

Section 10

Testing of Paints for Properties and Performance

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10 Testing of Paints for Properties and Performance National Grid Specification No PA9, lists a number of tests, (and required results), which a paint must be subjected to and comply with before acceptance as a material suitable for use on a National Grid site. BS 3900, methods of test for paints, is the British standard which details these tests, for method of test and equipment. It is subdivided into groups of tests from group A, tests on liquid paints (excluding chemical tests), through to group H, which covers defects and rating of. The following tests are to PA9 requirements.

10.1 Tests on paint

Determination of volatile, non-volatile This test, to BS 3900 part B2, can only be a guide and not 100% accurate, as it relies on solvent evaporation from a test sample. As soon as the can is opened the evaporation will start. A typical procedure would be: Select a clean, thoroughly dry glass stirring rod and watch glass and

weigh on a sensitive balance to the nearest milligram. Place onto the watch glass approximately 2gm of paint and weigh again. Place the watch glass with paint into a hot air oven, no naked flame or

element; repeatedly stir to drive away the volatile content. Take a final weight of the glass, rod and dry paint and simple

calculations will give volatile/non-volatile ratio by weight. Flash point determination As per BS 3900 part A8, using a closed Abel cup (as opposed to the open cup). Flash point is defined as being the lowest temperature at which solvent vapour from the product under test in a closed cup, gives rise to an air/vapour mixture capable of being ignited by an external source of ignition and is a safety factor. A high flash point material is safer than a low flash point material and would be determined as follows. Add solvent to the Abel cup, replace lid with thermometer and agitator in

place. Clamp the Abel cup onto a retort and lower into a water bath. Gently heat the water bath, which will in turn heat the solvent under test. Every ½°C rise in temperature activates the high frequency spark. The flash point temperature is reached when a blue flame flashes over

the solvent. An orange flame signifies that the flash point temperature has been exceeded and the test should be redone.



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Figure 10.1 Abel closed cup.

Agitator

Spark electrode

Retort

Water bath

Thermometer



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10.2 Paint density

Defined as being weight per unit of volume, density is calculated by weighing a known volume of material and using the formula:

Volume

WeightDensity

When imperial units were used density could be expressed as being pounds (lbs) per gallon. However metric units are now standard and the units for density are grams per cc. 1cc (cubic centimetre (cm3) of water weighs 1 gram. 1 litre (1000cc) of water weighs 1 kilogram. A density cup with a capacity of 100cc is used for measuring density of paint. Other names referring to the same cup are: 1 Relative density cup. 2 Specific gravity cup. 3 Weight per litre cup. 4 Weight per gallon cup. 5 Pyknometer. Figure 10.2 Density cup with lid chamfered to centre vent on underside.

100cc



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Procedure for use Weigh the clean, empty cup and the lid on a metric scale, sensitivity

0.1g. Fill the density cup with the paint, to within approximately 2mm of the

brim. Allow any trapped air bubbles to burst and replace the lid slowly and

firmly until it seats firmly on the shoulder of the brim. The chamfer in the lid allows air to be expelled as the lid is replaced, in

addition to paint in excess of the required 100cc volume. If no paint is expelled more paint can be added.

Wipe off any excess paint from the vent and weigh the filled cup. Deduct the weight of the empty cup from the final weight and divide by

100. The answer is the density in grams/cc. From information given on the materials data sheet and calculated density of the solvent it is possible, but difficult, to calculate the percentage of any added solvent, although better and easier ways exist. This piece of equipment however can be used in calculating if a two pack material has been mixed in the correct proportions. Relative density or specific gravity The density of distilled water is known to be 1gm/cc and the density of any other material can be calculated as above. Relative density or specific gravity is in effect comparing the density of another material with that of water using the formula:

waterofDensity

xofDensityRDorSG

Because relative density is comparing and giving a value of times heavier than, there are no units of value, but the digits will be exactly the same as density. Example If five litres of paint weight 7.2kg, what would be the density? Step 1 Convert units to grams and ccs. 5Lx1000= 5000 cc 7.2kg = 7200g Step 2

cc5000

gms7200

Vol

WTDensity

Step 3 Perform calculation = 1.44g/cc



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Step 4 Therefore SG or relative density would be 1.44 Example for two pack ratio calculation A two pack epoxy is mixed at a ratio of five parts base to two parts activator, the given densities of which are pack A 1.25g/cc and pack B 0.97g/cc. What is the density of the mixed paint? Five parts base at 1.25g/cc = 6.25g Two parts activator at 0.97g/cc = 1.94g Therefore total weight = 8.19g Total volume for weight = 7cc

Density of mix = 7

19.8

= 1.17g/cc

10.3 Hegman grind gauge

The Hegman grind gauge, also called a fineness of grind gauge, is used to measure the degree of dispersion of paint. Aggregates are present in all pigmented paints, but only the largest aggregates are of any concern. With gloss paint a perfectly smooth surface is required, so any aggregates within the paint should be substantially below the dimension of the film thickness. The Hegman grind gauge is a highly polished stainless steel block measuring approximately 17.5cm x 6.5cm x 1.4cm. Two grooves, (on some gauges a single groove), are precision machined, producing a taper of 100m deep to zero m along the length of the gauge. A 10m increment scale is engraved along the length of the groove, representing the depth at that point. Paint is added to the deepest point of the scale and drawn along to totally fill the groove using a specially profiled scraper bar. The specification BS 3900 requires that within three seconds of this operation the scale should be placed so that the eye looks almost parallel along the groove, very obliquely, to observe a point along the groove where, within a 3mm band, five to ten aggregates break through the surface of the paint. This actually, looking at the stated angle is the point where the surface will change from gloss, at the deep end, to matt at the shallow end.



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Figure 10.3 Hegman grind gauge. Figure 10.4 Cross-section of a Hegman grind gauge AA. Along the groove, at some point, the aggregates will rest along the bottom and protrude through the surface giving a result as below. Figure 10.5 Aggregates protruding and resting on the bottom.

3mm band

Gloss Matt

Aggregates protruding

Aggregates on the bottom

0 20 40 60 80

10 8 6 4 2

Cross-section below

AA

Gloss Matt



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10.4 Viscosity

Viscosity is a very important property for paint; it affects the manufacturing process and application and levelling properties. Viscosity is defined as being a fluid’s resistance to flow. Therefore a liquid described as being of a high viscosity is one which has a high resistance to flow, it will not run easily and conversely, a low viscosity fluid runs very easily. An increase in temperature (or decrease) can have a severe effect on a fluid’s viscosity and therefore comparative tests should be done at the same temperature. As the temperature increases the molecules within the paint gain more molecular freedom, move more easily and thus reduce viscosity. A typical recommended temperature is standard laboratory temperature of 20°C 0.5°C. There are several types of equipment available for measuring viscosity but they mainly fall into two categories. 1 Rotational viscometers. 2 Flow viscometers. Rotational viscometers Rotational viscometers rely on a paddle, disc or ball rotating in a liquid to measure the viscosity. The rotation can be driven by an electric motor, which gives dynamic viscosity measurements, or by falling weights which gives kinematic viscosity measurements. Dynamic viscosity For dynamic viscosity measurements a rotothinner can be used. Figure 10.6 Rotothinner. The rotothinner, a flat circular disc with four holes drilled transversely through it, is fixed into the chuck of the rotational viscometer (not unlike a pillar drill) and lowered into a 250 millilitre can containing the fluid under test. The can is magnetically attached to a spring loaded conical shaped base. When the disc enters the can, a micro-switch engages the motor and starts

Poises



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the disc rotating. When the rotating disc enters into the paint the frictional forces between the disc and the paint molecules and the can cause the can to rotate, which in turn tensions a spring in the base. When the two equalise the can will stop rotating and a reading can be taken from the pointer on the scale on the conical base. The Systems International (SI) units for dynamic viscosity are, newton-second per square metre (N.s/m2) although on many machines the poise is still used (cgs. unit). A poise has ten subdivision called centi-poise. Water has a viscosity of approximately one centi-poise. One poise is equal to one dyne second per cm2. Kinematic viscosity Figure 10.7 Krebs stormer viscometer. Kinematic viscosity is measured using a Krebs stormer viscometer. The weight is allowed to fall, which in turn causes the paddle to rotate in the paint. More weight added results in a higher rotation speed. Weights are added until the rotation speed is 200rpm as measured either with a stroboscope or digital display counter. A viscosity unit frequently used for kinematic viscosity is the stoke and centi stoke. A fluid having a viscosity of one poise and a density of 1g/cc has a viscosity density ratio of one stoke. (Krebs units or poise can also be used.) Flow viscometers (flow cups) There are various types of flow cups eg Zahn and Frikmar, used for hot fluids, Ford, ISO and DIN used for ambient temperature materials. The ford cup being the most widely used for industrial paints. The flow cup is machined from aluminium, has a capacity of 100cc and is fitted with a stainless steel nozzle at the bottom with various orifice sizes, in

Weight



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millimetres. For use with industrial paints a 4mm hole size is standard and known as A Ford Flow Cup No4. The cup is mounted on a special stand and has a lid with a bubble spirit level. The triangular base of the stand has one fixed foot and two screw adjustable feet, to facilitate the levelling of the stand and cup. A typical procedure for use would be: 1 Ensure that the equipment and paint temperatures are at 20°C 0.5°C. 2 Level off the equipment using the bubble level and adjustable screw

legs. 3 Put the lid to one side when levelling is complete. 4 Place a suitably sized receptacle under the orifice (greater than 100cc). 5 Place a finger over the nozzle orifice and fill with the paint to be tested,

up to the brim, leaving a convex meniscus. 7 Using a straight edge (a ruler) quickly scrape excess material into the

overflow rim on the top of the cup. 8 Simultaneously start a stopwatch (or use sweep second hand) and

remove finger from the nozzle. 9 The paint will run from the orifice in a continual stream. At the first

distinctive break in the stream ie when it drips, stop the watch. The time in seconds is recorded as the viscosity, at the measured temperature.

Thinners added to paint over and above recommended quantities could also be determined by viscosity. To do this a sample containing maximum amount permitted (by manufacturers TDS) is prepared and compared to samples taken from the operators at the point of application. Using the flow cup, if the operators sample runs through the cup faster than the reference sample, then more thinners than allowed has been added. To find the exact percentage added, small amounts can be added to the reference sample until operator’s sample and reference sample run through in the same time. Should the operator’s sample take longer than the reference sample, then there is no problem. Thixotropic paints cannot be measured using a flow cup.



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Ford flow cup, no.4 in stand Flow viscometer in use

Section 11

Film Thickness

Rev 2 June 201 Film Thickness


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11 Film Thicknesses Wet film thickness measurement From information given on a specification and the technical data sheets (TDS) correct application thickness can be calculated. If regular checks of wet film thickness (WFT) are carried out and found to be adequate, it gives added confidence that upon checking the following day, the dry film thickness (DFT) should meet specification requirements and hopefully eliminate major rectification. Wet film readings should be taken immediately after application, in order to obtain true readings (solvent starts to evaporate away as it exits the spray tip). WFTs can be measured by using either an eccentric wheel, or comb gauges. Eccentric wheel An eccentric wheel is a steel disc, machined to cut two grooves leaving three rims. The centre rim is machined smaller than an eccentric to the two outer rims. The inner rim is called the eccentric rim and the two outer, the concentric rims.

Figure 11.1 Eccentric wheel. A scale is engraved on the outer surface of one side of the wheel giving degree of eccentricity at any point. To use the wheel it should be placed on the surface with the zero at the six o’clock position, rolled through 180° in one direction, back to the zero and then 180° in the opposite direction, back to zero. The concentric outer rims will be wet for the full circumference, but the inner rim, The eccentric rim will only be wet for part of the circumference, having left and re-entered the film on two occasions. The wet film thickness value is taken by transferring (mentally) the interface between wet and dry on both



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sides of the eccentric rim into a value from the scale. The average of the two values is the WFT of the paint film. It should be noted that the eccentric wheel can only be used on flat plate. On a pipe, for example it would be used circumferentially.

11.1 Comb gauges

Comb gauges are supplied in many forms, square, rectangular and triangular, in metal and in plastic. Disposable plastic gauges will be supplied in small boxes containing several hundred. Stainless steel gauges are supplied in sets of four in a leather wallet. However all comb gauges are used in a similar manner. Assuming use of the SS gauges, four gauges will each have two working ends covering eight different WFT ranges. Above each tooth is engraved a value thou on one side and its equivalent in microns on the other side. This represents the value of the gap from tooth end to substrate when the gauge is place firmly, perpendicularly onto the substrate. When the gap under the tooth is full of paint it will wet the tooth. When not full it will not wet the tooth. A procedure for this operation would be: a Select the appropriate gauge with the smallest increment rise tooth to

tooth. b Apply the gauge firmly, perpendicular to the substrate into the paint film

ensuring that the two end lands are firmly on the substrate. c Withdraw the comb gauge and look at the teeth. d Two values should be recorded. The number above the last tooth wetted

by the paint and the value of the next highest not wetted. The WFT is not an absolute value but in between. Note: Comb gauges should be used longitudinally on curved surfaces eg pipes.



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Figure 11.2 Set of comb gauges. WFTs can be calculated by using the following formulae, according to information given.

DFTVS

100WFT

Area

Volume

A

VWFT

Figure 11.3 Contraction from evaporation.

