Page 147 - 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

• Reaction rate as current density: Partial anodic and cathodic
current densities cannot be measured directly unless they are
purposefully separated into a bimetallic couple. However, by
polarizing a metal immersed in an aqueous environment, it is
possible, with the use of simple assumptions and models of
the underlying electrochemical behavior, to estimate net
currents for both the anodic and cathodic polarizations from
which a corrosion current density can be deduced.
• Surface impedance: A corroding interface can also be modeled
for all its impedance characteristics, therefore revealing sub-
tle mechanisms not visible by other means. EIS is now well
established as a powerful technique for investigating corro-
sion processes and other electrochemical systems.
Corrosion potential or current produced by naturally occurring
or externally imposed conditions can be measured with a variety of
electrochemical techniques. Conversions of the measurements into
corrosion rates or other meaningful data use equations or algorithms
that are specific to each technique.
Limits of operation for field work are more serious than those
experienced in a laboratory environment, mostly for reasons of prac-
tical probe geometry. For example, capillary salt bridges (e.g., Luggin
capillary) commonly used in laboratory setups to reduce the solution
resistance interference are definitively too delicate or cumbersome
for field use [16].

Zero Resistance Ammetry
With this electrochemical technique galvanic currents between
dissimilar electrode materials are measured with a zero resistance
ammeter*. The design of dissimilarities between sensor elements may
be made to target a feature of interest in the system being monitored
(e.g., different compositions, heat treatments, stress levels, or surface
conditions). Zero resistance ammetry (ZRA) may also be applied to
nominally identical electrodes in order to reveal changes occurring in
the corrosivity of the environment.
The main principle of the technique is that differences in the
electrochemical behavior of two electrodes exposed to a process
stream give rise to differences in the redox potential at these electrodes.
Once the two electrodes are externally electrically connected, the
more noble electrode becomes predominantly cathodic, while the
more active electrode becomes predominantly anodic and sacrificial.
When the anodic reaction is relatively stable the galvanic current

* A zero resistance ammeter (ZRA) is a current to voltage converter that produces
a voltage output proportional to the current flowing between its input terminals
while imposing a “zero” voltage drop to the external circuit.