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.
Anatomy of a Pourbaix Diagram
The Pourbaix diagram provides much information on the behaviour of a system as the pH and potential vary.
The following animation explains how a Pourbaix diagram is built up from fundamentals.
Examples of a Pourbaix Diagram
Gold’s Pourbaix diagram explains why it is the most immune substance known. It is immune in all regions in which cathodic reactions can take place. So gold never* corrodes in an aqueous environment.
Immunity of aluminium only occurs at lower potentials. Therefore, unless under conditions that cause it to passivate, it is much more susceptible to corrosion than gold or zinc.
* provided that the water is pure; that no ion complexes are present to provide a cathodic half cell reaction that occurs at a potential higher than +1.5 V(SHE).
Constructing a 3D Pourbaix Diagram
A Pourbaix Diagram does not have to be limited to two dimensions. Three (or higher) dimension diagrams can be constructed by varying other parameters such as concentration or temperature.