Hydrogen Diffusion and EAC of Pipeline Steels Under Cathodic Protection

Pipeline steels can show Environmental Induced Cracking phenomena under slow straining with Hydrogen Embrittlement mechanism under cathodic protection. Hydrogen evolution can take place due to cathodic protection normally used in order to protect the pipeline against general corrosion. The steel is polarized at cathodic potentials in the range −0.8 to −1.1 V vs SCE, but very negative values could be reached in overprotected areas close to the impressed current anodes. The hydrogen ions reduction reaction takes place on the metal surface, forming adsorbed hydrogen. The adsorbed hydrogen diffuses into the metal owing its solubility in the metal lattice. Afterwards it can concentrate in plastic deformation areas, and promote brittle growth. Figure 1 shows the results of slow strain rate tests on a low alloyed steel. The loading curves obtained at −1050mV vs SCE is very different from the curve obtained in air. The differences are related to the presence of brittle areas on fracture surphace and secondary cracks (figure 1).