Se. Hornstrom et al., PAINT ADHESION AND CORROSION PERFORMANCE OF CHROMIUM-FREE PRETREATMENTS OF 55-PERCENT AL-ZN-COATED STEEL, Journal of adhesion science and technology, 10(9), 1996, pp. 883-904
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.