Solvent WFT Solvent

Binder

Pigment, extenders and others

DFT Volume Solids %

Solvent %



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11.2 Tests done on dry paint films

Dry film thickness The specification for a painting contract will state a DFT criteria for each coat of paint applied. As it is the inspector’s main function to ensure that work is carried out to specification, he/she should perform as many checks as needed to ensure that the specification criteria is met. The DFT value can be determined by one of four methods. 1 Test panels. 2 Calculations. 3 Destructive test gauges. 4 Non-destructive test gauges. Test panels Test panels are usually 150mm square plates of the same material as the component being processed. The plates undergo the same operations at the same time as the main components. Mainly used for destructive tests eg adhesion, they can also be used for DFT checks. Calculations Using certain formulae and information given on a materials data sheet, in conjunction with values determined from WFTs for example, calculations can give us the unknown values. Four formulae can be used according to information provided.

1 WFT = _V_ A

2 WFT = _100_ X DFT VS 1

3 DFT = WFT x VS 1 100

4 VS% = DFT x 100 WFT 1



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Examples of which are as follows:

1 WFT = V A

Question: If 12 litres of paint was used to cover an area of 10m x 10m what would be the average WFT? Answer:

Volume 12 litres 12L

Step 1 WFT = = =

Area 10 x 10 100m2

Step 2 Change to units common to volume and area = cm2 cm3

12L x 1,000 12,000cm3 = = 0.012cm

100m2 x 10,000 1,000,000cm2 Step 3 Multiply x 10,000(µm/cm) = 120µm

100 DFT

2 WFT = x

VS 1

Question: What WFT would be needed to give 50μm DFT using a paint with a VS% of 65%? Answer:

100 DFT 100 50 5000

WFT = x = x = = 76.92μm

VS 1 65 1 65



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WFT VS

3 DFT = x

1 100

Question: What would be the DFT if a paint with a VS content of 45% was applied at 120μm WFT? Answer:

WFT VS 120 45 5400

DFT = x = x = = 54μm

1 100 1 100 100

DFT 100

4 VS% = x

WFT 1

Question: What would be the VS% of a paint if it was applied at a WFT of 110μm and the DFT was 63μm? Answer:

DFT 100 63 100

VS% = x = x = 57.27%

WFT 1 110 1

5 Calculation for paint volume in litres is

WFT

V = Area x

1,000

11.3 Destructive test gauges

As the name implies these types of gauges cause damage to the film which then needs to be repaired. If a specification required a magnetic gauge to be used to measure a coating including micaceous iron oxide (MIO), in theory it can't be done, MIO is magnetic and would cause error in the reading. In this



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instance a destructive test gauge might be specified or it may be required to monitor closely the WFT and calculate (as above) the DFT. A PIG, (paint inspectors gauge) is a type of destructive gauge. A reference line of a contrasting colour is drawn on the painted surface to be tested. A blade is tightened into a special slot in the PIG, pressure applied to force the blade through the paint to the substrate and then cut across the reference line, leaving a damage about ⅜" it is then possible to examine the damage through a focusable microscope. Measurements can be taken by means of a graticule scale engraved on one of the lenses.

Figure 11.4 Paint Inspectors Gauge (PIG). The dimensions taken from the graticule scale at this point are not in any units. As the angle of the cutters used can change, so will the representations of the graticule. A chart is supplied with each gauge and blades of different angles. If for example the chart indicates Blade No3 will be ground to x angle, can be used on thickness less than 500m, multiply graticule reading by 1.8, 20 unit of graticule scale would then convert to 20 x 1.8 = 36m.



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Other commonly used destructive gauges are the Ericson test drill and Saberg thickness drill. The damage caused with this is circular.

11.4 Non-destructive test gauges

This category of gauges is the most widely used and can be subdivided into electronic and magnetic. Electronic The electronic gauges work mainly on two principles: electromagnetic induction and eddy current. The electromagnetic induction is suitable for ferro-magnetic substrates and the eddy current is suitable for non-ferro-magnetic substrates. Modern electronic gauges are sometimes supplied with probes suitable for both situations and the gauges automatically change function according to the fitted probe. Both types are for measuring non-ferro magnetic coatings. Accuracy ½%. Magnetic This classification of gauges works with permanent magnets, no batteries. The simplest of these is: The Tinsley pencil or pull- off gauge Sometimes called a foreman's gauge is suitable for spot checks and is not very accurate, even on modern gauges of this type 15 % accuracy is quoted. It looks very much like a pen and indeed is sometimes fitted with a pocket clip. It has a permanent magnet attached to a spring. The tension of the spring can be adjusted so that the gauge can be calibrated to work over a variety of thicknesses.

Screw to adjust tension

Spring

Cursor line

Permanent magnet

Scale



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Figure 11.5 Cross section of tinsley pencil often known as the foreman’s gauge.

Figure 11.6 Magnetic horseshoe gauge, ideal for TSA due to the hot substrate. The magnetic horseshoe gauge is a very old type of gauge still favoured for measuring hot surfaces such as metal spray. Accuracy often quoted as better than 10% and as for all magnetic gauges, it is suitable for use in hazardous areas. This gauge works by measuring the change in magnetic flux between two magnetic poles at the bottom of the gauge. The flux change is brought about by the thickness of the non-magnetic coating. The gauges are supplied in a wide variety of scales and are calibrated like all magnetic gauges. The magnetic coating thickness gauge, known colloquially as the banana gauge, measures non-ferromagnetic coatings over ferromagnetic substrates and can, according to the manufacturer even be used under water. This type of gauge relies on spring tension to break the magnetic attraction of a permanent magnet to a ferromagnetic substrate. Because spring tension doesn't have a linear function the scales on the gauges are in logarithmic increments. When calibrating for use it is therefore of paramount importance to calibrate using a shim as near as possible to the paint thickness. Modern gauges of this type often quote 5 % accuracy. Procedure for calibration to BS 3900 PT C5 (now ISO 2808) (National Grid specify calibration on a prepared surface, therefore a plate with the same substrate surface finish as that to which the paint is applied, should be used).



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It is extremely important to remember that should the gauge be calibrated on a flat plate, the reading on a blasted surface would take from approximately ⅔ of the depth of the profile, giving values of up to 50μm more than the actual 'over the peak' value. 1 Select a plastic shim (magnetically insulated) as near as possible in

thickness to that of the paint to be measured. 2 Place the shim centrally on the calibration plate, as detailed above. 3 Locate the magnet in the gauge onto the shim, apply a light pressure to

ensure that the heel doesn't wobble or rock and wind the scale wheel on the gauge fully forward to release all tension on the spring allowing the magnet to attach to the substrate.

4 Wind the wheel slowly back, clockwise, tensioning the spring until the magnet detaches. At this point the movable cursor on the gauge is adjusted so that the red line on top of the cursor is in line with the thickness value of the shim as shown on the scale wheel.

The gauge is now ready to use. Some banana gauges do not have a movable cursor. Instead these have a fixed cursor, moulded into the case and a movable scale and to calibrate these gauges, the value of the shim on the scale wheel has to be moved to the cursor.

11.5 Tests for mechanical properties on paint films

Abrasion resistance Ericson, Taber and Gardner are just three of many companies who manufacture specialist equipment for testing paint films. A material’s resistance to abrasion can be tested using a Taber rotary abraser. Discs painted with the material to be tested are rotated under special abrading wheels. The abrading wheels can be of various compositions, depending upon the degree of abrasion required. For example, sand paper or carborundum. Periodically the samples can be checked for thickness or damage inflicted. Hardness The hardness of a film can be tested by many methods including the Buchholz indentor and the Sward hardness rocker, but one of the most frequently used for hard coatings is the Koenig Albert. A pendulum with two spherical fulcra is free to swing on a plate painted with the material under test. The number of swings is counted electronically. (If the fulcra penetrate the surface, more resistance will reduce the number of swings).



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Flexibility BS 3900 E1 Standard panels are coated with material to be tested and bent around cylindrical mandrels of various diameters. The flexibility of a coating is expressed as the smallest diameter mandrel over which the paint will not crack when bent. A conical mandrel with a uniform taper from 3-37mm diameter is frequently used now. The conical type needs only one sample to achieve a result whereas the straight mandrels need a plate for each mandrel. Impact resistance Each generic type of coating material used has its own impact resistance requirements, as measured, in Joules. Tubular impact testers are commonly used for this test. A weight, typically 1kg, is lifted up the tube to the height required and held in place by a retaining collar. A painted sample is fixed under the tube. By rotating a ring within the collar the weight is released and falls onto the sample, which is then assessed for damage. Two types of test can be done, direct impact and indirect impact, direct being onto the painted side of the sample and indirect on the non-painted side.

11.6 Accelerated testing

Normal weathering tests are a simple process of hanging out painted panels, facing south, on an A frame and periodically testing for colour retention, chalking, water absorption etc. over a period of years. However new products ready to go on the market, cannot wait years for test results. The manufacturers may have spent many thousands of pounds on research and development of the product and will want some return. Accelerated tests can be done which reduce testing time to months by accelerating or intensifying the conditions to which the paint will be exposed. Some typical test cabinets used for testing specific conditions are: Humidity cabinets

For testing tropical conditions. Humidity is very high at 95% and elevated temperatures up to 55°C.

Salt spray cabinets

For checking paint ability to withstand salt laden environments. Water soak tests

Allowing painted panels to be submerged to test for water absorption, by weighing before and after submersion.

Temperature cycling

Painted samples are subjected to constant temperature cycles from hot to cold. Paints in common with most materials expand and contract



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according to temperature. Constant expansion and contraction can result in cracking. Maximum and minimum temperature can be set and cycle time, over a running period of 1000 hours, as an example.

Prohesion testing

Painted sample plates are cut with a pre-damage in the form of an X, 50mm each incision length. A 3% saltwater solution is sprayed onto the plate for 60 minutes and stopped for 60 minutes, at a constant 35°C. The cycle continues for 1000 hours. On examination after the 1000 hours, there shall be no blistering or undercut outside of a 3mm boundary on each side of the pre-damage.

11.7 Drying and curing tests

On the manufacturers data sheet for a paint it will invariably state a recommended over coating time, at a specific temperature, as a guide. The reason being that tests done to determine the drying/over coating time will have been done in a laboratory to that specified temperature. Higher ambient temperatures will shorten the stated time and conversely lower temperatures will need longer over coating time. Two tests to determine the drying time are: Ballotini test. Beck Koller Stylus test (BK trying time recorder). Ballotini test Ballotini, tiny spheres of glass, or sometimes sand is trickled onto a newly painted block graduated in hour of traverse, eg 24 hours for the block to traverse full length under the funnel. After a specified time the block is removed, tipped onto its side, tapped lightly and examined. The position of the last grain of sand or ballotini sticking to the surface is recorded as the drying time at that temperature eg 20oC 0.5°C. Figure 11.7 Ballotini test.

2 4 6 8 10 12 14 16 18 20 22 24



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11.8 BK drying recorders

The BK gives more information than the Ballotini, which purely indicates drying time. The BK defines also the stages of drying. eg solvent evaporation time, the SolGel transition, surface drying time and final dry time. Needles (stylus) are fixed to motor driven wires which then traverse over the full length of painted glass strips 300 x 25mm, in pre-set times of 6, 12, 24 hours. The needles can also be weighted if required. When the paint is wet the needle will penetrate through to the glass. As the solvent evaporates the needle will start to cut a continuous track in the film, as drying progresses it will cut an interrupted track, until finally dry when no scratch is visible.

11.9 Other tests

Mechanical thumb test This is a test for even through drying of paint. It simulates pressing a thumb onto a surface and applying a twisting motion. A cam drives a weighted shaft with a semi-spherical rubber end cap, allows it to drop onto the painted plate, rotate through 270o, then lifts it off again. The plate is then visually inspected for tearing, pulling, wrinkling etc.

Pencil scratch test (Wolff-Wilborn)

Pencils are graded in degree of blackness B and degree of hardness H. HB being the middle of the range. Higher the number and harder or blacker the lead is.

A sharpened pencil is fitted into a special steel block and pushed along the surface, starting with eg 3H and working up 4H, 5H etc. The first pencil to scratch the paint lends its hardness value to the paint eg 5H.

Mechanical scratch test

A stylus with various added weights is drawn across the painted surface. The weight that causes the surface to be scratched gives its value to the hardness eg 500gm.

Gold leaf test

A test for residual tack. A small square of gold leaf is lightly pressed onto the surface of the paint. The gold leaf is then peeled off and the area examined with a magnifying glass. No residual gold leaf should remain.

Thumbnail test

A quick test for hardness is to try to penetrate the paint film with the thumbnail. If the thumbnail penetrates, the film is cheesy.



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Opacity The opposite of transparency, a test to determine the ability to hide (cover) the substrate. The following are methods to determine the wet film opacity, a combined function of pigment concentration and refractive index, using cryptometers. Commonly used cryptometers are Pfund cryptometers and there are two types.

11.10 Trough type

A wooden block with a tapered sunken trough in the middle, the bottom of which is formed by chequered black and white glass squares. Paint is added at the deep end and scraped along to fill the trough. Looking perpendicular onto the trough, find the point where the underlying square can just no longer be seen. (Look at the squares offering the biggest contrast to the paint colour). A scale running along the groove will indicate the depth of the groove at that point and is recorded as a wet film thickness.

Figure 11.8 Pfund cryptometer, trough type.

11.11 Black and white fused plates

The second type is a black and a white glass square, fused together. On each square is an engraved scale starting at zero on the joint. Paint is applied onto the square with the most contrast and a top plate of clear glass placed in position with the tapered contact edge exactly on the fusion line. Small feet at the end of the top plate allow a tapered film of paint to form under the glass top plate. Look for a point along the film where the underlying black or white plate can just no longer be seen. Note the value on the scale and multiply by the constant on the top plate.

A A

Section on A - A



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Figure 11.9 Black and white fused plates.

