Protection From Corrosion

By Mahmood NaqviKarachi – Pakistan

Protection From Corrosion What causes rusting?

A process started by the interaction of air, moisture & metal

How do we prevent it?By putting on the metal a barrierImpermeable to air and moisture

Chief Principles for Prevention Barrier forming paints

Conventional paints, epoxy, vinyl paints Active rust preventing zinc rich paints

Red led paints,zinc chlorate primer Problems with zinc rich paints

Lead compounds make the paint poisonous

Linseed oil impractical because of long drying time

Surface Preparation Critical because different surfaces have

different requirements Sharp surface projections should be

ground smooth. Grinding should not effect thickness

Boreholes should be, drilled & smoothed Oil, grease & other contaminations should

be removed

Pretreatment Importance of Pretreatment

The Swedish standard sis 055900 is used to ensure quality

Effects the results obtained Effects the costs involved

Different Corrosion GradesA. Steel surface with little if any rust,

completely covered with mill scaleB. Steel surface has begun to corrode,

mill scale has begun to flakeC. Steel surface with visible pitting, mill

scale corroded awayD. Steel surface with considerable pitting,

mill scale has corroded away

Pretreatment For Corrosion Grades Pretreatment through wire brushing &

blast cleaning Bad (light brushing & scraping) Middle quality ( scraping with scraper,

wire brushing & blast cleaning) Scraping, wire brushing or blast

cleaning till clean grey steel

SIS 055900 Norms: ST: Wire brushing Sa: Blast cleaning ST1, ST2, ST3 & Sa1, Sa2, Sa3 Norm between Sa2 & Sa3 found ideal As a result Sa 2 ½ was made

Method & Tool Selection Depends on:

Degree of damage to the surface Which paint system is to be used The nature of the surface Previous treatment Results required

The Ideal Maintenance Job Removal of rust and mill scale Removal of grease,oil, dust Removal of unwanted paint Abrasion of sharp edges Malting of glossy paint Thorough removal of dust

Methods Of Paint Removal Method

Blast cleaning Mechanical wire

brushing Mechanical disc-

sanding Needle chipping Mechanical rust

chipping Hand brushing Hand scraping

Result Ideal Risk of polishing Risk of indentations Bad heavy indentations Good with other methods Poor Only use with other

methods

Classifications of Surface Preparation:

Solvent cleaning (SSPC-SP-1) Hand tool cleaning (SSPC-SP-2 st2) Power tool cleaning (SSPC-3, SIS st3) Brush-off blast (SIS Sa-1, SSPC-SP-7, NACE-4) Commercial blast (SIS sa2, SSPC-SP6, NACE-

3) Near white metal blast (SIS Sa-2.5,SSPC sp-10,

NACE-2) White metal blast (SIS Sa-3, SSPC SP-5, NACE-

1)

Rules for Pretreatment Use correct pressure 7-7.5 kg/sq.Cm (100-107 p.S.I ) Loose paint & rust must be removed Use the correct properties of sand Always use good quality sharp quartz sand Remove dust from the blasted surface Small area at a time Protect with primer Paint with selected paint system

Blasting Equipment Selection Compressors: ideal working pressure 90-100. Nozzle: should be matched with discharge

volume capacity.Compressor rated at 125cfm at 100 psi. One orif ice of ¼ inch dia, delivered 81 cfm. Two orif ice of 3/16 inch dia delivering 81 cfm. One orif ice of ½ inch dia plus one orif ice of ¼

inch dia.Compressor rated at 600 cfm at 100 psi. Both delivering a total of 419 cfm. One orif ice of 7/16 inch diameter. Plus one

orif ice of 3/8 inch diameter. Both delivering a total of 450 cfm..

Continued… Abrasive: quality of surface affected by

size and shape of abrasive used.Preferred abrasives.

16-20 mesh silica sand or mineral grit. 20-40 mesh garnet deep red color. Crush iron slag. G-50 irons grit.Have a surface profile 1.5-3.5.

