A comparison of the Corrosion behaviour of FeCrAlloy and MA 956 in simulated coal gasification atmospheres

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Abstract
The corrosion behaviour of two advanced Fe‐based alloys, FeCrAlloy and MA 956, in both as‐received and preoxidized conditions, has been compared during exposure to H2‐CO‐H2O‐H2S gas mixtures. The tests have been performed at 600°C in three H2S‐containing atmospheres for exposure periods up to 1000 h. Weight gain measurements, metallography, X‐ray diffraction and SEM‐EDS analysis of the scales were carried out on the corroded specimens. The results show that the alloys developed mainly Cr‐based oxide scales in the lower H2S‐containing gas mixture whilst in the higher H2S‐containing gaseous environments, both the alloys suffered sulphidation attack. The presence of yttrium in the two alloys, either as a metallic element (in FeCrAlloy) or as an oxide dispersion (in MA 956), was found to influence the character of the Al2O3 scales developed during pre‐oxidation treatments resulting in an improved resistance to sulphidation attack in all the environments investigated. The study has highlighted some differences between the corrosion resistance of FeCrAlloy and MA 956 in the preoxidized form, resulting from changes in the physical properties of the Al2O3 layers.