Corrosion mechanisms of phosphated zinc layers on steel as substrates for automotive coatings

The literature has been reviewed with regard to the mechanisms of cosmetic corrosion of zinc coated and painted automotive materials. The general consensus of opinion seems to be that degradation is by cathodic delamination, anodic undermining, mechanical delamination or a combination of these mechanisms, These variations are due to the many factors that influence the mechanism, such as the extent of physical damage, the type of metallic coating, the coating thickness, the pretreatment, the paint-metal adhesion, the test conditions and the paint system. There is a need to carry out a systematic study with strict control of all these variables in order to confirm the mechanisms of cosmetic corrosion of automotive materials.