Mechanism of CO2 SCC in Armor Wires of Flexible Pipes

Abstract This paper discusses the CO2 stress corrosion cracking (SCC) mechanism in carbon steel armor wires used in flexible pipes. Small-scale tests were conducted with high-strength carbon steel armor wire specimens stressed by four-point bending. Unmachined specimens were used to preserve characteristics inherent to the original surface of the wires. The experiments were carried out in artificial seawater supersaturated with FeCO3 to simulate the high confinement conditions in the flexible pipe annulus. CO2 partial pressures from 10 to 50bar were used in the experiments. Some specimens were precorroded in aerated seawater before exposure to a high CO2 environment. In some tests, the specimens were polarized to investigate the nature of the CO2 SCC mechanism. The duration of the tests varied between 30 and 45 days. The results showed that exposure to FeCO3 supersaturated solutions could reduce the testing time considerably when compared to the current practice in the flexible pipe industry for CO2 SCC testing. The protective properties of the iron carbonate scale are important for crack initiation. Precorrosion in aerated seawater increases the susceptibility to cracking due to local variations in iron carbonate protectiveness. The results indicated that CO2 SCC is an anodic dissolution SCC mechanism.