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Research Opportunities in Corrosion Science and Engineering 3 Research Opportunities Industry and government needs have been the primary drivers of corrosion research, the results of which have often led to new science, followed by further practical improvements in a continuing symbiotic cycle. Government has played many critical roles, including challenging industry with critical problems (such as the need for applications that can function well in extreme environments encountered, for example, during propulsion, and new synthetic-fuel, energy-storage, and fuel-cell concepts)1 and also performing and sponsoring research to address critical gaps in understanding. Future corrosion research priorities should continue to be guided by societal drivers and associated technological needs (top-down drivers), but progress in this area will also benefit from advances enabled by focusing on related areas of fundamental science (bottom-up drivers). The strong interactions between engineering-oriented corrosion grand challenges and the underlying fundamental science as discussed in Chapter 2 are illustrated by the iconic triangle shown there whose foundation is corrosion science. The impact of corrosion on everyday life is a major issue, given that corrosion and materials reliability affects public infrastructure, industrial complexes, and major areas of governmental endeavor and responsibility. The deleterious effects of corrosion and its societal impact are highlighted by growing concerns about public safety, endangerment of personnel, national security, energy security, national de- 1 Department of Energy, Basic Research Needs for Materials Under Extreme Environments, Report of the Basic Energy Sciences Workshop on Materials Under Extreme Environments, June 11-13, 2007, available at http://www.sc.doe.gov/bes/reports/files/MUEE_rpt.pdf.
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Research Opportunities in Corrosion Science and Engineering fense, industrial productivity, economic competitiveness, environmental protection and sustainability, and the standard of living and quality of life. Numerous what-if scenarios suggest ways that fundamental advances in corrosion science could have a positive impact on numerous problems facing society. What if fundamental science uncovered so-called silver bullets in materials or coating designs for mitigation of corrosion that could extend the use of cost-effective materials into more extreme environments or enhance materials capabilities for energy storage? The ability to effectively address many societal and technological challenges could benefit from game-changing advances in corrosion science. Corrosion science, a truly interdisciplinary field that includes aspects of physics, materials science, surface science, electrochemistry, and fracture mechanics, benefits directly from new developments not only in those associated fields of fundamental science, but also in others. One challenge for the corrosion science community is to pursue strategies to harvest those diverse benefits and apply them to corrosion-related problems. The multidisciplinary nature of corrosion research requires a balanced portfolio of single investigator and collaborative group activity. Group efforts at various government laboratories have addressed corrosion problems, and some continue at this time. In academia, however, funding for group efforts is difficult to find, particularly for fundamental and applied problems. National Science Foundation (NSF) funding, with the exception of that for large centers, tends to focus on single-investigator projects. A model of what is required is the DOD Multidisciplinary University Research Initiative (MURI) program, which supports research by small teams of investigators from more than one traditional science and engineering discipline in order to accelerate both research progress and the transition of research results to applications. Most MURI efforts involve researchers from multiple academic institutions and academic departments and include support for up to 5 years. Corrosion science remains a fertile scientific endeavor, poised for advances that will benefit society. As in the past, these advances will be enabled by progress in related fields, particularly in materials characterization and computation. Indeed, an overarching observation is that the amazing recent advances in these areas portend well for the future of corrosion science as capabilities for refining time and length scales allow modeling and experimentation to converge. While corrosion traditionally has been observed at the macroscale, recent scientific emphasis has shifted to understanding the processes at smaller length (and time) scales. For example, many corrosion processes are now known to be controlled by molecular-, submicrometer-, and micrometer-scale phenomena. Although much has to be learned regarding the nanoscale chemistry, structure, and dynamics at individual grain boundaries or other key material features, progress is also required at the granular scale to understand how networks of boundaries and
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Research Opportunities in Corrosion Science and Engineering arrays of defects behave under corrosive conditions for metallic and nonmetallic materials. Thus multiscale characterization and modeling will enable progress in understanding this phenomenon. This chapter highlights some of the research opportunities that hold great promise for corrosion mitigation, organized according to the four corrosion grand challenges (CGCs) identified by the committee: CGC I—Development of cost-effective, environment-friendly corrosion-resistant materials and coatings; CGC II—High-fidelity modeling for the prediction of corrosion degradation in actual service environments; CGC III—Accelerated corrosion testing under controlled laboratory conditions that quantitatively correlates to observed long-term behavior in service environments; and CGC IV—Accurate forecasting of remaining service time until major repair, replacement, or overhaul becomes necessary—i.e., corrosion prognosis. As indicated in each section, high-priority fundamental science issues are at the heart of the ability to predict corrosion damage, design new materials and coatings, and sense as well as predict corrosion. The section below is not intended to be an exhaustive compilation of all corrosion research opportunities. Instead, it highlights some of the challenges in the field of corrosion science and engineering in each of the important high-priority areas identified by the committee. An underlying theme is the need for participation by multidisciplinary and cross-disciplinary teams of researchers, in addition to the individual investigator, to address the above corrosion grand challenges, as well as the need to disseminate the knowledge acquired to the greater community. This chapter also includes a section on opportunities in instrumentation that briefly describes some of the analytical techniques that have enabled and will continue to enable ongoing advances in corrosion science and mitigation of corrosion. OPPORTUNITIES FOR RESEARCH CGC I: Development of Cost-Effective, Environment-Friendly Corrosion-Resistant Materials and Coatings Development of superior corrosion-resistant materials and coatings is the ultimate proactive corrosion challenge. While this has long been a goal, it has not been realized in many applications for a number of reasons, including the strategy of using trial-and-error approaches for material development, the high cost of achiev-
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Research Opportunities in Corrosion Science and Engineering ing an ultimate materials solution, the lack of fundamental knowledge about how to design a materials system expressly to resist corrosion while also meeting all the other mechanical and physical property requirements, and inadequate understanding of corrosion processes that degrade materials, including the very definition of the corrosive environment itself. The research opportunities identified are those that address key needs in the development of: Corrosion-resistant materials, Protective coatings, and Materials for active corrosion protective systems. Together with factors that impact the recyclability of materials, they suggest the type and scope of effort needed to make progress toward the goal of CGC I. Surface materials science is closely linked to corrosion behavior and should also be a focus of technologies in CGC I. Advances in corrosion mitigation will require better understanding of surface structure and properties. Development of Corrosion-Resistant Materials The design stage of a product or system is one of the first lines of defense against corrosion, and a designer should have the ability to prevent the onset of corrosion by choosing materials that are as intrinsically resistant to corrosion and environmental degradation as possible. By carefully considering materials choices, it is possible to affect the thermodynamic stability and/or alter the rate of corrosion kinetics, and thus appreciably impact performance of the system over time. As stated in the 2004 Defense Science Board Task Force report on corrosion,2 “an ounce of prevention is worth a pound of cure.” ICMSE will soon be possible,3 and there is the opportunity to tailor microstructure, composition, and processing to achieve corrosion properties to meet the needs of a design if certain scientific barriers are overcome. Increasingly, environmental concerns are driving the need for engineering materials with intrinsic corrosion resistance,4 but high-performance corrosion-resistant materials are often too expensive to use for applications where 2 See Defense Science Board, Corrosion Control, Final Report ADA428767, October 2004, available at http://www.acq.osd.mil/dsb/reports2000s.htm. 3 National Research Council, Integrated Computational Materials Science and Engineering: A Transformational Discipline for Improved Competitiveness and National Security, The National Academies Press, Washington, D.C., 2008. 4 For instance, large amounts zinc, and copper are exposed to the environment as parts of structures such as roofs, facades, and support beams; atmospheric corrosion of these structures results in unintended release of metal ions by run-off and then dispersion into the environment.
