Carbon steels are the cost-effective materials of choice for pipelines and casings for sour service in the oil and gas industry. In this work hydrogen permeation, absorption and trapping was studied in several line pipe and OCTG steels. It is shown that H absorption in line pipe steels is lower than in OCTG steels under both electrochemical and sour H charging. Thermal Desorption Spectroscopy (TDS) studies showed that the major trap sites for hydrogen in all steels were weak or reversible traps. Dislocations and carbides were identified as the majority traps in line pipe steels and OCTG steels respectively. The implications of these observations on sour performance is briefly discussed.
Sulfide Stress Cracking (SSC) degrades the mechanical properties of carbon steels, which is, and continues to be, a major technological challenge in the oil and gas industry (Fairchild et al., 2019, Salama et al., 2018, Baxter et al., 2018). SSC is a form of hydrogen embrittlement of carbon steels (Berkowitz and Horowitz, 1982, Berkowitz and Heubaum, 1984). A robust knowledge base on the mechanisms of SSC and hydrogen embrittlement is essential to the mitigation and control of this phenomenon. Based on the current understanding of SSC, one of the key steps involves the generation of hydrogen atoms in the service environment (i.e. sour corrosion in H2S containing production fluids) that is subsequently absorbed into the steel (Berkowitz and Horowitz, 1982, Berkowitz and Heubaum, 1984, Berkowitz et al., 1976). The hydrogen atoms then diffuse through the steel matrix as an interstitial solute atom and accumulate at various microstructural features (including inclusions, carbides, dislocations, grain boundaries etc) or regions of high hydrostatic tension strain (e.g. crack tip) (Hirth, 1980). The presence of dissolved hydrogen atoms in the steel is known to embrittle the steel and reduce the metal toughness dramatically (Hirth, 1980, Perez, 2013, Neeraj, 2017, Srinivasan and Neeraj, 2014, Neeraj et al., 2012, Li et al., 2015, Neeraj and Srinivasan, 2017). Therefore, it is important to understand the interaction of hydrogen atoms with the microstructure of casing and pipeline steels employed in sour service to better understand their performance. In this work we studied the interaction of hydrogen atoms with typical commercial grade line pipe (LP) and oil country tubular goods (OCTG) steels. Hydrogen permeation, absorption and trapping was studied in several LP and OCTG carbon steels using electrochemical charging. H absorption during electrochemical charging was compared to absorption under sour conditions on a subset of two steels. Hydrogen trapping states were studied using Thermal Desorption Spectroscopy (TDS) after electrochemical charging. It was observed that line pipe steels in general absorbed lower amounts of H (about 50% lower) than OCTG steels. This observation was consistent under both electrochemical and sour H charging conditions. TDS studies showed that the major trap sites in both LP and OCTG steels were weak or reversible traps. The microstructure of both LP and OCTG contained a variety of microstructural trap sites such as dislocations, cementite and alloy carbides, grain boundaries and inclusions. However, it was determined that dislocations and carbides were the majority traps in LP steels and OCTG steels respectively. It is important to note that the amount of trap sites was much greater in OCTG steels compared to LP steels. This was supported by the observations that LP steels consistently absorbed lower amount of H under similar H charging conditions, the effective diffusivity for H in LP steels was about 2X faster than in OCTG steels and more H was observed to evolve in TDS studies in OCTG steels as compared to LP steels. Finally, the implications of these observations on sour performance is briefly discussed.
