High temperature carburization behaviour of Mn–Cr–O spinel oxides with varied concentrations of manganese

Mn–Cr–O spinel often formed on austenitic alloys is an oxide phase that could be protective against high temperature carbonaceous attack. In this research, various Mn–Cr–O samples were tested in carburization environments with controlled oxygen partial pressures. The stoichiometric MnCr2O4 shows better resistance to carburization and coke formation than the Mn-rich Mn–Cr–O and the Cr-rich Mn–Cr–O samples because of its highest thermodynamic stability as compared with MnO and Cr2O3. (Mn,Cr)7C3 formed after carburization is catalytic to coke formation, and was found instable at higher levels of H2O/oxygen and may form volatile phases in the presence of H2O, leading to a continuous reduction in sample weight.

Research highlights
► High temperature carbonaceous attacks of Mn–Cr–O samples with varied Mn contents were examined.
► The stoichiometric MnCr2O4 shows the best resistance to carbonaceous attacks.
► Formation of (Mn,Cr)7C3 in Mn-Cr-O samples is catalytic to coke formation.
► (Mn,Cr)7C3 can be decomposed and react with H2O moisture to form volatile Mn(OH)2.
► High content of Cr stabilizes (Mn,Cr)7C3 phase and prevents it from its further decomposition.