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Corrosion and pH prediction in the annulus of flexible pipes under high pressure of CO2

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. This environment induces high super-saturation levels of dissolved iron, together with the establishment of pH values far above thermodynamic equilibrium. Nevertheless, it also induces difficulties to predict actual levels of pH in the annulus, since thermodynamic models do not apply. In order to overcome these difficulties, and to obtain trustful pH values for the design of flexible pipelines, lots of efforts were put both in the experimental and modelling directions. A kinetic model was thus developed a few years ago, taking into account electrochemical and chemical reaction rates which allowed to explain super-saturated pH levels. Specific experiments consisting of continuous in situ pH measurements in confined corrosion cells confirmed the validity of the model in a pressure range 1-10 bar and for short term exposures. In the present paper, the extension of experimental conditions to higher pressures of CO 2 is presented. A good agreement with the model prediction and the short term stabilization of in situ pH was still obtained. Furthermore, longer exposure tests were also performed. It appeared that the evolution of super-saturated pH with time went through a maximum value after a few days, but then decreased slowly to a new equilibrium. This trend was attributed to the slow build-up of an iron carbonate layer, and subsequent decrease of the corrosion rate of the metal. The first version of the kinetic model was well suited to predict the maximum pH reached after a few days, but it did not capture the slow decrease. The model was then modified, in order to take into account the impact of corrosion products precipitation on surface reactions. The new version of the model is now capable to simulate with a good accuracy the complete in-situ pH transient for durations of several weeks until stationary state is reached. It allows not only to predict stabilized super-saturated pH values, but also to quantify the steady-state corrosion rate, which usually stands below a few µm/year. 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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François Grosjean, Rémy Mingant, J. Kittel, L. Ke, Carol Taravel-Condat, et al.. Corrosion and pH prediction in the annulus of flexible pipes under high pressure of CO2. Eurocorr 2016, Sep 2016, Montpellier, France. ⟨hal-02464447⟩

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