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Materials and corrosion (2022). doi:10.1002/maco.202213140, Published by Wiley-VCH, Weinheim [u.a.]

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International audience; Equipments which operate in wet hydrogen sulfide (H 2 S) environment can be subjected to Hydrogen Embrittlement (HE) damages like Hydrogen Induced Cracking (HIC), Stress Oriented Hydrogen Induced Cracking (SOHIC), or Sulfide Stress Cracking (SSC). The resistance of steels to HIC and SSC is usually evaluated through standard tests (NACE TM0177 and NACE TM0284). These tests give operators and steel suppliers a pass/fail response for a given application. However, the detailed mechanisms of wet H 2 S cracking are not fully understood yet, although a lot of work has been carried out over the years on this subject. In particular it is of interest to better understand the initiation and propagation phases of HIC and SSC. For this purpose, some laboratory studies have used the acoustic emission (AE) technique to monitor the materials behaviour during HIC and/or SSC tests. Most of the results were promising, as some correlations were found between AE parameters and the cracking susceptibility of different materials. However, the most important gap stands in the analysis of AE data, which is always considered as a bulk, without any attempt to separate the contribution of other acoustic sources (such as general corrosion or hydrogen bubbling in the solution) from the cracking itself. A methodology was developed in this work to discriminate HIC signals from other sources. The aim was to isolate the cracking contribution, and compare HIC propagation mode for different non sour service steels. In addition tensile tests monitored by AE on two sour service steels are also presented. Difference between SOHIC and SSC mechanisms could be evidenced by AE.

The permeation of hydrogen in steel in the presence of acid gases is not a simple phenomenon as the steel may contain trapping sites and also because the permeation may be governed by surface reactions associated with corrosion. Recently, hydrogen permeation experiments carried out at corrosion potential have shown a constant flux for various membrane thicknesses in the range 0.05-0.8 mm. This is in apparent contradiction with the Fick's laws of diffusion, predicting a diffusion flux inversely proportional to the thickness of the membrane. Such results confirm the possibility for permeation to be governed by surface reaction and not by the diffusion through steel: this situation corresponds to a thin membrane case. For thicker membranes it is expected that diffusion mechanisms through the steel is the rate determining step, so that the flux proportionality with the inverse of the membrane thickness is recovered. Nevertheless, it seems also that the limit between thin membrane and thick membrane situations is not universal, but depends strongly on the hydrogenating media: previous studies in sour service environments showed that 1 mm membranes could still be considered as thin, i.e. controlled by the interfacial fluxes rather than by the diffusion flux. All these results reveal the difficulty to express the flux for thicker steel membrane (i.e. pipe) from laboratory studies on thin membranes, as the classical rule (flux proportional to the inverse of the membrane thickness) is not always applicable and not conservative. This paper presents new permeation results, obtained on steel membranes up to 10mm thick. The transition between thin and thick membrane is clearly established, and is in the millimeter range in sour conditions. The necessity to adopt a new interpretative framework to link permeation measurements and hydrogen cracking mechanisms is reinforced. For thin membranes, the permeation flux is constant and governed only by the charging flux crossing the entry face. This surface mechanism is probably correlated with surface cracking mode, like SSC. On the other hand, the traditional concept of diffusion can only be used in thick membrane situations. The diffusion flux is inversely proportional to the sub-surface concentration. This concentration is in direct relation with the hydrogen activity in the steel, which is probably correlated with internal cracking modes, like HIC.

International audience; In the oil and gas industry, it is known that equipments which operate in hydrogen sulfide (H 2 S) media can be subjected to damages like Hydrogen Induced Cracking (HIC) and Sulfide Stress Cracking (SSC). At the present time, material selection for sour environments is generally obtained using standard tests (NACE TM0177 and NACE TM0284) which give information about the resistance of steels in sour service. These tests are suitable for a quick assessment of candidate steels by operators or steel suppliers. However they cannot provide detailed information to better understand and quantify the damage occurring during the service life of steel components. Acoustic emission (AE) is an efficient technique to monitor degradation of materials. In the present work it is used for the early detection, characterization and time progress description of cracking phenomena caused by hydrogen embrittlement of steel in sour media. The methodology used for the identification of AE sources related to HIC (H 2 bubbles, FeS corrosion layer, cracking) is described. AE results are discussed and correlated with crack length ratio and AE performance for quantifying HIC damage is evaluated.