Tailoring plasma electrolytic oxidation through metallic cation addition: Insights from bipolar and unipolar electrical regimes

Presented research examines oxide layer formation on AA2024 alloy via plasma electrolytic oxidation (PEO) in a sodium silicate-potassium hydroxide electrolyte modified with zinc (Zn²⁺), cerium (Ce³⁺), nickel (Ni²⁺), cobalt (Co²⁺), and calcium (Ca²⁺) cations, introduced through highly soluble nitrates and acetates. By analyzing plasma discharge behavior, oxide layer composition, surface morphology and phase distribution, the influence of these cations on the coating properties was studied. Comparative testing between unipolar and bipolar regimes highlights the benefits of bipolar mode, where the “soft sparking” condition, achieved after approximately 4 minutes for all obtained samples, produces thicker, less porous coatings with superior structure. The experimental findings reveal that unipolar PEO primarily results in outward oxide growth, while bipolar PEO facilitates both outward and inward oxide formation, allowing for enhanced cation incorporation. Notably, the addition of nickel nitrate and zinc acetate under bipolar conditions initiates distinct oxide growth mechanisms compared to other cationic additives, underscoring the role of specific metallic cations and their characteristics.