Unveiling the interaction between corrosion products and oxygen reduction on the corrosion of Mg–4Nd–0.4Zr alloy under thin electrolyte layers

Although hydrogen evolution reaction (HER) is considered to be the main cathodic reaction of Mg corrosion, oxygen reduction reaction (ORR) has been recently confirmed to be a secondary cathodic reaction. The factors affecting ORR of magnesium (Mg) alloys are still unclear, especially in cases under thin electrolyte layers (TEL). In this work, the influence of the corrosion product films on the cathodic reactions of Mg alloys under TEL and in a bulk solution was investigated. ORR does not influence the hydrogen evolution rates in the corrosion of Mg alloys. Therefore, with the existence of oxygen, corrosion rates of Mg alloys measured by hydrogen evolution tests are not accurate under TEL. And weight loss test is a more accurate method to evaluate Mg corrosion rates under TEL. ORR was confirmed to participate in the corrosion of Mg–4Nd–0.4Zr, Mg–4Nd and Mg–0.4Zr alloys under TEL. In 100-μm TEL, the highest contribution of ORR in cathodic reactions for the corrosion of Mg–4Nd–0.4Zr, Mg–4Nd and Mg–0.4Zr alloys are 28.6%, 39.1%, and 35.8%, respectively. The more protective film on Mg–4Nd–0.4Zr alloy provides a stronger inhibition effect against the diffusion of oxygen, leading to decreased ORR contribution in cathodic reactions. In addition, it is suggested that the preparation of Mg alloys with protective corrosion product films can inhibit the corrosion induced by ORR in the atmosphere. This work emphasizes the effects of corrosion product films on ORR in Mg corrosion, especially under TEL.