Study On The Microstructure And Corrosion Resistance Of Zn Coating On AZ91D Alloy By Molten Salt Thermal Diffusion

Recently, under the social background of resources conservation and environmental protection, as the lightest metal structure material of the world, magnesium alloys have been widely used in automobile industry and biomedical field.but the poor corrosion resistance of magnesium alloy seriously constrains its more widely application, so the surface modification of magnesium alloy for improving the corrosion resistance is gradually becoming the hot topics in recent years, on the basis of the advantages and disadvantages of other surface modification methods, in this paper, molten salt thermal diffusion treatment is carried out in low temperature on the AZ91D magnesium alloy, and Mg-Al-Zn intermetallic layer was successfully achieved which improvd the corrosion resistance of the magnesium matrix effectively.Molten salt diffusion treatment was taken on the surface of AZ91D magnesium alloy with ZnCl2-NaCl system (ZnCl2:NaCl=0.677:0.323),and with the help of SEM、 EDX、 XRD and polarization curve tests to study the structure and corrosion resistance of diffussion layer,it shows that:With the same treatment time12h.the higher the temperature, the more thick the diffussion layer was, and the thickness increased about1.2μm; The thickness of diffussion coatings increased as the extension of time,with the same treatment temperature325℃,and the thickness was7.5μm、8μm、8.5μm、15.5μm respectively; And with the same treatment time12h as well as treatment temperature325℃.the thickness of the layer with3%Ce was about11μm,but there was obvious flaw with9%Ce, which shows that only the right amount of rare earth can play a positive role in the thermal diffussion process; The dynamic of the formation of diffusion layer was calculated, and the formation mechanism of diffussion layer was discussed, which was the active atomics reacted witn matrix through the grain boundary diffusion and reaction diffusion; The polarization curve testing results in3.5wt%NaCl solution indicated that,compared to untreated magnesium alloy,the treated ones showed higher electrode potential.which meaned the corrosion resistance of magnesium alloy was significantly improved.Good performance diffussion coatings could be obtained through thermal diffussion process in ZnCl2-NaCl molten salt, thermal diffussion temperature and time together with Ce content were of great importance in this technology. The advantages of this technology were simple operation.the metallurgical bonding connection between the layer and matrix and good corrosion resistance of coatings.In this experiment, the best diffusion temperature and Ce content time were325℃and3%respectively.