PASSIVITY AND PASSIVE FILMS - CASE OF STAINLESS-STEELS

The tendency of metals to oxidise is not necessarily a handicap with respect to their in service stability. Indeed, the more oxidizable metals are protected from their environment by a very thin oxihydroxide film, whose properties are determining with respect to both the surface reactivity and stability in corrosive environment. Based on electrochemical mechanisms and now standard concepts in physics, this article reviews the main properties of the so-called << passive films >>, with special emphasis paid to the iron-chromium alloys and especially to << stainless steels >>. The general behaviour of a metal/electrolyte interface, the oxide-reduction and acid-base phenomena occurring during anodic dissolution, and the polarization of ionic transport are briefly described. The basic mechanisms of passivation, the characteristic features of the passive films and of their interface with the aqueous solution, the conditions for establishing an electric field through the film, the ionic transport phenomena and the electronic properties of the film are then analysed. Last, one example of possible application of these principles to the films stability with respect to the pining corrosion in proposed.