1882
G. H. CARTLEDGE
rent densities used indicate that reduction of the passive film itself was unimportant kinetically by comparison with the reduction of oxygen. Incomplete studies in the benzoate system show that the generd relationships are similar to those in the phthalate system, with some differences in detail. For passivation to be maintained in the absence of an externally applied current it is necessary that the cathodic current density available from the effective passivator exceed the steady-state corrosion current density; that is, the polarization curve for reduction of the passivator must intersect the polarization curve of iron a t a potential more noble than the Flade potential.28 The steadystate corrosion rate varies with the nature and concentration of the electrolyte, as well as with pH, as may be seen by comparing the phthalate data of Weil and BonhoeffeF with Vetter's data for sulfate solutions.29 From the present experiments it is clear that at pH values appreciably below 6 in phthalate solutions of 5 X to 5 X low2j the oxygen current is inadequate for complete passivation, whereas it is ample a t pH 6 or higher. By extrapolation from Fig. 2 it is seen that, in the region of passive iron potentials, reduction of oxygen on the platinum electrode exceeded that on the iron electrode by a factor of about lo4. This is a demonstration of an effect similar to that of platinum on the passivation of stainless steel, as discussed by T o m a ~ c h o wand , ~ ~by Stern and Wissenberg3I for platinum and titanium. (28) This assunies the validity of Vetter's demonstration t h a t the paeaive film is free of poi=, BO t h a t both the anodic and cathodic prooesses operata over the same total area. Cf.K. J. Vetter, 2. h'lektrochsm., 55, 274 (1951). (29) K.J. Vetter, ibid., b9, 67 (1955). (30) N. D. Tomaschow, rbid., 62, 717 (1958). (31) Milton Stern and Herman Wissenberg, J . Electrochem. Soc., 106, 759 (1959).
Vol. 64
If the curves of Fig. 2 are extrapolated to the corresponding reversible oxygen electrode potentials, it may be estimated that tlhe exchange currents are of the order of 10-14 amp./cm.2 on the platinum surface and lo-'* amp./cm.2 on passive iron. These numbers may be compared with to 10-lo smp./cmq2found by Bockris and Huqa2 for smooth platinum in sulfuric acid of pH 1.25, and lo-*" ampJcm.2 obtained by extrapolation of the data of Wade and Hackermanas on passive iron a t 5' and a t pH 4. The present measurements therefore show that oxygen alone a t 1 atm. or less is reduced rapidly enough on a passive iron electrode to maintain the mixed potential in the passive region in spite of a continuing corrosion current densit'y, which is of the order of - lo-* amp./cm.2 in the phthalate system a t pH 6. The non-oxidizing inhibitor is essential, however, and its effectiveness is disturbed by the addition of foreign ions such as the sulfate ion, which sensitize the system to activation. Since it is difficult to see how such ions enter directly into the electrochemical processes a t the low concentrations involved, it seems most reasonable to assume that they compete with oxygen or inhibitor a t sites that are active in the electrochemical reactions. I n the following paper, the degree of participation of a reducible inhibitor in the total cathodic process will be examined. Acknowledgment.-It is a pleasure to acknowledge helpful discussions with my colleagues, E. J. Kelly, R. E. Meyer and Franz A. Posey in connection with this series of studies. (32) J. O'M. Bockris and A. K. M. S. Huq, Proc. Roy. Soe. (London), A231, 277 (1956,. (33) W. H. Wade and N. Hackerman, Trans. Puradag Soc.. 68, 1 (1957).
THE COMPARATIVE ROLES OF OXYGEN AND INHIBITORS IN THE PASSIVATION OF IRON. 11. THE PERTECHNETATE ION BYG. H. CARTLEDGE Chemistry Division, Oak Ridge National Laboratory, Operated by Union Carbide Corporation for the U. S. Atomic Energg Commisszon,Oak Ridge, Tennessee Received May 7, 1960
Galvanostatic and potentiostatic polarizations of passive iron electrodes have been made in solutions of a phthalate or pertechnetate and mixtures of them. By measuring the polarizations both in oxygen and in essentially oxygen-free helium the relative contributions of oxygen and the reducible inhibitor to the total cathodic current have been determined. An acceleration of the cathodic processes by the reduction product of the pertechnetate ion was demonstrated. It was shown also that, in all cases, reduction of oxygen is the principal cathodic process a t passive potentials.