Abrasives continued…

Abrasive Size Mils Microns

Large River Sand

US 12 – US 50 2.8 70

Medium Silica Sand

US 18 – US 40 2.5 62

Silica sand US 30 – US 80 2.0 50

Continued.. Miscellaneous

Use dry abrasive, free from dust, salt and other impurities

Keep moisture, organic matter of surface Apply coating as soon as possible to

prevent surface rusting Dry cleaning should not be done in rain,

mist, fog or humidity levels above 85%

Coating Material

General guidelines: Coating material should be protected from rain dust & sunlight. Should be inspected when opened for use. Surface should be rendered dust free prior to coating. Should be mixed thoroughly for sufficient time as specified by

manufacturer. Pot life of catalyzed coating should not be exceeded. No coating when surfaces are less than 5 degree above dew point. Manufacturers recommendations regarding curbing time should be

followed.

Coating Material Specifications Depends on service requirements

Vary from general atmospheric exposure to salt water immersion

Depends on the degree of surface preparation

Materials must be checked for health, safety and environmental standards

Coating Application Spray Equipment

Lines & pots must be cleaned Must be an adequate moisture trap Suitable working regulator & gauges for air supply to

pressure pot & pressure gun Spray equipment must be grounded For airless spray 28:1 or 30:1 pump ratio should be

used. Air supply should be 80-100 psi For conventional air supply equipment should include

pressure pot with mechanical agitator, separator atomizing and fluid pressure regulators

A supply of tips with varying spray angles and washers

Coating Application

Spray application Gun should be at right angles to surface For airless spray the spray gun should be held between

1 ½ to 2 feet For air supply the spray gun should be held between 6

to 10 feet Even and parallel passes should be made with spray

gun, each spray should overlap the other by 50% Coating should be applied uniformly and completely

over the surface Areas such as corners & welds should be precoated by

brush to ensure they get at leas the minimum film layer

Coating Application Brush application

Brushes should be of suitable size and quality Brushing should result in a smooth and uniformly thick

coat, there should be no brush marks Runs or sags should be brushed out Successive coats are applied by cross hatching the

previous coat Care must be taken that no lifting of former coats

occurs Areas such as corners & edges should receive extra

coating to ensure the minimum level is met

Inspection Good house cleaning

Clean and orderly work area Storage and issue area Mixing utensils and containers Wet coating safe grades

Surface preparation Prior to surface cleaning

Careful examination Recording of inspector observations Repair of damaged areas Surface must be free from hydrocarbon and water

Inspection

Surface Preparation Blast cleaning equipment

Correct nozzle size Minimum 2 inch internal diameter hose and

coupling Blast hose of sufficient size to maintain nozzle

pressure All couplings should be external type Remote control valves fitted for safety ?????????????????????

Inspection

Surface preparation Abrasive medium: grit/ sand should be inspected to

ensure it is clean Final inspection

All phases of surface preparation must be completed before final inspection

A time lapse of at most 2 hrs is allowable between completion of blast cleaning and final inspection

Cleaning clothe must not be used to inspect surface The inspector must must monitor the condition of freshly

blasted or cleaned areas Degree of cleanliness can be confirmed based on surface

texture, shade or by using the Swedish standard

Inspection Coating application

Application procedure Preliminary inspection Homogenous mixing Thinners Site control of material

Equipment Airless spray equipment Brush application

Inspection

Coating inspection Quality control measures

Wet film thickness Dry film thickness Holiday detection

Safety General: a safety meeting must take place prior to

job Good house keeping: unnecessary people and

equipment pose obstacles to workers Sand blasting:

Remote control valve for safety Dust evacuators and canvas screens for work area Properly trained certified personnel Unnecessary people kept away Adequate light Secure and adequate scaffolding

Safety Coating application

Adequate equipment grounds Non-conductive hoses Any other protective devices Keep clear from spray stream Handle spray gun carefully Never point at anyone Do not cover the fluid tip Use necessary safety gear for personnel

equipment Never exceed the recommended inlet pressure

Safety Coating application continued…

Know the working pressure of accessories used with the gun Never exceed 5000 psi working pressure Before removing equipment shut down the pump and

decrease system pressure Never to clean the tip when installed in the gun Never operate airless spray equipment until all static

electricity discharge grounds are in place and connected Never flush solvents into containers exposed to direct

sunlight When a tank is out of service or unattended all accesses

must be closed so that coating may dry up