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It is well known that the type and concentration of impurity ions in pressurized water reactor environments will significantly alter the corrosion behavior of materials. However, the effect of these water chemistry parameters on the corrosion behavior of candidate materials for future supercritical water-cooled reactor (SCWR) systems is still a lack of understanding. The type and concentration of impurity ions on the corrosion behavior of candidate materials for future supercritical water-cooled reactor (SCWR) systems is still a lack of understanding. In this study, the effect of Cr6+ concentrations on the corrosion behavior of 316LN stainless steel in 550 °C/25 MPa supercritical water (SCW) was investigated. The results show that the weight gain first decreased and subsequently increased with the increasing of Cr6+ concentration, which is 0.40±0.01 mg/cm2 for 0 ppm, 0.28±0.03 mg/cm2 for 0.1 ppm, 0.55±0.02 mg/cm2 for 1.0 ppm, and 0.59±0.01 mg/cm2 for 50 ppm, respectively. The Fe-rich magnetite oxides were gradually covered on the surface of samples, and its proportion on the surface followed a similar alteration trend as the weight gain, which is 44.3% for 0 ppm, 37.8% for 0.1 ppm, 67.7% for 1 ppm, and 87.6% for 50 ppm. A duplex structure of the oxide films, including an Fe-rich outer magnetite layer and a Cr-rich inner spinel layer, was developed on the regions covered with Fe-rich magnetite oxides. The alternation in the thickness of oxide film followed a similar trend as the weight gain, which is 7.67 μm for 0 ppm, 5.80 μm for 0.1 ppm, 9.97 μm for 1 ppm, and 11.23 μm for 50 ppm. However, a Cr-rich spinel oxide film was formed on the region without Ferich magnetite oxides. The thickness of this oxide film was unchanged regardless of Cr6+ ion concentration, which is about 2 μm, but the concentration of Cr in this oxide film is significantly higher than reference sample. These results indicate that the increasing of Cr6+ concentration in SCW will result in the corrosion rate to decrease first and then increase, which means that some measurements should be conducted during the operation of SCWR to keep a lower concentration of Cr6+.