Cathodic Protection Training Course



Module 13

Cathodic protection and corrosion currents

Current readings
Currents are moving electrical charges that are everywhere in the universe and studied by all scientists in one way or another.
We are interested in the currents resulting from the corrosion reaction itself and those that we generate in our CP systems.

Shunts
Shunts are conductive paths of a fixed resistance value, across which we measure the potential difference in volts. Ohms Law E = IR. E is energy I is current and R is resistance. Therefore I = E/R the charges trying to get accross the shunt are resisted by a known value in Ohms. The result is shown on a voltmeter as the value in amps of the current.

Multi-meters Analogue multimeters measure the pressure exerted on the elastic memory of a hair spring by the pressure of the current passing through the meter.
Digital multimeters measure the voltage across the two probes and only require a small amount of current to activate the display.

Magnetic field meters, Clamp meters measure the disturbance in the electric field created when th tongs are united. This measurement is in volts and these meters usually have input plugs for normal volt readings. The problem with this form of measurement is that the reading can be distorted by the angle of the conductor passing through the field.
The measurement can also be disturbed by external energy fields.

Current direction detection systems.
There was a practice in which we were required to weld cables to a pipeline span a specified distance apart. The theory was that a voltage across this span would make it possible to calculate the current vaue and direction through the pipeline. Anyone who tries to make this measurement with the most sensative voltmeter or even an oscilloscopeknows that this is a stupid idea made wup by somebody who has never done CP field work. The resistance in 10 meters of pipeline is almost nil (the resistance in 1 mile of 24" dia 1/2" thick pipline is 0.006 ohm according to a chart published by HACE). You might just as well hold the two probes together!