CATHODIC PROTECTION

Cathodic protection is the responsibility of the corrosion engineer or metallurgist. The subject is fundamentally reasonably simple to understand but can be extremely mathematical in its application. Direct current is arranged to flow out from the impressed anodes into the surrounding electrolyte, which is the sea water for offshore structures or the damp ground for onshore structures. The current returns through the structure itself and then back to the negative terminal of the impressed current source. The direction of current as described prevents the loss of metal from the structure into the electrolyte. This is opposite in direction to the natural current present due to corrosion action.

The electrical engineer is not usually involved in the chemistry of the system; his work is mainly associated with sizing the AC and DC cables, accounting for the power requirements and ensuring that the equipment satisfies any hazardous area requirements that may exist.

Impressed current systems require low-voltage high-current DC power. The voltages are typically 12, 25 and 50 volts. The currents are typically 100 to 800 amperes from one unit. The power is supplied by transformer rectifier units in which the transformer coils and the power rectifier are usually immersed in insulating oil to improve heat removal. The AC supply is usually three phase at LV voltage, e.g. 380 to 440 volts, and the supply power factor is about 0.75 lagging.

The output voltage is adjustable between +33% and −25% to take care of local site variations. The correct setting is determined at site during commissioning. Adjustments are often made periodically as the site conditions vary or if the installation is modified.

The anodes are made of various materials and the choice is determined by the physical conditions, the electric field pattern, current densities, and cost and anode corrosion. Anode current densities vary between 10 amperes per meter squared for silicon iron to more than 1000 amperes per meter squared for platinised and lead alloys. The electrical engineer needs to size AC and DC cables and to choose them to suit the physical environment.