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 degrees 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.