There are three primary gases that make up the air that we breathe: nitrogen, oxygen and carbon dioxide. When these three gases come in direct contact with water, a small percentage of each gas will dissolve in the water. For example, at atmospheric pressure and room temperature about 8 – 10 parts per million of oxygen will dissolve in fresh water.
In order for gases to become available to react with metals in the corrosion process, they must dissolve in the water. Water provides the necessary conduit to allow a metal to give up its electrons. In the process the metal is converted into a water soluble ion that leaves the metal surface and dissolves in the water which produces a corrosion by-product known as rust.
By increasing the percentage of nitrogen in the air space over water in a closed pipeline, it is possible to bring about two results:
In effect, by continuing to increase the percentage of nitrogen in the space over the water, oxygen can actually be stripped out of the water. When this is accomplished, the oxygen is no longer available to corrode the metal surfaces of the pipe. This phenomenon is at the heart of the process of using nitrogen gas to control corrosion in fire sprinkler systems.