Paint adhesion and corrosion performance of chromium-free pretreatments of 55% Al-Zn-coated steel

The adhesion and corrosion performances for several pretreatments of 55% Al-Zn-coated steels which were coil-coated with polyester paint systems were determined. The objective of this study was to evaluate new, silane-based metal pretreatments and to compare their performance with the conventional pretreatments based on chromates and with other non-chromate metal pretreatments such as Zr-based treatments. The silanes used were gamma-aminopropyltriethoxysilane, styrylaminoethylaminopropyl-trimethoxysilane, and bis-(triethoxysilyl)ethanesilane. In the comparison of the pretreatment performances, the standard chromate anticorrosive paint pigment was replaced with an ion exchange pigment. The painted panels were tested for adhesion by a 90 degrees paint pull-off test and by a crosshatch test. Corrosion rates were determined by electrochemical 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 terms of paint adhesion and corrosion protection, but the silane performance is dependent on the metal cleaning process. Further, the ranking of all treatments studied varied in the different tests that were compared. None of the systems studied matched the performance of the standard paint system with a chromate anticorrosion pigment, indicating that the effect of an anticorrosion pigment in determining the rate of corrosion 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 steel and the relative effects of metal pretreatment and paint pigment on the overall corrosion protection. This model is based on cross-sectional analyses by SEM/EDX of painted panels after exposure in the Volvo test.