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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 Cr 2 O 3 , which forms on the surface of oxidized austenite grains, reacts quickly to the MgCr 2 O 4 spinel, when it comes in contact with MgO. Longer oxidation times and higher oxidation temperatures facilitate the formation of Fe 2 O 3 and MgFe 2 O 4 , which partially intermix with Cr 2 O 3 and MgCr 2 O 4 . As the spinel phases are formed via interdiffusion and as their molar volume is larger than the molar volume of the original phases (MgO, Cr 2 O 3 /Fe 2 O 3 ), 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.
Mehdizadehlima et al. (Thu,) studied this question.