Effect of lanthanum nitrate on the microstructure and electrochemical behavior of PEO coatings on AZ31 Mg alloy

Plasma electrolytic oxidation coating was carried out in phosphate-based electrolyte embedded with different amounts of lanthanum nitrate (LN) on AZ31 magnesium alloy substrate in order to characterize the microstructure and protective properties of the prepared coatings. The SEM/elemental map results show that the additive lanthanum is positioning homogeneous within coating structure. Also, according to the obtained results and mechanism of PEO coating formation, the amount of additive replacement within the external porous layer was more than the replacement occurred within the internal dense structure. Results of potentiodynamic polarization (PDS) and electrochemical impedance spectroscopy (EIS) tests show that the corrosion resistance properties of PEO coatings in the presence of LN additives are greatly improved. Formation a structure with a higher density coupled with the barrier role of deposited lanthanum hydroxide due to the penetration of corrosion agents improves the corrosion resistance of the oxide coating. Finally, thermodynamic calculations prove that lanthanum hydroxide sediments were formed in respective immersion condition and also shows their stability and insolubility.