Carbon-induced decrease of the early-stage oxidation rate for a medium-Mn steel

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Medium-Mn steels are third generation advanced high-strength steels, receiving increased research attention due to their favorable combination of mechanical properties and a lean alloy composition. Two-step oxidation-reduction procedures to enable surface defect-free hot-dip galvanization of steel sheet have been proposed, facilitating sufficient corrosion protection for applications. The effect of carbon on the early-stage oxidation of medium-Mn steel is investigated at 600 °C by comparing a C-containing steel with a C-free Fe-base model alloy. A pronounced difference in oxidation kinetics was found, with a two to three times higher mass gain of the C-free model alloy. The difference in oxidation kinetics was also observed for quenched model alloy samples with a different microstructure, resembling that of the C-containing steel. Detailed characterization revealed a similar three-layered Fe-rich oxide on both alloys, with a fine-grained, inner spinel-type layer, an intermediate spinel-type and an outer corundum-type layer. However, the oxide microstructure exhibited pores leading to a loss of contact for the C-containing steel. A mechanism is proposed based on the oxidation of carbon from the steel substrate, forming CO and/or CO 2 , leading to pore formation and stabilization. The resulting oxide microstructure contains only small bridges connecting the outer and inner regions of the oxide, effectively restricting diffusion pathways through the oxide and thus decreasing oxidation kinetics.