Corrosion process hydrogen electrode reaction

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

In normal battery operation several electrochemical reactions occur on the nickel hydroxide electrode. These are the redox reactions of the active material, oxygen evolution, and in the case of nickel-hydrogen and nickel-metal hydride batteries, hydrogen oxidation. In addition there are parasitic reactions such as the corrosion of nickel current collector materials and the oxidation of organic materials from separators. The initial reaction in the corrosion process is the conversion of Ni to Ni(OH)2. 

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What is the mechanism of this phenomenon Very early during investigations of this field, it was realized that metals become embrittled because at some stage of their career, their surface was the scene of a hydrogen-evolution reaction either because the metal was deliberately used as an electron-source electrode in a substance-producing cell or because parts of the metal became electron-source areas in a corrosion process. In fact, the phenomenon has come to be known as hydrogen embrittlement. 

The results of such measurements are known as current density-potential curves. They represent cumulative curves given by the superimposition of the current density-potential curves of the individual reactions. For simple electrodes with defined electrode processes, these are the overpotential curves. For metals exposed to electrolytic attack, superimposition of several overpotential curves gives the actual current density-potential curves that are of significance in corrosion testing and research. Figure 20.9 shows the superimposition of the overpotential curves of a hydrogen electrode... 

The hydrogen evolution reaction is historically very important since its study has contributed much towards our understanding of electrode reactions. It is also met in corrosion and as a cathode reaction in water electrolysers, some chlorine cells and other oxidation processes. 

Fig. 10.8 Steady-state log (current)-electrode potential diagram for a metal M corroding via hydrogen evolution. Both electrode processes are under activation control. The diagram shows the definition of corrosion current i corr the corrosion potential Ecokr The reversible potential and corresponding to the exchange currents i and io for the single electrode reactions are also shown together with the cathodic polarization rj = corr E and the anodic polarization = corr E. ...

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