Cathodic Protection Training Course

Module 4

Dry cell battery as a corrosion cell

A dry cell battery is a corrosion cell. You cannot see the corrosion and you cannot feel it. It corrodes on demand when you complete the circuit and allow current to flow........ Faraday said it must and who can argue with that?

Two dry cell batteries are two corrosion cells and because they are not connected by a conductor they do not affect each other. Neither is corroding or giving off any energy.

The purpose of these pictures is to make you think differently about corrosion. The meter is only used because we cannot sense the elctrical status without it.

You need to be able to visualise the electrical potential within a battery in the same way as you visualise any EMF from any source. The meter allows you to gather electrical data so that you can form a fairly accurate picture of what is happenning. We can see by the polarity of the batteries that the corrosion reaction in one is in series with the corrosion reaction in the other. The result of this alignment is that the potential of each reaction is added together and the value on the meter confirms this.

In this picture we have less information as it is not clear how the batteries are arranged but we can see that there are three of them. This can be liked to detecting three coating faults on a length of pipeline but only having one voltage value to help in assessing the corrosion status of each of the three batteries.

In this next picture we can count that there are 5 batteries in line but we cannot be sure of their polarity. The value on the meter gives an indication of the electrical status of the system but we need much more information to make a proper assessment.

In this picture we have a bit more information in that we can see many other corrosion cells in the proximity of the system we are examining, but we can see that they are not connected, and are therefore unlikely to have any influence on the electrical stutus of the subject.

In this next series of pictures we have an assortment of corossion cells in circuits representing a pipeline under the influence of an impressed current cathodic protection system.

You can see each of the corrosion cells and the connections.

You can see the values on the meter with each configuration.

Visualise the configurations in terms of elecrical pressures (potentials and EMFs)

You should be able to imagine the exact flow of charges.

If this data is fed into a computer it is easy to write a piece of software that displays an equivalent electrical circuit showing all potentials and current values and direction at each node.

This is a very simplified demonstration of the evaluation of pipeline cathodic protection. It remains to define where all the batteries are and how the system is configured.

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