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Comparison
of EFM to LPR, EIS, and Tafel Plot for the Measurement of the Corrosion Rate of Iron in
Sulfuric Acid
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One of the nicer features of
the annual Corrosion Short
Course at Penn State University is the extensive laboratory
experiments that are designed for the course. The students are
exposed to a wide range of electrochemical techniques for real, hands-on
experience. For most of these experiments, the sample is iron and
the electrolyte is 0.5 N sulfuric acid. The iron is cast in epoxy
and polished to a mirror finish.
At the 2004 edition of the course, we took the opportunity to
compare Electrochemical Frequency Modulation, a
new technique for corrosion rate measurement, to the more-accepted
electrochemical techniques of Polarization Resistance, Tafel Plots, and
Electrochemical Impedance Spectroscopy. The results are shown in the
table below.
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| Technique |
Corrosion
Rate (milli-inches per year) |
| EFM |
64.3 |
| LPR |
61.9 |
| Tafel
Plot |
66.9 |
| EIS |
55.7 |
Most corrosion scientists would agree
that the reproducibility of the corrosion rate from these techniques is impressive, suggesting that
EFM may be confidently considered as a reliable tool for corrosion rate
measurements.
Each technique used a Density of 7.87 and an Equivalent Weight of 27.92.
Comments on each measurement follow.
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Electrochemical Frequency Modulation
EFM was run with a 10 mV
amplitude, 4 cycles, and frequencies of 0.2 and 0.5 Hz. The EFM data
acquisition plot is shown below. The results were:
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Corrosion
Rate
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64.24 mpy
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Anodic
Tafel Constant
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52.5 mV/decade
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Cathodic
Tafel Constant
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132.7 mV/decade
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Causality
Factory (2)
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1.943
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Causality
Factor (3)
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3.060
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The Causality Factors are very
close to the theoretical values of 2 and 3, suggesting that the reaction
is accurately described by the Butler-Volmer equation and the data is
valid. We will use the Anodic and Cathodic Tafel Constants with LPR
and EIS and compare the values obtained with the Tafel Plot.
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Electrochemical Impedance Spectroscopy
The EIS curve is shown
below. The data was fit with a Randles-like circuit that
incorporated a CPE instead of a capacitor. The Rp was 136.4 ohms.
The Anodic and Cathodic Tafel Constants from the EFM experiment were used
to calculate the Corrosion Rate of 55.7 mpy.
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Polarization Resistance
The Linear Polarization plot is below.
The Scan Rate was 0.125 mV/sec. The Anodic and Cathodic Tafel
Constants from the EFM experiment were used to calculate the Corrosion
Rate of 61.9 mpy. The blue X's mark the data region used to calculate the
slope and the red line is the fit to that data region.
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Tafel Plot
The Tafel Plot was run at a Scan
Rate of 1 mV/sec because of time constraints. Iron is characterized
by a cathodic Tafel Region that can only be described as stunningly
gorgeous! The red line on the curve below is the fit of the Gamry
DC105 Corrosion Techniques Software to the Butler-Volmer equation.
The calculated corrosion rate is 66.9. The calculated Cathodic Tafel
Constant is 0.139 V/decade, in excellent agreement with 0.133 from EFM.
The Anodic Tafel Constant is 0.050 V/decade, which is in remarkable
agreement with 0.053 from EFM. The agreement is remarkable because
we used a very limited portion of the anodic data. An insoluble film
on the electrode surface makes the anodic polarization curve complicated
and limits the useable data range. The Tafel Plot was run last in this series, since the more-extensive
polarization was likely to alter the electrode surface.
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