Corrosion of Fe-(2.25-11)% Cr-(1.6-2)% W-(0.1-0.5)% Mo Alloys at 600 and 700°C in (N₂, H₂S)-Mixed Gases

ASTM T23 steel (Fe-2.25Cr-1.6W-0.1Mo in wt%), T92 steel (Fe-9Cr-1.8W-0.5Mo in wt%), and T122 steel (Fe-11Cr-2W-1Cu-0.4Mo in wt%) alloys were corroded at 600 and 700°C for up to 70 h in 1 atm of N₂ gas containing 0.5% or 2.5%H₂S gas. The corrosion rates increased with a decrement in the Cr content and an increment of temperature and H₂S gas concentration. The T23 steel corroded fast, forming thick, fragile, and nonadherent scales that consisted primarily of an outer FeS layer and an inner (FeS, FeCr₂O₄, FeCr₂S₄)-mixed layer. Similar scales formed on the T92 and T122 steels, except that the scales became progressively thinner with an increment of the Cr content. Particularly, a large portion of the scales formed on the T122 steel was quite thin. During corrosion, mostly Fe with a small amount of W, Mn and Cu diffused outward to form the outer FeS layer, while oxygen and sulfur diffused inward to form the inner (FeS, FeCr₂O₄, FeCr₂S₄)-mixed layer.