Page 68 - Corrosion Engineering Principles and Practice

CHAPTER 4

Corrosion

Thermodynamics

4.1 Free Energy
In electrical and electrochemical processes, electrical work is defined
as the product of charges moved (Q) times the potential (E) through
which it is moved. If this work is done in an electrochemical cell in
which the potential difference between its two half-cells is E, and the
charge is that of 1 mol of reactant in which n mol of electrons are
transferred, then the electrical work (w) done by the cell must be −nE.
In this relationship, the Faraday constant F* is required to convert
coulombs from moles of electrons. In an electrochemical cell at
equilibrium, no current flows and the energy change occurring in a
reaction is expressed in Eq. (4.1).

w = ∆G = −nFE (4.1)
Under standard condition, the standard free energy of the cell
0
reaction ∆G is directly related to the standard potential difference
0
across the cell, E :
∆G = − nFE 0 (4.2)
0
For solids, liquid compounds, or elements, standard condition is
the pure compound or element; for gases it is 100 kPa pressure; and
for solutes it is the ideal 1 M (mol/L) concentration.
Electrode potentials can be combined algebraically to give cell
potential. For a galvanic cell, such as the Daniell cell shown previously
in Chap. 3, a positive cell voltage will be obtained if the difference is
taken in the way described in Eq. (4.3) and illustrated in Fig. 4.1.

E cell = E cathode − E anode (4.3)

* The Faraday constant is 96 485 C/ mol.
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