The Research On The Environment-friendly Plasma Electrolytic Oxidation Process Of Magnesium Alloy And Properties Of The Coating

Magnesium is the lightest of all metals used as the basis for constructional alloys,which is regarded as one of the most prospective metals in 21st century. Unfortunately,its poor corrosion resistance is the one of the main properties that has hindered its widespread use in many fields,particularly for outdoor applications. So it is valuable to strengthen its anti-corrosion performance through suitable surface treatment methods.Plasma electrolytic oxidation (PEO) is a new electrochemical surface treatment technology developed on the basis of the conventional anodizing,which employed higher energy density.The surface of valve metals,such as Al,Mg,Ti etc.,can convert to the ceramic-type coatings in situ under the conditions of plasma chemical,electrochemical,thermal-chemical effects and their anti-corrosion,wear-resisting and electrical-insulating performances can be improved greatly.To corrosion resistance of magnesium alloy poor and PEO existed a number of shortages,the electrolyte which is not contain Cr,F,P and environment-friendly of AZ91D magnesium alloys and prcess has been studied in this paper.In the experiment,orthogonal layout was applied and the dropping testing time, hardness were selected as the appraisal parameters,and the analysis method of the orthogonal layout experiment is innovated,and proposed a new method that combining the comprehensive balance method with desirability scale method.According to the results of the orthogonal layout experiment,we found the best content of the components for the electrolyte.The voltage-time curve,coating thickness-time,micro-appearance examination and thedropping testing were employed to study the coating-forming effects of the coating on magnesium alloy PEO process.The results indicated that the oxide film is similar to the surface under different processes,all are porous structure.With the current density increases,the extension of time,increasing of thickness.the surface roughness increase, the coating surface porosity Larger, and corrosion-resistant of coating increase. Comparing with the coating-forming effects under different parameters,found that the coating has the best overall performance in 15 mA/cm² current density and 30 min.Higher current density and the time is too long are not conducive to improve the corrosion resistance and surface roughness of coating, and the coating is more roughness,may be have micro-cracks.XRD,EDS,SEM were employed to study and analyse the ingredients,the film was mainly composed of MgO,MgSiO₃,MgAl₂O₄ and Al₂O₃.The oxide film composition and the composition of the electrolyte have great relations. Na₂SiO₃ enable the oxide coating contains MgSiO₃.Organic amine is joined to make Al increase in the content of the coating.Peroxide provides oxygen for the coating process. Magnesium alloy oxide coating is well-dense and surface-porous,and the formation mechanism of porous structure was discussed and found that the porous membrane structure has great relationship with the growth of coating and spark discharge process.Film and the base metal was Juchi shape,in different parts of the sample,the division of the coating thickness is not uniform.Between that oxide coating and the base metal have strong binding force,combined with binding force testing.The corrosion resistance of oxide coating of magnesium alloy was evaluated through different methods such as dropping test,total immersion test,and the same conclusion was acquired.It revealed that the corrosion resistance of magnesium alloy was to a corking extent improved through the new process.And,through hydrogen evolution measurement shows that changing process of corrosion rate with treatmen times before and after magnesium alloy AZ91D Optimization process.