Study On The Process Optimization And Performance Control Of The Electrodeposited Al Coatings On Mg Alloy

Mg and Mg alloys are widely used in various industries,such as military,aerospace,automobile,and electronic communication,owing to their desirable properties of low density,high strength to weight ratio,high impact resistance,low resistivity,recyclability and good electromagnetic shielding property.However,the poor corrosion resistance of Mg alloys seriously limits their wider applications.Surface protection and treatment have to be applied to Mg and Mg alloys.The Al coating is considered to be the best choice.The latest methods of Al coating deposition on Mg alloy are various,such as thermal spraying,liquid diffusion,sputtering and deposition.Electro-deposition Al coating is attracting more and more attention because of the simple equipment using in the process and low cost.Additionally,the thickness and the quality of the coating can be controlled by adjusting the plating parameters.However,in the process of preparing electrodeposited Al coating on Mg alloy surface,there are two problems that seriously affect the quality and corrosion resistance of the Al coating:（1）The quality of the Al coating obtained by electrodeposition is poor,and the smooth and compact coating cannot be obtained due to the serious dendrites.（2）In the pre-treatment process of Mg alloy,the surface oxide film is difficult to remove since the Mg alloy is too active,the adhesion between the coating and the Mg alloy matrix is poor,which is very easy to peel off.In order to effectively reduce the phenomenon of coating dendrite and improve the coating quality,the single additive Na I and the compound additive Na I and TMACl were added to the electrolyte to reduce the volatile rate of the electrolyte and obtain a very flat and compact coating,optimize and improve the coating quality.This paper mainly focuses on the coating quality and adhesion of the following three aspects:（1）Electrodeposition process:by adding a single additive Na I and composite additives Na I and TMACl to the electrolyte solution,reduce the volatile rate of the electrolyte,improve the density of the coating,get a smooth and compact coating,improve the quality of the coating.（2）Pretreatment:the basic pretreatment and zinc dipping pretreatment,as the metal substrate of electrodeposited aluminum coating,inhibited the formation of oxide film on the surface of Mg alloy and improved the bonding force of coating.（3）Heat treatment:after electrodeposition,through the heat treatment of the coating,the formation of Mg/Al intermetallic compounds,further improve the adhesion of the coating.The mechanism of electrodeposition of Al in Al Cl₃-Na Cl-KCl（80-10-10 wt.%）inorganic molten salt electrolyte was clarified,and the phase evolution rule of the heat treatment process and the corrosion protection mechanism of the coating on Mg alloy were explained.The main research contents and conclusions are as follows:The characteristics of Al Cl₃-Na Cl-KCl inorganic molten salt electrolyte and the reaction of the electrodeposition process were studied,and the influence of Na I on the cyclic voltammetry curve was studied.It was found that Na I could well enhance the cathodic polarization and inhibit the electrodeposition of Al,which was conducive to the formation of a better coating.The effect of additive Na I on the nucleation/growth mechanism of Al was studied by electrochemical analysis.The three-dimensional"instantaneous nucleation"mode of Al on the cathode surface could not be changed by additive Na I,but the morphology of electrodeposited Al coating could be improved effectively by Na I additive.After adding 10 wt.%of Na I,the particle size of the Al coatings became uniform,the particle size decreased from 5～8μm to 2～3μm,indicating that Na I had the effect of refining particles.The addition of Na I promoted the arrangement of Al atoms along the（220）plane,leading to the appearance of the preferred orientation of the（220）texture of the Al coating prepared by electrodeposition from Al Cl₃-Na Cl-KCl inorganic molten salt electrolyte.The effects of additive TMACl alone and combined additives Na I and TMACl on the electrodeposition process and microstructure of the Al coating were studied.The results show that the volatilization of Al Cl₃-Na Cl-KCl inorganic molten salt electrolyte was significantly reduced after the addition of TMACl,but the morphology of the electrodeposited Al coating had little effect.The composite addition of Na I and TMACl not only inhibit the volatilization of the electrolyte,but also optimized the morphology of the coatings,and significantly improved the coating quality.When 10 wt.%Na I and1 wt.%TMACl were added together,the diffusion rate and nucleation density decreased at the same potential value.The experimental results showed that the loss rate caused by electrolyte volatilization was reduced to 30%,the surface of Al coating became more compact and compact,showing a flat"bark-like"structure,and the particle size of Al coating was further reduced with uniform distribution.The average particle size was about 2μm.A lay of more compact,flat and smooth Al coating can be obtained by adding two additives together.The effects of mechanical grinding,alkaline deoiling,acid etching and activation treatments on the surface morphology and microstructure of the Al coatings electrodeposited on Mg alloy were studied.The results showed that the Zn layer obtained with different Zn immersion time had good uniformity and coherence.Smooth and compact Al coatings with average particle size of 4～6μm could be obtained by electrodeposition on the surface of Mg alloy after Zn immersion.Mg alloy substrates,Zn layers and electrodeposited Al coatings were closely bonded with each other and had good binding force.Compared with the Mg alloy substrate,the corrosion potential of electrodeposited Al coatings on different thickness Zn layer was positively shifted by about 0.24 V,and the corrosion current was decreased by about 60μA·cm^-2,which was an order of magnitude lower.The effect of different thickness of Zn layers on the morphology,microstructure and properties of Al coatings on Mg alloy surface after heat treatment was studied.Two different intermetallic compounds,Mg₂Al₃ and Mg₁₇Al₁₂,were formed on the surface of Mg alloy after heat treatment.The thermal shock experiments showed that there was a strong bonding force between the coating and the Mg alloy substrate after heat treatment.With the increase of Zn layer thickness,the thickness of Mg₁₇Al₁₂ decreased gradually,the thickness of Mg₂Al₃ increased gradually,and the overall thickness of the coating basically remained the same.With the increase of the thickness of the Zn layer,the nano hardness and elastic modulus of the coating after heat treatment gradually increased,and the corrosion resistance gradually became worse.Considering comprehensively,the coating after 4 min Zn immersion had the best comprehensive properties.The nano hardness was up to 3.58 GPa,and the elastic modulus was about65 GPa,which was about 3 times and 1.5 times of that of Mg alloy matrix,respectively.Compared with the Mg alloy matrix,the corrosion potential shifted by 0.14 V positively and the corrosion current decreased by about 55μA·cm^-2.The mechanical properties and corrosion resistance of Mg alloy were effectively improved.In this study,the effects of additives Na I and TMACl on electrolyte volatility and electrodeposited Al coating were studied in this paper,and the mechanism by which Na I and TMACl improved coating quality and inhibit electrolyte volatilization was revealed.The effect of pretreatment,heat treatment and Zn immersion time on the structure and properties of the coating were studied.The relationship between Zn immersion time and microstructure of the coating after heat treatment was established.A complete process route including Mg alloy pretreatment,electrodeposition process,and coating post-treatment was developed.Al coatings with good adhesion,corrosion resistance and mechanical properties were successfully prepared on the surface of AZ91D Mg alloy.It provides guidance for further research and development of electrodeposition of corrosion resistant metal coating of Mg alloy in inorganic molten salt electrolyte.