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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2023. Corrosion of Buried Steel at New and In-Service Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/26686.
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1

Introduction

Steel is a ubiquitous and important component of the nation’s infrastructure, especially infrastructure that is partially or fully constructed beneath Earth’s surface (i.e., the subsurface). Steel may be a primary construction material (e.g., as in fuel or water pipelines) or integrated into structures for support (e.g., as in structural foundations, retaining structures, and for roof control in tunneling and mining applications). Corrosion of steel often contributes importantly to infrastructure failure, which can result in loss of life and in adverse and prolonged effects on the environment, public health, safety, and the economy. Repairs associated with corroded steel can be costly (see Box 1.1). Problems associated with corrosion of steel are not new: protection against steel corrosion was the subject of one of the very first committees convened by the National Academy of Sciences (NAS) in 1863—the year the NAS was chartered. That committee considered methods to protect the bottoms of iron ships from corrosion (True, 1913).

Corrosion can be thought of as degradation caused by environmental conditions. Corrosion mechanisms are well understood in theory, and corrosion protection practices are often employed and effective. However, the subsurface environment (e.g., unconsolidated sediments—called “soils” by geotechnical and structural engineers) in which the steel is placed is spatially and temporally complex and heterogeneous. As such, it is impossible to predict with certainty where, when, and at what rates corrosion will occur, and therefore which corrosion protection measures might be most effective. When unexpected corrosion occurs, it is often impossible to determine whether the corrosion was the result of inaccurate site characterization and prediction of environmental corrosivity, poor choice of steel design or protection, material defects, poor construction or quality control practices, insufficient monitoring or maintenance, changes in subsurface conditions, or some combination of these factors. This report summarizes the deliberations of a National Academies of Sciences, Engineering, and Medicine (the National Academies) ad hoc committee of interdisciplinary experts convened to summarize mechanisms for corrosion of steel within earth materials. The committee was asked to assess current practices for characterizing the subsurface environment for corrosivity and to assess the use, efficacy, and uncertainties of methods used to predict, identify, and monitor corrosion of steel in earth materials at existing and new infrastructure (see Box 1.2). Several sponsors representing different interests, industry sectors, and types of infrastructure came together to support this activity including the American Society of Civil Engineers, Association for Mechanically Stabilized Earth, Federal Highway Administration, International Association of Foundation Drilling, National Science Foundation, U.S. Army Corps of Engineers, and the presidents of the National Academies. The breadth of interests held by the sponsors indicates the importance of the subject matter to the nation.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2023. Corrosion of Buried Steel at New and In-Service Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/26686.
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THE COMMITTEE’S CHARGE AND INTERPRETATION

The National Academies assembled an ad hoc committee of 11 volunteers (see Appendix A for the committee member biographies) to solicit input from the technical community; to examine critically the state of practice and of art in field, laboratory, and modeling methods for characterizing corrosion of steel buried in earth materials; and to identify sources of uncertainty in those practices. Committee members had expertise in civil, corrosion, geoenvironmental, geological, geotechnical, materials, and structural engineering, as well as in geophysical methods, metallurgy, microbially influenced corrosion, and risk assessment. To the committee’s knowledge, this study represents the first attempt to identify comprehensively the uncertainties associated with characterizing, modeling, and monitoring corrosivity and corrosion of buried steel across industries by a multidisciplinary group that includes not only experts in corrosion, corrosion engineering, and metallurgy but also experts in civil, geotechnical, and structural engineering.

