PASSIVITY AND PASSIVE FILMS - CASE OF STA INLESS-STEELS

The tendency of metals to oxidise is not necessarily a handicap with r espect to their in service stability. Indeed, the more oxidizable meta ls are protected from their environment by a very thin oxihydroxide fi lm, whose properties are determining with respect to both the surface reactivity and stability in corrosive environment. Based on electroche mical mechanisms and now standard concepts in physics, this article re views the main properties of the so-called << passive films >>, with s pecial emphasis paid to the iron-chromium alloys and especially to << stainless steels >>. The general behaviour of a metal/electrolyte inte rface, the oxide-reduction and acid-base phenomena occurring during an odic dissolution, and the polarization of ionic transport are briefly described. The basic mechanisms of passivation, the characteristic fea tures of the passive films and of their interface with the aqueous sol ution, the conditions for establishing an electric field through the f ilm, the ionic transport phenomena and the electronic properties of th e film are then analysed. Last, one example of possible application of these principles to the films stability with respect to the pining co rrosion in proposed.