Blending hydrogen in existing natural gas pipelines: Integrity consequences from a fitness for service perspective

Blending hydrogen in existing natural gas pipelines compromises steel integrity because it increases fatigue crack growth, promotes subcritical cracking and decreases fracture toughness. In this regard, several laboratories reported that the fracture toughness measured in a hydrogen containing gaseous atmosphere, KIH, can be 50% or less than KIC, the fracture toughness measured in air. From a pipeline integrity perspective, fracture mechanics predicts that injecting hydrogen in a natural gas pipeline decreases the failure pressure and the size of the critical flaw at a given pressure level. For a pipeline with a given flaw size, as shown in this work, the effect of Hydrogen Embrittlement (HE) in the predicted failure pressure is largest when a failure occurs by a brittle fracture. The HE effect on failure pressure diminishes with a decreasing crack size or increasing fracture toughness. The safety margin after a successful hydrostatic test is reduced and therefore the time between hydrotests should be decreased. In this work, all those effects were quantified using a crack assessment methodology (level 2, API 579-ASME FFS) considering literature values for KIH and KIC reported for an API 5L X52 pipeline steel. To characterize different scenarios, various crack sizes were assumed, including a small crack with a size close to the detection limit of current in-line inspection techniques and a larger crack that represents the largest crack size that could survive a hydrotest to 100% of the steel Specified Minimum Yield Strength (SMYS). The implications of a smaller failure pressure and smaller critical crack size on pipeline integrity are discussed in this paper

Effect of nickel on hydrogen permeation in ferritic/pearlitic low alloy steels

Nickel offers several beneficial effects as an alloying element to low alloy steels. However, it is, in the oil and gas industry, limited by part 2 of the ISO 15156 standard to a maximum of 1 wt% due to sulfide stress cracking resistance concerns. Hydrogen uptake, diffusion, and trapping were investigated in research-grade ferritic/pearlitic low alloy steels with Ni contents of 0, 1, 2 and 3 wt% by the electrochemical permeation method as a function of temperature and hydrogen charging conditions. Qualitatively, the effective diffusion coefficient, Deff, decreased with increasing Ni content. The sub-surface lattice hydrogen concentration, C0, decreased with increasing Ni content in all charging conditions while the trend between the sub-surface hydrogen concentration in lattice and reversible trap sites, COR, and Ni content varied with the charging conditions. Irreversible trapping, evaluated by consecutive charging transients, was not observed for any of the materials. Lastly, the possible influence of an increasing fraction of pearlite with increasing Ni content is discussed

Efficacy of Sterilization Methods and Their Influence on the Electrochemical Behavior of Plain Carbon Steel

Minimizing contamination of control treatments in microbiologically influenced corrosion (MIC) studies is of critical importance. Metal sterilization procedures should not alter the surface nor affect the inherent susceptibility of the metal to corrosion while adequately deactivating biological activity. However, there is no consensus in the literature regarding such procedures due to, in part, the lack of a universally accepted methodology. This investigation evaluates various sterilization methods for carbon steel concerning practicality, efficacy, and effects on the electrochemical response of the metal. Three sterilization procedures using i) dry heat, ii) ethanol, or iii) glutaraldehyde as sterilizing agents were evaluated. Even though all sterilization approaches were equally effective in eliminating microorganisms and spores from the metal surface, dry heating at 170°C in an inert atmosphere was identified as the most convenient sterilization method regarding practicality and consistency in the electrochemical response of the metal. Sterilization of carbon steels in 75 vol% ethanol and glutaraldehyde, as well as alcohol followed by flaming, is discouraged given the large dispersion in corrosion response caused by the exposure to the sterilization media

Effect of nickel in solid solution on the hydrogen embrittlement susceptibility of low alloy steels

© 2017 Chinese Society of Theoretical and Applied Mechanics. All Rights Reserved. In the oil and gas industry, the use of low alloy steels (LAS) in H2S containing environments is governed by ISO 15156-2. Nickel is limited to a maximum of 1 wt% due to sulfide stress cracking (SSC) resistance concerns. This work investigated the effect of solid solution nickel in the ferrite phase on hydrogen transport kinetics and hydrogen stress cracking (HSC) susceptibility. Ferritic/pearlitic research-grade LAS with nominal nickel contents of 0, 1, 2 and 3 wt% were examined. Electrochemical hydrogen permeability experiments were carried out to investigate hydrogen diffusion, solubility, and trapping in the steels. The relative HSC susceptibilities of the steels were determined by slow strain rate (SSR) testing with in situ hydrogen charging. Combining hydrogen permeability and SSR tests allowed for the quantification of the HSC resistance as a function of nickel content

Effect of thermal treatment on the localized corrosion behavior of alloy 718 (UNS N07718)

Nickel (Ni) based Alloy 718 (UNS N07718) is used extensively for Oil & Gas sour applications requiring yield strength levels in excess of 1000 MPa, mainly in the heat treated high strength condition for downhole and wellhead accessories and other components. It was of interest to determine how age hardening heat treatments affect the corrosion behavior of N07718. Alloy 718 was tested in the as-received condition and under the oil & gas API6A718 one step aging condition and the traditional two-step​ aging condition. The electrochemical tests were performed in 3.5% NaCl solution and in the NACE TM0177 Solution A (5% NaCl + 0.5% acetic acid). Immersion tests according to ASTM G 48D were used to determine the critical crevice temperature of the alloy. Laboratory results showed that the heat treatment performed to increase the strength of the alloy produced a slight decrease in the resistance to localized corrosion