Corrosion of Fe-(8.5∼36.9) wt% Cr Alloys at 600∼800°C in (N2, H2S, H2O)-Mixed Gases

Fe-(8.5, 18.5, 28.3, 36.9) wt% Cr alloys were corroded between 600 and soot for up to 70 h in a 1 atm gas mixture that consisted of 0.0242 atm of H2S, 0.031 atm of water vapor, and 0.9448 atm of nitrogen gas. Their corrosion resistance increased with an increment in the Cr content. The Fe-8.5%Cr alloy corroded fast, forming thick, fragile, nonadherent scales that consisted primarily of an outer FeS layer and an inner (Fe, Cr, 0, S)-mixed layer. The outer FeS layer grew into the air by the outward diffusion of Fe2+ ions, whereas the inner mixed layer grew by the inward diffusion of oxygen and sulfur ions. At the interface of the outer and inner scales, voids developed and cracking occurred. The Fe-(18.5, 28.3, 36.9)% Cr alloys displayed much better corrosion resistance than the Fe-8.5Cr alloy, because thin Cr2O3 or Cr2S3 scales formed.