A Systematic Study of the Role of Cathodic Polarization and New Findings on the Soft Sparking Phenomenon from Plasma Electrolytic Oxidation of an Al-Cu-Li Alloy

Understanding the role of cathodic polarization and soft sparking is critical for plasma electrolytic oxidation (PEO). In this study, PEO of an Al-Cu-Li alloy has been carried out under cathodic to anodic current density ratio (R) from 0 to 3.3. Controlled potential tests and electrochemical impedance microscopy are also adopted. The results show that increased cathodic polarization improves the long-term oxide growth efficiency until an optimum soft sparking regime is reached at R = 1.2, after that the efficiency decreases and damages to the coatings occur. Interestingly, anodic potential drop, which was considered one of the characteristics of soft sparking, is absent in some cases under R = 1.2, and the coatings under R = 1.2 is also featured by a white outer layer enriched with cations. Excessive cathodic polarization (R = ∼2.0–3.3) leads to the compact coatings with highest impedance values at the early PEO stage (300 s), but they deteriorated rapidly. The complex PEO behaviors with different cathodic polarization has been explained in terms of the intercalation of hydrogen species, mass transportation affected by H ₂ evolution, charge extraction and hydrogen induced stresses. Reciprocally, controlled potential tests indicate that anodic polarization also suppresses the subsequent cathodic hydrogen evolution.