The Flade Potential of Iron Passivated by Various Inorganic Corrosion Inhibitors

Abstract

The characteristic potential obtained just before decay of passivity (Flade potential) was measured for Fe passivated by ferrates, nitrites, chromates, molybdates, tungstates, concd. HNO3, and by anodic polarization in H2SO4. The Flade potential is linear with pH, of slope 0.059 up to approx. pH 4.5. Above this value up to pH 11.5, the potentials tend to follow an extension of the data in acids in the case of Fe passivated by chromates and nitrites, but reach a const. value for Fe passivated anodically or by concd. HNO3. The standard Flade potential (pH = 0) for various passivators is increasingly noble in the order: nitrite (-0.50 v.); chromate, ferrate, molybdate (-0.54 v.); tungstate (-0.61 v.); and anodic polarization or concd. HNO3 (-0.64 v). The av. of these values is in reasonable accord with previously reported values and only small observed deviations from the av. indicate that the structure and compn. of the passive film on Fe is largely independent of the passivation process. This is attributed to a primary passive film consisting essentially of a monolayer of chemisorbed O atoms through gaps of which a layer of O2 mols. is chemisorbed. The calcd. free energy of formation of such a film (-30.000 cal./mole adsorbed O), the chem. equivs. of passive film substance (0.01 coulomb/sq. cm.), and the oxidizing capacity (12 * 10-8 equivs./sq. cm.) are all consistent with independent data obtained by others for O adsorption on Fe, coulometry, and reaction of the passive film with CrO2- to form CrO4--. The mechanism of passivity, accordingly, is proposed to consist of: (a) adsorption of the passivator on the metal or metal oxide surface; (b) depolarization of cathodic areas by the passivator accompanied by anodic passivation of residual small areas of exposed metal; and (c) very slow formation of Fe oxides with continual repair of the adsorbed O film by electrochem. action, as described, at pores in the oxide. Oxide formation is accelerated by elevated temps. and presence of certain anions, e.g. Cl-. It is proposed that the greater stability of the passive film formed by some passivators is accounted for by supplementary adsorption of the passivator on the passive film, accompanied by shift of the Flade potential to a less noble value.