Effect of nickel in solid solution on hydrogen transport kinetics in low alloy steels

Abstract

The use of low alloy steels (LAS) in H2S-containing environments in the oil and gas industry is governed by the ISO standard 15156-2 (NACE MR0175-2). One requirement, which has been disputed over the years, is that the nickel (Ni) content shall not exceed 1 wt%. This work investigated the effect of Ni in solid solution on hydrogen diffusion, solubility, and trapping in ferritic/pearlitic research-grade LAS with nominal Ni contents from 0 to 3 wt%. Hydrogen permeability experiments were carried out in a Devanathan-Stachurski setup at 15, 45 and 70 °C. The effective diffusion coefficients, calculated by the tlag method, decreased with increasing Ni content. The sub-surface hydrogen concentration in lattice and trap sites increased with increasing Ni content. There was no difference between the first and subsequent hydrogen permeation transients, suggesting that Ni in solid solution forms reversible traps. The effect of Ni in refining the microstructure may be superimposed on the effect of Ni in solid solution, and should be investigated in future work.