POURBAIX DIAGRAM

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.   

 

Some limitations of such diagrams include:

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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