Steel pipelines are coated to stop corrosion by separating the metal from the corrosive chemicals in which they are submerged or buried.

Corrosion occurs at coating faults where the metal is in contact with the electrolyte in the ground.

The corrosion reaction requires electrical energy to pass from the metal into the electrolyte and complete a circuit back to the site of the corrosion. This is the circuit we use every day in our batteries and we control them with battery chargers and AC/DC power supplies.

In cathodic protection we use the same principles but there is a major difference in that there are millions of corrosion cells, equivalent to batteries, and we do not know where they are.

In a battery charger, or computer power supply, we can design a complete circuit that we can measure but in cathodic protection we put charges into the earth itself at a groundbed.

We can measure the electrical equilibrium between the whole of the pipeline metal and the groundbed anode(s) at the power supply itself and we can measure current and voltage drops in the metallic part of the circuit.

We CANNOT measure the effect that we have on the corrosion cells along the pipeline as these are all different in size and quality.

In a laboratory we can use a reference electrode in closed circuit condition to establish the electrical equilibrium at which the corrosion reaction is balanced so that no current flows and no corrosion takes place. This is shown in the Pourbaix diagram and in a variety of scientific equations.

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For many years it was believed that we could set a standard electrode in the ground and make a voltage measurement that would tell us if the pipeline was protected from corrosion. It was guessed that a potential of -0.850 volts relative to a copper/copper-sulphate electrode could be used as a criterion, but there were many real corrosion failures of pipelines that proved that this was not the case.

In the mid 1970's I went to a laboratory in Holland where I was shown the original experiment designed to examine how cathodic protection polarised steel and stopped corrosion. In making their voltage measurements it became apparent that the exact position of the reference electrode changed the value by an order of magnitude during the period in which current was flowing from the cathodic protection system.

In the laboratory in closed circuit conditions they were able to show that it is possible to eliminate the potential gradient in the electrolyte caused by the cathodic protection by making a recording voltmeter measurement that showed a 'kick' in the dowmward slope of the volts/time trace while the system was switched off.

It is this experimental demonstration that we first need to observe.

The experiment I witnessed consisted of a steel 'working electrode' immersed in a solution with a conductivity of about 50k ohm/cm. This was connected to a steel return electrode that was connected to a potentiostat to a DC variable power supply. The working electrode was connected to a recording voltmeter that had a needle tracing the voltages on a moving paper roll. The system was activated over a period of several minutes to show a curve/graph of the voltages (Y) related to time (X) and was run to show several wave forms.



It showed that the fall of voltage down to a certain level was a straight line before it began to curve and then there was a 'kick' that resulted in a ledge in the downward path before the downward curve continued logarithmically but never reached a straight baseline.

I no longer have access to an analogue recording voltmeter and this is an advantage to our experiment as it more accurate to use the digital instruments that enable repeated observation to be subject to calculations without approximation.

During my first run of these experiments I have been unable to produce a kick similar to that I witnessed in the 1970's.

Youtube video clip of first part of this demonsration

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The second experiment is set up to examine a corrosion cell consisting of a steel anode (nail 2) and a carbon cathode (C). This has similar properties to a corrosion cell on a steel pipeline between mill scale and bright steel that might be encountered at welded joints.



The transformer rectifier, power supply positive is connected through a timer, to nail 1 to act as the anode of an impressed current cathodic protection system and the negative is connected to nail 3, representing a protected pipeline. Nail2 and the carbon rod represent a corrosion cell embeded into the pipeline metalic mass.



A voltmeter is connected between the carbon rod and nail 3 to show the value and polarity of the potential differences and the behaviour of the charges in the metallic path.



A micro-ammeter is connected between the carbon rod and the steel nail 2 so that the corrosion current can be measured. This cannot be done in field work as we do not know the location of every corrosion cell or we would simply repair each coating fault before accepting completion of the constructed pipeline.



A voltmeter is connected between nails 1 and 2. This is equivalent to making a measurement between the transformer rectifier positive output and the pipeline at the drain point and should be the same value as the voltage shown on the transformer rectifier voltmeter. This measurement should always be carried out in the field to check the whole circuit.



A voltmeter is connected between nail 3 and the carbon rod to measure the voltage of the EMF of the corrosion reaction. The ammeter can be switched out of the circuit during this measurement.



An oscilloscope is set between the carbon and nail 2 to acquire the waveform achieved by switching the cathodic protection current off and on.

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The first measurement is that of the EMF of the subject corrosion reaction between the carbon rod and the nail 2. The voltmeter will show the reaction EMF and the micro ammeter will show the amount of current produced in this particular arrangement.

With all connections in place the meters show
meter 1 0.0 v
meter 2 0.0 v

meter 3 0.11 ma

meter 4 0.0 v

It is now seen that it is impossible to measure the potential difference between the EMF of the corrosion reaction because the charges are passing through many metalic paths in parallel but it is possible to measure the current passing between the carbon and the steel of nail 2. This is avery important observation as the measuring arrangement in this first test is identical to the measuring arrangement that is forced upon us during all field work.

with the power on

meter 1 0,0002 v
meter 2 -262 mv
meter 3 107.5 ma
meter 4 019 mv

We are now impressing charges into nail 1 and they are passing through the circuit according to Kirchoffs law through the lines of least resisrance as they do in our field work. The resistance has produced a measurable voltage between nail 2 and the carbon rod and the polarity of the potential between nail 1 and nail 2 shown on meter 2 indicates the tendancy of the current flow according to Ohms law. The resulting charge flow is shown on meter 3 and the polarity of the carbon potential in relation to that of the steel is shown on meter 4.

switching
meter 1 0.0 v to 0.002 v
meter 2 -0.237 v to -0.006

meter 3 -197.4 to -0.010 ma

meter 4 0.0 v to 0.018 v