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Research Opportunities in Corrosion Science and Engineering large amounts would be required.5 Clearly one opportunity lies in creating cheaper materials with high performance. Other engineering considerations—such as joining and fabricating—also play important roles in deciding whether a particular material can be used in an application. As described in Chapter 1, the traditional materials development process is long and drawn out, requiring multiple iterations in chemistry, microstructure, and processing methods to achieve desired material properties. In the past, optimization of corrosion properties has been achieved by trial-and-error, lessons learned, or—at best—by corrosion experts using a mix of empirical experience blended with some scientific intuition. One vision for the inclusion of corrosion in quantitative materials design is the development of focused tool-kits that can be used to optimize the development of materials, coatings, and treatments for mitigation of targeted corrosion processes—such as paint delamination, crevice corrosion, or high temperature selective alloy depletion of coatings. The lack of such a process for rapid, “intelligent” materials development in corrosion has been a major impediment to making significant improvements in the design of new products but also represents a significant opportunity to move this area forward when such tool-kits are developed. Integrated computational materials science and engineering (ICMSE) has shown the potential to optimize a new material relative to its required properties and cost through advanced computational tools and supporting databases.6 Figure 3.1 contrasts the traditional approach with the ICMSE approach. The ICMSE approach is based on computer modeling and simulations that have a high fidelity to physical experiments. Consequently multiple iterations equivalent to alloy development cycles can be conducted quickly, at low cost, by analysis (with selective physical experiments) compared to the traditional entirely physical materials development approach. This is revolutionizing materials development and reducing the time necessary to do so by more than 50 percent.7 Key to the advance of this process have been advances in other engineering tools and the rapid increase in available computational power. Clearly, development of new materials using this technology is strongly dependent on the availability of good models, which 5 For example, many superalloys are mainly used in very high value situations such as nuclear reactors, aircraft engines, equipment handling reactive, dangerous chemicals, or implantable medical devices. Because of this, much of our infrastructure is constructed using less-sophisticated materials such as ordinary concrete with carbon steel reinforcement. 6 National Research Council, Integrated Computational Materials Science and Engineering: A Transformational Discipline for Improved Competitiveness and National Security, The National Academies Press, Washington, D.C., 2008. 7 The time to develop new high temperature single crystal nickel-based superalloys with low rhenium content was reduced by more than 50 percent (see R. Schafrik, Accelerating materials and process development, International Association of Air Breathing Engines, ISABE2009 paper ISABE-2009-1167, September 2009).
