Corrosion and Passivity of a High Entropy Alloy in Sulfate Solutions at Various pH Levels

Abstract

As new alloys are developed, corrosion characterization studies are of key importance in understanding material degradation processes and the performance of the alloy under corrosive conditions. The mechanisms by which alloy composition, microstructure, and passive films combine to improve passivity-based corrosion resistance were identified and considered in the design and synthesis of a novel Ni-Fe-Cr-Mo-W-Ru high entropy alloy (HEA). HEAs are a class of materials which are composed of five or more elements in nearly equi-atomic concentrations, which may lead to unique properties in regards to corrosion resistance and susceptibility1. Necessary components to the design of this HEA were the consideration of both passive film stability and breakdown, and pit stabilization in developing the alloy’s corrosion resistance2. Establishing regions of stability of the protective passive surface film in sulfate solution as a function of pH was the main goal of this study on a Ni-Fe-Cr-Mo-W-Ru HEA. This paper focuses on investigating the composition of oxide films as a function of potential and pH. Tests were conducted in Cl--free sulfate solutions3,4,5. Materials and Procedures Electrochemical corrosion testing of the Ni-Fe-Cr-Mo-W-Ru HEA in sulfate solutions was conducted in varying potentials and pH levels in order to evaluate the effect of potential and pH on the corrosion behavior of the alloy. Air-formed oxides were reduced by cathodic reduction followed by step anodic passivation to potentials in the passive range. In some cases, buffer solutions were used to control pH. In other cases, pH was adjusted using H2SO4 or NaOH in a base solution of 0.1 M Na2SO4. Though most HEAs by design consist of elements in equal proportion, the slightly higher proportion of Ni in this particular alloy made it appropriate to consider the Ni-Fe-Cr-Mo-W-Ru HEA as Ni-based. As such, the corrosion characteristics of the HEA in this study were compared to commercial solid solution Ni-based super alloys such as C-22 (Ni-Cr-Fe-Mo-Co-C-Mn-Si-P-S-V-W) and Alloy 600 (Ni-Co-Cr-Fe-C-Mn-Si-S-Cu), as well as other Ni alloys containing varying proportions of the passivity-promoting elements Cr, Mo, and W. Potentiodynamic, potentiostatic, and galvanostatic techniques were employed to characterize the electrochemical passivation behavior of the HEA, including oxide growth, dissolution, and potential step repassivation current densities as a function of E-pH. In-situ measurements of changes in oxide thickness were measured by single frequency electrochemical impedance spectroscopy (SF-EIS). Ex-situ characterizations of the HEA surface at the atomic level were conducted by XPS and Raman spectroscopy and in the future will be explored by 3-D atom probe methods. Acknowledgements 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.