Reduction of Redox Systems

Corrosion in open circuit conditions requires a counter reaction to consume the electrons, which are produced by metal dissolution. Important reactions are hydrogen evolution and oxygen reduction. Fe + reduction is another. [Fe(CN)6] reduction is a simple outer sphere reaction, which may be used for basic corrosion studies. In most cases, one tries for simplicity to study metal dissolution and the redox reactions separately with the use of a potentiostat. In the following, the hydrogen evolution and the oxygen reduction reaction are discussed separately. 

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A very important part is the metal/elec-trolyte interface. This is the site where anodic metal dissolution and the compensating cathodic reduction of redox systems take place. At this site, adsorption and layer formation occur which might drastically reduce the corrosion rate by inhibition of the corresponding electrode reactions. The adjacent electrolyte is the medium where preceding or consecutive electrode reactions occur, and diffusion or migration of corro-... 

The detailed mechanism is often not fully understood. A blocking of active sites is often postulated for anodic dissolution or cathodic reduction of redox systems. However, the conclusions were mostly drawn indirectly from electrokinetic data, and a detailed investigation with surface analytical methods is still missing. The chemical reaction steps and their inhibition at surface sites should be further investigated systematically by methods like XPS, AES, STM, SFM, etc. Most of the knowledge is still empiric. However, the results of electrochemical investigations are very valuable for a better corrosion protection of materials in industrial applications. Some few principles of inhibition are discussed in the following sections. 

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