Cathodic Protection And Susceptibility To Hydrogen Embrittlement Of API X80 Pipeline Steels Under Simulated Deep-sea Conditions

The high strength pipeline steels gain more popularity in the engineering construction of petroleum and gas pipeline in deep-sea areas because of their excellent performance. In the deep-sea corrosion environment, cathodic protection is the most common method to suppress the corrosion of pipelines, but the high strength steels are susceptible to the hydrogen, so when the operation of cathodic protection applied on the pipeline of high strength steels is inappropriate, the pipelines tend to occur hydrogen embrittlement failure which will cause tremendous losses and untold suffering to the deep-sea engineering. Thus, to ensure the safety of pipelines in the long-term service, the work to study the cathodic process and susceptibility to hydrogen embrittlement of high strength steels systematically, and identify the appropriate cathodic protection range practically is of important scientific and technological significance. API X80 pipeline steel, which enjoys the vastest potential for future application, was chose for the work in the paper. In the simulated deep-sea conditions with the temperature of 4.0℃, dissolved oxygen content of 2.0 mg/L, the cathodic process and its susceptibility to hydrogen embrittlement of X80 pipeline were studied, also, an appropriate range of cathodic protection was proposed.Classic electrochemical test results showed that X80 steel had oxygen reduction and H2 O depolarization in the cathodic process, and as the cathodic potential turned negative, the oxygen reduction played an increasingly important role in the cathodic process, and its charge transfer resistance was decreasing; while H2 O depolarization obtained smaller and smaller proportion in the cathodic process and it was increasingly obvious that its reaction was controlled by diffusion.Hydrogen permeation current measurements results showed cathodic polarization promoted hydrogen permeation behavior significantly, that is, a linear relationship was found between stable hydrogen permeation current density and cathodic potential. The effective diffusivity D of H fluctuated between 2.615310-12 and 2.124310-11 m2/s, also both the steady-state permeation flux Jss and apparent solubility C0 of hydrogen increased with decreasing cathodic potential.Slow strain rate tensile test and fracture morphology analysis were applied to investigate the effect of cathodic polarization on the susceptibility of X80 pipeline steel in the simulated deep-sea condition. The results suggested that, X80 suffered decrease in plasticity, increase in brittleness and susceptibility to hydrogen embrittlement in the simulated deep-sea water. In the range of open circuit potential(OCP) to-900 mV(vs saturated calomel electrode), the mechanical property of X80 steel was better and showed obvious character of ductile fracture, hydrogen embrittlement coefficients F suggested that the steel was in safety zone, so above-900 mV was the optimum cathodic protection potential range; as the cathodic potential turned negative, the plastic deformation of the samples decreased, the calculated hydrogen embrittlement coefficients F increased significantly, suggesting the steel entering from dangerous zone to brittle fracture zone, in addition, its brittle fracture was more obvious, so the steel exhibited higher susceptibility to hydrogen embrittlement.