11.12 Hiding power charts and micrometer adjustable film applicator

Various designs of black and white A4 sized cards are used for this method. Chequered, striped, zigzag, half and half and cards of one’s own design can be used. The surface is coated with a solvent resistant lacquer to prevent immediate absorption. The applicator is a frame with an adjustable gate, which can be controlled by two micrometers for vertical movement. After zeroing on a flat surface the reading on the micrometers represents the gap under the gate. Paint is applied onto one chart and the bar applicator immediately drawn over it. If opacity is not achieved (as previous) the gate is adjusted 5m higher and the operation repeated on another card until the film thickness required for opacity has been attained.

11.13 Degree of gloss

Gloss is a measure of reflectivity. Light follows general rules and travels in a straight line. When light hits a surface it reflects off at the same angle as it strikes the surface. A modern gloss meter works on exactly this principle, a light source directs a beam of light onto the surface under test and a photo electric cell, set at the same angle, collects the reflected light and quantifies it and converts it digitally into a percentage of the incident light. On a perfectly smooth surface it would give almost 100%. On an uneven surface some of the light is deflected and so the percentage reading would be lower. A high percentage of reflection will be gloss and a low percentage will be matt. Gloss meters for general use have two common angles, typically 60° and 20° (both taken from the perpendicular), with the 60° angle being the most common.



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Figure 11.10 Degree of gloss. Any property, which can affect the paint surface formation, can affect the gloss factor. Main contributors are PVC, degree of dispersion, particle size, resin type (for polymer formation and RI, refractive index) and solvent type.

11.14 Adhesion

Inspection is defined as measuring, examining, testing, gauging, one or more characteristics etc. One of the properties required of a paint film is to provide adhesion to the substrate; therefore an inspector is expected to test to ensure the paint is performing this function. There are three main areas for adhesive failure within a paint system. Primer to substrate failure. Intercoat adhesion (between films). Cohesive failure (within a paint film). Primer to substrate failure Primer to substrate failure is the most serious. Failure here means no protection at all. This is a surface contamination problem mainly. Lack of adequate surface preparation, grease, oil, dirt and dust are the usual causes. Intercoat adhesion Caused by the problems above and others. Lack of observance of recommended over-coating limits and expansion/contraction differences between materials.

Incident light

Light scattered

Uneven surface

Incident light

Smooth surface

Photo electric cell

Reflected light



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Cohesive failure Over-thickness of a layer can entrap solvent during the drying process and thus stop polymerisation and the correct formation of the film, reducing cohesive strength. The main reason for cohesive failure is solvent entrapment, but incorrect ratio mix of a two pack can have exactly the same effect. These failure points can be detected in several ways, some costly, requiring equipment costing several hundred pounds and some requiring an outlay of a few pounds only. V cut test A craft knife is all that is required to perform this test. Cut through the paint, to the steel substrate, with two cuts forming an inclusive angle of approximately 30°, with leg length of approximately 13mm. Insert the tip of the blade into the tip of the V and try to lever off. The paint should chip across the tip of the V clearly and cohesively without following the line of any of the faults described. It should not expose any of the substrate. Cross cut (cross hatch test) Cut through the paint using six horizontal and six vertical cuts approximately 2mm spaces giving a 25 squared grid. Special profile cutters can be purchased for this, or a craft knife can be used. Apply an agreed tape to the area (different tapes have different degrees of stickiness and would give different results), rub smoothly onto the hatched area and then snatch off. The resulting areas of disbondment are then compared to diagrams shown in BS 3900 Pt E6 and classified according to percentage area of disbondment.

Dolly test The dolly test is more expensive to use, but unlike the above gives an answer in units of psi or newtons/m square, etc. and so is classed as a quantitative test.



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A typical procedure for the test would be: Ensure the test area is clean and oil/grease free, lightly abrade the area and apply mixed two pack heavy duty adhesive. Firmly place the aluminium alloy dolly in position onto the adhesive ensuring that the skirted flange is to the adhesive. Leave for manufacturers recommended cure time. Place the core drill supplied around the dolly and cut through the coating to the substrate (this ensures that only the area of the dolly flange receives the pull off forces). Apply the pull off gauge and apply pull off force, (some models use a ratcheted lever, others a knurled wheel) until failure occurs. This will usually involve a loud bang and the instrument will ‘jump’ from the substrate. Examine the face of the dolly and apportion adhesive failure according to areas exposed, at the pull off force indicated on the scale. For example with an aluminium metal spray, single coat, there could be: 1 Adhesive to dolly failure. 2 Adhesive to aluminium failure. 3 Cohesive failure within the aluminium. 4 Aluminium to substrate failure.

Pull off dolly tester. Hydraulic adhesion test equipment This is a much quicker test with a higher degree of accuracy. The HATE use cyano-acrylic impact adhesives and can usually be done approximately two hours after dolly/adhesive application, the dollys are mild steel and reusable because they are heated up to destroy the adhesive after use. The big downside for this test is the initial cost and usually high maintenance.



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Section 12

Specified Coating Conditions

Rev 2 June 2011 Specified Coating Conditions


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12 Specified Coating Conditions A manufacturers product data sheet will indicate under which ambient conditions a paint/coating can or cannot be applied. The clients specification may sometimes be a little stricter. However, in all cases, it is the specification which takes precedence, (it is common practice nowadays to include a phrase such as when these conditions do not prevail or similar, to allow coating to continue using special products). A typical specification used to be: It is not permissible to apply paints: 1 During rain, snow, or high winds: This clause would be sensible even in

modern specifications. 2 When the air or metal temperature is down to within 3°C above the dew

point temperature: Still common in specification now, but can be overridden by giving alternate systems.

3 When the air or metal temperature is below 5°C: Solvent evaporates very slowly at low temperatures and chemical cure rates used to be static.

4 When the relative humidity is more than 90%: Still a very common restraint and sometimes the benchmark for using moisture curing polyurethanes.

From the above, two very important phrases arise, relative humidity and dew point. Relative humidity

Defined as being The amount of water vapour in the air expressed as a percentage of the amount of water vapour which could be in the air at that same temperature. 100% humidity, saturation, is measured as being taken within 1 inch of the surface of a fast flowing river.

Dew point

This is the temperature at which water vapour in the air will condense. Condensation cannot occur unless the relative humidity is 100%. Recalling that every 11°C drop in temperature results in the airs capacity to hold water halving, even the smallest drop in temperature results in water being released from the air, in the form of condensation. So at 100% humidity the air temperature and dew point temperature and wet bulb temperature on the whirling hygrometer are all the same value.



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12.1 The whirling hygrometer, aspirated hygrometer or psychrometer

Commonly called the whirling hygrometer, this piece of equipment is widely used by coating inspectors to determine wet and dry bulb temperature readings, from which, using calculators or hygrometric tables, the relative humidity and dew point can be calculated. Two thermometers are mounted in a plastic frame, fitted with a handle so that the frame can be rotated through the air. One of the thermometers is fitted with a wick around the bulb. The wick passes through a hole in the end of the frame and into a small container with a screw lid, into which is put distilled water or clean rainwater ie de-ionised water. The water is drawn by capillary action all along the wick onto the area enveloping the thermometer bulb. This is referred to as the wet bulb and the second thermometer is the dry bulb. The frame with the thermometers mounted should be rotated quickly about a horizontal axis. (BS 2482 states in front of and to windward of the operator) so that the bulbs pass through the air at 4m/sec. If there is a wind the operator should face into the wind, if no wind then walk slowly into a clean air current. The frame should be rotated for 30-40 seconds, or as otherwise specified, as fast as possible (to meet requirement as above) and then read the values on the thermometer, always the wet bulb first, immediately on ceasing rotation. The water on the wet bulb uses heat energy from the air to change into water vapour, so the wet bulb will give a lower temperature reading than the dry bulb. When rotation stops, the aspiration rate slows and so the wet bulb temperature will slowly start to rise towards that of the dry bulb. This operation should be repeated as many times as is necessary until the following criteria is met. On two consecutive spins the readings should be within 0.2°C, wet bulb to wet bulb and dry bulb to dry bulb. The wet bulb and dry bulb temperatures recorded can then be used to determine the RH and DP from scales or tables. This operation should be carried out as near as possible to where the work is being done. Big difference in temperature can occur from N side to S side of a tank or down a trench and topside.

12.2 Steel temperature measurement

The air temperature (ambient) is the temperature recorded from the dry bulb thermometer. To measure the steel substrate temperature a magnetic gauge, known commonly as a limpet gauge is used, or a digital thermometer, thermocouple, sometimes called a touch pyrometer.



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Magnetic limpet gauge. Whirling or aspirated hygrometer.

Section 13

Cathodic Protection

Rev 2 June 2011 Cathodic Protection


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13 Cathodic Protection Cathodic protection is a secondary line of defence against corrosion, the primary defence being the coating. When damage to the coating occurs eg through impact on the coating during back filling on a pipeline, sling damage during the lowering in operation, or flotsam impact on an offshore platform leg, the underlying steel can then be in contact with electrolyte and corrosion can occur. But if these areas can become cathodic ie receive current, corrosion can be avoided. In order for cathodic protection to be applied, an electrolyte must be present. For example the external surface of a tank cannot have cathodic protection, but internal surfaces can if the tank is holding an electrolytic medium, but only up to the level of medium, not above. Underground and subsea pipelines can be protected, but steelwork above ground in an AGI needs painting. Cathodic protection can be applied in one of two ways: Sacrificial anodes systems. Impressed current systems.

13.1 Sacrificial anode systems

This system sometimes called, galvanic anode system, works on the principle of bimetallic corrosion, the natural potential between metals. Any metal which is more electronegative (less noble) or below steel on the galvanic list can be used as an anode. The choice of metal used would depend upon the potential required to protect the prescribed area. Sacrificial systems only protect small areas and the anodes need changing regularly as they corrode away. Figure 13.1 Sacrificial system.

13.2 Impressed current system

The impressed current system is used to protect long lengths of pipeline from one installation, a distance of approximately 10 miles. The current needed to run the system comes from the national grid and is connected through a transformer rectifier (TR). The national grid is very high voltage and very high amperage and also AC. Anti-corrosion currents need to be

Approximately 50m maximum Connecting wire of

copper. Minimum resistance

+

Aluminium zinc or magnesium or alloys of these



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DC. The TR rectifies the current to DC and transforms it to low voltage and amperage. The positive side of the TR is connected to a ground bed (anode system) and the negative to the pipe, making the pipe the cathode. The current is released into the electrolyte at the ground bed, passes through the electrolyte and is received at areas of coating damage on the pipe. A typical ground bed will be approximately 50m in length, at the same depth as and running parallel to the pipe. The cables carrying the current are of a substantial diameter and pure copper to produce a circuit of little or no resistance at the anode. The resistance encountered comes in the soil/clay/rock bearing the electrolyte and this will govern the driving voltage required and the number of anodes required to maintain negative potential on the buried pipe. The voltage required varies but is usually within the range of 10-50v at an amperage of around 0.15 amps. A CP system does not eliminate corrosion, it controls where corrosion occurs. Figure 13.2 Impressed current system.

13.3 Interference

When a buried steel structure is near to, or in the case of another pipeline, passes over or below a pipeline which is cathodically protected, problems can occur. This is interference but the term can be misleading. The offending structure does not adversely affect the CP system, but instead is affected by it.

Ground bed releases current into

To national grid supply

Current received at cathode. Protected.

TR. Transformer rectifier



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The interference structure picks up current released from the anode bed and conducts the current through a circuit of minimal resistance and releases the current again into the electrolyte near to the protected line. The interference therefore becomes a secondary anode and can suffer severe corrosion. If there is a possibility of a structure becoming interference then precautions need to be taken to avoid this eventuality. With the permission of the owner of the offending structure, three main methods can be employed. 1 Attach isolation joints one pipe length either side of the nearest point of

the offending line to the protected line. Join the two pipe lengths to the protected line with insulated wire and doubler plates, thus making them the same potential.

2 Attach isolation joints to both lines, one pipe length either side of the nearest point. Join the two isolated sections together and install a sacrificial anode to protect both sections.

3 Double wrap and contra-wrap the protected line giving four tape thicknesses with Cold Applied Laminate Tape for one pipe length either side of the nearest point.

The method chosen would be at the discretion of the engineer.

13.4 Monitoring CP

It is considered that -850mv will maintain a pipeline in a passive state but most CP engineers will require a more negative value, -1 to -2V being typical. To ensure that the required potential is being maintained, checks need to be carried out at regular intervals. One method of monitoring is known as half-cell reference electrode. The most commonly used half-cell electrode is the copper/copper sulphate half-cell electrode. It is used for measuring the pipe to earth potential, ie cathode to earth, the other half of the circuit being anode to earth. Periodically along the line, CP monitoring posts are installed, with a direct wire connection to the pipe, accessed from a stud on the CP post panel. A voltmeter is connected to the stud and to the copper/copper sulphate half-cell, which is then pushed into the earth directly above the pipe. This provides a circuit for electrons from the pipe, into the electrolyte, back to the anode bed.



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Figure 13.3 Monitoring CP.

