High-temperature corrosion of Co-15Mo alloys containing ternary additions of Al in mixed-gas environments

The corrosion behavior of Co-15 at.% Mo alloys containing up to 20at.% Al in gaseous H₂-H₂O-H₂S mixtures was studied over the temperature range of 600–900°C. The corrosion kinetics of all alloys followed the parabolic rate law over the temperature range of interest. Corrosion resistance increased with increasing aluminum content. Complex scales formed on the alloys, consisting of an outer layer of cobalt sulfide and a heterophasic inner layer. Al₂O₃formed only at high temperatures in alloys having aluminum additions of 15at.% or more. The absence of Al₂O₃in some cases is due to the small volume fraction of the intermetallic phase CoAl in the alloys and the nature of the slow growth rate of Al₂O₃.Improvement in corrosion resistance is attributed to the presence of a ternary sulfide, Al_0.55Mo₂S₄,and Al₂O₃in the inner layer.