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Corrosive environment in the annulus of flexible pipes: pH measurements in confined conditions and under high pressure

Abstract : Annulus of flexible pipelines represents a specific corrosive environment for the high strength carbon steel wires, mainly due to the low V/S ratio between the volume of electrolyte and the exposed steel surface. Corrosion experiments with continuous in situ pH measurements were performed under CO 2 partial pressure from 0.01 bar and up to 10 bar, and with a V/S ratio of 0.3 mL/cm². After only a few days immersion, supersaturated pH was measured at levels close to +1 pH unit above the saturated pH calculated corresponding to iron carbonate solubility. Good correlation was found between experiments and a pH and corrosion model specially developed for confined environment. This model predicts that the corrosion rate in super-saturated conditions is considerably reduced in comparison with bulk situation. Long term behavior was also examined. It appears that the evolution of super-saturated pH with time passes by a maximum value, then decreases slowly to a new equilibrium. It was postulated that the maximum corresponds to a fully active surface. Then, as precipitation occurs, corrosion rate is decreased by corrosion scale. The decrease of corrosion rate could be quantified from the final pH value: value as low as a few µm/year was found. Even with such low corrosion rates, high confinement still induced super-saturation effect, with final pH values at +0.5 pH units above saturated pH.
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Submitted on : Monday, February 3, 2020 - 12:57:54 PM
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J. Kittel, François Grosjean, L. Ke, Carol Taravel-Condat, N. Desamais. Corrosive environment in the annulus of flexible pipes: pH measurements in confined conditions and under high pressure. Eurocorr 2013, Sep 2013, Estoril, Portugal. ⟨hal-02464636⟩

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