13.5 Cathodic disbondment

Part of the electrical circuit of the corrosion reaction is the evolvement of hydrogen gas from the cathode. Hydrogen is a very powerful gas and can cause cracking in steel, (HICC). If hydrogen gas can penetrate underneath a coating it can easily disbond it. This is known as cathodic or hydrogen disbondment. Over protection of damaged areas on a pipe, results in over production of hydrogen and subsequent disbondment of more of the coating, resulting in a bigger area to protect, needing more current. All material used on a pipeline have to undergo tests to determine their resistance to cathodic disbondment. The test is done in the following manner. A 6mm diameter hole is drilled into a plate coated with the material to be tested, through the coating and into but not through the underlying steel. A short length, approximately 50mm of plastic tube approximately 50mm diameter is fixed in position, using typically araldite epoxy or elastomeric sealant with the drilled hole central to the tube. This is then part filled with 3% solution of common salt, sodium chloride and a lid fitted. The lid can be machined from a block of polyethylene with a suitable diameter hole drilled through. The plate is connected to the negative pole of a battery; an anode is connected to the positive pole and inserted through the hole in the lid into the salt solution. When the circuit is switched on the plate is the cathode and hydrogen (and chlorine) will be evolved from the steel and also at the interface of steel/coating. This enables hydrogen to penetrate under the coating, simulating areas of coating damage.

Pipe

CP post Voltmeter

Half cell reference electrode filled with copper sulphate solution

Porous plug

Ground level



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The circuit is stopped after 28 days stripped down, dried off and using a craft knife; two cuts are made at an inclusive angle of approximately 30° radiating from the centre of the hole, through the coating to the substrate. Where disbondment has occurred the coating will chip off as the cuts are being made. The distance from the edge of the hole to the extent of the disbondment is measured and should not exceed the stated requirements. For example FBE maximum 5mm after 28 days.

Figure 13.4 Cathodic disbondment.

Section 14

Holiday/ Pinhole Detection

Rev 2 June 2011 Holiday/Pinhole Detection Copyright TWI Ltd 2011

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14 Holiday/Pinhole Detection Holidays and pinholes in a paint film are defects which allow ingress of an electrolyte and therefore are detrimental to an anti-corrosion system and need repair. Not all defects of this nature are visible to the naked eye and we therefore need equipment to facilitate the detection. For coatings of thicknesses above 500m it would be necessary to use a high voltage holiday detector, but for coatings of less than 500m it is normal to use a wet sponge pinhole detector. Most paint systems on new steel fall into the latter. The wet sponge pinhole detector is a very simple piece of equipment and consists of a small control box, usually pocket size, with two terminals, positive and negative. The negative terminal is connected to bare steel on the structure to be tested. The positive terminal is connected to a hand stick with a sponge on the end. The operating power is provided by two, 1½v batteries in the control box. To use the detector the sponge electrode is wetted in water with a tiny amount of detergent/washing up liquid added and squeezed out to remove excess water. After switching on and selection of operating voltage, the sponge is traversed methodically over the area. On a vertical surface it is better to work upwards. On contact with a pinhole, the wetting agent (detergent) allows immediate penetration of the water, so providing a very low resistance circuit back to the control box. A high pitched bleep indicates the presence of a pinhole, the exact position of which is located by using a corner of the sponge. The position is then marked ready for repair.

14.1 Voltage setting

Basic models have two options for setting, 9 and 90V. More sophisticated models have an intermediate setting. For DFTs of less than or equal to 300m the 9V setting is normal. For DFTs of 300-500m 90v or 67½v intermediate sensitivity would be preferred.

Section 15

Paint Application

Rev 2 June 2011 Paint Application


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15 Paint Application Paint technology is advancing rapidly and specialised equipment and materials are being introduced into the industry on a regular bases. However, conventional paint application methods still apply. The three main basic methods are: Brush. Roller. Spray.

15.1 Brush application

Brushing is relatively slow, labour intensive, produces coating of uneven thicknesses, but is more environmentally friendly, results in less waste material and virtually no spotting or overspray damage to adjacent areas. Various types and quality of brushes are used, the most common being the flat brush, as opposed to the round headed variety called tar brushes, (and a variety of other names dependent upon geographical areas). The quality of the brush depends mainly upon the type of bristle, or filling used. Natural bristles have a scaly surface, taper along the length and split at the end. These factors allow the brush to hold more paint and spread it more evenly for a better finish. Synthetic fibres have smooth surfaces and are of uniform thickness for the full length. It was considered that brush application had a more personal shearing action and worked paint into the profile and any dust or other fine detritus present on the substrate worked into the film. Spray application deposits atomised droplets over the particles, entrapping air, which results in pinholes and loss of adhesion. Modern specifications usually state, as per manufacturers recommendations.

15.2 Roller application

Rollers are available in several materials eg mohair, lamb’s wool and sponge and in several different designs, jumbo rollers for large areas, radiator rollers for confined spaced, pressure fed rollers to avoid recharging and extension rollers which increase access. Curved rollers are supplied for pipe work and roller pile material is even made in glove form for areas of difficult access. However no matter what, they all have the same advantages and disadvantages. They enable paint to be applied quickly but do not give a uniform coating thickness and leave a distinct pattern known as roller stipple. Roller application doesn’t work the paint into the substrate and invariably is not mentioned as an approved application method on specifications. It is a method used at the discretion of the engineer and is certainly not suitable on internal corners, welds, toes, bolts, rivets and plate over laps. In areas of this nature a stripe coat must be applied.



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15.3 Spray application

Paint spray equipment can be divided into three distinctly different types. Conventional spray. Airless spray. Electrostatic spray. Conventional spray

Conventional spray systems can be subdivided into three different types of equipment which all have the same atomisation mechanism.

Suction feed

The paint container is underneath the gun; usually aluminium about one litre capacity and the paint is drawn up by venturi principle to the gun.

Gravity feed

The paint container is above the gun and paint feeds to the gun by gravity.

Remote pressure pot

Remote pots are supplied in several sizes and have the advantage of having a much greater capacity than the above and much bigger areas can be painted before refilling is required. A container (pot) is charged with paint and then sealed with a lid. Air from a compressor is fed into the top of the pot and the paint is forced out through a line to the gun.

Figure 15.1 Remote pressure pot. At the gun, when the trigger is operated, a tapered needle is drawn back opening the aperture, out of which the paint exits in a continual stream. Approximately 25mm in front of the aperture, two air channels, from lugs on the cap, diametrically opposed, blow air to converge at the paint stream. At this convergence the paint is atomised into very minute droplets and conducted onto the work piece.

Paint

Air in Pressurised air volume

Paint out to gun



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Figure 15.2 Conventional gun.

Airless spray

With an airless spray the fluid (paint), is pressurised by means of a pump. Electric motor pumps and hydraulic pumps are sometimes used but the most common is the pump operated by compressed air. These units operate by increasing the compressed air inlet pressure by a stated ratio, eg 35:1, by means of two pistons on a common shaft. For instance, if an air driven piston has a surface area of 35sq inches and is exposed to a pressure of 100psi, a piston at the other end of the shaft with a surface area of one square inch will exert a pressure of 3500psi. As the piston is driven down to pressurise the paint, the one way valve at the paint inlet is forced to close position and the paint out port is opened. When the piston reached the bottom of its stroke, the air circuit reverses and forces the piston back upwards. As this happens, the outlet port is closed and the inlet port opens to refill the cylinder with paint. At the top of the stroke the air circuit reverses again and drives the piston down again. The outlet pressure can be adjusted by reducing the inlet pressure from the compressor.

Air

AirNeedle

Trigger



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Figure 15.3 Airless spray. These systems are called airless because air is not used for atomisation. Atomisation occurs by forcing the paint at extremely high pressure, usually 2000-2500psi through a very small aperture, 12-23 thou diameter, into a volume of air offering a resistance to the paint flow. As the air and paint meet, the paint atomises. Most tips used on airless spray equipment have a facility for reversing the flow of paint through the tip. Blockages can then be cleared by turning the tip through 1800, triggering to ground or a container to clear the blockage, then reverse the tip again to its original position. A type of airless spray tip exists with an adjustable aperture size, called a Titan Tip. The aperture can be closed up or opened by turning a small knurled protrusion, which positions a small steel pin into the aperture to control the size. Pigments and extenders, especially MIO and metallic pigments can be quite abrasive and the tips are subject to wear. Some are sleeved with tungsten carbide to give a loner life. Data sheets for a product will recommend spraying pressure and tip sizes although each sprayer will have his/her own preference. Typical recommendations would be:

Lubricant packing

Paint pressurising piston

Paint inlet

One way ball valves

Paint to gun

Compressed air in

Air driven piston



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Paint type Tip size Pressure psi

Chlorinated rubber 13-21 thou inch 2400

High build epoxy 17-23 thou inch 3000

Zinc rich epoxy 17-23 thou inch 2800

Orifice sizes for conventional guns are quoted in metric. Airless spray application is much faster than conventional and more than one gun can be operated from a single pump. The manufacturers container can be used to supply the wet end, the inlet pipe, as there is no need for a pressurised container. Notable differences Conventional Airless

Slow application due to fluid delivery. Excellent application rates.

Low air pressure 40-75psiI Can need 100psi to operate the pump.

Delivery pressure greater than 20psi Delivery pressures greater than 6000psi, dependant on pump ratio.

Need special paint containers. Uses manufacturer’s containers.

Guns can be unwieldy, two lines to supply the gun.

Single line supplies pressurised paint.

Basic equipment needs very little maintenance.

Needs more maintenance due to high pressure and moving parts.

Easier to clean after use. Equipment needs flushing well to remove all traces of paint. Expensive replacement.

Safety considerations Always observe manufacturers recommendations. Wear recommended safety equipment. Always depressurise the system before even minor maintenance. Regularly check fluid lines for wear and leaks. Ensure that swivels and couplings are properly tightened. Always engage the safety catch when the gun is not in use. Never point the spray gun at yourself or other people.

Electro-static spray

Both liquid and powder paints can be electro-statically applied. For liquid paints a small air driven turbine is mounted on the gun and supplies a current to the tip. The current is usually on a thumb control for adjustment and operates in the region of 85kv. Powder paints in general are charged electro-statically by spraying the powder through an area of ionised air. In either case the component to be coated is earthed into the same circuit and thus becomes negatively charged. The coating material is positively charged and is attracted to the component. As the coating thickness increases it has an insulation effect and the coating material is then drawn



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to other charged areas. The voltage can control the thickness, especially when using powder coatings. Wastage is significantly reduced and it produces a more uniform coating. Electro-static application is widely used in industry for components such as kitchen white goods, office cabinets and line pipe. (When powers are used the components are either pre heated or post heated. Line pipe and other substantial section components can be pre heated, but thin steel plate components will not maintain sufficient heat and so are electro-statically coated and then post heated).

15.4 Other paint application methods

Industrial anti-corrosion systems are generally applied by the systems discussed previously; however various other accepted methods exist. Namely: Dip coating

A component is dipped into paint and hung to dry.

Padding Mainly DIY Pads of mohair or foam are used to apply paint. Large pads like plaster hawks for large areas and small ones (about 25mm square) for cutting in around door furniture and putty lines on windows.

Hot spraying

When paint is heated it reduces in viscosity (flows easier) and the cure or drying starts quicker. It is therefore easier to apply and wets out better and reduces the need for solvent addition.

Spin rotating

Usually called a spinner, the equipment, consisting of a three-legged frame, each with a wheel at the end and a centrally mounted spinner is drawn along dispensing paint from the spinner. Ideal for internal coatings on line pipe sections.

Flow/curtain coating

Bitumen and coal tar enamel pipeline coatings, when used, are applied hot, about 200°C, to the 12o’clock position of a pipe, the material flows down both sides to meet at 6o’clock. The material being thermoplastic hardens as it cools and coats the pipe.

Aerosols

Pressurised cans operated by push buttons, car paint touch up kits among others used.

Section 16

Metal Coatings

Rev 2 June 2011 Metal Coatings


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16 Metal Coatings

16.1 Galvanising

The coating of components with zinc. Many components both for offshore and onshore use are galvanised. Galvanising can give protection to steelwork for periods of up to 60 years dependant on exposure conditions. The components are chemically cleaned (acid), washed and fluxed, then totally immersed in a vessel containing molten zinc at approximately 450°C. When drawn out, the zinc solidifies at an average thickness of approximately 100m.

16.2 Sheradising

Nuts and bolts and other similar components are coated with this method. Galvanising threads would make a significant difference to the dimensions and workings of fixings and fasteners, so zinc powder, just below the melting point, is used instead. After cleaning the components are tumbled in the powdered zinc, impact fuses the zinc onto the components and in effect, cold welds the powder onto the metal.

16.3 Calorising

Calorising is coating with aluminium. Aluminium has a melting point of 625°C as opposed to 425°C of zinc so it is not really practical to tumble. One way of calorising a component is to dip it into molten aluminium. The resulting exothermic reaction is so severe that is alloys the aluminium with the steel. Calorising can also be done by immersing a component in a mix of fine sand and aluminium powder and heating.

16.4 Anodising

A treatment for aluminium, anodising is an electrolytic method of coating which results in the formation of a dense oxide. The component is immersed in a weak acid bath and oxidation is induced electrically.

16.5 Electro-plating

This is done by electrolytic deposition. If a current is released from an item into a metal salt solution through to a cathode, the metal salts ionise and deposit the positive metal ions on to the cathode bar.

16.6 Hot metal spraying

Any metal, which can be easily melted, can be sprayed. Zinc and aluminium are the most commonly used metals for spraying. They are both below steel on the galvanic list and so will provide cathodic protection to the steel and both metals have a reasonable low melting point. Both metals have advantages and a disadvantage, for instance zinc performs far better than aluminium in rural areas and alkaline environments. Aluminium is considered to be superior to zinc in slightly acidic environments and because of its higher melting point is more widely used on high temperature

Rev 2 June 2011 Metal Coatings


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surfaces such as exhaust stacks, compressor exhausts etc. where extremely high temperatures are encountered. It is specified for use on surfaces with working temperatures of up to 540°C. Application of metal sprayed coatings can be carried out by any of the following methods.

