Page 146 - 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

• Linearity: A system is said to be linear if the response to a
sum of individual inputs is equal to the sum of the indi-
vidual responses. Practically, this implies that the imped-
ance of a system does not vary with the magnitude of the
probing signal.
• Causality: The response of a system should be solely due to
the probing signal. Physically, this means that the system
being tested responds fully and exclusively to the applied
signal. This is an important consideration in electrochemical
systems, because charge transfer interfaces are often active
and do, in fact, generate noise in the absence of any external
stimulus [15].
• Stability: A system is said to be stable if it comes back to its
original state after a perturbation is removed. This
condition ensures that there is no negative resistance in the
system. This apparent restriction on the presence of
negative resistance has often been associated with the
presence of active-to-passive transitions in the system
being investigated [15].
• Finite value: The real and imaginary components of a complex
impedance must be finite over the entire frequency range
sampled.
If a system satisfies the conditions of linearity, stability, and
causality, it will a priori satisfy the KK transforms, provided that the
frequency range is sufficiently broad for the integrals to be evaluated.
However, a simple validity test is often sufficient to ascertain that the
EIS measurements are indeed meaningful. For measurements made
in real time, that is, by changing the frequency for each frequency
point, the validity of a forward frequency scan can be verified by
scanning in the opposite direction. Repeated measurements during
two successive scans in opposite direction should generally satisfy
KK transforms and represent the actual impedance of the system
being tested.

5.6.2 Corrosion Monitoring
Electrochemical monitoring methods involve the determination of
specific interface properties divided into three broad categories:

• Corrosion potential measurements: The potential at a corroding
surface arises from the mutual polarization of the anodic and
cathodic half-reactions resulting in the overall corrosion
reaction. Corrosion potential is intrinsically the most readily
observable parameter and understanding its behavior may
provide very useful information on the thermodynamic state
of a system.