Researchers have discovered rusting is an electrical process. When iron is exposed to the elements, such as being buried, a small electrical charge develops. This charge is minute, but it is constant. To prevent the charge from attacking the iron, the charge is redirected to a bundle of metal called a sacrificial anode. The anode is usually made of magnesium or zinc, and buried next to the pipe. The anode is connected to the pipe by a wire. The anode bundle attracts the electricity, and is corroded away instead of the pipe, hence the name sacrificial anode. When the anode bundle is completely dissolved, workers bury a new bundle of metal.
Impressed current cathodic protection is a simple yet effective system of corrosion control. The negative terminal of a battery or power supply is connected to the pipe to be protected, and the positive side of the battery is connected to a sacrificial anode, buried in the ground. The positive side of a battery is also called an anode. The pipe becomes the cathode, or the negative side of the battery. How far away the anode is placed from the pipe depends on local soil conditions, and a civil or design engineer usually runs soil tests and sets the distance. The theory behind this system is that electricity flows towards the anode and away from the cathode, so the anode corrodes instead of the pipe, which is the cathode.
Pipes runs for many miles, often hundreds of miles. One anode placed at the beginning of the run and calling it good will not do the job. Design engineers look at the run of the pipe, and place cathodic protection anodes and power supplies at different intervals, depending on the soil testing results. The United States Department of Transportation states a pipe must be protected throughout the complete run.
When engineers design a cathodic protection system, they look for stray electricity. For example, a gas pipeline may be buried next to underground electrical cables. Every time electricity flows through a wire, a magnetic field is developed around the wire. This field reacts with the pipe, causing it to be turned into an anode. This is not good, since anodes corrode quickly. The engineer has to account for the extra electrical field, and place more cathodic protection around areas of high stray electricity.