Study of the passive film layer semi-conductive properties and their impact on the corrosion resistance of stainless steels

摘要

Passives films of stainless steels are complex systems which require specific methods to be characterized. They are often described as bilayer films with an internal chromium oxide rich layer and an external iron oxide-hydroxide rich layer. The low thickness of the entire film and the high sensitivity of the film to its environment make difficult the distinction of the physical properties of each of these layers. In this study, the crossing of the results of several characterization methods makes possible to identify some properties of each layer. Indeed, the use of multi-frequency Mott-Schottky analyses and photo-electrochemical characterizations analyzed with a new mathematical model enables to identify for each slopes on the Mott-Schottky curves the characterized semi-conductive phases. Thus, it is possible to associate the found semi-conductive properties to the right layer and so to evaluate how surface treatments modify passive film properties. This approach also enables to study the modifications undergone by the passive film when it is in an aggressive media. The first results obtained with this approach give a great importance to the external part of the passive film in corrosion resistance mechanisms. They also help to understand the mechanisms implied in nitric immersion treatments which are known to improve corrosion resistance of passive films.