Hydrogen Induced Cracking (HIC) and Sulfide Stress Cracking (SSC) are major issues of line pipes exposed to sour environment. Severity of sour environment determined in accordance with EFC 16 and NACE MR0175 / ISO15156 for low-alloy steels has been proposed based on experimental SSC results. However, SSC susceptibility does not necessarily correspond to HIC susceptibility. The susceptibility of HIC and SSC, which are types of hydrogen embrittlement, would be accurately predicted if absorbed hydrogen content under every environment and critical hydrogen content are fully obtained. In this study, hydrogen permeability domains as a function of pH and H2S partial pressure were determined. In addition, the susceptibilities to these hydrogen-assisted environmental cracking modes under various combinations of pH and H2S partial pressure were also investigated. The severity of the sour environment was proposed based on the hydrogen permeability and the susceptibility for hydrogen assisted environmental cracking.
In oil and gas industry, oil country tubular goods (OCTG) or linepipes are exposed to wet H2S environments (Sour environments) in some cases. The presence of H2S in the sour environment enhances hydrogen entry into the steel due to catalytic action of H2S. The absorbed hydrogen enhanced by H2S affects hydrogen embrittlement. Hydrogen Induced Cracking (HIC) and Sulfide Stress Cracking (SSC) are hydrogen embrittlement caused forms of cracking occurring under sour conditions.
To evaluate susceptibility for this environmentally assisted cracking mechanisms, NACE TM0284 solution A, B, C and HLP solution, which comprises sodium chloride, acetic acid, and/or HCl dissolved in distilled water with various pH are used.1 However, various H2S partial pressures and pH values exist in actual oil and gas wells around the world. Therefore, corrosion tests often require solutions with H2S partial pressure and pH close to those of actual sour environments in European Federation of Corrosion (EFC) specifications.2 The severity of sour environment determined in accordance with EFC 16 and NACE MR0175 / ISO15156 for carbon and low-alloy steels has been proposed as indicated in Figure 1.2,3 Material properties such as yield strength and hardness are required depending on the severity of sour environment. The critical line of each region which was indicated as regions 0, 1, 2 and 3 in Figure 1 based on experimental SSC results of API grade N80 and P110.4 P110 is not applicable to the region below the upper diagonal (region 2 and 3) and N80 is not applicable to the region below the lower diagonal (region 3). Some researchers have investigated the susceptibility of HIC under various sour environments.5,6,7,8 However, HIC susceptibility does not necessarily correspond to SSC susceptibility. Hence, hydrogen assisted environmental cracking (HIC and SSC) domains as functions of pH and H2S partial pressure have not been fully generalized.