Coatings Basics

8/13/2019 Coatings Basics

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Corrosion Control


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Factors affecting the service Life


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Better Design


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Weld Design


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Avoid

Crevices


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Avoid Erosion Corrosion


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Better Material Selection

Material Selection Criteria Lowest Corrosion rate

Also falls within the project cost

If there is choice to select between steel and SS,you can better select SS

But if the choice is to select only steel, then one

has to make thorough investigations of varioussteels and find the best for application basedupon the chemistry of the steel


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Protection by Coatings


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Protection by CoatingsClassification of Coatings

Metallic / Ceramic/Organic

Room Temperature Application/ High

Temperature Application

Zn ( Galvanization), Cr, Sn, Ni, Cd

Aluminising, Al-Cr, Nitriding, carborizing


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Method of ApplicationHot Dip Method

Slurry method

Electroplating/Electroless Plating

Liquid route ( Cyanide bath)/Sol-gel

PVD/CVD

Thermal Spray / Laser

Coatings by Spray paint spray


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Paint Coatings

Though appears simplest method of

Corrosion Control, actually not so in

practice.

It is an art as well as science.

Requires good knowledge of paint

formulations, paint selection, applicationskill, suitable surface preparation method &

experience and inspection methods.


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Why paint Coatings ?

Aesthetics

Corrosion Protection

Both


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Paint System

Surface Preparation

Primer

Intermediate layer

Top layer

A technician is required to properly carry out

above jobs.

An inspector / supervisor is required to

continually monitor the requirement as per

specifications.


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Main Properties of a Coating

The coating material should be an effectiveelectrical insulator

It can be applied with no breaks The coating must be strong enough to resist

the impact of sharp objects.

It constitutes initially a perfect film whichremains so with time.


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Surface Preparation

Surface Cleaning

Conversion Coatings

Blasting

Dry

Hydroblasting


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Paint Application

Brush

Rollers

Air Spray

Airless Spray


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Coating Composition

Binder ( Resin)

Solvent Pigment

Additives


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Pigments

Several Functions as part of Coating Provide colour & Opacity

TiO2, C or lamp black

Reinforce the film structurally

Iron oxides

Corrosion resistance

Zn ( Galvanically)

Talc & mica act as extenders, reduce permeabilitythrough film as plate like particles block

permeation.

Lead chromate as inhibitive pigment


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Barrier Coating InhibitivePrimer Organic RichCoatings

Inorganic rich coating

Pigment

Role of Pigment


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Organic Zinc Rich Coatings They utilize an organic resin rather than an inorganic

silicate binder for film formation. They protect steel galvanically as well as by barrier

protection but do not provide a high level of galvanic

protection as do inorganic Zn rich coatings.

They are formed by simple solvent evaporation or bychemical reaction of components.

Film properties of these coatings are similar to that of free

zinc organic coatings. Organic Zn Coatings do not require high level surface

cleanliness while inorganic Zn coating require the best

cleaned surface.


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Inorganic Zn-rich Primer

Organic Zn-rich primers


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Additives

Trace or small quantities of these help in

modifying various properties:

Phenylmurcury, Zinc, Cuprous compounds actas mild inhibitors

Co and Mn Napthanates - help in drying

ZnO - protection of resin from heat and Sun


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What is required from a Paint

Coating Good Adhesion

Flexibility

Impact Resistance

Resistance to environment

Chemical

Moisture

Sunlight / Rains

T f C i


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Types of Coatings

Conventional

Lacquers, solutions of synthetic resins ( vinyl chloride,rubber and acrylic)

Water emulsion (latex) Coatings ( acrylics and Vinyls)

Oil based coatings

Epoxy Coatings Coal tar Epoxy coatings

Poly-urethanes

Polyester and Vinyl ester coatings

Organic Zn rich Coatings

Special

Solvent Less Coatings

Fiber Reinforced coatings


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Lacquers Lacquers are solutions of natural resins ( Shellac or bituminous materials) or

synthetic resins ( vinyls, chlorinated rubber and acrylics).

Upon application, solvent evaporates and the resin deposits unchanged.

Films have good water and chemical resistance but poor solvent and heat

resistance.

Require abrasive blast cleaning.

Applied in Multiple coats and easy to topcoats for repair. Have Good gloss Retention and resistance to weathering.

Advantages Limitaions

Rapid Drying & Recoating High in VOC

Good General Chemical Resistance Poor Solvent Resistance

Good in water immersion Low film build up

Good gloss Retention Blasted surface is necessary

Good Durability Occasional poor adherence


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Water Emulsion Latex Latex coatings are being successfully used to coat

wood and masonry structures.

Relatively porous nature of structure allows water

vapour to pass through them.

