Formation and stability of ultrathin organosilane polymers on iron

A combination of quartz crystal microweighing (QCM) and infrared reflection absorption spectroscopy (IRRAS) was used to investigate the formation of ultrathin polysiloxane films on iron oxide surfaces by the adsorption of alkyltrimethoxysilanes in a humid atmosphere. It was shown that in the absence of a catalyst, only the most reactive methyltrimethoxysilane led to film formation. These films are not completely polymerized and incorporate silanols and water. The addition of n-propylamine to the reactive gas phase as an alkaline catalyst enables one to use even the butyltrimethoxysilane as a starting material and leads to a much higher deposition rate. In addition, these films are completely condensed and very hydrophobic. Layers of both alkoxysilanes even with a thickness corresponding to only a few monolayers showed high stability in the presence of a corrosive atmosphere (SO2, H2O, O-2), leading to significant inhibition of the corrosion process. Corrosion started at local defects in the film, whereas the polymer itself showed no appreciable structural changes.