Page 132 - Corrosion Engineering Principles and Practice

106 C h a p t e r 5 C o r r o s i o n K i n e t i c s a n d A p p l i c a t i o n s o f E l e c t r o c h e m i s t r y 107

5.6 Examples of Applied Electrochemistry to Corrosion
Given the electrochemical nature of corrosion processes it is not
surprising to see that measurements and control methods based on
electrochemical principles are so extensively used across the whole
spectrum of corrosion science and engineering. The following sections
provide some examples to illustrate how these principles are applied
in practice.

5.6.1 Electrochemical Polarization Corrosion Testing
Corrosion testing with polarization methods basically consists in
forcing potential or current changes on a sample under study while
monitoring the resulting response in current or potential. This may
be achieved by using either a direct current (DC) or an alternating
current (AC) source. The instrumentation for carrying polarization
testing as illustrated in Fig. 5.1 consists of
• A potentiostat is needed that will maintain the potential of
the working electrode close to a preset value.
• A current measuring device for monitoring the current
produced by the applied potential. The ability of the current
measuring device to autorange or to change the scale
automatically is also important.
• The ability of the data to be stored directly in a computer, or
plotted out directly is also important.
• Polarization cells: Several test cells are available commer-
cially for making polarization measurements. Polarization
cells can have various configurations specific to the testing
requirements from testing small coupons, to sheet materials,
to testing inside autoclaves. In a plant environment the elec-
trodes may be inserted directly into a process stream. Some
of the features of a cell include [6]
a) The working electrode, that is, the test sample, which
may be accompanied by one or more auxiliary or counter
electrodes.
b) The reference electrode which is often separated from the
solution by a solution bridge or a capillary Luggin probe
(Fig. 5.1). This combination eliminates solution interchange
with the reference electrode but allows it to be moved
very close to the surface of the working electrode to
minimize the effect of the solution resistance.
c) A temperature monitoring device.
d) An inlet and outlet for gas to allow deaeration, aeration, or
the introduction of specific gases into the solution.