The corrosion behavior of Co-Cr-Al alloys exposed to mixed oxygen-sulfur atmospheres at 900 °C

Abstract

Three Co-8Cr-xAl alloys (x = 0, 3, 5 wt.%) were exposed to three mixtures (Gas 1: H2-CO2, Gases 2 and 3: H2-CO2-H2S) at 900 °C for 20 h. Gas 1 produced external chromia scales (plus internal alumina for Co-8Cr-3/5Al). Gas 2, with p(S2) below stability of Co sulfides, favored a transition from external chromia scales, observed in Gas 1, to exclusive external alumina scales for both Co-8Cr-3/5Al alloys. Conversely, Gas 3 was very aggressive for both Co-8Cr and Co-8Cr-3Al, forming a liquid Co-S phase, while with Co-8Cr-5Al it still formed external Al2O3 scales. Al additions improved the alloys resistance to the sulfur attack.