Evidence for formation of metallo-siloxane bonds by comparison of dip-coated and electrodeposited silane films

It has been amply demonstrated that thin films of organofunctional silanes deposited by dipping or spraying on metals, such as aluminum, can provide protection against various forms of corrosion. In this paper we show that denser films with higher pore resistance and better corrosion protection performance can be obtained if the silane film is produced by electrodeposition rather than by dipping. In such a process the silane reacts with the metal oxide in a different way, and in the case of aluminum, aluminate ions seem to be incorporated into the silane film. The resulting films can protect aluminum alloys against uniform and pitting forms of corrosion in a salt solution for more than 1000 h, which is comparable to the standard chromate-based treatments. This paper discusses the protection mechanism in some detail. An important aspect of this work is that the use of XPS and TOFSIMS to analyze electrodeposited silane films provided direct evidence for the presence of O2Al(O–Si–O) and OAl(O–Si–O)2 groups at the silane–aluminum oxide interface. Such bonds have been suggested, but they have largely remained elusive over the years.