Growth and surface morphology of hot-dip Al–Si on 9Cr-1Mo steel

The high-temperature oxidation behavior of a commercial cold-rolled 9Cr-1Mo steel sheet that was hot-dipped in Al–7 wt.% Si was studied. The oxidation kinetics showed a two-stage behavior. The oxidation rate was significantly faster during the first hour, followed by a decreased steady-state growth rate at longer times. The hot-dipped steel had a higher oxidation rate than has been reported for FeAl alloys. During oxidation, many large pores and needles or rod-shaped FeAl were formed beneath the oxide scale. The formation of pores was attributable to the consumption of the limited aluminum that is caused by the outward diffusion of aluminum to form γ-Al2O3 and the inward migration of aluminum by Fe/Al inter-diffusion. The formation of γ-Al2O3 and internal voids filled with oxides in the aluminide layer was responsible for the increase in the rate of oxidation.