Failure of coatings formed by micro-arc oxidation on titanium under cathodic conditions in H2O2 solution

Abstract

In the existing chemical stripping methods for micro-arc oxide (MAO) coatings, the application of strong acids (HF, HNO3) or alkali increases the difficulty in post-treatment for stripping solutions. In this study, the failure of MAO coatings on titanium under cathodic conditions was observed in a 6 % H2O2 solution for the first time. The MAO coatings exfoliated from the TiO2/Ti interface in a visible form within 60 min at a pulsed voltage of 1.6 V (i < 10 mA·cm−2), when the specimen was the cathode directly. The necessary conditions for failure were determined through single-factor experiments, and the mechanism was summarised based on Ti corrosion. The presence of both voltage and H2O2 is the necessary condition; the failure mechanism is as follows: large amounts of OH− generated by H2O2 reduction near the cathode create a micro-regional alkaline environment for the reaction of amorphous TiO2 and OOH−, and Ti enters the solution in the form of titanium complex. In particular, the electrolyte is almost neutral (pH ∼ 6.9) and has a good recyclability (Ti release at mg·L−1 levels). This treatment is experimentally effective for MAO coatings containing Ca, Si, P, or Al. This paper provides an alternative for non-conforming coatings and retired workpieces treatment.