Page 145 - Corrosion Engineering Principles and Practice

120 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 121

Complications with Polarization Methods
The widespread use of DC or AC polarization methods in corrosion
studies does not mean that they are without complications. Both
linear polarization and Tafel extrapolation need special precautions
for their results to be valid. The main complications or obstacles in
performing polarization measurements can be summarized in the
following categories:

• Effect of scan rate: The rate at which the potential is scanned
may have a significant effect on the amount of current
produced at all values of potential [6]. The rate at which the
potential is changed is an experimental parameter over
which the user has control. The goal is to set the polarization
scan rate at a slow enough rate to minimize surface
capacitance charging. If not, some of the current being
generated serves to charge the surface capacitance in addition
to the corrosion process with the result that the measured
current can be greater than the current actually generated by
the corrosion reactions alone.
• Effect of solution resistance: The distance between the Luggin
probe and the working electrode is purposely minimized in
most measurements to reduce the effect of the solution
resistance. In solutions that have extremely high resistivity,
for example, concrete, soils, and organic solutions, this can be
an extremely significant effect.
• Changing surface conditions: Corrosion reactions take place
at the surface of a metallic material. When the surface
condition is modified, due to processing conditions, active
corrosion, or any other reasons, the corrosion potential is
usually also changed. This can have a strong effect on the
polarization curves [6].
• Determination of pitting potential: In analyzing polarization
curves the presence of a hysteresis loop between the
forward and reverse scans often indicates that localized
corrosion, for example, pitting or crevice corrosion, is in
progress. This feature can be advantageously used to detect
the susceptibility of an alloy to pitting in certain
environments. However, it also means that the results
obtained cannot be used with confidence to estimate
general corrosion rates.

A critical problem in EIS, as well as any other scientific
measurements, is the validation of the experimental data. The use of
Kramers-Kronig (KK) transforms has been proposed to assess the
quality of the measured impedance data [15]. These integral
transforms were derived assuming four basic conditions: