硼含量對AlCoCrFe2NiMo0.5Bx高熵合金腐蝕性質之影響

The Effect of Boron Content on the Electrochemical Corrosion Behavior of AlCoCrFe2NiMo0.5Bx High-entropy Alloys

高熵合金首先打破傳統合金以單一元素為主要成份的觀念，藉由5種以上的主要元素相混合，各種元素之元素含量界於5 ~ 35原子百分數（%）之間。過往研究指出，AlCoCrFe2NiMo0.5Bx高熵合金具有極佳的硬度與抗高溫潛變能力，且若是在合金中添加不同莫耳比的硼元素時，可以有效提升其耐磨耗能力。為了進一步瞭解AlCoCrFe2NiMo0.5Bx合金的腐蝕行為，將其與316不銹鋼相互比較，探討在合金中硼成分的多寡對其腐蝕與電化學性質之影響。由動電位極化曲線得知：AlCoCrFe2NiMo0.5Bx高熵合金在0.5 M的硫酸溶液中隨著硼成分的增加其腐蝕電位（Ecorr）降低、腐蝕電流密度（icorr）上升且鈍態區域也會明顯變窄。即使AlCoCrFe2NiMo0.5Bx合金中的硼含量到達1莫耳，對均勻腐蝕的抵抗能力依舊勝於316不銹鋼。在1 M的氯化鈉水溶液中，當添加微量硼元素時（0.1莫耳），對高熵合金的抗蝕有些許助益（Ecorr升高、icorr降低），但是一旦添加硼含量超過0.5莫耳時，則其抗蝕能力會快速的衰減。由EIS分析顯示：當AlCoCrFe2NiMo0.5Bx合金中硼含量增加超過0.1莫耳，其Rct （charge transfer resistance）會逐漸減少，且當硼含量超過0.5莫耳時，該合金之Nyquist圖在低頻時會出現明顯的電感迴圈。

High entropy alloys are a newly developed family of multi-component alloys composed of several major alloying elements of 5 at.% to 35 at.%. Literature data indicated that the AlCoCrFe2NiMo0.5Bx alloy would bear very good hardness and creep resistance, as well as wear resistance, when the boron content was increased in the alloy. This paper discusses the effect of boron on the electrochemical properties of the boron-containing AlCoCrFe2NiMo0.5Bx alloys. In addition to analyzing the corrosion behavior of this high entropy alloy, we also studied that of commercially available Type 316 stainless steels for comparison purpose. Polarization analyses on the AlCoCrFe2NiMo0.5Bx alloy in a 0.5 M sulfuric acid solution clearly indicated that the corrosion potential decreased, the corrosion current density increased and the passive region decreased as the amount of boron increased. However, the general corrosion resistance of this alloy was still better than that of Type 316 stainless steels even if the boron content in this alloy reached 1 mole. In a 1 M NaCl solution, the corrosion potential moved toward the noble direction and the corrosion current decreased when the content of boron was reduced to 0.1 mole. When the content of boron was increased to more than 0.5 mole, the pitting resistance would be impaired. On the other hand, the electrochemical impedance spectra indicated that the Rct (charge transfer resistance) decreased when the boron content in the AlCoCrFe2NiMo0.5Bx alloy increased. In the meantime, when the boron content was more than 0.5 mole, the Nyquist plot showed a clear inductance loop.