CO2 corrosion of high strength steel wires in the annulus of flexible pipes

Abstract

Corrosion of carbon steel may occur at near neutral pH in deoxygenated solutions with high bicarbonate concentration when the corrosion reaction produces elemental hydrogen. Such conditions are encountered in the annulus of flexible pipes when it contains water with dissolved CO2, which produces high concentration of ferrous ion (Fe2+) and bicarbonate (HCO3-). The objective of the present study was to investigate if high levels of Fe2+ and HCO3- can persist for the time required to induce SCC in armor wire steels. It implies that the nucleation and growth of siderite (FeCO3) is so slow that the solution remains supersaturated for several days. The role of surface finishing and cathodic areas composed of cementite were also evaluated, as well as the salinity, temperature, and pCO2. The results show that, at temperatures lower than 40 °C and CO2 partial pressure of 5 to 10 kPa, the saturation ratio (SR) of siderite can persist at values much higher than 1 and maintain a near neutral pH for several hundred hours. The uniform corrosion rate of armor wire steel is low if pH soon achieves near neutral range. The siderite forms a layer on the steel surface and the cathodic reaction becomes limited by diffusion through it. The corrosion rate increases with increasing salt content and exposed cementite, being even more pronounced at 25 °C. In such cases, siderite could not completely inhibit localized corrosion, which may be a great site for SCC cracks to start. That is related with a join action of the higher solubility of siderite and the effect of ion chloride. At 25°C and brine, the pCO2 is the main parameter to concern in active surfaces of armor steels.