Module 5 : Electrochemistry
Lecture 25 : Corrosion
  25.2

Pourbaix Diagrams

The electrode potentials of the metal, E and the pH of the surrounding medium are the main factors or parameters that determine the stability of the solid phases and the ionic species produced during redox reactions of metals. When iron metal is exposed to acidic/neutral/basic solutions, several reactions can occur. Some of the reactions involved, their Eo values (with respect to SHE) and the Nernst equation associated with each of the reactions is given in the following table.
 
Table 25.2 : Reduction reactions, emfs and Nernst equations for redox processes in corrosion.
 

 Reaction

 Eo / V

 Nernst Equation E = Eo– (2.303RT/ nF) *  log{ [Products] / [Reactants]}

 1) Fe 2+ + 2e = Fe

 -0.440

 -0.440 + 0.0295 log [ a Fe2+]

 2) Fe3 O4 + 8H + + 8e - = 3Fe + 4 H 2O

 -0.085

 -0.085 – 0.0591 pH

 3) Fe3 O4 + 8H + + 2e - = 3Fe2+ + 4 H 2O

 0.980

 0.980 – 0.2364 pH - 0.0886 log (a Fe 3+ )

 4) Fe3+ + e- = Fe2+

 0.771

 0.771 + 0.0591 log ( a Fe 3+/ a Fe 2+ )

 5) 2Fe3+ + 3H2O = Fe2O3 + 6H+

 Use  (aH+)6/(aFe3+)2    =100 (This is not  an electrode  reaction)

 pH = { - log (a Fe3+ ) - 1} / 3

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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