The Flade Potential of Iron Passivated by Various Inorganic Corrosion Inhibitors

The characteristic potential obtained just before decay of passivity (Flade potential) was measured for iron passivated by ferrates, nitrites, chromates, molybdates, tungstates, conc. , and by anodic polarization in . The Flade potential is linear with , of slope 0.059 up to approximately 4.5. Above this value up to 11.5, the potentials tend to follow an extension of the data in acids in the case of iron passivated by chromates and nitrites, but reach a constant value for iron passivated anodically or by conc. . The standard Flade potential ( ) 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 conc. (−0.64 v). The average of these values is in reasonable accord with previously reported values and only small observed deviations from the average indicate that the structure and composition of the passive film on iron is largely independent of the passivation process. This is explained by a primary passive film consisting essentially of a monolayer of chemisorbed oxygen atoms through gaps of which a layer of O 2 molecules is chemisorbed. The calculated free energy of formation of such a film (−30,000 cal/mole ads. O), the chemical equivalents of passive film substance (0.01 coulomb/cm 2 ), and the oxidizing capacity are all consistent with independent data obtained by others for oxygen adsorption on iron, coulometry, and reaction of the passive film with to form . 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; the adsorbed oxygen film is formed at anodes by discharge and combination of OH − at current densities above approximately 17 amp/cm 2 in accord with data of Franck; and ( c ) very slow formation of iron oxides with continual repair of the adsorbed oxygen film by electrochemical action, as described, at pores in the oxide. Oxide formation is accelerated by elevated temperatures 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.