| Magnesium alloy has excellent properties,such as low density,high specific strength and specific rigidity,good damping performance,good electromagnetic shielding performance and so on.Their potential applied fields include automobile,aerospace and consuming electronics,etc.But the poor corrosion resistance of magnesium alloy which has greatly limited its extensive application.A ceramic coating can be in situ formed on the surface of AL,Mg,Ti and their alloys by using Micro-arc oxidation,which improves both the corrosion resistance and abrasion resistance of alloy.In this paper,through a great deal of experiments that AZ31B magnesium alloy being treated by micro arc oxidation in silicate electrolyte,and a coating with good properties were formed on the surface of AZ31B.Then micromorphology,phase composition,thickness,and corrosion resistance of the coating were studied.3.5%NaCl solution was chosen as the corrosive liquid to study the corrosion abrasion behavior of coating.The main work and result obtained are asfollows:(1)According to SEM results,the surface of micro-arc oxidation ceramic coatings was consisted of numerous micro-porous,and the vertical cross-section is composed of an outer loosed layer,an inner compact layer and a transition layer.Though X-Ray Diffraction analysis,the micro-arc oxidation ceramic coating is mainly consisted of MgO and Mg2SiO4.The results showed that processing parameters,such as electrolyte component concentration,current density and processing time could influence the performances of micro-arc oxidation coatings.It can be used to improve the micromorphology and the thickness of micro-arc oxidation coatings.(2)The corrosion resistance and corrosion mechanism of the coating were analyzed by polarization curves and immersion corrosion test.The results indicated that the micro-arc oxidation coatings have excellent corrosion resistances in corrosive liquid,and could protect magnesium alloy matrix effectively.With the applied current density of 6A·dm-2,the processing time is 20 min and the electrolyte optimum value is 14 g L-1 Na2SiO3,12 g·L-1 KF,4g L-1 NaOH,9mL L-1 glycerol,the corrosion resistance property of AZ31B magnesium alloys is best.NaOH and glycerol have little influence to corrosion resistances of the coatings.(3)The corrosion abrasion resistance and its mechanism of the coating were analyzed by friction-wear test machine in corrosive liquid.The results showed that micro-arc oxidation ceramic coatings were seriously damaged under the together action of mechanical wear and chemical corrosion,and then magnesium alloy matrix and cylinder pin combined into corrosion cell,which accelerated the process of corrosion abrasion.Processing parameters and electrolyte component concentration,especially the friction conditions,could influence the property of micro-arc oxidation coatings.(4)Based on micro-arc oxidation,a process of MAO combined with closed field unbalanced magnetron sputtering ion plating(CFUBMSIP)method was used to prepare composite coatings on AZ31B magnesium alloy.It is found that,after the closed field unbalanced magnetron sputtering ion plating(CFUBMSIP)treatment,diffractive peak of CrN appeared in the X-ray diffraction patterns.CrN coating had a developed columnar microstructures,the surface of CrN coating had a round cell like morphology.There were some pinholes in the coatings.The through holes in the CrN coating and the micro-arc oxidation ceramic coatings were the prime positions for galvanic corrosion.The release of H2,which took place at these positions,accelerated the damage. |
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