Research Opportunities in Corrosion Science and Engineering (2011) / Chapter Skim
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3 Research Opportunities
3 Research Opportunities
Pages 53-120
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From page 53... ...
. 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.
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scales. For example, many corrosion processes are now known to be controlled by molecular-, submicrometer-, and micrometer-scale phenomena.
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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 corrosionresistant 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.
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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 Trans formational Discipline for Improed Competitieness and National Security, The National Academies Press, Washington, D.C., 2008.
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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.
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From page 58... ...
• Identifying the elements that act in synergy with other major alloying elements to enhance the intrinsic effects of corrosion mitigating elements. For instance, addi tions 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.
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From page 59... ...
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 • Deeloping an affordable, manufacturable, high-strength pipeline steel that is highly corrosion resistant.
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Corrosion resistance is often secondary, although major alloying elements have been incorpo rated 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.
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Protectie Coatings The ICMSE approach is also applicable to the development of corrosionresistant 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.
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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 protec tion 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 aggres sive environments.
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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.
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The development of such scale should incorporate detailed understanding at the molecular scale in order to fine-tune properties appropriately.19 Corrosion Enironments -- Properties of the Ideal Coating for Aqueous Corrosion Resistance An ideal coating (Figure 3.2) should have a very hydrophobic surface and should form high barriers to water and electrolytes.
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If one can get cross-diffusion across an interface without creating excess chemical reactivity or mechanical stress, one gets true improvements in adhesion. If the coating is applied as a liquid layer, no defects in the application and film formation processes are acceptable (this is why electro-deposited paints such as those used in automobiles are so excellent at corrosion protection)
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Khatak, and B Raj, Anodic and cathodic protection, Chapter 5, Section 5.1 in Encyclopedia of Electrochemistry: Volume , Corrosion and Oxide Films (A.
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Hence, research in developing a new robust, accurate reference electrode is needed in order to extend active corrosion protection to areas such high-temperature systems.22 Another research need for anodic protection is alloys designed to be effectively self-driven to a stable, self-healing passive state under ambient conditions. This is the current mechanism for the corrosion resistance of stainless steels in oxygenated near-neutral water.
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However, there is a need for additional research to determine promising materials combinations. CgC II: High-Fidelity Modeling for the Prediction of Corrosion Degradation in Actual Service Environments Corrosion grand challenge II entails developing a better understanding of cor rosion mechanisms and morphologies, coupled with the environments in which they operate, and the fusion of the knowledge, data, models, and tools available into more accurate -- and widely descriptive -- models of materials and environments for all types of corrosion.
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However, the emphasis to date has been on tools to optimize mechanical and physical properties within a target cost range, and corrosion resistance has rarely been directly considered in ICMSE except by extension of physical properties. Figure 3.4 shows
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The outputs might include initiation, propagation, and arrest of selected corrosion phenomena. Advances in computational methods and enhanced understanding of stages of corrosion processes create an opportunity for solid advancement, especially if gaps in the fundamental knowledge of corrosion are pursued.
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Such prediction is now possible in a limited number of alloy systems but is in its infancy -- for instance, the effects of all alloying elements are not incorporated, and broad applicability to a range of environments in uncertain. Moreover, although some progress has been made in grain boundary engineering using percolation theory and in interface engineering for single species on simple boundaries, the design of materials via tailoring of the composition of major, minor, and trace elements is not yet fully developed.
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• Modeling at multiple length scales ranging from atom to component scales, and modeling of the damage from corrosion in three dimensions. • Further scientific progress required to completely understand the details of corrosion mechanisms at multiple length scales.
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Because corrosion phenomena are complex, deterministic models evolve continually as restrictive hypotheses are eased when additional, empirical knowledge is acquired. In essence, it is the scientific method that nudges a model to reality.
