Forming characteristics and mechanisms of micro-arc oxidation coatings on magnesium alloys based on different types of single electrolyte solutions

Abstract

To systematically investigates the forming ability and properties of micro-arc oxidation (MAO) coatings on magnesium alloys using different types of single electrolytes and further enhance the understanding of the coating formation mechanism from multiple perspectives. MAO treatments were performed in three types of single electrolyte solutions containing either sodium silicate, potassium fluoride, or sodium hydroxide, which were severally designed in terms of main salts, additives, and alkaline substances. The results show that MAO coatings could still be formed in the three single electrolyte solutions; however, the sparks and observable phenomena on the sample surface differed obviously from those in multicomponent electrolytes. In potassium fluoride and sodium hydroxide electrolytes, sparks occurred continuously at fixed positions on the sample surface, forming concave defects with uneven distributions of Mg, F, and O on the coating surface. In addition, a larger response current was observed in the sodium hydroxide electrolyte during MAO; this is disadvantageous in reducing energy consumption. The coating obtained in the potassium fluoride electrolyte had the highest thickness and number of micropores because of the synergy between the electrochemical process and the spark state on the sample surface. In contrast, the coating prepared in the sodium silicate electrolyte exhibited better corrosion resistance because of a certain thickness and good compactness.