The growth and stability of passive films

Abstract

This paper will consider the growth and breakdown of passive oxide films on metals and alloys. Emphasis is placed on the use of surface-analytical techniques, particularly secondary ion mass spectrometry (SIMS) to characterize oxides formed on Fe and Fe-Cr alloys in 18O-containing solutions, and to determine the oxide's stability to subsequent air exposure. It is found that the stability of passive films towards air exposure decreases with increasing Cr content of the alloy. Films formed on Fe in the passive potential range in borate buffer solution are stable towards ex situ exposure, whereas films formed on Fe-26Cr alloys are not stable to air exposure after passivation at any potential in the passive region. This result has implications for examining the thickness and composition of passive films on high-Cr alloys using ex situ techniques. Passive films play a critical role in the initiation of pitting and the processes which influence film breakdown on Fe in Cl −-containing solutions are discussed. The results indicate that pitting is associated with a critical stage in the development of passive oxide films.