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Research Opportunities in Corrosion Science and Engineering FIGURE 3.1 Comparison of existing or traditional approach (left) and desired approach (right) to the design of corrosion-resistant materials. SOURCE: John R. Scully, Department of Defense Corrosion Conference, 2009. is discussed later in this chapter as part of CGC II. A significant opportunity for research consists of developing and integrating corrosion models (discussed under Corrosion Grand Challenge II) with other materials models so that high-fidelity predictions can be made regarding corrosion behavior for new materials and new corrosion environments. With this integration, the following types of problems are among those that could be addressed by the ICMSE approach. Identifying the elements that act in synergy with other major alloying elements to enhance the intrinsic effects of corrosion mitigating elements. For instance, additions of molybdenum and minor amounts of nitrogen to stainless steels, copper to weathering steels, and arsenic as well as tin to brass have been found to be incredibly potent strategies to improve aqueous and atmospheric corrosion resistance. The expectation is that other such combinations of elements are soon to be discovered for other materials systems. Improving the properties of ultra-high-strength stainless steels that are desired for critical applications within aerospace, such as highly durable bearings, power transmission shafts, and aircraft landing gear structures. Advanced modeling tools have the potential to guide the selection of an optimum chemistry balance in which general corrosion resistance is improved without increasing the susceptibility to at the expense of other modes of corrosion such as environmental cracking. Because of their ability to form protective chromia layers at elevated temperatures, stainless steels also proffer good high-temperature corrosion resistance—up to a point. As new applications demand higher temperatures and introduce more aggressive reactants, often for energy or process efficiency, conventional stainless steels do not
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Research Opportunities in Corrosion Science and Engineering have sufficient strength or corrosion resistance. However, advances in capabilities to do accurate thermochemical modeling and prediction, combined with principles of selective oxidation of alloys and mechanistic knowledge of creep, provide new and unique pathways to producing future stainless steels with higher temperature capabilities.8 Developing corrosion-resistant materials for use in concrete reinforced structures often deteriorated by corrosion of the reinforcing steel. Solid stainless steel and stainless steel-clad rebar materials have demonstrated the ability to extend the chloride induced corrosion initiation threshold in concrete to over 100 years when compared with plain carbon steel currently such a material change is quite costly.9 There is a possibility to design new low cost, intrinsically corrosion-resistant reinforcing materials without resorting to the use of expensive alloying elements,10 which will ultimately enable their use not only in concrete but in other environments. Other innovative mitigation strategies that can also be investigated, including developing concrete microstructures that have lower permeability to moisture or contain corrosion inhibitors as part of their intrinsic chemistry.11 Developing an affordable, manufacturable, high-strength pipeline steel that is highly corrosion resistant. Pipelines for deep-water oil and gas production and recovery present severe corrosion challenges: hydrogen sulfide, carbon dioxide, oxygen, and mineral salts (especially chlorides) can lead to material degradation by hydrogen embrittlement, sulfide cracking, and localized corrosion. Even though modest progress has been made with corrosion-resistant nickel-based superalloys and supermartensitic stainless steels with Ni and Mo in limited applications the former are currently too expensive for widespread use and thus pose an excellent opportunity for research. The need for material development for pipelines for CO2 sequestration must also be considered as this is a possible solution to excess atmospheric CO2 in global warming control. However there is a shortage of true data and engineering knowledge, especially on potential corrosion problems in such a large undertaking. 8 Y. Yamamoto, M.P. Brady, Z.P. Lu, P.J. Maziasz, C.T. Liu, B.A. Pint, K.L. More, H.M. Meyer, and E.A. Payzant, Creep-resistant, Al2O3-forming austenitic stainless steels, Science 316:433-436, 2007. 9 M.F. Hurley and J.R. Scully, Threshold chloride concentrations for localized corrosion on selected corrosion resistant rebar materials compared to carbon steel, Corrosion Journal 62(10):892-904, 2006. 10 F. Presuel-Moreno, J.R Scully, and S.R Sharp, Literature review of commercially available alloys that have potential as low-cost corrosion resistant concrete reinforcement, NACE Corrosion Conference 2009, Atlanta, Georgia, Paper 09204, 2009. 11 National Institute of Standards and Technology, International Workshop on Fire Performance of High-Strength Concrete, NIST, Gaithersburg, MD, February 13-14, 1997, Proceedings, NIST SP 919, September 1997.
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Research Opportunities in Corrosion Science and Engineering The related “add-on” challenge is to optimize materials for conjoint failure modes when conjoint, nonlinear and coupled corrosion processes occur, including mechanically induced modes (wear, fretting, fatigue, and creep). Another need is the ability to handle or anticipate changes in solution or processes with time and transitions in corrosion modes. The IDEAL Corrosion-Resistant Alloy for Aqueous Environments Alloys are often designed with properties other than corrosion resistance in mind, such as mechanical strength. In the case of structural materials strength, ductility, toughness and joining issue are often dominant properties. Corrosion resistance is often secondary, although major alloying elements have been incorporated for many years to increase corrosion resistance. The question arises as to the ideal attributes of an alloy for maximizing corrosion resistance whilst keeping its originally intended properties. It should be recognized that there are trade-offs with a range of properties and that all of those below cannot likely be realized simultaneously. A corrosion-resistant alloy ideally would have the following properties: Form a homogeneous solid solution alloy lacking structural and chemical non-uniformities. Contain a critical amount of beneficial alloying elements in homogeneous solid solution that are readily enriched in the passive film, in amounts exceeding the thresholds for passivation. With respect to the key alloying element, be preserved by having the means to avoid interface or surface depletion. The passive films developed on the surface might function in other roles through engineering of the properties of these films (which are, in essence, semi-conducting oxide films) to suppress electron transfer reactions or create ion selective membranes. Avoidance of depassivating alloying elements would be desirable. However, if they cannot be avoided, ideally these elements would not be hydrolysable so that the pH would not be lowered at local corrosion sites. Embody beneficial synergy among the alloying elements listed above. Contain alloying elements that make the passive film a poor substrate for electron transport reactions such that reactions like oxygen reduction are suppressed. Have attributes for maintenance of low interfacial stresses and possess sufficient ductility to avoid cracking and spallation. Perhaps the oxide would be graded to avoid a classical interface. Contain elements that enable fast repassivation rate at scratches and flaws on multiple occasions. Lack negative impurities to the extent possible or otherwise sequester them so that they cannot be swept or collected at surfaces and interfaces (interface engineering).
