Sulfidation and sulfidation-oxidation of nickel- and cobalt-base alloys containing dispersions of stable oxides

Abstract

The presence of a fine dispersion of a stable oxide is known to have a beneficial effect on the oxidation resistance of nickel- and cobalt-base heat-resisting alloys. This paper presents some preliminary experimental results relating to the hot-corrosion resistance of these alloys. Alloys forming Cr ₂O₃ scales appear to be resistant to oxidation when coated with sodium sulfate, whereas an alloy normally forming an Al ₂O₃ scale suffers accelerated attack. During sulfidation some of the alloys suffer an accelerated degradation, with sulfur penetrating rapidly along what appear to be grain boundaries. The same effect is noted in sulfidation-oxidation experiments, when the Cr ₂O₃-forming alloys suffer accelerated oxidation, the effect of the dispersoid being apparently removed. An Al ₂O₃-forming alloy resists this form of attack well. The sodium sulfate-coated test is probably a good guide to the behavior under weakly corroding conditions, whereas the sulfidation-oxidation test may give a better indication of the behavior under highly aggressive conditions.