Hydrogen absorption, diffusion, and equivalent fugacity of Ni steels exposed to thiosulfate solutions

The allowable nickel content in low alloy steels is restricted in standards used for selection of pipelines and structural materials resistant to hydrogen embrittlement. ISO 15156/NACE MR0175 limits Ni to ≤1 wt.% for sour service, motivated by reported decreases in sulfide stress cracking (SSC) resistance with increasing Ni; SSC is a hydrogen‑assisted failure caused by absorbed hydrogen in hydrogen sulfide (H₂S) solutions. Hydrogen permeation through nickel‑containing steels was measured using the Devanathan–Stachurski permeation cell, with 10 -3 M thiosulfate acidified brine at the charging side, reproducing sulfide chemistry without H₂S bubbling. Steels with 0–5 wt.% Ni were quenched and tempered to comparable microstructures and hardness near the 22 HRC limit. Nickel increased the corrosion potential, reduced the thermodynamic driving force for the hydrogen evolution reaction, and promoted the formation of sulfide films. The resulting lattice hydrogen concentration on the charging side (C₀) decreased monotonically with Ni, by up to an order of magnitude at 5 wt.% Ni (≈0.06 to 0.004 ppm). Trapping was not significantly enhanced by Ni, as trapped hydrogen in 3–5 wt.% Ni steels was lower than in 0–1 wt.% Ni steels. The implications of these observations for the nickel effect in SSC and other hydrogen‑assisted stress‑cracking modes of low alloy steels are discussed.