Adhesion and corrosion behaviour of electrodeposited metallic coatings on rare-earth magnets: Role of interfacial electrochemical interactions

Rare-earth permanent magnets are widely used in industrial applications due to their high magnetic performance; however, their poor corrosion resistance severely compromises long-term reliability. Protective coatings are therefore employed to isolate the substrate from the environment and suppress the microgalvanic processes responsible for degradation. The presence of a metallic coating alone does not ensure effective corrosion protection on Fe-based rare-earth (Fe-RE) magnets, as their heterogeneous microstructure and high electrochemical reactivity may promote interactions with the plating electrolyte during the earliest stages of electrodeposition, before continuous surface coverage is achieved, thereby affecting the formation and integrity of the resulting coating. A comprehensive assessment must therefore consider the corrosion response of the deposited layer together with its adhesion and coating/substrate morphology, while also accounting for the initial substrate–electrolyte interactions. To address these interconnected aspects, this work compares electrodeposited Ni, alkaline HEDP-Cu, and Zn coatings, selected as representative systems with different bath chemistries, metal-ion nobility, and protection mechanisms. The corrosion mechanism of the Fe-RE substrate was examined after industrial service exposure and controlled chloride-based testing. Coating performance was evaluated by open-circuit potential monitoring, potentiodynamic polarization, electrochemical impedance spectroscopy, pull-off testing, and cross-sectional SEM analysis. Open-circuit immersion tests followed by SEM/EDX characterization were also performed in the deposition electrolytes to examine substrate modification before the application of external current. Preferential intergranular corrosion was associated with the rare-earth-rich phases of the magnet. Ni exhibited the lowest corrosion current density and highest charge-transfer resistance but also showed predominantly interfacial failure and a markedly altered coating/substrate region. Zn produced less severe substrate modification and a more regular coating/substrate morphology, despite its active sacrificial response. Alkaline HEDP-Cu showed intermediate barrier performance and mixed failure, although spontaneous Cu-related reactions occurred during initial immersion. These findings demonstrate that the electrochemical response of the final coating is not necessarily indicative of coating/substrate integrity. Corrosion resistance, adhesion, and initial substrate–electrolyte interactions must therefore be considered together when selecting and optimizing electrodeposition systems for highly reactive Fe-RE magnets.