The Sulphidation Properties of Fe-10wt% Al Alloys at 1173 K in H₂S/H₂ Atmospheres

Abstract

<p>The sulphidation properties of Fe-10wt% Al alloys at 1173 K in H₂SH₂ atmospheres were studied in the range of PS₂ = 1 x 10⁻² atm to 1 x 10⁻⁶ atm with special reference to the morphological development in the sulphide scale. Nodular growth of Fe₁₋ᵪS has been observed at Ps₂ = 1 x 10⁻⁴ atm to 1 x 10⁻³ atm with extensive internal sulphidation beneath these nodules.</p> <p>Sulphidation kinetics of the Fe-10wt% Al alloys were approximated to a two-stage parabolic rate. Aduplex scale is observed with an outer layer of Fe₁₋ᵪS, an inner layer composed of Fe₁₋ᵪS + FeAl₂S₄ having a pearlitic appearance and Al₂S₃ platelets or needles at the alloy/scale interface. At long exposure, a film of FeAl₂S₄ + Al₂S₃ develops at the alloy/scale interface. Two models are advanced for the growth of this film. The presence of Al₂S₃ platelets or needles beneath the continuous Al₂S₃ film is interpreted to result from morphological breakdown of the Al₂S₃ alloy interface and internal sulphidation. Platinum markers were found between the outer and inner layers suggesting growth of the outer layer by outward iron migration and growth of the inner layer by inward sulphur migration.</p> <p>A schematic ternary Fe-Al-S isotherm is constructed using the alloy sulphidation results and available thermodynamic data with respect to the binary sulphide phases. Various virtual diffusion paths representative of the sulphidation kinetics and scale microstructures for Fe-10wt% Al and Fe-15wt% Al alloys are indicated on this schematic diagram.</p>