Abstract
Passive films formed on SUS304 stainless steel that were covered with thin water layer containing various kinds of salt and exposed to atmospheric corrosion environment of 90％ relative humidity and 60℃ were characterized using X-ray photoelectron spectroscopy (XPS). Thickness and Cr content of passive films were affected by a minute amount of salts provided on the surface as a thin water layer. Thickness of the thin water layer on the surface of the steel seemed to be controlled by the equilibrium relative humidity of saturated solution of chlorides supplied on the specimen. Namely, the thickness of the thin water layer containing chloride with low equilibrium relative humidity was larger compared to that containing the chloride with higher equilibrium relative humidity. As a result, the selective dissolution of Fe is stimulated under the thicker water layer to be the more Cr containing passive films. Chlorides enhanced the formation of more Cr enriched passive films, but more concentrated chloride caused localized corrosion. In particular, passive films formed with the thin water layer containing suitable amount of sodium chloride occasionally exhibited high localized corrosion resistance. Sulfates also accelerated the selective dissolution of Fe and increased Cr content in passive films, resulting in stable growth of passive film without localized corrosion.
