Abstract:Plasma electrolytic oxidation (PEO) of brass was carried out in aluminate electrolytes with the addition of NaH₂PO₄ (S1) and Na₂SiO₃ (S2), respectively, with the aim to investigate the effect of additives on the coating formation and corrosion resistance. For the PEO in S1 electrolyte, a mixed layer of AlPO₄ and Al₂O₃ is formed at the initial stage, which leads to fast plasma discharges and formation of black coatings with the compositions of Al₂O₃, CuO, Cu₂O and ZnO. However, in S2 electrolyte, plasma discharges are delayed and the coatings show a reddish color due to more Cu₂O. Mott-Schottky tests show that the S1 coatings are p-type semiconductors; while the S2 coatings can be adjusted between n-type and p-type. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests show that the PEO treatment can significantly improve the corrosion resistance of brass, with protection efficiency up to 91.50% and the largest charge transfer resistance of 59.95 kΩ·cm² for the S1 coating.

Key words: plasma electrolytic oxidation; brass; corrosion resistance; aluminate; sodium dihydrogen phosphate; sodium silicate