| Thermodynamics: Through the use of thermodynamic theory (the Nernst equation), so-called Pourbaix
diagrams can be constructed. These diagrams show the thermodynamic stability of species as
a function of potential and pH. Although many basic assumptions must be considered in
their derivation, such diagrams can provide valuable information in the study of corrosion
phenomena. The diagram below represents a simplified version of the Pourbaix diagram for
the iron-water system at ambient temperature. For the diagram shown, only anhydrous oxide
species were considered and not all of the possible thermodynamic species are shown.
click on image to enlarge
Some limitations of such diagrams include:
- No information on corrosion kinetics is provided by these thermodynamically derived
diagrams.
- The diagrams are derived for specific temperature and pressure conditions.
- The diagrams are derived for selected concentrations of ionic species (10-6 M
for the above diagram).
- Most diagrams consider pure substances only - for example the above diagram applies to
pure water and pure iron only. Additional computations must be made if other species are
involved.
- In areas where a Pourbaix diagram shows oxides to be thermodynamically stable, these
oxides are not necessarily of a protective (passivating) nature.
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See also:
Thermodynamic corrosion cycle
Water stability diagram
References/Literature:
M. Pourbaix: "Atlas of Electrochemical Equilibria in Aqueous
Solutions", Pergamon, New York, 1966.
E.D. Verink: "Simplified Procedure for Constructing Pourbaix
Diagrams", in Uhlig's Corrosion Handbook 2nd Edition, Ed. R.W. Revie, Jon
Wiley, New York, 2000.
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