| Magnesium alloy has been widely used in automotive parts,space devices,electronic products and military fields because of its advantages of low density,high strength,high weight ratio,low modulus of elasticity and green environmental protection.However,magnesium alloys are highly reactive,and are corroded easily in the environment,which limit their application.Therefore,surface treatment of magnesium alloys is needed to improve its corrosion resistance.At present,chemical conversion is the most commonly used surface treatment technology,which has been widely applied in the coating treatment or corrosion protection of magnesium alloys.Phosphate conversion treatment is a good substitute for chromate conversion coating because of its characteristics of no pollution,low cost,simple operation and corrosion resistance,but the phosphating process of magnesium alloy is not mature enough,and the protective effect of the coating is not very good.Therefore,the conversion of phosphate on AZ91 D magnesium alloy surface is discussed in this paper,the characteristics,properties and coating forming mechanism of the conversion coating are also analyzed.The main experimental results are as follows:(1)By adding additives in the phosphate solution,the anticorrosive coating was prepared on the surface of die-casting AZ91 D magnesium alloy.And the effect of p H value of the phosphating solution on the formation process,microstructure and corrosion resistance of the phosphate conversion coating was studied.The study shows that the formation of phosphate conversion coating is the result of the hydrolysis equilibrium movement of manganese dihydrogen phosphate and the hydrolysis and condensation of ammonium vanadate.With the increase of pH value of phosphating solution,the decrease of hydrogen evolution rate and the reaction of vanadate will promote the deposition of phosphate conversion coating.However,when the pH value is too high,the electrochemical reaction cannot be carried out and the phosphating coating is difficult to form.The surface quality and the corrosion resistance of the conversion coating obtained at different p H values is totally different.The quality of the conversion coating is the best when the pH value of the phosphating solution is 4.0 and the distribution area of the surface micro-crack are all minimum which means it has the best corrosion resistance.(2)As ammonium metavanadate has an significant influence on the coating forming effect and corrosion resistance of the conversion coating during the process of chemical conversion of phosphate,the effect of ammonium metavanadate concentration on the performance of phosphate conversion coating is further analyzed.The results show that by adding ammonium metavanadate into the phosphate conversion solution,the quality and corrosion resistance of the conversion coating on the surface of magnesium alloy are better than that of non addition.As the concentration of ammonium metavanadate increased,the corrosion resistance of phosphate conversion coating increased at first and then decreased.When the concentration is 0.5 g/L,the self corrosion current density of the conversion coating is low,the resistance arc is large,the corrosion rate reaches the lowest,the surface quality of the coating is better,the thickness is the largest,the micro crack size is the smallest,and the protective effect of the matrix is better.(3)After determining the best process of phosphate conversion treatment,the application of phosphate conversion coating in different forming magnesium alloys was discussed,and the morphology and properties of the coating were compared and analyzed.The experimental results show that the morphologies of conversion coatings obtained on three different forming magnesium alloys,as cast,die-casting and solid state,are quite different.The conversion coating prepared on the surface of the die-cast specimen is more uniform and dense,the micro-cracks on the surface are narrower.The surface cracks of the cast coating are the widest,there are some oxide particles.The solid solution conversion coatings are more flat,but the coatings are thin and the cracks are wide.Three layers ars mainly composed of Mg,Mn,P,O,Al and V elements.The conversion coating of the diecast specimen has the best corrosion resistance due to largest thickness of the conversion coating,least self-corrosion current density and the lowest efficiency of long-term corrosion. |
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