Simultaneous chromizing and aluminizing using chromium oxide and aluminum: (II) on austenitic stainless steel

The codeposition of Cr and Al on 304 stainless steel has been investigated, using the conversion reaction of Cr2O3 to halide. An increase in ratio of Cr2O3 to Al in the pack composition tends to form a chromide coating, which is very poor in high temperature oxidation resistance. A codeposited coating layer, which shows high oxidation resistance, is mainly characterized by three zones: an outer layer of iron aluminide, a nickel-rich iron aluminide, and an interdiffusion zone consisting of alpha ferrite and nickel aluminide precipitates. Oxidation resistance increased as the thickness of the outer iron aluminide layer increased. This means that the aluminum in the outer layer, rather than that in nickel aluminide precipitates or alpha ferrite, acts as a strong aluminum source forming a protective Al2O3 scale at the coating surface. Using a pack mixture containing 10 wt.% Cr2O3 and 10 wt.% Al, a coating, which shows excellent oxidation resistance at 1100°C, was obtained after codeposition of aluminum and chromium on 304 stainless steel at 1050°C for about 6–8 h.