High-temperature corrosion of iron-chromium alloys in oxidizing-chloridizing conditions

Art investigation has been carried out into the effects of 0.1 to 1.0% HCl on the oxidation of Fe-28%Cr and Fe-28%Cr-1%Y in argon-20%O-2 at 600 and 700 degreesC. Ar the higher temperature, the additions of HCl to the gas caused considerable increases in corrosion of the binary alloy, with the rates of metal loss actually being greater than those of iron in the 0.5 and 1% HCl-containing environments. Thick and multilayer ed scales were observed; these were oxides, particularly FeCr2O4 and Fe2O3, that developed following formation and vapor phase transport of chlorine-containing species from the metal surface. The main metal-loss processes were evaporation of FeCl2, CrCl2, and CrO2Cl2, with the first two of these reacting with oxygen to form solid oxides in the scale, while the third was lost mainly to the environment. The addition of 1% Y to the alloy resulted in a marked improvement in corrosion resistance at 700 degreesC, because of the reactive element facilitating rapid establishment of a protective Cr2O3-rich layer and promoting the formation of condensed chlorides rather than the more volatile CrO2Cl2 phase. At 600 degreesC, additions of HCl to argon-20%O-2 caused formation of some localized condensed chlorides on both alloys, but the corrosion rates were relatively low, because of protection by a Cr2O3-rich oxide scale.