Hydrogen embrittlement theories

In considering the various theories it is also apparent that many of them may be considered as alternative descriptions of essentially the same physical process, or as descriptions of parallel processes which collaborate in the failure. Thus a complete description of hydrogen embrittlement in a given situation will almost inevitably incorporate aspects of several of the following theories. 

The role of microstructure has been treated in terms of the trap theory of hydrogen embrittlement [65, 133]. The theory assumes that, regardless of the mechanism of embrittlement (see Sect. 2.2.6.3), the concentration of trapped hydrogen... 

Internal Pressure This theory considers hydrogen embrittlement to be caused by the formation of molecular hydrogen and the resulting buildup of pressure at internal voids or other internal surfaces [142, 143]. It provides an explanation for blistering in low-strength steels but does not adequately account for other forms of degradation such as HSC. 

One important prototypical study in this area is that on the Pd (111) surface and the interaction of hydrogen with this surface.This system is of great interest because of its fundamental importance in chemisorption theory and in technological areas such as catalysis, hydrogen storage, and hydrogen embrittlement. 

It is postulated that specific ions are absorbed and interact with strained bonds at the surface of the crack tip, thus reducing the bond strength, and permitting continued brittle fi acture. This theory has been supported by observations in SCC. By chemisorption of the environmental species on the crack tip, the local fracture stress of the metal lattice is reduced. The theory has been applied to hydrogen embrittlement and liquid metal embrittlement. The adsorption phenomenon may be used to interpret the crack propagation mechanism of alloys which fail by hydrogen embrittlement, such as aluminum alloy 7075. 

Until the end of the 1960s, the theory of hydrogen embrittlement was considered inapplicable to aluminium alloys, for two reasons ... 

A model study of hydrogen interaction with the lattice of Ta has been carried out by Grena et al. through the Density Functional Theory (DFT), which allows us to describe the modifications of the Ta lattice induced by the presence of interstitial hydrogen. In particular, the DFT evaluated the elastic constants, which play an important role in the embrittlement of the hydrogenated material. The elastic constants evaluated in this work are shown in Fig. 4.16. The main result of the study was to show that the bulk modulus is unaffected by the presence of hydrogen, while the shear modulus presents a minimum (at H/M = 0.5, that is, moles of hydrogen per mole of metal) that could induce brittle behaviour. 

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