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Research Opportunities in Corrosion Science and Engineering Contain other beneficial trace alloying elements such as bond promoters, passivity promoters, glass formability, or elements that serve as gettering agents to sequester harmful species. Exploit the full capabilities of defect engineering to avoid one-, two-, and three-dimensional defects of critical sizes, and spacings that trigger certain corrosion modes and their spreading. This is especially necessary in classes of alloys where heterogeneity is unavoidable, as in the case of precipitation age-hardened alloys. Exhibit avoidance of grain boundaries or incorporate clean grain boundaries with controlled application of a number of boundaries with low CSL. Avoid segregation or depletion of the alloying element during heating. Exhibit diffuse, not co-planar, plastic deformation that occurs in grain interiors and is not focused at grain boundaries. Have alloying elements that, once the oxide film was broken down or penetrated, would have a slow dissolution rate, resist noncongruent dissolution that is detrimental, and collect beneficial alloying elements at easy low coordination dissolution sites. The alloying element might also be engineered to alter surface diffusion rates. Inspection of this list suggests that amorphous metallic alloys satisfy many of these criteria. This is partially true. Amorphous materials are a possible choice in some applications, and this list is growing with the advent of bulk metallic glasses. However, in many cases where conventional crystalline alloys must serve as the best available choice, they are either unavailable in the product form needed or lack some other desired properties. A slightly different set of attributes might be desired for resistance of hydrogen embrittlement. The ideal corrosion allowance material might: Exploit species in the solution to make an insoluble tenacious oxide or corrosion product that slows the corrosion rate, Dissolve in a extremely predictable manner, Avoid stress buildup that can spall or crack oxide, or Corrode in a uniform, predictable manner. Protective Coatings The ICMSE approach is also applicable to the development of corrosion-resistant coatings. An advantage of coatings is that they offer the potential of a hybrid structure in which the function of the coating can be specialized for corrosion resistance while not affecting the key properties of the underlying substrate material. (However, this is a particular challenge at high temperatures, where inter-
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Research Opportunities in Corrosion Science and Engineering diffusion puts additional requirements on the coating-substrate system.) Coatings can also provide a lower cost solution than using a higher grade substrate material. Such coatings are often solely physical and chemical protective barriers between the corrodible substrate and its environment. Barrier protection from corrosion occurs when a physical layer is constructed to prevent damaging environmental species from reaching an object or system. In the simplest sense, humans use buildings as barriers between the contents of the building and the outside environment, thus preventing weathering attack on the interior objects. This form of barrier is passive, because the barrier material does not depend on a chemical process to provide protection, but rather simply stops the passage of environmental threats—such as acid rain—to the object to be protected. Passive barriers, coatings, and barrier layers include the following: Electro-deposited metal layers such as gold, silver, chromium, and others that put a relatively inert material between the corrodible substrate and its environment; Vapor-phase-deposited metals, alloys, oxides, or other materials for protection of reactive metal substrates; Spray-applied layers, including flame-sprayed metals, reactive sprays, high velocity sprays, cold sprays, and plasma-deposited layers of all sorts; Zinc- and zinc/aluminum-based galvanized layers for steel that combine a cathodic protection layer and a barrier layer; and Organic coatings (paints) that insulate reactive metal surfaces from aggressive environments. The following is a selection of areas focusing on coatings that the committee identified as high-priority opportunities: Coatings possessing high adhesion, mechanical property matching, interfacial compatibility, and low interfacial impurities. This is applicable to a wide range of films from high-temperature ones of the type used on turbine blades to those on electro-coated metal connectors. As an example, for coatings designed to proffer high temperature oxidation resistance, it is often critical to tightly control rare earth (RE) and light element (e.g., carbon and silicon) concentrations at the less than 0.1 atomic percentage level, both respectively and with respect to the RE-C and RE-S ratios.12 These concentrations can be quite difficult to achieve without advances in understanding and controlling processing approaches. If there were 12 B.A. Pint, Optimization of reactive-element additions to improve oxidation performance of alumina-forming alloys, Journal of the American Ceramic Society 86:686-695, 2003.
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Research Opportunities in Corrosion Science and Engineering new deposition monitoring tools capable of accurately determining composition, direct feedback could achieve the needed accuracy in the composition. Robust, defect-free cost-effective barrier films, available for large surface areas. Nanoscale thin films for aqueous corrosion resistance show much promise; however, the results have been based on films produced in the laboratory. Further assessing the feasibility of these barrier films’ properties requires considerable process scale-up. Recently, paints containing zinc oxide powder, which have long been utilized as anticorrosive coatings for various metals and alloys, have been seen in a new light. These paints are especially effective for galvanized steel, which is difficult to protect because its reactivity with organic coatings leads to brittleness and lack of adhesion. Zinc oxide paints retain their flexibility and adherence on such surfaces for many years. Progress has been made recently13 with ZnO highly n-type doped with Al, Ga, or In which is transparent and conductive and can be used as heat protection if applied to windows. Another exciting ongoing development is the incorporation of additives such as Mg in the zinc oxide. This Mg addition renders the oxide inactive by a change of semiconductor type from n-type to p-type for the reduction of oxygen. This inactivation of oxygen-reduction capability is a necessary step in mitigating corrosion and coating degradation. The n-type oxide is detrimental because it reduces oxygen too easily, whereas conversion to the p-type makes that process less probable and as such improves the protective capability of an already-flexible and highly adherent sacrificial coating.14 Self-sensing and self-healing films and coatings. These smart films can be externally interrogated or can self-sense their own health and respond by either self-healing or actuating an external healing response.15 A number of strategies that can be used to trigger healing and release.16 However, more research is needed to determine what approaches works best. One example is a material that contains small spheres with a sealant; if a crack forms, some spheres will break open and release the sealant into the crack. This sealant can be designed so that it reacts upon release and solidifies, effectively repairing the crack. Thorough understanding of corrosion protection mechanisms for nontoxic corrosion inhibitors and conversion-coating procedures. Over the past several years, considerable effort has been spent in replacing chromate coatings, which pose a 13 A. Kalendová, D. Veselý, and P. Kalenda, Pigments with Ti4+ -Zn2+, Ca2+, Sr2+, Mg2+-based on mixed metal oxides with spinel and perovskite structures for organic coatings, Pigment and Resin Technology 36(1):3-17, 2007. 14 R. Hausbrand, M. Stratmann, and M. Rohwerder, Corrosion of zinc-magnesium coatings: Mechanism of paint delamination, Corrosion Science 51:2107-2114, 2009. 15 E.J. Barbero, and K.J. Ford, Characterization of self-healing fiber-reinforced polymer-matrix composite with distributed damage, Journal of Advanced Materials 39(4):20-27, 2007. 16 D.Y. Wu, S. Meure, and D. Solomon, Self-healing polymeric materials: A review of recent developments, Progress in Polymer Science 33:479-522, 2008.
