Corrosion of Buried Steel at New and In-Service Infrastructure (2023) / Chapter Skim
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4 Corrosion of Buried Steel
4 Corrosion of Buried Steel
Pages 35-48
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From page 35... ...
GENERAL CORROSION Figure 4.1 illustrates the reactions occurring at anodic and cathodic sites on a buried steel component. Iron dissolution occurs at anodic sites, and reduction reactions occur at closely located cathodic sites.
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From page 36... ...
. The fundamental understanding of corrosion kinetics as expressed by the Tafel equation can be used in an accurate determination of corrosion rate (corrosion current density)
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From page 37... ...
LOCALIZED CORROSION MECHANISMS FOR BURIED STEEL As shown in Figure 4.1, anodic and cathodic sites can continually move across the surface with time, causing general corrosion or uniform steel loss. However, when the anodic and cathodic sites are spatially fixed (see Figures 4.3–4.7)
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From page 38... ...
Plain carbon steels commonly used in underground environments may form a protective iron oxide layer if the local pH is relatively high. As the oxide layer begins to break down, for example, through reactions with dissolved ions such as chlorides, the steel becomes susceptible to pitting corrosion.
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From page 39... ...
The occlusion causes a physical separation of anodic and cathodic sites because of the limited supply of oxygen (the common cathodic reactant) in the crevice.
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From page 40... ...
The zinc will corrode preferentially to the steel and the underlying steel will not be consumed until the zinc coating is exhausted, thus affording corrosion protection to the steel. Macrocell Corrosion In a variety of steel structures including piles, soil nails, or pipelines (see Table 2.1 for descriptions)
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From page 41... ...
They often encounter buried metallic structures including buried utility pipes and cables, underground storage vessels, and reinforced concrete structures. This type of corrosion is most commonly observed on structures that have large dimensions in one direction such as pipelines, and, in fact, much of the experience with stray-current corrosion is from the observed performance of pipelines.
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From page 42... ...
However, because the electrical resistivity of the steel pipe is much less than the soil, the lowest resistance follows a path through the pipe. The current enters the pipe at a location near the CP anode, resulting in a localized cathode where oxygen is reduced (cathodic reaction)
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From page 43... ...
. MICROBIALLY INFLUENCED CORROSION MIC of buried steel is the result of microbial activities, typically within biofilms that are bound to the surface of buried steel (see Chapter 3 for a description of microorganisms in the subsurface)
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From page 44... ...
Once electrical contact is established, a galvanic couple develops with the steel surface as an anode and electron transfer through the cathodic iron sulfide. Introduction of oxygen causes conversion of the sulfide back to an oxide and an immediate increase in the corrosion rate (Blackwood, 2020)
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From page 45... ...
Both structural and geotechnical engineers, with the assistance of a corrosion engineer, developed the remediation of the five piers, which included specific corrosion protection measures that would not require the removal of the suspect fill material. The investigation and remediation of the Leo Frigo Memorial Bridge required the specialty knowledge of struc tural engineers, geotechnical engineers, a hydrogeologist, a cathodic-protection specialist, and corrosion engineers, and specialty testing for the presence, amount, and kind of microbes and chemical analysis of fill and soil materials.
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From page 46... ...
. The corrosion rate of plain carbon steel in water does not vary with pH values between 4.5 and 9.5 (Coburn, 1978; Uhlig and Revie, 1985)
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From page 47... ...
has, to date, obscured any differences between steel types, future studies will benefit from significant focus on the environment, not the alloy. Statistically designed experimental studies with thorough characterization of the soil, moisture conditions, and seasonal variability in climate may allow more sensitive identification of the differences in corrosion rates between different steel alloys, but until those data are available, the type of steel is a secondary consideration in corrosion.
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