PAINT ADHESION AND CORROSION PERFORMANCE OF CHROMIUM-FREE PRETREATMENTS OF 55-PERCENT AL-ZN-COATED STEEL

The adhesion and corrosion performances for several pretreatments of 5 5% Al-Zn-coated steels which were coil-coated with polyester paint sys tems were determined. The objective of this study was to evaluate new, silane-based metal pretreatments and to compare their performance wit h the conventional pretreatments based on chromates and with other non -chromate metal pretreatments such as Zr-based treatments. The silanes used were gamma-aminopropyltriethoxysilane, styrylaminoethylaminoprop yl-trimethoxysilane, and bis-(triethoxysilyl)ethanesilane. In the comp arison of the pretreatment performances, the standard chromate anticor rosive paint pigment was replaced with an ion exchange pigment. The pa inted panels were tested for adhesion by a 90 degrees paint pull-off t est and by a crosshatch test. Corrosion rates were determined by elect rochemical impedance spectroscopy (EIS) and by accelerated tests: salt spray, GM Scab, Cleveland, and the new Volvo indoor test. The results indicate that silanes are comparable to the standard chromate in term s of paint adhesion and corrosion protection, but the silane performan ce is dependent on the metal cleaning process. Further, the ranking of all treatments studied varied in the different tests that were compar ed. None of the systems studied matched the performance of the standar d paint system with a chromate anticorrosion pigment, indicating that the effect of an anticorrosion pigment in determining the rate of corr osion propagation in a painted panel is much greater than that of the metal pretreatment. A model is presented which explains the mechanism controlling the edge corrosion behaviour of painted 55% Al-Zn-coated s teel and the relative effects of metal pretreatment and paint pigment on the overall corrosion protection. This model is based on cross-sect ional analyses by SEM/EDX of painted panels after exposure in the Volv o test.