A corrosion inhibitor is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy.

The effectiveness, or corrosion inhibition efficiency, of a corrosion inhibitor is a function of many factors, including but not limited to: fluid composition, quantity of water, and flow regime. If the correct inhibitor and quantity is selected then it is possible to achieve high, 90-99%, efficiency.

Some of the mechanisms of its effect are formation of a passivation layer, that is a thin film on the surface of the material that stops access of the corrosive substance to the metal, inhibiting either the oxidation or reduction part of the redox corrosion system (anodic and cathodic inhibitors), or scavenging the dissolved oxygen.

Some corrosion inhibitors are hexamine, benzotriazole, phenylenediamine, dimethylethanolamine, polyaniline, sodium nitrite, cinnamaldehyde, condensation products of aldehydes and amines (imines), chromates, nitrites, phosphates, hydrazine, ascorbic acid, and others. The suitability of any given chemical for a task in hand depends on many factors, from the material of the system they have to act in, to the nature of the substances they are added into and their operating temperature.

http://en.wikipedia.org/wiki/Corrosion_inhibitor