Surface Modification of Biomedical MgCa_4.5 and MgCa_4.5Gd_0.5 Alloys by Micro-Arc Oxidation

The aim of this work was to characterize the structure and corrosion properties of the MgCa_4.5(Gd_0.5) alloys surface treated by the micro-arc oxidation (MAO) process. The MgCa_4.5 and MgCa_4.5Gd_0.5 alloy samples were processed by MAO in an electrolyte composed of NaOH (10 g/dm³), NaF (10 g/dm³), NaH₂PO₄ (5 g/dm³), Na₂SiO₂·5H₂O (10 g/dm³) and water. Two different voltages (120 V and 140 V) were used in the MAO process. The alloys protected by an oxide layer formed in the MAO were then the subject of corrosion resistance tests in an environment simulating the human body (Ringer's solution). After the experiments, the resulting samples were investigated using SEM, XPS and EDS techniques. The addition of Gd affected the fragmentation of the coating structure, thereby increasing the specific surface; higher voltages during the MAO process increased the number and size of surface pores. Corrosion tests showed that the MgCa_4.5Gd_0.5 alloys were characterized by low polarization resistances and high corrosion current densities. The studies indicated the disadvantageous influence of gadolinium on the corrosion resistance of MgCa_4.5 alloys. The immersion tests confirmed lower corrosion resistance of MgCa_4.5Gd_0.5 alloys compared to the referenced MgCa_4.5 ones. The MgCa_4.5 alloy with the MAO coating established at voltage 140 V demonstrated the best anticorrosion properties.