Role of the electrolyte composition in establishing plasma discharges and coating growth process during a micro-arc oxidation

Micro-arc oxidation (MAO) coatings were prepared on Al substrate in alkaline sodium hexametaphosphate, sodium silicate and sodium aluminate electrolytes, the breakdown voltage and responsive current respectively decreased and increased with increasing the electrolyte concentration. The behavior of electrolyte during MAO processes was analyzed and discussed based on the data of the coating microstructure, elemental and phase composition, and the carrier density and impedance of the coating. The behavior can be explained by a mechanism in which anions incorporated into the coating reduced the anion vacancies, thus reducing the carrier density and improving the impedance. •Breakdown voltages decreased with increasing the concentration of electrolytes.•MAO coating's integrality increased with increasing the electrolyte concentration.•Anion vacancies decreased with increasing the concentration of electrolytes.•A mechanism for explaining the behavior electrolytes during an MAO process was proposed.