XPS characterization of passive films formed on Type 304 stainless steel in humid atmosphere

Cr enriched passive films were hardly formed on Type 304 stainless steel under humid atmosphere. Cr fraction in passive films under relative humidities (R.H.) in the range 30–70% was similar to the alloy composition. However, slightly Cr enriched passive films were formed under R.H. 90%. This is attributed to the thin continuous water layer into which Fe ions are selectively dissolved. When chloride is in the water layer, the Cr enrichment was enhanced by accelerated dissolution of Fe as chloride complex. At lower R.H., pitting corrosion occurred because of the more concentrated chloride in the water layer.

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► We characterized passive films formed on Type 304 stainless steel exposed to humid atmosphere.
► Cr enrichment in the passive film hardly occurred in the humid atmosphere.
► Only in atmosphere of 90% relative humidity, slight enrichment of Cr was observed.
► Small amount of chloride which was put prior to the test enhanced the Cr enrichment.