In Situ Electrochemical-AFM Study of Local Dissolution Processes of Al x Co Cr Fe Ni High-Entropy Alloys

Abstract

The multiple compositionally-equivalent high-entropy alloys (HEAs) have shown remarkable potential in structural and energy-related applications. Their intrinsic random arrangement of elements results in homogeneous chemical distribution, which is expected to be beneficial for improving the corrosion resistance. However, with the increased Al content in the objective Al x CoCrFeNi (x = 0.3, 0.5, and 0.7, molar ratios) HEAs, the multi-phase microstructure with chemical segregations is inevitable. The microstructure changes from single FCC solid-solution to FCC + disordered/ordered BCC phases. The FCC and disordered BCC phases are (Cr, Co, Fe)-rich, while ordered BCC phase is (Al, Ni)-rich. This chemical segregation would lead to the localized corrosion, which evidently impact the corrosion resistance. To investigate the detailed localized corrosion mechanism, the in-situ electrochemical-AFM (EC-AFM) measurements are performed at consecutively applied anodizing potentials in a 3.5 wt.% NaCl solution in the present study. The topography changes on the surface at micro/sub-micro scale are monitored in real-time. The results show that in the single phase Al0.3CoCrFeNi alloy, at the passive region, the surface roughness increases slightly due to the formation of oxide film. While increasing the potential continuously, deposition of the corrosion products results in increased sample roughness. At the active potential region, severe local dissolution is observed as the onset of pitting. For the Al0.5CoCrFeNi alloy, the FCC matrix performs similar corrosion behavior as the single FCC phase of the Al0.3CoCrFeNi alloy. However, the active potential is easier to reach due to the preferential formation of pits at the FCC/BCC phases boundaries. For the Al0.7CoCrFeNi alloy, the critical breaking down potential of passive film decreases dramatically. No pitting-like corrosion is observed, but a large amount of selective dissolution occurred at the FCC and BCC phase boundaries at the active potential region. Local selective dissolution of the (Al, Ni)-rich ordered BCC phases is observed as well. The XPS results indicate that the oxide film with the higher Cr content is more protective, therefore, pitting and selective corrosion prefer to initiate at the (Al, Ni)-rich phases. Moreover, the trenches formed along the phase boundaries indicate that the galvanic cell formed between the FCC and BCC phases accelerate the alloy dissolution as well. Overall, in situ EC-AFM is shown to be a useful tool to explore details of the anodic oxidation/dissolution behavior of the Al x CoCrFeNi HEAs. The in-depth understanding of the local dissolution activity and corrosion behavior obtained in the present study is of importance for the future design of high performance HEAs. Figure 1. In situ EC-AFM images of (a-c) Al0.3CoCrFeNi, (d-f) Al0.5CoCrFeNi, and (g-i) Al0.7CoCrFeNi HEAs at increasing potentials. Figure 1