The Combined Effect of O₂ and CO₂ on Corrosion of Flexible Armour Wires

A static flexible pipeline lost containment before the expected field lifetime despite being operated well within established limits. In the following failure investigation, it was found that the failure most likely was the result of significant corrosion of the armour wires, although verification of the failure mode was not possible as the source of the leak from the pressure sheath was not found. It was also discovered that the outer sheath had been punctured during the installation process. At the time of installation, the annulus was filled with air, originating from manufacturing and from annulus pressure testing with air. When the outer sheath was damaged, seawater entered the annulus creating pockets of compressed air at local high points. This resulted in oxygen corrosion in the air/water interface initially, but after the flexible pipe was taken into operation it gradually changed to CO₂ corrosion. The combined oxygen and CO₂ corrosion phenomenon was studied by laboratory experiments in simulated annulus conditions.

The experiments showed that in a normal situation with CO₂ containing annulus gas without oxygen (air), the armour wires would quickly be passivated by the build-up of protective iron carbonate products on the metal surface. However, if the wires were exposed to oxygen first, this resulted in increased corrosion rates, formation of localised corrosion attacks and build-up of porous corrosion products. After the purge gas was changed from O₂ to CO₂, it took long time for the wires to build up protective iron carbonate layers and more importantly the resulting corrosion rate was much higher than for the wires that only had been exposed to CO₂. Thus, it was experimentally demonstrated that pre-exposure to O₂ before CO₂ may result in significantly enhanced corrosion since the process of protective iron carbonate formation is disturbed.