高亂度合金的金相與電化學極化之研究——在室溫下與其他鋼材作比較

Metallography and Electrochemical Polarizations of High Entropy Alloys － a Comparison with Steels at Ambient Temperature

高亂度合金是一種由數種常用的合金元素如銅、鎳、鋁、鈷、鉻、鐵、矽、以及鈦等所組成的新型多元合金。由X光繞射的結果顯示高亂度合金的基地相幾乎為一完全無序的非晶質相；而這種擁有高度原子無序的類玻璃合金其機械性質如高強度和高硬度可與玻璃相比擬，不過此合金的腐蝕行為對我們來說仍是陌生的。為了要進一步去了解高亂度合金的電化學行為，故有必要拿它與其他鐵系合金如304型不銹鋼和普通碳鋼作比較，並利用電化學極化的技術來研究高亂度合金在室溫下水溶液中的陽極極化行為。而在此實驗中主要影響高亂度合金陽極極化曲線的因素為電解質的種類如氯化鈉與硫酸、溶液的pH值和溫度。由較高的腐蝕電位和較低的腐蝕電流密度之實驗結果很清楚地顯示：高亂度合金在室溫下不論電解質的濃度是高（1M）或低（0. 1M），其對均勻腐蝕發生的初期抵抗力均優於304不鋼及普通鋼。另一方面，由於較低的孔蝕電位與較窄的鈍態區域可知高亂度合金在較高氯鹽的環境下對孔蝕的抗性不如304不銹鋼。在含有不同氯離子濃度的濃硫酸中，高亂度合金在接近海水的氯離子濃度下有一個對均勻腐蝕最差抵抗力的臨界現象產生。而由循環極化法中因為沒有發現遲滯環，故可證實高亂度合金與304不銹鋼一樣，在1N的硫酸中均不會發生孔蝕。

High entropy alloy (HEA) is a newly multi-component alloy, which is composed by several popular composition elements such as copper, nickel, aluminum, cobalt, chromium, iron, silicon, and titanium etc. The matrix phase of HEA is almost a fully disorder non-crystalline phase proven by the X-ray diffraction analysis. This kind of glassy alloy has a high degree of atomic disorders that its mechanical properties is comparable to that of glass and is therefore of high strength but brittle. And its corrosion behavior is relatively new to us. In order to further realize the electrochemical behaviors of HEA, it is necessary to compare it with other ferrous alloys such as type 304 stainless steels and mild steels. The anodic polarization curves of HEA in aqueous environments at room temperature were studied using electrochemical polarization techniques. The major factors affecting the anodic polarization curves of HEA are the type of electrolyte, e.g. NaCl and H2SO4, solution pHs and the bath temperature. The results clearly indicated that the corrosion resistance to general corrosion for HEA at ambient temperature (~25℃) is superior to that of the 304 SS and the mild steel whether the concentration of electrolyte is concentrated (1M) or low (0.1M), because of the nobler corrosion potential and the lower corrosion current density. On the other hand, the resistance to pitting corrosion of HEA in high Cl environment is inferior to 304 SS in view of the lower pitting potential and the narrower extend of passive region. In the concentrated sulfuric acid containing different concentration of chloride ions, there is a critical phenomenon that HEA has less resistance to general corrosion in the chloride ion concentration near seawater. By cyclic polarization, it is confirmed that both HEA and 304SS are resistant to pitting in 1N H2SO4 due to no hysteresis were observed.