Influence of hydrogen-containing environments on fatigue crack extension resistance of metals

The meaning of the thresholds of hydrogen-assisted cracking

“…Thus, local environments may destroy K-driven autonomy. At the same time, in gases, the difference between the local and bulk activities of hydrogen (the drop of its partial pressure at the crack tip (pCT < p) caused by the delay of the gas-phase transport in a crack [35,36]) exists for a quite short period of time Z~H due to the high mobility of gases, and parity is rapidly attained: pCT ___> p. For the process of electrolytic hydrogenation in various systems, it was demonstrated [26] (but not yet proved as a general rule) that ~CT CT ,,~ --AE~E, where the (cathodic) hydrogen-evolution overpotential AE CT = Ev CT -E v (pH CT) {with the following bound of thermodynamic stability for water: E v (pH) = -0.014 -0.059 pH (V) [26,27] } varies within a broad range and attains the asymptotically stable level of the most severe hydrogenation (Fig. 6) ~CT(t) --9 ~HC~* as t---> ~…”

“…The reliability of the threshold SIF as an invariant characteristic requires the K-controlled autonomy of the crack-tip zone for both mechanical and physicochemical events of HAC [26][27][28]. This problem is discussed in what follows.…”

“…It must be well applicable to the case of HAC in corrosive systems including both cathodic hydrogenation and anodic mechanisms activating different factors of the environmental effect on fracture such as hydrogen degradation, oxide film creation and rupture, and anodic dissolution [26,28,[36][37][38]. Their competition runs in time and is affected by the dynamics of loading, so that the extreme damage may be caused in a transient manner for quite short periods of time.…”

“…The latter depends on environmental flows, diffusion-like transport, and chemical reactions on the crack faces and in the volume of the crack [26][27][28][35][36][37]. The list of variables controlling the crack-tip environment starts with crack depth a determining the transport distance toward the tip.…”

“…For environmentally assisted cracking in a stationary system of a material and bulk surroundings, it was recognized that the local crack-tip environment is variable and significantly differs from the global one and that the envi-ronmental effects on cracking are governed by the local physicochemistry [26][27][28]. The latter depends on environmental flows, diffusion-like transport, and chemical reactions on the crack faces and in the volume of the crack [26][27][28][35][36][37].…”