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Research Opportunities in Corrosion Science and Engineering resolve fine microstructural and compositional details of an oxide film on stainless steel formed under pressurized-water-reactor conditions (see Figure 3.15).54 Such findings indicate the potential of state-of-the-art atom probe techniques to advance basic corrosion science knowledge and help understand origins of failures due to environmental degradation. Time-of-Flight SIMS Secondary ion mass spectrometry (SIMS) has had an important role in corrosion science (as shown, for example, in the work of Bishop et al. 55 and Marriott et al.56). For high-temperature oxidation, it has been particularly powerful in determining transport mechanisms in protective oxide scales grown on metallic specimens using O18 exposures.57 Recently, time-of-flight SIMS has been used to generate detailed images through sections of protective oxide scales, revealing details of the distribution of various alloying elements/phases (Figure 3.16).58 Further application of this technique to corrosion science and engineering seems well justified. Advanced Electron Microscopy For many years, transmission electron microscopy has been of great benefit to corrosion science and engineering by revealing the relationships among microstructure (and, now, nanostructure), composition, and different corrosion behaviors (dissolution, crack growth, scaling, and so on)—see, for example, Przybylski et al.59 and Gertsman and Bruemmer.60 Advances in analytical election microscopy for high-resolution chemical analysis and focused ion beam techniques to provide high-quality thinned sections of oxidized material from precise areas of a corroded specimen (for example, Haynes61) have truly made electron microscopy an essential 54 S. Lozano-Perez, D.W. Saxey, T. Yamada, and T. Terachi, Atom-probe tomography characterization of the oxidation of stainless steel, Scripta Materialia 62:855-858, 2010. 55 H.E. Bishop, D.P. Moon, P. Marriott, and P.R. Chalker, Applications of a high spatial resolution combined AES/SIMS instrument, Vacuum 39:929-939, 1989. 56 P. Marriott, S.B. Couling, and P.R. Chalker, High spatial resolution SIMS investigation of oxides formed on stainless steel under PWR conditions, Applied Surface Science 37:217-232, 1989. 57 J. Jedlinski and G. Borchardt, On the oxidation mechanism of alumina formers, Oxidation of Metals 36:317-337, 1991. 58 D.B. Hovis and A.H. Heuer, unpublished data. 59 K. Przybylski, A.J. Garratt-Reed, and G.J. Yurek, Grain boundary segregation of yttrium in chromia scales, Journal of the Electrochemical Society 135:509, 1988. 60 V.Y. Gertsman and S.M. Bruemmer, Study of grain boundary character along intergranular stress corrosion crack paths in austenitic alloys, Acta Materialia 49:1589-1598, 2001. 61 J.A. Haynes, B.A. Pint, K.L. More, Y. Zhang, and I.G. Wright, Influence of sulfur, platinum, and hafnium on the oxidation behavior of CVD NiAl bond coatings, Oxidation of Metals 58:513, 2002.
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Research Opportunities in Corrosion Science and Engineering FIGURE 3.15 (a) Atom probe tomography reconstruction showing the presence of Li atoms within the cap and sub-interface oxides. The arrows indicate the location of the cap-oxide-to-metal interface. The Li atom distribution is superimposed on the oxide atom maps. (b) Top view of the sub-interface region showing the distribution of oxides (cap oxide removed). The oxide regions beneath the cap are interconnected. (c) Sub-volume (5 × 15 × 18 nm; ~40,000 detected atoms) taken from the cap-oxide-to-metal interface showing selected species. (d) Concentration profile across the oxide–metal interface generated using the proxigram technique. The presence of Li is represented by an atom count because its concentration is very low. Uncertainties in the data points are comparable to the marker size. SOURCE: Reproduced from S. Lozano-Perez, D.W. Saxey, T. Yamada, and T. Terachi, Atom-probe tomography characterization of the oxidation of stainless steel, Scripta Materialia 62:855-858, 2010. part of corrosion research and development and failure analysis. Currently, there are numerous efforts to develop ways to conduct in situ experiments inside the columns of high-resolution electron microscopes, including the introduction of liquids (see Figure 3.17).62 Such developments have obvious relevance to advancing the state of fundamental knowledge about corrosion processes and how such processes are controlled by structure and composition. 62 N. de Jonge, D.B. Peckys, G.J. Kremers, and D.W. Piston, Electron microscopy of whole cells in liquid with nanometer resolution, Proceedings of the National Academy of Sciences 106:2159-2164, 2009.
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Research Opportunities in Corrosion Science and Engineering FIGURE 3.16 Cross-sectional view of multilayer scale formed on an oxidized NICrAl (YHf ) alloy (spinel overlaying α-alumina). The α-alumina data have been removed to show the morphology of the embedded Y-rich oxide phases. SOURCE: D.B. Hovis and A.H. Heuer, unpublished data. Neutron Scattering Neutron scattering has not found wide applicability in corrosion science but is proving scientifically powerful in unraveling the structure and dynamics of oxide-H2O interfacial regions of relevance to geochemistry and the fundamental science of aqueous solutions in contact with solids.63,64 The greater penetrating power of neutrons, compared to other energetic beams used for analysis, can allow subsurface (or undercoating) structures and processes to be probed and could prove to be of particular value for in situ experiments. Defect distributions, structural fluctuations, and film growth can be monitored as a function of depth into a solid using various neutron scattering techniques, and the sensitivity of neutrons to protonic species is of particular relevance to many corrosion processes. 63 E. Mamontov, L. Vlcek, D.J. Wesolowski, P.T. Cummings, W. Wang, L.M. Anovitz, J. Rosenqvist, C.M. Brown, and V. Garcia Sakai, Dynamics and structure of hydration water on rutile and cassiterite nanopowders studied by quasielastic neutron scattering and molecular dynamics simulations, Journal of Physical Chemistry C 111:4328, 2007. 64 E. Mamontov, D.J. Wesolowski, L. Vlcek, P.T. Cummings, J. Rosenqvist, W. Wang, and D.R. Cole, Dynamics of hydration water on rutile studied by backscattering neutron spectroscopy and molecular dynamics simulations, Journal of Physical Chemistry C 112:12334, 2008.
