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.

► The hypothetical growth mechanism for Al2O3 and Fe2O3 whiskers has been proposed. ► The NaCl-induced hot corrosion of aluminide steel has been described in detail. ► NaCl acted as a catalyst to increase oxidation rate of the oxides in one direction. ► Grain boundary of scale provides a rapid diffusion path for oxide whiskers to grow. ► Temperature and concentration of NaCl dominate the morphology of oxide whiskers.