Discoloration Process of Minted Copper–Nickel Alloys in Chloride Ion-Containing Environments: Experimental and DFT Research

A corrosion discoloration model for copper–nickel alloys in Cl− environments was established using CIE-Lab, UV–VIS absorption spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The corrosion discoloration process and the corresponding main corrosion products can be summarized as follows: silver-white (Cu + Ni) → green (NiO) → reddish-brown (NiO + Cu2O) → black (NiO + Cu2O + CuO). Density functional theory was employed to explain the corrosion process of copper–nickel alloys and the detrimental effect of Cl−. The results indicate that adsorbates preferentially bind to nickel, leading to the preferential formation of NiO, which imparts a green appearance to the surface. Furthermore, the difficulty in forming nickel cation vacancies and the higher diffusion barrier for nickel inhibit the migration of species within the oxide layer. Notably, nickel also suppresses carrier migration within the oxide layer, reducing the charge transfer rate. In contrast, the promotion of corrosion by Cl− is primarily attributed to the reduction in surface work function and the formation energy of cation vacancies.