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.