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Spectroscopic observations on the nature of passivity
Corrosion Science, Vol. 29, No. 2/3, pp. 291-312, 1989 Printed in Great Britain
SPECTROSCOPIC
OBSERVATIONS PASSIVITY*
0010-938X/89 $3.00 + 0.00 Pergamon Press plc
ON THE NATURE
OF
J. O ' M . BOCKRIS Surface Electrochemistry Laboratory, Texas A&M University, College Station, TX 77843, U.S.A. Abstract--Spectroscopic characterizations of passive films on iron are reviewed. Some results obtained from Mossbauer, Auger, XPS, ISS, SIMS, ellipsometry and radiotracer measurements are discussed in the light of understanding the nature of passive films. It is evident from the spectroscopic investigations that the amorphous character due to the presence of water is an essential part of iron passivity. INTRODUCTION
THE THEMEof the present paper is that spectroscopic investigations of passive layers may lead to information which is difficult to obtain from purely electrochemical (current-voltage or impedance) examinations. The work which is reviewed in this paper started in 1973, but has been mainly carried out in the last three or four years. It includes a description of the first e x situ high-vacuum measurements made in electrochemistry (Revie et al., 1975). 1 One of the themes which comes out of the present work is the importance of the time elapsing between breakdown and the taking of measurements on the degraded film; and the conditions under which such measurements are made (e.g. vacuum, but under conditions of very low temperature, etc.). MOSSBAUER
It is best to start by saying a few words about the use made of the M6ssbauer approach, the use of which was suggested by graduate student W. O'Grady. 2 The apparatus that was used in our work is shown in Fig. 1. It consists of an emitter and absorber. The emitter consists of a piece of iron which is enriched to about 2% with iron 57. The absorber is the specimen on which passive film is produced, and it has the same isotopic composition (with respect to iron) as the emitter. In M6ssbauer spectroscopy, we deal with an isomer shift and a quadrupole shift. The 'isomer' shift refers to the two nuclei in different states. In the one state, the nucleus has a certain environment (e.g. iron in iron), and in the other, the iron nucleus is surrounded by, for example, oxygen atoms corresponding to the structure of a certain oxide. The nuclear resonance is shifted from the original value to a new value, and the major point of the M6ssbauer approach is that the high nuclear energies result in an extremely narrow line width, so that the tuning of the spectra is extremely sensitive, and very small changes in the environment, namely in the energy of the nucleus, can be detected.
* This paper was given at the German-American Colloquium on Electrochemical Passivation, which was held in Niimbrecht in the Federal Republic of Germany from 8-13 September 1987. Manuscript received 20 June 1988. 291
292
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Spectroscopic observations on the nature of passivity
293
The way one retunes the spectrum into resonance, and thus measures the isomer shift, is to impart kinetic energy to the absorber so that the absorber is shifted up and down with velocities reaching a few mm s -~. At some velocity, between 0 and, say, 20 mm per second, resonance will be reestablished, absorption will occur, and a minimum will be obtained on the intensity vs velocity graph. Isomer shifts are given in terms of millimeters per second. Correspondingly, the quadrupole shift refers to the fact that the nucleus itself may be a quadrupole, and the quadrupole interaction with the surroundings also gives rise to a measurable quantity which is characteristic of the splitting which occurs in the frequencies associated with the nuclei as a result of the interaction of the nuclear quadrupole with the surrounding ligand field (actually, it is the ligand field gradient which is involved). In Fig. 2, there are some raw data which involve measurements at the liquid helium temperature. In Table 1, the basic data on the isomer shift and the quadrupole shift are given. It is important to look closely at this table because it was interpretation of the data given here which first led us to a suggestion which became the basis of a lot of other work we have done in the area of passive film formation. In the research carried out by W. O ' G r a d y at the University of Pennsylvania, we first compared the MOssbauer frequencies and patterns we had obtained from the passive film measured in situ with the results of corresponding measurements made
TABLE 1.
M~SSBAUER
Compound Fe(OH)2 FeO Fe(OH)3 a-Fe~O3 Fe203" nH20 Fe203- 2H20 F-Fe~O3 a-FeOOH fl-FeOOH y-FeOOH 6FeOOH 6FeOOH Fe304
Amorphous Fe(OH)3- 0.9H20 Amorphous iron (III) oxide (thin film) Amorphous iron (III) oxide (thick film) Passive film (insitu) * Isomer shift relative to SNP.
