Preparation And Corrosion Resistance Of MAO Black Ceramic Coating Of Magnesium Alloy Under Copper Salt System Electrolyte

Magnesium alloys have the performance of specific strength and modulus of elasticity,excellent casting and cutting properties and good dimensional stability.Magnesium alloys obtained by color treatment can be widely used in electronic products,optical instruments and other fields,and can be used for spacecraft external camera and optical instrument housing due to its excellent extinction and heat absorption,etc.The formation of a black ceramic film on the surface of magnesium alloys by means of micro-arc oxidation（MAO）has become a hot research point in the field of surface modification of magnesium alloys.In this paper,zirconium salt and copper salt additives were selected as the base electrolyte and coloring salt,respectively,which were used to prepare the black ceramic films on the surface of AZ91D magnesium alloy via the microarc oxidation technique.The effects of additive type,additive concentration,oxidation time and termination voltage on the blackness,microscopic morphology and phase composition of the ceramic film were investigated,and the growth mechanism of the black ceramic film by micro-arc oxidation was investigated.The corrosion resistance and ageing resistance of the micro-arc oxidation ceramic film were analyzed by immersion corrosion,electrochemical corrosion and UV ageing tests.The results show that the addition of copper chelate and copper pyrophosphate additives to the electrolyte effectively reduced the Lab value of the ceramic film,and that the additive concentration,termination voltage and oxidation time all had an effect on the performance of the micro-arc oxide ceramic film.By comparing the Lab value,thickness,roughness,hardness and wettability of the ceramic films,the copper pyrophosphate additive was selected as the preferred coloring salt.The optimum preparation parameters were as follows:copper pyrophosphate additive concentration of 6 g/L,termination voltage of 400 V,and oxidation time of 14 min.The ceramic film prepared in the optimum parameters has the minimum Lab value of 19.71,the maximum thickness of 50.4μm,the maximum surface roughness of 1.17μm,the maximum microhardness of 605 HV,and the maximum wetting angle of 45.26°.The ceramic film has a typical"crater"microporous appearance with a low number of surface cracks and no obvious defects.XRD analysis shows that the main phases of the ceramic film are Mg O,Zr O₂,Mg F₂ and Cu O,with the Cu O phase giving the film a black color.XPS analysis further determined the presence of elemental Cu in the form of Cu O.With the concentration of copper pyrophosphate increasing,the corrosion resistance of the ceramic film first increased and then decreased.The best corrosion resistance of the micro-arc oxidized black ceramic film prepared at a copper pyrophosphate concentration of 6 g/L showed an increase in corrosion potential of 0.15 V and a reduction in corrosion current of approximately four times compared to the micro-arc oxidized ceramic film prepared without the copper salt additive.After electrochemical corrosion the surface microform of ceramic film B3 has the smallest corrosion area and a few cracks appear in the cross-sectional microform.The concentration of copper pyrophosphate was 6 g/L and the Lab value of the ceramic film B3increased to 31.12 after 168 h immersion,with a slight reduction in weight,no discoloration of the surface and no corrosion spots.The UV aging test showed that the overall color difference of ceramic film B3 was less than 3,and that the Lab value of the ceramic film increased slightly and that the ceramic film B3 has good stability and durability under UV conditions,which was attributed to the micro-arc oxidation coloring mechanism of ceramic film B3 and the generation of CuO phase.