Cathodic protection theory depends on a voltage measurement taken between the subject metal and a reference electrode placed at the interface between the metal and the electrolyte. In the laboratory this is achieved by using a capillary of inert, but conductive material, such as agar-agar.
The isopotential cell is based on the same principle but adapted for cathodic protection field use. Several versions of this have been patented in continental Europe, but the simple version used by me at Rapele (Nigeria) utilises only established arrangements in common use, and is not believed to contravene any Patent Conventions.
It has now been recognised that there are inherent errors in voltage measurement obtained by the established techniques. The 'immediate off potential' method is one attempt to remove these errors
Attempts have been made to practice the laboratory technique in the field, the first being made in Nigeria in 1976 when I was ENGE18 engaged by Shell International.
I also made attempts to carry out this procedure when engaged on the original CIPS development team with the Gas Council of the UK and North Thames Gas in the London area.
It can be seen as impossible to obtain meaningful records showing the required 'kick' during cathodic protection field work.
I therefore followed the developments in Germany and continental Europe as reported in the paper published in Australasia during the 1980's.
I made a similar arrangement and trialed it on site.
THE NAME 'ISOPOTENTIAL CELL'The term 'isopotential cell' is adopted in this study as a convenient expression of the workings of the arrangement, which creates a position, in which to place the electrode, that is at the same potential as that at the interface between the electrolyte and the subject metal.
A porous plug is fitted into the hole in the centre of a steel coupon which is used as the base of a plastic tube containing an inert fill.
The inside surface of the coupon is coated with an insulating material and there is provision for electrical connection to the coupon metal.
The exposed metal of the coupon is placed firmly on the ground, as the base of the cell, and connected to the pipeline, via a conductor.
The pipeline is connected to a high impedance voltmeter the other pole of which is connected to an electrode.
The porous plug of the electrode is placed in the open top of the plastic tube containing the inert fill, thus completing the measuring circuit.
THE EQUILIBRIUM OF THE CELL
The reaction of the metal of the coupon to the electrolyte surrounds the porous plug which is saturated by and directly connected to the inert content of the plastic tube.
The potential of the porous plug is reflected by the potential of the inert fill instantaneously, but not further current flows through this fill after equilibrium is reached.
This is an identical status to that of a Lugin Capillary in a laboratory and the electrode placed in this situation can be regarded as a 'reference electrode', whereas in normal cathodic protection work it is only an earth contact electrode.
PROCEDURE TO USE THE CELL
- The bottom of the coupon is cleaned to bright metal and placed on the ground as close to the pipeline as possible.
- The high impedance voltmeter is connected between the pipeline and the electrode which is placed in the top of the plastic tube.
- The coupon is connected to the pipeline and the voltage on the meter is noted.
It has been acknowledged that the true interface potential of the pipe can only be obtained from an isopotential cell placed at the pipe position and it is recommended that facilities are permanently installed for this purpose.
Voltages measured with the isopotential cell are invariable much lower than conventional 'pipe-to-soil potentials' with the cathodic protection system on. The reading will be similar to an 'immediate off potential' reading, as the use of the isopotential cell removes the errors caused by the passage of the cathodic protection current.
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