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

A THEORETICAL MODEL for the delamination of organic coatings from zinc-phosphated steel surfaces will be presented. It will be shown that under ambient conditions and in neutral NaCl media delamination occurs as a result of the formation of a surface-tension gradiant during oxygen depolarization and, subsequently, the disruption of adhesion at the coating-phosphate interface. Dissolution of the phosphate coating occurs only after adhesion is lost. An accelerated cathodic delamination test showing good correlation with the long-term GM-scab test will also be introduced.