Spinel coatings produced via oxidation of an AISI 316L-MgO composite

Abstract

The possibilities of producing protective spinel coatings on the surface of an AISI 316L-MgO composite via high-temperature oxidation at 800 °C, 900 °C and 1000 °C were explored using a combination of structure and microstructure analyses, and thermodynamic calculations. The structure and microstructure of the coatings were analyzed in situ and ex situ using high-temperature and conventional X-ray diffraction, scanning electron microscopy, electron backscatter diffraction and X-ray spectroscopy. The ex situ analyses identified the oxidation and reaction products and revealed their spatial distribution within the surface coating. The in situ analyses helped to describe the oxidation and reaction kinetics. It was found that Cr₂O₃, which forms on the surface of oxidized austenite grains, reacts quickly to the MgCr₂O₄ spinel, when it comes in contact with MgO. Longer oxidation times and higher oxidation temperatures facilitate the formation of Fe₂O₃ and MgFe₂O₄, which partially intermix with Cr₂O₃ and MgCr₂O₄. As the spinel phases are formed via interdiffusion and as their molar volume is larger than the molar volume of the original phases (MgO, Cr₂O₃/Fe₂O₃), they overgrow the surface of the MgO grains. This mechanism provides a basis for controlled growth of protective spinel coatings on the surface of the AISI 316L-MgO composites.