Corrosion mechanism and characteristic processes

Based on the electrochemistry of Mg discussed in the prior sections, the corrosion mechanism and characteristics of Mg and its alloys can be illustrated as follows. 

Self-corrosion is basically an anodic dissolution process at a rate equal to that of the coupling cathodic process on an electrode. Under a steady state self-corrosion condition, all the electrons generated by anodic reactions are consumed by cathodic reactions on an elecrode. On Mg the overall cathodic process is reaction (1.77) or (1.78) and the overall anodic process is reaction (1.43) or (1.44), respectively. When they are coupled, ie. having the same rate, the overall corrosion of Mg can be written as  

The detailed anodic and cathodic reactions under the self-corrosion condition are schematically presented in Fig. 1.13. In general, anodic reactions occur mainly in film-free areas whereas in film-covered areas anodic dissolution is negligible. Cathodic reactions with hydrogen evolution as a main process, can take place in both the film-free and film-covered areas, although in a film-free area much faster than on a film-covered surface. 

The corrosion mechanism of Mg discussed above is applicable to both pure and the matrix phase (a single phase Mg alloy) of Mg alloys. The matrix phase has the same crystalline structure as Mg and similar electrochemical behavior, the presence of alloying elements in the matrix phase may alter 

13 Schematic illustration of anodic and cathodic reactions involved in the self-corrosion of Mg. 

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