PECVD Organosilicon thin films for corrosion protection of metals

This paper deals with a systematic investigation of the electrochemical behaviour and of the corrosion performance of different substrates, typically mild steels, magnesium alloys and silver alloys, coated with PECVD organosilicon thin films performed in different conditions, starting from organosilicon precursors in order to assess the effect of several deposition parameters and of substrate pre-treatments with the ultimate goal of optimising the performances of the deposited films. An analysis of the environmental impact of these innovative coating processes has also been carried out by means of Life Cycle Assessment (LCA) methodology, in order to compare them with the traditional coating techniques. The coatings 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 suitable pre-treatment step in fluorine, hydrogen and oxygen- containing plasmas. Surface and morphological characterisation of coatings was performed by means of X-Ray Photoelectron Spectroscopy (XPS), Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). In order to evaluate the protective effectiveness, the electrochemical characterisation of the coatings was carried out in sulphate- containing solutions by means of Electrochemical Impedance Spectroscopy (EIS). Moreover, SiOx-like films deposited on silver-based alloys were subjected to tarnishing tests carried out in Na2S 0.1 M aerated solution at room temperature. Notwithstanding their low thickness, about 1000 nm, the organosilicon films show in any case protective effectiveness properties against corrosion. An increase in the protective effectiveness has been observed on film deposited in O2 rich plasma at increasing input power. As shown by FT-IR data, this behaviour may be related to the progressive increase of the inorganic nature of deposited coatings, i.e. increase in SiOx groups amount and decrease sylanol groups content. Furthermore XPS analysis allows evaluating the effects of plasma pre-treatment performed as a function of the substrate typology. The good corrosion behaviour observed on mild steel specimens coated after O2-plasma treatment may be attributed to the enhancement of the surface oxidation degree. On the contrary, H2 plasma discharges allow increase the adhesion of SiOx film deposited on magnesium and silver alloys, and consequently ameliorate their protective performances.