Interpretations on the hydrogen-induced cracking mechanism in sour media using H-pressure probe studies

Steel pipelines can fail by hydrogen-induced cracking (HIC) on exposure to environments where hydrogen sulfide (H₂S) is present, and corrosion is accompanied by hydrogen entry into the material bulk.  HIC cracks originate at abnormalities within the matrix such as inclusions and precipitates, sites where diffusing hydrogen may collect, react, and form localized high pressure H₂ gas atmospheres.  There remains some doubt in the literature and operators’ belief concerning HIC in sour media.  For instance, one parameter often quoted is the absolute 'hydrogen content' contained within a sample, to gauge a rule-of-thumb HIC risk.  Simply the value of the total hydrogen content acquired (or could be acquired) within the steel is correlated to the likelihood of HIC.  However, we have reason to believe that the importance of the rate of hydrogen entry (flux) should not be understated.     

Classical electrochemical permeation (Devanathan-Stachurski) and thermal desorption techniques (TD), accompanied by numerical analyses, have advanced to the extent that the hydrogen content can be measured and even partitioned into diffusible and trapped hydrogen. Considering experimental methods, neither electrochemical permeation nor TD methods constitute a simple, simultaneous in situ diffusible hydrogen collection and flux methodology which could be useful as a screening tool in materials testing and alloy development against HIC.  Indeed, both techniques require skill to implement, interpret and analyze.   

The H-pressure probe technique, where a hollow probe fabricated from the alloy of interest is connected to a sensitive pressure sensor and installed within a cell or an autoclave, may offer a safer and direct alternative – comparable to industrial corrosion and cracking testing.  As hydrogen adsorbs at the external environment/H-probe interface and is absorbed, a hydrogen diffusion flux is set up towards the sealed internal cavity which continues to “collect” the diffusing hydrogen. Practically, such a setup permits the testing of several types of metallic test specimens.  In this study, simultaneous to HIC tests across the pH-PH₂S diagram, H-probes were used to measure H-diffusion fluxes across the steel wall as well as attaining a value for the lower limit of total accumulated (inner cavity-collected) hydrogen. Crack aspect ratio (CAR) measurements using ultrasound inspection indicate a far closer correlation between the H-flux and cracking severity, rather than the total hydrogen pressure inside the cavity of the hollow sensor. We discuss these findings in the context of hydrogen flux as a bulk defect accumulator that may facilitate HIC.