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

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 C02 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. lt appears that the evolution of super-saturated pH with time passes by a maximum value, then decreases slowly to a new equilibrium. lt 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.