Effect of nickel in solid solution on the hydrogen embrittlement resistance of low alloy steels
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The ever-increasing demand and consumption of energy in the world is depleting conventional oil and gas reserves. Nickel alloying improves mechanical and technological properties of low alloy steels (LAS), but is limited by the ISO 15156 standard to a maximum of 1 wt% in H2S containing environments due to controversial sulfide stress cracking concerns. Lifting the 1 wt% Ni restriction in the ISO standard could greatly benefit the development of sour oil and gas reservoirs with severe temperature and pressure conditions. The goal of this work was to investigate the effect of solid solution Ni in the ferrite phase on hydrogen stress cracking (HSC) resistance. Electrochemical hydrogen permeation tests and slow strain rate testing (SSRT) were performed on ferritic/pearlitic research-grade LAS with nominal Ni contents of 0, 1, 2 and 3 wt%. Hydrogen uptake, diffusion, and trapping were investigated as a function of Ni content at different temperatures and in three different electrochemical charging environments. Ni alloying did not form irreversible hydrogen traps, but the effective diffusion coefficient, Deff, decreased with increasing Ni content at all tested temperatures and charging environments. The sub-surface lattice hydrogen concentration, C0, reflecting the hydrogen charging severity, decreased with increasing Ni content in all environments, while the trend between the sub-surface hydrogen concentration in lattice and reversible trap sites, COR, with Ni content varied with the charging conditions. SSRT was performed with cathodic hydrogen charging to potentials of -1.05 and -2 VAg/AgCl in 3.5 wt% NaCl solution. Ni alloying did not change the fracture morphology of the LAS, but the plastic elongation ratios and reduction in area ratios decreased monotonically with increasing Ni content when tested at -2 VAg/AgCl. The fraction of pearlite in the LAS increased with increasing Ni content and deconvoluting the direct effects of Ni in solid solution from the indirect influence of Ni alloying – altering the phase equilibria of the LAS – on hydrogen permeation and SSRT results was difficult. Samples of pure Fe and Fe alloyed with 1 wt% Ni were subject to in situ electrochemical nanoindentation to investigate the effect of Ni alloying on homogeneous dislocation nucleation (HDN), detected by the pop-in load, in the presence of hydrogen. In the hydrogen charged condition, the pop-in load decreased to about 50% of the values obtained in air for both alloys. The decrease in pop-in load showed that hydrogen lowered the activation energy for HDN and that the body centered cubic iron was susceptible to hydrogen embrittlement (HE), but 1 wt% Ni in solid solution did not reduce HE resistance. The in situ electrochemical nanoindentation work should be expanded to Ni contents above 1 wt% and, ideally, testing should be done in the ferrite phase of LAS containing interstitial carbon. Further work to elucidate the effect of Ni in solid solution should focus on micromechanical test methods that effectively circumvent the indirect effects of Ni alloying.