Corrosion Control & Protection
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.
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.
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.
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