Oxidation Behavior of Fe-Ni-Cr Alloys at 700°C in Air and Steam

High ︲ temperature oxidation behavior of Fe ︲ Ni ︲ Cr alloys with different Ni contents in air and steam at 700 ℃ was investigated. Oxidation mass gain in steam was higher than that in air. In both atmospheres oxidation mass gain tended to decrease with increas ing Ni content. However, the Ni concentration dependence of oxidation mass gain in air was complex, i.e., oxidation mass gain increased with increase in Ni content up to 35 ％ then decreased with higher Ni content. But oxidation mass gain of binary Ni ︲ 20Cr significantly increased. Such a complex oxidation behavior was not observed when alloys were oxidized in steam. The internal oxidation zone was locally formed in air, and a Cr 2 O 3 scale was found to develop on the areas without internal oxidation. The size of area, where an internal oxidation zone was developed, changed depending on Ni content. In steam, most of the surface area of alloys was covered by an internal oxidation zone. Cr content, 20 ％ , was considered to be insufficient but close to the critical Cr content for exclusive Cr 2 O 3 scale formation in air. The critical Cr content for exclusive Cr 2 O 3 scale formation was found to depend on alloy Ni content due to lower oxygen permeability in alloys with higher Ni content and the cross ︲ term effect for Cr outward diffu -sion. ［ doi:10 . 2320 / jinstmet.JB201705 ］ ）