In situ atomic emission spectroelectrochemical (AESEC) investigation of plasma electrolytic oxidation (PEO) treatment of AA2024 aluminum alloy in different electrical regimes

Plasma electrolytic oxidation (PEO) of AA2024 aluminum alloy was investigated under unipolar and bipolar electrical regimes using in situ atomic emission spectroelectrochemistry (AESEC) to explain material transport mechanisms during coating formation. The evolution of Al and Cu species concentration in the electrolyte was monitored in real time and correlated with voltage response, microdischarge behavior, and coating characteristics. Under unipolar conditions, a gradual increase in elemental concentration reflects a transport regime dominated by localized, high-energy microdischarges and outward oxide growth. In contrast, bipolar processing induces a transition to the soft sparking regime, particularly at a cathodic-to-anodic current density ratio of R = 1.3, resulting in a pronounced increase in Al release associated with redistribution of discharge activity and enhanced material exchange. Morphological and compositional analyses reveal that bipolar regimes produce thicker, less porous, and more alumina-rich coatings with a significant inward growth contribution. The absence of crystalline aluminosilicates under bipolar conditions further supports reduced thermal intensity and modified plasma characteristics. The results demonstrate that the transition to soft sparking corresponds to a shift from ejection-dominated transport to a regime involving also field-assisted dissolution, and ionic migration.