Cathodic protection is said to be achieved when all the local action cathodes, on a metallic surface in contact with an electrolyte, are polarized negatively to at least the voltage of the most active local action anode. This can be accomplished in many ways which is the art of cathodic protection design.
Cathodic polarization requires the application of DC current to the metallic structure. The DC current can come from a metal that sacrifices itself through corrosion to provide current to the metal being protected. A common example of this is galvanization of steel. The zinc coating will corrode to protect the steel layer beneath. Sacrificial anodes, such as zinc, magnesium and alloyed aluminum are often used where powered systems are not available.
Powered system are called impressed current cathodic protection. These have a supply of DC current from an external source of power such as from the rectification of AC power, from thermo-electrical generators or from solar panels and batteries. Typically, these system use a type of non-consuming anodes as opposed to sacrificial anodes.
Current requirement for the protection of a metallic object depends on the metal alloy, the surface area exposed to the soil or electrolyte, the temperature of the metal-electrolyte interface, the pH of the electrolyte, the presence and concentration of cations and anions in the electrolyte and several other parameters. It is important to provide enough current to polarize all of the cathodic areas, but not too much current that coatings are damaged or excess hydrogen is generated causing hydrogen issues in the metallic object. With aluminum, it is important not to create an excessively basic environment at the metal-electrolyte interface, or pitting corrosion can occur.
For larger facilities such as storage tanks or pipelines, even application of cathodic protection current becomes an issue. Northern Grounding uses specialized design tools created by others to assess the spread of current on tank bottoms, pipelines and complex interconnected piping systems.