INFLUENCE OF TENSILE STRAIN ON THE CORROSION OF PREOXIDIZED MA-956 INAN S-O-C-BEARING ATMOSPHERE

This paper deals with an investigation into the protective limit of an oxide scale preformed on the ODS alloy MA 956. The experiments were c arried out under creep conditions for different strains and strain rat es at 600 degrees C up to 8000h exposure. The possible consequences of scale rupture on the quality of protection have been analysed by expo sing the alloy in an S-O-C-bearing gas of low oxygen and medium sulfur potentials which was intended to simulate a coal-gasification atmosph ere. The results have shown that MA 956 forms an adherent Al(2)0(3) sc ale about 1.5 mu m thick after preannealing for 2h at 1100 degrees C. This scale exhibited full protection even after long-term exposure. Sc ale 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 ob served in the range 10(-10)-2 x 10(-7) s(-1). Cracking could be identi fied by surface examination due to the external corrosion taking place along the cracks. The onset of corrosion resulting from scale crackin g was characterized by the formation of Cr-rich (Cr, Fe)-sulfides on t he outside of the scale. With progressing time, external Fe-rich (Fe, Cr)-sulfides were also observed, and internal oxidation and sulfidatio n occurred in parallel. With larger strains, e.g. more than about 1%, indications of scale spallation were found.