Study On Hydrogen Permeation And Hydrogen Embrittlement Of X80 Pipeline Steel And Its HAZ Caused By Cathodic Protection

Cathodic protection is one kind of long effective anticorrosion methods which are widely used in offshore structure and deep-sea facilities. According to resent reports, when the polarization potential is much negative, hydrogen will be generated on the surface of metal and diffuse into metal substrate to cause the hydrogen embrittlement. The mechanical performance of heat affected zone(HAZ) is much different from the X80 steel for the microstructure difference which caused by heat input in welding. Whether the cathodic protection parameters of X80 steel are suitable for HAZ or not is unclear.In this paper, welding HAZ was obtained by welding thermal simulation technique. The welding thermal effect to microstructure and crystal defects of X80 steel were analyzed by optical microscope, X-ray diffraction, scanning electron microscopy, electron probe microanalysis and transmission electron microscopy. The mechanism of welding thermal effect to X80 steel hydrogen diffusion and hydrogen embrittlement sensitivity were studied by electrochemical hydrogen permeation test, electrochemical impedance spectroscopy(EIS) tests and slow strain rate tensile test(SSRT).The result show that the microstructure and crystal defects density which determine hydrogen diffusion and hydrogen embrittlement sensitivity will be changed under welding thermal cycle of different peak temperature. Fine grain zone with fine and uniform ferrite has the highest hydrogen diffusion coefficient, secondly is coarse grain zone with lath martensitic structure, X80 steel with highest crystal defect density has the lowest parameter. Under the same cathodic polarization potential, Coarse grain zone is more sensitive to hydrogen embrittlement for its higher diffusion coefficient of hydrogen and hydrogen trap density. The parameters of cathodic protection according X80 steel is not suitable for the welding joint. Two theory models of crack initiation which caused by hydrogen and dislocation are proposed. One model is micro crack at the second phase particle and the other is brittle crack at grain boundaries due to the hydrogen and dislocation collaboration.