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Electrochemical Frequency
Modulation (EFM) is a new technique for corrosion rate measurement. It was
developed at the University of Leuven in Belgium with support from the
Electric Power Research Institute. EFM can be used in the laboratory
or in the field.
In EFM, two AC voltage waveforms are summed and applied to a corrosion
sample. The frequencies of the two sinusoidal waveforms must share no
common factors. For example, 2 Hz and 5 Hz are suitable frequencies for
EFM.
While simple in concept, EFM yields an impressive amount of information on
the corrosion process:
- Corrosion Rate
- Tafel Constants
- Causality Factors
The Corrosion Rate is calculated from the
corrosion current that is measured by EFM. It is worth emphasizing
that the Tafel Constants are not needed to measure the corrosion current.
On this point, EFM enjoys a significant practical advantage compared to Polarization
Resistance.
EFM provides an independent measure of the anodic and cathodic Tafel
Constants. The Tafel Constants can be used in Polarization
Resistance if you wish. We believe it will be very interesting to
monitor the Tafel Constant values as a function of time in long
experiments. A change in the Tafel Constant may signal a
fundamental change in the corrosion mechanism.
The Causality Factors are used to validate the data. The Causality
Factors are similar to an internal check of the consistency of the
measurement process. There are two Causality Factors -- 2 and
3. If the calculated Causality Factors are not 2 and 3, suspect the
quality of your data.
Additionally, EFM may be less susceptible to errors in applied potential
from iR effects than Polarization Resistance. This issue is being
evaluated at the present time.
Finally, it may be possible to measure the corrosion rate of a structure
that is under Cathodic Protection (CP). This would be an extremely
useful characteristic with a huge potential benefit for corrosion
engineers.
The two references below are the original publications describing EFM:
“Electrochemical
Frequency Modulation: A New Electrochemical Technique for Online Corrosion
Monitoring”, R.W. Bosch, J. Hubrecht, W. F. Bogaerts, and B.C. Syrett,
Corrosion, 57 (2001): p. 60.
“Instantaneous
Corrosion Rate Measurement with Small-Amplitude Potential Intermodulation
Techniques”, R. W. Bosch and W. F. Bogaerts, Corrosion, 52 (1996): p.
204.
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