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

•Three Co-8Cr-xAl (x = 0,3,5 wt.%) alloys were exposed at 900 °C to one H2/CO2 (Gas 1) and two H2/CO2/H2S mixtures (Gases 2,3).•P(O2) was always below the stability of CoO, but above that of Cr2O3 and Al2O3, P(S2) was either below (Gas 2) or above (Gas 3) that of cobalt sulfide.•The small sulfur activity of Gas 2 promoted a transition from internal to external oxidation of Al in Co-8Cr-3Al and Co-8Cr-5Al in Gas 2.•Alloys Co-8Cr and Co-8Cr-3Al corroded heavily in Gas 3, forming a liquid Co-S phase.•5 wt.% Al was sufficient to form protective external alumina scales in all the three gas mixtures. 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.