Growth Process of Passive Films on Austenitic Stainless Steels under Wet-dry Cyclic Condition

Abstract

Passive films formed on Type 304 and 316 stainless steels under a wet-dry cyclic condition were characterized using X-ray photoelectron spectroscopy (XPS). Cr enrichment in the passive films during early stage of the wet-dry cycles occurred due to the preferential oxidation of Cr and the selective dissolution of Fe. However, the Cr fraction of hydroxide layer in passive film gradually decreased with time, because Fe(OH)_aq dissolved in the thin water layer were accumulated into the outermost hydroxide layer of a passive film during the dry condition. Chloride in thin water layer facilitated selective dissolution of Fe ions. On the other hand, the Mo in Type 316 stainless steel suppressed the dissolution of Fe ions. Consequently, Cr fraction in passive film on Type 316 became smaller than that of Type 304 stainless steel.