corrosion in metal: local cell model

Our understanding of corrosion caused to metal by water has been systemised using the localised corrosion cell model. This model provides a description of the electro-chemical process that causes metals to dissolve. This model is based on the Wagner & Traud "mixed potential theory" with its two principles :

• every electrochemical reaction can be broken down into at least two partial oxidation and reduction reactions ;
• there can be no net accumulation of electrical charges during an electrochemical reaction.

In this model, metal destruction through oxidation and dissolution is shown as the following reaction :

This oxidation reaction has been defined as the "anodic" process. In order to meet the electro-neutrality requirement, the oxidation and reduction reactions must take place at the same time, thus consuming the same number of electrons :

The reduction reaction is called the "cathodic" process. The combined oxidation reduction process, at equilibrium, is written as :

Figure 1 illustrates the overall process.

The global potential of the combined oxidation reduction process will determine whether or not the reaction can take place and, consequently, the likelihood of corrosion.

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