Cracking mechanism in API 5L X65 steel in a CO₂-saturated environment – Part II: A study under cathodic polarisation

The aim of this work is to achieve a better understanding of hydrogen effect in CO₂ environments, isolating its contribution by imposing cathodic polarisation on Hydrogen Permeation (HP) and Slow Strain Rate Tests (SSRTs). The influence of Fe₃C and FeCO₃ layers on hydrogen embrittlement (HE) susceptibility was a specific focus of this study. The results indicate that CO₂ environment generates hydrogen, which permeates through steel, although in lower amount compared to H₂S environments. Moreover, in Fe₃C-rich surface, the HP current achieves values higher than in wet-ground surface. Furthermore, in FeCO₃-filmed surface HP current is higher at the beginning of the test but decreases over time. The results of SSRT show the loss of ductility of the steel under cathodic polarisation that was driven by hydrogen and the embrittlement effect magnitude depends on the surface condition, indicating that a pre-corroded steel surface can raise the HE susceptibility in a CO₂ environment.