AZ系列鎂合金錳酸鹽化成皮膜結構與性質研究

Abstract

鎂合金具有優異的比強度和低密度，廣泛應用於輕量化的產品，包括3C殼件、自行車、汽車等。但鎂是極為活潑的元素（標準還原電位－2.37 V vs. SHE），因此鎂合金需要適當之防蝕處理；傳統的化成處理大量使用了六價鉻（致癌物質，RoHS指令已限制其使用），發展替代六價鉻化成處理製程技術實是刻不容緩。 本研究以AZ91D與AZ31B鎂合金作為研究底材，並選用業界進行表面處理時，常使用的錳酸鹽化成處理作一基礎性的討論。利用掃描式電子顯微鏡(SEM)及穿透式電子顯微鏡(TEM)執行微觀顯微結構，並以半定量成份分析(EDS)和擇區繞射分析皮膜成份，搭配化學分析電子光譜儀(ESCA)鑑定皮膜組成；以極化曲線、電化學阻抗頻譜(EIS)與鹽霧試驗分析皮膜抗蝕能力；另以百格試驗評估化成皮膜附著性及以Loresta歐姆計測量電阻率。 實驗項目共分為兩個部分，第一部分探討錳酸鹽化成皮膜之製程參數。研究結果顯示，過錳酸鉀溶液中添加磷酸鹽及硝酸錳的化成處理具有良好的抗蝕性與附著性，相較於傳統磷酸鹽/錳酸鹽化成皮膜，可大量抑制裂紋生成。主要藉由Guyard reation的作用，在鎂合金表面生成緻密的鈍化膜，其厚度約200~400nm，此化成皮膜由外至內為緻密層和多孔層，以Mg、O、Mn為主要組成。 第二部分則針對AZ31B鎂合金經最佳參數鈍化後皮膜之腐蝕行為分析，實驗中以EIS進行長時間(48 h)量測及搭配微結構分析(SEM、TEM)，進一步提出鈍化膜的腐蝕行為。此部分之研究，同時採用具有抗蝕表現之鉻酸鹽化成處理(Dow 1)作比較分析。由電化學性質量測及鹽霧試驗結果顯示，經最佳參數的錳酸鹽化成處理試樣，其耐蝕性已明顯獲得改善，但仍劣於鉻酸鹽化成皮膜，可能原因為鉻酸鹽化成皮膜具有自我癒合的能力。 本論文最後則針對錳酸鹽化成皮膜提出可能的成長機構。並探討雙相結構對化成皮膜成核的影響及化成溶液老化的問題。

With excellent specific strengh and low density, magnesium alloys are extensively used in light products, including electrical appliances, bicycles and automobiles. However, since magnesium is chemically reactive (standard reduction potential -2.37 V vs. SHE), surface conversion coating treatments are therefore indispensable for improving the corrosion resistance of magnesium alloys. Hexavalent chromium conversion treatment was widely used in conventional conversion coating treatment, but has been limited by RoHs due to its high toxicity. Thus, the development of alternatives to hexavalent chromium conversion coating treatment is now an urgent necessity. In this study, AZ91D and AZ31B magnesium alloys were used as experimental materials. Meanwhile, permanganate coatings, which are widely used for surface treatment in industries, are adapted to have fundamental discussions. The surface morphology of the coating was investigated by scanning electron microscopy (SEM). The microstructure and thickness of the coating were characterized by cross-sectional transmission electron microscopy (TEM). The compositions of the coating were investigated by energy dispersive spectrometry (EDS) and x-ray photoelectron spectrometry (XPS). Moreover, the corrosion resistance of the coating was measured by polarization test, electrochemical impedance spectroscopy (EIS) and salt spray test (SST). The adhesion of the coating was measured by the tape adhesion test according to ASTM D3359-97 standard. Finally, the resistivity of the coating was measured by Loresta Meter. The Experiments are divided into two parts. In the first section of the dissertation, an optimal preparation condition of permanganate conversion treatment is studied. The excellent properties of the permanganate conversion coating is obtained by adding phosphate (KH2PO4) and manganese nitrate (Mn(NO3)2) in the permanganate conversion solution. The Guyard reaction provides a new route to form a thin, nearly crack-free MnO2-rich conversion coating on magnesium alloys. The conversion coating exhibited a two-layered structure: a compact layer major overlay and a porous layer directly contacting with the substrates. Dense passive films were generated on the surface of the magnesium alloy with the interaction of MnO4-and Mn2+, which the thickness of about 200~400 nm and consist of the Mg, O, and Mn composition. Experiment results indicated that the coating formed on magnesium alloys may provide enhanced corrosion protection. The second section of the dissertation investigated the properties of the optimal permanganate conversion coating proposed in the first section. Thus, with increasing immersion time (48 h), the coating’s lifetime would be definitely responded. Based on the EIS measurement, structure and composition analysis of the conversion coated AZ31 was proposed for understanding the corrosion process of passive film. With the form of the impedance spectrum could be determined the mechanism of the corrosion process. An acid chromate bath, Dow 1, was also used for comparison. The results of the electrochemical measurements and the salt spray tests demonstrate that the corrosion resistance of the AZ31 alloy has been markedly improved by the permanganate conversion treatment. However, the optimal permanganate conversion coating still has corrosion resistance inferior to the chromate (Dow 1) conversion coating. This is likely due to the unique self-healing capability of the chromate conversion coating. The formation mechanism of permanganate conversion coating was discussed in detail, which emphasis on the evolution of the coating. Moreover, we discussed how the Mg17Al12 (β) phase affects the microstructure of the coating and the aging time of conversion solution.