UREX+ Centrifugal Contactor Corrosion in HNO3-HF Aqueous Solutions

The UREX+ is an advanced solvent extraction separation process under development to separate uranium from the nuclear waste. This operation utilizes 304L stainless steel (SS) centrifugal contactors. The contactor is exposed to two different fluids that flow continuously; these are aqueous and organic solutions which are identified also as phases. These two solutions are ultimately separated at the end but during the process they form different mixtures at several regions under various Hydrodynamic conditions. Our investigation focuses on the corrosion of the stainless steel in the aqueous solution. The critical operational parameters considered in this study are acid solutions and temperatures.

      In this investigation 304L SS coupons were exposed to three types of corrosion tests that mimic the corrosion environments in a specific region of the contactor. The corrosion tests have been conducted in a 5M HNO₃ aqueous solution with a maximum concentration of 0.1M HF, which is the main corrosive agent. The third solution type includes the addition of zirconium ions. The three types of corrosion tests are immersion tests, potentiodynamic polarization and electrochemical impedance spectroscopy.

      A rotating cylinder was used in a three-electrode cell configuration to obtain corrosion data based on potentiodynamic polarization and electrochemical impedance spectroscopy techniques in order to assess the performance of 304L stainless steel. The corrosion tests were carried out at three temperatures of 25, 40 and 80^oC. The results showed that the mixture of Nitric and Hydrofluoric acid is the most corrosive environment and addition of Zr reduces the damage caused by Hydrofluoric acid by forming Zirconium Fluoride complexes.  Besides, in all the three solutions of 5M HNO₃, 5M HNO₃ + 0.1 M HF and 5M HNO₃ + 0.1 M HF + 0.1 M Zr⁴⁺, the increase of temperature leads to higher corrosion rate.