16.7 Powder system

Powdered metal is fed into a heat source (usually butane or propane and pure oxygen burning) and propelled onto the substrate. Using this method a relatively low proportion of the metal powder is actually deposited on the substrate.

16.8 Electric arc system

This method is ideal for production line type facilities such as gas bottle production and lamp standards etc. where components are of a uniform shape and the process can be mechanised. As in a welding process the metal (to be sprayed) acts as an electrode in a metal carbon arc circuit and the electrode melts. The molten metal is atomised and blown onto the component by means of a heated air jet. This system gives a superb fine grain finish.

16.9 Wire and pistol system

By far the most common and widely used method for site application of metal spray. The metal wire, of a very high degree of purity, greater than 99.5%, is driven through a gun by means of two knurled wheels powered by compressed air. As the wire, 3-5mm passes through to the front of the gun it passes through a ring of burners, with the flames focused about 35mm from the exit point. The fuel gases used are butane/propane and pure oxygen. The flames melt the wire and droplets of metal are propelled to the steel by the combustion gasses and compressed air. The coating is usually applied at a thickness of 100-125m and is about 85-95% density of the original wire. This is because the resulting film is in an open cell structure due to individual particles forming a fish scale like structure, the interstices between the particles are not all filled. If the coating is to be subjected to high temperature services it will need sealing with a silicone sealer, aluminium or carbon pigmented. If however the metal spray is applied to give an extended major maintenance free life to an anti-corrosion system, then either an epoxy sealer or etch primer would be applied prior to the specified system.

Section 17

Coating Faults

Rev 2 June 2011 Coatings Faults


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17 Coating Faults As defined in BS 2015, Glossary of Paint and Related Terms. Bittiness

The presence of particles of gel flocculated material or foreign matter in a coating material, or projecting from the surface of a film. The term seedy specifically denotes the presence of bits that have developed in a coating material during storage. The term peppery is sometimes used when the bits are small and uniformly distributed. Bleeding

The diffusion of a soluble coloured substance through a coating material from beneath, providing an undesirable staining or discolouration. Examples of materials, which may give rise to this defect, are certain types of the following materials: bituminous paints, wood preservatives, oleoresins from wood knots, organic pigments and stains and coal tar. Bitumen and coat tar enamels also.



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Blistering

The formation of dome shaped projections or blisters in the dry film of a coating material by local loss of adhesion and lifting of the film from the underlying surface. Note - Such blisters may contain liquid, vapour, gas or crystals. Chalking

The formation of a friable, powdery layer on the surface of the film of a coating material caused by disintegration of the binding medium due to disruptive factors during weathering. Note - Chalking can be considerably affected by the choice and concentration of pigment. Cissing

The formation of small areas of the wet film of a coating material where the coating material has receded leaving holidays in the film.



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Cracking

Generally the splitting of the dry film of the coating material usually as a result of ageing. Specifically a break down in which the cracks penetrate at least one coat and which may be expected to result ultimately in complete failure. Hair cracking

Cracking that comprises of fine cracks, which may not penetrate the top coat, they occur erratically and at random.

Checking

Cracking that comprises of fine cracks, which do not penetrate the top coat and are distributed over the surface giving the semblance of a small pattern.

Crocodiling/Alligatoring

A drastic type of crazing producing a pattern resembling the hide of a crocodile or alligator.

Mud cracking

A network of deep cracks that form as the film of a coating material dries, especially when it has been applied to an absorbent substrate. Mud cracking is associated primarily with highly pigmented water borne paints.

Cratering

The formation of small bowl shaped depressions in the film of a coating material, caused by escaping solvent or gases. High viscosity paint will not flow to fill any depressions, resulting in small bowls and craters.



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Curtaining/sagging

A downward movement of a coat between application and setting (caused by over-application), that results in an uneven area of coat having a thick lower edge. The resulting sag is usually restricted to a local area of a vertical surface and may have the characteristic appearance of a draped curtain. Run

A narrow downward movement of a coat that may be caused by the collection of excess quantities of paint at irregularities in the surface eg cracks and holes, the excess material continuing to flow after the surrounding material has set.

Tear

A small run resembling a teardrop.

Flaking

The coating material ages, becomes brittle and starts to detach from the substrate in the form of flakes or scales. Oxidising paints are especially vulnerable (natural oils and resins).



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Holidays

This is caused by faulty application techniques, seen as areas where the film of a coating material is of insufficient thickness or where there is a complete absence of coating materials on random areas of the substrate. Lifting

Softening, swelling or separation from the substrate of a dry coat, caused by a lack of observance of the stated over coating time.

Orange peel

The uniform pock marked appearance, in particular of a sprayed film, resembling the peel of an orange due to the failure of the film to flow out to a level surface. This is caused by paint viscosity problems or by applicator technique.



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Ropiness (Brushmarks)

Pronounced brush marks that have not flowed out because of the poor levelling properties of the coating material, caused by brushing when film is almost dry, or when paint viscosity is too high.

Wrinkling/rivelling

The development of wrinkles in the film of a coating material during drying. Usually due to the initial formation of a surface skin, trapping solvent below the surface. Rash (or Spot Rusting)

This is caused by rogue peaks or by contamination of the paint film by foreign matter, such as grit or dirt etc.

Section 18

Colour

Rev 2 June 2011 Colour


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18 Colour When considering the aesthetics of a final coat of a paint system, colour is an important property, as well as gloss and opacity. Colour can affect mood and perception and can create illusions. White light, light emitted from the noonday sun, is a combination of electromagnetic wavelengths from 400-700 nanometres, blue through to red. When white light strikes an object, certain frequencies are absorbed and others reflected. It is the reflected frequencies that the human eye translates into colour. Colour has three attributes, which are: 1 Hue

Refers to the basic colour eg red, yellow, green and blue. Can be represented in circle form, clockwise, red yellow green blue red.

2 Brightness

Sometime called lightness, it refers to the amount of lightness or darkness of the colour. The degree of reflectivity of the surface receiving the light governs this property and is sometimes called value or reflectance value.

3 Saturation

How vivid colour appears. It is measured in terms of the difference of a colour from the neutral grey with the same degree of brightness. Lower saturation, greyer the colour. The terms chroma and intensity and sometimes weight, are also used.

Black and white and the greys in between are called achromatic colours, they lack hue and saturation. Anything perceived as having colour is chromatic.

The three attributes can be related to a three dimensional model of a helix.

Rev 2 June 2011 Colour


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Figure 18.1 Three dimensional helix. The Munsell colour system An American system, which identifies colour by its three attributes, hue chroma and value (Reflectance value). In the Munsell system, Hue is divided into five basic colours red, yellow, green, blue and purple each identified by its initial letter, with a second dimension between each, giving ten basic colours. Value is defined in eleven steps from white to black and chroma has fifteen steps. The BS 4800 colour system This BS specifies 100 colours selected from 237 used in the BS 5252. The BS 4800 uses the same basic colours but expands to thirteen, including a neutral. The colours are numbered from 02-24, 00 being neutral, achromatic, using even numbers only. Lightness is identified by capital letters A-E, where A is maximum lightness and E is minimum lightness. The chroma is given by number, the third part of the coding, from 01, in single digit rises to 56. The higher the number, the stronger the colour. The BS 5252, framework for colour co-ordination for building purposes The BS 5252, framework for colour co-ordination for building purposes, selects a framework of 237 colours as a source for all building colour standards and a means of co-ordinating them. It is not itself a range of colours.

Red

Red purple Purple Purple blue

Blue

Blue green

Green

Green yellow Yellow

Yellow red

Saturation

Brightness

Hue

White

Black

Section 19

Health and Safety

Rev 2 June 2011 Health and Safety


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19 Health and Safety Control of Substance Hazardous to Health Regulations 1988 generally abbreviated to COSHH regulations. These regulations provide a framework to help to protect personnel at the workplace against health risks from substances, which are hazardous. For the purpose of COSHH regulations, substances hazardous to health include. a Substances or preparations listed as being toxic, very toxic, harmful,

corrosive or irritant in part 1A of Chemicals (Hazard Information for Packaging) Supply.

b Substances with MEL or OES as detailed in schedule one of COSHH or if Health and Safety Commission has approved an OEL.

c Harmful micro-organisms. d Dust of any kind in substantial concentrations. e Any other substance creating comparable hazards to people’s health

such as pesticides and other chemicals used on farms.

19.1 Hazard warning symbols

- Black symbol of skull and crossbones on an orange square with the words Toxic or Very Toxic printed below

- Black diagonal cross on an orange square with the words Harmful or Irritant printed below.

- Black symbol showing a tilted test-tube dripping onto a hand with a chunk out, adjacent to a test tube dripping onto a stone flag. Orange background with the word corrosive printed below.

Corrosive

Harmful or Irritant

Toxic or Very Toxic



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19.2 Responsibilities

It is the employer’s duty to assess the risk to employees on his/her premises and any other premises, which might be visited during the execution of duties. Training establishments are responsible for trainees. It is an employer’s duty to prevent, where ever possible, exposure to hazardous substances, but if it is not reasonably practical to totally prevent exposure then protective clothing, masks etc. should be issued to minimise exposure. COSHH regulations require that regular monitoring should be carried out and records kept, particularly in situations where there could be serious risk to health if control measures were to fail or deteriorate. Guidance note EH 40 (occupational exposure limits), is a document published by the HSE, which lists all substances known to be hazardous to mankind. It gives details in table form of common names, chemical formulae and chemical names of hazardous substances. The Hydrocarbon solvents used in modern paint formulations are hazardous to health and are listed in EH 40. Xylene is one such solvent and has an Occupational Exposure Limit (OEL) of 100ppm (parts per million). This means that air containing more than 100ppm would be considered to be a hazard to the health of personnel exposed to it. There are two categories of OEL.

19.3 Maximum exposure limit (MEL)

‘The maximum concentration of an airborne substance, averaged over a reference period, to which employees may be exposed by inhalation under any circumstances and is specified, together with the appropriate reference period, in Schedule one of COSHH.’

19.4 Occupations exposure standard (OES)

The concentration of an airborne substance, averaged over a reference period, at which, according to current knowledge, there is no evidence that it is likely to be injurious to employees if they are exposed to inhalation, day after day, to that concentration and which is specified in a list approved by HSE. When referring to reference periods above, long term exposure limits are averaged over an eight hour reference period and short term exposures over ten minute reference periods. If the EH 40 specifies that a substance has an MEL then the quoted figure must not be exceeded at any time, but kept as low as is reasonably practical.



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With an OES it is permissible to exceed the stated figure provided that the average over a reference period is below the stated figure. Exposures OEL examples of some solvents

Solvent Name OEL in ppm

Alcohols Methanol Ethanol

200 1000

Ethers Ethyl ether Isopropyl ether

400 250

Esters Methyl acetate Ethyl acetate

200 400

Ketones Acetone Methyl ethyl ketone

750 200

Aromatics Xylene Toluene

100 50

Aliphatics White spirit Hexane

100 500

Chlorinated hydrocarbons Trichloroethane Trichloroethylene

350 ab 100 ac

Note: a = MEL. b = maximum short term exposure 450. Dräger tube and Dräger bellows Figure 19.1 Dräger tube. One way of monitoring the toxicity of the air is by dräger tube and bellows. The dräger tube is a glass tube about 110mm long with moulded nipples at each end. One half of the tube is filled with chemical crystals (sensitive to the material testing for) and are held in position by fine wire mesh plugs. A cellophane sleeve, incorporating a scale in ppm is wrapped around the tube. There is also an arrow on the sleeve indicating the way in which the tube is to be inserted into the bellows. The bellows are hand operated and are a one way air system, as the bellows are depressed, air is expelled from a slot at the back, when released, air is drawn in through a small rubber grommet like aperture at the front. The bellows incorporate two compression springs and stops and two

N

=

5

Dräger



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retaining chains, so that every depression and release exchanges an air volume of 100cc exactly. Figure 19.2 Cross-section of Dräger bellows. Using the tubes and bellows Using a special fitting situated on the bellows, the nipples are snapped off both ends of the tube, which is then inserted into the aperture on the bellows in the direction indicated by the arrow. The crystals should be adjacent to the bellows. The bellows are then depressed and released according to the number expressed as n =, as written circumferentially around the centre of the tube. Each depression and release slowly draws 100cc of air through the open end of the tube, through the crystals and into the bellows. As the air containing the hazardous material passes into the crystals, a chemical reaction takes place, resulting in a colour change in the crystals. The extent of the colour change along the scale is recorded in ppm. Note: Many variation of crystal combinations exist for monitoring a variety of different toxicants, all have a different requirement for number of depressions and different colour changes. The tube for monitoring the concentrations of xylene needs five depressions and the colour change is from white to reddish brown. Some materials in common use in the coatings industry do not evaporate into gas or fumes; they remain instead as tiny particles of solids suspended in the atmosphere. Materials of this nature cannot therefore be detected by Dräger tube. They are quantified by the units milligrams per cubic metre rather than ppm.

Limiting chain

Discharge valve

Front plate

Break-off husk

Pump head

Sieve



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Three materials, which fall into this category, are asphalt, coal tar and isocyanates. Asphalt is considered to be fairly safe with an OEL of 5mg/ m3. Isocyanates are very toxic with an MEL of 0.02mg/ m3.

