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Some of the factors responsible for good corrosion resistance of select polycrystalline and emerging alloys in chloride solutions are discussed with a goal of providing some perspectives on the current status and future directions. Traditional metallic glass alloys, single phase high entropy alloys (HEAs), early metallic glasses, and high entropy metallic glasses are all emerging corrosion-resistant alloys (CRAs) that utilize traditional strategies for improved corrosion resistance as well as take advantage of some other novel beneficial attributes. These materials enjoy many degrees of freedom as far as choice of both composition and structure, providing great flexibility in the pursuit of superior corrosion resistance. The new materials depart from classical solvent-solute type polycrystalline binary or ternary alloys. Thus, such emerging materials provide significant opportunities to achieve even greater improvements in corrosion resistance in harsh environments. Several examples of the unique corrosion properties of selected materials in the context of modern theories of corrosion are discussed herein. Discussion is restricted to solid-solution binary or ternary polycrystalline alloys, several metallic glass alloys, and single phase HEAs. A common feature of many CRAs is that composition and microstructure often affect both passivity and resistance to localized corrosion that can be divided into initiation, stabilization, and propagation stages. Enormous complexities in protective oxide structures and chemistries and the large number of combinatorial possibilities in newer materials such as HEAs preclude trial-and-error approaches and perhaps even combinatorial experimental design. Computational materials methodologies will be required in the search for new corrosion-resistant alloys in these material classes. The search must consider the best scientific insights available regarding how major and minor alloy additions, as well as various microstructural attributes, contribute to corrosion mitigation. Additional scientific insights, as they emerge, will enable choices beyond the reliance on high concentrations of alloying elements that are known to affect passivity breakdown and pit stabilization. A challenge is to connect the “basic attributes” of an alloy with its properties. The strength of this connection will likely require new scientific principles enabling deep multiphysics insights in order to link feature(s) such as composition and metallurgical phases to the desired corrosion properties. Application of data informatics will likely also play a role given the plethora of variables that are important in corrosion and the difficulty in assessing all relationships. The opportunity exists to accelerate the design of emerging materials for high corrosion resistance.

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Nucleation and growth of metastable oxides on polycrystalline, solid solution Ni-22 wt % Cr alloy were studied in slightly acidic chloride solution and compared to sulfate solutions at the same pH. A combination of ac and dc electrochemical methods, photoelectron spectroscopy, and atomic force microscopy was utilized to characterize the passivation and local breakdown of oxides, yielding information on oxide morphology at the nanometer scale, thickness, composition, and global rates of formation. Oxide nucleation occurs in the form of islands, which form on newly passivating surfaces within seconds of the establishment of an electrochemical potential driving force. Oxides tend to grow first as islands and coalesce to form nm-scale coverages on the NiCr alloy surface, which is consistent with the sharp decrease in oxidation rates commensurate with thin film-field-driven passivation. At the same time, it is found that chemical driving forces favor simultaneous oxide dissolution, especially in the case of Ni 2+ oxides. The balance between these two processes is argued to govern morphologies and thickness evolution. Roughness and topographical heterogeneity evolve with time and depend critically on anion identity. Cl − is shown to enhance the spread in oxide island radii, heights, and Cr/Ni ratio compared to SO 4 2− . The possible impacts of morphology and chemical composition on oxide instability and breakdown are discussed.

Ni-Cr-based alloys are considered amongst the most corrosion-resistant when utilized in aqueous marine applications. The superior corrosion resistance of these alloys is known to depend on the presence of a homogeneous solid solution and the desirable attributes of their oxide films. However, the scientific basis for excellent passivation and protection against localized corrosion initiation remains uncertain and insights are often based on short term laboratory testing under limited conditions even though exposures are long-term. The engineering basis for alloy choice often relies on lab testing, field exposures or empirical metrics such as the PREN that do not even consider exposure time. Both highly engineered legacy and emerging alloys lack information on the precise attributes of protective passive films after long-time exposures. However, relevant field exposure periods are nearly infinite in time. The focus of the current work was on solid solution Ni-22Cr, Ni-22Cr-6Mo, and Ni-22Cr-6Mo-3W alloys. Potential step passivation studies in 0.1 NaCl pH 4 solutions in the passive range were investigated over 10s to 10 days at select potentials. To characterize the formation and exposure aging of oxide films, electrochemical methods were used as well as XPS and three-dimensional APT. Improvements in passive films protectiveness was correlated Cr3+ enrichment and aliovalent cation accumulation (Figure 1), and minimal morphological roughening. There is possibility of either oxide doping and/or phase separation promoted by aliovalent cations associated with alloying with Mo and W. Ni2+ rich oxides and hydroxides were preferentially dissolved with time. The most corrosion resistant alloys possess passivating films that are efficacious not only just after formation, but that self-healing capabilities and improve over long exposure times as evident from impedance spectroscopy and metastable pit studies. Such alloys were observed to be the most resistant when interrogated for local corrosion after various periods of exposure compared to other alloys whose impedance response degraded over exposure time. The latter alloys eventually arrived at conditions favoring breakdown. This work contributes to the scientific understanding of the long-term passivation process in Ni-based alloys in acidic chloride-containing environments. The presence of Mo and W as minor alloying elements on aqueous passivation and localized corrosion resistance initiation and stabilization correlated with certain features of electrochemically grown oxide films. Oxide exposure aging time, potential, and oxide “transformation” diagrams are introduced and shown to provide valuable insights on alloy. Passivated films may form as solute captured solid solutions after short term passivation or when grown quickly but are gradually enriched in Cr, Mo and W over long time. Can such Cr enrichment continue during exposure without limits? Factors affecting and limiting Cr enrichment are discussed as time allows. This work was supported by the Office of Naval Research under MURI “Understanding Atomic Scale Structure in Four Dimensions to Design and Control Corrosion Resistant Alloys” through Northwestern University #SP0028970-PROJ0007990-MURI ONR N00014-16-1-2280. Figure 1