The committee interprets its Statement of Task (see Box 1.2) as dictating an objective assessment of practices associated with identifying and mitigating corrosion of steel in contact with earth materials throughout the life

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2023. Corrosion of Buried Steel at New and In-Service Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/26686.
×

cycle of infrastructure. The practices described are not necessarily state of the art. Materials may be “buried” (e.g., covered with earth material) or “placed” (e.g., drilled, hammered, or otherwise installed) in soils or rock. The relevant infrastructure serves a variety of purposes within different sectors, and, as such, is designed, constructed, and operated by various experts and governed by standards created by bodies serving different industries. Similarly, the extent and consequences of corrosion can vary greatly in different applications. In some civil applications (e.g., the use of steel to support a small embankment), the placement of steel components may be quite localized (e.g., a 10-meter span). In such cases, site characterization and monitoring may be relatively simple, and protection measures against corrosion may be designed specific to the site. Failure of a single component might represent minimal risk to life and safety, and may represent only short-term disruption of infrastructure operation. In other applications—for example, a 100 kilometers-long steel pipeline transporting combustible fuels—the infrastructure may be buried in numerous kinds of earth materials under different environmental conditions. Simple logistics prevent detailed characterization along the length of the pipeline, and failure at any location on the pipeline could result in, for example, loss of life, destruction of property, damage to the environment, and disruption of energy transport serving large populations. Given the array of circumstances in which steel comes in contact with earth materials, an assessment by the committee of all practices for all applications was not a reasonable undertaking.

Early in its deliberations, the committee recognized little common language surrounding key issues related to corrosion, corrosion rates, corrosivity, and corrosion protection among those from different sectors and expertise. Important terms are understood and defined differently across sectors and by professions within sectors, practices and guidance vary greatly between sectors, and knowledge does not often transfer easily across sectors. As a result, the committee found it necessary to identify and define the relevant types of steel, the different subsurface environments and conditions that affect corrosivity, the range of buried-steel applications and protection measures, and even the fundamental types of steel corrosion so that committee members had a common basis of understanding. Much of the early text of this report is primer material developed by the committee so that its own members could deliberate effectively and meaningfully. Readers of this report will need familiarity with this material to understand and apply the report recommendations and should not assume that their familiarity with specific vocabulary and concepts is consistent with terms and concepts as defined in the report.

To focus the broad task assigned to the committee, the report does not cover “best practices,” steel in concrete (except reference to cementitious grouts), steel in marine environments, or interior pipeline corrosion. Steels protected with grouts are considered because grouts form a low-volume matrix that is integrated into the soil in contact with the buried steel. Stainless steels are mentioned in this report but not considered extensively.

COMMON TYPES OF STEEL USED IN BURIED APPLICATIONS

Steel is a metal composed of iron mixed with various metallic (e.g., manganese) and nonmetallic (e.g., carbon) elements in low concentrations to create alloys fit for different purposes. With careful control of composition and processing, steels can possess a wide range of properties such as strength, ductility, and corrosion resistance. The choice of steel for a particular application is based on the properties required, cost, and availability. The wide variety of steels available can be classified differently, including based on composition, properties (e.g., strength and ductility), and application. Steels of interest in this report are those placed commonly in the subsurface. These include plain carbon steel (including plain low-carbon or mild steel, plain medium-carbon steel, and plain high-carbon steel), high-strength low-alloy steel, and cast iron/ductile iron (see Table 1.1 for descriptions). These classifications are based on the composition, including the content of carbon and other alloying elements. Steels may also be classified according to yield strengths before nonrecoverable deformation occurs. Steels may be assigned grade numbers to reflect the minimum yield strength in units of kilopounds per square inch (ksi), regardless of composition.1

The inclusion of carbon in steel strengthens it but increases its brittleness and decreases its weldability. This makes optimization of the carbon content dependent on the application. Most structural applications use plain low--

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1 Common steel grades used currently in subsurface applications are Grades 50, 65, and 80. Grade 36 steel was used historically for some buried steel components.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2023. Corrosion of Buried Steel at New and In-Service Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/26686.
×