Section 20

Duties of an Inspector

Rev 2 June 2011 Duties of An Inspector


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20 Duties of an Inspector BS 4778 Pt1 (EN 28402, ISO 8402) Quality Vocabulary – International Terms, defines inspection as Activities such as measuring, examining, testing, gauging, one or more characteristics of a product or service and comparing these with specified requirements, to determine conformity. Documents available to an inspector could include, but not be limited to: a Job specification. b Data sheets for the paints/coatings. c Procedures. d Quality plans. e Plant drawings. f Site plans. g British Standards eg 7079 Pt. A. h Waste management, duty of care document. i Relevant local regulations. The job specification is the main tool of the inspector and should be observed at all times. It is not the inspector’s responsibility to rewrite the specification and permission for any deviation should be given in writing and retained by the inspector. An inspector should keep adequate and accurate records of all stages of the work being carried out, materials used, ambient conditions etc so that in the event of illness or any other situation requiring a replacement, the new inspector will be in full possession of all relevant information. Paint/coatings inspector’s daily report sheets need to be completed and passed on to the engineer, containing all information requested and a copy retained by the inspector. The format of daily report sheets varies but in general will require the following information. 1 Details about the contract and contractor, including plant on site and

number of personnel. 2 Ambient conditions applicable during the work period, to be monitored as

near as possible to the task location. 3 For surface preparation activities the information required will include,

method used, original substrate condition, abrasive type, degree of cleanliness achieved, profile achieved, identity of plant and times of starting and completion.

4 For materials, the information required will include manufacturer, product reference number, expiry date, batch number, colour, reference number of thinners, WFT and resulting DFT, time of application and identity of plant. In the case of labour only contracts it will be required to record quantity used.



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5 The comment part is a space left for the inspector to report on any irregularities, non-conformance or deviation from specification.

In addition to the daily reports it may also be a requirement to complete a weekly summary, detailing progress and any other information, such as repeated deviation from specification, for the engineer. Typical examples of situations to report would be: 1 Substituting approved products with unapproved products. 2 Substituting new materials with out of date materials. 3 Using solvents other than those approved by the manufacturer. 4 Not observing induction times when specified. 5 Using untrained personnel. 6 Re-using expendable abrasives. 7 Not observing recommended over coating times. 8 Continuing with the next stage of operations without inspection of the

substrate and approval. 9 Painting/coating over areas of inadequate surface preparation. 10 Working in conditions outside of specified requirements.



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Job: Report No. Contractor: Date: Location: Specification: No. of men: Start time: Finish time:

Weather a.m. p.m.

Time Ambient temp.oC

Relative humidity%

Dew point oC

Steel temp oC

Time Ambient temp.oC

Relative humidity%

Dew point oC

Steel temp oC

Surface preparation: Initial condition: Blast clean Abrasive: Type: Profile Sa2 Sa2½ Sa3 Hand clean

Method St2 St3

Paint application: Manufacturer’s name: Coat No. Manufacturer’s description Ref. No. Colour Wft Dft Quantity

used-ltrs.

Item coated Coat No. Ref. No. Batch No. Time interval Wft Total Dft Comments: Signature:

Transmission Department PAINTING INSPECTION FORM

WALES GAS NWY CYMRU



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Example

PAINTING CONTRACTOR

PAINT SUPPLIER

ABRASIVE SUPPLIER

AREAS TREATED SURFACE PREPARATION

PAINTS APPLIED

METHOD OF APPLICATION

WFT DFT

TIME DRY BULB TEMP

WET BULB TEMP

REL HUM

DEW POINT

STEEL

TEMP

TYPE OF

WORK

TIME DRY BULB TEMP

WET BULB TEMP

REL HUM

DEW POINT

STEEL

TEMP

TYPE OF

WORK

WEATHER No OF MEN ON SITE

COMMENTS INSPECTOR PRINT SIGNED INSPECTOR SIGNED ENGINEER

PAINTING INSPECTION REPORT

CUSTOMER REF NO CONTRACT REF No

CLIENT

LOCATION

REPORT No DATE

Section 21

List of Specification and British Standard (BS) Numbers

Rev 2 June 2011 List of Specifications and British Standard (BS) Numbers


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21 List of Specification and British Standards (BS) Numbers List of Specification and British Standards (BS) Numbers

BS 410 Specification for test sieves.

BS 2015 Glossary of paints and related terms.

BS 2569 Pt 2 Specification for sprayed metal coatings.

BS 3900 Methods of test for paints.

BS 4800 Schedule of paint colour’s for building purposes.

BS 5252 Framework for colour co-ordination for building purposes.

BS 5493 Code of practice for protective coating of iron and steel structures.

BS 7079 Preparation of steel substrates before application of paints and related products.

BS 7079 Group A Visual assessment of surface cleanliness. (was ISO 8501)

BS 7079 Group B Methods of assessment of surface cleanliness. (was ISO 8502)

BS 7079 Group C Surface roughness characteristics of blast cleaned steel substrates.

BS 7079 Group D Methods for surface preparation. (was ISO 8504)

SIS 055900 Pictorial surface preparation standards for painting steel surfaces.

T/SP/PA 5 Notes for guidance, painting inspection.

T/SP/PA 7 Stoved paint finishing.

T/SP/PA 8 Internal coating for steel small bore pipe.

T/SP/PA 9 Paint properties and performance requirements.

T/SP/PA 10 New and maintenance painting at works and site for above ground pipeline and plant installations.

IGE SR 21 Code of practice for safety during blast cleaning operations.

BGC PS PWC1 Acoustic cladding

BGC PS PWC2 Thermal insulation of above ground pipe work and equipment.

BS 1710 Specification for identification of pipelines and services

Section 22

Quality

Rev 2 June 2011 Quality


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22 Quality Quality assurance The definition of quality assurance in BS 4778 Pt1, Quality Vocabulary is All those planned and systematic actions necessary to provide adequate confidence that a product or service will satisfy given requirements for quality. Quality assurance is regarded as being a management tool, a method of maintaining and improving quality whilst controlling costs. A quality system operates on the theory that if there is time and budget allocation to rectify mistakes within a process, then it is preferable to allow a little time to get it right first time at reasonable cost. (By using mathematical tools like Pareto analysis on a production line, eliminate the most frequent fault and reparation is often halved, then eliminate the next most frequent.) Companies employing quality systems produce procedures for every task performed, if everyone works in a formalised way to achieve the requirements of the specification, then consistency of quality should automatically follow. If quality is improved and costs reduced, then a company can be more competitive and consequently improve its position in the market place. Quality assurance is not solely operated by production, but is throughout an organisation and deals with every aspect of a company’s operations from planning and design and training through to packing the final product, transport and marketing. Quality control BS 4778 Pt1 definition Operation techniques and activities that are used to fulfil requirements for quality. The inspection function provides information in order that quality control can be maintained by adjusting the process to eliminate any deficiency. Quality related standards BS EN ISO 9000 series Quality systems.

BS 4778 Quality vocabulary (EN 28402 ISO 8402).

BS 7229 Quality systems auditing.

BS EN 30011 Guidelines for auditing quality systems.

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Quality related definitions Code of practice Document that recommends practices or procedures for the

design, manufacture, installation maintenance or utilisation of equipment, structures or products.

Instruction Provision that conveys an action to be performed.

Normative document A document that provides rules guidelines or characteristics for activities or their results.

Procedure A specified way to perform an activity.

Regulation A document providing binding legislative rules that is adopted by an authority.

Specification The document that prescribes the requirements with which the product or service has to conform. NB. A specification should refer to or include drawings, patterns or other relevant documents and should also indicate the means and the criteria where by conformity can be checked.

Standard Document, established by consensus and approved by a recognised body, that provides, for common and repeated use, rules, guidelines or characteristics for activities or their results, aimed at the achievement of the optimum degree of order in a given content.

Technical specification A document that prescribes technical requirements to be fulfilled by a product, process or service. NB. A technical specification should indicate, where ever appropriate, the procedure(s) by means of which it may be determined whether the requirements given are fulfilled. A technical specification may be a standard, a part of a standard or independent of a standard.

Section 23

Revision Questions

Rev 2 June 2011 Revision Questions


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23 Revision Questions Corrosion OP – Monday 1 Is the electrical circuit in a corrosion reaction AC or DC? 2 Does corrosion occur at the cathode or at the anode? 3 Name the three factors needed for corrosion to occur. 4 What is meant by the term electrolyte? 5 What is corrosion? 6 In the corrosion circuit do electrons flow from anode to cathode? 7 Which gas is released at the cathode when the electrolyte is water? 8 Which is the more noble metal, steel or aluminium? 9 Which is more electronegative, steel or aluminium? 10 If steel and copper were in contact in an electrolyte which would

corrode? 11 Name two common hygroscopic salts. 12 Name three metals used as sacrificial anodes on a steel pipeline 13 What is the approximate thickness of millscale? 14 Which of the two metals would corrode if steel and zinc were coupled? 15 Which other names relate to the Galvanic List? 16 In which environment are you likely to encounter chloride salts? 17 Which three compounds together form millscale? 18 If magnesium was coupled with zinc, which would corrode? 19 In which environment would sulphate salts be found? 20 What is an osmotic blister? 21 What is an ion? 22 What is meant by polarisation? 23 Is an anode positive or negative? 24 Can corrosion occur without an electrolyte? 25 Name a sub atomic particle. 26 What is millscale and when and where does it occur? 27 Name three factors, which can accelerate corrosion reactions. 28 Why is it considered essential to remove millscale prior to painting? 29 Why does an un-coated steel plate corrode? 30 If corrosion occurs at anodic areas, why does steel corrode evenly all

over the surface?



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Surface preparation - Monday 1 Which British standard would be used in determining the size of copper

slag abrasive? 2 Which British standard would be used in determining the size of metallic

abrasives? 3 Which regulations prohibit the use of sand for blasting steel? 4 What is meant by the term key? 5 Why is it important to have good surface preparation? 6 What is meant by the term sliver? 7 What is a hackle? 8 Name two other terms that could be used for anchor pattern? 9 What are the main advantages of using Testex papers for measuring

profiles? 10 What is meant by the term grade, relating to a blast finish? 11 What are the main factors governing the grade of a blast finish? 12 Can the grade of a blast finish be determined by using the surface

comparators to BS 7079 Pt C3? 13 What profile range can be measured using X coarse Testex? 14 What profile range can be measured using ‘coarse’ grade Testex? 15 What are the two theories of adhesion? 16 Briefly describe the mechanisms of the two theories of adhesion. 17 How many microns are in 1thou? 18 Give three different names for the cross section of a blast. 19 What is the approximate speed of abrasives leaving a venturi nozzle? 20 What is the most common cause of flash rusting on a blasted substrate? 21 What would be considered to be an ideal shot grit mix? 22 What is the purpose of mixing shot and grit? 23 Which abrasive would have the effect of work hardening a substrate? 24 Name three methods of measuring or assessing a profile. 25 What is the most common cause of rogue peaks on a substrate? 26 In what situation would it be better to use steel grit in preference to

copper slag abrasives? 27 If cracks or laminations are found on a substrate after blasting what

steps should be taken? 28 Using comparators to ISO 8503, what are the three main profile

assessments? 29 What are the other two assessments when the above three are not

appropriate? 30 What would be size of copper slag needed to give a profile of 50 to

75m?



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Surface preparation – Tuesday 1 What is the title of the BS 7079? 2 What are the four characteristics of an abrasive? 3 Why are blast hoses carbon impregnated? 4 Name the gauge used for measuring pressure at the blast nozzle? 5 Name four advantages of centrifugal blasting over open blasting. 6 According to BS 7079 is it possible to blast clean to an A Sa1? 7 Is there any difference between an A Sa1 and B Sa1? 8 Could you tell the difference between rust grades A and B blasted to

Sa3? 9 Could you tell the difference between rust grades C and D blasted to

Sa3? 10 What would be a typical speed of abrasives leaving a wheel abrator? 11 What is considered to be the most efficient blasting pressure? 12 What is meant by the term burnishing? 13 What would be the equivalent to St2 in the Sa grades? 14 What is the neutral figure on the pH scale? 15 What is the meaning of pH and how is it measured? 16 Why are inhibitors sometimes added to water in wet blasting? 17 Name two typical areas where needle guns might be used? 18 What is the duplex process of surface preparation? 19 Which pH range covers acid and which range covers alkalis? 20 Name two areas on a structure where flame cleaning cannot be done. 21 Which three basic operations are performed during flame cleaning? 22 How does BS 7079 define flame cleaning standards? 23 What is a Jason’s Hammer? 24 What is meant by St2 and St3? 25 Two alloys are used to render wire brushes spark free, what are they? 26 Why should burnishing be avoided? 27 Name two major disadvantages of using a needle gun. 28 After phosphating, what would be a typical pH requirement prior to

coating? 29 What is understood by the term knock out pot? 30 If an operator was blasting with a nozzle pressure of 80 psi. What would

be his approximate efficiency? 31 Which solvents are commonly used for degreasing? 32 What is a ‘deadman’s handle’? 33 How is abrasive cleansed in a wheelabrator system? 34 What is the main disadvantage of high pressure jetting compared to

other systems? 35 Name five methods of wet blasting. 36 What are the typical temperatures and concentration of sulphuric acid in

the pickling process? 37 Describe the duplex process. 38 What would be a maximum pressure for high pressure water jetting? 39 What are the disadvantages of wet blasting over dry blasting? 40 What would be considered to be advantages of wet blasting over dry

blasting? 41 Why is the phosphating or chromating of steel done?



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42 What would be an acceptable remedy for burnished areas? 43 Would burnishing be expected on areas of St2 preparation? 44 How many photographs of blast cleaning standards are shown in BS

7079 Pt A? 45 Do the plates shown in BS 7079 Pt. A relate to grit blasting or shot

blasting?



