Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Patent
1990-11-14
1992-08-18
Weisstuch, Aaron
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
204404, G01N 1702
Patent
active
051396270
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to corrosion monitoring.
It is known that corrosion is an electrochemical phenomena, and as such, measurements of electrochemical parameters associated with corrosion processes may be used to estimate the rate of corrosion attack. Methods Which have been used in the past and are known and well documented include the linear polarisation resistance method in which a dc signal is applied to a corroding cell consisting of two or three electrodes and the resulting dc polarisation is monitored. Provided the applied current is small so that the potential shift is less than 20 mV the response is linear in most cases and the measured resistance (termed the polarisation resistance) may be related inversely to the rate of the uniform corrosion attack. Other techniques include the application of electrochemical impedance in which a sine wave current is applied, in a similar manner to the linear polarisation technique, and the sine wave potential resulting from the applied current is monitored. Alternatively, a pseudo-random noise signal can be applied and the electrochemical impedance obtained by time domain transformations. Again, provided the current is small enough such that the resulting cell potential shift is not more than .+-.20 mV, the response is linear. Corrosion includes capacitive components, and therefore if the frequency of the applied signal is varied the resulting output signal is also varied: hence by inputting a range of frequencies a range of outputs may be obtained and these may be suitably analysed to provide information corresponding to the solution resistance, the corrosion kinetics and other aspects including electrochemical double layer capacity, diffusion effects and absorption of intermediates on the surface. The corrosion rate is then estimated by use of the calculated charge transfer resistance from the electrochemical impedance data in a manner essentially the same as for linear polarisation resistance.
Both the above electrochemical techniques, whilst widely used, suffer severe limitations in that they can only provide information on uniform corrosion conditions, that is by their very nature they provide an average signal for the surface of the electrodes being monitored. However, depending upon the type of environment, the metallic material and type of corrosion occurring, it is often found that the assumption that the corrosion rate is proportional to the measured charge transfer or polarisation resistance is invalid since the corrosion is of a localised nature. A further complication arises from the interference effect of solution resistance which may be variable during the measurement period or of a high value thereby altering the observed measurements
In order to overcome the above limitations the use of electrochemical noise analysis has been applied and shown to be successful particularly for localised corrosion involving breakdown of a passive film. Essentially, electrochemical noise analysis consists of the measurement and analysis of small frequency, small amplitude random fluctuations in the corrosion potential of a corroding electrode. Suitable analysis is by means of frequency response analysis using Fast Fourier Transforms or Maximum Entropy Spectral Analysis, or by suitable filtering of the signal through a band pass system centred at 50 mHz.+-.10 mHz and the subsequent calculation of the root mean square of the filtered signal. U.S. Pat. No. 4,575,678 describes such techniques. Previous experience has shown that this analysis may yield information as to both corrosion type and corrosion rate.
Another electrochemical measurement system which has been successfully applied in corrosion studies is the monitoring of coupling currents between dissimilar electrodes, for example, copper and steel electrodes, for measurement of corrosion phenomena. However, useful information can be obtained using similar electrodes of identical material, particularly when the system is moving from a general to a localised corrosion form of attack. In this instance tw
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patent: 3660249 (1972-05-01), Townsend
patent: 3878064 (1975-04-01), Weisstuch et al.
patent: 4238298 (1980-12-01), Tsuru et al.
patent: 4395318 (1983-07-01), Tait et al.
patent: 4575678 (1986-03-01), Hladky
Dawson John L.
Eden David A.
John David G.
Capcis Limited
Weisstuch Aaron
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