TABLE 1.1 General Classification of Steels

Steel Type Common Name Alloy Composition Underground Use Comments
Plain carbon steela Plain low-carbon steel or mild steel <0.3 wt % carbon Common; structural, pipelines Often has a protective coating in geo-civil industries.
Plain medium-carbon steel 0.3–0.6 wt % carbon Pipelines, structural bolts, bolted connections, splices of structural components Pipelines are typically coated with bonded dielectric coatings and protected by cathodic protection.
Plain high-carbon steel 0.6–1 wt % carbon High-strength steel bars used in anchorages High strength but low ductility, prone to brittle-type failure.
Cast iron Gray iron 2.5–4.0 wt % carbon, 1.0–3.0 wt % silicon Pipelines installed in the United States circa 1810–1970s Corrosion of iron occurs preferentially to carbon leaving a graphite skeleton. It may come with black asphaltic coating.
Ductile iron 3.0–4.0 wt % carbon, 1.8–2.8 wt % silicon Pipelines introduced in the United States circa 1955 as an improvement to gray iron Usually comes with black asphaltic coating and wrapped in polyethylene encasement (ANSI/AWWA C105/A21. 2018).
Alloy steel High-strength low-alloy steel Low carbon and small amounts of cobalt, nickel, chromium, molybdenum, vanadium, nitrogen, etc. Pipelines More corrosion resistant in some environments (Fletcher, 2005; Shreir et al., 1994) but susceptible to hydrogen embrittlement.
Stainless steel >11 wt % chromium and often other elements Rare Very corrosion resistant in some environments.

a Often contain small amounts of manganese, phosphorus.

SOURCES: Stefanescu (1990); Washko and Aggen (1990).

carbon steel (also known as mild steel). This type of steel contains less than 0.3 percent carbon. Higher-strength steel such as high-strength low-alloy steels may sometimes be used. Cast iron often has a carbon content above 2.5 percent and is commonly used for water distribution pipelines. Ductile iron is an innovative type of graphite-rich cast iron, with graphite incorporated into the metals in nodules rather than flakes, as is the case in cast iron. Carbon included in this manner provides improved impact and fatigue resistance.

“Alloy steels” are those that contain major percentages of alloying elements in addition to carbon. Stainless steel, for example, results when chromium is included in percentages higher than 11 percent. There are numerous types of stainless steel distinguished by concentrations of carbon, chromium, nickel, molybdenum, manganese, titanium, and other elements. Stainless steels are sometimes used in buried-steel applications, but initial costs are prohibitive, and there is little guidance on the selection of stainless steel type given for the environment and application. None of the common stainless steels are completely resistant to corrosion, and failures have occurred when used improperly. Some governing bodies do not recommend the use of stainless steels (e.g., AASHTO, 2020).

Other alloy steels are used in an array of buried infrastructure applications, from general structural to welded pipe, welded wire, high-temperature applications, welded and bolted connections on buildings and bridges, and soil reinforcements (see Table 1.1). Many civil engineering application designs generally specify alloys that have performed well previously and are offered “off the shelf” by manufacturers. In contrast, pipeline industries often use steels developed for specific applications. Except high-strength steels, which are susceptible to hydrogen embrittlement, the committee could find no evidence in the literature of significant differences in corrosion rates of the different types of plain carbon steels in buried applications (see Chapter 4). Alloy composition is considered of secondary importance to corrosion rates when compared to variable corrosivity found in natural and engineered subsurface environments.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2023. Corrosion of Buried Steel at New and In-Service Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/26686.
×

INFORMATION GATHERING AND REPORT ORGANIZATION

The study committee was convened in 2020, and all committee information gathering and deliberations were conducted remotely as a result of the ongoing COVID-19 pandemic. Although the committee only met remotely, members were able to draw information from multiple resources including from many experts with a variety of expertise. The committee held two information-gathering sessions, including a 2-day virtual workshop. Agendas for those meetings can be found in Appendix B. In addition to information gathered through talks and panel discussions during those meetings, the committee relied on its collective and extensive expertise, held one-on-one discussions with experts on a variety of topics, and consulted the published and unpublished literature, instrumentation manuals, and the standards and regulations established for multiple industries.

