The multi-effects of K2TiF6 additive on the properties of PEO coatings on AZ31 Mg alloy

Plasma electrolytic oxidation of AZ31 Mg alloy was performed in an alkaline silicate-based solution containing various concentrations of K2TiF6 using unipolar waveform at a constant voltage. The surface morphology of all coatings was rough and contained a micro-pore network, micro-cracks, and granules of oxide compounds in different diameters and micro-pore levels. The coating is composed of MgO, MgF2, Mg2SiO4, SiO2 (amorphous), and TiO2 (crystalline and amorphous) phases. The average thickness of the coatings was increased by adding the K2TiF6, where EDS results showed that a skinny fluoride-rich passive layer forms. With increasing time and reaching the final potential, the TiO2 colloidal particles absorbed physically by the oxide erupted into the discharge channels allowing the formation of a thicker and more compact layer coating along with the pore sealing. Incorporation of silicate ions develops amorphous SiO2 in the coating that also partially seals the pores in the coating. Long-term corrosion performance of the coatings was investigated using EIS during 7 days of immersion in 3.5 wt% NaCl solution. It was found that the barrier performance of the PEO coatings was improved by adding K2TiF6 up to 5 g l−1. The coating produced in the presence of 5 g l−1 K2TiF6 displayed the lowest porosity percent (13.04%) with an appropriate thickness, which provided the highest barrier performance at both short- and long-immersion times.