Elucidation of corrosion failure mechanisms of coated phosphated steel substrates

Abstract

Coatings are commonly believed to protect metal surfaces from corrosion based on some combination of their barrier properties and electrochemical properties. Various physical and electrochemical tests were performed on seven different coatings (latex, alkyd, 2-PK epoxy, and electrocoat) to determine which properties were the main determinants of corrosion resistance in continuous and cyclic corrosion tests. Physical property tests and AC electrochemical tests were all related to barrier behavior, while DC electrochemical tests were related to electrochemical behavior. DC electrochemical properties are commonly associated with inhibitor chemistry, but can be broadly related to all components of the paint. These test results were compared with corrosion test results from both continuous (B-117) and cyclic (GM9540) accelerated tests. The best correlation was seen with a model emphasizing barrier behavior with a secondary component relating to electrochemical protection.

Zinc hypophosphite: a suitable additive for anticorrosive paints to promote pigments synergism

“…In fact, after paint removal of panels exposed to the salt spray chamber, it was noticed that thick black iron oxide films developed under blisters and a nonexpansive oxide layer grew in the remainder of the surface. A similar dark oxide was reported previously by Amin et al, 43 who attributed an exceptional corrosion resistance to this layer constituted by a divalent cation, a trivalent one, and anionlike chloride. So, it was speculated that the main function of zinc hypophosphite was to prevent the oxidation of ferrous species into ferric ones to yield the formation of this dark oxide.…”

“…Alkyds showed an improved anticorrosive performance because their E corr was higher than -0.200 V during the first week of immersion and was more positive for the highest pigment content. 2,7,12,13 As was said previously, this behavior was due to the interaction of the binder with zinc oxide [42][43][44][45][46][47] (Fig. 5).…”

“…The unusual behavior of alkyds was attributed to the interaction of the binder with zinc oxide, which may result in a more impervious film. [42][43][44][45][46][47] As it occurred in other cases, waterborne paints did not exhibit a significant barrier effect; ionic resistance was too low from the very beginning of the test period (Fig. 4).…”