Mechanism of Corrosion and Delamination of Painted Phosphated Steel During Accelerated Corrosion Testing

Paint delamination from electrocoated automotive steels during atmospheric corrosion. Part I. Hot-dip galvanized and electrogalvanized steel

“…The phosphate crystals were only partly attacked in the delaminated zone, but had disappeared completely in the region containing the corrosion products. This proposed mechanism is entirely different from that reported for phosphated and painted cold-rolled steel [3][4][5]12]. In the latter, the locus of failure at the delamination tip is usually at the phosphate-paint interface and the phosphate crystals dissolve completely in the electrolyte of the delamination zone as a result of the very high pH.…”

“…The need for improving the corrosion performance of painted precoated automotive steels in recent years has led to several publications on the mechanism of corrosion propagation in accelerated and atmospheric conditions [1][2][3][4][5][6][7][8]. The reported mechanisms seem to be markedly different for the different types of metallic coatings [7][8][9] and are sometimes conflicting for the same type of coating from different sources [7,8] or in different corrosion tests [10].…”

“…In the latter, the locus of failure at the delamination tip is usually at the phosphate-paint interface and the phosphate crystals dissolve completely in the electrolyte of the delamination zone as a result of the very high pH. There is a consensus that for steel the delamination region is cathodic in nature [3][4][5]11]. The anodic nature proposed for zinc coatings seems to result in a slightly acidic environment in the delamination zone [6,8].…”

Spinels – reactive pigments affecting the diffusion of corrosive substances through the coatings

“…As has already been shown, each organic coating exhibits a definite permeability to corrosive substances such as water, oxygen, Cl ± , SO 4 2± , and the like. These substances penetrate to the substrate metal, at the surface of which the corrosion processes are initiated (Knaster and Parks, 1986;Appleman and Campbell, 1982). The corrosive medium components can penetrate through the film in several ways.…”

Adhesives, Sealants, and Coatings for Space and Harsh Environments

“…Thus, in some cases, the addition ofa coupling agent 1 to a coating system can improve adhesion and reduce the chances of disbondment. Two papers on t~e §~rrosion of steel in a salt solution (107) and in an acidic solution 10 have been published. In a salt solution, the loss of adhesion has been attributed to oxygen depolarization, while in an acid solution, the delamination or blister-formation is caused by the evolution of hydrogen.…”