Preparation and characterisation of black ceramic coating on AZ91D magnesium alloy by plasma electrolytic oxidation with reduced energy consumption.

A black ceramic coating was prepared on AZ91D magnesium alloy by plasma electrolytic oxidation (PEO) with reduced energy consumption in an electrolyte containing sodium silicate (Na₂SiO₃), sodium hexametaphosphate ((NaPO₃)₆), ammonium metavanadate (NH₄VO₃) and sodium citrate (C₆H₅Na₃O₇). The results showed that with the increase of NH₄VO₃ concentration, the surface color of the PEO ceramic coating turned darker. There was no more change observed in the coating color as the NH₄VO₃ concentration reached beyond 10g/L. With the optimization of PEO parameters and the electrolyte composition, the average energy consumption of the PEO coating was successfully reduced to 0.632 kw·h/(m2·μm) after 8 minutes process, which was only 30% of the energy consumption of the normal PEO coloring process. The black color of the ceramic coating was resulted from the mixture of V₂O₃ and V₂O₅. [ABSTRACT FROM AUTHOR]