The process of corrosion requires four elements: an anode, a cathode, an electrolyte and a metallic path. Corrosion is a natural process that cannot be prevented, but intervention with the correct measures can control it. Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell.
Sacrificial anode cathodic protection
The simplest method to apply cathodic protection is by connecting the metal to be protected with another more easily corroded metal to act as the anode.
When two metals are electrically connected to each other in an electrolyte e.g. seawater, electrons will flow from the more active metal to the other, due to the difference in the electrical potential. When the most active metal (anode) supplies current, it will gradually dissolve into ions in the electrolyte, and at the same time produce electrons, which the least active (cathode) will receive through the metallic connection with the anode. The result is that the cathode will be negatively polarized and hence be protected against corrosion.
Impressed current cathodic protection (ICCP)
The impressed current cathodic protection (ICCP) system uses an external source of electrical power provided by a regulated dc power supply. The power supply provides the current necessary to polarize the hull. The protective current is distributed by specially designed inert anodes, generally a conductive material which is not consumed. A typical ICCP anode consists of titanium substrate coated with a noble metal or metal oxide catalyst.
The main advantage of an ICCP system is its automatic control feature, which continuously monitors and varies the current required for corrosion protection.