The Inhibition of the Corrosion of Iron by Some Anodic Inhibitors

Inhibitors having relatively strong oxidizing anions, such as sodium chromate and nitrite, passivate iron both in the presence of air and in deaerated solutions. Much weaker oxidizing agents such as sodium tungstate and molybdate behave similarly to chromate and nitrite in the presence of air but do not prevent corrosion in deaerated solutions despite the fact that potential/time and polarization curves indicate that slow film formation is occurring. Tungstate ions, however, are effective oxidizing agents toward iron when discharged anodically at high current density. Solutions of sodium acetate, benzoate, carbonate, hydroxide, orthophosphate, and silicate, which do not contain oxidizing anions, passivate iron only when they contain dissolved air. When these solutions are deaerated they attack iron very slowly, potential/time and polarization curves indicating that this attack is mainly under cathodic control. It is postulated that oxygen dissolved in solution is mainly responsible for passivity by virtue of its heterogeneous reaction with surface iron atoms to form a thin film of , approximately 200 Å thick, in a manner similar to that by which oxide films are formed in air. This film, if kept in constant repair, prevents iron ions from the metal passing into solution. It is considered that, in inhibitors containing oxidizing anions, the passivity film is formed mainly by dissolved oxygen, whereas in other inhibitors, which do not contain oxidizing anions, the oxide is formed entirely by dissolved oxygen.