The preceding paper in this series' demonstrated that passivation of iron can be achieved under the oxidizing action of oxygen alone a t atmospheric pressure, provided a suitable non-oxidizing inhibitor is present. When a reducible inhibitor is available, it may supplement the cathodic current due to reduction of oxygen, and some reduction of such inhibitors is generally observed. The purpose of this study is to determine the extent to (1) G.H.Cartledge, THISJOURNAL, 64,1877 (1960).
which the inhibitor itself is reduced under passivating conditions, in comparison wit,h the reduct,ion of oxygen under a total pressure (including water vapor) approximating 1 atm. For this purpose, the pertechnetate and chromate ions and osmium(VII1) oxide have been used as inhibitors. The pertechnetate ion differs in important ways from the chromate ion with respect to properties that are important for theories of inhibition which assume oxidizing and precipitating power as the
Dec., 1960
COMPARATIVE ROLESOF OXYGENAND PERTECHNETATE IN IRON PASSIVATION 1883
essential requirement for an inhibitor. The pertechnetate ion, Tc04-, is univalent and derives from a strong acid. It therefore does not have the buffering action possessed by the chromate ion. The normal oxidation-reduction potential of the 4Hf 3e-F?Tc(OH)4(ppt.), is couple, Tc040.738 v. noble to the normal hydrogen electrode,2 in comparison with a calculated value of 1.246 v. for the couple, Cr045H+ 3e- a Cr(OH), HzO. For 10-3 f solutions at pH 6, the calculated reversible potent,ials for these couples are 0.20 and 0.59 v. for technetium and chromium, respectively. The chromate ion is therefore energetically a more vigorous oxidizing agent under conditions in which inhibition is very efficient. It also possesses good buffering action in the Cr04H+$ HCr04- equilibrium, and is therefore able to precipitate ferrous ions possibly arising from corrosion, as shown by Hoar and Evans.3 Similar action by the pertechnetate ion is not possible thermodynamically under conditions which yet permit inhibition in aerated sol~tioiis.~ For an inhibitor to be effective in maintaining pnssivit'y by virtue of its own oxidizing action it is necessary that its reduction rate on the passive surface and at passive potentials be adequate to balance the corrosion still occurring. In the present experiments, therefore, cathodic polarizations of a passivated electrode have been made to establish the relative reduction rates of oxygen and the inhibitors mentioned. Experimental
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The measurements involved essentially the establishment of cathodic polarization curves for passive iron electrodes in inhibited systems when fully oxygenated and again when oxygen was reduced t o a very low concentration by passage of a rapid stream of "oxygen-free'' helium through the closed cell. In view of the results of the preceding paper,' 0.0100 f phthalate a t about pH 6 was used as a medium for establishing the reliability of each electrode used. All phthalate solutions were pre-elect,rolyzed. as previously described. The potassium or ammonium pertechnetate used had been repeatedly recrystallized and was very pure spectroscopically, but for obvious reasons could not be pre-electrolyzed. Solutions of these salts were prepared in triply distilled water. Measurements were made as in the previous work. The electrodes were from the same batch of electrolytic iron and the abraded and air-oxidized surfaces were activated by treatment with 1 N HzSOc just before passivation. Polarizations were measured only in solutions that had been introduced t o the cell after completion of passivation. The temperature was 23-24'. Helium, ,when used, was passed through freshly reduced copper turmngs held a t 425', then through Ascarite, a cotton filter and a water saturator. Measurements were begun only when the electrode potential had become stable 16 hr. or more after the beginning of passivation. In most c,sses the galvanostatic procedure was used to estab!ish the entire polarization curve, but. in later (2) G. H. Cartledge and Wm. T. Smith, Jr., ibid., 69, 1111 (1955). (3) T. P. Hoar and U. R. Evans, J. Chem. SOC.,2476 (1932). (4) This conclusion was verified in a n experiment for which the author is indebted to Dr. R. F. Sympson. The reactants were thoroughly deaerated and mixed in a vacuum system. No precipitate formed in five hours at room temperature, although the concentrations and pH value were such t h a t corrosion of iron would have been inhibited. Slight precipitation occurred after opening the mixture t o air for five days, b u t beta counting showed the well-washed precipitate t o contain only of the order of 0.003% of the amount of technetium t h a t would have been reduced had the total precipitate been due to reduction of pertechnetate ions. T h e experiment makes i t clear t h a t inhibition by the pertechnetate ion, which occurs a t lower concentrations than are required in the case of rliromate, cannot be ascribed to any power of precipitating ferrous ions.
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9 L A 1 I I .-.-i--i -200 -150 -100 -50 0 450 t400 +150 ELECTRODE P O T E N T I A L (mv vs SC.E.).
Fig. 1.-Cathodic polarization of passive iron in a pertechnetate solution in oxygen or in helium. E. and j , are the observed open-circuit otential and indicated corrosion current density, respectiv3 E t h is the calculated reversible potential of the Tc(VI$-(IV) couple for the conditions prevailing. 1 0 - ~ ~
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