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Effect of Carbon Steel Composition and Microstructure on CO2 Corrosion

Akeer, Emad S.
http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1399315460

Abstract Details

2014, Doctor of Philosophy (PhD), Ohio University, Chemical Engineering (Engineering and Technology).
The environmental conditions encountered in oil and gas wells and pipelines can cause severe localized corrosion to mild steel. The utility of carbon steel in oil and gas pipelines depends on formation of protective corrosion product layers. However, the microstructure and chemical composition of steel are considered to be important variables that affect the ability of these layers to protect steel from corrosion. The present study investigated the effect of alloying elements and metallurgy of five different pipeline steels, with different chemical composition and microstructure, on CO2 corrosion in flowing conditions with focus on the iron carbonate layer formed and related corrosion phenomena that could lead to localized corrosion. The microstructure of tested steels was examined using optical microscopy and etching. Preliminary experiments were conducted using a glass cell, which is a very well known and widely used apparatus. Then a comparison was done with the newly developed thin channel flow cell (TCFC) to validate whether the TCFC can be used instead of glass cell in this study, which required very high velocity and wall shear stresses. It was found that there are no significant effects of alloying elements and steel microstructure on corrosion rate in experiments done at pH 4.0 at 25°C and 80°C. Further experiments were then conducted in the TCFC to study the effect of alloying elements and microstructure under conditions where a protective FeCO3 corrosion product layer forms, using very high liquid flow rates. For each of the studied steels, an FeCO3 corrosion product layer was formed within two days of exposure at low wall shear stress at 80°C, pH 6.6, and partial pressure of CO2 of 1.5 bar (1.5 bar pCO2). For all tested steels, the FeCO3 layer reduced the general corrosion rate to less than 1.0 mm/y. These "pre-formed" FeCO3 layers were then exposed to high liquid flow velocity and wall shear stress (535 Pa) for 3 days. This caused partial loss of the protective FeCO3 layer which was probably related to the local increase in shear stress and the changes in pressure caused by turbulence at the high flow rates. Although all steels suffered from pitting corrosion to different degrees, the FeCO3 layer formed on normalized steel was more protective than the one formed on quenched and tempered steel (Q&T). This can be attributed to microstructure, because the pearlite structures present in the normalized steel conferred superior FeCO3 adherence to the steel surface. On the other hand, X65II steel, which has metallurgical characteristics consistent with a normalized hot rolled material, suffered pitting corrosion, which initiated even before increasing wall shear stress. This type of localized corrosion was related to inclusions and phase distributions within the ferrite/pearlite microstructure. In a separate series of experiments, the formation mechanisms of the FeCO3 corrosion product layer were challenged for each steel at high wall shear stress (535Pa) and at 80°C, pH 6.6, and 1.5 bar pCO2. It was observed that the FeCO3 corrosion product layers did not form at this high wall shear stress, even under conditions that were supersaturated with respect to FeCO3. This was related to mass transfer behavior, where the fast movement of species from and toward the steel surface contributed to removal of generated ferrous ions and prevented the formation of an FeCO3 layer. High local shear stresses may have also mechanically interfered with formation of any FeCO3 layer on the steel surface. At high wall shear stress, the general corrosion rates of normalized steels (X52, A106GRB) are higher than for Q&T steels. This can be related to the amount of iron carbides in the steel. There was no localized corrosion observed at high wall shear stress since no FeCO3 formed on the steels.
Srdjan Nesic (Advisor)
187 p.
Chemical Engineering
CO2 corrosion; wall shear stress; iron carbonate; steel micro-structure; normalized; quenched; tempered

Recommended Citations

Citations

  • Akeer, E. S. (2014). Effect of Carbon Steel Composition and Microstructure on CO2 Corrosion [Doctoral dissertation, Ohio University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1399315460

    APA Style (7th edition)

  • Akeer, Emad. Effect of Carbon Steel Composition and Microstructure on CO2 Corrosion. 2014. Ohio University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1399315460.

    MLA Style (8th edition)

  • Akeer, Emad. "Effect of Carbon Steel Composition and Microstructure on CO2 Corrosion." Doctoral dissertation, Ohio University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1399315460

    Chicago Manual of Style (17th edition)

ohiou1399315460
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This open access ETD is published by Ohio University and OhioLINK.