Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 287

where Δ F is the difference in energy (actually, the amount of chemical
energy per mole of reactants converted to electrical energy in reactions
in which electrons are either released or consumed) between the reac-
tants and products, n is the number of electrons transferred by oxida-
tion per atom of metallic reactant, ℱ is a constant called the Faraday
constant (1 mole of electrons, equal to 96,520 coulombs of charge), and
E 0 is the half-cell potential. Values of E 0 have been determined for virtu-
ally all chemical reactions under standard conditions (25°C, atmospheric
pressure, 1 M concentrations of reactants and products). These electrical
potentials, like all potentials, are relative and are referred to a standard
reaction taking place at a carbon or platinum electrode:

1/2 H 2 (dissolved) → H + + e −

whose E 0 is set equal to zero. This is often written in shorthand fashion
as

H/H + E 0 = 0

Ideal electrochemical series

If we use this notation and rank the most favored ionization reactions by
the value of E 0 relative to H/H + , we obtain a very useful table called an
electrochemical series (also called “electromotive force” or “galvanic”
series, although galvanic series are usually practical rather than ideal
series) (Table 12.2).
The metals with large negative oxidation potentials, such as gold, are
called noble and are corrosion resistant. Those that have large positive
oxidation potentials, such as aluminum, are called base and are signifi-
cantly prone to corrosion. Iron and vanadium each appear twice in this
series, since the half-cell potential depends on, among other things, the
valence of the resultant ion. Differences in pH and pO 2 affect the valence
of the dissolved ions, as will be seen later (other ionic valences are pos-
sible but, for simplicity, are not included). The actual values of the half-
cell potentials, as given in common reference sources, are small, not
exceeding 2 V.

The galvanic cell

Table 12.2 is an ideal electrochemical series and predicts the outcome
of an experiment as shown in Figure 12.2. Two electrodes of the same
shape and size are made from different metals, placed in water con-
taining ions of both metals, and connected externally by an electronic
conductor or wire. This arrangement is called a galvanic cell in honor of
Luigi Galvani's discovery of the electric potential difference produced
by a bimetallic pair (“couple”) in contact with an electrolyte (moist frog
sartorius muscle).