THE EFFECT OF AGING ON PASSIVE FILMS FORMED ON STAINLESS-STEELS BY ANNEALING IN HYDROGEN ATMOSPHERES

Passive layers formed on a ferritic stainless steel by annealing in moist H-2 atmospheres are studied using various surface analysis techniques (GDOS, AES. and XPS), electrochemical tests in various corrosive environments, impedance and photocurrent measurements in a phosophate buffer. The respective influence of the annealing and ageing conditions is treated. Wet annealing increases the film thickness, its degree of hydration and the concentration of oxidizable elements, Cr and Si (mainly in the outmost layer for Si). Wet annealing results in a better corrosion resistance to acid, but a poorer resistance to pits initiation in neutral chloride solutions, with respect to dry annealing. Short-term (1 month) water ageing does not modify significantly either the film thickness or its composition. slightly decreases the film hydration, while, at the same time, it increases both acid and pitting corrosion resistances. These results are consistent with a change in the film polarizability with water ageing. Whatever the film formation and ageing conditions. non-linear Mott-Schottky plots are deduced from impedance measurements. In all cases. a Fermi level pinning is observed over a large potential region. Long-term (up to 6 months) water ageing effects are checked by photo-electrochemical measurements. Water ageing causes a large increase in the photocurrent response. on a time scale depending on the water content of the annealing atmosphere: the exposure time to water is shorter for wet than for dry annealing. These results are consistent with a change in the electronic structure of the films induced by sufficient water ageing. An aged state of the films is revealed by photo-electrochemistry