What is passive film

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 behavior 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.
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.

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