Insights into the mechanism of Mo protecting CoCrFeNi HEA from pitting corrosion-A quantitative modelling study on passivation and repassivation processes

Mo has been widely reported as a conducive element for the corrosion resistance of massive alloy systems. However, the mechanism of Mo optimizing the corrosion resistance is complicated, and in-depth studies are still required. The present work comprehensively and quantitatively studied the critical influences of Mo on the passivation and repassivation behavior of CoCrFeNi HEA based on the dissolution diffusion-deposition model proposed in our previous work. The experimental results indicated that Mo remarkably eliminated the metastable pitting corrosion, significantly improved the breakdown potential and perfectly protected the CoCrFeNiMo0.2 HEA from pitting corrosion. The modelling and X-ray photoelectron spectroscopy (XPS) results both show that in the passivation process, MoO2 was the last product to deposit, thereby existing in the outer layer of the passive film. Mo addition increased the Cr content by weakening the deposition of Fe2O3 and Fe3O4 and also improved the Cr2O3/Cr(OH)3 ratio by promoting deprotonation of Cr(OH)3, thus enhancing the quality of passive film. Besides, when pitting corrosion occurred, MoO2, MoO3, and FeMoO4 were the first products to deposit and accelerated the repassivation process of HEA by timely covering the matrix in the pit cavity, thereby preventing further corrosion of the matrix.(c) 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.