THE ANODIC PASSIVATION OF IRON

Abstract

A study was made of the anodic passivation of iron in solutions of 0.1 M sodium sulphate at a pH of 3.0. A mass balance of iron equivalent to coulombs required for passivation and iron found in solution after passivation showed that approximately 0.002 coulomb/cm2 is involved in the passive layer. À Flade arrest does not appear in decay traces of anodic potential unless a potential of at least 1.2 v (EH) is achieved on charging the electrode, indicating incomplete coverage by the passive layer below 1.2 v. Partial cathodic reduction followed by anodic reformation of the passive layer at the Flade arrest showed that the first charging current involved in reformation is highly efficient even when as much as 70% of the Flade arrest is reduced. These results support the view that only a fraction of the passive layer is represented by coulombs involved in the Flade arrest and that after partial reduction the remainder of the passive layer disintegrates spontaneously. Addition of chelating agents to the electrolyte has an adverse effect on passivation by chelation of ferrous ion.