The role of grain boundaries in the initial oxidation behavior of austenitic stainless steel containing alloyed Cu at 700 °C for advanced thermal power plant applications

•We study the role of grain boundary on high temperature oxidation in austenitic steel.•Grain boundaries contribute to the oxidation resistance but special boundaries do not.•Increased Cr flux from grain boundaries induces high Cr content oxide formation.•Areas within 4 μm from grain boundary form protective Cr2O3 and thin (Fe,Cr)3O4 oxide.•Areas 4 μm away from grain boundary form thick non-protective Fe-rich spinel oxide.

The role of grain boundaries during the early stages of oxidation in austenitic stainless steels containing alloyed Cu was investigated using APT, TEM, EBSD, EPMA, and XRD. The oxidation experiments were performed at 700 °C in air with 20% water vapor. Within 4 μm from the grain boundaries, the oxide layer exhibits a dual-layer structure consisting of a thin Fe-rich spinel oxide on a protective Cr2O3 oxide. Away from the grain boundaries, non-protective spinel oxide layers are formed as the outer and inner oxide layers. A critical grain size that prevents the formation of fast-growing spinel oxides is discussed.