Analysis of the corrosion protective ability of PACVD silica-based coatings deposited on steel

Silica-based coatings are well known for their high performance in terms of mechanical, tribological, and electrochemical properties. The aim of this paper is to analyze the protective character of SiOx coatings deposited on M2 tool steel. Coatings were deposited by r.f. plasma assisted chemical vapour deposition using tetraethoxysilane as precursor. Surface chemical and morphological characterization of coatings was carried out by X-ray Photoelectron Spectroscopy, Atomic Force and Transmission Electron Microscopies, while emerging porosity was evaluated electrochemically. Intrinsic electrical behaviour of the film was determined by electrochemical measurements performed in Hg-pool. The corrosion protective properties of deposited films were assessed by means of Electrochemical Impedance Spectroscopy in aerated NaCl solution.Amorphous SiOx films provide excellent corrosion protection of steel, which is explained, on the one hand, by a tiny porosity rate, and on the other hand, by intrinsic insulating properties of the coating. This barrier effect avoids any galvanic coupling deleterious to the uncoated metallic surface, and is correlated with the SiO2-like character of the film.