Assessment of the degradation of metals and alloys in air-2% chlorine at high temperature

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Abstract
The degradation of a number of high-temperature alloys in air-2.13% chlorine at 900°C under isothermal conditions has been studied. Despite the formation of a Cr203 or Al2O3 scale, all the alloys underwent considerable corrosion, which involved penetration of chlorine-containing species through the scale to the scale-alloy interface, and reaction of these species with appropriate elements in the alloy. Some of the chlorides formed had high vapour pressures and diffused through the scale towards the scale-gas interface. This resulted in a general thinning of the substrate if the element was a major constituent of the alloy, or to the formation of voids in the substrate, particularly at the alloy grain boundaries, if the element was a minor constituent. In some cases, particularly for iron-rich alloys, the volatile species (eg FeCl2) reacted with oxygen at or near the scale-gas interface to form the corresponding oxide, leading to the development of a thick, porous scale. When chlorides were formed in the alloy by an internalcorrosion process, reaction of the chloride with oxygen could occur to form internal oxide, the released chlorine diffusing deeper into the alloy to form further internal chloride. Alloys rich in elements that form volatile reaction products, such as iron, chromium and molybdenum, were particularly susceptible to corrosion. Development of an Al20 3 scale gave better protection than a Cr203 scale, but the overall corrosion resistance was largely determined by the resistance of the underlying substrate.

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