Corrosion and adhesion study of polyurethane coating on silane pretreated aluminum

In this paper, the effect of silane pretreatment pH on adhesion strength and corrosion protection of subsequent polyurethane (PU) coating on aluminum alloy substrate is evaluated. Practical adhesion of the coating on substrate was measured in dry, wet and recovered states via pull-off method for desmutted, chromated and vinyltrimethoxysilane (VTMS) pretreated aluminum alloy (AA1050). VTMS resulted in good adhesion performance in dry, wet and recovery states for VTMS treated specimens at pH < isoelectric point (IEP) which is related to the silanol groups orientation toward aluminum surface. Adhesion performance at pH > IEP is lower than desmutted specimens which can be related to presence of unreacted VTMS molecules on the surface. Vinyl groups orientate toward the aluminum surface at pH above IEP, results in prevention of VTMS reaction on the surface. Corrosion protection of PU coating was studied with electrochemical impedance spectroscopy (EIS) and salt spray in the presence of silane layer; then, compared to coated desmutted and chromated specimens. Water sensitivity of unreacted VTMS molecules leads to poor protective performance of specimens treated at pH > IEP. However, at pH < IEP protective performance is considerably higher than desmutted and comparable with chromated specimens.