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Home & Garden Articles
Corrosion Control & Protection Part 4
Thu 27 Aug 2009 - 22:27
Corrosion Control & Protection Part 4
By [http://ezinearticles.com/?expert=C._C._Ang]C. C. Ang
Protective Coatings
Anti corrosive pigments are:-
a) Red Lead
b) Zinc Chromate
c) Zinc Phosphate
d) Zinc Dust
Preventing corrosion involve minimizing the flow of corrosion current so that if negligible current, negligible corrosion result. Resins with highest electrical resistance are, catalyzed epoxy, phenol, vinyl and chlorinated rubber. Addition of pigments such as talc, china clay, mica and iron oxide also assist in increasing the resistance.
The removal of soluble deposits on the steel surface is necessary as their presence will short circuit the resistance of the paint film to stage where rusting can occur. The thicker the coating the higher will be the electrical resistance. Coatings limit the access of oxygen, water and salts and therefore control corrosion in this way by a purely barrier effect. Catalyzed epoxies, coal tar epoxies, polyurethane, vinyl's and chlorinated rubber coatings have the lowest permeability to water and oxygen.
Alkyd and oil based coatings are more permeable to corrosive agents than high performance coatings specified to ensure all corrosive substances are removed from the steel surface and maximum adhesion is achieved between the coating and steel. The coatings chosen or specified should be conducive to the environment, characteristic and purpose the substrate is subjected.
For purpose to prevent and resist causes by:-
a) Chemical spillage
b) Chemical fumes
c) High temperature
d) Skid resistance
e) Abrasive resistance
f) Oil resistance
g) Fire retardant
Types of Coatings
Normally coatings are classified as conventional coating systems or high performance systems. The decision on the type of coating to be used depend on the cost and budget one have and also the expected life or duration of the coatings and the required purpose of the coatings, either for the protection, decoration or coding purposes, etc.
For coding and decorative purposes, conventional systems are usually recommended but for protection purposes, high performance systems are recommended. Conventional coatings are usually one pack product whilst high performance coatings come in two packs. Single pack product are usually air drying, two pack products normally cure by chemical reaction on the base by the catalyst or hardener.
For best results, it is best to consult the manufacturers who are specialised in manufacturing certain range of coatings. Some are specialised in decorative coatings for houses, or protective coatings for industrial purposes or marine coatings vessels and ships or automotive coatings for vehicles or special coatings for appliances or air crafts. Go to the right manufacturer for the right type of coatings needed.
Type of Application
Application can be done by brush, roller, conventional spray or airless spray. Brush and roller are used where condition does not allow air or airless spray. Not suitable for high build coatings or achieve high film thickness. Slow in process and coverage is about 180m2 to 380m2 per man day. Conventional spray is suitable for decorative and coding as only thin film can be achieved.
For protection purposes, airless spray is recommended and is the quickest method of application. Covers 750m2 to 1100m2 per man day. Control of application is by tip size and input and output pressure. Can be used for most paint types and can achieve high film thickness.
Application Condition
Paint should not be applied in adverse conditions, like temperature below 5 degrees C and not above 35 degrees C. Not in a windy condition or when surfaces are wet due to rain, snow, ice, fog or condensation. Allow painting only when surface temperature is 3 degrees C above the dew point.
Paint Mixing
Every new tin of paint opened must be mixed thoroughly with a stirrer to disperse the solids. In two pack products, to ensure that the base and hardener can chemically react together and cure, always use a mechanical mixer. Always mix base and hardener in the correct ratio.
Paint Thinning
Use thinners only when necessary. Use the correct thinner for the product. Don't use more than 5% by volume of thinners. Excessive use will cause solvent entrapment, runs, sags, slumping, retard cure and affect wet film thickness and dry film thickness ratio.
Pot Life
All two pack products have a pot life. That is time period between mixing and curing when the paint is workable and can be applied. Pot life is temperature related. Never mix more than can be used before pot life expires. Exceeding pot life will waste paint and will block up airless spray equipment.
Airless Spray Procedures
Check tip size and output pressure. Hold spray gun 30 cm to 45 cm from the surface. spray gun to be parallel to surface 90 degrees from it. Horizontal pass 50 cm to l meter. Overlap previous pass by 50% if possible to ensure even film thickness. Trigger to be released at the end of each pass. Cross hatching recommended for even film thickness.
Wet film thickness/dry film thickness
Apply paint at the correct wet film thickness (WFT) to give the specified dry film thickness (DFT) (Check product data) Check WFT as soon as paint has been applied - before solvent evaporation, and adjust the spray or application technique until correct WFT is applied. The WFT:DFT ratio depends upon the volume solids. 50% volume solids WFT of 100 microns = DFT of 50 microns. 70% volume solids WFT of 100 microns = DFT of 70 microns. Check dry film thickness with electronic gauge or banana gauge if necessary but remember this will read total film thickness i.e. old paint and new paint. For wet film thickness use wheel or comb gauge.
Paint Losses
Actual loss from container, equipment, over spray, spillage, pot life and windy conditions. Apparent loss from surface roughness and variable film thickness distribution.
Paint Coverage
Coverage = Theoretical spreading rate TSR in m2/ltr
Area to be painted m2 over TSR = Theoretical consumption in litres.
Practical spreading rate PSR = TSR - Loss factor %
Area to be painted m2 over PSR = Practical consumption in litres.
Coating Defects
The major defects found in coatings are:
1. Slow or Incomplete Drying which means coating does not become touch or hard dry within specified time.
The causes are conventional coatings have not been properly mixed prior to application, drying additives have not been properly incorporated.
Two pack products have not been mixed in correct ratio, lack of curing agent leads to incomplete cure.
For remedy, leave coating for further 24 hours, observe if full drying occurs.
If coating is still soft or tacky, remove by mechanical means such as scraping until hard dry substrate is reached.
Re-apply coatings, ensuring material is properly stirred and mixed and checking correct thinners are added. Check temperatures.
2. Surface Discoloration.
The coating surface has a different appearance from that when originally applied.
Causes could be dirt settlement on rough coating surface. Resin breakdown as a result of ultra violet or chemical attack, usually yellowing Fading due to breakdown of colouring pigment. Rust staining due to rust water running down from corrosion sites.
Remedy is to remove discoloration by fresh water washing with water based detergent if required or by solvent washing if stains are not removed by above, abrade lightly with fine sandpaper until original colour is exposed. Patch prime and re-coat with finish.
3. Corrosion
Appearance of iron corrosion signs on steel surface.
Cause by removal of coatings by detachment of mechanical forces. Low film thickness, inadequate removal of salt contamination before coating application.
Remedy by scraping off rust scale and loose coatings. Wire brush or needle gun corroded areas to remove as much corrosion as possible. Feather edges of intact coatings to give smooth overlap. Patch prime and apply touch up of finish.
4. Defect of sagging or curtaining.
Coating has slumped on vertical surface during drying process.
Cause by coating being applied at excessive wet film thickness.
Remedy by allowing paint to dry to a hard finish. Remove runs with sandpaper to produce smooth surface. Re-apply finish to ensure thin areas above runs are also re-coated to film thickness required.
The author CC Ang who is the owner of http://engineeringsolutioncentre.com
and has over 60 years of experience in corrosion control & protection, is important for all metal structures. He has given consultations to countless organizations in the industry.
To know more about this industry you can visit his blog http://engineeringsolutioncentre.com
Article Source: http://EzineArticles.com/?expert=C._C._Ang http://EzineArticles.com/?Corrosion-Control-and-Protection-Part-4&id=2765178
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