Effects of surface states on the oxidation behavior of 316LN stainless steel in high temperature pressurized water

Nuclear grade 316LN stainless steel with different surface states are prepared: electropolished and ground to 240-grit. Surface topography, surface Volta potential distribution, and nanohardness distribution on cross-sectional surface are investigated, respectively. Oxide films on the ground and electropolished samples after immersion in high temperature pressurized (HTP) water for 0.1, 168, and 366-h were studied using various methods. Distinct from electropolished surface, the ground surface is featured by a rugged surface and a heterogeneous distribution of surface Volta potential. A thicker (∼28-μm) cold-worked layer was introduced by grinding. More crystallites were precipitated on the ground surface, resulting from the existence of high-energy sites with higher residual strain and higher electrochemical activity and a less protective inner layer. The oxidation rate was reduced by electropolishing and a thinner oxide film was formed, due to the formation of a more protective oxide film with higher content of chromium (Cr). The mechanism of oxide film formation was further discussed.