Cathodic protection is used to avoid corrosion in buried metal pipes by making the pipe the cathodic side of an electrochemical cell. It is commonly used to protect steel natural gas pipes since corroded pipe is subject to failure that can result in gas leaks or serious emergency incidents.
The simplest method for applying cathodic protection is connecting the metal to be protected with another more easily corroded metal that then acts as the anode of the electrochemical cell. This is called a passive or galvanic system. In this case, anodes made of aluminum, magnesium, or zinc are buried next to a gas pipe. These anodes are sometimes called sacrificial anodes since they are designed to corrode instead of the pipe. The anode is electrically connected to the pipeline, causing a positive current to flow from the anode to the pipe so that the whole surface of the steel becomes negatively charged making it the cathode.
An alternative is to apply a small DC electric circuit to an inert anode using a rectifier and a connection to an AC source of power. This is called an active or impressed current system. The impressed DC flow again causes the pipe to become a cathode. Where AC power is not available, a thermal generator power supply that converts heat energy from natural gas combustion into electricity can be used to create the impressed current.
Galvanic systems are easy to install, have low operating costs and minimal maintenance requirements, and they do not need an external power supply. However, they do not work in all soil types and provide a lower level of protection with larger pipes. Thus, impressed current systems are also commonly used.
Because cathodic protection is critical to maintaining gas pipes, utilities and pipelines must carry out regular measurements to verify the operation of each system. This is required at sacrificial anodes in galvanic systems and at rectifiers and tests points in impressed current systems. Sacrificial anodes must be replaced periodically.