Investigations of the degradation of high-temperature alloys in a potentially oxidizing-chloridizing gas mixture

The degradation of high-temperature alloys in argon-5.5% oxygen-0.96% hydrogen chloride-0.86% sulfur dioxide at 900°C under isothermal and thermal cycling conditions has been investigated. All the alloys showed reasonable resistance under isothermal conditions, although the Al₂O₂ ***-forming material, alloy 214, gave the lowest amount of corrosion, consistent with Al₂O₃ being a more effective barrier than Cr₂O₃ to inward penetration of chlorine or sulfur-containing species from the environment. Significant internal corrosion was observed for some alloys. Degradation of all the alloys was much more severe under thermal cycling conditions because of the failure of the protective scales. In all cases, formation of volatile chlorine-containing compounds was observed. Degradation of the alloys resulted from the penetration of chlorine-containing species through the initially formed oxide scale and formation of chlorides or, possibly, oxychlorides at the alloy-scale interface or in the subjacent alloy. The sulfur dioxide did not play any obvious role in the process.