Effects of Zn and Ni addition on the oxidation behaviour of 316 SS under simulated BWR conditions

The properties of the oxide formed on 316 Stainless Steel (SS) in the presence of metal cations injected into the water (Zn and/or Ni) in low-level Hydrogen Water Chemistry (HWC) of modern Boiling Water Reactors (BWRs) (H:O molar ratio ∼8) at 288°C were evaluated via high spatial resolution microscopy to understand the role of metal cations incorporation into the oxide. These conditions have not been extensively studied in the literature, yet are very important to understand the oxide that forms before the application of the Online NobleChemTM (OLNC). A baseline test was conducted in the absence of metal cation injection into the water. The other tests were conducted in the presence of metal cations: Zn test (Zn = 10 ppb), Ni test (Ni =10 ppb), and Zn+Ni test (Zn =10 ppb, Ni =10 ppb). The study showed that the simultaneous injection of Zn and Ni formed the most protective oxide, followed by the oxide produced under Zn injection. Conversely, the injection of Ni produced an oxide with intermediate properties between those exhibited in Zn-dosed and non-dosed conditions. These results are also validated by Electrochemical Impedance Spectroscopy (EIS) measurements, which provide more macroscopic properties. The increase in protectiveness in the presence of both Zn²⁺ and Ni²⁺ cations in the water was attributed to the higher degree of spontaneity of the substitution reaction of Ni²⁺ with Zn²⁺ in the NiCr₂O₄ inner oxide compared to the substitution reaction of Fe²⁺ with Zn²⁺ in the FeCr₂O₄.