Pitting at the δ/γ Boundary of Type 304 Stainless Steel in NaCl Solution: The Role of Oxide Inclusions and Segregation

Abstract

The role of inclusion compositions and the pit initiation mechanism at the δ/γ boundary of cast Type 304 stainless steel was investigated in 1 M NaCl. Three types of inclusions were detected: large CaO-SiO2-MgO-Al2O3 slag inclusions (Type I) and small SiO2-Al2O3-MnO-Cr2O3-TiO2 complex oxide inclusions that were located at the δ/γ boundaries (Type II) or found in the γ phases (Type III). In Types II and III, a small amount of Ca was occasionally detected. In micro-scale polarization, the size of the electrode area was changed from 0.01 to 10 mm2, and the number densities for the stable and meta-stable pit initiation sites were estimated to be 0.1 and 1 site mm−2, respectively. Type II inclusions acted as the pit initiation sites at the δ/γ boundary. At the δ/γ boundary without an inclusion, a meta-stable pit was initiated. The segregation of P and S was detected at the boundary where the pit was initiated, but no accumulation of Si was observed. At a randomly selected boundary of δ/γ on the steel, no clear segregation of P and S was observed, but the Si concentration was two to three times higher than the average Si concentration in the steel.