Nitrate ions induced transpassive degradation and corrosion mechanism shifts in 304L/C25 stainless steels within simulated spent nuclear fuel reprocessing environments

The effects of nitrate ion (NO₃^-) concentration on corrosion behavior of 304L and C25 stainless steels in 6 mol/L boiling nitric acid (simulating spent nuclear fuel reprocessing) were explored. Increasing NO₃^- to 5 mol/L accelerate the corrosion of both 304L and C25 steels by enhancing the cathodic reduction reaction drastically, as evidenced by the increased corrosion current density and mass loss rate, positive shifts in corrosion potential, and a decrease in cathodic Tafel slope. These observations suggest a transition from activation-controlled to diffusion- or mixed-controlled corrosion mechanisms. Meanwhile, passive films degraded significantly with the reduced Cr(OH)₃/Cr₂O₃ content. 304L stainless steel undergoes intergranular corrosion at low NO₃^- concentrations (0.5 mol/L) and transitions to uniform corrosion at 5 mol/L NO₃^-.In contrast, C25 stainless steel exhibits pitting corrosion at NO₃^- concentrations of 3 mol/L or higher, with the formation of Mo-oxide precipitates observed at 5 mol/L.