Under unbalanced bipolar operation, the HVDC transmission system can generate a large unbalanced current that flows through the grounding electrode. As a consequence, interference current up to several hundred amperes will be produced on the pipelines near the grounding electrode. As shown by on-site tests, the pipe-to-soil potential of the pipelines under grounding electrode discharge interference can be up to 300 V, which is far beyond the potential range that is required to achieve cathodic protection. In this situation, it would be expected that pipeline steels would suffer from a loss of ductility, strength and/or toughness due to the absorption of hydrogen produced from hydrogen reduction reaction under excessive cathodic polarization with a large overpotential. In the present paper, the accumulation and diffusion behaviors of hydrogen in API 5L X80 steel, as well as the evolutions of ductility and strength of X80 steels, were investigated in simulated soil environment under cathodic current interference. The results show that the hydrogen concentration in X80 steel increased with the increase of cathodic current density and interference duration. Based on the hydrogen embrittle susceptibilities determined by slow strain rate tensile test under different interference conditions, the risk threshold of hydrogen embrittlement for X80 steels and its welded joint was defined. Furthermore, a safety boundary diagram was depicted based on the obtained results, which can provide reference for the safe operation of HVDC transmission system.

HVDC, interference, ground electrode, pipeline, security boundary