Plasma electrolytic oxidation of tantalum in an aluminate electrolyte: Effect of cathodic polarization and frequency

The effect of cathodic polarization on plasma electrolytic oxidation (PEO) of the refractory metal of tantalum is investigated in 10 g/l NaAlO2+2 g/l KOH under 1000 and 100 Hz. Ceramic coatings are formed under various cathodic-to-anodic current densities ratios (R = jc/ja) of 0, 0.6, 1.3 and 2.3. For PEO under 1000 Hz, the plasma discharges intensify with the increase of R from 0 to 1.3, with optical emission from Ta being detected at later stage of PEO under cathodic polarization. However, excessive R ratio of 2.3 has caused early extinguishment of the plasma discharges. Correspondingly, thicker coatings are formed as the R ratio increases from 0 to 1.3, and the coating under R = 1.3 exhibits a most porous outer layer. A thin and compact coating is formed under R = 2.3. It is also found that the application of cathodic polarization increases the Na content in the resultant coatings, with a new crystalline phase NaTaO3 being formed under R = 2.3. Cathodic polarization also decreases the corrosion resistance of the coatings. Surface wettability tests show that the coatings formed under R = 1.3 are the most hydrophilic. PEO under 100 Hz shows some differences in discharge and coating formation. Larger sized discharges are also observed at the later stage under R = 0.6, but Ta species are detected at a delayed time. For the R values of 1.3 and 2.3, patches of soft sparking occur on the sample surface, leading to non-uniform coatings. This study shows that both cathodic polarization and frequency significantly affect the PEO process. Moreover, compared with the PEO of Al, tantalum responds quite differently to cathodic polarization. Mechanisms of coating formation have been illustrated: Protons and other cations are attracted by cathodic electric field into the coatings, where they undergo a series of reactions to significantly affect the PEO behaviors.