11631 Mechanisms of HE

There are three postulated mechanisms responsible for hydrogen embrittlement:

1. Internal pressure. Here molecular hydrogen is said to accumulate at sub-critical crack sites leading to a pressure enhancement of existing residual or applied tensile stresses. The subsequent effect is that the fracture stress is exceeded, that is, a + p > a0, where a is the applied/residual stress, p is the internal pressure, and a0 is unembrittled fracture stress [33]. This mechanism is generally invoked to explain the phenomenon of delayed fracture [34].

2. Surface adsorption. In this case adsorbed H atoms lower the surface energy of the cracks [35].

3. Decohesion. Absorbed hydrogen atoms at interstitial sites weaken the lattice cohesion causing a lowering of the cleavage fracture stress [36].

The basic process resulting in hydrogen adsorption involves hydrogen entering the metal interstitially via natural corrosion reactions or through cathodic polarization. Two possible pathways are available for the atomic H formed (Fig. 11.16): (i) interstitial diffusion into the metal lattice and (ii) the combination of adsorbed species to form molecular H, which leaves the metal surface in gaseous form.

FIGURE 11.16 Possible pathways available for hydrogen atom recombination or interstitial adsorption.

H2(gas)

FIGURE 11.16 Possible pathways available for hydrogen atom recombination or interstitial adsorption.

Which of these reactions will dominate and the rate at which they proceed will depend upon factors such as alloy composition, temperature, metal potential, catalytic activity of the metal surface, and solution chemistry.

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