Corrosion studies of stainless steel 304 L in nitric acid in the presence of uranyl nitrate: effect of temperature and nitric acid concentration

In order to better understand the role of uranium on the corrosion behavior of stainless in aqueous nuclear fuel reprocessing and decontamination operations, 304L stainless steel coupons were treated with nitric acid media at 45 degrees C and 90 degrees C in the presence of 0.3 M of uranyl nitrate. The impact of time, temperature, nitric acid concentration and uranyl nitrate concentration has been studied over a 183 days period and corrosion rates determined. After 183 days, corrosion rates (mu m y(-1)) at 90 degrees C are: 33.7 +/- 3.4 for 8 M HNO3 with [U] = 0 M; 39.7 +/- 0.1 for 8 M HNO3 with [U] = 0.3 M; 5.3 +/- 0.1 for 3.5 M HNO3 with [U] =0 M and 5.9 +/- 0.2 for 3.5 M HNO3 with [U] = 0.3 M. After 183 days, the corrosion rates at 45 degrees C are all below 2 mu m y(-1) for the nitric acid concentration and uranyl nitrate concentration studied. Analysis of the data indicate that the corrosion domain remains passive, no uranium uptake onto stainless steel has been observed but only an increase of corrosion rate with temperature (major effect) and nitric acid concentration (minor effect) were reported. When both temperature and nitric acid concentration increased and drive the system into a transpassive corrosion domain, the presence of uranium promote the increase of corrosion rate after 3 months. Analyses of the coupon by scanning electron microscopy indicates that at the first stage low homogeneous dissolution of the steel occurs; intergranular corrosion appears at 10 days and finally grain loss is observed after 6 months treatment.