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Research Opportunities in Corrosion Science and Engineering FIGURE 3.17 Schematic of scanning transmission electron imaging of metallic nanoparticles in a liquid. SOURCE: Reproduced from N. de Jonge, D.B. Peckys, G.J. Kremers, and D.W. Piston, Electron microscopy of whole cells in liquid with nanometer resolution, Proceedings of the National Academy of Sciences 106:2159-2164, 2009. X-ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy (XPS) is a surface analytical approach that provides detailed chemical information from the top 1 to 10 nm of a sample surface. The surface is irradiated with an x-ray beam and the kinetic energy of the emitted electrons is analyzed. This technique is well established in the corrosion field because it has great utility to measure thin protective films and corrosion product layers. The latest technical developments in XPS instrumentation enhance the usefulness of the technique. One trend is a decreasing x-ray beam size. X-ray beams are much more difficult to focus than electron beams, which are used in many other analytical techniques. As a result, XPS has relatively poor lateral resolution. However, XPS tools now provide x-ray beams less than 10 micrometers in diameter, which allows for the analysis of surfaces on small microstructural features
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Research Opportunities in Corrosion Science and Engineering such as second phases and inclusions. Another development in XPS technology involves the ion beams used to sputter samples for depth profiling. Standard ion beams such as Ar ion beams can damage samples, particularly organic samples such as paint coatings, which is relevant to corrosion. In recent years beams of large ion clusters have been offered for use in sample sputtering. Ions of C60, so-called buckyballs, allow for gentler sputtering. The large ions impact the surface with a lower speed so that there is much less penetration and much less damage compared to beams of single ions. Very recent reports have shown that large gas cluster ions caused very little damage during sputtering of polyimide, which is very sensitive to ion irradiation damage.65 In this work the gas cluster ion gun generated an Ar ion cluster distribution centered at Ar2500. The polyimide XPS spectra before and after sputtering with this beam exhibited very little change. Electron Backscatter Diffraction Although the fundamentals of the technique can be traced back to the work of Kikuchi in the late 1920s, electron backscatter diffraction (EBSD) emerged in the early 1980s as a method for analysis of local crystallographic texture in materials.66 Automation and computerization of the analysis of backscattered electron diffraction patterns led by Adams and co-workers in the 1990s further led to the development of commercial software and hardware that has enabled EBSD to become a relatively mature technique today for characterizing microtexture.67 It is well known that the crystallographic texture of a material influences its thermophysical properties. Manipulating texture through heat treatment or forming techniques such as rolling or pilgering may lead to improvements in corrosion resistance or resistance to environmentally influenced cracking if applied properly. A few groups have used EBSD to inspect growth processes at crack tips during stress corrosion cracking and other crack-growth processes in structural alloys68 65 T. Miyayama, N. Sanada, M. Suzuki, J.S. Hammond, S.-Q.D. Si, and A. Takahara, X-ray photo-electron spectroscopy study of polyimide thin films with Ar cluster ion depth profiling, Journal of Vacuum Science and Technology A 28:L1, 2010, doi: 10.1116/1.3336242. 66 A.J. Schwartz, M. Kumar, and B.L. Adams, Electron Backscatter Diffraction in Materials Science, Kluwer Academic/Plenum Publishers, New York, 2000. 67 B.L. Adams, S.I. Wright, and K. Kunze, Orientation imaging: The emergence of a new microscopy, Metallurgical and Materials Transactions A 24A(4):819-831, 1993. 68 A.F. Gourgues, Electron backscatter diffraction and cracking, Materials Science and Technology 18:119-133, 2002; G.S. Rohrer, D.M. Saylor, B.E. Dasher, B.L. Adams, A.D. Rollett, and P. Wynblatt, The distribution of internal interfaces in polycrystals, International Journal of Materials Research 95:197-214, 2004; A. King, G. Johnson, D. Engelberg, W. Ludwig, and J. Marrow, Observations of intergranular stress corrosion cracking in a grain-mapped polycrystal, Science 321:382-385, 2008; J. Burns, “High Temperature Fatigue Crack Growth Behavior and Microstructural Evolution in Alloy 230,” M.S. thesis, Boise State University, 2010.
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Research Opportunities in Corrosion Science and Engineering FIGURE 3.18 Electron backscatter diffraction image of a fatigue crack in alloy X. Bar is 200 µm. Colors represent specific crystal orientations. SOURCE: Courtesy of J. Burns and M. Frary, Boise State University, from J. Burns, “High Temperature Fatigue Crack Growth Behavior and Microstructural Evolution in Alloy 230,” M.S. thesis, Boise State University, 2010. (see Figure 3.18). Although only a small number of materials have been analyzed under a narrow range of conditions, studies suggest that the propensity for environmentally induced crack growth can be strongly affected by changes in the orientation from one grain to another. For example, Arafin and Szpunar found that low-angle and special coincident site lattice boundaries in API X-65 pipeline steel were more resistant to crack propagation compared to high-angle boundaries.69 The use of EBSD, particularly when combined with other tools such as FIB and 69 M.A. Arafin and J.A. Szpunar, A new understanding of intergranular stress corrosion cracking resistance of pipeline steel through grain boundary character and crystallographic texture studies, Corrosion Science 51:119-128, 2009.
