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UBC Theses and Dissertations

External oil pipeline corrosion of near-fusion heat-affected zones simulated from high-strength steels Eliyan, Faysal Fayez


The thesis evaluates the external pipeline corrosion at the near-fusion Heat-Affected Zones (HAZs). The HAZs were simulated by Gleeble© thermal cycles by which specific thermal conditions during pipeline welding are applied. They were applied on the high-strength steels API-X100 and API-X80. The thermal cycles were set to simulate the HAZs that are in direct adjacency to the fusion zone, where the temperature is high enough to transform the starting microstructure of the base metal into the single phase austenite. In the thermal simulations, the peak temperatures, cooling rates, and peak-temperature time periods are the parameters by which the different HAZ microstructures are simulated. For API-X80 and API-X100, the HAZ microstructural features are quantitatively and qualitatively linked, respectively, to the corrosion behavior. The slowly-cooled ferritic HAZs were found to corrode faster than the faster-cooled bainitic and martensitic HAZs. They, however, showed a tendency to develop thicker corrosion products, which are in the long run protective against underlying dissolution. It was found that bicarbonate in low concentrations exacerbate corrosion and prevents the passivation – regardless of the environmental conditions. In higher concentrations, bicarbonate promotes the passivation indefinitely, and that was also the case with carbonate, but regardless of the concentration. The laboratory conditions contain the corrosive constituents that drive the external pipeline corrosion upon exposure to the ground water or hydrated soils. The test solutions contain different levels of bicarbonate, carbonate, and chloride in mediums naturally-aerated, de-oxygenated, and CO₂-saturated. The corrosion behavior is investigated by electrochemical methods that measure the corrosion rates, evaluate the kinetics of dissolution and cathodic reduction, and model and evaluate the growth, protectiveness, and reactivity of passive films.

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Attribution-NonCommercial-NoDerivs 2.5 Canada