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For example, if an empirical model is developed based on observation of the corrosion of iron in an acidic medium, the use of that model for the corrosion of the same iron in a basic medium will give inadequate results. Empirical corrosion models can benefit by adapting techniques from the field of artificial intelligence (AI)
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28 D.D. Macdonald et al., The deterministic prediction of corrosion damage, in L.L.
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Model Integration Model integration and multiscale modeling are critical if modeling is to be useful in predicting the behavior of materials in specific environments. Opportu nities exist to integrate models across length and time scales, and to describe each step in the corrosion evolutionary path, beginning with initiation of corrosion damage, in order to produce macroscopic predictions from nano- and microscale observations.29 One example of model integration is mixed deterministic-statistical modeling where fundamental laws of corrosion are expressed in the model where they are known.
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There is no reason why this approach cannot be extended to the corrosion field to benefit both the understanding of material behavior and prognosis. Additionally, because materials degrade through processes other than corrosion, such as fatigue and creep, a complete set of modeling tools would incorporate corrosion modeling with these other degradation models.
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that has been the mainstay of structural mechanics for the past several decades.31 The stress intensity factor was developed to predict the behavior of pre-existing flaws in structural materials and the even tual life of a component under conditions in which the flaw develops into stable cracks. The power of K is in the concept of similitude: well-defined cracks and crack tips that are different in size or shape but possess the same K (as determined by geometry, loading, and the theories of linear-elastic fracture mechanics)
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Many industries now regularly use fracture mechanics in the design and maintenance of their engineering structures. Moreover, damagetolerant design can specify the size and shape of defects, plus load levels that can or cannot be tolerated according to a specification of critical stress intensity factors for crack growth or sudden fracture.
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A science-based parameter would allow quantitative connections between corrosion rates observed in laboratory exposures and those found in field environments and might also allow databases on materials in one environment to be translated into equivalent corrosion behavior in another environment, such as one with lower concentrations of aggressive species. The 32 The corrosion intensity factor describes the product of driving force and time in one parameter such that the equivalent CIF would produce the same mode and amount of damage whether expo sure occurred in a laboratory test, a field test, or during component service.
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It should also be pointed out that a corrosion intensity factor is not a replacement for deterministic or some other form of modeling to predict behavior and performance. In fact, deterministic models of sulfidation exist, but there is still the acute need for relevant accelerated testing.
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Understanding the Corrosion Processes That Occur During Accelerated Testing Current accelerated testing methods are empirically correlated with field per formance, and acceleration factors are often deduced from relative differences in the evolution of damage over time. However, there is often little understanding of why a given method succeeds or whether it will be effective on another coating, material, or in another environment, because the rules that determine its ap plicability to other situations are not understood.
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Scientists are currently working on accelerated laboratory tests that do crack these latter materials and are developing statistical methods to predict when and how they will eventually fail in service. CgC IV: Accurate Forecasting of Remaining Service Time until Major Repair, Replacement, or Overhaul Becomes Necessary -- i.e., Corrosion Prognosis Corrosion grand challenge IV addresses the critical need for better methods to monitor the actual deterioration of a component once it is placed in service in a corrosive environment, analyze that information, and, based on decision-making algorithms (or "reasoners")
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Effective corrosion mitigation and control require appropriate action at the correct time. Since many engineered structures include multiple components and corrosion processes are cumulative, periodic inspection is often not the optimal procedure for identifying areas requiring timely action to mitigate corrosion.
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Electrochemical methods of monitoring suffer drawbacks and limitations related to deposition of corrosion products and changes in a corrosion medium's conductivity that may affect the sensor output. Furthermore, most corrosion sensor technologies are not capable of accurately identifying localized corrosion or clearly evaluating cumulative corrosion processes.
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SOURCE: Courtesy of J.R. Scully, Evaluation of State-of-the-Art and Emerging Innovative Corrosion Rate Sensors for Monitoring Steel Bridges in Various Corrosive Environments, ERDC, 2009.
