High entropy alloys (HEAs) contain five or more alloying elements typically in equimolar concentrations. In contrast, traditional solid solution alloys typically consist of one majority solvent element and several solute elements in comparatively low atomic proportions. HEAs are of interest for development of high performance materials particularly corrosion resistant alloys, because they can be engineered to form a single solid solution phase which reduces compositional and microstructural heterogeneity. Several hypotheses have been proposed to explain why HEAs form solid solutions, including a high entropy of mixing, and a near-zero enthalpy of mixing1, 2. Many desired properties have been reported to arise in HEAs such as high hardness, superior wear resistance, high-temperature strength, and structural stability1, 2. Excellent corrosion and oxidation resistance has been proposed but testing is limited1-3.
HEAs are expected to perform superior to conventional alloys due to their unique combination of elements as well as an increase in the degrees of freedom in design which can be used to regulate the structure and properties of these alloys through alloying process control, and which cannot be achieved in typical binary and ternary systems3. This enhanced control over process variables in HEA design enables a systematic scientific examination of the effect alloy composition on corrosion properties, particularly passivity, metastable pitting, and repassivation.
Characterization of the protective film formed on the HEA surface will provide an understanding of how alloy composition, microstructure, and oxide properties combine to improve passivity-based corrosion resistance and prevent film breakdown4.
The focus of this talk will be to investigate alloy passivation as a function of chloride concentrations in an effort to elucidate the mechanism of aqueous corrosion in nickel-based HEAs, with the overarching goal of predictive corrosion-resistant alloy (CRA) design.
In the present study a single HEA of composition 33.2Ni-16.85Cr-16.3Fe-8.57Mo-9.56Ru-5.48W, wt%, was designed and synthesized. The HEA demonstrated a stable FCC structure. The HEA was tested and compared to commercially pure Ni as well as the crystalline corrosion-resistant Ni-based alloy, C22 (Ni-21Cr-3.9Fe-13.3Mo-0.72Co-0.0035 C-0.23Mn-2.9W-0.026Si-0.011P-0.013V-0.0039S, wt%).
Cyclic potentiodynamic polarization (CPP) scans were performed in solutions of increasing chloride concentration to assess the stability of the passive film. These tests enabled the identification of the passive current density, pitting, and repassivation potentials. A single frequency sinusoidal potential perturbation of 20 mV at 1 Hz was superimposed on the CPP scan to provide electrochemical impedance spectroscopy (EIS) data as a function of applied potential within the passive region. The electronic properties of the oxide film were also assessed using Mott-Schottky analysis of the EIS data.
Ex-situ characterization was conducted to determine the corrosion morphology after electrochemical testing. The molecular identity of the oxide was determined using Raman and X-ray photoelectron (XPS) spectroscopies.
Preliminary results indicated good corrosion resistance and a broad potential range for the passive region. Results were compared to pure Ni and C22. The passive range was compared to E-pH stability diagrams that were constructed using the CALPHAD approach and provided by Questek®.
Acknowledgements
This work was supported as part of the Center of 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
References
1.Y. Qiu, M. A. Gibson, H. L. Fraser and N. Birbilis, Mater Sci Tech-Lond, 31, 1235 (2015).
2.M. H. Tsai and J. W. Yeh, Materials Research Letters, 2, 107 (2014).
3.Z. Tang, L. Huang, W. He and P. K. Liaw, Entropy, 16, 895 (2014).
4.E. McCafferty, Introduction to Corrosion Science, p. 1, Springer, New York, NY (2010).