Models hydrogen embrittlement

Figure 6.49 Schematic presentation of (a) the slip dissolution and (b) the hydrogen embrittlement models 02...

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. 

Setting aside the specifics of these localized fiacture mechanisms in the process zone for the present, it is apparent that hydrogen embrittlement models predict discontinuous crack propagation at an average rate of... 

As approaches the crack can also propagate by hydrogen embrittlement processes during the higher load parts of the stress cycle. This forms the basis of various models which have been developed to describe corrosion fatigue, probably the best-known of which are the superposition models due to Wei"". In its most recent version this model takes the form ... 

This model is considered to be useful to improve the knowledge of the role played by the factor of hydrogen accumulation in prospective rupture sites by stress-assisted diffusion, one of the key items in hydrogen embrittlement, a very dangerous phenomenon that frequently accompanies structural metals and alloys in service. 

Proposed computational model seems to be a promising tool as an aid to develop the life-prediction analyses for metallic components and structures subjected to any king hydrogen embrittlement in service. 

In alkaline solution, the discharging entity for the cathodic reaction is water. The cathodic reaction has been studied quite extensively 73, 74). Besides its practical importance (in hydrogen embrittlement, corrosion, hydrogen oodeposition, etc.), its study should serve as a model for elucidating the mechanism of other electrode reactions. 

The proponents of this model reasoned that similar fracture processes occur in liquid-metal embrittlement, hydrogen embrittlement, and S.C.C., with chemisorption facilitating the nucleation of dislocations at the crack tip and promoting the shear processes that result in brittle, cleavage-like fracture. It was found that cleavage fracture occurs by alternate slip at the crack tip and formation of voids ahead of the crack tip [35-37]. 

Thus far, we have charted in this review a progression that began with the structure and reactivity of water at a metallic surface and moved on to the adsorption of other ions and neutral species and next to the response of a system to coadsorption and surface factors that control hydrogen embrittlement. As a final instance of how first-principles modeling can illuminate... 

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