Role of Sour Environments on the Corrosion Fatigue Growth Rate of X65 Pipe Steel

Corrosion fatigue of C-Mn steels in sour environments is a concern in various offshore applications, particularly for flow-lines and risers. The commonly accepted mechanism of the enhanced fatigue crack growth rate (FCGR) behavior in these environments is hydrogen embrittlement. Hydrogen enters in the metal exposed to a sour environment through corrosion reactions occurring on the metal surface (referred to as bulk-charged hydrogen). During crack propagation, the fresh exposed metal at the crack tip under fatigue loading can also react with the corrosive environment and therefore generate hydrogen (referred to as crack tip hydrogen). Potentially, both of these two sources of hydrogen can impact the FCGR of the pipe steel. In this work, it was found that the bulk-charged hydrogen played a more dominant role in enhancing the FCGR of carbon steel in sour environments. The contribution to the FCGR from the crack tip hydrogen is not appreciable. The interaction of the freshly exposed metal with the corrosive environment could lead to the formation of iron(II) sulfide (FeS) film at the crack tip. This could lead to crack closure and decrease the effective ΔK and thus decrease the FCGR at low frequency, particularly at high H2S concentration and low pH environments, which is likely the cause for the observation of a plateau in FCGR in the low-frequency regime for carbon steel in the sour environment. Additionally, the experimental results suggest that although diffusible hydrogen was primarily responsible for the enhanced FCGR in the sour environment, the trapped hydrogen also played some role.