Silanization of adhesively bonded aluminum alloy AA6060 with γ-glycidoxypropyltrimethoxysilane. II. Stability in degrading environments

Alkaline etched aluminum alloy AA6060 treated with of γ-glycidoxypropyltrimethoxysilane was investigated. The stability of the silane films in degrading environments was investigated by exposing them to acidic and alkaline solutions at 40°C, followed by analysis with reflection-absorption infrared spectroscopy. Desorption of the silane from the surface occurred at pH 4, and at a much slower rate at pH 7, while the silane film was stable at pH 8. Two models for the degradation of the silanized aluminum surface in an acidic environment were proposed: one involving simultaneously hydrolysis of the siloxane network and corrosion of the underlying aluminum surface, and the other involving only corrosion of the aluminum surface. The durability of the silane treated surface was determined using wedge tests on joints made with XD4600 one-component epoxy adhesive. The durability was significantly reduced in highly acidic environment (pH 2), but no significant differences in durability were observed in the pH range from 4 to 11, except that the durability was slightly higher in higher alkaline environment during the initial period of testing. Better durability in an alkaline environment is connected to a better stability of both the siloxane network and the aluminum surface in this environment.