Effect of Si on morphology of alumina scales

Hot dip aluminised coatings with pure Al, Al–3Si and Al–7Si melts had been produced on 9Cr–1Mo steels and subsequently heat treated at 900°C for 5 h under oxidising conditions to generate alumina films over Fe–Al diffused case. The characterisation of these samples by X-ray diffraction and SEM-EDX techniques indicated reduction in the post-heat treated aluminide case depth with increasing Si concentration. Further, it was observed that after oxidation heat treatment, pure Al coated samples indicated γ-Al₂O₃ and θ-Al₂O₃, while Al–7Si coated samples revealed transformation of metastable θ-Al₂O₃ phase to stable α-Al₂O₃ phase. The present paper describes change in morphology of oxide after thermal oxidation from γ to θ and θ to α as a function of the Si content of Al melt in hot dip aluminised 9Cr–1Mo steels. The effect of Si on the diffused aluminide case depth is also discussed.