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Paint technology (1) - Wednesday 1 Name a third type of paint other than solvent free and solvent borne. 2 An epoxy resin would use which solvent? 3 Name four or more advantages of chlorinated rubber paints. 4 What are the three main disadvantages of chlorinated rubber paint? 5 Which solvent could be used with a phenolic resin? 6 Chlorinated rubber paint would contain which solvent? 7 Would it be good practice to apply chlorinated rubber over alkyd resin? 8 Which solvent would be used with an alkyd resin? 9 How was the word alkyd derived? 10 What is meant by opaque? 11 What is meant by vehicle? 12 Would it be acceptable practice to apply an alkyd over chlorinated

rubber? 13 Would it be acceptable practice to apply chlorinated rubber over

phenolic? 14 Would it be acceptable practice to apply phenolic resin over chlorinated

rubber? 15 Would it be acceptable practice to apply epoxy over linseed oil base? 16 Would it be acceptable practice to apply chlorinated rubber over epoxy? 17 Would it be acceptable practice to apply epoxy resin over alkyd resin? 18 What is another name for an un-pigmented paint? 19 What are the natural properties of a resin? 20 What are the natural properties of an oil? 21 How does paint using the barrier principle work? 22 How does paint using the passivation principle work? 23 How does paint using cathodic protection principle work? 24 Give another name for solvent free two packs. 25 Name six properties of a binder. 26 Name three natural resins used in paints. 27 Name five natural oils used in paints. 28 What does oleoresinous mean? 29 Name an Inorganic high temperature service binder. 30 Name two pigments likely to be used for high temperature service.



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Paint technology (2) - Wednesday 1 By what name would you call the basic unit of a polymer? 2 What is polymerisation? 3 Name three types of polymers. 4 What would be the characteristics of a short oil paint? 5 What would be the characteristics of a long oil paint? 6 What is meant by the term opaque pigment? 7 What is a typical size of a pigment particle? 8 Briefly describe the difference between saturated and unsaturated when

referring to oils or polymers. 9 Name two drying oils, which are unsaturated. 10 What is the main difference between a dye and a pigment? 11 What are the sources of pigments? 12 If titanium dioxide was used in paint, what would be the colour? 13 Name three rust inhibitive pigments considered to be toxic. 14 Name four commonly occurring minerals used as extender pigments. 15 Name three laminar pigments. 16 If pigment was added way below the CPVC, how would it affect the film? 17 The abbreviation CPVC means what? 18 Why are thixotropes added to a paint formulation? 19 If carbon was used as a pigment what would be the paint colour? 20 Name four properties that a binder contributes to a paint film. 21 Describe how a basic inhibitor works. 22 Which of the common extenders could not be used in whites and

pastels? 23 How would the film be affected if pigment was added above the CPVC? 24 Which of the rust inhibitive pigments is the most common? 25 Why are extenders used in paint formulation? 26 If chromium was used as a pigment, what colour would the paint be? 27 Why are plasticisers added to paint? 28 Two metals are commonly used as galvanic pigmentation, name them. 29 Why are driers added to oil based paint? 30 What is meant by the term thixotropic? 31 What is meant by the term aggregate when referring to paint? 32 If an antioxidant was added to paint, what would it do? 33 Give the names of two plasticisers. 34 What is meant by the term solution? 35 Give two examples of a solution. 36 What is meant by the term dispersion? 37 There are two types of dispersion, what are they? 38 If paint cures by chemical reaction is it reversible or convertible? 39 What type of polymerisation occurs in chemically curing paint? 40 Name a paint, which dries solely by solvent evaporation. 41 What is meant by non-convertible? 42 What is meant by non-reversible? 43 Name four drying mechanisms. 44 In a coating, which dries by solvent evaporation, what type of

polymerisation occurs? 45 What is another term for Fineness of Grind?



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46 Which generic types of paint dry by solvent evaporation followed by oxidation?

47 What type of polymer forms during oxidation? 48 What term applies to paint drying at ambient temperatures? 49 What is meant by the term coalescence? 50 What is meant by the term pot life? 51 Name three curing agents used in epoxies? 52 Is paint a solution or dispersion, qualify? 53 What is an exothermic reaction? 54 What is meant by the term induction period? 55 What is the difference between ‘thermoplastic’ and ‘thermosetting’? 56 With a chemically curing paint, what type of polymerisation occurs? 57 Two other terms relate to induction period, what are they? 58 Does a phenolic resin have an induction period? 59 Which of the following binders are reversible? a) Epoxy d) Urethane g) Cellulose b) Phenolic e) Chlorinated rubber h) Silicone c) Vinyl f) Alkyd 60 Is an epoxy powder paint thermoplastic or thermosetting? 61 If a coating is permeable, what does it mean? 62 What is meant by cross-linking, give two binders as an example. 63 What is the opposite to exothermic? 64 What is the term used for paints needing temperatures in excess of

65oC to cure? 65 What would be a typical induction period for chlorinated rubber paint? 66 Name a material used as a dryer in paint formulation. 67 Why would bentones or waxes be used in paint formulation? 68 Name two materials used as plasticisers. 69 What generic type of paints would use anti-oxidants? 70 How does a single pack, epoxy ester paint dry? 71 How is dew point defined? 72 How is relative humidity defined? 73 When using a whirling hygrometer which bulb should be read first and

why? 74 At what speed should the thermometer bulbs pass through the air? 75 What should be used when wetting the wick on whirling hygrometer? 76 By what other name can we refer to a whirling hygrometer? 77 When the air temperature rises does the air’s capacity to hold water

increase or decrease? 78 What is the stated criterion for acceptance, prior to calculations, on a

whirling hygrometer? 79 Name two pieces of equipment used for taking steel temperature. 80 Is it possible for a wet bulb temperature to be higher than the dry bulb?



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Paint testing – Thursday 1 Define viscosity. 2 What is meant by high viscosity? 3 Approximately, what is the viscosity of water? 4 Name the cgs and SI units of dynamic viscosity. 5 Name three different flow cups. 6 When using a flow cup which unit of viscosity would be used? 7 In Ford Flow Cup No 4 what does 4 relate to? 8 Give the names of three different rotational viscometers. 9 Give a reason for performing a viscosity test on site. 10 Which viscometer would not be used on thixotropic paint? 11 Why is temperature very important when doing viscosity tests? 12 What is the main difference between the rotathinner and Krebs Stormer? 13 Describe how to use a Ford Flow Cup. 14 Give another name for a Fineness of grind gauge. 15 Is a low flash point safer than a high flash point? 16 How and for what is a Hegman grind gauge used? 17 Briefly describe how to do the volatile, non-volatile test to BS 3900 Pt.

B2. 18 Name the equipment used to determine the flash point of a solvent. 19 What colour should the flame be at the flash point? 20 What formula is used to calculate the density? 21 What formula is used to calculate specific gravity? 22 What is relative density? 23 What are the other names for a density cup? 24 What is a stoke, the unit for? 25 Which test is used to determine abrasion resistance? 26 Which equipment would be used to determine flexibility? 27 Which equipment would be used to measure impact resistance? 28 For what reason would the Koenig Albert apparatus be used? 29 For which two reasons could a density cup be used on site? 30 Name four accelerated test boxes. 31 Why would a tropical box be used? 32 Would a paint be higher or lower density than water? 33 How would the density be affected if solvent was added to paint? 34 What is the capacity of a density cup? 35 What difference is there between SG and density? 36 What information could be obtained from a water soak test? 37 What information could be obtained from a temperature cycling test? 38 What information could be obtained from a cold check test? 39 Name four drying and curing tests. 40 What stage of the BK test would be recorded as the drying time? 41 Name three methods of determining opacity. 42 What effects the opacity of a paint film? 43 Why would a Pfund cryptometer be used? 44 Give one reason why an inspector would use a PIG gauge? 45 Why are wet paint film thicknesses needed? 46 Name two methods of measuring WFTs 47 What is the reason for taking WFTs immediately after application?



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48 Where could an inspector find information to determine if a two-pack paint was mixed in the correct proportions, using a density cup?

49 Can a banana gauge be used on non-ferromagnetic substrate? 50 Could an eddy-current gauge be used on ferromagnetic substrates? 51 Can a horseshoe gauge be used on non-ferromagnetic substrates? 52 As part of which test would a bar applicator be used? 53 Which instruments would be used to measure reflectivity? 54 How does a gloss meter work? 55 Which factors in paint govern the degree of gloss? 56 In a primer/mid coat what would be the expected degree of grind? 57 In a gloss paint what would be a typical degree of grind? 58 What percentage reading would be expected when measuring gloss on a

glass panel? 59 Using a gloss meter a reading of 25% would signify what? 60 If an aggregate size of 35m was present in a paint of 30m DFT what

would be a likely result when using a gloss meter? 61 Name three common tests for determining adhesion of a paint film? 62 Which adhesion tests are quantitative? 63 Intercoat adhesion and primer to substrate adhesion are two adhesion

faults name the third? 64 What chemical solution is used to conduct a cathodic disbondment test? 65 Which gas evolved at the cathode causes disbondment? 66 What criterion is used when assessing a cathodic disbondment test

panel? 67 Name the two methods of applying cathodic protection. 68 What is used to determine the potential of a pipeline? 69 Would it be advisable to refill a pipe trench with carbonaceous backfill? 70 Does a cathodic protection system eliminate corrosion? 71 Can the external surface of a tank be protected? 72 Could a crude oil tank be fully protected internally? 73 What voltage would be used on a 250m thick paint using a sponge type

pinhole detector? 74 What voltage would be used on a 450μm thick coating with a sponge

type pinhole detector? 75 When using a wet sponge, what other liquid is added to the water? 76 What function does the above additive perform? 77 Would it be advisable to do wet sponge detection on galvanising? 78 Why work upwards on a vertical surface with a wet sponge? 79 Does a sponge detector work on AC or DC current? 80 Other than the wet sponge, which other equipment could be used to

determine the presence of pinholes/holidays?



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Revision questions general – Friday 1 Name two categories of paint mill. 2 What is the main reason for processing paint in a mill? 3 Briefly describe how a ball mill works. 4 Briefly describe how an attritor mill works. 5 When would steel balls not be used in a ball mill? 6 A bead mill is sometimes called by which other names? 7 How does a colloid mill work? 8 Name eight items of information listed on a materials data sheet. 9 What do you understand from the term halogenated hydrocarbon? 10 How can we determine the viscosity of a high viscosity paint? 11 Briefly describe the principles of CP. 12 What function does a primer have in a paint system? 13 In a mordant primer what is the main working constituent? 14 What advantages do electrostatic application methods provide? 15 Which is the most expensive type of brush filling? 16 What is cohesive failure in paint, give the main cause? 17 Why does a zinc rich paint need a strong binder? 18 Why are etch primers not spray applied? 19 What do you understand by the term over spray? 20 Name four methods of determining DFTs. 21 What is a psychrometer used for? 22 What colour should a galvanised surface be after application of T wash? 23 How soon can a T washed substrate be coated? 24 Other than pigment, base and curing agent name two other constituents

of FBE powder paint. 25 Give the main differences between airless and conventional spray. 26 Brush application has advantages over spray application, what are they? 27 What is the main consideration when selecting a metallic pigment for a

sacrificial paint? 28 What is meant by sheradising? 29 Name three types of paint feed for a conventional spray. 30 What is the calorising process? 31 Why would a sealer be applied to aluminium metal spray? 32 What is the BS 2015 term for skipped or missed areas? 33 A colour has three properties, what are they? 34 Why would paint be applied by hot spray? 35 On an airless spray tip how are blockages cleared? 36 How is atomisation achieved using conventional sprays? 37 How is atomisation achieved using airless sprays? 38 What is dip coating? 39 What do you understand from the term ropiness? 40 What is efflorescence and how does it occur? 41 Name two ways of melting aluminium to enable it to be sprayed. 42 What is flocculation? 43 What could be the cause of bittiness in a paint film? 44 What is a tie coat? 45 How many depressions of the bellows are needed for the Dräger test? 46 What are the hazard signs for Toxic, Very Toxic, Harmful and corrosive?



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47 What is saponification? 48 What units are used for measuring toxicity? 49 Which material would have to be used on a perpetually damp surface? 50 What is padding? 51 What air inlet pressure is needed to give 2500 psi delivery with 35:1

pump? 52 What causes lifting of a paint film? 53 What is cissing and how is it caused? 54 What is meant by the abbreviations: OES, OEL, MEL, UEL, LEL and

RAQ? 55 Why would a paint inspector use potassium hexacyonoferrate? 56 What would be an average thickness for galvanising? 57 How can you tell the difference between blooming and chalking? 58 What could be the reasons for inter coat adhesive failure? 59 How would you determine quality of added thinners in thixotropic paint? 60 Why are manufacturers developing solvent free, water borne and

powders? 61 What would be the cause of grinning on a paint film? 62 How can bleeding be avoided? 63 In less than 30 words, explain the duties of a painting inspector. 64 Name five documents, which a painting inspector might need on a

contract. 65 What information should be given on a daily report sheet? 66 Curtains, Sags, Runs and Tears are a result of what? 67 Some binders can be modified to use water as a solvent, name four. 68 What is meant by the term stripe coat? 69 How many cm3 are there in 4.5 litres? 70 A paint data sheet provides a wealth of information, name eight items.



