corrosion in metal: local cell modelReading time:
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.