Study On Hydrogen Permeation Behavior Of X80 Pipeline Steel Caused By Cathodic Protection And Stress

In recent years, many countries around the world lay much emphasis on the research and exploitation of ocean resources, so the service life of facilities used in the sea or deep-sea is particularly important. Engineering practices show that cathodic protection technology can effectively protect the offshore structures and deep-sea facilities from corrosion. However, if the protection potential is too low or the metal anticorrosion coating is partially damaged and peeled off, the hydrogen evolution will occur easily, then, the failure of steel structure will be induced by hydrogen embrittlement with the generated hydrogen gathering and infiltrating into the steel. At the same time, stress on the marine steel structure are very complex, it will inevitably influence the transport and aggregation of hydrogen in steel. Nowadays the research on hydrogen embrittlement behavior of high strength steel in sea water, such as X80 steel, is popular. But a special study of X80 steel hydrogen permeation behavior is relatively rare. In this paper, the author evaluates the hydrogen permeation behavior of X80 steel caused by cathodic protection and stress, also has a discussion about the combined mechanism of cathodic protection and stress working on the hydrogen permeation behavior in the steel.In this study, the hydrogen permeation behaviors of X80 pipeline steel under the conditions of different cathodic protection potential and stress were evaluated by using a modified Devanathan-Stachurski cell combined with WDML-5 slow strain rate tensile machine. And then the hydrogen permeation parameters under each condition were calculated accurately. The results showed that no matter under what stress level, the steady-state hydrogen penetration current density increased as cathodic polarization potential decreases and the steel’s sensitive hydrogen precipitation potential was lower than-1000 m V(vs. SCE).When the cathodic polarization potential was-1100 m V(vs. SCE), the hydrogen permeation flux of X80 steel would change during dynamic tensile process. The flux increased with theincrease of stress at the beginning, then began falling after it reached steady condition, and later increased significantly when the tensile sample was completely going into the plastic deformation, finally the flux began to decreasing at the necking stage. When the stress was among the scope of 0 ~ 110% σs,the apparent hydrogen diffusion coefficient D increased first and then decreased with the increase of stress, on the other hand, the sub-surface hydrogen concentration C0 decreased first and increased later on.According to the standard tensile curve of X80 steel, the deformation process under tensile stress can be divided into pure elastic stage, elastic-plastic deformation stage and fully plastic deformation stage. When only pure elastic deformation occurred, it could not produce new lattice defects in a certain range. However, the lattice expanded, and the increase of lattice energy could prompted a large number of hydrogen entering into the metal, and at the same time the space between metal atoms increased which make it easier for hydrogen to penetrate, and this can explain why the hydrogen permeation flux and diffusion coefficient increased as the stress level increases. The calculation of hydrogen trap energy confirmed that when X80 steel appeared a small amount of plastic deformation, the new dislocations acted as the hydrogen trap to capture hydrogen, so the hydrogen permeation flux and diffusion coefficient decrease with the increase of plastic deformation.