Corrosion inhibitors

Corrosion inhibitors:

Corrosion inhibitors are substances which when in small concentrations to a corrosive environment decreases the corrosion rate. The inhibitors provide protection against corrosion by retarding either anodic or cathodic reactions.

They are broadly classified as

          (a)  Anodic inhibitors  (b) Cathodic inhibitors

(a) Anodic inhibitors: During corrosion of metals, oxidation occurs at anodic side and if this oxidation is prevented, cathodic reactions also stop there by retarding corrosion reaction. This is achieved by addition of anions such as chromate, tungstate, molybdate, phosphate etc. These anions combine with metal ions formed at the anodes forming sparingly soluble respective salts. These salts get deposited on the anodic side and acts as a barrier between the metal surface and the corrosive environment. Thus further anodic reaction is prevented. Anodic inhibitors are found to be effective only when sufficient quantities of inhibitors are added.

(b) Cathodic inhibitors: The two important types of cathodic reactions are liberation of H₂ gas and absorption of O₂.Therefore there are two distinct methods in achieving inhibition by cathodic reactions i.e.1) Preventing liberation of H₂ gas 2) Preventing absorption of O₂.