Localized corrosion hydrogen-induced cracking

Several testable models for stress-corrosion cracking (SCC) of metals are discussed in terms of the main experimental variables stress, metallurgy, and environment. Slip-dissolution, film-induced cleavage, and hydrogen embrittlement models are all shown to be consistent with experimental data in particular systems. Other models that cite effects of corrosion (without a film) or adsorption on crack tip deformation, leading to microcleavage or plastic microfracture, are less easy to test. No model can be universal in view of the demonstrable multiplicity of mechanisms. In many cases the atomistic mechanism is unknown, yet cracking can be controlled or predicted via the localized corrosion process that precedes SCC. 

The determination of the crack tip environment is not easy because of the numerous electrochemical reactions and the associated mass transport and thermodynamic criteria that govern this environment. The nature of the solution (composition, pH, species, corrosion products that can induce roughness, etc.) can be very different at the crack tip and in the bulk solution. Figure 12.16 shows the possible electrochemical reactions at the crack tip. Both anodic dissolution effects and a coupled hydrogen reaction must often be taken into account. The mathematical modeling of the localized crack tip chemistry is therefore possible [19]. 

---