| Mg-4Y-4Al magnesium alloy has excellent mechanical properties,but its poor corrosion resistance and wear resistance restrict its further development.Compared with the electroless plating,anodizing and laser surface modification,the packed powder diffusion coating?PPDC?treatment can not only improve the corrosion resistance and wear resistance of magnesium alloy at the same time,but also has the advantages like good bonding force between the substrate and coating,simple process steps,low pollution and the low cost of equipment.However,the current PPDC treatment on magnesium alloy has the disadvantages of high processing temperature,long processing time and need of protective atmosphere.Aimed at the problems above,from the component design of the zincizing agent,preparation of the zinc coating with excellent performance on the basis of traditional zincizing agent for magnesium alloy by adding KHF2 as the activator and CeO2 as the catalyst.In this work,the microstructure,corrosion resistance and wear resistance of the zinc coatings prepared by the different zincizing agent were investigated by using various analysis methods of OM,SEM,EDS,XRD,electrochemical tests,microhardness test and friction-wear test.The conclusions are as follows:With 5%KHF2 and 3%CeO2 added in the zincizing agent,a dense and uniform zinc coating was obtained by the zinc PPDC treatment on the as-cast Mg-4Y-4Al magnesium alloy at 400°C for 8 h without protective gas or vacuum.The microstructure of the zinc coatings is that the black primary block?-Mg solid solution distributes on the continuous white matrix composed of MgZn,Mg0.97Zn0.03and intergranular?-Mg solid solution.The absolute value of the formation enthalpy of the MgZn phase is the largest.The addition of CeO2 in zincizing agent can increase the diffusion efficiency of Zn atoms into the alloy substrate.The thickness and compactness of zinc coatings increases first and then decreases with the increase of CeO2 addition from 0 to 9%,and the zinc coating has the maximum thickness?about 1200?m?when the CeO2 content is 3%.With much more CeO2 added in the zincizing agent,the zinc coating appears to be of low quality with the presence of many cracks.With the increase of CeO2 content from 0 to 9%,the corrosion potential Ecorr and polarization resistance Rp are first increased and then decreased.While,the corrosion current density Icorr is first decreased and then increased.When the CeO2 content is 3%,the Ecorrorr is the most positive,the Rp value is the biggest,and the Icorr value is the smallest.With the increase of CeO2 content,the radius of the capacitive loop,charge transfer resistance Rct,inductive reactance RL and dispersion coefficient n are first increased and then decreased.While,the phase angle constant Y0 and capacitance value C are first decreased and then increased.When the CeO2 content is 3%,the radius of the capacitive loop,Rct,RL and n are the biggest,and the Y0 and C are the smallest,indicating the corrosion resistance of the C2 zinc coating reaches the best.As the extension of soaking time,the Mg-4Y-4Al magnesium alloy matrix was rapidly corroded,and the presence of the zinc coating significantly slowed the corrosion rate.The continuous dense zinc coating prepared by adding 3%CeO2 in the agent can isolate the magnesium alloy matrix and the corrosive medium,effectively hinder the hydrogen evolution corrosion,and inhibit the occurrence of pitting corrosion.However,excessive CeO2 addition would cause defects such as cracks and pores on the zinc coating,and accelerate corrosion rate.When 3%CeO2 added in the agent,the microhardness of the zinc coating reached the maximum value(about 210 HV0.05),which is mainly related to a large amount of high-hardness intermetallic compounds such as MgZn in the zinc coating.When the CeO2 content is 3%,the surface friction coefficient of the zinc coating under the load of 330 g and 530 g is both the smallest,which is obviously lower than that of the matrix.The value of the width and depth of wear scar,wear volume,specific wear rate and wear rate of the C2 zinc coating is significantly lower than that of the matrix.The wear mechanism of the zinc coating under the load of 330 g and 530 g is mainly abrasive wear and oxidative wear,and the C2 zinc coating has the slightest wear and the best wear resistance. |
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