Effects of Zn and Ni addition on the Co incorporation in 316 SS oxide formed under simulated BWR conditions

Advanced microstructural and microchemical analyses provided critical insights into the mechanism of Co incorporation into the oxide formed on 316 Stainless Steel (SS), when exposed to simulated Boiling Water Reactor (BWR) conditions. The study showed that the presence of Zn and/or Ni injected into the water significantly influenced Co incorporation. Coupons of 316 SS were exposed to multistage oxidation tests under low-level Hydrogen Water Chemistry (HWC) (H:O molar ratio ∼8) at 288°C, both in the absence and the presence of metal cations injection (Zn, Ni, Co). Moreover, Online NobleChem™ (OLNC) was applied in an intermediate exposure stage on pre-oxidised coupons to show how the transition from a non-catalytic to a catalytic oxide surface impacts the oxide properties. The application of OLNC did not substantially influence the incorporation of metal cations in the oxide. Notably, the study showed that Co was preferentially incorporated into the normal spinel oxides, thus suggesting that Co2+ prefers the tetrahedral sites of these spinel, similar to Zn2+. This translated into a Co uptake mitigation effect when 10 ppb of Zn was injected. Conversely, in the presence of 10 ppb of Ni in the water, Co uptake was not mitigated, suggesting that Ni2+ does not compete with Co2+ for the same interstitial sites of the spinel oxides. However, a mitigation effect on Co uptake was observed when Zn and Ni were injected simultaneously, indicating a synergistic effect of these metal cations on the Co incorporation mechanism.