The mechanism of oxide whisker growth and hot corrosion of hot-dipped Al-Si coated 430 stainless steels in air-NaCl(g) atmosphere

The mechanisms of oxide whisker growth and hot corrosion of 430 stainless steel (430SS) and aluminide 430 stainless steel hot-dipped in a Al-10 wt.%Si molten bath (430HDAS) were studied at 750 and 850 °C in air mixed with 500 and 990 vppm NaCl_(g). The results showed that the loose Cr₂O₃ scale which formed on the 430SS could not prevent the corrosion of 430SS in a 500 vppm NaCl_(g) atmosphere, resulting in the formation of Fe₂O₃ scale. Fe₂O₃ whiskers grew at the grain boundary of the Fe₂O₃ scale. However, no Fe₂O₃ whiskers formed on the Fe₂O₃ scale while 430SS was exposed in a 900 vppm NaCl_(g) atmosphere. During the initial high-temperature corrosion of 430HDAS in a 500 vppm NaCl_(g) atmosphere, a dense Al₂O₃ scale formed on the surface of the specimens. Also, Al₂O₃ whiskers grew on the Al₂O₃ scale. As exposure time increased, cyclic chlorination/oxidation degraded the protective aluminide layer and caused the formation of Fe₂O₃ scale and Fe₂O₃ whiskers. The morphology of Fe₂O₃ whiskers formed at 750 °C is more slender than those formed at 850 °C. The formation and growth of both Fe₂O₃ and Al₂O₃ whiskers may be attributed to the chloridation of both the steel substrate and aluminide layer, accelerating the diffusion rate of metallic ions in the oxide scales.