Effect of membrane thickness on hydrogen permeation in steels during wet H2S exposure

• Published in: Corrosion (Houston, Tex.) Tex.), 2008-10, Vol.64 (10), p.788-799
• Main Authors: Kittel, J., Ropital, F., Pellier, J.
• Format: Article
• Language: English
• Subjects: Chemical Sciences
• ISSN: 0010-9312
• DOI: 10.5006/1.3278446

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 the corrosion potential have shown a constant flux for various membrane thicknesses in the range 0.05-0.8 mm. These results revealed 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 10 mm thick. The transition between thin and thick membranes 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.