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.[1] The effectiveness of a corrosion inhibitor depends

on fluid composition, quantity of water, and flow regime. A common mechanism for inhibiting corrosion

involves formation of a coating, often a passivation layer, which prevents access of the corrosive

substance to the metal. Permanent treatments such as chrome plating are not generally considered

inhibitors, however. Instead corrosion inhibitors are additives to the fluids that surround the metal or

related object.

Corrosion inhibitors and their role

The nature of the corrosive inhibitor depends on (i) the material being protected, which are most

commonly metal objects, and (ii) on the corrosive agent(s) to be neutralized. The corrosive agents are

generally oxygen, hydrogen sulfide, and carbon dioxide. Oxygen is generally removed by reductive

inhibitors such as amines and hydrazines:

O2 + N2H4 → 2 H2O + N2
In this example, hydrazine converts oxygen, a common corrosive agent, to water, which is generally

benign. Related inhibitors of oxygen corrosion are hexamine, phenylenediamine, and dimethylethanolamine,

and their derivatives. Antioxidants such as sulfite and ascorbic acid are sometimes used. Some corrosion

inhibitors form a passivating coating on the surface by chemisorption. Benzotriazole is one such species

used to protect copper. For lubrication, zinc dithiophosphates are common - they deposit sulfide on

surfaces.

The suitability of any given chemical for a task in hand depends on many factors, including their

operating temperature.