Influence of tensile strain on the corrosion of preoxidized MA 956 in an S-O-C-bearing atmosphere

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Abstract
This paper deals with an investigation into the protective limit of an oxide scale preformed on the ODS alloy MA 956. The experiments were carried out under creep conditions for different strains and strain rates at 600°C up to 8000 h exposure. The possible consequences of scale rupture on the quality of protection have been analysed by exposing the alloy in an S-O-C-bearing gas of low oxygen and medium sulfur potentials which was intended to simulate a coal-gasification atmosphere. The results have shown that MA 956 forms an adherent A12O3 scale about 1.5 μm thick after preannealing for 2h at 1100°C. This scale exhibited full protection even after long-term exposure. Scale cracking was first observed in the range 0.5−0.6% total strain. No influence on the critical strain value due to strain rate could be observed in the range 10−10−2 × 10−7 s−1. Cracking could be identified by surface examination due to the external corrosion taking place along the cracks. The onset of corrosion resulting from scale cracking was characterized by the formation of Cr-rich (Cr, Fe)-sulfides on the outside of the scale. With progressing time, external Fe-rich (Fe, Cr)-sulfides were also observed, and internal oxidation and sulfidation occurred in parallel. With larger strains, e.g. more than about 1%, indications of scale spallation were found.