Corrosion Behavior of Type 304L Stainless Steel in UREX+ Nitric Acid Solutions Using Immersion and Rotating Cage Techniques

UREX+ is an advanced separation process for treatment of spent nuclear fuels where the separation of radioactive ions is accomplished in centrifugal contactors made of Type 304L (UNS S30403) stainless steel. This research work was focused on corrosion behavior of Type 304L (UNS S30403) stainless steel at experimental parameters simulating the environment of a centrifugal contactor during its operation. Corrosion studies were completed using the immersion and rotating cage (RC) corrosion techniques. In this investigation corrosion coupons were exposed to three different nitric acid (HNO₃) solutions. Corrosion rates in 5 M HNO₃ and 5 M HNO₃ + 0.1 M hydrofluoric acid (HF) + 0.1 M Zr⁴⁺ solutions were lower than 0.0127 mm/y. The relatively high corrosion rates in 5 M HNO₃ + 0.1 M HF solutions were reduced by the addition of Zr⁴⁺ ions that behave as corrosion inhibitors forming Zr-F complex ions. The inhibitor efficiency of Zr⁴⁺ ions reached an average of 99.61%. Corrosion results were mainly a function of the corrosive characteristic of each solution and lesser degree dependent on solution temperature. The effect of wall shear stresses generated in the rotating cage technique was not observed in these tests. Corrosion test results also were discussed based on surface chemistry, electrochemical impedance spectroscopy (EIS) Nyquist plots, and surface appearance changes, which included microstructural analysis.