The Influence of Contaminants on the Failure of Protective Organic Coatings on Steel

Abstract

While the ability of the organic coating to conduct corrosion currents may sometimes come into play, primarily the corrosive deterioration of protective organic coatings is brought above by the flow of corrosion currents through a film of electrolyte beneath the organic coating. Any of several specific mechanisms may be involved. The influences of contaminants lie in their abilities to: (1) draw water through the organic coating to establish a conductive film of moisture, (2) provide ions to carry the corrosion currents, and (3) provide ions of specific effect on underfilm corrosion mechanisms. Specific attention is given here to the effects of underfilm salt contaminants on atmospheric corrosion. Filiform corrosion observed is interpreted as a form of anodic undermining corrosion, which involves transport of the anion and the effect of the anion on the dissolution of metal from beneath the organic coating. Cathodic detachment, also observed, is interpreted in terms of cationic transport and the ability of the specific cation to form a strongly basic solution, resulting in loss of organic coating adhesion by water displacement.