The Alexander Cell will allow you to watch corrosion being stopped by the application of cathodic protection.


In the picture above the Alexander Cell is placed on the electrolyte and connected to a micro-ammeter. The meter shows zero as the circuit is not complete.


In the next picture a sample of the electrolyte bridges the two steel electrodes on top. One of these electrodes has been recently polished, making it less noble than the other. A corrosion reaction causes current to flow into the unpolished electrode, through the base electrodes and through the meter.



The Alexander Cell is then connected to the pipeline in such a way that the cathodic protection current has a path through both the anode and the cathode. The current on the meter is a result of the effect of the cathodic protection. It can be seen in this case to have stopped corrosion as the reading on the meter has a negative sign denoting that the current has reversed.



The cathodic protection is switched off and the corrosion can be seen to resume.



When the cathodic protection is on, it can pass onto either the anode or the cathode of the Alexander Cell. It is the current density in the electrolyte that determins if the cathodic protection is effective and this depends on the potential of the electrolyte at the anodic interface.

The base electrodes of the Alexander Cell are in the same area of electrical potential as the pipeline and subject to the same current density. However, the current can only pass into a corrosion cell if the potential in the electrolyte is greater than the EMF of the corrosion reaction. This is always the case at the cathode where corrosion current is already passing onto the metal. Cathodic protection current will always pass onto the cathode at a coating fault and this clouds the issue when conducting CIPS and DCVG surveys.

If a simple coupon is used in these circumstances it would be impossible to ascertain that the reaction at the anode had stopped. The Alexander Cell demonstrates this fact very simply as cathodic protection current can pass through the cell onto the pipeline without stopping the corrosion, and this can be seen on the meter. The current simply passes through the cathodic interface if there is insufficient potential in the electrolyte at the anodic interface.

The confusion of electrical currents (and resulting potentials) that are always present in field conditions are sensed by the Alexander Cell in the same way that they influence the pipeline itself.

It is the ability to measure corrosion current in closed circuit condition that makes the Alexander Cell suitable to monitor the effects of electronic control of corrosion noise.

The Alexander Cell is a corrosion cell that, when connected to the pipeline, is subject to the influence of the cathodic protection system in the same way as any corrosion cell on that pipeline. The corrosion current can then be seen to react to the adjustment of the CP system, making it possible to ascertain that corrosion has, in fact, stopped.

This eliminates the need to switch the impressed current cathodic protection system off before making the measurement. The normal state of the pipeline is to have the cathodic protection switched on and there are some corrosion engineers who believe that switching the CP on and off can actually accelerate the corrosion reaction.

The Alexander Cell has the unique quality that it can be used on hybrid CP systems where a mixture of Impressed Current and Sacrificial Anodes are used.

The demonstration pages on this website explain the complexities that made the development of the Alexander Cell necessary.

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The Alexander Cell was developed and tested in the field as a means of assessing the risk of corrosion to steel pipelines, it is simple to use, satisfies all electro-chemical theories and is the only measuring technique that can be modelled on a computer. Other methods include too many variables in the formulae used for their mathmatical resolution.
Software that uses the advance features of the Alexander Cell Technology is already in the advanced stage of development and interested parties should contact Roger Alexander who can arrange their involvement.

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The Alexander Cell was named by Tim ffrench-Mullens, the founder member of the Institute of Corrosion Science and Technology (UK) and has been scrutinised by leading academic institutions and practicing cathodic protection engineers who now recognise that it is the only method which can be adopted as an international criterion for cathodic protection.

All inquiries to roger.alexander3@ntlworld.com

Training in the use of the Alexander Cell is available through the Cathodic Protection Network who are the sole distributors.

A limited number of Alexander Cells are now available for field trials by pipeline operating companies worldwide.

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