The Role of Grain Boundaries during the Initial Oxidation Stages in Cu-Added Austenitic Stainless Steel at 700 °C Studied at the Atomic Scale

The early stages of the oxidation behavior (≤ 12 h, in air with 20% water vapor at 700°C) of Cu-added austenitic stainless steels with various grain sizes was investigated using atom probe tomography (APT), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), electron probe microanalyzer (EPMA) and X-ray diffraction (XRD). A thin Cr-rich oxide layer is formed over the entire surface of a small-grained (8 µm) sample, which is similar to that formed near grain boundaries of medium-grained (17 µm) and large-grained (27 µm) samples. Oxidation of all samples proceeds via the lattice and grain boundary diffusion of Cr, leading to the formation of a protective Cr-rich oxide first at the grain boundaries and then via lateral growth toward the grain interiors. APT and TEM studies clearly reveal that within 4 μm of the grain boundaries, the oxide layer exhibits a duplex-layer structure consisting of a thin Fe-rich (Fe,Cr)₃O₄ oxide (~55 nm) above and a protective Cr₂O₃ oxide (~40 nm) below as a diffusion barrier. In contrast, further away from the grain boundaries, a non-protective Fe-rich (Fe,Cr)₃O₄ oxide (~160 nm) and a Cr-rich (Fe,Cr)₃O₄ oxide (~40 nm) are formed as the outer and inner layers, respectively. The oxidation kinetics was studied in terms of mass gain measurements. The critical grain size (< 8 µm) to prevent the formation of fast-growing, non-protective Fe-rich oxides is discussed based on the experimental findings.