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• A type 430 stainless steel is coated by Mn-Co spinel by a slurry method. • Continuous Cr 2 O 3 formed by pre-oxidation before coating reduces a scaling rate. • Pre-oxidation helps reduce Cr loss by volatilisation. • Coating structure e.g. pore size affects the flux of Cr loss. • Dependence of gas diffusivity on the coating structure is graphically proposed. An AISI 430 stainless steel was coated by Mn-Co spinel using a slurry method. Pre-oxidation before the coating helped reduce the oxidation rate of the coated steel at 800 °C in O 2 -5%H 2 O, relating to the formation of the continuous chromia layer which inhibited the outward diffusion of iron to the coating layer. It also helped reduce the mass flux of Cr loss due to the volatilisation. The combined molecular and Knudsen diffusion of the volatile species through the coating layer was suggested to explain the reduced volatilisation rate of the coated sample with pre-oxidation. [ABSTRACT FROM AUTHOR]