Effect of Nb-content on the corrosion resistance of Co-free high entropy alloys in chloride environment

This study investigated the effect of Nb-content on the corrosion behavior of the Al7Cr20Fe35-xNi35Mo3Nbx\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Al}}_{7}{{\text{Cr}}}_{20}{{\text{Fe}}}_{\text{35} - {{x}}}{\text{Ni}}_{35}{{\text{Mo}}}_{3}{{\text{Nb}}}_{{x}}$$\end{document} (x = 0, 1 and 2) high-entropy alloys in a Cl- environment. The results indicated that only the face centered cubic phase existed in the Nb0 and Nb1 alloys, while the emergence of Laves phase was observed when Nb-content reached 2 at.%. The Nb0 and Nb1 alloys exhibited pitting corrosion, while the NiAl-rich phase experienced localized preferential corrosion in the Nb2 alloy. The electrochemical results revealed that the corrosion current density of the Nbx\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Nb}}_{{x}}$$\end{document} (x = 0, 1 and 2) alloys fell within the range of 10-8-10-7 A<middle dot>cm-2, with a pitting corrosion potential exceeding 700 mVSCE. Notably, the Nb1 alloy exhibited the most impressive corrosion resistance, as its corrosion current density (7 x 10-8 A<middle dot>cm-2) was merely half that of 316LN stainless steel. This is attributed to its rapid passivation process, resulting in the development of a protective film characterized by an increased Cr2O3/Al2O3+Cr(OH)3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Cr}}_{2}{{\text{O}}}_{3}{\text{/Al}}_{2}{{\text{O}}}_{3}{\text{+Cr(OH)}}_{3}$$\end{document} ratio. This study highlighted that the addition of Nb contributed to grain refinement, decreased corrosion current density, and elevated pitting potential, favoring passivation and improving corrosion resistance. However, excessive Nb-content resulted in the emergence of the Laves phase, inducing galvanic corrosion and diminishing resistance to corrosion.