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Corrosion behavior of API X100 steel in near-neutral pH bicarbonate environments : experimental and modelling studies Gadala, Ibrahim M.


API X100 is a new high-strength low-alloy steel which has garnered great interest from the pipeline industry due to the economic benefits it offers in terms of lower material, transportation, and fabrication costs. However, buried pipeline steels suffer from external corrosion and cracking, the mechanism of which in near-neutral pH environments is not yet fully understood. This dissertation presents and discusses the results of several electrochemical studies and numerical models conducted on X100 steel, contributing to a more complete understanding of the fundamental corrosion processes occurring in these environments. Improved simulation accuracy for corrosion rates and pipeline integrity is achieved. Applications of this research are strongest within the buried oil and gas transmission pipeline field, yet are extendable to other infrastructural and engineering applications such as utility piping systems and steel reinforcements of buried or concrete structures. Results of this work elucidate the criticality of even minor pH variations within the near-neutral pH environment on the corrosion and passivation of X100. Conflicting impacts of [HCO^-₃], versus %CO₂ on corrosion rate are revealed, attributed to the gradual dominance of hydrogen evolution cathodic reactions involving HCO^-₃ species at higher pH. A finer three region subdivision of the near-neutral pH range is proposed based on the dissolution, dissolution-adsorption, adsorption-diffusion, and diffusion-controlled behaviors which appear as pH, [HCO^-₃], [Cl^-], [SO24^(2-)], and temperatures are changed. Dynamic electrochemical impedance spectroscopy identifies the role of Cl^-/ SO24^(2-) in increasing corrosion rate, decreasing pre-passive stage diffusion, and decreasing passive layer protection. Increased porosity of FeOOH tubercle structures formed in low dissolved [O₂] environments augments diffusion therein, spurring the exclusive formation of Fe₂O₃ underneath instead of only Fe₃O₄. Hydrogen diffusivity in X100 steel is found to be 4.4x10-⁷ cm²/s. During free corrosion, a diffusible hydrogen concentration of 1 atomic ppm is also measured in the alloy. Simulations in the Finite Element models developed indicate ideal anode placements and applied voltages for an underground cathodic protection system. Additionally, the transient evolution of an external corrosion defect which reaches 3 mm deep within 3 years is visualized, wherein the pipeline’s structural integrity is diminished by 8% of the intact pipeline strength.

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