PARAMETERS OF THE
Isomer shift* (ram/s) 1.44 1.18 0.59 0.61 0.64 0.62 Td 0.535 Oh 0.675 0.70 0.640 + 0.006 0.648 + 0.006 0.64 + 0.06 0.76 + 0.2
WELL-CHARACTERIZEDOXIDES Quadrupole split (ram/s) 2.92 0.8 0.65 0.42 0.62 0.64 0.84 0.68 0 0.700 -4- 0.006 0.594 _+ 0.006 0.48 + 0.06
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in situ for all the oxides which were available to us. We found that the M6ssbauer patterns for the passive layer were substantially different, both in the isomer shift and the quadrupole shift, from those we were obtaining for any of the oxides. For example, passive iron gives a value of 0.71 for the isomer shift and 1.00 for the quadrupole shift. When the Fe203 is dry (or after drying the passive layer for a full hour at about 150°C), then there is a distinct change in the M6ssbauer spectrum corresponding to the passive layer, and the value of the isomer shift becomes 0.66 (the accuracy of these shifts is about 0.01, so that the difference is very significant). Having made these detailed comparisons, we were disturbed by the fact that we could not get agreement of the results for passive layers with those of any of the iron oxide with the passive layer. Then, O'Grady happened to come across a paper by Prados and Good 3 which was on the M6ssbauer spectrum of amorphous hydrate polymers of iron in the solid state containing dioxy and trioxy bonds, and in this paper some of the values, both of the isomer shift and of the polymer hydrate, were very similar to those which O'Grady had obtained with the passive layers. This was the origin, then, of the original suggestion made in 1973 in a brief note, but elaborated on in 1980 in a paper by O'Grady, 4'5 in which the structure of the passive layer as a polymer hydrate was put forward for the first time. This structure is given in Fig. 3. AUGER
SPECTRA
The M6ssbauer measurements were made in the Chemistry Department at the University of Pennsylvania in 1971,2 but the Auger measurements were made at the Flinders University in Adelaide, Australia, in 1973,* and published in 1975. At this time, Auger techniques were new, and had not been applied to electrochemistry at all, largely because, of course, at first the requirements of a vacuum and conditions appeared to put electrochemical situations at a disadvantage. One of the reasons the author had chosen to go to the Flinders University was because it contained a strong team in high-vacuum surface chemistry led by Bruce Baker. Winston Revie, a former co-worker of Herb Uhlig, came to work at Flinders * The apparatus devised by Revie e t al. ~ was different from that devised by Hubbard, but the principle was the s a m e - - t r a n s f e r from the electrochemical situation to that of the high v a c u u m without exposure to outside air. In the Hubbard apparatus, however, the specimen was not frozen.
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Univeristy in Adelaide, collaborating with Baker and the author in the first Auger measurements made in an electrochemical situation. This point is worth emphasizing because the work of Hubbard in the area of ex situ measurements in electrochemistry has become so well known. In fact, unknown to the authors in Australia, Hubbard 6 was working in parallel with them and had published a summary of a talk which he had given and which was referred to in the first journal paper on ex situ measurement in electrochemistry which was published by Revie et al. in 1975.t The apparatus devised by Revie et al. is shown in Fig. 4. The essential objective, of course, is to carry out the electrochemical part of the measurements and then transfer of the specimen without contact with the atmosphere to the high vacuum situation, where it can be subjected to electron irradiation, and therefore the Auger measurements made. This is no small task because one has to extract the specimen from the electrolytic solution inside the compartment, and then obtain the removal of the surface water, without subjecting the specimen to a high vacuum which would be likely to eliminate the internal water, then raising the specimen into the high-vacuum chamber and bringing it under very strict and rigid control to avoid the destruction of the water (for it had been shown in the M6ssbauer work that if the specimen was allowed to dry, the passive structure was destroyed).
