The degradation of metals in a hydrogen chloride-containing gas at high temperature

Abstract

A study has been made of the corrosion of metals which form the basis of high temperature alloys in an argon-25%H2-10%HCl-5%CO-1%CO2 gas mixture at 900°C. The aim has been to ascertain the possible importance of the formation of volatile chloride-containing species of these metals in the degradation of alloys under similar conditions. Considerations of the thermodynamics of the system enable the chlorine, oxygen and carbon potentials of the mixed gas to be ascertained and thus the possible reactivity of the metals to be estimated. The situation is complicated by the development of phases with high vapour pressures. Thus, iron is corroded rapidly due to the formation of FeCl2. The chlorides of nickel and cobalt are less volatile and, hence, these metals undergo much less corrosion. Chromium is able to form a Cr2O3 scale in this environment but penetration of chlorine-containing species to the metal-scale interface enables liquid chloride phases to develop and disrupt further the protective scale. Molybdenum and tungsten are reasonably inert in the gas mixture.