Fe50Mn30Co10Cr10 中熵合金於酸性環境中電化學行為與鈍化膜特徵之研究

Abstract

高熵合金設計基於「雞尾酒效應」，將每種元素能提供的特性同時兼併合於同一合金。雖然過去研究強調高熵合金傾向形成單相，但僅限於理想狀況才能擁有最 高混合熵，進而形成單相合金。Fe50Mn30Co10Cr10 中熵合金為近年提出之新穎熵合 金，其仰賴「相變誘導塑性 (Transformation induced plasticity, TRIP)」效應而發揮 高強度伴隨高延展性之塑性變形行為，優於傳統的 CoCrFeMnNi 等莫耳高熵合金 之拉伸性質。然而 Fe50Mn30Co10Cr10 擁有優異的機械性能，若期待將其應用於管線或工件中，抗蝕能力評估成了重要環節。本次研究以 0.1M 稀硫酸模擬酸性環境， 在如此嚴苛環境下量測 Fe50Mn30Co10Cr10 及其鈍化膜之電化學行為，並與 CoCrFeMnNi 進行比較，以評估該新穎材料之抗蝕性，並針對鈍化膜分析成分，探 究成分與抗蝕性之關係。 極化曲線顯示在此環境條件下，雖然 Fe50Mn30Co10Cr10 的腐蝕電流密度高於 CoCrFeMnNi，兩者的鈍化電流密度卻無明顯差異，而透過極化過後的表面得知前者整體而言呈現均勻腐蝕，但可看出沿著麻田散體呈現嚴重的溶解效應;而後者呈現粒間腐蝕，又因有少量的介在物嵌在試片中，進而介在物周圍發生伽凡尼腐蝕效應。另外，兩合金在高電位的狀況下，表面有二氧化錳薄膜沈澱導致電流密度再次 下降，發生二次鈍化行為，透過 X 射線光電子能譜分析(XPS)驗證二氧化錳薄膜之 存在。透過恆定電位生長之鈍化膜後，進行該膜之綜合性評估。高解析穿透式電子 顯微鏡(HRTEM)和歐傑電子能譜之縱深剖面分析顯示 Fe50Mn30Co10Cr10 之鈍化膜 略厚於 CoCrFeMnNi，但 CoCrFeMnNi 之鈍化膜有著較優異的穩定性及抗蝕性，從點缺陷模型分析，兩者鈍化膜擁有高密度之點缺陷，但施體參雜濃度較高的 Fe50Mn30Co10Cr10 應證其鈍化膜較不穩定，以上可以歸因於鈍化膜有較高的氧化鉻含量和較低的錳含量，伴隨較佳的抗蝕性。因此本研究認為，在第一次鈍化(低電位)仰賴鉻產生氧化物或氫氧化物造成鈍化行為，然而當電位上升，第二次鈍化行 為必須透過錳形成氧化物或氫氧化物致使電流密度下降，因此錳添加雖不利第一 次鈍化膜的穩定，卻可在高電位發揮極佳的保護作用。

The design of high-entropy alloy is based on the “cocktail effect”, which can yield the alloy with superior properties for the combination of mechanical performance and anti-corrosion resistance because every element can provide their specific natures. Although past studies have emphasized that high-entropy alloys tended to form a single-phase solid solution, they are limited to the ideal case of the maximum mixing entropy. The Fe50Mn30Co10Cr10 medium entropy alloy (MEA) is a novel entropy alloy proposed in recent years, whose plastic deformation behavior revealed high-strength and high- ductility because of “transformation induced plasticity (TRIP)' effect. It was superior to equiatomic CoCrFeMnNi high entropy alloy (HEA). Regarding MEA applied to pipelines or workpieces, it is necessary to establish the corrosion resistance of MEA in acid solutions. In this study, the dilute sulfuric acid with a molarity of 0.1 molars was used to simulate the acidic environment, and then the electrochemical behavior of MEA and its passive film were investigated to evaluate the corrosion resistance of the material, compared with HEA. Finally, the composition of the passive film was characterized and relationship between the composition and corrosion resistance was explored. The polarization curve showed that MEA had relatively higher corrosion density and lower corrosion potential than HEA, however, they displayed the similar passive current densities. After polarization, the surface of both alloys exhibited intergranular corrosion attacking, especially MEA occurred intense corrosion obviously distinguishable along the ε-martensite plates. Besides, the Galvanic corrosion effect occurred around the inclusions embedded in HEA. On the other hand, under the condition of high potential, the manganese dioxide film precipitated on the surface of both alloys led to the current density decreasing again, which is known as “secondary passivation behavior”. The existence of the MnO2 film was verified by XPS. After passivation two hours at passive potential, a comprehensive evaluation of the anodic passive films was performed. Both HRTEM and the depth profile of the AES revealed the passive films on MEA is slightly thicker than that on HEA, but the passive films on HEA has higher both stability and corrosion resistance from the result of potential decay, and EIS. High point defects densities of MEA and HEA were carried out through the Mott–Schottky analysis, and MEA with higher doping concentration demonstrated the unstable passive film on the surface. These phenomena were the grounds that passive films had higher chromium oxide content and lower manganese content. Therefore, this study suggests that the first passivation (at low potential) depended on chromium oxides or hydroxides causing passivation behavior. When the potential rose, the second passivation behavior pertained to the generation of manganese oxides or hydroxides to cause current density reducing. In short, addition of manganese is unfavorable to the stability of the first passive film, but it is dominant species of secondary passivation behavior at high working potential.