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#072 Corrosion Protection Measures - Painting - 2

Category : Corrosion - Corrosion Protection
March 4, 2011

Painting has been in long use for isolating metal surfaces from various corrosive environments. Painting here means to coat metal surfaces with paint and create layers to protect the metal surfaces from corrosive environments.

Painting is applied to virtually everything from ground structures such as bridges, buildings, towers, chemical plants, and marine structures such as shore protections, off-shore structures, as well as various vehicles and home appliances. It is said that 60% of all corrosion protection costs are spent on painting.

Paints are comprised of mixtures of insoluble pigment particles suspended in "vehicle" components that form the coating layers. Generally, solvents are added to the former to give fluidity. This is applied on metal surfaces and let naturally or heat evaporate the solvents for the coating layer to form by chemical reactions.

Natural oils such as linseed and tung oils are customarily used as the "vehicle'. These belong in the drying oil category where oxidation and polymerization occur and harden when exposed to air.

The pigments added to the vehicle are: titanium oxide for white, carbon black for black, and Prussian blue for blue coloring. For the adjustments of glossiness, strength, and volume of the coatings, barium sulfate, calcium carbonate, clay, and diatomaceous earth are used.

The most important Corrosion protect element is the Corrosion protect pigment. Lead tetroxide Pb3O4 and zinc chromate ZnCrO4 are historically being used. They function as corrosion inhibitors by dissolving into water that passes through the coating layer.
The important trait for these pigments would be the dissolvability in water. If the dissolvability is insufficiently low the corrosion protection would be ineffective, and if the dissolvability is excessively high the premature runoff would occur.

Oil based corrosion protection paints have been in wide use. These are comprised of lead tetroxide added to natural vehicles such as linseed oil, and were commonly used due to good economy offered. But recent advancements in high polymerization technology resulted in more use of excellent synthetic resin based paints. The current mainstream choice are phthalic acid, chlorinated rubber, urethane, epoxy, and vinyl chloride resins.

Paint coating method is the easiest and applicable to virtually all shapes and sizes and relatively lower in cost compared other methods. However, the Corrosion protection effectiveness can vary largely depending on the coating application techniques used. This means that it is relatively difficult to always maintain the quality level at a constant.

The quality variations may come from application specific coating layer formation processes, base metal preparation condition where the surfaces are more prone to result in rusts as well as some conditions that may prohibit the coating from adhering perfectly. The pre-coating preparation largely affects the final outcome.