A kinetic model of CO2 corrosion in the confined environment of flexible pipe annulus
Abstract
Flexible pipes used in the offshore industry are composed of steel wires enclosed in an annulus formed by inner and outer thermoplastic sheaths. The CO 2 corrosion of the carbon steel wires located in the annular space occurs within restricted volumes of electrolyte. For instance, the typical V/S ratio between the volume of electrolyte and the exposed steel surface is in the order of 0.03 mL.cm-2. In such confined environments, corrosion measurements clearly show that the results classically obtained in bulk conditions (infinite electrolyte volume) do not remain valid. From a theoretical point of view, the effect of confinement is usually ignored by available CO 2 corrosion models, so that experimental results strongly deviate from predictions when the V/S ratio decreases. Thus, the corrosion rates measured in the annulus conditions are commonly 2 or 3 orders of magnitude lower than those predicted and the measured pH are significantly higher than the saturation pH predicted by thermodynamic models such as CORMED. In this context, a new kinetic model of the physico-chemical processes occurring during the corrosion of steel covered by a thin liquid film and exposed to a pure CO 2 atmosphere was developed. From a qualitative point of view, this model allows to explain theoretically some unusual physico-chemical phenomena classically observed in such corrosive and confined environments: iron supersaturation, high values of pH, low corrosion rates, and their dependence with the V/S ratio. The paper also presents the results of tests and measurements performed in the annulus of flexible pipes and shows that the model agrees well with the measurements.
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