PSI - Issue 13

Zahreddine Hafsi et al. / Procedia Structural Integrity 13 (2018) 210–217 Hafsi et al. / Structural Integrity Procedia 00 (2018) 000–000

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In fig. 7 the hydrogen concentration was expressed in ppm (parts per million i.e. number of mg of solute per kg of solution). In fact, ppm is convenient unit for very small concentration, as in our case of hydrogen concentration in the metal. Considering molar mass to hydrogen M H =0.5 x 2.02 g/mol=1.01 g/mol (as the diatomic molecule of hydrogen H 2 dissociates prior its dissolution) and a density of steel material of 7850 kg/m 3 , one gets 1mol/m 3 =0.12866 ppm ). A comparison of the concentration profiles over the pipeline wall for the three considered materials is plotted in Fig.8. It’s observed that X80 and nitrided X52 steel materials are more resistant to hydrogen penetration in the lattice structure. Furthermore, a zoom on the concentration profile in the vicinity of the outer surface of the pipe demonstrates that, contrarily to the use of X80 and nitrided X52 steel material for which one is still working on safe conditions (no hydrogen leakage), for API X52 base steel, traces of hydrogen are existing that means the gas has already escaped from the pipe wall with a small amount. The latter may lead to installation failure and even catastrophic incidences.

Fig. 8. Numerical and analytical solution of hydrogen concentration (in ppm ) at t=24h for unnitrided API X52 steel

Such first findings illustrated by Fig.8 have to be more investigated by looking for to what extent hydrogen has permeated into the lattice. Hence, the value of permeation depth  for each material at t=24 h is to follow up. Actually,  can be obtained through the COMSOL model or, as the simulation is already validated by the analytical solution, the latter may be calculated by calculating the value of x that causes the RHS expression of equation (6) to vanish for t=24h . Table 2 summarizes calculated permeation depths for the three tested materials as well as correponding characteristic diffusion lengths

Table 2. Permeation depths and characteristic diffusion lengths at t=24 hours Material  ( mm )

Dc t ( mm )

Unnitrided API X52 Nitrided API X52

13.5237 6.3117 8.4836

1.932 0.901 1.212

API X80

It’s well confirmed through the values of  that under the simulation conditions, precautions have to be taken once using X52 natural gas pipelines to transport hydrogen. In fact, added to overpressure values likely to cause installations failure, the diffusion of such flammbale gas had led to a leakage (the permeation depth  exceeds the wall thickness e ) that may yield unfortunate consequences. Table 2 confirms also that the alternative of using API X80 steel material for gas conduction makes installations more resistant to hydrogen embrittlement. Furthermore, it’s observed that the nitriding process of X52 steel makes the latter more resistant to hydrogen diffusion as such treating process acts on the surface hardeness and reduces considerably the material diffusion coefficient. Besides the studied cases of material, another alternative that was also used for gas installation mainly in power plants scale, is the coating of internal wall of steel pipelines. Coating is a protective measure against corrosion and it’s also eager to reduce hydrogen embrittlemnt effect as hydrogen needs to diffuse through the coating layer that will