Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Frequency of cyclic current or voltage
Patent
1983-06-15
1986-04-22
Eisenzopf, Reinhard J.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Frequency of cyclic current or voltage
G01R 3100, G01R 3112
Patent
active
045845212
DESCRIPTION:
BRIEF SUMMARY
CROSS REFERENCE TO RELATED APPLICATION
The present application is a national phase application of PCT/CH82/00109 filed Oct. 11, 1982 and based in turn upon a Swiss national application No. 6619/81-9 filed Oct. 16, 1981 under the International Convention.
FIELD OF THE INVENTION
The present invention relates to a method of detecting the defects in a dielectric coating at the surface of an electrically conductive underlayer. This method is especially applicable to the detection of defects in the coating of submersed pipelines.
BACKGROUND OF THE INVENTION
It is known that these pipelines are provided with double protection against corrosion, one being the protection afforded by a coating layer of plastic material and the other the protection provided by cathodic polarization of the steel of the pipeline with respect to the sea water, this cathodic polarization being obtained, for instance, by arranging zinc anodes at given intervals around the pipeline, the polarization taking place between the zinc bracelets and the portions of the steel pipe not protected by the coating as a result of the deterioration of this coating.
Although it is rather easy to detect and correct coating defects occurring before the positioning of the pipeline, no provisions are made to correct damage suffered during laying of the pipeline. Under these conditions the zinc bracelets are arranged at intervals based on a statistical evaluation of the defects.
It has been already proposed to control the cathodic protection after the positioning of the pipeline, either by measuring the potential of the pipe with respect to the surrounding medium, or by measuring the protection currents circulating either in the pipe or in the surrounding medium. The measurement of the protection current can not be done during the positioning because of the perturbations generated by the parasitic currents originating from the numerous electrical machines used during the positioning of a pipeline, such as the welding currents.
There are methods already known in electrochemistry to study the condition of the surface by measuring the impedance of a surface submerged in a conductive medium by using an electrode excited by an alternating-current generator of average frequency and a potential-measuring electrode located very closely to the studied surface. The impedance is a result of the relation between the measured potential and the excitation current delivered. Such a method of measurement is applicable only to an examination taking place very close to the surface (only a few tens of microns), due to the fact that when the distance is of the order of millimeter, the low resistivity of the medium especially when this medium is sea water has a tendency to equalize the potential between the excitation electrode and the substratum, so that it is no longer possible to measure the variations in impedance.
OBJECT OF THE INVENTION
The object of the present invention is to measure the surface impedance at a distance from the underlayer sufficient to allow relative displacement of the underlayer and an excitation electrode, in order to measure continuously big and small defects, even if they are close to each other.
SUMMARY OF THE INVENTION
To accomplish this object the invention provides a method of detecting defects in a dielectric coating at the surface of an electrically conductive underlayer, according to which an excitation electrode is positioned at a distance from said underlayer on the side thereof covered by said coating, this electrode and this underlayer being placed in a common electrically conductive medium and the potentials corresponding to said underlayer and said electrode are carried to the terminals of an alternating-current generator. This method is characterized by the fact that a catcher coil is oriented essentially perpendicular to the electric field lines and an electrode for measuring the potential is positioned in the opening of said coil and one determines the impedance resulting from the voltages and currents detected by said mea
REFERENCES:
patent: 1297929 (1919-03-01), Taylor
patent: 2651021 (1953-09-01), Hays
patent: 2860304 (1958-11-01), Hall
patent: 3210655 (1965-10-01), McGlasson
patent: 3526831 (1970-09-01), Smith
Gaud Jacques V.
Kornmann Michel
Battelle (Memorial Institute)
Dubno Herbert
Eisenzopf Reinhard J.
Ross Karl F.
Solis Jose M.
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