This procedure 7a is another method of 'two-half-cell' survey which is being marketed as DCVG. This was described in the paper New Developments in Measuring the Effectiveness of Cathodic Protection.

Fig 4 clearly shows a voltage being measured between to half-cells (electrodes) and throughout the paper there is a discussion about the effects of ground potentials.



The above paper was written to draw attention to facts which make it necessary to find a new method of establishing the corrosion status of the pipeline metal.
It shows that 'potential gradients in the ground' have sound scientific credibility and that the two-half-cell is a valid tool for field engineers.

Procedure 7a is merely a method to locate coating faults with great degree of accuracy. There seems to be no evidence that DCVG can indicate the state of the metal at the location of a coating fault or whether the cathodic protection current is effective. However, it does indicate that cathodic protection current is entering the pipeline but it must not be assumed that this is necessarilly entering at the anodic interfaces. It is likely that cathodic protection current is entering at the cathodic interface of the corrosion cells and a question of doubt whether the electrolyte potential at the anodic interface has been raised to balance the corrosion reaction EMF.

The 'two-half-cell' survey is a powerful tool which can be used by any field worker using instruments and equipment which is available in every Cathodic Protection organisation and department.

Connecting a 'half-cell' on each pole of a meter clearly demonstrates neither can be regarded as a 'reference electrode'. A reference potential must be fixed in relation to other reference potentials and this survey shows that the voltage between the two is variable. The reaction within each electrode being identical, it must therefore be other potentials within the measuring circuit which differ between one point of contact and another.

INSTRUMENTS

1. High resistance voltmeter.
2. 2 Cu/CuSO4 electrodes.
3. Trailing conductor reel and mounting.

METHOD

7a.1.1. Each electrode should be fixed into a holder which enables the operative to place it in firm contact with the ground without stooping.

7a.1.2. The electrode conductor wires should be connected to the positive and negative poles of a volmeter.

7a.2.1. Using the two electrodes in a 'stepping action' they should be placed in contact with the ground at 1 m intervals as close as possible to the centre of the pipeline route.

7a.2.2. The meter reading should be noted as each electrode changes position.

7a.2.3. The operator will see that the polarity sometimes changes and this indicates that the ground potential is higher with the electrodes in one direction that the oposite.

7a.3.1. It is useful but not essential to record the polarity, as the operator can then visualise that the electrodes are reacting to an increase or decrease in the ground potential.

7a.3.2. The locations of highest voltage should be marked. These are normally before and after a very small voltage due to the electrodes being in positions on either side of a 'potential depression'.

7a.4.1 At such positions the operator can 'feel' around and identify the exact location of the lowest potential. This should be pegged or sprayed for later examination.

7a.4.2. The 'half-cells' should be stepped across the the pipeline route to confirm the exact position of the lowest ground potential. These 'transverse' readings should extend as far as possible, or from "remote" to "remote".

INTERPRETATION

The largest voltages are obtained where the IR drop in the soil is caused by the CP current returning to the pipeline. It follows that the marked locations are over coating faults which allow contact between the backfill and the pipe metal.

It has been found that transverse readings can indicate the severity of the coating fault and help to eliminate errors caused by other IR drops.

An 'interupter' is often placed in the cathodic protection transformer/rectifier so that the cathodic protection current can be identified. This has some advantages during the survey, but it slows down the location of coating faults on a known pipeline.
The 'off' potential has no significance during this type of survey as it cannot be related to the equilibrium of the pipeline metal.

The direction of the 'corrosion current' changes during the 'off' period of the cycle, would not reach the ground level at the locations of corrosion cells on the pipeline.

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click to read my paper published in the early 1980s