電性參數對AZ31B鎂合金微弧氧化膜顯微結構及抗蝕性質之研究

Abstract

本研究旨在探討雙脈衝模式下微弧氧化 (Micro-arc oxidation, MAO) 膜層的生長機制，並據此調整製程參數以提升能源使用效率與膜層性能。實驗採用矽酸鹽系統之電解液，雙脈衝定電流模式，並以商用 AZ31B 鎂合金為基材。研究分為兩個階段：第一階段探討陽極與陰極停滯時間對膜層的影響；第二階段則針對調整陰極電流密度進行膜層生長分析。 在第一階段中，於固定陽極與陰極輸入條件下調整停滯時間，製程時間設定為 10 分鐘。結果顯示，具有陰極停滯時間的試樣橫截面缺陷較多，其交流阻抗值約為 105 Ω·cm2，明顯低於僅具陽極停滯時間試樣的 106 Ω·cm2，顯示陰極停滯時間對膜層品質具有負面影響。基於此觀察，第二階段將原有的四個時間區段波形簡化為三個時間區段，移除陰極停滯時間；同時，考量第一階段中推測氫氣相關效應可能與近界面之膜層中缺陷有關，因此近一步探討陰極對膜層特性的影響。 在移除陰極停滯時間的新波形下，第二階段實驗分別對陰極電荷量與陰極電流密度進行討論，並在陰極電荷量實驗中發現，其輸入量與陽極相同時，MAO膜層有較好的抗蝕能力，因此將陰極與陽極電荷量固定等量的條件下，討論陰極電流密度所帶來的影響。結果顯示，當陽極電流密度大於或等於陰極電流密度時，膜層表現出相似的表面形貌與良好的抗蝕性；但當陰極電流密度超過陽極時，膜層孔洞減少卻在基材與膜層界面形成非典型結構，導致阻抗值下降至少一個數量級。 綜合兩階段實驗結果可知，MAO 製程中的電性參數對膜層的生長機制與耐蝕性能具有關鍵影響。本研究提出一個新的波形設定準則，用以兼顧效能與性能。陰極停滯時間宜自波形設計中移除，以避免提供非成膜物質擴散時間而引發缺陷累積與抗蝕能力下降之情況；陰極電荷量與陰極電流密度皆不宜超過陽極，以避免界面非典型膜層形成，維持膜層之抗蝕能力。此準則可讓MAO製程在不增加輸入能量的條件下，即可實現更穩定的屏蔽腐蝕因子效果，為鎂合金雙脈衝MAO製程應用與發展提供新的方向。

This study investigates the underlying mechanisms of micro-arc oxidation (MAO) coatings produced under pulsed bipolar, constant-current mode and uses those insights to tune electrical parameters for better energy efficiency and coating performance. Experiments were carried out in a silicate-based electrolyte on commercial AZ31B magnesium alloy. This study proceeded in two stages. In the first stage, the effects of anodic and cathodic pause times—defined as the periods following anodic and cathodic pulses, respectively—were systematically examined. Due to the complex interactions associated with bipolar pulse power in MAO, clarifying these mechanisms is essential for further process optimization. The results showed that cathodic pause time had a detrimental effect on coating growth and properties. Based on this finding, a modified three-section waveform, excluding the cathodic pause period, was proposed to improve the MAO process. This adjustment was followed by a focused examination of cathodic input, as hydrogen-related processes during the cathodic input half-cycle were suspected to contribute to near-interface defect formation. Using this three-section waveform, the second stage first evaluated the effect of cathodic charge quantities and observed that corrosion resistance was maximized when the cathodic and anodic charge quantities were matched. Accordingly, subsequent experiments fixed the anodic and cathodic charge quantities to be equal in order to isolate the influence of cathodic current density. Under these conditions, when the anodic current density was greater than or equal to the cathodic current density, the coatings exhibited uniform morphology and superior corrosion resistance. In contrast, when the cathodic current density exceeded the anodic current density, smaller surface pores were observed, but atypical interfacial structures formed at the coating/substrate interface, resulting in a reduction in impedance by at least one order of magnitude. These findings indicate that electrical parameters critically govern the growth mechanisms and corrosion resistance of MAO coatings on magnesium alloys. Excluding cathodic pause time helps prevent defect accumulation, while careful control of cathodic current density is required to avoid structural degradation. The proposed bipolar pulse strategy effectively balances energy efficiency and coating performance, offering new opportunities for the surface modification of magnesium alloys.