Study on Microstructure and Properties of Black Micro-Arc Oxidation Coating on AZ31 Magnesium Alloy by Orthogonal Experiment

Abstract

The effects of CuSO4 concentration, voltage and treating time on the hemisphere emissivity and corrosion resistance of AZ31B magnesium-alloy black micro-arc oxidation coatings were studied by orthogonal experiment. The microstructure, phase composition, corrosion resistance and hemisphere emissivity of the coating were investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, electrochemical test and infrared emissivity spectrometer, respectively. The results showed that the influences of each factor on corrosion current density and the hemisphere emissivity are as follows: voltage > treating time > CuSO4 concentration. The black MAO coatings are mainly composed of WO3, MgAl2O4, CuAl2O4, MgO, CuO and MgF2. The CuO and CuAl2O4 phases are the main reasons for blackness of the coatings. The coating exhibits the best corrosion resistance under the conditions of CuSO4 concentration 1.5 g/L, oxidation voltage 500 V and treating time 10 min. Additionally, the variation trends of hemispherical emissivity and roughness of the black MAO coating are the same when the composition of the coatings is similar. When the concentration of CuSO4 is 1.5 g/L, the oxidation voltage is 450 V and the treatment time is 10 min, the coating with the highest hemispherical emissivity of 0.84 can be obtained.

Improved Corrosion Properties of Mg-Gd-Zn-Zr Alloy by Micro-Arc Oxidation

“…The non-inductive steel tank was used as the cathode in the micro-arc reaction with a circulating cooling system, and the Mg alloy was used as the anode. It has been reported that the anodizing is shortened in this voltage range, the MAO coating grows quickly, and the thickness is uniform [23,24]. A study has shown that when the voltage is less than 450 V, the thickness of the film increases slightly with the increase in voltage; when the voltage is greater than 450 V, the thickness of the film increases significantly with the increase in voltage, and the thickness of the film will greatly affect the corrosion resistance of the alloy; and as the applied voltage rises from 400 V to 500 V, the surface porosity will be relatively stable [25].…”