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imbedded film, long for imbedded type noble electrode type imbedded Maybe Yes Yes Yes Yes Yes Yes Yes Yes Yes Maybe Maybe Yes Yes
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Adapted from Defense Science Board, Corrosion Control, Final Report ADA428767, October 2004, available at http://www.acq.osd.mil/dsb/reports2000s.htm, and based on information in Ammtiac Quarterly, Vol.
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From page 89... ...
Recent developments include the use of fiber optic sensors to monitor pipeline corrosion and stress corrosion cracking, the development of both surface and penetrating sensors aimed at detecting cracks and corrosion in bridges, laser scanner systems that are being tested for evaluating urban structures' integrity and fluorescing nanomaterial coatings to reveal when and where corrosion is occurring in the substrate. Needless to say, there are many more advances that will be required in order to develop reliable sensor technologies.
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From page 90... ...
Thus, past performance and degradation of the coating up to the point of installation is challenging to match, and the rate of corrosion of the sensor surrogate material must be correlated with the corrosion rate of the structure. Work to determine atmospheric corrosion rates of metallic materials under coatings in the atmosphere is thus a high priority.
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From page 91... ...
Until researchers investigate these opportunities and materials scientists apply the results to design better materials with the needed corrosion resistance, it will never be possible to control or eliminate corrosion simply by selecting the right material. Theories from other fields of science and exciting new investigative tools can also have a great impact on the corrosion field, as the advances described in Box 3.1 and some of the success stories in Chapter 1 indicate.
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Traud, On the interpretation of corrosion processes through super position of electrochemical partial processes and on the potential of mixed electrodes, Z Electrochem.
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Advances in understanding in these areas are needed to achieve the goals of all the corrosion grand challenges. Synchrotron Radiation An irony of the study of corrosion is how remarkably difficult it is to directly measure the rate of corrosion since it is often highly heterogeneous and takes place in cavities such as pits shielded by metal in wet environments.
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Marrow, Observations of intergranular stress corrosion cracking in a grain-mapped polycrystal, Science 321:382-385, 2008.
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, scanning probe techniques, and low-voltage electron microscopy. Advances in first-principles theory, modeling approaches, and simulation provide new opportunities to generate needed understanding over important time and length scales consistent with those being accessed experimentally.
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chemical microscopy; electrochemical capillary probe microscopy; and electro chemistry mapping combined with other techniques such as micro-Raman and atomic force microscopy, scanning Kelvin probe, or scanning tunneling microscopy will allow understanding beyond that enabled by global electrochemical methods traditionally used. Societies such as the Electrochemical Society and the Interna
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From page 97... ...
Moreover, lack of understanding of processing and heat-to-heat variations or the effects of trace alloying elements further limits the predictability of the onset or growth of stress corrosion cracks. Similar considerations can apply to environmental effects on high-temperature crack growth in gaseous environments.
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Corrosion Mechanisms -- Stress and Strain Effects There is a lack of fundamental understanding of the effect of elastic tensile stress or strain and plastic strain on dissolution by either thermodynamic or kinetic interpretations. The roles of stress and/or strain in aqueous corrosion reactions occurring at the atomistic level close to room temperature have been modeled for micrometer-scale descriptions of stress corrosion cracking (SCC)
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In particular, there is no quantified understanding of how stress affects some well-known anodic dissolution SCC mechanisms, such as for pre-existing, activepath corrosion. For example, the distinction between stress-assisted intergranular corrosion and intergranular stress corrosion cracking (IGSCC)
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Because these driving forces are not well understood, the concept of accumulation of damage under stressed conditions cannot be used as a basis for quantitative life prediction. Another area that is not well understood is the role played by dynamic plastic strain in the case of hydrogen embrittlement that occurs during hydrogen produc tion taking place in either aqueous or gaseous hydrogen environments.
