Hydrogen Effects in Gas Transmission Pipeline Steels

Six steels were obtained from excavations of in-service gas transmission pipelines, and subjected to hydrogen charging by electrochemical means. The aim was to assess the susceptibility to hydrogen damage of these materials, which were not expected to encounter hydrogen-containing environments when they were originally put into service, but which are now found to be vulnerable to hydrogen exposure under certain external environmental conditions. For all the steels, the H concentration was found to increase as the applied current density increased, but each material developed different concentrations at a given current density. In addition, all the steels exhibited increasing hydrogen blistering as the H concentration was increased, but each material displayed its own characteristic propensity to blistering. It was possible to identify a threshold concentration for the onset of blistering in each steel, and on this basis, two grade X46 pipes and a grade X70 pipe were found to be the most susceptible to hydrogen damage, while a X52 and a X56 pipe were the least susceptible.