AbstractTo understand the formation and growth of plasma electrolytic oxidation (PEO) coatings in presence of different secondary phases in a substrate, PEO treatment was carried out on AlSi9Cu3 alloy at different treatment times ranging from 15 s to 480 s. The coating formation and evolution process was traced by surface and cross-sectional observation of the layers on the different phases of the alloy. The results demonstrated a sequential involvement of the different phases in the plasma discharges: firstly, discharges start on the α-Al matrix, then on the intermetallic Al2Cu and β-Al5FeSi phases at the same time and finally on the eutectic Si. The presence of intermetallic Al2Cu remarkably affects the initial dissolution, the deposition of conversion products and the ignition of discharges at the early stages of processing. Eutectic Si in the substrate exhibits the highest electrochemical stability at all stages and contributes in the beginning to a distinct coating morphology eventually. The resultant PEO coating tends to be uniform if processing times are longer and a double-layer structure appears in the coating.