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3 Experimental Methods in Characterization of Nanosystems
correspond to the same reaction proceeding in opposite directions, the anodic and
cathodic arms belong to different reactions in the corrosion studies: oxidation of the
metal (anodic dissolution) and reduction of the oxidizer (mostly O 2 or H
+ ). Obviously, the Tafel slopes of the two partial reactions are independent of each other. The
corrosion potential is set to the value where the anodic and cathodic partial reactions
take place at an equal rate. In an ideal case, the two Tafel lines have the intercept at
the corrosion potential, and the current density corresponding to the intercept yields
the corrosion rate.
The reason why one has to be particularly cautious with the large-amplitude perturbation method is that it may induce irreversible changes in a material of complex
structure, which makes correct corrosion rate estimation impossible. A few possible
pitfalls of the large-amplitude polarization studies are as follows. (i) When the polarization curve is recorded with a potential sweep, the capacitive current contribution
causes a current misestimating, which has the highest relative error around the corrosion potential where the current is presumably zero. Therefore, the corrosion potential as assessed from a sweep is often misestimated. (ii) If the polarization curve is
recorded with a single sweep from the cathodic end potential to the anodic one, the
accumulation of the cathodic reaction product may be oxidized in the anodic part,
hence falsifying the current measured. (iii) If the cathodic and the anodic parts of
the polarization measurement are performed on an identical specimen, the cathode
reaction may irreversibly modify the surface, and the anodic part is recorded for a
surface much different from the original surface to be tested. A great care must be
taken so that the measurement conditions indeed refer to the material of interest in
the state to be studied.
While the small-amplitude perturbation methods eliminate a few of the above
mentioned problems, some others may arise. A common method is to record a slowrate sweep in the narrow (say, ±10 mV) neighbourhood of the corrosion potential
and to approximate the polarization curve with a line. The inverse slope of the line,
(dj/dE)
−1 , is called the polarization resistance. If the Tafel slopes of the cathodic and
anodic partial reactions are known, the corrosion rate can be calculated (even though
the polarization resistance of similar materials is also suitable to estimate a relative
order of the corrosion rate of various specimens).
Another small-amplitude perturbation method is the measurement of electrochemical impedance that involves sinusoidal potential perturbation of various frequencies.
For electrochemical impedance spectroscopy (EIS), the high-to-medium frequency
part of the impedance spectrum yields a semicircle on the Nyquist plot whose diameter can be associated with the charge transfer resistance. Even though the charge
transfer resistance,
∂j
∂E
c i, θ,..
−1
, has a different meaning than polarization resistance, it is often used for quantification of the corrosion resistance. While the application of the charge transfer resistance if more or less correct for rating the relative
corrosion rate of samples of the same family, the entire impedance spectrum is often
fitted with the parameters of some equivalent circuits that cannot be validated. Hence,
the border between the correct application of the impedance method and the arbitrary
and heuristic “equivalent circuit engineering” is very narrow. It is to be remembered
3 Experimental Methods in Characterization of Nanosystems
correspond to the same reaction proceeding in opposite directions, the anodic and
cathodic arms belong to different reactions in the corrosion studies: oxidation of the
metal (anodic dissolution) and reduction of the oxidizer (mostly O 2 or H
+ ). Obviously, the Tafel slopes of the two partial reactions are independent of each other. The
corrosion potential is set to the value where the anodic and cathodic partial reactions
take place at an equal rate. In an ideal case, the two Tafel lines have the intercept at
the corrosion potential, and the current density corresponding to the intercept yields
the corrosion rate.
The reason why one has to be particularly cautious with the large-amplitude perturbation method is that it may induce irreversible changes in a material of complex
structure, which makes correct corrosion rate estimation impossible. A few possible
pitfalls of the large-amplitude polarization studies are as follows. (i) When the polarization curve is recorded with a potential sweep, the capacitive current contribution
causes a current misestimating, which has the highest relative error around the corrosion potential where the current is presumably zero. Therefore, the corrosion potential as assessed from a sweep is often misestimated. (ii) If the polarization curve is
recorded with a single sweep from the cathodic end potential to the anodic one, the
accumulation of the cathodic reaction product may be oxidized in the anodic part,
hence falsifying the current measured. (iii) If the cathodic and the anodic parts of
the polarization measurement are performed on an identical specimen, the cathode
reaction may irreversibly modify the surface, and the anodic part is recorded for a
surface much different from the original surface to be tested. A great care must be
taken so that the measurement conditions indeed refer to the material of interest in
the state to be studied.
While the small-amplitude perturbation methods eliminate a few of the above
mentioned problems, some others may arise. A common method is to record a slowrate sweep in the narrow (say, ±10 mV) neighbourhood of the corrosion potential
and to approximate the polarization curve with a line. The inverse slope of the line,
(dj/dE)
−1 , is called the polarization resistance. If the Tafel slopes of the cathodic and
anodic partial reactions are known, the corrosion rate can be calculated (even though
the polarization resistance of similar materials is also suitable to estimate a relative
order of the corrosion rate of various specimens).
Another small-amplitude perturbation method is the measurement of electrochemical impedance that involves sinusoidal potential perturbation of various frequencies.
For electrochemical impedance spectroscopy (EIS), the high-to-medium frequency
part of the impedance spectrum yields a semicircle on the Nyquist plot whose diameter can be associated with the charge transfer resistance. Even though the charge
transfer resistance,
∂j
∂E
c i, θ,..
−1
, has a different meaning than polarization resistance, it is often used for quantification of the corrosion resistance. While the application of the charge transfer resistance if more or less correct for rating the relative
corrosion rate of samples of the same family, the entire impedance spectrum is often
fitted with the parameters of some equivalent circuits that cannot be validated. Hence,
the border between the correct application of the impedance method and the arbitrary
and heuristic “equivalent circuit engineering” is very narrow. It is to be remembered