WE43-T5鎂合金二次相對pH值驅動化成皮膜的影響

Abstract

鎂合金有低密度及高比強度等特性，被廣泛應用在各個領域，包括生醫、航太、汽車、電子等領域。其中添加稀土元素的鎂合金因其能提高強度、高溫抗潛變能力、耐燃性，以及抗蝕性，因此近年來有許多研究。而鎂合金具高活性，故為了進一步提高應用時的抗蝕性，對其做化成處理。 WE43中有兩種兩種微米級二次相，分別是富Y和富Zr二次相，其中富Zr二次相會造成WE43的伽凡尼腐蝕，故研究此兩種二次相對化成膜成膜的影響為一重要課題。 本研究使用過錳酸鹽化成進行第一道化成處理，由電子顯微鏡分析得知膜層的主要組成為 MnO2/Mg(OH)2/MgO，並觀察到在晶界處未成膜，以及當浸泡時間較長，膜層上會有脫水裂紋，這些缺陷造成抗蝕性提升有限。為填補這些缺陷，在第一道膜層上，再進行第二道六氟鋯酸鹽化成處理，由電子顯微鏡觀察到經第二道 化成後，晶界未成膜處被 ZrO2 所填補。電化學量測結果顯示，相較於只有一道次處理的，兩道次化成處理的腐蝕電流小了一個數量級，且大幅提升了交流阻抗測試中等效電路的總阻抗值。而鹽霧測試結果符合電化學量測，在24h測試後，經兩道次化成處理的腐蝕面積僅有2%。經 KMnO4/K2ZrF6 兩步驟化成處理的式樣呈金黃色，同時達到美觀及抗蝕性提高的效果。 根據微結構分析，化成處理過的式樣，在鹽霧後的巨觀腐蝕白點源自於富Zr顆粒，因 Cl-會攻擊富 Zr顆粒周圍的缺陷處，引起Zr顆粒與鎂基材間的伽凡尼腐蝕。而富Y顆粒對成膜並未有顯著的影響，在經過錳酸鹽化成處理後會被原位氧化為Y2O3。

Magnesium and its alloys, characterized by low density and high specific strength, are widely utilized in various fields, including biomedical, aerospace, automotive, and electronics. In recent years, there has been significant research interest in magnesium alloys containing rare earth elements due to their ability to enhance strength, high-temperature creep resistance, flammability, and corrosion resistance. Magnesium alloys are highly reactive, thus requiring surface treatments such as conversion coatings to further improve their corrosion resistance for practical applications. In the case of WE43, two types of micron-scale secondary phases exist Y-rich and Zr-rich phases. Among them, the Zr-rich phase is known to cause galvanic corrosion in WE43. Therefore, studying the effects of these two secondary phases on the formation of conversion coatings is an important research topic. This study employed permanganate-based conversion coating as the first step treatment. Electron microscopy analysis revealed that the main composition of the coating layer is MnO2/Mg(OH)2/MgO. It was observed that no conversion film forms at grain boundaries, and prolonged immersion led to dehydration cracks on the coating. These defects resulted in a limited enhancement of corrosion resistance. To address these defects, the second K2ZrF6 conversion treatment was applied on top of the first coating. Electron microscopy observations revealed that after the second treatment, the areas where the film doesn’t form at grain boundaries are sealed with ZrO2. Electrochemical measurements showed that compared to the single-step treatment, the corrosion current decreased by an order of magnitude with the two-step treatment. Additionally, EIS results show a significant increase in the total impedance of the film. The salt spray test results were consistent with the electrochemical measurements. After a 24-hour test, the corrosion area of the samples treated with the two-step conversion process was only 2 %. Samples treated with the KMnO4/K2ZrF6 conversion process exhibited a golden color, achieving both aesthetic appeal and improved corrosion resistance. According to microstructural analysis, macroscopic corrosion spots observed after salt spray testing originated from Zr-rich particles. Cl attacked the defects around these particles, leading to galvanic corrosion between Zr particles and the Mg matrix. Y-rich particles do not significantly affect the formation of the coating and are oxidized in situ to Y2O3 after permanganate-based conversion treatment.