[en] Highlights: • 316L SS treated by magneto-electropolishing (MEP) under a magnetic field. • About 2 nm thick oxide film on as-treated mechanically polished (MP) and MEP surfaces. • A double layer sub-micrometer thick oxide on exposed MP surface in PWR water. • A nanometer-thick oxide of high Cr content on exposed MEP surface in PWR water. • High Cr/(Fe + Ni) ratio of MEP surface contributed to its excellent oxidation resistance. - Abstract: The surfaces of 316L stainless steel with mechanical polishing (MP) or electropolishing (EP) in the presence of a magnetic field (MEP) before and after exposure to simulated pressurized water reactor (PWR) primary water were characterized by various techniques such as SEM, Raman spectroscopy, XPS and TEM measurements. The surface microhardness was lower and the surface roughness was higher for the as-treated MEP specimen than for the as-treated MP specimen. The oxide films that formed on the as-treated MP and MEP surfaces were nanometer-thick and rich in chromium. The Cr/(Fe + Ni) ratio of the as-treated MEP surface film was higher than that of the as-treated MP surface film. After exposure to simulated PWR primary water at 310 °C for 1050 h, a dark double oxide layer that was approximately 662 nm thick formed on the MP surface, while the MEP surface retained its original brightness as before exposure with a nanometer-thick oxide layer with a high Cr content. The MP and MEP specimens showed different oxidation kinetics when exposed to high temperature water. The high Cr/(Fe + Ni) ratio of the surface film on the MEP 316L SS contributed to its excellent oxidation resistance in simulated PWR primary water.