The Electrochemical Study on Anodic Treatment of Mg Alloy (AZ 61)
Dr. Chuen-Guang Chao
|關鍵字:||鎂合金;電化學;陽極處理;化成處理;直流極化;交流阻抗;Magnesium alloy;Electrochemistry;Anodic Treatment;Chemical conversion treatment;Polarization;AC impedance|
|摘要:||本實驗係利用化成處理及陽極處理法改善AZ61鎂合金表面之抗腐蝕性，以化成處理法增加鎂合金基材與陽極處理膜間之附著性，化成處理主要以磷酸為主，再以矽酸溶液封孔；陽極處理液則以非鉻系之鹼金族矽酸溶液為主。在施以外加電壓條件下進行強迫氧化反應。經電化學動態極化曲線及靜態交流阻抗於3.5%NaCl水溶液中測試，可得知反應形成之矽酸鹽氧化膜具有良好之抗腐蝕性。實驗結果顯示鎂合金原材之腐蝕電壓為-1.536V (相對於飽和氯化銀參考電極)，腐蝕電流為1.613 mA/cm2，表面電阻值為204.6 ohm，經表面處理後試片之腐蝕電壓提升為
-0.182V，腐蝕電流降低為1.401E-6 A/cm2，表面電阻增為48724 ohm，較鎂合金原材明顯增加許多，達表面改質之效果。以顯微鏡觀察陽極處理表面可得知為多孔狀結構，孔徑約為50微米。成份分析結果顯示陽極氧化膜表面之主要成份為Mg、Al、O、Si及鹼金族元素。以EPMA 量測鈉系列陽極處理膜，厚度約為35微米，鉀系列陽極處理膜厚約為17微米。經表面處理後試片之硬度值由鎂合金原材之69.5 HV提升為132.1 HV，顯示經表面改質後抗磨耗性質亦增加。|
In this study, the corrosion resistance of AZ61 Magnesium was improved by chemical conversion treatment and subsequent anodic treatment. Chemical conversion treatment can enhance the adhesion between substrate and the following anodic films. Phosphoric acid was main component used in chemical conversion treatment. Aqueous slicate solution was applied for sealing. The electrolyte of anodic treatment was mostly composed of alkali silicates but non-chromate salts. External applied voltages forced Mg alloys to oxidization. The resulting anodic oxides showed good corrosive resistance by employing potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) measurements in 3.5% NaCl solution. The experimental results implied that the corrosion potential of untreated Mg alloy was -1.536V vs saturated Ag/AgCl electrode, the current density was 1.613 mA/cm2，and the passive resistance was 204.6 ohm. After the surface treatments, the corrosion potential of the treated Mg alloy was raised to -0.182V vs saturated Ag/AgCl electrode, the current density was lowered to 1.401E-6 A/cm2，and the passive resistance was increased to 48724 ohm. However, these two-step treatments improved the surface properties of Mg alloys. The surface morphology of the anodic film was examined by microscopes. The film exhibited porous structure and the diameter of the pores was about 50 micron meter. The composition analysis suggested that the anodic films were mainly composed of Mg、Al、O、Si and alkali metals. From the observation of Electron Probe X-Ray Microanalyzer (EPMA), the thickness of coating, forming in sodium-containing solution and in potassium-containing solution were about 35 micron meter and 17 micron meter, respectively. The hardness of the alloy was increased from 69.5 HV to 132.1 HV after anodizing. It is obvious that the abrasive resistance of Mg alloy with anodic coating can also be improved.
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