Dr. Dmitri Kopeliovich
Pourbaix diagram (Electrode potential / PH diagram) is a graphical presentation of the thermodynamic equilibrium states of a metal-electrolyte system.
Pourbaix diagrams are plotted in the axes Electrode potential of the metal vs. PH of the electrolyte.
Oxidizing conditions are described by the top part of the diagram (high positive electrode potential).
Reducing conditions are described by the bottom part of the diagram (high negative electrode potential).
Acidic solutions are presented in the left side of the diagram (PH lower than 6).
Alkaline solutions are presented in the right side of the diagram (PH higher than 6).
The lines of the diagrams dividing different zones of the equilibrium states are calculated by the Nernst equation:
E = E0 - (0.059/n)*lnCion
E0 - Standard electrode potential, V;
n - number of electrons transferred;
Cion - molar activity (concentration) of ions.
Pourbaix diagrams allow to determine the corrosion behavior of a metal in water solutions i.e. the direction of electro-chemical processes and the equilibrium state of the metal at a certain electrode potential in a water solution at a certain value of PH.
Normally the Poubaix diagrams are built for the water solutions with the concentrations of metal ions 10-6M and at the temperature 298K (77ºF/25ºC).
An example of a Pourbaix diagram for the system iron-water is presented in the picture.
The diagram defines the following zones of the equilibrium states:
Horizontal lines of the Poubaix diagrams correspond to the redox reactions, which are independent of PH.
Vertical lines of the Poubaix diagrams correspond to the non-redox reactions (electrons are not involved), which are dependent on PH.
Diagonal lines of the Poubaix diagrams correspond to the redox reactions, which are dependent on PH.
Here are some of the reactions and the corresponding lines of the Fe-H2O Pourbaix diagram:
Limitations of Pourbaix diagrams: