Hydrogen diffusion into three metallurgical microstructures of a C–Mn X65 and low alloy F22 sour service steel pipelines

•Hydrogen diffusion in two pipeline steels (C-Mn API 5L X65 and low alloy F22) in three different metallurgical microstructures.•New insights on H permeation test to investigate the effect of pure diffusion and trapping.•Criticism of different methodologies to represent better the transport of H in industrial metals.

This paper describes the process of hydrogen diffusion in API 5L X65 and ASTM A182 F22 steels with three different metallurgical microstructures (quenched and tempered, quenched and annealed) by an electrochemical technique which is a modified Devanathan and Stachurski's cell. The analysis of experimental results compares different measurement methods for hydrogen diffusion coefficients: 1) charge method; 2) partial charge and partial discharge method; 3) discharge method. For the steel, cathodically polarized in acidic medium (pH ≈ 4.2), the less dispersed and more meaningful results are obtained by partial charge and partial discharge method after about one hundred hours of polarization, i.e., when a stationary regime was reached. The discharge method, carried out after one hundred hours of polarization, provides analogous results by extrapolation of the experimental results obtained during the initial phase of hydrogen desorption. The hydrogen diffusion coefficients measured would appear dependent only on the hydrogen migration processes in the crystal lattice regular sites and corresponding values DL increase as follows: quenched < quenched and tempered < annealed.