LZ91鎂鋰合金之腐蝕行為與鈦酸鹽化成處理

Abstract

鎂合金為一輕量化金屬，具有高比強度、良好的電磁遮蔽效應與生物相容性等優點，與鋰形成之鎂鋰合金更能大幅改善延展性與韌性，使其成為未來節能減碳趨勢下綠能產業中的一大發展重點。然而鎂鋰合金之活性大，腐蝕速率高，使得工業應用上難以推廣，故發展一環保、有效的防蝕處理實為鎂鋰合金研究中最重要的議題。本研究秉持此一原則，探討LZ91鎂鋰合金之腐蝕行為，並施以鈦酸鹽化成處理以期有效降低LZ91之腐蝕速率，研究內容主要包含三個部分：其一，系統性的探討LZ91之腐蝕行為，釐清LZ91本身的腐蝕機制和防治腐蝕的重點；其二，探討鈦酸鹽化成的參數，找出可於LZ91表面形成最具防蝕能力之化成膜的操作條件；其三，比較LZ91與鈦酸鹽化成後試樣之防蝕能力和腐蝕行為，以了解鈦酸鹽化成膜對防蝕的幫助，並獲取足夠的資訊以進一步改善鈦酸鹽化成系統。 研究發現，LZ91合金中腐蝕傾向優先發生於雜質（如鐵、錳、鈣等）聚集處，而與雙相效應較無關聯性，且在施以適當的鈦酸鹽化成處理後可於底材表面生成一附著性良好的雙層化成膜，外層主要由二氧化鈦／二氧化矽組成，內層則為氟化鎂／氫氧化鎂。此化成膜可有效降低動電位極化曲線中的腐蝕電流密度與提升電化學交流阻抗中的總阻抗值，除此之外，透過電化學交流阻抗之等效電路模擬與浸泡腐蝕即時攝影觀察可發現，鈦酸鹽化成膜雖並未改變LZ91的腐蝕機制，但其存在延緩了腐蝕的發生與進行。

Magnesium alloys possess high strength-to-weight ratio, good electromagnetic shielding properties, and biocompatibility. Alloying with lithium to form magnesium-lithium alloys can further reduce weight and enhance ductility along with toughness; however, magnesium-lithium alloys are susceptible to corrosion and can lead to disastrous failure, so developing an effective and environmentally friendly anti-corrosion treatment is essential and imperative. This study investigates the corrosion behavior of LZ91 alloy with and without titanate conversion coating treatment in order to clarify the mechanics behind the corrosion and suggest a valid choice to passivate the alloy. The results indicates that LZ91 alloy suffers little from galvanic corrosion; nevertheless, the corrosion of LZ91 actually begins at impurities, e. g. iron, manganese, and calcium rich phases. By applying proper titanate conversion treatment, a two layer conversion coating can be formed on LZ91. The existence of the coating results in lower corrosion current density and higher total impedance in dynamic polarization test and electrochemical impedance spectroscopy seperately. In addition, the in-situ photographing and electrochemical impedance spectroscopy both agree on that although the titanate conversion coating doesn’t drastically change the corrosion behavior of LZ91, it successfully delays the corrosion and lowers the corrosion rate.