Passive films on stainless steels--chemistry, structure and growth

The outstanding corrosion resistance of stainless steels results from the presence of a thin oxide - 'passive' - film on the metal surface, typically 1-3 nm thick. The characterisation of the composition and structure of such thin films and the study of their interaction with corrosive environments requires a combination of sophisticated experimental techniques. This paper reviews progress in the characterisation and understanding of passive films on stainless steels achieved over the past two decades. During this period, ex situ surface analysis methods have made substantial progress and new in situ methods for the study of passive films with atomic resolution have been introduced, giving real time information on film chemistry and growth. It has been found that whereas passive film growth occurs in seconds or minutes, long range film ordering is a considerably slower process that takes several hours. In situ investigations indicate that at short times, charge transfer at the metal/film or the film/solution interface limits the rate of film growth on stainless steels. In situ estimates of film composition confirm previous data obtained with ex situ techniques