 Cathodic Protection Study 5

SIMPLE MODELS OF CORROSION CELLS

Corrosion control requires a short term measurement for design, commissioning and maintenance activities and this can be achieved by an electrical measurement because metal dissolves in proportion to the electrical current discharged by the corrosion reaction.

The corrosion reaction causes an electro-motive force (EMF) to discharge current into the electrolyte at the anode and this current must return to the metal at the cathode, for the reaction to continue.

In the laboratory the voltage between a standard reference electrode and the interface EMF of a corrosion reaction, is measured using a capillary of non-reactive material. There are many scientific papers confirming that this experiment requires exact setting up.

The computer models of corrosion cells in the worksheet CPSTUDY5.XLS show the difficulty in making the measurement in the laboratory. The worksheet calculates the voltages that would be shown on the meter resulting from the different positions of the measuring probe.

The metal is shown as a continuous bar at the top of the sheet and the different contact electrolytes are shown as four sections below the bar.

THE CORROSION CIRCUIT

The EMF of the corrosion reaction can be varied by the model user as this is an indefinable variable in real life.

The metal side of the corrosion reaction is zero potential for the purposes of this model and the metal itself is assumed not to have any resistance as the value would be so small by comparison with the other values in the model.

The electrolyte, the earth, has a resistance which can be varied by the user at two points. This is to takes into consideration the fact that the corrosion current must pass through a resistance to get to 'mass earth', which is considered to be of infinitely low resistance, but from there it must then pass through the resistance at the cathode, to complete the circuit.

THE MODEL

The effect of the passage of the corrosion current is to create a voltage gradient in which 'shells' of the electrolyte surrounding the anode and cathode obtain increasing and decreasing potentials, when measured in relation to the potential of the metal.

The potential values in the model have been simplified to represent those resulting from the total resistance towards the cathode and the total resistance from the anode through the electrolyte to remote earth, in relation to the zero of the metal.
The corrosion cell on the left of the model shows the actual values in a typical corrosion circuit as calculated using the most simple laws of electricity.

Moving to the right, across the spread sheet, the model shows the reading which would be obtained if a capillary were placed at the actual anodic interface, as in a Daniel Cell, which is used in laboratories.

The next corrosion cell to the right shows the result of a reading taken when the capillary is placed close to the cathodic interface between earth and the metal.

The last corrosion cell, on the far right, shows the result which would be obtained from a measurement taken when the electrode is placed in 'remote earth'. This could conceivably be from a few centimeters to many meters distant from the interface, depending on the resistance value of the electrolyte itself.

In field work, cathodic protection is superimposed over the natural corrosion cell, and stray currents from other sources cause potential variations in the electrolyte., the measurement becomes increasingly difficult, if not impossible as there is no way to determin the exact ELECTRICAL position of the electrode.