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Issues Impacting Bridge Painting: an Overview
Appendix A. Description of Electrochemical Techniques
Table of Contents
Given below is a short description of the three electrochemical
techniques utilized in this study: electrochemical impedance
spectroscopy (EIS), linear polarization, and potentiodynamic
polarization.
Electrochemical
Impedance Spectroscopy
EIS is a technique where a small amplitude
signal, usually a voltage between 5 to 50 mV, is applied
to a specimen over
a range of frequencies. Normally for corrosion systems, the
frequency range is 0.001 Hz to 100,000 Hz, since most of
the relevant information regarding the corrosion reaction
occurs over this range. The EIS instrumentation records the
real (resistive) and imaginary (capacitive) components of
the impedance response, Z' and Z", respectively. FIGURE
85A shows an idealized Nyquist plot for a metal coated
with a porous coating. The high-frequency limit on the left
side of the plot gives the ohmic resistance of the electrolyte,
Rs. At lower frequencies, two semicircles representing
the corrosion reaction at the metal/electrolyte interface
can be seen. One represents the coating resistance and capacitance,
Rc and Cc, and the other represents
the polarization resistance and capacitance, Rp and
Cp. At even lower frequencies, a straight line
is sometimes observed that is related to the mass transfer
resistance (Zd), of the process. In the case of
a coated metal surface, this parameter is related to the
diffusion of electrolyte through the coating. Thus, from
one experiment, relevant physical characteristics of a corrosion
system (such as coated steel) can be found.
An important part of the EIS analysis is to
create an "equivalent
circuit" of the system using resistors and capacitors in
series and in parallel. The physical behavior of the corrosion
system can be simulated and quantified with this circuit
to gain insight into the important processes in the corrosion
system. FIGURE 85B shows an
equivalent circuit simulating the ideal Nyquist plot for
the metal coated with a porous coating shown in FIGURE
85A.
The EIS spectrum can also be presented in the
Bode plot form, which gives the logarithm of the impedance,
|Z|, and
the phase angle, í, versus the logarithm of frequency. FIGURE
86C shows an idealized Bode plot for a metal coated with
a porous coating. The Bode plot is useful when determining
the frequency at maximum phase angle, wmax,
because the maximum phase angle is immediately apparent on
the plot. In the case of a metal with a porous coating, there
are two maxima, one representative of the coating and the
other representative of the corrosion reaction. The change
in frequency at maximum phase angle can then be followed
as the coating degrades and corrosion begins to occur.
Linear Polarization
Linear polarization is a well-established electrochemical
technique where a potential scan of 20 mV, positive and negative,
of the free-corrosion potential (the open-circuit potential)
is imposed on a metal sample and the current is recorded.
The current/potential relationship is linear in this voltage
range, and the slope (þE/þi) is the polarization resistance
(Rp). Polarization resistance is defined as the
resistance of the metal to oxidation during the application
of an external potential. The corrosion rate is directly
related to Rp and can be calculated from it by
knowing the anodic and cathodic Tafel slopes, which can be
obtained from potentiodynamic polarization measurements.
The equation for calculating the corrosion rate is:
where (E.W.) is the equivalent weight of the metal, A is
the area, d is the metal density, and þa and þc are
the anodic and cathodic Tafel slopes, respectively.
Potentiodynamic Polarization
Potentiodynamic polarization is a well-established electrochemical
technique where a potential scan of 250 mV, positive and
negative, of the free-corrosion potential is imposed on a
metal sample and the current is recorded. The current in
this potential range varies logarithmically with potential. FIGURE
86 shows a typical potentiodynamic polarization scan
for a mild steel rod in electrolyte. The Tafel slopes of
the anodic and cathodic reactions are obtained from the linear
portions of the scan and together with the corrosion current,
icorr, the corrosion rate can calculated. The
corrosion current is obtained from the plot where the two
slopes join, which is at the free-corrosion potential. In
addition, the anodic and cathodic Tafel slopes can be used
with Rp obtained from linear polarization, and
a corrosion rate also be calculated. |
