Cathodic Protection Training Course

Module 01 page 4

It is now time to examine the recommendations on the use and maintenance of the copper/copper-sulphate electrode. I do not have one that is provided by a manufacturer as they are too expensive and I have used many of these in all situations in the field in many countries.

CPN is concerned with the analysis of data gathered in relation to the cathodic protection of buried and submerged, coated, steel pipelines that carry most of the worlds energy supplies from source to the consumer.
This is a very specialised study that must begin at the interface between the pipeline metal and the electrolyte in which it is submerged or buried.

This old photograph was taken during the construction of the network of gas pipelines that have been buried in the UK for over 30 years. This particular stretch was coated with coal tar enamel and was handled by heavy construction machines. The coating was often damaged and repaired before back-fill.

It is clear that coating faults were sometimes missed.

The pipe metal at these coating faults is in contact with the ground (the electrolyte) , which gets 'charged up' with electricity at the anode of the corrosion cell.
We can set this up on the bench with instruments to measure the data that will confirm the theories. You will not that the reaction measurments must be made at the interface between the metal and the electrolyte at the anode and the cathode as the EMF is different at each.

We are setting it up so that the anode and cathode are at seperate coating faults as in this picture.

There are four coating faults in this experiment and we are measuring the corrosion on two corrosion cells. An anode and a cathode being one corrosion cell.

This pictures shows the visible effect of metal going into solution at one of the anodes.

The electrical potential of this bit of electrolyte is increased to a higher electrical 'pressure' than the surrounding electrolyte and so the electricity 'radiates' into the system.

The metal that is disolving at the 'anode' forces electrical current into the electrolyte and this corrosion current returns to the cathode of this corrosion cell.

The metal at other coating fault is the 'cathode' into which the current passes from the 'charged up' electrolyte, because the electrical pressure must be balanced out. (everything tries to equalise it's electrical potential with everything around it).

It can be seen that current passing into the metal stops it from corroding.
In sacrificial anode cathodic protection systems the disolving metal is provided by a less noble metal in the corrosion circuit... the system is known as 'Sacrificial Cathodic Protection'.

There are limits to which sacifical cathodic protection can be used but the same principle can be used by causing a manufactured electrical pressure which is 'impressed' into the electrolyte. The electricity is then 'drained' out of the subject metal....... boat hull or pipeline.... and this interfers with the natural tendency of the metal to disolve.(rust)

Students should try to form a mental image of electrical potential (pressure) and the resulting flow of 'charges'. Do not get confused by the flow of electrons as we cannot see this on our meters. It might be important to the academics but it is irrelevant to field engineers

Impressed current cathodic protection

Alternating Current Electricity is generated by a 'pumping' action which causes it to flow backwards and forwards in 'waves', but this is no use for our purposes so we have to get it going in one direction through a circuit known as a 'rectifier'. At the same time we can control the amount of current by transforming it, so the apparatus is know as a transformer-rectifier.

A transformer-rectifier can be regarded as an electrical pump which is sucking the electricity out of the pipeline (etc) and pumping it into the ground (or sea ... or swamp... or wherever else you want to pump it).

The effect of this is amazing. It stops rust! And it's cheap!

But there are some snags.

Because it's so good, it gets installed .... then ignored...... well most people don't even know it exists... and because it's cheap some people don't think it's important.