Ar+ irradiation improves passivation of molybdenum in aqueous solution

Long-term oxidation of metals is often accelerated by simultaneous ionizing irradiation. However, this may not always be the case in the early phase of oxide formation, particularly in an aqueous environment. In this study, we observed that Ar⁺ irradiation enhances molybdenum (Mo) passivation in an aerated 3.5 wt% NaCl solution, slowing further corrosion of Mo. This enhancement of passivation was confirmed by electrochemical measurements, where the potentiodynamic polarization curve for irradiated Mo exhibited well-defined passivation region, in contrast to the dissolution-like behavior observed in non-irradiated Mo. Characterization of the oxide revealed a thin (tens of nanometers), uniform passive film on irradiated Mo, compared to a micron-thick, non-uniform oxide formed on non-irradiated Mo. We attribute this irradiation-enhanced passivation to the irradiation-induced defects near or on the surface, which provide excess nucleation sites for oxide and promote rapid initial oxide formation, thereby leading to a more protective passive film that mitigates further oxidation. Both experiments and DFT calculations confirm that oxygen adsorbs more strongly to the surface of Mo after Ar⁺ irradiation, indicating its enhanced oxidation tendency. These findings provide valuable insights into the effects of irradiation on passivation of metals, particularly those that do not naturally exhibit strong passivation.