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Revision questions PA 10 specific 1 What is the specified course of action for grit inclusions? 2 The term long term protection refers to what? 3 What is the difference between new and weathered galvanising? 4 What criterion determines which paint system should be used? 5 What is the total DFT of the compliant epoxy system? 6 What is the total DFT of the water borne system? 7 When can ladders and other means of access be removed? 8 Two materials are specified for used on damp surfaces, what are they? 9 After removal of a non-drying paint, which type of primer is

recommended? 10 Some non-ferrous substrates are painted for aesthetics only, name four. 11 Which three non-ferrous substrates are painted for anti-corrosion

purposes? 12 According to PA 10 in which situations would T wash be used? 13 How many coats of primer are specified on surfaces at 100-149°C? 14 Give preferential order of coating systems for surfaces 150-340°C. 15 Is it mandatory for a contractor to produce a test area? 16 List four items needing masking off prior to blasting and painting. 17 Which Aluminium substrate would not be sweep blasted? 18 Which three paint systems are specified for use on aluminium? 19 What differences are there in new and maintenance painting

specifications for substrates below 100°C? 20 Toxic coatings need special considerations for removal from substrates,

name two methods which comply. 21 In which situations is a Permit to Work required? 22 Which primers are specified for non-weathered galvanising? 23 Which primers are specified for weathered galvanising? 24 According to PA10 is flame cleaning allowed? 25 According to PA10 is thinning of paint allowed? 26 What temperature range is covered by ‘hot duty service’? 27 Does PA10 cover internal coatings on pipes? 28 What is the specified overlap on repair areas? 29 What would be the specified surface preparation and coating system for

aluminium cladding? 30 What would be the procedure for removal of algae and mould? 31 What would be the procedure for degreasing prior to surface

preparation? 32 What would be the procedure for degreasing after to surface

preparation? 33 When blast cleaning on an AGI what precautions are taken? 34 Is it permissible to prepare paint by stirring? 35 What would be the surface preparation method for new galvanising? 36 When would it be necessary to apply a sealer to inorganic zinc silicate? 37 How could areas of a paint breakdown be prepared for repainting? 38 What information should be on a paint can label for BG? 39 When measuring DFTs over galvanising what allowances are made? 40 What is the first coat applied to galvanised substrates and why?



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41 Properties and Performances of paint are covered in which BG specification?

42 What are the considerations when selecting a paint system? 43 According to PA 10 which two coats are applied at works? 44 Give the criterion for when and when not, painting can take place. 45 What should be the substrate reaction when T wash is applied to a newly

galvanised substrate? 46 Which two materials are specified for use on damp surfaces? 47 What is the maximum time lapse from surface preparation to coating? 48 Which is the most common pigment used in high temperature paints? 49 What would be the result of over thick application of zinc silicate? 50 According to PA10 is roller application permissible?



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B Gas 3.2 Maths Exercises WFT (Wet Film Thickness) calculations 1 What WFT would need to be applied to give a DFT of 45μm using a paint

of 56% VS? 2 What WFT would need to be applied to give a DFT of 60μm using a paint

of 40% VS? 3 A paint of 38% VS was used to give a DFT of 45μm what would be the

WFT? 4 A DFT of 55μm was obtained from a paint of 55% VS, what was the

WFT applied? 5 What WFT would be applied to leave a DFT of 65μm using a paint of

49% VS? DFT (Dry Film Thickness) calculations 1 What would be the DFT if 20 litres of paint, 45%VS covered an area of

9m x 12m? 2 25 litres of paint, 65%VS was used to cover a circular area of 10m

diameter. What would be the resulting DFT? 3 What DFT would be obtained if a paint VS content of 42% was applied at

a WFT of 84μm? 4 With a WFT of 130μm, using a paint containing 83% VS, what would be

the resulting DFT ? 5 A paint, 65%VS was applied at a WFT of 130μm, what would be the

resulting DFT? VS (Volume Solids) calculations 1 A DFT of 53m was obtained from a WFT of 110m, what was the %VS

of the paint? 2 A paint was applied at 120m WFT. The resulting DFT was 65m, what

was the %VS? 3 What would be the %VS of a paint if it was applied with a WFT of 120m

and a DFT of 68m was obtained? 4 What was the %VS of a paint with a DFT of 36m, when the WFT was

108m? 5 A DFT of 62m was measured, from a WFT application of 100m, what

would be the %VS of the paint used? Volume calculations 1 What volume of paint would be required to cover an area of 300 square

metres, to a specified DFT of 65m, using a paint of 45% VS? 2 How much paint would be required to coat a tank, roof and side sheets

to a DFT of 100m? The tank is 5 metres diameter and 6 metres high. The paint to be used is solvent free.

3 How much paint would be needed to cover a circular area of 10 metres diameter, using a paint 65% VS to a DFT of 60m?

4 A circular area of 7 metres radius is to be coated to a DFT of 45m. What volume of paint would be required if the VS content was 48%?



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5 How much paint would be needed, at 55% VS, to coat an area of 250 square metres to a DFT of 60m?

Density and SG exercise 1 What would be the weight of 16.5 litres of paint with a SG of 1.45? 2 What is the density of a paint if 7.5 litres weighs 9.75kg? 3 What would be the relative density of paint in question two? 4 If the weight of 25 litres of paint is 37.5kg, what would be the SG? 5 A two-pack epoxy should be mixed at one part base to one part

activator, the base has a density of 1.4gm/cc and the activator 0.9gm/cc. What would be the density of the mixed components?

6 A two-pack paint is mixed at a ratio of six parts pack A (density 1.3gm/cc) to one part pack B (density 0.9gm/cc). What would be the density of the combined parts?

7 A mixed two-pack paint has a density of 1.35gm/cc. The density of the base was 1.5gm/cc and the activator 0.9gm/cc. The mixing ratio was 3:1. Has the paint been mixed correctly?

8 A mixed two-pack paint has a density of 1.35gm/cc. Mixed at a ratio of 6:1, base density 1.45gm/cc, activator density 0.95gm/cc. Has the paint been mixed correctly?



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RH and DP exercise WB DB DP RH Steel temperature Y/N

1 10 12 13

2 9 10 11

3 4 6 6

4 5 7 6.5

5 11 12 12

6 14.5 15.5 16

7 9.5 10.5 11

8 12 16 17

9 12 13 13

10 13 13.5 14

11 17.5 21 23

12 14 17.5 17

13 11 11 11.5

14 7.5 8.5 8

15 7 6 7

16 6.5 8 11

17 2 3 3

18 13 15 16

19 8 8 8

20 16 18.5 19

21 17 18 18

22 8 9.5 10

23 22 24.5 24.5

24 16 16.5 19

25 3 4 5

26 7 8 9

27 19 18 20

28 12 12.5 13

29 14 16.5 16.5

30 8.5 11 11

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Appendix 1 - Insulation General The following text deals with acoustic cladding and thermal insulation. The materials used for both applications may be split into three separate types: 1 Insulating materials. 2 Protective coverings. 3 Fixing materials. All insulating materials must be stored in dry conditions under cover. During installation, weatherproof sheeting must be used during inclement weather and after each day’s application. Installation must be performed generally to standards normally accepted as first class workmanship. The finished cladding must be of good appearance and free from dents and sharp edges. Nameplates, code inspection plates and stampings on equipment must be left permanently visible and the cladding must be properly sealed around them. If the above requirement is impracticable, a second plate supplied by British Gas and permanently marked with the same information and with the work ‘DUPLICATE’ must be fixed on the outside of the cladding in the most convenient, adjacent position.

Acoustic cladding

General The relevant British Gas standards are: BGC PC PWC1 - Acoustic cladding. Part 1 - Cladding of gas pipe and equipment. Part 2 - Notes for guidance. Cladding is defined as an assembly comprising porous insulation material with a metal outer jacket. The purpose of the cladding in this application is to cut down noise, typically by 10-20 dB. Materials The insulation is usually in the form if semi-rigid sections. For small bore pipework, 25mm diameter and below, flexible wool or fibrous materials may be used. The insulation is typically 50-100mm thick and must comprise of non-toxic materials including materials which release non-toxic fumes during a fire.



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The metal jacket must be aluminium alloy, or galvanized mild steel sheet where greater rigidity is required. The thickness of the jacket must be related to the durability and strength required, ease of fitting and costs but will normally be between 0.71 and 1.6mm thick. Galvanized materials, or any materials which contain zinc, must not be used on stainless steel due to the danger of causing zinc embrittlement. Insulation banding may be metallic or non-metallic. If metallic, it must be of the same material as the jacket. Mastic sealants and rubber or neoprene bedding strips used must be suitable for use at temperatures between -20-50°C with occasional increases up to 80°C. Nuts, bolts, screws and washers must be either stainless steel or zinc metal coated mild steel

Application of materials

The basic sequence for the application of acoustic cladding: 1 Preparation. 2 Insulation plus fixing. 3 Repeat insulation and fixing if required. 4 Metal cladding plus fixing. All surfaces to be clad must be clean and dry. For pipework up to approximately 300mm, preformed section of insulation must be cut and profiled to fit, and secured at 450mm intervals with banding strip. For greater diameters, flexible flat forms of insulation, eg mattresses, must be used. The jacket should not be allowed to come into direct contact with the noise radiating structure or with its supports, but should cover the whole noise radiating area without gaps or voids. The finished insulation must be even, solid, tightly joined and well secured. Where two layers of insulation are specified, then both the longitudinal and circumferential seams must be staggered. The insulation must be completely covered by a metal jacket. All overlapped joints must be at least 25mm, bonded with mastic sealant, and must be arranged to shed water. The metal jacket must never touch the pipe or equipment.



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Clearance between the metal jacket and branches should be approximately 6mm and filled with mastic. Potential metallic contact at other similar locations is also important to consider. The metal jackets of all acoustic cladding must be continuously bonded together by a strip of jacketing metal and connected to the pipe; however, cladding must not act as an electrical bridge over isolation joints.

Thermal insulation General The relevant British Gas standard is: BGC PS PWC2 - Thermal insulation of above ground pipework and equipment. The insulation in this application is used for heat conservation, cold conservation, personnel protection, anti-condensation, frost protection and for the maintenance of operating temperatures. Temperatures applicable are from –200-1000°C depending on the application. Materials Insulating materials The insulating materials proposed for any application must be selected from the following types: a Glass fibre. b Foamed glass. c Rock wool. d Modified slay wool. e Expanded perlite. f Vermiculite (loose granular fill). g Calcium silicate. h Phenolic foam (not within buildings – toxic during combustion). i Polyisocyanurate (not within buildings – toxic during combustion). The insulating materials used must not contain substances which support pests or encourage the growth of fungi. They must not cause a known hazard to health from particles or toxic fumes, during application, whilst in use or on removal. The insulating materials used may be applied in layers depending on the total thickness required; in some cases up to approximately 400mm total thickness may be specified.



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Protective coverings

Protective coverings include: Vapour seals – vapour sealing compound, preferably trowelling grade,

which has an interposed scrim cloth made of woven glass cloth. Vapour sealing tape may also be used. Vapour seals are used for cryogenic applications.

Metal cladding – galvanized mild steel must be used, unless stainless

steel is being insulated, in which case aluminium alloy sheet is used. Hard-setting composition/self-setting cement and glass cloth may also

be used as protective coverings as an alternative to metal cladding in certain circumstances.

Fixing materials To hold down insulating materials and protective coverings, the following may be used: 1 Wire netting – used to hold down insulation but only used with hard-

setting or self-setting cement as a protective covering. 2 Binding wire. 3 Binding tape. 4 Fixing bands. 5 Self-tapping screws. 6 Nuts, bolts and other fastenings. 7 Adhesives. 8 Anti-abrasion compound. 9 Joint sealant Application of materials The basic sequence for application for general heat conservation and protection: 1 Preparation. 2 Insulation plus fixing. 3 Repeat insulation and fixing if required. 4 Metal cladding plus fixing, or self-setting or hard setting cement. The basic sequence for application for general cold conservation and cryogenic service: 1 Preparation. 2 Insulation plus fixing. 3 Repeat insulation and fixing if required. 4 Vapour seal. 5 Metal cladding.



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As far as possible, insulation of pipework must be performed with preformed sections of insulating materials not exceeding 1m in length. Each layer of insulation is secured with binding wire every 150mm on pipework; the wire must not be allowed to cut into the insulation. If insulating equipment, fixing bands are used every 300mm. Metal cladding must be applied so that all overlapped joints must be at least 75mm (40mm on 40mm diameter pipe and below). The overlapped joints must be arranged to shed water. The metal cladding must never touch the pipe or equipment. Fixing bands must be used every 450mm.

Rev 2 June 2011 Appendix 2 - Data Sheet Examples


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Appendix 2 – Data Sheet Examples



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Other documents National Grid Specification for New and Maintenance Painting at Works and Site for Above Ground Pipeline and Plant Installations - T/SP/PA/10 (August 2004) Institution of Gas Engineers (IGE) Recommendations* National Grid Specification for Paint Systems - Properties and Performance Requirements - T/SP/PA/9 (August 2004)



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All of previous coatings. Part Al: - Supplement 1 - Representative photographic examples of the change of appearance imparted to steel when blast-cleaned with different abrasives.



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Institution of Gas Engineers (IGE) Recommendations* *These Recommendations are available from 21 Portland Place, London WIN 3AF. IGE/SR/3 - Electrical equipment in gas production, transmission, storage and

distribution IGE/SR/4 - Low pressure gasholders storing lighter-than-air gases IGE/SR/5 - Opening of gas works plant and working in confined spaces IGE/SR/12 - Handling of methanol IGE/SR/21 - Blast cleaning operations IGE/TD/6 - Handling, transport and storage of steel pipes, bends, tees, valves

and fittings National Grid specifications G11 - Notes for guidance on the issue of Permits to Work CW5 - Code of Practice for the selection and application of field applied

external pipework coatings PA9 - Technical specification for paint properties and performances

requirements DIS 3.1 - Engineering Procedures - Safety - Health and Safety at Works DIS 3.5 - Engineering Procedures - Health, Safety and Environmental

Protection Other National Grid publications Handbook on Safe Handling of Substances in Use within the Gas Industry Computerised Information System for Substances in Use in National Grid (CISSUB).



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