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Research Opportunities in Corrosion Science and Engineering microstructural modeling, may lead to the discovery of textured, low-cost materials resistant to SCC and other environmentally induced crack processes. Terahertz Acoustical and Electromagnetic Spectroscopy One of the real measurement issues facing the corrosion scientist and engineer involves detecting and characterizing “hidden” corrosion, which ranges from corrosion in structures underlying the tiles on the space shuttle and corrosion of the steel reinforcement bars used in concrete bridges, roads and structures, to corrosion and blistering under paints and other types of protective and decorative coatings. An approach whose use has been growing recently is the use of both electromagnetic (EM) methods in the terahertz frequency (often between 300 GHz and 3 THz) range and acoustic spectroscopy/microscopy in related frequency ranges, as well as mixed-mode methods sometimes identified as pulsed laser acoustics or photoacoustic near-infrared spectroscopy. Reviewed in several references,70 the EM methods occupy a niche in EM nondestructive evaluation (NDE) used in studies of corrosion under tiles in the space shuttle.71 Acoustic methods, often described under acoustic microscopy,72 have been used recently in studies of corrosion-related blistering in organic coatings.73 Laser-induced acoustics to study corrosion at interfaces has also been developing quite rapidly.74 All of these methods seek to identify hidden sites of corrosion, characterize the events and processes occurring at these hidden interfaces, and provide images, numerical characterization, or mechanistic interpretation of results. Blistering, adhesion loss, and degradation of protective coatings have been studied in detail by these methods. 70 D.L. Woolard, R. Brown, M. Pepper, and M. Kemp, Terahertz frequency sensing and imaging: A time of reckoning future applications, Proceedings of the IEEE 93:1722-1743, 2005; M.C. Martin, U. Schade, P. Lerch, and P. Dumas, Recent applications and current trends in analytical chemistry using synchrotron-based Fourier transform infrared microspectroscopy, Trends in Analytical Chemistry 29:453-463, 2010. 71 E.J. Madras et al., Application of terahertz radiation to the detection of corrosion under the shuttles thermal protection system, pp. 421-428 in Review of Progress in Quantitative Nondestructive Evaluation, Volume 27, 2008. 72 A. Briggs, Acoustic Microscopy, Clarendon Press, Oxford, U.K., 1992. 73 M. Doughtery and J.M. Sykes, A quantitative study of blister growth on lacquered food cans by scanning acoustic microscopy, Corrosion Science 50:2755-2772, 2008; I. Alig, S. Tadjbach, P. Krüger, H. Oehler, and D. Lellinger, Characterization of coatings systems by scanning acoustic microscopy: Debonding, blistering and surface topology, Progress in Organic Coatings 64:112-119, 2009. 74 J. Vollmann, D.M. Profunser, A.H. Meier, M. Döbeli, and J. Dual, Pulse laser acoustics for the characterization of inhomogeneities at interfaces of microstructures, Ultrasonics 42:657-663, 2004; A. Blouin, C. Neron, and L.P. Lefebvre, Nondestructive structure characterization by laser-ultrasonics, pp. 441-444 in MetFoam 2007—Proceedings of the 5th International Conference on Porous Metals and Metallic Foams, DEStech Publications, Inc., Lancaster, Pa., 2008.
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Research Opportunities in Corrosion Science and Engineering Combined Techniques and Tools One of the great hurdles in the study of corrosion is how remarkably difficult it is to make direct measurements of the rate of corrosion. Direct measurement is especially problematic because corrosion is often highly heterogeneous and sometimes takes place in cavities such as pits shielded by metal in wet environments. Electrochemical measurements cannot give spatial information: the traditional method is serial sectioning, but this causes the pit or intergranular corrosion site to cease to exist, so that it is not possible to obtain time-dependent information. The second greatest need beyond the need for information on aspects of hidden corrosion is the acute need to acquire information linking the electrochemical properties and other damage-related phenomena—such as cracking, pitting, locally dissolving, dealloying, and even hydriding—to the chemical and structural characteristics of the metallic alloy in question with spatial resolution. Traditionally this has been done with separate yet similar sites or separate specimens analyzed using separate techniques. In other words, no one corrosion site can be simultaneously subjected to multiple characterization probes. Another way to state this need is that there is an acute need for several types or channels of information (structural, chemical, electrochemical, hydrogen, and information from electrical and chemical spectroscopy) at the same time and at the same location on a surface with nanometer-scale resolution in situ. Connected to this need is the desire to look at more than one corrosion site, such as several grain boundaries, so that results from one isolated boundary or crack tip are not accidentally taken to represent the average or most prevalent behavior in the entire material. This has been a traditional shortcoming of STEM work capable of structural and chemical characterization of a slice of a crack tip. The effort expended is enormous to section and characterize just a few crack tips. Recent advances in three-dimensional microtomography as discussed below present the opportunity to map hidden damage in three dimensions and in real time.75 Advances in the three-dimensional atom probe tomography, three-dimensional secondary ion mass spectroscopy, and three-dimensional techniques focused ion beam sectioning are all helpful, but the need exists to combine these approaches into a supertool than can raster over large areas and then focus on sites of interests with high resolution. Electrochemical Impedance In the last several decades there has been an explosion of techniques that can probe instantaneous corrosion rates, including electrochemical impedance, har- 75 S. Lozano-Perez, D.W. Saxey, T. Yamada, and T. Terachi, Atom-probe tomography characterization of the oxidation of stainless steel, Scripta Materialia 62:855-858, 2010.
