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.