Analysis of the various processes downstream cathodic hydrogen charging III: Mechanistic issues on charging, degassing and sulfide stress cracking

Abstract

The elementary steps of hydrogen charging and degassing are identified from a combination between reaction orders and acoustic emission, and the higher degassing rate on the H 2 S side makes the natural permeation in the field still more different from its laboratory model. In sour service, airstream measurement devices are certainly able to measure a permeation flux, but the true difficulty is to subsequently derive reliable information on the actual level of hydrogen charging throughout the steel wall. Regarding sulfide stress cracking (SSC), all what is known since 70 years is both consistent and contradictory, i.e. with very deep trends, like the involvement of a surface directly exposed to cathodic charging, or the combination of shear strain, dislocation glide and hydrogen drag, but also approximate, oversimplified or obsolete assumptions. Hence, the systematic failure of standard mechanics to explain all aspects of SSC. Similarly in metallurgy, a mere steric effect associated with the outdated atomic hydrogen cannot be at the same time sufficiently large or small for simultaneously explaining Cottrell clouds and dislocation drag. Nevertheless, the reality of the SSC mechanism should not be basically different from the previous thoughts, but substantially more complicated, and this will be addressed in Part IV.