Advantages LimitationsReduced level of VOC Limited Durability

Easy to apply, topcoat & repair Poor chemical resistance

Fast to dry for recoating Poor wetting of surface

Excellent Flexibility Poor immersion service

Low Cost Best cure above 50 C.


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Oil Based Coatings Coatings based upon drying oil ( linseed oil, tung, soyabean,

fish oil).

Cure by reaction with oxygen.

Though complete dry less than in one day, complete curing

takes much longer.

Alkyd coatings use resin formed by the reaction of polyhydricalcohols ( glycerin) and polybasic acid ( phthalic acid) followed

by modification with drying oils. These cure much faster than

unmodified alkyds..

Silicon alkyd coatings were developed by modifying alkyd resin

with silicon ( 30%) to provide greater gloss retention.

Epoxy ester coatings are another modification of drying oils to

improve performance, particularly chemical resistance.

Uralkyd coatings are formed with polyurethane. These coatings

are hard.


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Epoxy Coatings

The Most Common two component thermosetting product. An epoxy resin is based on a reaction product of phenols,

commonly bisphenol F or Cresol with epichloridehydrin.

Available in solvent free, with solvent or water containing

formulations.

The two components are called base and the curing agent. Latteris used to polymerise the epoxy resin which has major influence

on the mechanical and chemical resistance properties. The most

common curing agents are aliphatic amines, ketamines and

polyamides.

Epoxy coatings bond well to the abrasive cleaned steel and cleanconcrete.Their films are hard and relatively inflexible. They

chalk in sunlight.


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Epoxy Coatings

We make the prepolymer using bisphenol A and

epichlorohydrin

Diepoxy molecule

Bisphenol A Epichlorohydrin


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Polyurethane Coatings

R-N = C = O + OH--R = R- NH- C - O -R

Very important Class of Coatings

Urethanes are perhaps the most common

materials used in our day to day life.

Soft, flexible to rigid materials are

availbale.


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Polyester and Vinyl ester Coatings

Unsaturated polyester and vinyl ester thermosetting resins are used as

base for a useful group for a high performance corrosion resistant lining

for steel, concrete and other surfaces

Polyesters and vinyls contain unsaturated groups which are reacted with

unsaturated monomers such as styrene.The combination of resin and

monomer forms a relatively low viscosity material to which when added

some catalysts such as peroxides accelerate curing.

A great skill is required to ensure proper propotionating and mixing

Limited pot life

Blast surface required when used as primer.

More expensive than epoxies.

To the final lining product, one can add reinforcements and fillers to

minimise the effect of shrinkage and enhance properties such as lowering

coefficient of friction and increasing tensile strength and decrease

permeation.

Polyesters and vinyls are applied in large thicknesses from 500 um to 5

mm.

Natural and Synthetic


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Natural and Synthetic

Rubber Linings Natural and synthetic rubber linings are used in such industries as

steel, chemical, paper, plating, mining, agriculture and power The physical properties of these steel linings allow them to

protect steel from erosion, corrosion, abrasion and chemical

attack.

These linings are applied tovarious types of equipments such asflue gas desulphurisation systems, pipe, tanks, railway tank cars,

rolls and pulleys.

Typical thickness of Rubber linings are from 3.2 mm to 6.4 mm.

Surface should be blast cleaned SA21/2 with a profile of 40-50

um.

The surface before linings must be primed with suitable primer.


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R-N = C = O + OH--R R- NH- C - O -R

Chlorinated rubber

Polyester Coating

Vinyl Acetate

Urethane

- CH - CH2-

Epoxy

--- CH -- CH --

O = C - OCH2- - CH3Acrylate


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Paint Application

1. Brush or Rollars

2. Spray

Air Spray Airless Spray

Electrostatic Spray


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2000+ psi fluid pressure

Transfer efficiency = 50 - 60%

Airless, Theory of Operation

Airless spray atomization- hydraulic force through

a cats eye shaped orifice

2000+ psi fluid pressure

Fastest and heaviest spray

finish and is measured in

liters per minutes

Transfer efficiency =

50-60%

Protective coatings

Click to see spray


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Transfer Efficiency

Transfer efficiency (TE) = (paint deposited on a part)/(total paint

sprayed)

TE = (Wp/Ws) x 100%

Wp= Weight of wet coating on the part

Ws= Weight of liquid coating sprayed


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Electrostatic Technology


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High Performance Coatings

1. One those which are functional specific

2. Can be applied in highly aggressive

environment.3. Have high Corrosion resistance

4. High Erosion & abrasion resistance

5. Can dry very fast

6. Can give large thickness in one coat


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Solventless Systems

Can achieve very high thickness inone coat

Can have Drying time of a few

minutes

Life can further enhanced by

addition of glassfalkes


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Fibreglass Reinforced Linings Fibergalss-reinforced plastic (FRP) linings are unstaurated

polyester and vinyl ester thermosetting resin linings intowhich fibre glass are incorporated to optimize the lining

performance.

