Multi layered Surface Oxides within Crevices of Type 316L Stainless Steels in High-Temperature Pure Water

Surface oxide layers were formed within crevices of Type 316L (EMS S31603) stainless steels in pure water at 288 degrees C and 8 MPa. Cross-sectional structures of the surface oxides were analyzed using transmission electron microscopy. In the condition of dissolved oxygen concentration of 2 ppm, the properties of the surface oxide layer changed with position and dual- or triplex-layered oxides were formed at a certain distance from the crevice mouth. The multilayered oxides were composed of Fe-based oxide in the core and a high-Cr content in the outer layer, which had not been observed on a boldly exposed surface. On the contrary, in deaerated conditions, the surface oxide layers were composed of a magnetite (Fe3O4)-based outer and a Cr-enriched inner oxide layer, regardless of the crevice position. Electrochemical condition within the crevice was identified by using a E-pH diagram. It was suggested that, at 400 mu m distance from the crevice mouth, the potential lowered at the early stage of exposure, and then shifted to the noble direction with a decrement of pH. Consequently, even within a narrow crevice with a gap size of a few mu m, the uniqueness of the crevice electrochemistry, characterized by the position and time dependence of both the potential and the pH, has been exhibited.