Page 148 - Corrosion Engineering Principles and Practice

122 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 123

monitors the response of the cathodic reaction to the process stream
conditions. When the cathodic reaction is stable, it monitors the
response of the anodic reaction to process fluctuations [16].
This technique has been found particularly useful to study
depolarization effects of the cathode of a galvanic pair of electrodes to
obtain feedback of low levels of dissolved gases, particularly oxygen,
or the presence of bacteria, which depolarize the cathode of the
galvanic pair and increase the coupling current. When used for
detection of low levels of oxygen, other dissolved gases may interfere.
Calibration against a dissolved oxygen meter is usually required if
quantitative values are needed.

Electrochemical Impedance Spectroscopy
The measurement cycle time with EIS is quite critical and depends
on the frequency range used, especially the low frequencies.
A single frequency cycle at 1 mHz, for example, takes 15 minutes.
A high-to low-frequency scan going to such a low frequency would
take more than two hours. In order to make routine corrosion
monitoring with EIS certain simplifications are needed to maximize
the use of high frequency data and drastically shorten the
measurement time. It is also important to simplify the data
processing and analysis to make the technique user friendly for
field corrosion monitoring.
In order to simplify the analysis of field EIS results, a method was
developed which consists of finding the geometric center of an arc
formed by three successive data points on a complex impedance
diagram (Fig. 5.28) [17;18]. This technique was designed as an
improvement over the two-point method based on the comparison of
high and low frequency data points for which the impedance would
be proportional to the R at the high frequency point and the
s
summation of R and R at the low-frequency point [19]. In real world
p
s
situations, one difficult assumption to satisfy with the two-point
method is that data points should contain negligible imaginary
components (i.e., 0 phase shift), a condition usually hard to achieve in
a meaningful manner at low-measuring frequencies.
The three-point analysis technique was further developed by
permuting the data points involved in the projection of centers in
order to obtain a population of projected centers. This improvement
has permitted to automate the data analysis while providing some
information concerning the adherence of the results with the
resistive-capacitive (RC) behavior described in Eq. (5.26) that is
assumed for the evaluation of the parameters associated with
uniform corrosion [18].
More recently, a full-spectrum, relatively low-cost EIS corrosion
monitoring system has been developed, which is wireless, small (5 cm
diameter, 1.2 cm height), requires nominally 10-mW power during its
200-second measurement period, and has an electronic identifier,