Localized Corrosion Behavior of a Ni-Based High-Entropy Alloy

Abstract

Because of the costs of corrosion and implications of failure, the development of corrosion-resistant alloys (CRAs) with improved properties is of great interest. Recently, metallic alloys of a new type, known as high-entropy alloys (HEAs), have been developed and studied. HEAs contain five or more principal elements with near-equimolar elemental fractions. It is possible that the combination of many elements at high concentration inherent in HEAs can offer enhanced levels of corrosion performance. Furthermore, HEAs provide numerous degrees of freedom in the alloy design process, so the whole approach for the design of CRAs must be reconsidered. This is the focus of a new large multi-team project. The approaches embodied in the integrated computational materials engineering (ICME) methodology are being used for the first time to address the design of CRAs. This work, which represents the first iteration at the design process, employed a simplified design approach that combined historical knowledge of alloying effects with CALPHAD thermodynamic predictions. This paper will discuss the localized corrosion behavior of a particular Ni-based HEA. A Ni-based HEA button was successfully prepared using arc-melting. The composition was Ni38Cr21Fe20Mo6W2Ru13, which was predicted by the ThermoCalc program to be single phase FCC. The as-cast sample was indeed a single FCC phase, but it exhibited a cored dendritic structure with dendrites enriched in Ru and inter-dendritic regions enriched in Mo and Cr. Homogenization of the sample was accomplished through a subsequent heat treatment at 1250oC for 120 h. Potentiodynamic polarization of the HEA samples in 3.5 wt.% NaCl at 30oC showed that both as-cast and homogenized samples exhibited a high corrosion resistance with spontaneous passivity and transpassive breakdown at high potentials, around 1.1 VSCE. Potentiostatic polarization at 0.7 VSCE in the same electrolyte was applied with a temperature scan to test the critical pitting temperature (CPT). The CPT of the HEA sample is >85oC, which is much higher than that of conventional CRAs. Additionally, no obvious metastable pitting was observed during the whole test, indicating that a very protective passive film was formed on the sample surface. Interestingly, the as-cast sample exhibited pitting in the transpassive region (above 1.1 VSCE) at 30oC. This “transpassive-induced pitting” evolved as follows. Transpassive dissolution started from inter-dendritic regions and then spread to dendrites. Cracking of surface of inter-dendritic regions and then delamination of the “transpassive film” from inter-dendritic regions left fresh surface exposed. Finally, pitting ensued in the cracked inter-dendritic regions. In contrast, no pitting was observed on the homogenized sample under the same condition. The corrosion behavior in more aggressive environments will also be presented. Acknowledgments: This work was supported as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584.