高熵合金Co1.5CrFeNi1.5Ti0.5Mox在氯鹽環境中之電化學孔蝕研究

Electrochemical Studies on Pitting Corrosion of the High Entropy Alloy Co1.5CrFeNi1.5Ti0.5Mox in Chloride Environments

周永隆(Yung-Lung Chou)

指導教授 ： 葉均蔚 ； 施漢章

國立清華大學/工學院/材料科學工程學系/博士(2011年)

摘要

高熵合金是一種由五個以上主元素相混合所組成的多元合金。這類合金具有高度原子無序排列的特性，造成許多獨特的磁性、機械及電化學性質。先前的研究指出Co1.5CrFeNi1.5Ti0.5 合金為單一 FCC固溶體結構、高硬度、耐高溫氧化及大氣腐蝕能力。另一方面，鉬 (Mo) 具有固溶強化、抗氯鹽孔蝕的作用，因此添加Mo於高熵合金內。本研究利用動態極化試驗於室溫及稍高的水溶液，評估不同Mo成分莫耳比的高熵合金Co1.5CrFeNi1.5Ti0.5Mox (x = 0, 0.1, 0.2, 0.5, 0.8) 之腐蝕行為。
首先評估Mo的添加對Co1.5CrFeNi1.5Ti0.5Mox合金分別在酸性、鹼性和中性環境下的腐蝕阻抗影響(25 ℃)。由極化曲線清楚顯示在氫氧化鈉及硫酸溶液中，隨著Mo含量的增加，高熵合金的抗均勻腐蝕能力隨之下降；另一方面，由循環極化掃瞄及SEM表面腐蝕型態結果得知在Co1.5CrFeNi1.5Ti0.5 合金中添加Mo有助於提升孔蝕阻抗。
接著探討Mo含量對Co1.5CrFeNi1.5Ti0.5Mox合金在氯鹽水溶液中(由25 ℃升至80 ℃)的孔蝕行為。結果顯示Co1.5CrFeNi1.5Ti0.5 合金分別在0.001、0.01、0.1 與1 M 氯化鈉之臨界孔蝕溫度為80 ℃、 50 ℃、30 ℃及低於 25 ℃。合金的孔蝕電位隨著氯鹽濃度上升而下降、臨界孔蝕溫度隨著Mo含量的增加而上升。
最後評估無機/有機腐蝕抑制劑添加於氯鹽中對Co1.5CrFeNi1.5Ti0.5Mo0.1 合金的孔蝕電位及臨界孔蝕溫度影響。結果顯示該合金分別在0.1、0.5和 1 M 氯化鈉之臨界孔蝕溫度為70 ℃、60 ℃和60 ℃; 孔蝕電位與氯鹽濃度成線性關係。當硫酸鹽與氯鹽比超過0.5，硫酸鹽對合金孔蝕電位及臨界孔蝕溫度有正面效應。孔蝕抑制能力依序為硝酸鹽≒苯甲酸鹽＞醋酸鹽≒過氯酸鹽＞草酸鹽＞鉬酸鹽。

關鍵字

高熵合金 ； 氯鹽 ； 電化學 ； 孔蝕

並列摘要

A high-entropy alloy (HEA) is a multi-component alloy containing n major alloying elements (n≧5), which has a high degree of atomic disorder that leads to various unique magnetic, mechanical and electrochemical properties. The Co1.5CrFeNi1.5Ti0.5 HEA, evaluated previously, possessed a single FCC structure, high hardness, excellent resistance to oxidation and atmospheric corrosion. Molybdenum is an element known to enhance the resistance to pitting corrosion in chloride-containing solutions. Another important contribution of molybdenum to a given alloy is its solid-solution strengthening effect, due to its large atomic volume in the matrix to pin dislocation. Therefore, the purpose of this thesis is to investigate the corrosion behavior of the Co1.5CrFeNi1.5Ti0.5Mox (x = 0, 0.1, 0.2, 0.5, 0.8) HEA in aqueous environments at room and elevated temperatures using the potentiodynamic polarization technique.
First, the corrosion resistance of Co1.5CrFeNi1.5Ti0.5Mox alloys was conducted in three different environmental conditions；1 M NaOH, 0.5 M H2SO4 and 1 M NaCl, which will be referred to as base, acid and salt solution, respectively henceforth at 25 ℃ under atmospheric pressure. The potentiodynamic polarization curves of the Co1.5CrFeNi1.5Ti0.5Mox alloys, obtained in aqueous solutions of H2SO4 and NaOH, clearly revealed that the corrosion resistance of the Mo-free alloy was superior to that of the Mo-containing alloys. On the other hand, the lack of hysteresis in cyclic polarization tests and SEM micrographs confirmed that the Mo-containing alloys are not susceptible to pitting corrosion in NaCl solution.
Second, we report the effect of molybdenum from 25 to 80 ℃ on the pitting corrosion of the Co1.5CrFeNi1.5Ti0.5Mox high-entropy alloys in chloride solutions. The values of critical pitting temperature (CPT) for the Mo-free alloy in 0.001, 0.01, 0.1 and 1 M NaCl are 80, 50, 30 and below 25 ℃, respectively. The values of pitting potential (Epit) decrease with increasing chloride concentrations and the values of CPT increase with the increase of Mo content.
Finally, we investigate the effect of inorganic/organic inhibitors on the Epit and CPT of the high-entropy alloy Co1.5CrFeNi1.5Ti0.5Mo0.1 in chloride solutions. The results indicate that the values of CPT for the alloy in 0.1, 0.5, and 1 M NaCl are 70, 60 and 60 ℃, respectively; the values of Epit are linear with the logarithm of chloride concentrations at 70 and 80 ℃; and the addition of SO42- ions to chloride solutions has a positive effect on both Epit and CPT as the ratio of SO42-/Cl- is higher than 0.5. Six anions all retard the occurrence of pitting corrosion; the inhibitive effect decreases in the order: nitrates≒benzonates＞acetates≒perchlorates＞citrates＞molybdates.

並列關鍵字

high entropy alloy ； chloride ； electrochemical ； pitting corrosion

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