Passive film of 18-8 stainless steel structure and its function

Abstract

A number of studies have reported polarization curves for stainless steels, relating them to their resistant property in various media. The structure and composition of the passive film has not been examined in detail, although the presence of such a film on the steels determines almost their corrosion resistance. Comparison of the polarization behaviour and the structure of the film is hence desirable for a clear understanding of the basic principle controlling the low corrosion rate of the passive steel. This paper discusses the structure and composition of the passive film formed in sulphuric acid solution at definite potentials in the passive region. The resistance of the passive steel to pitting in acidic solution containing chloride ion is examined and the function of the passive film in determining the corrosion resistance of the steel is discussed. The structure, composition, and thickness of films were examined by infra-red spectroscopy, tracer technique, ellipsometry, and electron spectroscopy (ESCA), all of which techniques are specially designed to detect a small change in the property of the very thin film. The presence of two kinds of film depending on their formation potential is disclosed, and the most important parameter controlling the corrosion resistance of the steel is concluded to be the amorphous nature of the film in which bound water is included.