Thermodynamic Probability Analysis of the Effects of Rb on the Corrosion Susceptibility of Cr-Containing Steels for Nuclear Materials Canisters

Rubidium (Rb) generated from the β-decay of Kr-85 has been theorized to be corrosive toward steel, specifically in the storage of Kr-85 nuclear waste streams. In the present study, the phase equilibria of Rb_xCr_yO_z with Rb in dry oxygen and water are investigated to understand a possible pathway to unusual deterioration of the corrosion resistance of canister steels in the presence of Rb. It was found that, in dry oxygen environments, the accumulation of Rb (more than 0.01 mol) can completely consume the Cr in 1 mol of AISI 4130 steel by forming α-Rb₂CrO₄ and Rb₃CrO₄ and prevent the formation of protective Cr₂O₃ scale. In aqueous environments, Rb_xCr_yO_z are metastable species. In order to investigate their role, the probability of forming various oxides is invoked in order to avoid the all-or-nothing approach to oxide formation typical of E-pH diagram, which only predicts the most stable species dissolved, ionized, or solid ionized. Thus, the probability of forming Rb_xCr_yO_z was considered and reported herein. It was found Rb_xCr_yO_z can possess a larger than 7% probability of forming over Cr₂O₃ in the Rb-rich case and 15% in the Cr-rich case, indicating that it is expected to find a small amount of Rb_xCr_yO_z in the thermodynamically formed reaction products. Even though Cr₂O₃ is more stable than Rb_xCr_yO_z, the protective Cr₂O₃ scale is likely to have some vulnerability to Rb, leading to one possible route for the decline in the corrosion resistance of steel canisters in aqueous environments. Therefore, from a thermodynamic perspective, the current study supports the hypothesis that Rb can thermodynamically react with Cr in steels and can lead to the formation of Rb_xCr_yO_z at certain potentials and pH levels, showing the Rb influence of steel corrosion cannot be discounted. The paper considers experimental mixed potential and pH levels observed and their relationship to thermodynamic probability. From this relative corrosion resistance can be assessed in a preliminary way in aqueous environments.