Magnesium-based structural materials are excellent candidates for weight-saving applications in aerospace, space, sports, automotive, electronics and healthcare. Especially, for biomedical applications, Mg alloys have opportunities in the medical market as biodegradable implants. However, they are highly susceptible to corrosion due to high electrochemical activity. To control the corrosion the surface modification methods like conversion, spraying, electrochemical deposition, and plasma, could be used. Plasma electrolytic oxidation (PEO) is one of the most studied processes, enabling the preparation of anticorrosive, biocompatible and bioactive ceramic coatings with a tailored porous layer and microstructure. Electrical parameters such as electrolyte composition, PEO time and temperature, and sample pretreatment are crucial to improve the magnesium alloy corrosion resistance through the PEO. Extensive research activities have been currently focused on the impact of the use of basic solutions with the addition of electrolyte additives on PEO properties.
In this work, various additives such as sodium silicate (SO), trisodium orthoborate (BO), or sodium bicarbonate (CO) were added to the base electrolyte composed of sodium hydroxide and tri sodium phosphate (PO) to obtain protective oxide layers on AZ31B Mg-based alloy via PEO process. The morphology, chemical, and phase compositions of the plasma electrolytic oxidation (PEO) coatings were evaluated by scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction analysis, respectively. The corrosion behavior of the coatings was assessed by electrochemical impedance spectroscopy using Hanks and NaCl solutions. SEM results revealed that the porous structure of the coatings was influenced by the electrolyte: pore size of 0.7 µm for CO, 1.2 µm for BO, 1.4 µm for SO, and 1.6 µm for PO, respectively. The coating thickness was not affected by the electrolyte solution. The electrochemical behavior of the coatings in NaCl and Hanks solutions showed that the best corrosion resistance of CO coatings, which was attributed to its their small pore size. The addition of carbonate (CO) to the base electrolyte solution is more environmentally friendly and yields more durable oxide layers, making it more advantageous for use in a wide range of applications.

Acknowledgments
This work was supported by European Union´s Horizon 2020 research and innovation programme under grant agreement No 739566.