E-pH Diagrams of Metals

The E-pH diagram of aluminum and zinc are quite similar and surely amongst the simplest E-pH diagrams of all metals. The Pourbaix diagram of aluminum will be used here to demonstrate how such diagrams are constructed from basic principles. In the following discussion, only four species containing the aluminum element will 

Since there is no change in valence of the aluminum present in the two ionic species considered, the associated equilibrium is independent of the potential and the expression of that equilibrium can be derived in Eq. (4.43) for standard conditions. 

Assuming that the activity of H O is unity and that the activities of the two ionic species are equal, one can obtain a simpler expression of the equilibrium in Eq. (4.43) based purely on the activity of Eq. (4.45) and its logarithmic form, Eq. (4.46). 

By using the standard thermodynamic data from the literature [3], it is possible to calculate that the free energy of reaction in Eq. (4.42) is in fact equal to 120.44 kj moP when both [AP ] and [AlO -] are equal. Equation (4.47) then becomes Eq. (4.48). 

This is represented, in the E-pH diagram shown in Fig. 4.11, by a dotted vertical line separating the dominant presence of AP+ at low pH from the dominant presence of AlO at the higher end of the pH scale. 

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