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© 2018 by NACE International. Super duplex stainless steels are ferritic-austenitic stainless steels with 25 wt% Cr and a pitting resistance equivalent (PRE) = 40. Even though other elements, such as Cr and Mo, have been studied in much more detail than W, research has shown that an optimal composition exists, in which W in solid solution improves localized corrosion resistance. Outside this range, W is ineffective or even detrimental. However, the mechanisms by which W improves localized corrosion resistance in stainless steels are unclear. For example, debate still exists as to whether W enhances passivity or facilitates repassivation. In this work, two super duplex stainless steels were investigated: a W-free (UNS S32750) and a 2.1 wt% W-containing (UNS S39274) grade. The first goal was to study differences in the overall localized corrosion resistance; while the second objective was to gain an insight into the mechanisms by which W affects the localized corrosion performance of super duplex stainless steels. The work was divided into two parts. Firstly, anodic potentiodynamic polarization curves were conducted in different simulated pit environments at various temperatures. Two simulated pit solutions were tested: 1 M HCl and 7 M LiCl adjusted to a pH of 0. In the second part, crevice repassivation potentials were measured using the potentiodynamic-galvanostatic-potentiodynamic technique in 3.5 wt% NaCl pH = 6.5 solution. This investigation will provide evidence regarding the influence of W in passive film stability and repassivation kinetics.

Nickel additions to low alloy steels improve mechanical and technological properties. However, Part 2 of ISO Standard 15156 limits the nickel content to a maximum of 1 wt% in oil and gas environments containing H2S because of controversial concerns regarding sulfide stress cracking. The objective of this work was to investigate the effect of nickel in solid solution in the ferrite phase on hydrogen stress cracking resistance. Ferritic/pearlitic research-grade low alloy steels with nominal nickel contents of 0, 1, 2, and 3 wt% were tested by the slow strain rate test method with cathodic hydrogen charging to -1.05 VAg/AgCl and -2 VAg/AgCl. No difference in fracture mode or morphology was found between the alloys. However, the plastic elongation ratios and reduction in area ratios decreased with increasing nickel content when tested at -2 VAg/AgCl. The direct and indirect effects of nickel, such as the influence of an increasing fraction of pearlite with increasing nickel content, are discussed.

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.

© 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.

Crevice corrosion affects the integrity of stainless steels used in components exposed to seawater. Traditionally, crevice corrosion testing involves the use of artificial crevice formers to obtain a critical crevice potential, which is a measure of the crevice corrosion resistance of the alloy. The critical acidification model proposed by Galvele predicts that the critical crevice potential is the minimum potential required to maintain an acidic solution with a critical pH inside either a pit or a crevice. Application of Galvele’s model requires an estimation of both the diffusion length and the i vs. E behavior of the metal in the solution inside the crevice. In this work, the crevice corrosion resistance of a 22%Cr duplex stainless steel (UNS S31803) and a 25%Cr super duplex stainless steels (UNS S32750) was investigated. The i vs. E response of the two stainless steels was determined in acidified solutions of various chloride concentrations, which simulate those found in an active crevice. Critical potentials predicted by the critical acidification model were compared with critical crevice potentials measured in simulated seawater. Results showed that despite the various assumptions and simplifications made by Galvele, the model correctly predicted the occurrence of crevice corrosion of both 25Cr super duplex stainless steel and 22Cr duplex stainless steel close to room temperature in a 3.5 wt% NaCl environment. Critical potentials obtained by Galvele’s model were similar if assuming that the chloride concentration of the simulated crevice solutions was between 7 M and 12 M acidified to a pH of 0.

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.

Low alloy steels (LAS) are extensively used in oil and gas (O&G) production due to their good mechanical properties and low cost. NACE MR0175/ISO15156 specifies a maximum of 1wt% nickel (Ni) in LAS. That is, the NACE/ISO restriction excludes a significant number of high-strength and high-toughness alloys, such as Ni-Chromium (Cr)-Molybdenum (Mo), Ni-Mo and Ni-Cr-Mo-Vanadium (V) grades, for sour service applications. Ni is desirable in LAS because it: a) increases hardenability, strength, and fracture toughness, b) has a low penalty on weldability, and c) lowers the ductile to brittle transition temperature (DBTT) below -100°C. The aim of the current research was to melt eight experimental steels containing 0.2%C + 0.8% Mn and a varying concentration of Ni between 0 and 5%, and test their electrochemical response in deaerated NACE TM0177 solution A. Preliminary experimental results showed that Ni did not have a linear effect on the electrochemical behavior of the experimental steels at the corrosion potential and under cathodic polarization. In the anodic potential range, additions of 1% Ni or higher reduced the anodic dissolution current.

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.

Low-alloy steels (LAS) are extensively used in oil and gas (O&G) production due to their good mechani- cal properties and low cost. Even though nickel improves mechanical properties and hardenability with low penalty on weldability, which is critical for large subsea compo- nents, nickel content cannot exceed 1-wt% when used in sour service applications. The ISO 15156-2 standard limits the nickel content in LAS on the assumption that nickel concentrations above 1-wt% negatively impact sulfide stress cracking (SSC) resistance. This restriction excludes a significant number of high-strength and high-toughness alloys, such as Ni-Cr-Mo (e.g., UNS G43200 and G43400), Ni-Mo (e.g., UNS G46200), and Ni-Cr-Mo-V grades, from sour service applications and can be used only if suc- cessfully qualified. However, the standard is based on controversial research conducted more than 40 years ago. Since then, researchers have suggested that it is the micro- structure that determines SSC resistance, regardless of Ni content. This review summarizes the advantages and dis- advantages of nickel-containing LAS in terms of strength, weldability, hardenability, potential weight savings, and cost reduction. Likewise, the state of knowledge on the effect of nickel on hydrogen absorption as well as SSC ini- tiation and propagation kinetics is critically reviewed.

Magnesium and magnesium alloys are susceptible to stress corrosion cracking in various environments, including distilled water. There is compelling evidence to conclude that SCC is assisted, at least in part, by hydrogen embrittlement. This paper reviews the thermodynamics of the Mg-H system and the kinetics of hydrogen transport. Aspects of magnesium corrosion relevant to hydrogen absorption are also discussed. Crack growth mechanisms based on delayed hydride cracking, hydrogen adsorption dislocation emission, hydrogen enhanced decohesion, and hydrogen enhanced localized plasticity have been proposed and evidence for each of them is reviewed herein.

Infrared and high resolution Raman spectroscopic probes of thin films of C60 and C70 are presented and discussed in terms of previous measurements, semi-empirical calculations and plausible molecular geometries.