Hydrogen-induced plasticity

Hydrogen-induced plasticity. Another approach is related to the fact that hydrogen increases local plasticity. Two models have been proposed based on the adsorption and absorption of hydrogen. 

Figure 5-55. Schematization of adsorbed hydrogen induced plasticity model for CF cleavage-like cracking.

Other mechanisms have been proposed, particularly the hydrogen-induced plasticity model for precipitates containing materials such as Al-Zn-Mg alloys. This model is shown in Figure 19 [5]. 

Schematization of the hydrogen-induced plasticity model for CF cleavage like cracking. (From Lynch, S.P., Acta Metall, 36,2639,1988.)...

Figure 2. The dipole moment of the absorbed water molecules varies from approximately 1.8 to 0.9 for the polyimides and from 1.1 to 0.7 for the polyamide-imides corresponding to fractional polarizabilities of l.O/i -0.4/x. The low values of p (<0.5p ) as seen in all the amide-imide po ymers and several of the ° polyimides, indicate restricted mobility of the water molecules. In the amide-imide polymers, we believe this is due to increased water-polymer interactions such as hydrogen bonding. Other evidence of hydrogen bonding in polyamide-imides is the water-induced plasticization and Tg lowering frequently observed.

Water-induced plasticization of polymers by disruption of intermolecular hydrogen bonding between polymer chains that are generally considered to be hydrophobic is rather common. Moy and Karasz [30] show that the lowering of Tg for an epoxy-diamine resin is proportional to the amount of water in the system. 

Nair and Tien [91] explained the effects of yield strength on Kiscc through hydrogen-induced fracture resulting from the plastic flow on a localized scale in the crack tip plastic zone. The suggested cause of SCC was isolated and unstable local fracture events in the crack plastic zone controlled by the local embrittlement due to adsorbed hydrogen [92]. 

The specific role of stress and strain in hydrogen embrittlement and cracking mechanisms has been established in the case of only a few models such as hydrogen-induced decohesion (HIDE) and hydrogen-induced local plasticity (HELP). 

In cases of plastic deformations within a critical rate range, however, even relatively soft structural steels in seawater may be at risk for hydrogen-induced stress corrosion cracking [25]. 

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