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Module Four of CCE 281 Corrosion: Impact, Principles, and Practical Solutions

E-pH Diagram of Metals

Building a Pourbaix or E-pH diagram to represent the stability of a metal or an alloy in a given environment is not an insurmountable task. However it could take a few hours of your precious time to produce what is commonly a universally accepted tool to discuss the expected behavior of metals, many Thanks to the well known Belgium scientist that gave his name to these diagrams. The process of building these diagrams should always follow the following steps:

  1. Study background reference material on the metal/environment of choice. For the Iron-water system we found four acceptable references.

  2. Decide on the species that will be considered. For the Iron-water system the data representing the species considered is abundantly available.

  3. Decide on the target state of the species considered. For many metals and alloys there are different levels of hydration in the scale of stability. The Iron-water system is typically described in two states of hydration, i.e. wet and dry. The addition of extraneous soluble species such as commonly present chloride and sulfate ions can greatly complicate the thermodynamic picture.

  4. Write down the equations interrelating the chemical species corresponding to the state chosen. The twenty three equations representing the wet state of iron species in pure water are shown here.

  5. Well, now that the easy part is done, one has to go over a few sleepless nights to come to the diagram presented here.

Fortunately a few software systems are available to compute E-pH diagrams.

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See also: Equilibrium reactions of iron in water, Pourbaix diagram of iron, Rust chemistry

References

  1. Le HH, Ghali E: Interprétation des diagrammes E-pH du systeme Fe-H2O en relation avec la fragilisation caustique des aciers. Journal of Applied Electrochemistry 1993;72-77.

  2. Silverman DC: Presence of Solid Fe(OH)2 in EMF-pH Diagram for Iron. Corrosion 1982;38:453-455.

  3. Townsend HE: Potential-pH Diagrams at Elevated Temperature for the System Fe-H2O. Corrosion Science 1970;10:343-358.

  4. Biernat RJ, Robins RG: High-Temperature Potential/pH Diagrams for the Iron-Water and Iron-Water-Sulphur Systems. Electrochimica Acta 1972;17:1261-1283. (back)