They are commonly used to control the internal corrosion

of above ground petroleum storage tank bottoms. The main reinforcement used with polyester and vinyl ester

resin liner are E glass mat and 250 um glass veil ( very thin

fibre glass mat) which has electrical properties, strength

and durability appropriate for most applications. C glass mat is chemical resistant, e.g. resistant to most

acids. A 3000g/m2 is sometimes used instead of choopede

strand mat.


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SubstrateSubstrate

Coating

Glass Flakes

Moisture Penetration


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Offshore

Structures

Glass Flake Reinforced

Polyester Coating to achieve

750um in one coat

A li ti


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Application

Underwater:

Brushes - Using vigorous

circular motion.

Underwater pump withround brush - using

vigorous circular

motion


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Once we have made diepoxy prepolymers, we tie

them all together by adding a diamine.


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How Epoxies Work The well known adhesion of epoxies is due to the

strong polar bonds it forms with the surfaces it comesin contact with.

On dry surfaces the bond between the surface and the

epoxy displaces the air, which is a fluid. The same is true underwater. As on dry surfaces, the

polar bond attraction is strong enough to displace thefluid, in this case the water, and produce an strong

bond even underwater.

Thus, painting underwater is, in theory, no differentthat painting above the water. The cross linkingreaction of epoxies should be independent of thesurrounding environment.

POWDER COATINGS


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POWDER COATINGS Type I Thermoplastic- are high molecular weight products

that melt, flow and form a film without change in their

chemical composition. Because they are difficult to grind

they are often applied in fluidized beds and used in thick

films. : polyethylene, polypropylene, PVC, nylon.

Type 2 Thermosets- are lower molecular weight polymers

that cross link during heat curing to form higher molecularweight. They can be ground finer and thus applied in thinner

films 25-200 um. They chalk in presence of UV light.

FBE is one of the best known thermoset Coatings used in

large applications for external and internal pipelinescorrosion protection.

Application methods : Fluidized bed, electrostatic spray, hot

flocking and thermal spray.

Fl oroelastomeric Coatings


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Fluoroelastomeric Coatings Fluoroelastomeric coatings are among the most expensive and

highest performance coating materials in present Day.

Application, surface finishing, material mixing are of paramount

importance

Fluoroelastomeric coatings are chemical resistant coatings that can

withstand elevated temperatures and attack by acids and alkalies,

hydrocarbons, gases and solvents and many harsh industrial

chemicals.

They are generally two component catalysed systems that cure at

room temperature.

Physical properties can be controlled by varying the amount of

catalysts.

They are commonly used as heat and acid resistant linings in power

station flues and duct work. And also in tanks, sumps, ditches,

hazardous waste dikes and immersion zones.

Fl l B d L Lik l t B k


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The C-F bond is one of the strongest known(PVDF shown below)

Fluoropolymer Bonds are Less Likely to Break


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Fluorine Content of PVDF

Chemical Spot Test


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Chemical Spot Test

PVDF

Polyester


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Tapes & Wrappers TAPES are finally prepared coating material ready to apply

onsite. They may be either thermoplastic or felt impregnated with

petrolatum or petroleum wax.

Success of tapes depends upon the application. The most

important problem is the improper overlapping that causesgaps where shielding corrosion may take place.

The most common application of tapes is for immediate

repair of underground pipelines.

One of the failure areas of tapes is in above ground araessuch as soil to air pipe transition zones

Experienced tape applicators or especially with machine

wrapping one can eliminate failure problems.


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Thermal Spray Coatings

The most latest and easy to apply and

provide much longer life.

Flame spray, Arc Spray, Detonation Gun orPlasma Spray guns are used.

They are also called hybrid systems with

internal metal coating with a top sealcoating of epoxy or PU.

Life of such coatings is 25- 50 years.


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Various Thermal Spray SystemsFlame Arc

HVOF Plasma

Actual Application


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Actual Application

Flame Spray

Thermal Spray by Robots


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Summary Paint Coating is one of the most effective and known

methods of corrosion protection at the surface. To get an excellent corrosion protection, several factors

which include, proper surface preparation, correct

application technique, suitable environmental

conditions, inspection during surface preparation and

application are required.

Paint selection is very important. Depending upon the

environment, a suitable system is selected. Most modern paint coating systems such as

Floroelastomers, glass flake systems are used for

selective applications are now readily available.


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Coatings Basics