Modulation of Electrochemical Tunnel Etching of Aluminum Foil through Phosphoric Acid-Based Passivation Pretreatment

This study investigates the role of phosphoric acid-based pretreatment in optimizing the tunnel etching process of aluminum foils for electrolytic capacitor applications. Electrochemical impedance spectroscopy (EIS) and polarization analyses reveal that phosphoric acid induces the formation of a micro-cracked AlPO4 passivation layer, which significantly enhances corrosion resistance while guiding uniform tunnel nucleation. At optimal concentrations, the passivation film suppresses random pitting by directing chloride ions to preferentially attack at controlled micro-crack sites, thereby reducing tunnel merging and improving pore distribution uniformity. Compared to foils without phosphoric acid-based pretreatment, the modified surface exhibits enlarged tunnel dimensions and enhanced structural homogeneity under constant charge conditions, leading to increased specific surface area. These findings demonstrate that phosphoric acid contained pretreatment effectively balances corrosion inhibition with controlled tunnel etching, offering a promising approach for manufacturing high-performance capacitor foils with improved capacitance and stability. The results provide valuable insights for industrial-scale optimization of aluminum foil etching processes.