Evaporation of Cr2O3 in Atmospheres Containing H2O

Stainless steels in atmospheres containing H2O form a Cr2O3 scale in the early stage of oxidation. However, the Cr2O3 scale gradually degrades with time. In order to determine the effect of H2O on the deterioration of a Cr2O3 scale, the evaporation behavior of Cr2O3 in N2–O2–H2O atmospheres was investigated. The rate of mass loss in an N2–O2–H2O atmosphere was found to be one order of magnitude higher than the rates in N2–O2 and N2–H2O atmospheres, indicating that deterioration of the Cr2O3 scale is likely to occur in mixed atmospheres of oxygen and water vapor. Volatilization of Cr2O3 is probably based on the following reactions: 1/2Cr2O3(s)+3/4O2(g)+H2O(g)=CrO2(OH)2(g). However, it is also speculated that the reaction, Cr2O3(s)+2/3O2(g)=2CrO3(g), affects the evaporation of Cr2O3 at temperatures higher than 1323 K. The evaporation rate of Cr2O3 is roughly comparable to the growth rate of the Cr2O3 scale. Therefore, a Cr2O3 scale can be degraded by the evaporation of Cr2O3.