AbstractThe corrosion of high temperature alloys in a gas mixture (Ar-25H2–10HCl–5CO–1CO2, vol.-%), which is oxidising to chromium and aluminium, but reducing to the base metal and containing a high concentration of hydrogen chloride, has been studied under isothermal and thermal cycling conditions. The rates of attack were fairly low with Al2O3forming alloys, which were more resistant to attack than Cr2O3forming alloys. There was relatively little metal loss during isothermal exposures at 900°C, but, in some alloys, there were significant depths of internal corrosion. Some volatile chlorides were formed following penetration of chlorine containing species through the oxide scale to the alloy substrate: FeCl2was the major volatile species detected. The extent of corrosion was increased by thermal cycling, as a result of further disruption of the protective oxide scales. The corrosion processes can be accounted for in terms of the effectiveness or otherwise of the oxide scale as a barrier between the alloy substrate and the environment.MST/882