As discussed earlier, corrosion scientists, geotechnical engineers, and structural and civil engineers have developed their knowledge of corrosion of buried steel and corrosion protection almost independently, and their vocabularies have evolved in response to practices in each of the different fields. Because the committee had to develop a common vocabulary to deliberate its charge and prepare this report, it was necessary for the committee to go back to the very basics. This report reflects that need and provides definitions of specific terms that may not be consistently defined in different sectors.

Chapter 2 of this report provides a description of the fundamentals of corrosion and the committee’s observations about the general approaches taken by different industries to address corrosion of buried steel. The text describes some fundamental vocabulary associated with corrosion, the understanding of which was critical to committee deliberations. The text also describes different buried-steel applications, and the sources of data that inform industry knowledge and decision making related to the design and protection of buried steel infrastructure. Chapter 3 describes the complexities of the natural and engineered subsurface environments that contribute to the corrosivity of the environment and the difficulties in characterization. Chapter 4 describes different mechanisms for the corrosion of buried steel and how that corrosion is manifested on the steel surface, while Chapter 5 describes different industry methods to protect against that corrosion. Chapter 6 provides a description of different laboratory- and field-based methods available to characterize the subsurface for corrosive conditions, and Chapter 7 provides a description of standard infrastructure monitoring techniques and practices. Chapter 8 provides descriptions of different modeling techniques applied in different industries. Chapter 9 provides an assessment and conclusions regarding current and emerging practices, and provides recommendations regarding how practice could be improved and research that might be undertaken to increase basic understanding of corrosion of buried steel that leads to better infrastructure-related decision making.

Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2023. Corrosion of Buried Steel at New and In-Service Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/26686.
×

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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2023. Corrosion of Buried Steel at New and In-Service Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/26686.
×
Page 11
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2023. Corrosion of Buried Steel at New and In-Service Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/26686.
×
Page 12
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2023. Corrosion of Buried Steel at New and In-Service Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/26686.
×
Page 13
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2023. Corrosion of Buried Steel at New and In-Service Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/26686.
×
Page 14
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2023. Corrosion of Buried Steel at New and In-Service Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/26686.
×
Page 15
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2023. Corrosion of Buried Steel at New and In-Service Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/26686.
×
Page 16
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Steel is a common component of U.S. infrastructure, but that steel can corrode when buried in soil, rock, or fill. Steel corrosion is estimated to cost the United States 3-4 percent of its gross domestic product every year, and it can lead to infrastructure failure, loss of lives, property, disruption of energy and transportation systems, and damage to the environment. Although the mechanisms of steel corrosion are well understood, limited data on subsurface corrosion and the inability to measure corrosivity directly make accurate corrosion prediction through modeling a challenge. When hazardous levels of corrosion does occur, it is difficult to determine whether the cause was related to site selection, engineering decisions, changes in subsurface conditions, or a combination of these factors.

This report explores the state of knowledge and technical issues regarding the corrosion of steel used for earth applications (e.g., for ground stabilization, pipelines, and infrastructure foundations) in unconsolidated earth or rock in different geologic settings. The report summarizes mechanisms of steel corrosion, assesses the state of practice for characterizing factors in the subsurface environment that influence corrosion and corrosion rates, and assesses the efficacy and uncertainties associated with quantitative, field, and laboratory methods for predicting corrosion.

The industries and experts most involved with managing buried steel should collaborate to improve multidisciplinary understanding of the processes that drive buried steel corrosion. Developing a common lexicon related to buried steel corrosion, generating new data on corrosion through collaborative long-term experiments, sharing and managing data, and developing new data analytical techniques to inform infrastructure design, construction, and management decisions are key. Industries, experts, and regulators should collaboratively develop decision support systems that guide site characterization and help manage risk. These systems and new data should undergird a common clearinghouse for data on corrosion of buried steel, which will ultimately inform better and more efficient management of buried steel infrastructure, and protect safety and the environment.

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