Page 201 - Corrosion Engineering Principles and Practice

176 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 177

The bimetallic driving force was discovered in the late part of the
eighteenth century by Luigi Galvani in a series of experiments with
the exposed muscles and nerves of a frog that contracted when
connected to a bimetallic conductor. The principle was later put into
a practical application by Alessandro Volta who built, in 1800, the
first electrical cell, or battery: a series of metal disks of two kinds,
separated by cardboard disks soaked with acid or salt solutions. The
principle was also engineered into the useful protection of metallic
structures by Sir Humphry Davy in the early part of the nineteenth
century. The sacrificial corrosion of one metal such as zinc, magnesium,
or aluminum has become a widespread method of cathodically
protecting metallic structures.
Galvanic Series
The potential of a metal in a solution is related to the energy that is
released when the metal corrodes. Differences in corrosion potentials
of dissimilar metals can be measured in specific environments by
measuring the direction of the current that is generated by the
galvanic action of these metals when exposed in a given environment.
An arrangement of metals in a galvanic series based on observations
in seawater, as shown in Fig. 6.31 [17], is frequently used as a first
approximation of the probable direction of the galvanic effects in
other environments.
In a galvanic couple involving any two metals in a galvanic series,
corrosion of the metal higher in the list is likely to be accelerated,
while corrosion of the metal lower in the list is likely to be reduced.
Metals with more positive corrosion potentials are called noble or
cathodic, and those with more negative corrosion potentials are
referred to as active or anodic. Note that several metals in Fig. 6.31 are
grouped. The potential differences within a group are not likely to be
great and the metals can be combined without substantial galvanic
effects under many circumstances.
Values of potential can change from one solution to another or in
any solution when influenced by such factors as temperature,
aeration, and velocity of movement. Consequently, there is no way,
other than by direct potential measurements in the exact environment
of interest, to predict the potentials of the metals and the consequent
direction of a galvanic effect in that environment. As an example,
zinc is normally very negative or anodic to steel at ambient
temperatures, as indicated in the galvanic series shown in Fig. 6.31.
However, the potential difference decreases with an increase in
temperature until the potential difference may be zero or actually be
reversed at 60 C [18;19].
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Area Effects
Another important factor in galvanic corrosion is the area effect or the
ratio of cathodic to anodic area. The larger the cathode compared