Pourbaix VII

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Pourbaix VII
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Pourbaix Diagram

• Chemical potential
• Pourbaix diagram

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Pressure Dependence of Gibbs Free Energy-Ideal Gas
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Application of the Chemical Potential to an Ideal
gas

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Pourbaix Diagrams

• Marcel Pourbaix provided the brilliant means to
  utilize thermodynamics more effectively in
  corrosion science and electrochemistry in
  general. This development resulted in four
  important books that interpret his work
  Thermodynamics of Dilute Aqueous Solutions, Atlas
  of Electrochemical Equilibria in Aqueous
  Solutions (solid-aqueous equilibria) Lectures on
  Electrochemical Corrosion (a teaching text) and,
  in his last years, Atlas of Chemical and
  Electrochemical Equilibria in the Presence of a
  Gaseous Phase (solid-gaseous equilibria). His
  outstanding work in thermodynamics provided one
  of the main underpinnings of electrochemistry,
  especially in corrosion science.

Marcel Pourbaix b. 1904, Myshega, Russia d.
September 28, 1998, Uccle (Brussels), Belgium
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Pourbaix Diagrams

• 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 on the right 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.   
• How do we construct this diagram for water?

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Pourbaix Diagrams

• Use Nernst Equation

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Pourbaix Diagrams
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Pourbaix Diagrams
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Pourbaix Diagrams

• We will consider Cu in an aqueous solution as the
  next exercise five different reactions are
  involved.

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Pourbaix Diagrams
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Pourbaix Diagrams
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Pourbaix Diagrams

• The diagram shown here shows how the potentials
  for reduction and oxidation of water vary with pH
  for natural waters. These are the inner two lines
  that slope downward from low pH to high pH. Note
  that the pH scale only runs from 2-10 (we are
  talking here about natural waters). For both
  oxidation and reduction of water, an additional
  line is shown that lies 0.6V above (for oxidation
  of water) or below (for reduction) the
  theoretical E. This pair of lines represents the
  potentials including an approximation for the
  overvoltage. Lastly, there is a pair of vertical
  lines at pH4 and 9. These are reflective of the
  fact that most natural waters have a pH somewhere
  between these limits. .

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Pourbaix Diagrams

• A Pourbaix diagram is an attempt to overlay the
  redox and acid-base chemistry of an element onto
  the water stability diagram. The data that are
  required are redox potentials and equilibrium
  constants (e.g. solubility products). On the
  right is the Pourbaix diagram for iron. Below
  that is the same diagram showing only those
  species stable between the water limits.

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Pourbaix Diagrams

• Equilibrium Reactions of iron in Water
• 1. 2 e- 2H  H2
• 2. 4 e- O2  4H  2H2O
• 3. 2 e- Fe(OH)2  2H  Fe 2H2O
• 4. 2 e- Fe2  Fe
• 5. 2 e- Fe(OH)3-  3H  Fe 3H2O
• 6. e- Fe(OH)3  H  Fe(OH)2 H2O
• 7. e- Fe(OH)3  3H  Fe2 3H2O
• 8. Fe(OH)3-  H  Fe(OH)2 H2O
• 9.  e- Fe(OH)3  Fe(OH)3-
• 10. Fe3  3H2O  Fe(OH)3 3H
• 11. Fe2  2H2O  Fe(OH)2 2H
• 12.  e- Fe3  Fe2
• 13. Fe2  H2O  FeOH H

• 14. FeOH  H2O  Fe(OH)2(sln) H
• 15. Fe(OH)2(sln)  H2O  Fe(OH)3- H
• 16. Fe3  H2O  FeOH2 H
• 17. FeOH2  H2O  Fe(OH)2 H
• 18. Fe(OH)2  H2O  Fe(OH)3(sln) H
• 19. FeOH2  H  Fe2 H2O
• 20. e- Fe(OH)2  2H  Fe2 2H2O
• 21.  e- Fe(OH)3(sln)  H  Fe(OH)2(sln)
  H2O
• 22. e- Fe(OH)3(sln)  2H  FeOH 2H2O
• 23.  e- Fe(OH)3(sln)  3H  Fe2 3H2O

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Pourbaix Diagrams

• Some limitations of Pourbaix 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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Pourbaix Diagrams