Both highly engineered legacy and emerging alloys lack information on the precise attributes of protective passive films especially after long-time exposures when it really matters. Relevant field exposure periods are nearly infinite in time but both accelerated testing and high-fidelity experiments are often very short term in duration. Recent studies have explored passivation and protection of a number of different MPEAs connecting oxide chemistry to corrosion electrochemistry but most reports are after reality brief exposure periods.

The goals of this work are to (a) review the nature of passive films (i.e., solid solution oxides, phase separated single element oxides, and/or complex oxides) and (b) determine the effect of exposure aging on the evolution of oxide features such as composition, structure, phase separation and physical attributes such as thickness in MPEAs; and to correlate these features with both protectiveness in the passive range and local corrosion resistance. Furthermore, we aim to understand what factors limit beneficial alloying element enrichment in the oxide, and further insights on potential third element effects.

The focus of the current work extends from the solid solution Ni-22Cr, Ni-22Cr-6Mo, and Ni-22Cr-6Mo-3W family to a variety of emerging MPEAs containing “d-block” elements. AC and DC electrochemistry during potential sweeps and potential step passivation studies were conducted in 0.1 M NaCl pH 4 (HCl) as well as other solutions in the passive range. Exposure and high-fidelity characterizations were investigated over 10 s to 10 days at select potentials.

Improvements in passive films protectiveness were correlated with the enrichment of certain elements and depletion of others. Mn oxides were found to be detrimental when formed over the long term in the outer layer of oxides on MPEAs. The most corrosion-resistant alloys possess passivating films that are efficacious not only just after formation, but that self-heal as well as improve over long exposure times as evident from impedance spectroscopy and metastable pitting resistance. Such alloys were observed to be the most resistant when interrogated for local corrosion compared to inferior alloys whose impedance response degraded over time as the passive film evolved. The latter alloys eventually arrived at conditions susceptible to local corrosion. Time-potential-transformation diagrams based on long-term exposures in Cl^- followed by high fidelity characterization is suggested to be an approach to improve scientific understanding of long-term passive-protection features relevant to chloride exposures.

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; the Office of Naval Research under MURI ONR N00014-16-1-2280 and N00014-19-1-2420.