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Corrosion Behavior of Buried Pipeline in Presence of AC Stray Current in Controlled Environment

Ghanbari, Elmira
http://rave.ohiolink.edu/etdc/view?acc_num=akron1475677625895234

Abstract Details

2016, Doctor of Philosophy, University of Akron, Chemical Engineering.
Cathodic protection (CP) and other corrosion mitigation strategies to preserve the integrity of pipelines are reported have failed in the presence of AC. This type of failure is a threat for catastrophic failure and there is a lack of well-agreed mitigation criteria for it. AC grounding systems as well as "consensus" standards that recommend limits on the maximum allowable AC current density and critical ratios of AC and CP currents are the only available techniques for preventing AC corrosion. Unclear mechanism of AC corrosion is the main reason for the uncertainty on the CP criteria in the presence of AC interference. AC interference can occur by conduction or induction mechanism where pipelines share right of way with some interference sources such as a high-voltage transmission line typically fed by a high voltage line at 50 or 16.7 Hz frequency. The goal of this work is to understand the mechanism of AC induced corrosion by studying the effects of AC interference on steel at different DC potentials. This work consists of two phases. In the first phase, the influence of simultaneous presence of different AC and DC potentials on corrosion rate of API grade X65 pipeline steel by performing both electrochemical techniques and weight loss analysis in sodium chloride solution was studied. Experimental test results showed that the effect of AC is most pronounced near the open circuit potential; at more positive potentials the rates approach those of the ohmic drop/mass transport-limited DC rates. Correspondingly, at negative potentials the rates decrease. Surprisingly, at all potentials, it was found that the AC corrosion rate was equal to the average AC current in the system. The data generated from weight loss experiments were compared with the results from a model for AC corrosion that was developed using a modified Butler-Volmer approach. The model considers the anodic and cathodic Tafel slopes, diffusion limited oxygen transport, interfacial capacitance and solution resistance. The model was used to explain the observation that the AC corrosion rate was equal to the average AC current in the system. Both experimental and model results showed the importance of the solution resistance and interfacial capacitance on the rate of AC corrosion, especially at the frequency of 60Hz or higher. As it relates to the pipeline, one of the factors that influences this interfacial capacitance is the composition of the soil around the pipeline. For example, alkaline earth mineral content of the soil such as calcium or magnesium base minerals produce calcareous deposits on the coating fault. In the second phase of this project, the influence of the solution resistance and interfacial capacitance on the rate of AC corrosion was investigated in more detail. This was done by performing AC experiments in both extracted soil environment and soil simulated solution. Additionally, artificial scales were grown on X65 pipeline steel, which were then exposed to different AC and DC potentials in a soil-simulating solution. These films were meant to simulate those that might deposit on the surface of the steel in calcium and carbonate rich soils during long-term exposure to cathodic protection (CP). In these experiments films in the order of 2µm were deposited onto X65 carbon steel. After the deposition process, the properties of these films were characterized extensively including film capacitance. Results showed that the interfacial capacitance is one of the key parameters that ties together different factors controlling the magnitude of AC induced corrosion, such as the magnitude of AC voltage, AC current density, AC frequency and soil properties. In the light of the developed model and the experimental results, some misconceptions about AC corrosion were discussed. For example, it was shown that the current density is not a good candidate as a basis of AC mitigation criteria.
Robert Scott Lillard, PhD (Advisor)
Dmitry Golovaty, PhD (Committee Member)
Jie Zheng, PhD (Committee Member)
Nathan Ida, PhD (Committee Member)
Qixin Zhou, PhD (Committee Member)
199 p.
Chemical Engineering; Engineering; Materials Science
AC interference on pipelines, corrosion, carbon steel, scale formation, interfacial capacitance

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Citations

  • Ghanbari, E. (2016). Corrosion Behavior of Buried Pipeline in Presence of AC Stray Current in Controlled Environment [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1475677625895234

    APA Style (7th edition)

  • Ghanbari, Elmira. Corrosion Behavior of Buried Pipeline in Presence of AC Stray Current in Controlled Environment . 2016. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1475677625895234.

    MLA Style (8th edition)

  • Ghanbari, Elmira. "Corrosion Behavior of Buried Pipeline in Presence of AC Stray Current in Controlled Environment ." Doctoral dissertation, University of Akron, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=akron1475677625895234

    Chicago Manual of Style (17th edition)

akron1475677625895234
669
© 2016, some rights reserved.Creative Commons License Corrosion Behavior of Buried Pipeline in Presence of AC Stray Current in Controlled Environment by Elmira Ghanbari is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by University of Akron and OhioLINK.