Page 157 - Corrosion Engineering Principles and Practice

132 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 133

intermediate cases the results of the measurement of Z would show
n
a mixed behavior and be more difficult to interpret. For example, if
hydrogen bubbles are evolving on the cathode while the anode
undergoes generalized corrosion, the noise of the cathode is orders of
magnitude larger than that of the anode, so that Z becomes equal to
n
the impedance modulus of the anode, |Z |. In these conditions, while
a
the time records appear to show only the cathodic processes, the
impedance measured is that of the anode, using the noise of the
cathode as input signal.
An opposite case would be a cell where the anode is undergoing
pitting, while the cathodic reaction is the reduction of dissolved
oxygen or an imposed galvanic situation. Since the anodic noise is
preponderant, Eq. (5.27) shows that Z is equal to the impedance
n
modulus of the cathode, |Z |. The anodic noise is the internal
c
signal source utilized for the measurement of the impedance of
the cathode.
Coupled Multielectrode Array Systems and Sensors
The use of multielectrode array systems (CMAS) for corrosion
monitoring is relatively new. The advantages of using multiple
electrodes include the ability to obtain greater statistical sampling of
current fluctuations, a greater ratio of cathode-to-anode areas in order
to enhance the growth of localized corrosion once initiated. CMAS
also provide the ability to estimate the pit penetration rate and obtain
macroscopic spatial distribution of localized corrosion [25].
Figure 5.38 shows the principle of the CMAS in which a resistor is
positioned between each electrode and the common coupling point
[26]. Electrons from a corroding or a relatively more corroding
electrode flow through the resistor connected to the electrode and
produce a small potential drop usually of the order of a few microvolts.
This potential drop is measured by the high-resolution voltage-
measuring instrument and used to derive the current of each electrode.
CMAS probes can be made in several configurations and sizes,
depending on the applications. Figure 5.39 shows some of the typical
probes that were reported for real-time corrosion monitoring.
Because the electrode surface area is usually between 1 and
0.03 mm , which is approximately 2 to 4 orders of magnitude less
2
than that of a typical LPR probe or a typical electrochemical noise
(EN) probe, the prediction of penetration rate or localized corrosion
rate by assuming uniform corrosion on the small electrode is realistic
in most applications. CMAS probes have been used for monitoring
localized corrosion of a variety of metals and alloys in the following
environments and conditions:
• Deposits of sulfate-reducing bacteria
• Deposits of salt in air
• High pressure simulated natural gas systems