An EIS study of the corrosion behaviour of PECVD coated brass substrate

The electrochemical impedance spectroscopy (EIS) was used to analyse the corrosion behaviour of the Ni plated brass (OT59) substrate covered by SiO2 coatings of different thickness. The specimens were immersed for long time (up to 168h) in a corrosive solution (synthetic swear). Plasma Enhanced Chemical Vapour Deposition (PECVD) technique was used to deposit SiO2 coatings: the aim was to obtain a surface treatment that prevents the release of Ni, an allergenic metal. The deposition was performed in RF (13.56 MHz) plasma with hexamethyldisiloxan (HMDSO) and O2 mixture at near room temperature without any solvents and without particular surface pretreatments. Different thickness of SiO2 film was obtained varying the time of plasma exposure. The EIS technique was demonstrated to provide a wealth of information on the behaviour of the coated substrate over time, on condition that the obtained data are carefully interpreted. In this study the attention was focused on the equivalent circuit (EC) that models the electrode/solution interface. The electrical circuit elements were interpreted in terms of influence of the coating defects on the substrate corrosion, and to choose the best PECVD treatment.