The Study Of Rare Earth Conversion Coating On AZ63Magnesium Alloy Suface

Magnesium is a perfect anode material because of the low standard electrodepotential （-2.36V vs SHE） and the large theoretical specific capacity （2205mAh g-1）.Magnesium and most magnesium compounds are non-toxic or low toxic,they areenvironmentally friendly. It is obvious that magnesium and its alloys can be idealbattery materials. Because the magnesium alloy corrosion potential is low, its chemicalactivity is high. It can react with many media,the surface is easily corroded to formmagnesium oxide. Poor corrosion resistance become one of the key problems to restrictthe application of magnesium alloys.Rare earth conversion coating treatment,an environment-friendly treatmentmethod,could effectively improve corrosion resistance of magnesium alloy. The processand operation is simple and convenient. The current use of cerium and lanthanum salt isto form the cerium and lanthanum conversion coating. This paper studies the ceriumlanthanum nitrate treatment on the surface of AZ63magnesium alloy. Inhibition ofcerium and lanthanum conversion coatings on AZ63magnesium alloy surface wasinvestigated by scanning electron microscopy （SEM）, energy dispersive X-rayspectroscopy （EDS）, Tafel polarization curves （LSV）,cyclic voltammetry curves （CV）and electrochemical impedance spectroscopy （EIS）.The results show that corrosion resistance of AZ63magnesium alloy by the ceriumand lanthanum conversion treatment is improved obviously. Rare earth conversiontreatment can improve the corrosion resistance of AZ63magnesium alloy. When theconcentration of lanthanum nitrate is0.06mol/L, the corrosion current is minimum andthe inhibition rate reaches94.76%. Comparing the three kinds of film-forming solutioneffect on the corrosion resistance of magnesium alloys, the result follows by0.02mol/LCe（NO₃）₃+0.06mol/L La（NO₃）>0.06mol/L La（NO₃）₃>0.02mol/L Ce（NO₃）₃. Compositefilm of cerium and lanthanum is more uniform and denser than single rare earth film.With the extension of immersion time, the surface becomes smoother and cracks on themagnesium alloy surface are gradually restored. The corrosion potential graduallyincreases, the corrosion current increases first and then decreases. The corrosionresistance of dual rare earth conversion film reaches highest at30min in NaCl solution.Due to Ce3+and La3+embedded in conversion coating cracks, extension of aging timecontributes to the oxidation of cerium and lanthanum, makes the film more compact and the corrosion resistance is improved. The experiment proves that the best aging time is48h.