Hydrogen embrittlement of API 5L X65 pipeline steel in CO2 containing low H2S concentration environment

Abstract

The basic corrosion mechanisms in CO2 environments are widely investigated and reported by scientific community. However, cracking failure mechanisms in CO2 environment or in environments where CO2 coexist with other gases need a better understanding. Cracking susceptibility of API 5L X65 in pure CO2 was recently reported and the basic mechanism was attributed to hydrogen embrittlement. The influence of hydrogen permeation and the detrimental effect of cathodic polarisation support the assumption of hydrogen embrittlement as the determining factor for the loss of plasticity and strength. The loss of strength of an API 5L X65 steel when CO2 and H2S coexist in the same environment is the focus of the present work. Hydrogen embrittlement (HE) and hydrogen induced cracking (HIC) of pipeline steels are failures modes expected to occur in CO2 with traces of H2S environments. The present work aims to approach the performance in laboratory of an API 5L X65 steel in CO2 with traces of H2S environment (slightly sour), using as baseline the performance of the material in pure CO2 environment (sweet), already reported. Hydrogen permeation and Slow Strain Rate Tests (SSRT) were carried out in CO2 environment containing low concentration of H2S. It was observed that traces of H2S in CO2 environment promote much more intense hydrogen permeation, much higher than in purely CO2 environment. Loss of ductility was also much more severe than in CO2 environment, with and without the influence of cathodic polarisation. Hydrogen generation enhancement promoted by traces of H2S in solution, corresponding to an average content of 21 ppm in solution, had a heavy effect on hydrogen embrittlement of the steel, confirmed by the lower ratio of reduction in area (RRA) and time to fracture in comparison with sweet service conditions.