Effect of boiling phenomena on the corrosion of austenitic stainless steel (R-SUS304ULC) in nitric acid solution containing chromium ions

Abstract

The redox behavior of chromium was investigated under three conditions for the nitric acid solutions considering the operating conditions of the reprocessing plant-atmospheric-pressure boiling, reduced-pressure boiling, and atmospheric-pressure non-boiling-to determine the relationship between the corrosion of austenitic stainless steel (R-SUS304ULC) and this behavior. The oxidation of Cr(III) to Cr(VI) in 8 M boiling nitric acid was increased over that in non-boiling nitric acid, even at the same temperature by measuring the time dependence of the chromium concentration and thermodynamic calculations. These results show that Cr(III) is more likely to oxidize under boiling than non-boiling conditions. On the other hand, the apparent rate of Cr(VI) reduction, as consumed by corrosion of the stainless steel, was more than 10 times greater than the rate of Cr(III) oxidation in 8 M nitric acid solution. This is evidence that the rate of Cr(VI) reduction by corrosion is kinetically faster than the rate of Cr(III) oxidation. In addition, the apparent rate of Cr(VI) reduction and the corrosion rate of R-SUS304ULC were positively correlated. Therefore, these indicate that the corrosion of R-SUS304ULC apparently follows Arrhenius' law in 8 M nitric acid solution containing Cr(VI), regardless of the boiling or non-boiling conditions.