Water Vapor and Oxygen/Sulfur-Impurity Effects on Oxidation and Nitridation in Single- and Two-Phase Cr–Nb Alloys

Abstract

The oxidation and nitridation behavior of Cr(Nb) solid solution and Cr2Nb Laves phase was studied individually and in combination at 1100°C and 950°C in dry air, humid air, and N2–4H2 for alloys containing high or low levels of oxygen and sulfur impurities. At 1100°C, humid-air exposure of alloys in which the Cr2Nb phase was the matrix resulted in greatly increased subscale nitridation as compared with exposure in dry air. At 950°C, little difference between dry vs. humid-air exposures was observed; however, greatly increased subscale nitridation was observed in the Cr2Nb-matrix eutectic alloy when the levels of oxygen and sulfur impurities in the alloys were reduced. In contrast, unalloyed Cr and Cr(Nb) solid-solution alloys were relatively insensitive to water vapor or impurity oxygen/sulfur effects under the conditions studied, although the addition of Nb as a solute in Cr did increase the extent of subscale nitridation. Possible synergistic effects between sulfur impurities and water vapor in the Cr2Nb alloys and the possibility for similar effects in other intermetallic systems are discussed.