Corrosion of Buried Steel at New and In-Service Infrastructure (2023) / Chapter Skim
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6 Standard and Evolving Subsurface Characterization
6 Standard and Evolving Subsurface Characterization
Pages 57-76
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From page 57... ...
This chapter explores current field and laboratory subsurface characterization techniques that inform the prediction, assessment, and modeling of soil corrosivity and corrosion rates of steel structures in the subsurface, as well as how those techniques are evolving. Although there are numerous techniques used to characterize the subsurface, this chapter focuses on those associated with understanding corrosivity of the subsurface environments and the corrosion rates of steel placed in them.
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From page 58... ...
Most soil corrosivity testing is conducted traditionally through laboratory testing of bulk samples collected by disturbed sampling at the site and not from in situ field testing. BOX 6.1 Definition of Terms Characteristic: Observation of a material that does not vary regardless of testing condition (e.g., grain size)
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From page 59... ...
In some cases, especially for large geo-civil infrastructure projects or oil and gas pipelines, in situ field corrosivity testing may also be conducted to complement laboratory testing of field samples. The principal reason is that steel may penetrate multiple soil strata, and the lateral character of the soil conditions may also vary significantly over the project site.
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From page 60... ...
For example, resistivity and pH can be measured via AASHTO T 288-12 (2016) and AASHTO TABLE 6.1 Example Laboratory Test Methods Consistently Cited as Important in the Identification of Corrosive Soils Characteristic/ Property Importance Method Example Standards Soil Unified Soil Classification System used to Determine soil grain size using sieve ASTM D422-63 (2016)
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From page 61... ...
indicates that there are roughly 20 soil characteristics and properties that have been identified as contributors to corrosion of buried steel. Of those, the following seven are consistently cited as important in the identification of corrosive soils: soil type, groundwater table, moisture content, resistivity, pH, chlorides, and sulfates (NCHRP, 2017)
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From page 62... ...
. TABLE 6.4 American Water Works Association Point System for Evaluating Soil Corrosivitya Soil Characteristic or Property Values Points Resistivity (ohm-cm)
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From page 63... ...
Grain size X Plasticity Index X Vertical/horizontal X X homogeneity Water table level X X X X X X X Resistivity X X X X X X X X X X Moisture content X X X X X X pH X X X X X X X X X X Carbonate X Total alkalinity/acidity X (buffer capacity) Chloride X X X X X X X X Sulfate X X X X X X X Sulfides/hydrogen sulfide X X X X Cinder and coke X X X X Redox potential X X Sulfate-reducing bacteria X Contamination by deicing X X salts, manure, fertilizers, leaking sewer, industrial pollution Stray currents X X X a Rows in gray are those that are commonly cited as important across classification and rating schemes.
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From page 64... ...
Therefore, will result of the in different soil absence of supporting information for all applications of electrical resistivity (Binley and Slater, including two soils same electrical electrical conductivity measurements if all other properties remain successfulthe same.determining As a result,a corrosion soil electrical conduc resistivity may exhibit widely varying soil corrosivity. One thing is certain, electrical resistivity has never been at accurately rate.
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From page 65... ...
Electrical resistivity and EMI are most commonly used as field methods when considering soil corrosivity, likely because both measure electrical resistivity and because there are laboratory techniques that can be conducted in parallel. Because the subsurface is highly heterogeneous, field electrical resistivity methods require different considerations and may produce different results from those applied in the laboratory.
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From page 66... ...
The data then are converted to a two-dimensional plot of apparent resistivity, and the true resistivity distribution of the subsurface is obtained through an inversion process. Note that there are inherent limitations to directly using multielectrode field resistivity measurements to determine soil corrosivity,
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From page 67... ...
(2019) supported this finding using field electrical resistivity measurements col lected with more electrodes over a larger area and supplementary electrical resistivity tests on soil boxes at varying degrees of saturation.
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From page 68... ...
or drag a sensor via a sled behind a vehicle and set the device to collect data linked with global positioning system locations every few seconds along a pipeline corridor and rapidly collect continuous apparent electrical conductivity measurements. Assessing Impact of Stray Currents As discussed in Chapter 2, stray electrical currents may exist around direct current (DC)
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From page 69... ...
Examples of characterizing subsurface stray currents include identifying the distribution of potential gradients between the current source and buried steel of interest. The source might be stray-current leakage from, for example, DC-powered transit systems (Sankey and Hutchinson, 2011)
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From page 70... ...
Despite these standard practices, assessing the likelihood of MIC remains difficult because stratified biofilms can often create highly localized conditions that may not reflect highly averaged field surveys of heterogeneous subsurfaces or even samples of soils taken for laboratory analysis. Additionally, the type of infrastructure may affect the susceptibility to MIC (see Box 6.5)
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From page 71... ...
Leaks in water distribution systems, which may be ignored from a safety perspective, can contribute oxygenated, treated water to otherwise dry, anaerobic pipe–soil interfaces. STANDARD PRACTICE DURING OPERATIONS AND AFTER FAILURE Subsurface characterization after the initial site investigation is not routinely performed as part of operation and maintenance of facilities with buried steel within the geo-civil industries.
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From page 72... ...
Resistivities, chlorides, sulfates, and microorganisms are examples of the types of properties that are evaluated to determine whether cathodic protection and a reapplication of protective coating is needed. Standard Practice for Characterizing Microbially Influenced Corrosion During Operations and After Failure Detailed test methods for diagnosing MIC after it has occurred are available (NACE TM0106, 2016)
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From page 73... ...
ASPIRATIONAL BIOCEMENTATION METHODS FOR CONTROL OF ENVIRONMENT Biocementation is an aspirational technology that could impact corrosion of buried steel structures. Bacteriamediated calcium carbonate (CaCO3)
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(b) FIGURE 6.5 Measurements of resistivity and galvanized steel corrosion rates (estimated from weight loss and thickness measurements)
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From page 75... ...
STANDARD AND EVOLVING SUBSURFACE CHARACTERIZATION 75 FIGURE 6.6 Simplified version of a decision flowchart uses grain size to dictate test methods and interpretation. This figure describes the decisions related to coarse-grained materials.
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From page 76... ...
76 CORROSION OF BURIED STEEL AT NEW AND IN-SERVICE INFRASTRUCTURE FIGURE 6.7 Simplified version of a decision flowchart uses grain size to dictate test methods and interpretation. This figure describes the decisions related to fine-grained materials.
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