Assessment of Hydrogen Trapped by Microalloyed Precipitates in Steel Using Electrochemical Permeation Tests

A numerical method to solve the McNabb-Foster model is proposed to estimate the H trapping-detrapping rates and H-trapping abilities of alloy carbides in steels using permeation transients. Three model microalloyed steels with one of Nb, Ti, and V were prepared in the form of thin sheets and aged suitably for getting precipitates of alloy carbides (NbC, TiC, or VC) in ferrite matrix. The results of the analysis show that the apparent H diffusivity in the aged Nb-added alloys, is lower (2-4.3 × 10⁻¹⁰ m²/s) than in the aged V-added alloys (9.3-25.6 × 10⁻¹⁰ m²/s). Slower diffusion of H through Nb-steel was attributed to the higher H-binding energy of Nb-carbonitride in the Nb-added steel (13-17.9 kJ/mol) compared to the other two steels having lower binding energies (Ti-added steel: 6.7-8.0 kJ/mol, V-added steel: 3.9-5.9 kJ/mol) and therefore faster H permeation. The Nb-added and Ti-added steels in aged conditions showed a similar range of H trapping ability (0.9-2.5 ppmw) and lower apparent diffusivity compared to the V-added steel. The permeation exponent (\(p\)), measured from the experimental transients, was found to have linear correlations with trapped-H contents and H trapping and detrapping rates during the permeation tests with different sensitivities depending on the type of H-trap sites, i.e., NbC, TiC, or VC.