REVISITING HYDROGEN IN STEEL, PART I: THEORETICAL ASPECTS OF CHARGING, STRESS CRACKING AND PERMEATION

摘要

This article first of all summarizes recent changes in the understanding of hydrogen charging and hydrogen stress cracking (HSC) mechanisms: In a completely unexpected manner, hydrogen charging is found to coexist with significant degassing of the metal towards the corrosive medium. The surface reaction 2 H_(ads) ->H_2 21 -> thus extremely rapid, even in sour media. It is therefore in no way inhibited, contrary to former belief. Charging probably involves a cathodic reaction in which a proton is transferred directly from the electrolyte into the metal, without requiring the intermediate formation of an adsorbed hydrogen atom. Charging and degassing can therefore coexist. Charging is thus itself a faradaic process and is consequently extremely powerful. Damage due to so-called "internal" hydrogen (blistering, hydrogen-induced cracking, stepwise cracking) always involves bulk metallurgical processes related to the concentration of dissolved H. In contrast, damage due to so-called "external" hydrogen (HSC, SSC) involves surface processes related only to the charging rate. The forced injection of protons locally expands the crystal lattice, thus inducing high surface stresses that add to the working stresses and cause surface crack initiation. Finally, the coexistence of charging and degassing explains why the severity of the medium varies with P_(H2S) in a different way depending on whether "external" or "internal" hydrogen is concerned. The consequences on permeation measurements and their interpretation are then discussed. In particular, the coexistence of charging and degassing radically modifies permeation conditions, with the need to separate thick membranes, for which the concentration c_0 beneath the entry face is constant and the diffusion flux inversely proportional to the diffusion distance d, and thin membranes, for which the diffusion flux is constant and the concentration c beneath the entry face proportional to d. All these theoretical predictions have the advantage that they can be verified experimentally, and Part II of the present study shows that this is indeed the case.