Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2000-04-11
2001-11-20
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
Reexamination Certificate
active
06320395
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method of monitoring aqueous corrosion of metals, and more particularly to a method of monitoring employing electrochemical frequency modulation with the capability of validating data quality.
BACKGROUND OF THE INVENTION
Aqueous corrosion of metals is an electrochemical process involving anodic oxidation of a metal in a solution and cathodic reduction of species from the solution. This process is often monitored using an electrochemical technique, such as Linear Polarization Resistance (LPR), Tafel extrapolation and Electrochemical Impedance Spectroscopy (EIS). However, all of these techniques suffer from one or more significant drawbacks. For example, the LPR and EIS techniques can be used for instantaneous corrosion rate measurements only if the anodic and cathodic Tafel parameters (b
a
and b
c
, respectively) are known. The Tafel extrapolation technique permit determination of the corrosion rate and the Tafel parameters but is not suitable for instantaneous corrosion rate measurements because the system must be polarized over a wide potential range such that the measurement is time consuming and the surface of the metal is affected by the measurement.
Other electrochemical techniques rely on the fact that the corrosion process is non-linear, and that applying one or more sinusoidal signals will generate a response current at more frequencies than the frequencies of the applied signal. Thus, the corrosion rate can be determined by measuring a response current to the sinusoidal signals. One of these techniques, known as the Faraday rectification technique, involves measuring the response current at a “zero” frequency, that is measuring a direct current (DC). The Faraday rectification technique can be used if at least one of the Tafel parameters is known. Another technique, known as harmonic analysis, enables the corrosion rate and both Tafel parameters to be obtained with one measurement by analyzing the harmonic frequencies. Harmonic analysis has been used for corrosion rate measurements in acid media with and without inhibitors. A special application of harmonic analysis is Harmonic Impedance Spectroscopy (HIS), where the harmonic current components are transformed to harmonic impedances. HIS has been used to measure corrosion rates of polarized systems.
In corrosion research, virtually no attention has been given to intermodulation techniques. With the intermodulation technique one or more sinusoidal signals of different frequencies are applied to a corroding system and response currents measured. The alternating current (AC) responses include response currents at harmonics or multiples of the frequencies of the applied signals (&ohgr;
1
, 2&ohgr;
1
, 3&ohgr;
1
, . . . , &ohgr;
2
, 2&ohgr;
2
, 3&ohgr;
2
, . . . ), and response currents at the intermodulation frequencies (&ohgr;
1
±&ohgr;
2
, 2&ohgr;
1
±&ohgr;
2
, 2&ohgr;
2
±&ohgr;
1
. . . ). With the intermodulation technique, just as with harmonic analysis, it is possible to determine a corrosion rate without prior knowledge of the Tafel parameters. The intermodulation technique as such has been used satisfactorily in semi-conductor research. However, until now the intermodulation technique has never been successfully used for monitoring corrosion rates.
A further problem with all of the above techniques is that they do not provide a way to validate the measured response currents. Small currents have to be measured to determine a corrosion rate. These currents are easily influenced by background noise or any other kind of (electrical) disturbance. Using improperly measured data for the calculation of a corrosion rate can result in misleading results. In case of under estimation of corrosion rate, without a mechanism for validating the measured data an error in determining the corrosion rate is often not discovered until considerable damage has been done.
Accordingly, there is a need for a method and apparatus for quickly and continuously monitoring a corrosion rate of a corroding system without prior knowledge of Tafel parameters of the system. There is also a need for a method and apparatus for determining the Tafel parameters of the system. There is a further need for a mechanism for validating the measured data used to calculate the corrosion rate.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and apparatus for monitoring a corrosion current density to determine a corrosion rate of a corroding system. The method including the step of applying two sinusoidal signals having different frequencies to a portion of the system, measuring a response current in response to the sinusoidal voltages, and analyzing the response current measured at zero, harmonic and intermodulation frequencies to obtain the corrosion rate.
Generally, the method includes the steps of applying two sinusoidal signals having different frequencies to a portion of the system, measuring a response current in response to the sinusoidal signals, and analyzing the response current measured at zero, harmonic and intermodulation frequencies to obtain the corrosion rate. According to one embodiment, the two sinusoidal signals have angular frequencies &ohgr;
1
and &ohgr;
2
and the step of analyzing the response current includes the steps of transforming the response current from a time domain to a frequency domain, and calculating the corrosion rate using one of the following equations:
i
corr
=
i
ω
1
ω
2
2
2
8
i
ω
1
ω
2
i
2
ω
2
±
w1
-
3
i
ω
2
±
ω
1
2
,
i
corr
=
i
ω
1
ω
2
2
2
i
ω
2
±
ω
1
Optionally, the step of analyzing the response current includes the steps of analyzing the response current to obtain at least one Tafel parameter and to validate the quality of the measurements made. The step of validating the quality of the measurements includes the steps of calculating a causality factor using the response current measured at an intermodulation frequency and at a harmonic frequency of one of the two applied sinusoidal signals, and comparing the causality factor to a predetermined value. In one embodiment, the predetermined value is 2, and the causality factor is calculated using the following equation:
Causality
factor
2
:
i
ω
2
±
ω
1
i
2
ω
1
In another embodiment, the predetermined value is 3, and the causality factor is calculated using the following equation:
Causality
factor
3
:
i
ω
2
±
ω
1
i
3
ω
1
In another aspect, the present invention is directed to a computer program for monitoring a corrosion rate of a corroding system. The computer program includes a sinewave-generator module for generating and applying at least two sinusoidal signals having different frequencies to a portion of the system. A measured-waveform module for measuring voltages of the sinusoidal signals and a response current generated in the system in response to the signals. A frequency-response-magnitude module for transforming the measured response current from a time domain to a frequency domain. And a frequency-response-analyzer module for analyzing the response current to obtain the corrosion rate. Generally, the response current is measured at zero, harmonic and intermodulation frequencies of the sinusoidal signals. In one embodiment, the frequency-response-analyzer module is further configured to analyze the response current at harmonic and intermodulation frequencies of the sinusoidal signals to validate the quality of measurements made by the measured-waveform module. In another embodiment, it is also configured to analyze the response current to obtain one or more of the Tafel parameters for the system.
REFERENCES:
patent: 4658365 (1987-04-01),
Bogaerts Walter F.
Bosch Rik-Wouter
Flehr Hohbach Test Albritton & Herbert LLP
Katholieke Universiteit Leuven
Kerveros J
Metjahic Safet
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