Oxidation-Driven Lithium Uptake in Nickel-Base Alloys – Smoking-Gun Evidence for an Inner Cathode under LWR Conditions

• This study demonstrates that the inner-cathode mechanism, previously proposed for protons, also applies to lithium cations. • The mechanistic investigation reported here addresses lithium enrichment within the inner oxide scale, as documented in the literature. • Alloy uptake and lithium enrichment are attenuated because the hydrogen evolution pathway is not applicable to lithium, unlike hydrogen. • Regarding the consequences of enrichment, this work supports further exploration of alternative pH-control buffers for PWR operation. Lithium hydroxide is widely used for pH control in pressurized water reactors, yet lithium incorporation in chromia-forming Ni-base alloys has been linked to degraded corrosion resistance. Using lithium as a tracer for protons, we show that Li⁺ uptake is consistent with an inner-cathode mechanism for oxidation by water, in which hydroxylated oxide grain boundaries transport molecular water equivalents toward a cathodic region near the alloy/oxide interface. First-principles calculations demonstrate that both H⁺ and Li⁺ promote chromium oxidation by NiO and catalyse metallic nickel precipitation within the oxide scale, compromising scale integrity. Unlike hydrogen, lithium cannot be removed by molecular evolution and therefore accumulates at the inner cathode. This work provides a novel elucidation of mechanisms for experimentally observed lithium enrichment that has indeed been suggested to promote stress corrosion cracking in LiOH-containing reactor water.