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

This study deals with the surface and electrochemical characterization of mild steel surfaces after plasma-enhanced chemical vapour deposition of organosilicon films, performed in order to increase corrosion resistance. Coating deposition was performed in a home-made reactor with hexamethyldisiloxane-oxygen-argon mixtures of different compositions and at different input power values on as-received specimens and after a pretreatment step in oxygen-containing plasma. Surface and morphological characterization of coatings was performed by x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy. To evaluate the protective effectiveness, electrochemical characterization of the coatings was carried out in sulphate-containing solutions by electrochemical impedance spectroscopy. Notwithstanding their low thickness of ∼1000 nm, the organosilicon films show protective effectiveness properties against corrosion. An increase of the impedance values, related to an increase in the protective effectiveness, has been observed on films obtained at increasing input power and after O 2 plasma pretreatment of the substrate. As shown by FTIR data, the positive effect of increasing input power may be related to the progressive increase of the inorganic nature of the deposited coatings, i.e. increase in SiO x groups and decrease of sylanol groups. Furthermore, as shown by XPS analysis, the good corrosion behaviour of mild steels coated after O 2 plasma pretreatment may be attributed to removal of the surface contamination, which improves adhesion of the SiO x film, and to enhancement of the surface oxidation degree.