APPLICATION OF SURFACE-ANALYSIS TECHNIQUES TO THE STUDY OF PAINT METAL INTERFACES RELATED TO ADHESION AND CORROSION PERFORMANCE

Interfaces between galvanized steel and automotive cathodic electrocoat paint systems were characterized by imaging XPS and static SIMS, using time-of-flight SIMS and quadrupole SIMS with an SF6 primary source. Variables were the pretreatment of the metal (phosphate coating) and the cure state of the paint system. Analyses were carried out prior to and following exposure in an accelerated cyclic corrosion test. In systems without phosphate pretreatments, severe delamination was observed in non-corroded areas. In such areas, zinc hydroxycarbonate was found at the interface. In corroded areas, adhesion was retained, probably as a result of an interaction between cathodically generated OH- ions and the paint. In phosphated systems, an undercure of the paint led to an increase of the underfilm corrosion. Static SIMS analysis of the paint/metal interface demonstrated that the state of cure of cathodic electrocoat systems can be detected by this technique and, furthermore, that certain paint constituents are preferentially adsorbed on the metal surface. The degree of cure of the paint was shown to change in going from the air/paint surface to the bulk and on to the paint/metal interface. The adhesion behaviour of epoxy electrocoat systems to metallic substrates can be described in terms of acid-base interactions.