Abstract
The permeation of hydrogen in steel in the presence of acid gases is not a simple phenomenon because the steel may contain trapping sites and 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 from 0.05 mm to 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, since 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 a surface cracking mode, like sulfide stress cracking (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 subsurface concentration. This concentration is in direct relation with the hydrogen activity in the steel, which is probably correlated with internal cracking modes, like hydrogen-induced cracking (HIC).