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For instance tomographic transmission electron microscopy methods offer the possibility of mapping dislocation structures at crack tips, and diffraction contrast tomography and three-dimensional atom probe methods can help to advance understanding of cracking phenomena inside materials. Corrosion Mechanisms -- Self-stress in the Breakdown of Passie Films and Protectie Scales Although externally applied stresses are certainly important, many corro sion processes generate in situ local stresses.
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Although there has been substantial progress in rigorously defining the chemical compositions and electrochemical conditions of crack tip solutions, there are a number of opportunities for further development of the model, including a lack of understanding of the exact mechanical proper ties of passive films at crack tips, uncertainty over bare area created by straining, and the inability to properly inventory bare surface dissolution rates once the 42 M Schütze, Protectie Oxide Scales and Their Breakdown, Wiley, West Sussex, England, 1997.
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From page 103... ...
As with stress corrosion cracking, the effects of heat-to-heat variations cannot be predicted. Consequently, there are no quantitative models or fundamental knowledge to guide the design of high-performance engineering alloys with good fracture toughness and slow crack growth rates for use in extreme environments.
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From page 104... ...
It is not always appreciated that chromium oxide is a very good electrical insulator, so that in theory one could enrich chromium to such an extent that the reduction of oxygen on top of the passive film on stainless steel would cease, thus eliminating localized corrosion in salt water. In practice, not much iron content is required to make the film rather conductive, but still, manipulation of cathodic reaction kinetics on passive films has to be considered an important challenge for the future.
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Corrosion Morphologies -- Protectie Layers Formed at High Temperature The key to achieving high-temperature corrosion resistance is the development of a continuous, protective surface layer.
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. The presence of water vapor in the environment often promotes higher oxidation rates -- particularly for chromia- or silica-formers where water can dramatically enhance oxide volatiza tion; impact oxide structure, stability, and/or solid-state transport properties in a negative way; and decrease corrosion lifetimes.48 While there has been substantial progress in finding alloys that are more resistant to water vapor or coatings to pro tect silicon-bearing ceramics in water-bearing environments,49 there is still much to be learned about effects on protective scale stability and the underlying defect mechanisms controlling transport, particularly for the more extreme environments envisioned for future technologies.50 48 E.J.
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are controlled. Models for defect engineering may be expanded to include the effects of defect size, spacing, and intrinsic defect properties or extended to the case of grain boundaries where the most corrosion-prone boundaries, their distribution, connectivity, and grain shape can all be explored computationally and experimentally to optimize resistance to intergranular corrosion.
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From page 108... ...
Time-of-wetness is known to be an important parameter in outdoor exposure, given that water associated with pre cipitation or condensation is critical to the corrosion processes. However, accurate prediction of corrosion rates depends on knowing how the water on the surface affects the concentration of all the important corrosive species.
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From page 109... ...
The committee felt strongly that it should not try to indicate which techniques and approaches hold the greatest potential for addressing the corrosion grand challenges, not only because of its inability to cover every one of the vast array of experimental and computational advances, but also, and more importantly, because the research community has historically proved adept at taking advantage of such advances when and where scientifically and technically appropriate. Examples of Relevant Techniques and Tools Atom Probe Tomography Atom probe tomography and its ability to reveal the three-dimensional atomic structure of materials has become a very powerful tool in modern materials science.51 However, its application to corrosion science has been quite limited despite the ability to finely resolve phenomena such as elemental segregation that can play such an important role in sensitization and associated stress corrosion cracking or in internal oxidation.
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Bruemmer, Study of grain boundary character along intergranular stress corrosion crack paths in austenitic alloys, Acta Materialia 49:1589-1598, 2001.
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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.
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structures and processes to be probed and could prove to be of particular value for in situ experiments. Defect distributions, structural fluctua tions, 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.
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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.
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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
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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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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.
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From page 117... ...
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, threedimensional 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.
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The method has application to organic coatings, bare metals, passive films, and other corrosion-related applica tions. Challenges still remain, however, such as determination of corrosion rates under coatings and at defects.
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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 combina tion 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.
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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 exten sion 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.
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