Effects of Annealing and Oxidation on the Microstructure of Hot-Dipped Aluminum–Silicon Coating of 316L Stainless Steel

In this study, a continuous and dense FeAl/Al₂O₃ gradient coating was prepared on the surface of 316L stainless steel by hot-dipping aluminizing, vacuum annealing and low-oxygen pressure oxidation. The results showed that when a hot-dipping pure aluminum was applied, the alloy compound layer was composed of (Fe, Cr, Ni)₂Al₅ and (Fe, Cr, Ni)Al₃ phases. When the Si content in the bath was higher than 2.5 wt.%, a new intermetallic compound, Al₇(Fe, Cr)₂Si, appeared at the interface, and the thickness of the metal compound layer decreased continuously upon increasing the silicon content in the melt pool. When the hot-dipped sample was annealed in vacuum at 900 °C for 3 h, the brittle (Fe, Cr, Ni)₂Al₅ phase was transformed into a ductile (Fe, Cr, Ni)Al phase. As low-oxygen pressure oxidation progressed, FeAl/Al₂O₃ gradient coatings were formed on the surface layer. The oxide film formed by the Al-2.5 Si wt.% sample was flatter and denser, which significantly improved its high-temperature oxidation resistance. Reducing the oxygen partial pressure promoted the external oxidation of Al element and the formation of Al₂O₃ oxide film.