Corrosion of iron exposed to H2S saturated solution at pH 4 was studied by electrochemical impedance spectroscopy, weight loss coupons and surface analysis. Hydrogen permeation was also used as indirect means of evaluating the intensity of the proton reduction reaction leading to hydrogen entry into the metal. Since corrosion in this type of test solution results in the rapid build-up of a conductive and highly porous iron sulfide scale, a specific contribution of the film has to be considered. An impedance model was thus proposed. The faradaic anodic impedance consists of a two-step reaction with charge transfer and adsorptiondesorption. An additional contribution, associated with the conductive and highly porous iron sulfide film was added in parallel. This contribution, mostly visible in the 2 low frequency domain, presents a 45° tail associated with a porous electrode behavior. This model was well adapted to describe impedance diagrams measured at various exposure times, up to 620 hours. Charge transfer resistance determined from impedance analysis allowed calculating the evolution with time of the corrosion current density. A very good correlation was found between this corrosion current density and the hydrogen permeation current density. As expected in our experimental conditions, a permeation efficiency close to 100 % is demonstrated. Corrosion rate of 490 µm/year was measured by weight-loss specimens, confirming the validity of the impedance analysis, which resulted in a calculated corrosion rate of 530 µm/year.

Materials selection in the oil and gas industry relies on engineering standards, such as NACE TM0177 and NACE TM0284, which stipulate that oxygen pollution should be avoided during materials testing in H 2 S-containing media. In this paper, we explore the manner in which traces of oxygen can modify the test solution chemistry and the corrosion of/ hydrogen permeation across iron membranes in H 2 S-containing solutions. Oxygen pollution is shown to strongly influence solution chemistry, through the introduction of sulfur-oxygen reaction products resulting in bulk acidification. Weight loss, electrochemical methods, and solution chemistry measurements conclude that iron corrosion rates in the presence of oxygen pollution are doubled, when compared against the control system (without oxygen pollution). Unexpectedly, despite a lower pH and higher corrosion rates in the oxygen-polluted H 2 S-containing solutions, the hydrogen permeation rate decreases monotonically, relative to the control. We discuss how this observation is most likely related to a disruption of sulfur adsorbates involved in hydrogen entry promotion.

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Materials selection in the oil and gas industry relies on engineering standards, such as NACE TM0177 and NACE TM0284, which stipulate that oxygen contamination should be avoided during materials testing in H 2 S-containing media. In this second paper, as part of a series of articles that evaluates how traces of oxygen modify the corrosion of pure iron and hydrogen permeation across iron membranes in H 2 Scontaining solutions, the impact of changing the H 2 S partial pressure from 100 kPa to 0.1 kPa was investigated. It was found that bulk solution chemistry for all H 2 S partial pressures changes with time, due to the formation of H 2 SO 2 reaction products (sulfates, sulfites, and thiosulfates), which results in bulk solution acidification. Electrochemical and weight-loss measurements confirm that Fe corrosion rates in baseline well-deaerated H 2 S-containing solutions decrease with decreasing H 2 S partial pressure, although these are observed to be much higher under continuous oxygen contamination. With decreasing H 2 S partial pressure, hydrogen uptake in Fe also decreases, due to lower and lower concentrations of dissolved H 2 S and the associated increase in pH. However, even at 1 kPa and 0.1 kPa H 2 S, permeation effciencies remain close to 100% when no O 2 contamination is present. The hydrogen uptake is always relatively lower in Fe exposed to oxygen-polluted H 2 S solutions. Permeation efficiencies decrease continuously. From electrochemical data and surface characterization, these observations at lower H 2 S partial pressures are attributed to the disruptive effect of oxygen on the nature of sulfide corrosion products, and hydrogen entry promotion, along with the contribution of an additional cathodic reaction that does not result in hydrogen entry into the metal.

This paper highlights the importance of considering the magnitude of acetate (ethanoate) species concentration on corrosion and hydrogen permeation rates, important factors associated with cracking initiation in steels for sour service qualification. Materials selection relies on standards, such as NACE TM0177 and NACE TM0284, which stipulate that oxygen pollution should be avoided during testing in H2S-containing media. The 5% NaCl test solutions in current standards are buffered using acetic acid (CH3COOH)/sodium acetate (CH3COONa) to fix the solution pH over long periods. In this third paper, as part of a series of articles that evaluate how oxygen entry modifies the corrosion of (and hydrogen permeation across) low alloy steel membranes in H2S-containing solutions, we investigate the effect that changing the solution chemistry has through testing X65 steel in different concentrations of acetic acid and sodium acetate in H2S-saturated 5% NaCl solutions, i.e. Solutions A and B (NACE TM0177-2016), and the HLP solution of NACE TM 0284-2016. Increasing the total acetic acid + acetate concentration encourages a higher average X65 corrosion rate and longer-sustained hydrogen charging flux, assigned to the cathodic reaction rate enhancement by acetic acid and the iron solubilizing effects of acetates. Introducing 300 ppb of dissolved oxygen does not push the solution pH outside of the permitted error range but increases average X65 corrosion rates and, again, helps sustain hydrogen permeation flux for longer. Through an evaluation of the surface structure and electrochemical impedance spectroscopy data, this appears to be down to an increase in the permeability and porosity of the troilite FeStroilite dominant scale. The HLP solution (at pH 3.5), with the highest acetic acid and acetate concentration, is the most aggressive. In this electrolyte, an iron sulfide overlayer structure is attained with an oxygen-rich inner layer between the metal substrate and a thick iron sulfide film.

We demonstrated recently that polyelectrolytes with cationic moieties along the chain and a single anionic head are able to form physical hydrogels due to the reversible nature of the head-to-body ionic bond. Here we generate a variety of such polyelectrolytes with various cationic moieties and counterion combinations starting from a common polymeric platform. We show that the rheological properties (shear modulus, critical strain) of the final hydrogels can be modulated over three orders of magnitude depending on the cation/anion pair. Our data fit remarkably well within a scaling model involving a supramolecular head-to-tail single file between cross-links, akin to the behaviour of pine-processionary caterpillar. This model allows the quantitative measure of the amount of counterion condensation from standard rheology procedure.