Comparison of Short-Term Oxidation Behavior of Model and Commercial Chromia-Forming Ferritic Stainless Steels in Dry and Wet Air

Abstract

A high-purity Fe–20Cr and commercial type 430 ferritic stainless steel were exposed at 700 and 800 °C in dry air and air with 10% water vapor (wet air) and characterized by SEM, XRD, STEM, SIMS, and EPMA. The Fe–20Cr alloy formed a fast growing Fe-rich oxide scale at 700 °C in wet air after 24 h exposure, but formed a thin chromia scale at 700 °C in dry air and at 800 °C in both dry air and wet air. In contrast, thin spinel + chromia base scales with a discontinuous silica subscale were formed on 430 stainless steel under all conditions studied. Extensive void formation was observed at the alloy–oxide interface for the Fe–20Cr in both dry and wet conditions, but not for the 430 stainless steel. The Fe–20Cr alloy was found to exhibit a greater relative extent of subsurface Cr depletion than the 430 stainless steel, despite the former’s higher Cr content. Depletion of Cr in the Fe–20Cr after 24 h exposure was also greater at 700 °C than 800 °C. The relative differences in oxidation behavior are discussed in terms of the coarse alloy grain size of the high-purity Fe–20Cr material, and the effects of Mn, Si, and C on the oxide scale formed on the 430 stainless steel.