ImpressedCurrent

Impressed current is a method of cathodic protection in which an external DC power source supplies electrical current to a metal structure to drive its surface potential negative with respect to the surrounding soil or seawater. The intent is to suppress corrosion by making oxidation less favorable at the metal surface. The amount of current is controlled to maintain adequate protection under varying environmental conditions.

A typical system includes a rectifier or dedicated DC power supply, anodes embedded in the surrounding medium (often inert materials such as MMO-coated titanium, graphite, or mixed-metal oxide), insulating connections, conductors, and reference electrodes to monitor potentials. The protected structure becomes the cathode, and the external current from the supply is adjusted to keep the structure at a selected protective potential.

Impressed-current CP is distinguished from galvanic or sacrificial CP, which relies on more active anodes in the soil or water. Impressed-current systems can deliver higher and more controllable protection currents, enabling protection of large structures or environments with high resistance. They require reliable power, regular monitoring, and can cause stray currents that may affect nearby structures; overprotection can cause hydrogen embrittlement of certain steels.

Common applications include pipelines, tanks, offshore platforms, ships hulls, and reinforced concrete elements in marine or buried environments. Design and operation involve monitoring potentials with reference electrodes, balancing current output, ensuring proper electrical isolation when needed, and performing routine maintenance on rectifiers and anodes.