Regulation Ni²⁺ concentration gradient and diffusion layer for ultra-smooth electrochemical polishing of Ni-P microstructured surfaces

Amorphous nickel-phosphorus (Ni-P) alloys have been widely used in precision optical and microstructured components, owing to their excellent corrosion resistance, wear resistance, and favorable mechanical properties. As the surface quality requirements become increasingly demanding, electrochemical polishing (ECP) has attracted considerable attention due to its non-contact nature and superior surface leveling capability. However, the mechanisms governing interfacial evolution, material removal uniformity, and defect suppression remain insufficiently elucidated. In this study, the anodic dissolution behavior, ion transport characteristics, and surface integrity evolution of amorphous Ni-P alloys during ECP were systematically investigated. Planar and microstructured surfaces were first fabricated by ultra-precision machining (UPM), and ECP was then performed under controlled processing conditions. Numerical simulations were carried out to predict the spatial distribution of Ni²⁺ near the electrolyte-electrode interface. Corresponding experiments and comprehensive surface characterizations were conducted to validate the coupling effects of current density, mass transport, and interfacial reactions. Increasing the current density significantly enhances Ni²⁺ generation, leading to rapid ion accumulation and accelerated formation of a high-concentration diffusion layer. At low current densities, slow ion accumulation resulted in weak diffusion control, thereby promoting localized dissolution and corrosion pit formation. In contrast, elevated current densities facilitated diffusion-controlled dissolution, reduced current densities localization, and effectively mitigated corrosion pit growth. The high-concentration diffusion layer was further demonstrated to regulate interfacial stability by mitigating concentration perturbations and interrupting the positive feedback associated with surface defects. This study clarifies the intrinsic relationship among ion transport, interfacial regulation, and surface evolution, thereby providing theoretical guidance for achieving high-quality finishing of amorphous Ni-P alloys, particularly for microstructured surfaces with stringent surface-quality requirements.