A Pourbaix diagram plots the equilibrium potential (Ee) between a metal and its various oxidised species as a function of pH.
The extent of half-cell reactions that describe the dissolution of metal
M = Mz+ + ze-
depend on various factors, including the potential, E, pH and the concentration of the oxidised species, Mz+. The Pourbaix diagram can be thought of as analogous to a phase diagram of an alloy, which plots the lines of equilibrium between different phases as temperature and composition are varied.
To plot a Pourbaix diagram the relevant Nernst equations are used. As the Nernst equation is derived entirely from thermodynamics, the Pourbaix diagram can be used to determine which species is thermodynamically stable at a given E and pH. It gives no information about the kinetics of the corrosion process.
Constructing a Pourbaix Diagram
The following animation illustrates how a Pourbaix diagram is constructed from first principles, using the example of Zinc.
Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.
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