Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1993-11-12
1995-06-20
Niebling, John
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204404, 204406, 204412, 204434, 324 711, 324 712, 324347, 324348, G01N 2726
Patent
active
054258677
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for producing electrochemical impedance spectra.
It is well known that useful information concerning electrochemical processes can be obtained from impedance data. Typically an electrochemical impedance spectra is obtained from a system under investigation using perturbative techniques. For example a sine wave, pulse or random noise signal is applied to an electrochemical cell under investigation and the cell response is correlated with the applied signal. The resultant data is typically presented as either a Bode plot, a frequency spectra of impedance and phase angle against frequency, or a Nyquist diagram of real impedance (resistive) and complex impedance (capacitive) with frequency as a parameter. The article "A Review of Impedance Plot Methods used for Corrosion Performance Analysis of Painted Metals" by G. W. Walter, Corrosion Science, Vol 26, No. 9 PP681-703, 1986 describes Bode and Nyquist plots in some detail. There are however a number of earlier publications presenting similar information, see for example "Electrochemical Impedance of Pure Iron at various Potentials in Sulphuric Acid", (M. Keddam, O. R. Mattos and H. Takenouti, J. Electrochem. Soc. 128, 257 (1981)).
The interpretation of electrochemical impedance spectra is a subject of continuing scientific debate. Nevertheless, it is generally agreed that information on electron transfer processes, diffusion, absorbed species and electro-crystallization can be obtained from analysis of the loops that appear on the Nyquist-type impedance diagrams. These loops, sometimes referred to as relaxations, may be either of capacitive or of inductive type (negative capacitance) and, depending on the electrochemical process or reactions involved, each loop can be ascribed to the presence of an adsorbed species or a reaction rate constant. The reaction rate constant may also involve consideration of the surface coverage and/or surface concentration of an electroactive species.
The use of the known perturbative techniques to generate electrochemical impedance spectra can provide useful information and as a consequence such techniques have become widely accepted. It is the case however that the application of the perturbation signal may itself produce impedance spectra which are a function of the perturbation signal itself, for example due to the electrode shape and cell configuration. Furthermore, at low frequencies the time taken for the measurement from which the spectra is derived can lead to errors in the data due to spontaneous changes which occur naturally within the system being studied.
It is known to monitor the corrosion of metal components by reference to spontaneously generated fluctuations in the potential of an electrode. In one known system, as described in European Patent Specification EP 0084404, the electrochemical potential of an electrode is monitored and the monitored signal is filtered to remove DC and higher frequency AC components. This provides a measure of the rate of corrosion of the electrode.
It is also known from European Patent Specification EP 0302073 to measure electrochemical current noise originating in an electrode array and to compare the electrochemical current noise with a coupling current between two electrodes of the array to generate an output which is indicative of the degree to which corrosion is localised. In addition, electrochemical current noise and electrochemical potential noise are compared to provide an output which is indicative of the overall rate of corrosion. This output does not however present the detailed information available from electrochemical impedance spectra.
It is an object of the present invention to obtain electrochemical impedance spectra from an electrochemical system without it being necessary to apply a perturbing signal to the system. It has surprisingly been found that such spectra can be generated from spontaneously occurring noise.
According to the present invention, there is provided a method for producing electrochemical imp
REFERENCES:
patent: 4575678 (1986-03-01), Hladky
patent: 4631116 (1986-12-01), Ludwig
patent: 5139627 (1992-08-01), Eden et al.
Walter, G. W., "A Review of Impedance Plot Method used for Corrosion Performance Analysis of Painted Metals", Corrosion Science, vol. 26, No. 9, pp. 681-703, 1986. No month presently available.
Keddam, Michel., "Reaction Model for Iron Dissolution Studied by Electrode Impedance", J. Electrochem Soc. Electrochemical Science and Technology, pp. 257-267, Feb. 1981.
Anderson, N., "On the Calculation of Filter Coefficients for Maximum Entropy Spectral Analysis", Geophys, vol. 29, pp. 69-72, Feb. 1974.
Carr Richard N.
Dawson John L.
Eden David A.
Bell Bruce F.
Capcis March Limited
Niebling John
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