Abstract
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.