Fatigue and Static Crack Growth Rate Study of X-65 Line Pipe Steel in Gas Transmission Pipeline Applications

摘要

Hydrogen embrittlement phenomenon has been investigated in the current study by performing crack growth rate (CGR) measurements under static and fatigue loading conditions on fracture mechanics-based specimen extracted from X-65 line pipe steel. The source of hydrogen inside the line pipe could be the green/blue hydrogen blending that is being considered in an effort to reduce the carbon footprint of the oil and gas industry. Other environments could be aqueous impurities such as NaCl in presence of the transported CO_2 and hydrogen charging conditions created by a possible use of sacrificial metallic coatings. The effect of cyclic loading frequency on fatigue crack growth rate (FCGR) has been investigated in a simplified hydrogen charging environment - 3.5% NaCI solutions containing CO_2 at room temperature, with imposed CP. The applied hydrogen charging conditions were electrochemical potentials associated with mixed potential of Zn and/or Al based coatings, being considered for sacrificial metal coatings. FCGR in environment was found to be significantly higher than in-air values and was also found to increase with decrease in frequency of applied fatigue loading, suggesting susceptibility to environmentally assisted cracking. A segmented crevice electrode setup was designed to simulate electrochemical conditions experienced by crack tip for a given bulk chemistry and cathodic polarization (CP) applied to freely exposed surface. It was found that the CP level experienced by electrodes inside the crevice was not the same as that of the freely exposed surface in solution. These experiments supplement the results of the CGR measurement tests.