Future Frontiers in Corrosion Science and Engineering, Part I

In 2008, the National Academy of Science identified 14 Engineering Grand Challenges facing civilization in the 21st Century.1 Of the 14 challenges and subsequent goals and engineering triumphs sought, a case could be made that over 1/3 of them face corrosion issues that are major obstacles to achieving the outcomes recommended. Two centered on the need for clean abundant energy, for instance, which is a grand challenge that requires significant advancements in materials whether in the application of solar, geothermal, nuclear, or other approaches. The Department of Energy in 2017 conducted a workshop on the basic research needs and priority research directions in advanced nuclear energy.2 By all indications, the challenges in corrosion science are daunting with regard to this single focus issue. But I see great hope. This optimistic view was shared in another National Academy study, “Research Opportunities in Corrosion Science and Engineering.”3 Indeed, corrosion science is on the threshold of great advances in high fidelity, in-operando experimental methods4-5 that can probe the chemical, electronic, and structural nature of materials on a local scale,6 in computational modeling from the atomic scale to application, in theory, data analytics, as well as their integration. Many challenges remain but the future is very bright. In this three part series, I will comment on some of the needs, gaps, and opportunities that define some of the frontiers in corrosion science and engineering.Corrosion is a complex process involving coupled chemical, electrochemical, biological, and solid-state reaction dynamics, materials science, surface science, and environmental chemistry. The multi-scale nature of corrosion processes and the need to connect across length (and time) scales to advance our knowledge of the factors governing corrosion are daunting. Yet, the corrosion community rapidly embraces advances in other fields of science and engineering; these are quickly adapted, enabling great progress.3 High-resolution studies reveal that processes that seemed uniform at the mesoscale are often extremely heterogeneous at the local scale. Phenomena at each length scale are often regulated by underlying elementary processes at a smaller scale that in concert produce damage of engineering consequence. Moreover, corrosion is hierarchical. For instance, key processes within the substrate material, in the surface-reaction layer, and at both the layer/substrate and layer/environment interfaces often all play a role in dictating reaction rates as well as the extent of protectiveness by oxides, coatings, and conversion layers. Structural and chemical heterogeneities in any one of these layers and interfaces may regulate corrosion as they do in catalysis.7 The reactivity of isolated sites may differ significantly from the global average. Thus, the size, behavior of each site, fractional coverage of such sites, and their interactions are all critical.8 Moreover, the effects of microstructural heterogeneities, grain boundary nano-chemistry, and structure at the 1-1,000 nm scale, as well as grain texture often come in to play.9 Thus, processes at the materials science length scale must be factored into atomic scale analysis to gain a complete picture. Drivers, dependencies, and triggers for all of these processes must be understood,10 including mechanical driving forces in the case of environmental cracking or film rupture.The corrosion field can gain deeper understanding beyond “look and see” forensic type characterizations by first interrogating and then connecting science from each scale to inform the next level. Focus on one or two length scales in isolation without much attention to connection or integration has dominated past work. Empirical studies producing practical data of technological significance are sometimes lacking in microscopic understandings. Similarly, excellent surface science and atomic scale studies are often weakly connected to engineering application. This challenging range of length scales is portrayed for copper and for a variety of microstructurally complex alloys in Figure 1. Atomic behavior is often the focus of surface scientist (dark red), investigations ranging from the nanometer to micrometer “materials length” scale is often central to corrosion scientist (blue), while studies by corrosion engineers (green) take place at the meso, macro, and component length scales. The far right shows culmination in the form of tangible damage such as intergranular corrosion in an Al-Mg alloy and finally pin hole pitting in a copper pipe.The corrosion field can advance at an accelerated pace by further connecting and integrating information at each scale. We are beginning to see this happen. In some recent multi-investigator corrosion projects,11 teams of researchers have been assembled with expertise in metallurgy, ceramics, oxidation, corrosion, surface science, and computational modeling to undertake such an approach.12 Quantitative modeling includes ab initio quantum chemistry methods, Monte Carlo approaches, phase field techniques, and granular and continuum scale modeling. These comprehensive projects seek to integrate experimental and computational approaches within each stage of corrosion, at the key governing interfaces and with respect to crucial unit processes. This effort will enable design of better materials and coatings, as well as better models of corrosion processes and prognosis.3 This paradigm can be applied to metallic materials, ceramics, and glasses alike as each material class has distinct similarities as well as differences. Increased understanding of the corrosive environment and its evolution, especially in confined spaces such as pits, crevice, and cracks and under coatings, will also be an important aspect necessary to achieve this surge in progress.The future is bright and holds great opportunity for future advances in the science of corrosion. Parts II and III will address the some of the other challenges and opportunities facing frontiers of corrosion science.Dr. John R. ScullyTechnical Editor in ChiefJohn R. Scully was supported in part by 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 No. DE-SC0016584. He was also supported by ONR MURI “Understanding Atomic Scale Structure in Four Dimensions to Design and Control Corrosion Resistant Alloys” on Grant No. N00014-16-1-2280.