Lecture 7: Theory for Aqueous Corrosion (V)

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4.10 POTENTIAL-pH DIAGRAMS

Also called POURBAIX DIAGRAMS

are charts based on thermodynamic calculations that can be used to distinguish a corroding condition from a non-crroding condition.

CORRODING CONDITION: [Mn+] > 10-6 M

IMMUNITY CONDITION: [Mn+] < 10-6 M

Possible regions of a E-pH diagram: immunity, corrosion or passivation.

Each domain indicates a region in which one species is the most thermodynamically stable. If the metal is the most stable species then it is considered immune to corrosion, but if a soluble ion is most stable then the metal should corrode. A region in which an insoluble corrosion product is the most stable species is considered passive.

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Calculation and Construction of E-pH diagram

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Application and Limitation

It is the E-pH diagram that made the study of corrosion a reputable science. Its applications include:

Practical limitations in the use of E-pH diagram are:

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Summary

E-pH diagram is a very useful chart that maps out the different regions of a corroding system. These possible regions are immunity, corrosion, passivation or cracking. It can be used to formulate corrosion comtrol methods and to predict the possible corrosion products. However, its practical application is limited by its lack of kinetic information and thermodynamic nature of origin.

Reading Assignments

To reinforce learnings in this lecture read pages 106-110 (textbook)
To prepare yourself for the
next lecture read pages 130-143 (textbook)

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