For larger structures, or where electrolyte resistivity is high, galvanic/sacrificial anode cathodic protection system cannot economically deliver enough current to provide protection from environmental corrosion. In these cases, impressed current cathodic protection (ICCP) systems are used. These consist of anodes connected to a DC power source, often a transformer-rectifier powered by AC supply. In the absence of an AC supply, alternative power sources, such as solar panels, wind power or gas powered thermoelectric generators etc.
Anodes for ICCP systems are available in a variety of shapes and sizes. Common anodes are tubular and solid rod shapes or continuous ribbons of various materials. These include high silicon cast iron, graphite, mixed metal oxide, platinum and niobium coated wire and other materials.
For pipelines, anodes are arranged in ground beds either distributed or in a deep vertical hole depending on several design and field condition factors including current distribution requirements.
Cathodic protection transformer-rectifier units are often custom manufactured and equipped with a variety of features, including remote monitoring and control, integral current interrupters and various type of electrical enclosures. The output DC negative terminal is connected to the structure to be protected by the cathodic protection system. The rectifier output DC positive cable is connected to the anodes. The AC power cable is connected to the rectifier input terminals.
The output of the ICCP system should be optimized to provide enough current to provide protection to the target structure. Some cathodic protection transformer-rectifier units are designed with taps on the transformer windings and jumper terminals to select the voltage output of the ICCP system. Cathodic protection transformer-rectifier units for water tanks and used in other applications are made with solid state circuits to automatically adjust the operating voltage to maintain the optimum current output or structure-to-electrolyte potential. Analog or digital meters are often installed to show the operating voltage (DC and sometime AC) and current output. For shore structures and other large complex target structures, ICCP system are often designed with multiple independent zones of anodes with separate cathodic protection transformer-rectifier circuits.
Electrode potential is measured with reference electrodes. Copper-copper sulfate electrodes are used for structures in contact with soil or fresh water. Silver/silver chloride/seawater electrodes or pure zinc electrodes are used for seawater applications. The methods are described in EN 13509:2003 and NACE TM0497 along with the sources of error in the voltage that appears on the display of the meter. Interpretation of electrode potential measurements to determine the potential at the interface between the anode of the corrosion cell and the electrolyte requires training and cannot be expected to match the accuracy of measurements done in laboratory work.