Université de Lyon, INSA de LYON
Several metals and alloys used in many industrial applications are protected with a very thin protective oxide layer, known as a passive film. This layer is formed on the surface of the metallic material by physicochemical reactions with the surrounding medium (air or aqueous solution). Protected by this passive layer, these materials then exhibit good resistance to uniform corrosion; hence interest in them for industrial purposes. Unfortunately, the presence of this passive film also renders these materials susceptible to localized corrosion under certain physicochemical conditions. As a result of discrete instabilities from the passive steady state, passivity breakdown and underlying material damage may occur, sometimes leading to component failure.
The recent advances in experimental procedure and interpretations of electrochemical impedance spectroscopy (EIS) results allows us to accurately describe the pysical properties of passive films, including their electronics properties (Mott-Schottky(MS) theory) and to understand the passive film susceptibility to localized corrosion (pitting corrosion). In the present study, the passive ability of materials was firstly characterized using electrochemical techniques and X-ray photoelectron spectroscopy. Then, the pitting phenomenon was then investigated using polarization studies in terms of metastable pitting and stable pitting.
This methodology was applied on i) binary Nickel-Chromium alloys and ii) TiZrCuPd(Sn,Si) bulk metallic alloys. In both cases, the understanding of the pitting mechanism point out some key factors that have to be taking into account to improve the passive film integrity.