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An Assessment of the Susceptibility to Corrosion from Alternating Current of Cathodically Protected Steel Pipelines in Soils

Moran, Andrew J.
http://orcid.org/0000-0002-3712-2082
http://rave.ohiolink.edu/etdc/view?acc_num=akron1590695228893218

Abstract Details

2020, Doctor of Philosophy, University of Akron, Chemical Engineering.
In this work, the phenomenon of AC corrosion is investigated through theoretical modeling and experimentation to gain a better understanding of its mechanisms as they relate to cathodically protected steel pipelines. First, a comprehensive synopsis of AC corrosion is given with historical background and the motivations for studying this phenomenon, these being, primarily, the utility of accurate AC corrosion rate predictions to pipeline integrity management. Then, a theoretical model based on Butler-Volmer kinetics and interfacial electrochemical processes is introduced as a mechanistic explanation of AC corrosion. The AC corrosion model can be used to identify the parameters implicated in producing high corrosion rates from alternating currents. The model serves as a guide to narrow the focus of study to certain measurable factors both in laboratory and field experiments. Special emphasis is given to the interfacial capacitance of steel as this parameter is predicted to have a considerable influence on the ratio of faradaic-to-capacitive current. Long-term mass loss tests were conducted as a first means of assessing the impact of AC corrosion. Literature is either sparse or contradictory about the influence of AC on corrosion rates with some researchers reporting negligible effects and others reporting corrosion rates far in excess of acceptable standards, even at low AC densities. In this work, tests in aqueous solution produced modest corrosion rates on cathodically protected samples, only exceeding 1 mpy at AC densities greater than 600 A/m^2. At or below the commonly cited thresholds of AC density (30 A/m^2 and 100 A/m^2, the corrosion rate of cathodically protected steel is only slightly increased compared to a zero-AC condition. However, greatly increased corrosion rates are observed on non-cathodically protected steel, emphasizing the need for proper cathodic protection on pipelines. Field studies were also conducted as a way to validate the observed trends in laboratory experiments. Samples were bonded to test pipelines and subjected to AC and cathodic protection voltages. Corrosion rates tend to be modest for cathodically protected samples with some samples showing corrosion rates greater than expected for their CP level and others showing full protection from increased corrosion. Data suggest that AC-induced corrosion can present a moderate-risk problem on cathodically protected pipelines at AC densities greater than 100 A/m^2. Then, study of the interrelated effects of AC and DC on pipeline steel is advanced using a combined modeling and experimentation analysis designed to demonstrate their relationship to each other. Results show that AC can greatly influence DC on a steel electrode, but the effect is dependent on the DC potential in relation to the OCP of the steel. Results can be predicted with modeling of electrochemical kinetics and are backed up by experiment. DC can also influence AC but the effect is not electrochemical but rather due to local chemistry changes in the immediate solution environment. The implications of these effects is discussed as it relates to pipelines and cathodic protection. Finally, the interaction between the interfacial capacitance of steel and its soil environment is explored. Experiments were conducted with a multitude of synthetic soils and with solutions of varying chemistries of which steel would be expected to encounter in pipeline applications. Measurements of interfacial capacitance were conducted on steel electrodes in these environments through the use of electrochemical impedance spectroscopy and equivalent circuit modeling. Evidence from these investigations indicates that interfacial capacitance on actively corroding steel derives, in large part, from the conductive magnetite corrosion product film. This porous magnetite imparts a uniquely large capacitance to the steel so that most AC is composed of capacitive current rather than faradaic current which would lead to corrosion. Soil minerals and dissolved ions do not influence this property with the exception of scale-developing cations. However, while deposited mineral scales and passive oxide layers can greatly decrease interfacial capacitance, they are unlikely to increase corrosion rates for AC-influenced steel due to their concomitant protective nature.
Robert Lillard (Advisor)
Teresa Cutright (Committee Member)
Dmitry Golovaty (Committee Member)
Rajeev Gupta (Committee Member)
Hongbo Cong (Committee Member)
Nathan Ida (Committee Member)
338 p.
Chemical Engineering
AC; Corrosion; steel; pipelines; cathodic protection; AC corrosion; corrosion rates; DC; capacitance

Recommended Citations

Citations

  • Moran, A. J. (2020). An Assessment of the Susceptibility to Corrosion from Alternating Current of Cathodically Protected Steel Pipelines in Soils [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1590695228893218

    APA Style (7th edition)

  • Moran, Andrew. An Assessment of the Susceptibility to Corrosion from Alternating Current of Cathodically Protected Steel Pipelines in Soils. 2020. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1590695228893218.

    MLA Style (8th edition)

  • Moran, Andrew. "An Assessment of the Susceptibility to Corrosion from Alternating Current of Cathodically Protected Steel Pipelines in Soils." Doctoral dissertation, University of Akron, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=akron1590695228893218

    Chicago Manual of Style (17th edition)

akron1590695228893218
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This open access ETD is published by University of Akron and OhioLINK.