text: Different types of corrosion inhibitor exist. Among them, oxidizing species such as chromate () and nitrite () were the first used to re-establish the state of passivation in the protective oxide layer. In the specific case of steel, the  cation being a relatively soluble species, it contributes to favor the dissolution of the oxide layer which so loses its passivity. To restore the passivity, the principle simply consists to prevent the dissolution of the oxide layer by converting the soluble divalent  cation into the much less soluble trivalent  cation. This approach is also at the basis of the chromate conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys. 
  As hexavalent chromate is a known carcinogen, its aqueous effluents can no longer be freely discharged into the environment and its maximum concentration acceptable in water is very low.
  Nitrite is also an oxidizing species and has been used as corrosion inhibitor since the 1950's.
  Under the basic conditions prevailing in concrete pore water nitrite converts the relatively soluble  ions into the much less soluble  ions, and so protects the carbon-steel reinforcement bars by forming a new and denser layer of γ- as follows:
  Corrosion inhibitors, when present in sufficient amount, can provide protection against pitting. However, too low level of them can aggravate pitting by forming local anodes.