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. Anodic inhibitors
An example of an anodic inhibitor is chromate which forms a passivation layer on aluminum and steel surfaces which prevents the oxidation of the metal.
Nitrite and Nitrate are further anodic inhibitors. If anodic inhibitors are used at too low concentration, they can actually aggravate pitting corrosion, as they form a nonuniform layer with local anodes.
Other types of anodic inhibitors are based on phosphates.

2. Cathodic inhibitors
An example of a cathodic inhibitor is zinc oxide, which retards the corrosion by inhibiting the reduction of water to hydrogen gas.
One very good example of a cathodic inhibitor is a volatile amine present in steam; these are used in the boilers used to drive turbines to protect the pipework in which the condensed water passes.