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The Role of Iron Sulfide Polymorphism in Localized Corrosion of Mild Steel

Ning, Jing
http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1476660078407962

Abstract Details

2016, Doctor of Philosophy (PhD), Ohio University, Chemical Engineering (Engineering and Technology).
H2S localized corrosion, occurring at discrete sites on a steel surface, can result in fast penetration of pipeline walls and loss of containment. This mode of metal attack is generally considered to be the main cause for catastrophic corrosion failures of facilities in the oil and gas industry. Hence, the prediction and control of H2S localized corrosion is a significant challenge for assuring asset integrity in oil and gas fields containing H2S. The purpose of this dissertation project is to explore, heretofore poorly understood, localized corrosion mechanisms related to the formation of iron sulfide polymorphs and related phases. In order to make this investigation possible, a thermodynamic model was initially developed as a tool to determine experimental conditions that could potentially replicate localized corrosion associated with iron sulfide polymorphism in an aqueous H2S system. Equilibrium expressions for H2S solubility and dissociation constants were reviewed and compared. Models to predict water chemistry of an H2S-H2O system were built and verified against experimental measurements. In order to predict the formation and dissolution of iron sulfides, their solubility limits were experimentally measured in an H2S-H2O-Fe2+ system. At 25oC, the measured pKsp,2 values were observed to change with time as the identity of the observed iron sulfide type changed. The pKsp,2 of mackinawite at 25oC was measured as 3.6 ± 0.2. Pyrite and greigite were observed at 60oC. Greigite was dominant around pH 5 with a pKsp,2 9.8 ± 0.5, while pyrite was dominant around pH 3.5 with pKsp,2 6.5 ± 0.5. The electrochemical thermodynamics of an H2S-H2O-Fe system were then investigated, with iron sulfides selected in relation to the oil and gas industry. Mackinawite, pyrrhotite, greigite, and pyrite were taken into consideration for Pourbaix diagram generation, accompanied by a complete accounting of all the assumptions, underlying thermodynamic data, and reaction mechanisms. Generated Pourbaix diagrams were validated by long-term experiments at different temperatures (25oC and 80oC) and by adjusting solution pH. Following the establishment of the thermodynamic model, experimental conditions leading to the formation of different iron sulfides as corrosion products in a sour environment were established. A strong correlation between the formation of greigite and/or pyrite on a steel surface and onset of localized corrosion was observed. Localized corrosion was absent when neither greigite nor pyrite formed. Consequently, the formation of greigite and/or pyrite was hypothesized to play an important role in the initiation of localized corrosion. Novel experiments involving deposition of pyrite on the steel surface were then designed and conducted. It was found that the galvanic coupling between pyrite particles and steel is the dominant mechanism for this type of localized corrosion. Finally, a descriptive model was built to answer when, where, and how this type of localized corrosion occurs in a sour environment. This model can provide guidance for the mitigation of localized corrosion in field conditions.
Srdjan Nesic (Advisor)
306 p.
Chemical Engineering
Hydrogen sulfide corrosion; carbon steel; Thermodynamics; Pourbaix diagram

Recommended Citations

Citations

  • Ning, J. (2016). The Role of Iron Sulfide Polymorphism in Localized Corrosion of Mild Steel [Doctoral dissertation, Ohio University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1476660078407962

    APA Style (7th edition)

  • Ning, Jing. The Role of Iron Sulfide Polymorphism in Localized Corrosion of Mild Steel. 2016. Ohio University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1476660078407962.

    MLA Style (8th edition)

  • Ning, Jing. "The Role of Iron Sulfide Polymorphism in Localized Corrosion of Mild Steel." Doctoral dissertation, Ohio University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1476660078407962

    Chicago Manual of Style (17th edition)

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This open access ETD is published by Ohio University and OhioLINK.