Investigation On Preparation And Corrosion Resistance Of Yttrium-based Conversion Coating On AZ91D Magnesium Alloy

Magnesium alloy has many excellent properties,such as high specific strength,high specific stiffness,goodelectromagneticshieldingproperty,goodcastingability,favorable bio-compatibility,recycling,etc.The disadvantages of it is their poor corrosion resistance,which seriously limits their popularization and applications.Corrosion and protection research on magnesium alloys has important theoretical and practical significance.In this dissertation,firstly,the yttrium-based conversion coating was obtained on the surface of AZ91D magnesium alloys,and optimizing the preparation process.The formation mechanism and corrosion mechanism of yttrium-based conversion coating were investigated.Secondly,according to the characteristics of multi cracks of the yttrium conversion film,the silica sol sealing and phosphating process were studied.Firstly,yttrium nitrate were chose to be the main elements in conversion solution.The optimized process was determined by orthogonal experiment design.The micro-morphology,composition,phase structure and electrochemical features of the coatings were investigated by scanning electron microscopy?SEM?,energy dispersive spectroscopy?EDS?,X-ray photoelectron spectroscopy?XPS?and the electrochemical station,respectively.And the formation mechanism and corrosion mechanism of yttrium based rare earth conversion coating on AZ91D magnesium alloy were analyzed and proposed.Secondly,the sealing treatment with silica sol solution and the phosphating process were achieved on the basis of the yttriun conversion coating prepared at the optimized process.Then the morphology,composition and corrosion resistance of the treated coating were tested.The following results were achieved:?1?The optimization processing conditions of the orthogonal test were composed of solutions of yttrium nitrate,Y?NO₃?₃ with concentrations of 10 g/L at 30?for 50 min.The results of phase analysis showed that the coating mainly composed of Y₂O₃,YOx/y,Al₂O₃ and MgO.The coating with a large amount of crack was thicker and homogenous,and it can improved thecorrosion performance of magnesium alloy.?2?The formation process of the yttrium conversion coating mainly include:Y³⁺,Mg²⁺,Al³⁺exist in the liquid layer of alloy surface,the cathode reaction result in an increased OH^-concentration and pH value of the local cathodic sites between the substrate and the solution,which lead to the forming of the metal hydroxides.When the pH value in the solution rises to a certain number,hydrates coating will deposit on sample surface.During reaction with OH^-,Al³⁺,Y³⁺ions and Mg²⁺ions to form hydroxides and deposit.During drying,the decomposition will occur to form the oxide.?3?The yttrium conversion coating can improve the corrosion performance of magnesium alloy substrate.The main corrosion mechanism of the coating include:when the yttrium conversion coating dipped in sodium chloride solution,Cl^-has selective preferential adsorption in the the defect position such as film hole and micro cracks on the conversion coating.Once chloride ions entered into the defect,then reacted with magnesium oxide to generate soluble MgCl₂ at a certain probability.All of these made the magnesium ions get into the solution,which cause the film of adsorption area tend to thin.The coating dissolving and matrix exposing leads to forming corrosion point.As the extension of etching time,the accumulation of the corrosion products covered on the surface of the coating,which inhibited the anodic reaction and increased the resistance of the coatings to some extent.When the immersion time further extended,the chloride ion penetrated through the film of corrosion products to the substrate.The corrosion reactions expanded to a broader area,until the coating suffered serious destruction.?4?The processing parameters of the sealing treatment of silica sol solution is 250?+2 h.After sealing treatment,the number of the micro cracks on the surface of yttrium conversion coating were reduced obviously,which protected the magnesium alloys from the corrosion medium in ambient and dramatically improved the corrosion-resistance capability of the magnesium alloys.Besides,the treatment also improved the adhesion strength between the coating and substrate.The E_corrorr of the silica sol-treated sample is significantly shifted positively by about230 mV,and the silica sol-treated coating decreased the corrosion current density about two orders of magnitude compared with the bare alloy.?5?The parameters of the chemical densification process on the yttrium conversion coating are as follows:mass fraction of ammonium dihydrogen phosphate is 1.5%,treating time is 90s,and treating temperature is 80°C.The number and size of the cracks reduced obviously after the post-treatment with phosphate.The phosphated composite coatings are mainly consisted of Y₂O₃,YOx/y,MgPO₄,AlPO₄ and YPO₄.The compactness of the yttrium conversion coatings after phosphaing is improved,which benefits to reduce the number and size of the cracks.Follow phosphating,the corrosion potential of the coating was shifted positively about 180m V compared to the uncoated one,and the corrosion current density decreased about one order of magnitude.