Cathodic Protection Training Course

Module 4

Physical modelling of CP measurement techniques.

It is very useful for students to actually model the cathodic protection that they are studying.

This helps them to visualise the electrical paths and effects that they are dealing with.

It allows them to understand that potentials are electrical pressures that are similar to water pressure and gas pressure, but have other qualities that must be taken into consideration.

Water pressure is often shown in drawings but CPN uses actual models with real water to show some of the aspects of the application of cathodic protection.

We have four glass flower vases, each being three tubes joined so that water can level out in each tube.
We have placed these vases in compartments of a transparent container, each supported at a different level and the fourth section being filled with sand so that we cannot see the bottom part of the flower vase. This shows that we are limited in the information that we can gather about the status of each tube and each vase. Our meter allows us to 'see' the potential of one tube by comparison to each of the other tubes. In the case of our model, our eyes allow us to evaluate the water levels in each of the tubes but not in the parts buried in sand.

Each glass flower vase represents a coating fault, with the water level in each tube being the electrical potential (or EMF) of that particular surface reaction. If the 'circuit' of each tube is completed through the connecting glass tube and atmospheric pressure, then the water will reach equilibrium and the surfaces will be level.

If the 'potential' of each tube is too low then the water will not flow between them .... no current will flow and there is no metal or energy lost.

We can use the whole model to show the difficulties in measuring the relative potentials of all of the coating faults on a length of pipeline.

A side view of the water model shows that the level of each main compartment is different and the level of each of the tubes might be different in relation to the bottom of that tube.

Now look at the sand filled partition and we cannot know anything about the botoms of the tubes or the water levels that are out of our sight. All we can see is the water level that is visible above the sand. This is exactly the situation we are in when measuring the voltage between a half-cell and the pipeline in cathodic protection work.

It is necessary to play around with this model or the notion of this model so that you understand that all 'potentials' have to be related to other potentials to be significant. It is no good puting a potential value into a formula if it is not related to a common zero value within that formula.

You can apply Ohms Law to a single corrosion cell because that is a single circuit but you cannot apply it to two corrosion cells unless they are integrated.

You can relate this notion of modeling to that of using dry cell batteries. A dry cell battery has a potential difference from one pole to the other and can be compared to the tubes in each of the flower vases.