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.