Surface analysis of PECVD organosilicon films for corrosion protection of steel substrates

The surface and electrochem. characterization of mild steel surfaces were done after plasma-enhanced chem. vapor deposition of organosilicon films, performed to increase the corrosion resistance. The coating deposition was performed in a home-made reactor with hexamethyldisiloxane-oxygen- argon mixts. having different compns. and at different input power values on as-received specimens and after a pretreatment step in an oxygen- contg. plasma. Surface and morphol. characterization of coatings was performed by XPS, Fourier transform IR spectroscopy and SEM. To evaluate the protective effectiveness, electrochem. characterization of the coatings was carried out in sulfate-contg. solns. by using electrochem. impedance spectroscopy. The organosilicon films .apprx.1000 nm thick showed protective effectiveness against corrosion. An increase in impedance values, related to an increase in the protective effectiveness, has been obsd. for films obtained at increasing input power and after O2 plasma pretreatment of the substrate. As shown by FTIR data, the pos. effect of increasing the input power may be related to the progressive increase in inorg. nature of the deposited coatings, i.e., increase in SiOx groups and decrease in silanol groups. Furthermore, as shown by XPS anal., the corrosion resistance of mild steels coated after O2 plasma pretreatment may be attributed to removal of the surface contamination, which increases adhesion of the SiOx film, and to enhancement of the surface oxidn. degree.