| As the lightest metal structural material,magnesium(Mg)alloy has many important applications in automotive,aerospace,electronics,biomedical materials and other fields due to its high specific strength and specific stiffness,good electrical and thermal conductivity,excellent anti-electromagnetic interference and biological compatibility properties.However,the poor corrosion resistance of Mg alloy is one of the main disadvantage for its wide application in industrial products.By adjusting the composition of micro-arc oxidation(MAO)electrolyte and the corresponding electrical parameters,an in-situ MgO ceramic layer with controllable thickness and good bonding strength with substrate could be formed on the surface of Mg alloy.However,the surface of the MAO ceramic coating layer with hydrophilic properties has structural defects such as micro-pores and micro-cracks,which can only provide passive corrosion protection for Mg alloy matrix.On the other hand,the good conductivity of Mg alloy is sacrificed due to the high insulation of MAO ceramic layer,which limits its application in electronics,aerospace and other fields.In order to further improve the corrosion resistance of MAO ceramic layer,steam treatment and simple hydrothermal treatment were used to post-treat the MAO ceramic layer coated on Mg alloy in this paper.By adjusting the process parameters of subsequent treatment,the composite coating with better corrosion resistance was obtained.Then,Cu layer was prepared on the surface of MAO ceramic layer by electroless plating and magnetron sputtering in order to obtian the surface of MAO ceramic layer with excellent conductivity.Finally,ZnO-LDH(Layered double hydroxide)/MAO with corrosion resistant and conductivity were designed and prepared on Mg alloy.By characterizing the microstructure and electrochemical properties of the coating prepared on MAO ceramic layer,the formation mechanism of the coatings prepared on porous MAO ceramic layer was revealed,the relationship between the preparation process and the microstructure of the coating was clarified,and the physical mechanism of corrosion failure and corrosion mechanism of various composite coatings were expounded.The corrosion resistance of MAO ceramic layer on Mg alloy can be further improved after steam treatment.Mg(OH)2 and Mg-Al LDH mixed phase were grown in-situ on the surface of MAO ceramic layer with the steam treatment time of 24 h,which had obvious sealing effect on the inherent defects such as holes and cracks on the surface of MAO ceramic layer.The dense coating could act as a barrier to the infiltration of corrosive chloride ions into the matrix,and the corrosion current density(3.39×10-8 A·cm-2)decreased compared with that of MAO ceramic layer(6.29×10-7 A·cm-2).Different hydrothermal treatment could also further improve the corrosion resistance of MAO ceramic layer.It was found that Mg(OH)2,Mg-Al LDH and Mg-Al-Zn LDH were formed on the surface of MAO ceramic layer after hydrothermal treatment in different concentrations of NaOH solution,alkaline Al(NO3)3 solution and alkaline solution containing Al3+and Zn2+,respectively.With increasing NaOH concentration,prolonging hydrothermal treatment time and increasing Al3+concentration,more hydroxides were formed on the surface and in the pores of the MAO ceramic layer during the hydrothermal process,which had the better sealing effect.The obtained composite coatings had an obvious barrier effect on the corrosive chloride ions to penetrate into the matrix.Therefore,the composite coatings achieved better corrosion resistance than the single MAO ceramic layer.Especially,the active protective LDH coating formed in the hydrothermal process has a more significant contribution to the improvement of corrosion resistance of MAO ceramic coating.The corrosion current density of LDH/MAO composite coatings(?10-9 A·cm-2)decreased by two orders of magnitude compared with that of MAO ceramic layer.The Cu/MAO composite coating prepared on the surface of Mg alloy by MAO-electroless plating treatment or MAO-magnetron sputtering treatment had not only good conductivity,but also changed the color of MAO ceramic layer because of the presence of the metal copper layer.However,the corrosion resistance of Cu/MAO composite coating was lower than that of MAO ceramic layer,and its corrosion current density(?10-5 A·cm-2)was two orders of magnitude higher than that of MAO ceramic layer.In order to obtain a highly corrosion resistant and conductive composite coating on the surface of Mg alloy,ZnO-LDH composite coating was prepared on MAO coated Mg alloy by three-step method.Mg-Al-CO LDH coating was prepared on MAO ceramic layer after MAO ceramic layer post-treated by hydrothermal treatment.Mg-Al-Co LDH nanosheets formed during the hydrothermal treatment grown in-situ on MAO ceramic layer in the way of nests and mastoids,which can seal the inherent defects such as micro-pores and cracks in the MAO ceramic layer.Subsequently,ZnO coating was prepared on the surface of Mg-Al-CO LDH/MAO composite coating combined sol-gel and hydrothermal epitaxial growth method.It was the presence of the ZnO seed layer prepared by sol-gel spin method that caused ZnO to grow in the form of nanorods perpendicular to the seed layer during the hydrothermal epitaxial growth.Meanwhile,ZnO nanorods growing in the gaps between Mg-Al-Co LDH nanosheets further improved the surface compactness of the coating.ZnO-LDH/MAO composite coating not only had better corrosion resistance than MAO ceramic layer and Mg-Al-Co LDHMAO composite coating(its corrosion current density was 2.29 × 10-9 A·cm-2),but also endowed the composite coating with better surface conductivity due to the presence of ZnO semiconductor coating.ZnO/LDH coating prepared on the surface of MAO ceramic layer by multi-step method meet the requirement of corrosion resistance and electrical conductivity. |
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