The Anodic Passivation of Iron in Solutions of Inhibitive Anions*

Abstract

The anodic passivation of iron in air-saturated and de-aerated solutions of a number of inhibitive anions has been studied by the determination of potentiostatic polarisation curves. The critical passivation potential, critical passivation current density and leakage currents through the passive films have been found to depend on pH and on the nature of the anion present in the solution. The critical passivation potential (N.H.E. scale) in solutions of anions, where passivation occurs relatively easily, varies with pH in the range 6–12 according to E_p = + 0.09 – 0.06 pH volt For the anions studied, the critical passivation current densities in de-aerated 0·1 M-solution increase (and the difficulty of passivation increases) in the order: nitrite < hydroxide < chromate < borate < phosphate < carbonate < benzoate < bicarbonate < nitrate.