Modification of PEO coatings on AZ31 magnesium alloy via TiO2-enriched electrolyte and PLA polymer sealing

: The aim of this study is to improve the corrosion resistance of AZ31 magnesium alloy using two-step surface modification based on barrier effect of the combined coating system. Prior to the sealing, plasma electrolytic oxidation (PEO) was carried out in a phosphate based electrolyte containing 1 g/l TiO 2 particles producing more compact coating (E1). The resulting E1 coating is characterized by smaller pores compared to the TiO 2 -free coating (E0). The TiO 2 agglomerates present within the E1 coating ensured partial sealing of the pores and defects. Presence of the TiO 2 in form of the anatase phase was confirmed by XRD analysis. Additionally, the polylactic acid (PLA) was used to seal the PEO coatings. The presence of PLA polymer sealing layer was confirmed by SEM and Raman spectroscopy. The polymer penetrated and filled the finer pores of the E1 coating more effectively, resulting in the formation of a continuous protective film. On the other hand, incomplete filling was observed in the E0 samples. Corrosion characteristics of the examined surface conditions were evaluated by electrochemical impedance spectroscopy (EIS) measurements during 168 h of exposure at the laboratory temperature in 0.1 M NaCl solution. The performed optimization of combined coatings system improved corrosion resistance of both PEO coatings.