Investigation of hydrogen concentration and hydrogen damage on API X80 steel surface under cathodic overprotection

During the application of the API X80 grade pipeline steel in long distance gas transportation, the risk of hydrogen induced degradation for this kind of high strength material needs to be clarified from a safety point of view. Commercial X80 steel samples were hydrogen charged electrochemically at different current densities and immersion time. Hydrogen damage in the form of blisters and cracks was observed by optical microscope and scanning electron microscopy. Average hydrogen content in the sample measured by thermal desorption method and the sub-surface concentration of diffusible hydrogen detected by hydrogen permeation cell were both employed to indicate the correlation between hydrogen and surface damage. The influence of hydrogen on the mechanical properties of X80 steel was also characterized by slow strain rate tests. The results showed that the hydrogen damage can be detected on the steel surface if the hydrogen content could reach a certain threshold value during the process of rapid hydrogen uptake. It is suggested that there exists a critical hydrogen level before blisters initiate on X80 steel and that the level is 2-3 ppm, under the test conditions used in this work.