Cathodic protection

Cathodic protection is used to prevent or reduce corrosion rates. Corrosion is an electrochemical process resulting from the formation of cathodes and anodes at different positions on the metal surface. Cathodic protection works by connecting the reinforcement to another material that is anodic in relation to it. The reinforcement becomes a cathode and its corrosion is substantially reduced.

There are two systems: sacrificial anode and impressed current.

In sacrificial anode systems, the material of the anode is more reactive than the metal in the structure, e.g. zinc or magnesium; the anode is consumed in preference to the reinforcement.

In impressed current systems, the anode material itself may be inert but is maintained in an anodic state relative to the structure by being connected to the correct pole of a direct current power supply. An outline of the process is shown in the Figure. The anode is distributed over the surface (either an inert metal mesh or a conductive coating) or many discrete anodes are fixed in drilled holes in the concrete.

It is usual to carry out a commissioning survey once the system has been activated to check that the reinforcement is achieving the required degree of polarisation. Often a permanent monitoring system is installed to check that the system is continuing to operate successfully.

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