Hydrogen evolution potential

Over the past several years, Gruen and coworkers have examined the SH response from iron electrodes in alkaline solutions [45, 53, 172]. In their work on polycrystalline iron, they concluded that the potential dependent SH response which was observed during surface oxidation could be attributed to two intermediate phases on the electrode surface between the passive film at oxidative potentials and the reduced metal at hydrogen evolution potentials [53]. They have recently extended this work to Fe(110). In this study [172], they examined the SH rotational anisotropy from this crystal under ambient conditions. They found that the experiments reveal the presence of both twofold and threefold symmetric species at the metal/oxide interface. When their data is fit to the theory of Tom et al. [68], they conclude that the measured three-fold symmetric oxide is found to be tilted by 5° from the Fe(110) plane. The two-fold symmetric structure is aligned with the Fe(110) surface. 

Very different results can be obtained in the case of a potentiostatic or potentiodynamic program within the hydrogen evolution potential region. For example, Figure 12.5 shows the anodic potentiodynamic profile of the platinum/acid aqueous solution interface after 60 min potential holding at —1.5 V. 

Fig. 6.4 Efb versus pH for p-GaPN and for a p-GaPN tandem. Both materials show Nemstian behavior. The desired fb for n-type materials (not shown) should be above the solid line for the hydrogen evolution potential to enable majority electron carriers to drive the HER. Similarly, the fb for p-type materials (shown here) should be below the dashed line for the oxygen evolution potential to enable majority hole carriers to drive the OER...

The problems of hydrogen embrittlement and of gas evolution are usually controlled by limiting the potential of the steel to below the hydrogen evolution potential. However, in acidic pits or crevices it may be possible for the potential to exceed the hydrogen evolution potential without being sensed by measuring electrodes. The cathodic protection of prestressed structures should only be undertaken with great care and input from experienced corrosion experts. An excellent state-of-the-art report on the cathodic protection of prestressed has been published (NACE, 2001). 

The steel should not exceed the hydrogen evolution potential, especially for prestressed steel to avoid hydrogen embrittlement. 

Ideally, we would like to depress the potential sufficiently to reach the immune zone. However, that is very close to the hydrogen evolution potential (lower dotted line) at pH 12 which is where steel in concrete lies. For the reasons discussed earlier in this section, we want to avoid hydrogen evolution, so we aim for the area below the pitting potentials. This is discussed in Appendix 1 of BSEN12696 (2000) and also in the section on criteria 7.4. 

Because of the risk that if the potential exceeds the hydrogen evolution potential hydrogen embrittlement could occur with potentially... 

To extend the hydrogen evolution potential in an aqueous media, a fluorine-terminated diamond surface is prepared in a radio frequency plasma reactor of CF-He [124]. Fluorine-terminated diamond is usually used to study electrode reactions that require high over potential. For covalent attachment of different biomolecules, hydrogen-terminated diamond electrodes are also treated with ammonia plasma to prepare the surface with terminal amine groups [125]. 

Most standards state that cathodic protection should not be applied to prestressed concrete structures. This is because of the risk that if the potential exceeds the hydrogen evolution potential, hydrogen embrittlement could occur with potentially catastrophic failure of the steel These problem, were discussed in Sections 6.2.1 and 6,5. High strength steel may trap atomic hydrogen which weakens grain boundaries and the crystalline structure. There are several issues that can be considered when considering cathodic protection of prestre,ssed concrete structures ... 

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