In the early days, hydrogen damage had been categorized as respectively "external" or "internal" hydrogen when the damaging process does or does not require the continuous presence of the corrosive medium. Stress dependent cracking belong to the former type, and stress independent cracking to the latter. Due to the large variability of corrosive media and cracking types, it has been recently attempted to directly compare the severity of media through their ability to dissolve hydrogen into steel. in order to avoid any risk of experimental or interpretative artefact, a very direct experimental characterization was chosen. The principle is to measure the back pressure P-H2 developing in a hollow sensor immersed in each medium. In the steady state, P-H2 measures the hydrogen activity in the steel. In the calibration stage with respect to the severity of sulfide stress cracking (SSC), as displayed in the EFC 16 diagram, P-H2 was surprisingly found to depend mainly on pH, and nearly not on P-H2S. Degassing towards the hydrogenating medium was also observed in parallel to charging. This is contradictory to the hitherto commonly accepted mechanism of hydrogen charging in sour service, based on a supposed inhibition of the recombination of adsorbed hydrogen atoms H-ads into H-2 molecules. in addition, SSC severity also appears to be related to the kinetics of hydrogen charging rather than to P-H2. Again, this is inconsistent with the conventionally acknowledged metallurgical processes. Moreover, the traditional wording of "atomic hydrogen" is misleading, since like any metallic alloying element, hydrogen dissolved in the crystal lattice is dissociated into a proton and a conduction electron. In order to resolve these three inconsistencies, two new mechanisms were proposed for both hydrogen charging and hydrogen stress cracking (HSC) : Hydrogen charging is considered to involve a direct proton transfer H-electrolyte(+) --> H-metal(+), acting as a "protonic" cathodic reaction. This is not exclusive of the classical electronic cathodic reduction of H+ ion into H-2. Both contribute to the cathodic current i(K) and are compatible with simultaneous degassing. A key issue is that the new charging process is driven by the hundreds of mV of the electrochemical potential, instead of the few tens of the desorption energy of H-ads. HSC is a consequence of this much stronger driving force. In a reverse manner to Troiano's mechanism involving the effect of triaxial tensile stresses upon local H content, the high energy charging should generate high surface stresses capable of initiating cracks at the surface. Their combination with working stresses shall then lead to a stress dependent cracking process. Altogether, damages related to "internal hydrogen" should depend on the concentration of dissolved hydrogen and those related to "external hydrogen" on the entry rate.
