氟離子對AZ91D錳酸鹽化成皮膜結構與抗蝕性之影響

Abstract

鎂合金具有低密度、高質比強度、良好電磁遮蔽率、高散熱率以及生物可相容等多項優點，其中，鎂鋁合金因具有良好的機械性質與抗蝕性，而為廣泛應用之鎂合金，但鎂鋁合金之高腐蝕速率仍限制了其工程應用範圍，因此鎂鋁合金需作表面防蝕處理增進其抗蝕性。故本研究旨在以添加氟離子改良一錳酸鹽化成處理，使其能應用於AZ91D雙相鎂合金上提升抗蝕性，由於所欲改良之錳酸鹽化成溶液具有不穩定性，本研究之內容分為兩部分，其一為氟離子對化成溶液穩定性之影響，其二為氟離子對化成皮膜微結構與抗蝕性之影響，此外亦探討化成時間與氟離子濃度之效應。 研究結果指出，未添加氟離子之化成溶液由於不穩定性而使得溶液內之過錳酸根濃度隨時間快速下降，而添加氟離子則有效提升化成溶液穩定性；未添加氟離子之化成皮膜由於受到雙相伽凡尼效應之影響，化成皮膜形成不均，於β相上有大量二氧化錳沉積及伴隨出現之脫水裂紋，因此具有較差之抗蝕性；由於氟離子可於α相上形成較多之氟化鎂，鈍化α相減少雙相間之活性差距，抑制雙相伽凡尼效應，因此添加氟離子之化成皮膜形成均勻，且相當薄亦不具有脫水裂紋等缺陷，大幅提升皮膜抗蝕性；隨著化成時間的增加，化成皮膜厚度增長會使脫水裂紋增加，亦會使化成皮膜開始溶解而產生孔洞等缺陷，因而使抗蝕性劣化，60秒為最佳之化成時間；若添加至化成液之氟離子濃度太低，則氟離子不足以於α相上形成足夠之氟化鎂，因此失去抑制雙相效應之效果，又使得β相上有大量二氧化錳沉積及脫水裂紋，使得化成皮膜抗蝕性下降，相對的，若添加至化成液之氟離子濃度太高，雖同樣有抑制雙相效應之能力，但氟離子可與β相之鋁反應溶出形成孔洞，使得該處化成皮膜為薄弱區，因此亦使化成皮膜抗蝕性下降，0.02 M為最佳之氟離子濃度。動電位極化曲線之極化阻抗與電化學交流阻抗之總阻抗值兩者間具有一致性，且動電位極化曲線之腐蝕電流值與析氫試驗換算之電流值亦兩者接近，故本研究之抗蝕性分析方式間彼此互相映證，證明分析結果之正確性；此外，根據本研究電化學交流阻抗之等效電路模擬結果，發現可直接以高頻容抗圈大小評估化成皮膜抗蝕性。

Magnesium alloys have low density, high specific strength, good magnetic shielding, good heat dissipation, and biocompatibility properties. Among them, due to good mechanical properties and proper corrosion resistance, magnesium-aluminum alloys are widely used. However, the applications of magnesium-aluminum alloys are still limited by their high corrosion rate. Surface modifications are thus required to enhance their corrosion resistance. Therefore, this study aimed at improving a permanganate conversion solution by addition of fluoride ion to make it effectively enhance the corrosion resistance of AZ91D magnesium alloy. Due to the instability of the original permanganate conversion solution, this study investigated the effect of fluoride ion on the solution stability and on the microstructure and corrosion resistance of the permanganate conversion coatings. Moreover, the effect of conversion treatment time and concentration of fluoride ion were also investigated to find the optimal treatment condition. The results show that the permanganate concentration in the conversion solution without addition of fluoride rapidly decreases due to the instability, and the addition of fluoride ion reduces the decrease in permanganate concentration and effectively enhances the solution stability. Because of the dual-phase galvanic effect, the conversion coatings formed without fluoride are heterogeneous, and plenty of MnO2 forms on the β phase accompanied by dehydration cracks, which leads to poor corrosion resistance. In contrast, because more MgF2 forms on the α phase and therefore inhibits the dual-phase galvanic effect. As a result, the conversion coating formed with fluoride is homogenous, thin, and nearly crack-free, which significantly improves the corrosion resistance. With longer conversion treatment time, the thicker coatings induce more dehydration cracks, and the dissolution of the coatings leaves pore on the surface. Both degrade the corrosion resistance. Hence, the best treatment time is 60 s. With lower fluoride ion concentrations, MgF2 formation is inadequate and dual-phase galvanic effect is not inhibited, and once again, plenty of MnO2 forms on the β phase accompanied by dehydration cracks. With higher fluoride concentrations, although dual-phase galvanic effect is inhibited, the β phases are etched by fluoride ion leaving pores on the surface. Hence, the coatings formed with high and low fluoride ion concentrations are both defective and have inferior corrosion resistance, and the best fluoride concentration is 0.02 M. The polarization resistance deduced from potentiodynamic polarization (PD) is consistent with the total impedance estimated from electrochemical impedance spectroscopy (EIS). Moreover, the corrosion current measured from PD is close to the equivalent corrosion rate measured from hydrogen evolution test, which indicates the corrosion analyses are related to each other and prove the accuracy. In addition, based on the equivalent circuit used in the EIS analysis, the simulated total impedance at frequency approaching zero can be directly used to evaluate the corrosion resistance.