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Research Opportunities in Corrosion Science and Engineering monic and electrochemical frequency modulation methods, and electrochemical noise methods. One approach to determining rates of corrosion and gaining mechanistic insight about corrosion of a metal involves the electrochemical impedance (sometimes known as AC impedance) method. Over the past 30 years this method has received much attention in corrosion and has shed a good deal of additional light on corrosive processes. It also has applicability to corrosion sensors. In this technique, typically a small-amplitude sinusoidal potential perturbation is applied to the subject alloy at a number of discrete frequencies, ω. At each one of these frequencies, the resulting current waveform will exhibit a sinusoidal response that is out of phase with the applied potential signal by a certain amount (Φ) and has a current-amplitude that is inversely proportional to the impedance of the interface. The electrochemical impedance, Z(ω), is the frequency-dependent proportionality factor that acts as a transfer function by establishing a relationship between the excitation voltage signal and the current response of the system. The electrochemical impedance is a fundamental characteristic of the electrochemical system it describes and contains information on the resistance to charge transfer, mass transfer, and ohmic resistive processes. Knowledge of the frequency dependence of impedance for a corroding system allows a determination of an appropriate equivalent electrical circuit describing that system. The method has application to organic coatings, bare metals, passive films, and other corrosion-related applications. Challenges still remain, however, such as determination of corrosion rates under coatings and at defects. Both harmonic and electrochemical frequency modulation (EFM) methods takes advantage of nonlinearity in the E-I response of electrochemical interfaces to determine corrosion rate. A special application of harmonic methods involves harmonic impedance spectroscopy. The EFM method uses one or more AC voltage perturbations in order to extract corrosion rate. In the most often used EFM method, a potential perturbation by two sine waves of different frequency is applied across a corroding metal interface. The E-I behavior of corroding interfaces is typically nonlinear such that such a potential perturbation in the form of a sine wave at one or more frequencies can result in a current response at the same and at other frequencies. The result of such a potential perturbation is various AC current responses at various frequencies such as zero, harmonic, and inter-modulation. The magnitude of these current responses can be used to extract information on the corrosion rate of the electrochemical interface or conversely the reduction-oxidation rate of an interface dominated by redox reactions, as well as the Tafel parameters. This is an advantage over polarization resistance and electrochemical impedance spectroscopy (EIS) methods. A special extension of the method involves harmonic impedance spectroscopy whereby the harmonic currents are converted to harmonic impedance values at various frequencies through knowledge of the magnitude of the AC perturbation. Electrochemical noise analy-
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Research Opportunities in Corrosion Science and Engineering sis can provide a parameter called the electrochemical noise resistance, Rn. It is desirable to utilize this parameter in an analogous fashion as the polarization resistance or EIS. This method also has application in sensors, but interpretation is often controversial. The future direction in these techniques is to extend them to heterogeneous corrosion processes on smaller length scales, and in combination with other spectroscopies such as local atom and molecular spectroscopies in such a way that spatial and temporal information is given from the same surface at once. Still, hidden corrosion remains a challenge, as do sensor interpretation and subsequent decision-making algorithms. Near-Field Scanning Optical Microscopy76 Near-field scanning optical microscopy (NSOM) is a technique for nano-structure investigation that breaks the far-field resolution limit by exploiting the properties of evanescent waves. This is done by placing the detector very close (at a distance much smaller than wavelength λ) to the specimen surface, thus allowing for inspection of the surface with high spatial, spectral, and temporal resolving power. With this technique, the resolution of the image is limited by the size of the detector aperture and not by the wavelength of the illuminating light in the traditional wave optic limit. In particular, lateral resolution of 20 nm and vertical resolution of 2 to 5 nm have been demonstrated. As in optical microscopy, the contrast mechanism can be easily adapted to study different properties, such as refractive index, chemical structure, and local stress. Dynamic properties can also be studied at a subwavelength scale using this technique. It is possible to take advantage of the various contrast techniques available to optical microscopy through NSOM but with much higher resolution. By using the change in the polarization of light or the intensity of light as a function of the incident wavelength, it is possible to make use of contrast-enhancing techniques such as staining, fluorescence, phase contrast, and differential interference contrast. Staining and fluorescence have large applications in corrosion, especially if the staining or fluorescence indicates key chemicals that are significant to corrosion reactions. These methods have been applied to corrosion already but seldom are coupled to an array of methods. It is also possible to provide contrast using the change in refractive index, reflectivity, local stress, and magnetic properties, among others. Instead of performing imaging of a surface, various near-field spectroscopy methods can be applied to the study of corrosion, such as micro-Raman and surface-enhanced micro-Raman or other atomic and molecular microscopy techniques. 76 Paragraphs one and two reproduced from http://en.wikipedia.org/wiki/Near-field_scanning_optical_microscope.
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Research Opportunities in Corrosion Science and Engineering Scanning Probe Microscopy Scanning probe microscopy (SPM) makes it possible to form images of surfaces using a physical probe that scans the specimen. An image of the surface is obtained by mechanically moving the probe in a raster scan of the specimen, line by line, and recording the probe-surface interaction as a function of position. SPM began with the invention of the scanning tunneling microscope in 1981. Many scanning probe microscopy techniques can image several interactions simultaneously. The manner of using these interactions to obtain an image is generally called a mode. The resolution varies somewhat from technique to technique, but some probe techniques reach a rather impressive atomic resolution, owing largely to the ability of piezoelectric actuators to execute motions with a precision and accuracy at the atomic level or better on electronic command. One could rightly call this family of techniques “piezoelectric techniques.” The other common denominator is that the data are typically obtained as a two-dimensional grid of data points, visualized in false color as a computer image.77 A number of methods have applications in studies of corrosion, such as Kelvin probe atomic force and chemical force and scanning tunneling methods. Of these techniques, atomic force microscopy and scanning tunneling microscopy are the most commonly used for roughness measurements. NSOM and SNOM are scanning probe methods used to obtain optical imaging or some form of contrast. Summary Observations on Instrumentation Some of the techniques and tools outlined above have been used in corrosion research over the past 20 years. The future direction for these techniques is extension to heterogeneous corrosion processes on smaller and smaller length scales, and combination with other spectroscopies such as local atom and molecular spectroscopies in ways such that spatial and temporal information is given from the same surfaces at once. Hidden corrosion remains a challenge, as do sensor interpretation and subsequent decision-making algorithms. Pan and Leygraf78 have combined atomic force microscopy with scanning electrochemical methods to obtain a coordinated x-y surface view of metrology and electrochemical reactivity. The future for instrumentation is to expand the number of channels of information and to ensure sufficient dynamic range to sample large enough areas so as to obtain a clear picture about corrosion in complex materials. 77 Paragraph reproduced from http://en.wikipedia.org/wiki/Scanning_probe_microscopy. 78 A. Davoodi, J. Pan, C. Leygraf, and S. Norgren, Integrated AFM and SECM for in situ studies of localized corrosion of Al alloys, Electrochimica Acta 52(27):7697-7705, 2007.