Analytical and Electrochemical Study of Passive Films in Stainless Steels Subjected to Aqueous Solutions

The objective of this research is to study the corrosion behavior of the stainless steel centrifugal contactor used in the spent nuclear fuel treatment process called UREX+ process. AISI type 304L stainless steel was suggested as the material of construction for this contactor. Corrosion of 304L stainless steel in three acidic aqueous solutions of 5.0M HNO3, 5.0M HNO3 + 0.1M HF, and 5.0M HNO3 + 0.1M HF + 0.1M Zr4+ was studied. Immersion, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) corrosion tests were conducted at test temperatures of 25, 40, and 80°C and three different rotational speeds (0, 1000, 2000 rpm) in order to mimic the operating conditions of the centrifugal contactor. The results showed that the 5.0M HNO3 + 0.1M HF solution was the most corrosive environment as the fluoride ions dissolved the passive film present on the surface of the stainless steel. The addition of 0.1M Zr4+ ions to this acidic mixture reduced the corrosion caused by HF to levels similar to those found in HNO3 solutions and allowed the stainless steel to preserve its passive film. Further addition of zirconium ion did not result in better corrosion resistance of the stainless steel. Besides, higher corrosion rates were obtained as the solutions temperatures increased while the hydrodynamic conditions had less significant effect on corrosion rates.