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

Asako Otake, Izumi Muto, Aya Chiba, Yu Sugawara, Nobuyoshi Hara
2017 Journal of the Electrochemical Society  
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-SiO 2 -MgO-Al 2 O 3 slag inclusions (Type I) and small SiO 2 -Al 2 O 3 -MnO-Cr 2 O 3 -TiO 2 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
more » ... cro-scale polarization, the size of the electrode area was changed from 0.01 to 10 mm 2 , 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. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 207.241.231.83 Downloaded on 2018-07-20 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 207.241.231.83 Downloaded on 2018-07-20 to IP
doi:10.1149/2.0561714jes fatcat:knoykh5syffdzk6urngonp7gqa