Cathodic protection continued anode requirement

The cost and economics of cathodic protection depend on a variety of parameters so that general statements on costs are not really possible. In particular, the protection current requirement and the specific electrical resistance of the electrolyte in the surroundings of the object to be protected and the anodes can vary considerably and thus affect the costs. Usually electrochemical protection is particularly economical if the structure can be ensured a long service life, maintained in continuous operation, and if repair costs are very high. As a rough estimate, the installation costs of cathodic protection of uncoated metal structures are about 1 to 2% of the construction costs of the structure, and are 0.1 to 0.2% for coated surfaces. 

The anode is fixed to the concrete using non-metallic fixings and may be supplied as a prefabricated mesh or more often as a continuous anode strand which is laid over the surface of the structure to be protected. The spacing between the anode strands may be adjusted to give the required current distribution and current density per unit area of concrete necessary to provide cathodic protection to a particular structure. 

A single sacrificial anode attached to one end of a pipe can normally, and in freshly piped seawater, protect the internal surfaces of a pipe only up to 2-3 diameters in depth and installation of a continuous strip anode may be required. In stagnant seawater, however, even 12-50 mm (0.5-2 inches) nominal diameter stainless steel (e.g. Type 304) and copper pipes can obtain effective cathodic protection after an initial polarization period, which can vary between four days, for 5 cm (2 in) diameter stainless steel pipe, and 186 days for 12mm (0.5 inch) diameter copper pipe, by a single sacrificial anode fitted to one end to a depth of approximately 6 m (20 ft). 

Theoretically, it would be possible to hold the potential at a value where the metal is thermodynamically immune from corrosion. In practice, however, this Is likely to be too expensive. The continuous passage of current requires the consumption of power (j cell 0 tid, particulary if the medium between the protected surface and the anode is not highly conducting, the use of anything but a very low current will entail an unacceptably high energy consumption, The voltage required of the power supply also depends upon the active area of Che structure, the nature of the electrode reactions and resistance of the anode. The cathode potential is kept within the desired limits by ... 

The potential control in anodic protection installations has two functions. First, the potential must be measured and compared to a desired preset value. Second, a control signal must then be sent to the power supply to force the dc current between the cathode and vessel wall. In early systems, this control function was done in an ON-OFF method because of the high costs of electronic drcuitiy. The recent progress in power electronics has resulted in all systems having a continuous proportional-fype control. Packaging these electronic components occasionally involves special requirements because most of the installations are made in chemical plants. Explosion-proof enclosures are sometimes required, and chemically resistant enclosures are necessary in other installations. 

---