Corrosion is a fact of life. Things rust and wear away, especially if they are in a corrosive environment, like sea water. It was actually within the shipping environment that some people first began to notice that the careful placement of some dissimilar metals seemed to protect other metals from deteriorating. Since then, the science of cathodic protection using sacrificial anodes (metals of lower electric potential) has made an appearance in a variety of industries with further advances using impressed current instead of sacrificial anodes.
One widespread use of sacrificial anodes is to protect water and gas pipelines. By burying a sacrificial anode near an underground pipeline that could be subject to corrosion, the pipeline is protected by acting as the cathode in a connected system. The anodes are generally made of aluminum, magnesium, or zinc. All these metals have a more negative electric potential than steel or copper, which the sacrificial anodes are used to protect.
Another form of cathodic protection for pipelines used is impressed current using rectifiers that deliver DC current instead of AC. This forces the current towards the sacrificial anode in cases of high resistivity. As cathodic protection systems continue to operate, the metallic lump of aluminum, magnesium, or zinc deteriorates and thus protects the other metals within the loop by behaving as a sacrificial anode. The system does need to be calibrated to maintain optimal operating conditions.
Water tanks, including your own home hot water, can also make use of an aluminium sacrificial anode. An aluminum, magnesium, or zinc anode is often built right in and the homeowner is not even aware that replacing it can significantly increase the life of their tank. This can keep the water from degrading the internal surface of the tank holding the water. For large commercial applications, it’s also important to have sacrificial anode systems installed to protect water and fuel tanks that are subject to corrosion.
Sacrificial anodes eventually wear out and corrode away – just as they were meant to do. That means that any metal surface may lose its protection over time. Careful monitoring is required to make sure the system is working properly. At some point the protection will fail and new anodes will need to be added. However, by monitoring the electrochemical potentials in a system, a tester can figure out when a system may be exhibiting problems or needs new anodes installed. However, this takes some training and knowledge on how to properly implement cathodic protection.