The effect of different cathodic potentials applied to the X70 pipeline steel immersed in acidified and aerated synthetic soil solution under stress using a slow strain rate test (SSRT) and electrochemical impedance spectroscopy (EIS) was studied. According to SSRT results and the fracture surface analysis by scanning electron microscopy (SEM), the steel susceptibility to stress corrosion cracking (SCC) increased as the cathodic polarization increased (Ecp). This behavior is attributed to the anodic dissolution at the tip of the crack and the increment of the cathodic reaction (hydrogen evolution) producing hydrogen embrittlement. Nevertheless, when the Ecp was subjected to the maximum cathodic potential applied (−970 mV), the susceptibility decreased; this behavior is attributed to the fact that the anodic dissolution was suppressed and the process of the SCC was dominated only by hydrogen embrittlement (HE). The EIS results showed that the cathodic process was influenced by the mass transport (hydrogen diffusion) due to the steel undergoing so many changes in the metallic surface as a result of the applied strain that it generated active sites at the surface.
An approximate analytical solution to solve the nonlinear diffusion-reaction transport problem has been developed. The system considered is a liquid membrane where the facilitated transport or carrier-mediated transport occurs. The approximate analytical solution methodology is based on the Taylor series expansion of the reaction rate around the two limiting surfaces of the liquid membrane. The methodology leads to analytical expressions for the concentration profiles of the species in the membrane. Predictions for the facilitation factor have been obtained for a wide range of Damko ¨hler number values, from the physical diffusion regime to the equilibrium chemical reaction regime, considering cases of equal and unequal carrier and complex diffusivities and cases of zero and nonzero downstream solute concentration. For the example presented here, the difference between the approximate predictions and those obtained from the numerical solution is not greater than 5.0%.
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