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New insights into hydrogen permeation in steels : measurements through thick membranes

Abstract : 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.
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J. Kittel, F. Ropital, J. Pellier. New insights into hydrogen permeation in steels : measurements through thick membranes. Corrosion 2008, Mar 2008, New-Orleans, United States. ⟨hal-02475533⟩

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