Mechanism and corrosion behavior of PEO coatings on die-cast AM50A in phosphate-fluoride electrolyte

• Uncovering the Growth Mechanism of the Plasma Electrolytic Oxidation Coating. • Uncovering the Correlation between Substrate Second Phases and Discharge Behavior. • Uncovering the Film-Forming Mechanism of Electrolyte Elements. • Establishing the Correlation between Different Oxidation Times and Coating Corrosion Resistance. Plasma electrolytic oxidation (PEO) can significantly improve the corrosion resistance of magnesium alloys, yet the structural evolution of coatings under constant voltage, the role of substrate second phases, and their correlation with final performance remain unclear, limiting controlled coating preparation. This study systematically investigates the growth process and corrosion resistance of PEO coatings on AM50A magnesium alloy under constant voltage. By analyzing microstructure, phase composition, and electrochemical behavior from 20 s to 15 min, the dynamic growth mechanism and performance evolution are revealed. Results show that coating growth undergoes four stages: anodic oxidation, spark discharge, PEO, and arc extinction, ultimately forming a composite structure with a low-porosity outer layer and a dense inner layer. The filling effect of amorphous Mg 3 (PO 4 ) 2 and MgF 2 , together with the influence of substrate second phases (β-Mg 17 Al 12 and Al 8 Mn 5 ) on initial discharge and local film formation, critically affects morphology and integrity. The coating at 15 min exhibits optimal performance, with the lowest porosity (4.4%), a corrosion current density of 5.6564 × 10 -8 A·cm −2 (two orders lower than at 20 s), and polarization resistance of 4.75 × 10 5 Ω·cm 2 (three times higher than at 8 min). This work offers theoretical and process support for regulating and optimizing PEO coatings on AM50A.