| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系所 === 107 === This work investigates the corrosion behavior of annealed equimolar AlCoCrFeNi tri-phase high entropy alloy (HEA) in 0.5 M H2SO4 and 3.5 wt% NaCl aerated aqueous solution. Through homogenization annealing treatment at 1100℃, the micro-structure composed of nano-sized BCC particle and ordered B2 matrix for the as-cast has transformed into three-dimensioned plate wall BCC embedded in B2 matrix and interconnected grain boundary FCC. Compared with the as-cast, the corrosion resistance is improved, the passivate current density decreasing from 5.54 to 2.12 μA/cm2 in 0.5 M H2SO4 and the corrosion current density decreasing from 7.26 to 0.55 μA/cm2 in 3.5 wt% NaCl. After polarization tests in 0.5 M H2SO4 and 3.5 wt% NaCl aeration solution at room temperature, the B2 matrix is preferentially corroded away while BCC and FCC phases are remained. Furthermore, in 0.5 M H2SO4 solution, the stable passive current density is related to lower constant of solubility product ksp values of Co(OH)3, Fe(OH)3, and Cr(OH)3, while the annealing effect on enhancing corrosion resistance is ascribed to the more homogenized composition and then the more uniformity of the passive hydroxide film. In 3.5 wt% NaCl solution, the corrosion resistance enhancement is attributed to the annealing where the formation of 3-D plate walls composed of BCC in the matrix and FCC on the grain boundary complicate the corrosion path and then retards the penetration. The results of the study show that annealing does effectively improve the corrosion resistance.
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