Hydrogen diffusivity in X65 pipeline steel: Desorption and permeation studies

Hydrogen diffusivity can play a crucial role in hydrogen-assisted degradation of steels. Herein, we investigate the hydrogen diffusion behavior in two X65 pipeline steels using desorption and permeation techniques. Gas charged samples were used for desorption experiments, while permeation tests were conducted using electrochemical charging. The Vintage steel, having a banded ferrite-pearlite microstructure, exhibits a significant influence of strong traps. A modified solution of Fick's 2nd law is proposed to account for the strong traps. At room temperature, the effective diffusivity in the Modern steel, which has homogenous ferrite-bainite microstructure, has a scatter factor of three, whereas for the Vintage steel, the factor is 17. The difference is explained by considering a concentration-dependent effective diffusivity and a tortuous diffusion path. Comparable values of the highest measured effective diffusivity at room temperature were obtained for the Modern and Vintage steels: 2.70 x 10-6 and 3.34 x 10-6 cm2/s, respectively.