Effects of current density on the corrosion resistance of micro-arc oxidation coatings on Ti6Al4V welded joints

The effects of different current densities in micro-arc oxidation (MAO) on the microstructure and corrosion resistance of electron beam welded Ti6Al4V alloy joints are investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD). The electrochemical corrosion behavior is studied in a 3.5% NaCl solution by polarization and electrochemical impedance spectroscopy (EIS). The thickness of the BM-MAO coating (base metal region) increases gradually with current densities and reaches a maximum at 16 A/dm2. In comparison, the thickness of the WZ-MAO coating (welded zone) increases initially and reaches a maximum at 12 A/dm2 before decreasing gradually. When the current density is 10-14 A/dm2, the WZ-MAO coating is thicker than the BM-MAO coating. However, the thickness of the WZ-MAO coating is smaller at 16 A/dm2. The BM-MAO and WZ-MAO coatings show decreased porosity with increasing current densities, and the porosity of the latter is lower. Dynamic potential polarization tests reveal a significant reduction in the self-corrosion current density at the BM area and welded joint (WJ), by an order and two orders of magnitude, respectively, after the application of MAO coatings with different current densities. The BM-MAO and WJ-MAO coatings deposited at a current density of 12 A/dm2 exhibit optimal corrosion resistance. The corrosion resistance of BM-MAO coating is better than that of WJ-MAO at different current densities according to the equivalent resistance and dielectric behavior.