Self-assembled monolayer and multilayer thin films on aluminum 2024-T3 substrates and their corrosion resistance study

Aluminum alloy 2024-T3 substrates were modified with self-assembled monolayer or multilayer thin films from four different alkylsilane compounds, octadecyltrichlorosilane (C18SiCl3), octadecyltrimethoxysilane (C18Si(OMe)3), octyltrimethoxysilane (C8Si(OMe)3) and octadecyldimethylchlorosilane (C18SiMe2Cl). The water contact angle of the aluminum surface increased substantially after modification by the two trimethoxysilane compounds and trichrolosilane C18SiCl3, but decreased after the modification with monochlorosilane C18SiMe2Cl. Potentiodynamic polarization test, prohesion exposure, and electrochemical impedance spectroscopic studies revealed that the corrosion resistance of the C18SiCl3-modified aluminum alloy substrates improved significantly compared to the unmodified and the two trimethoxysilane-modified aluminum alloy samples. The C18SiMe2Cl-modified substrate also exhibited an improved corrosion resistance compared to the unmodified ones. While extensive corrosion appeared on the unmodified aluminum alloy samples after only 20-30 h of prohesion exposure, early stage corrosion was observed from C18SiCl3-modified samples after 150-200 h of prohesion exposure and C18SiMe2Cl-modified samples after 80 h of prohesion exposure. Based on the results obtained so far, it is believed that a mixed monolayer or multilayer thin film with opposite molecular orientations was formed through the self-assembling of alkylsilane molecules on the aluminum 2024-T3 surface.