LZ91與AZ31鎂合金微弧氧化製程與耐腐蝕性質研究

Abstract

鎂合金是最輕的工程金屬材料，其高比強度在汽車、航太和消費性電子商品產業都有很大的發展性，但因為鎂合金的耐腐蝕和耐磨耗性質差，而嚴重影響其商業用途。微弧氧化為鎂合金的電化學表面改質技術之一，其特色是以高電壓在鎂合金表面生成一氧化膜，可以大幅提升其耐腐蝕與耐磨耗特性。 本研究以LZ91和AZ31鎂合金作為材料，搭配矽酸鈉和磷酸鈉基礎之電解液，透過調整電解液成分、電流密度、佔空比、工作頻率和工作時間來達到提升耐腐蝕性質的目的。本研究利用極化曲線、交流阻抗和鹽霧試驗來評估膜層的耐腐蝕性質，並利用掃描式電子顯微鏡、X光光電子光譜儀和X光繞射儀分析膜層之微觀形貌、化學成分和結晶組成。 研究結果發現選擇適當的微弧氧化製程能提供鎂合金很好的保護性，其中，陰極電流密度和佔空比對耐腐蝕性質的影響較小，而工作頻率和工作時間的影響較大；在固定電流密度和佔空比的實驗中，工作頻率1000 Hz和工作時間8分鐘的微弧氧化膜層有較好的耐腐蝕性質，在96小時鹽霧試驗後，LZ-SAc 10% 1000 8和 AZ-SAc 10% 1000 8分別只有2和1個蝕點，且腐蝕面積分別為0.009%和0.061%。此外，透過腐蝕形貌觀察發現微弧氧化膜層在腐蝕反應中的剝離效應(Flaking effect)，此效應有可能使有微弧氧化膜層的鎂合金之腐蝕反應速率增加。 另一方面，在實驗中發現LZ91的微弧氧化膜層會隨著大氣放置時間而有顏色上的變化，透過XRD、SEM和XPS發現在結晶度或是表面膜層的結構和成份上都沒有變異，但EIS結果指出極化阻抗會隨時間下降，故此顏色和腐蝕性質的變異應該是因為膜層內部成分或化學組態的變化而造成。

Magnesium is the lightest structural metal and its alloys are attractive to the automotive, aerospace and electronic industries for their high ratio of strength to weight. Unfortunately, magnesium alloys exhibit a very poor corrosion resistance and wear resistance. These weaknesses seriously influence the development and application of magnesium alloys. Micro-arc oxidation, which can generate oxide coatings on alloys’ surface, is a high voltage electrochemical surface treatment for magnesium alloys. This technique can remarkably promote the corrosion resistance and wear resistance of magnesium alloys. In this study, LZ91 and AZ31 magnesium alloys are choosed to be anodized in silicate and phosphate based electrolytes. The effect of various process parameters, including electrolytic composition, current density, duty ratio, frequency, and working time, on MAO coatings was evaluated. Salt Fog Test, Potentiodynamic Polarization Test, and Electrochemical Impedence Spectroscopy (EIS) were conducted for corrosion analysis; Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and X-ray Diffraction (XRD) were used to study the morphology, microstructure and composition of coatings. Test results show that MAO treated magnesium samples with proper process parameter exhibit excellent corrosion resistance. The effect of cathodic current density and duty ratio is smaller than the one of frequency and working time. In all experiments, samples produced by 1000 Hz and 8 minutes were always rated better corrosion resistance. After 96 hours salt spray test, LZ-SAc 10% 1000 8 and AZ-SAc 10% 1000 8 had only 1 and 2 pits with total corrosion area 0.009% and 0.061%, respectively. Besides, by studying the microstructure of corroded smaples, flaking effect was observed. MAO treated samples may suffer from faster corrosion rate for this effect. On the other hand, a color changing phenomenon was observed on MAO treated LZ91 samples. The test results of XRD, SEM and XPS indicate that the degree of crystallinity, morphology and surface composition of coatings remain no change; however, decaying corrosion resistance is showed by EIS data on 15 days idling sample. Therefore, the conversion of color and corrosion resistance may be contributed to the change of composition or chemical state in MAO coatings.