296
J. O'M. BOCKRIS
How this was achieved in the work by Revie et al. is shown in Fig. 4. The essence of the arrangement is the windlass, which is shown at the top. It is, in fact, a little crane which is operated by an external magnet. On winding the crane downward, a hook was arranged which could be brought by an external magnet to take hold of the iron specimen and draw this up to the first section (the one below the through-hole valve). In this section, the specimen could be subjected to a low vacuum, about 10-3 torr, which easily removed the surface water in about 1 min (later a washing train was involved). After the removal of the superficial water (at room temperatures), the specimen was moved up through the through-hole valve, which was then closed, and the specimen brought up into contact with the liquid air-containing vessel, freezing the specimen and putting it in a position to be subjected to the stream from the electron gun, and the Auger-measuring apparatus. In this way, we sought to prevent disruption of the water content of the passive layer which, in the Mfssbauer measurements, we had found vital, for when we removed the water by gently heating the specimen (i.e., drying it), we had found that the spectrum given by the Mrssbauer moved back from that characterizing a passive layer (and similar to those of Prados and Good in the hydrated polymer oxides) and became simply that of y-Fe203. There are two diagrams which I would like to put forward here for the results in the Auger work. In the first (Fig. 5a), one sees the diagram for pure iron, where the Auger energy peak is 46, and then when one exposes the iron to oxygen at 10-3 ton" for 1 min, two peaks arise, 42 and 50. The 50 turns out to the oxygen Auger line. The passive layer is shown in the third diagram (Fig. 5c), and here one sees that a peak at 50 is maintained, but the other character of the spectrum is changed. When one heats the passive layer, it goes back to be similar to the oxide (perhaps ~-Fe203, though this was not verified). The important thing to see here is that the passive layer has an Auger spectrum which is different from that of the oxide caused by a reacting iron with oxygen in the dry. The important part of the Auger work is from the spectra, which we examined at high energy. The high energy Auger line for oxygen is at 508 eV, and it is a ls O KLL transition. The iron lines under the high energy section are at 590, 645 and 695 eV. In order to obtain some information about what we were dealing with (which was our objective), one has to take into account the ionization cross-section, i.e. one cannot simply take the area of the peaks and compare them to obtain the total amount of iron and oxygen present. We first carried this exercise out for 'the oxide', that is to say the entity we had produced by exposing the evaporated iron film to oxygen. Here, we found that the area of the O-peak, multiplied by the ionization cross-section for this O, divided by the sum of the iron peaks multiplied by the ionization cross-section for iron, gave 1.5. The critical point was the corresponding value for the passive layer, and here we carried out six runs and got, by similar calculations to those which we had made for the oxide, the value 1.8 + 0.2. Obviously, this stretches up (at 2.0) to the ratio value which we would expect for Fe203 • I-I20 and, but only qualitatively, it agrees with the description we had originated on the basis of the Mrssbauer peaks and their comparison with Prados and Good's iron hydroxy polymers. Perhaps more significant than this suggestive numerical approximate agreement was the fact that when we heated the passive layers, and then re-examined the Auger
Spectroscopicobservationson the nature of passivity
297
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spectra after we had heated them, we found that the O : F e ratio had gone down to 1.2 + 0.2. This seemed in agreement with the indication from the M6ssbauer measurements that removal of the water (which we assumed was a reasonable interpretation from the effect of mild heating) gave an oxide back again, and we interpreted the value of 1.2 + 0.2 as being consistent with Fe30 4. In the A u g e r experiments, we did not bring the passive layer into contact with concentrated chloride and depassivate it--as we did later on with XPS and ISS---but we did subj ect the evaporated iron sheet to the passivation potential, in the presence of strong KCI. Of course, we did not expect to form a passive film under these conditions, and sure enough the A u g e r peaks which we now obtained had the ratio o f O : F e o f l . 2 + 0.1. Thus, it seemed that the Auger measurements gave rise to a picture in which the
298
J. O'M. BocKms
passive layer was different from the oxide which one forms by subjecting the iron to oxygen, that this difference corresponded to something which had a higher degree of oxygen in it than would correspond to Fe203, and that both heating and exposure to KC1 gave rise to films which have a smaller O : Fe ratio, and perhaps corresponded to the Fe304. Note there is a slight difference here from the M6ssbauer results, which seemed to indicate the dried sample corresponding to Fe203. Qualitatively, however, the agreement was good, and encouraged us in the concept that an essential feature associated with passive layer was hydration. These M6ssbauer and Auger measurements were lacking in one respect. As we contacted colleagues and told them our results, their general reaction was that it was agreed that water was a part of the passive layer structure, but only at the surface, and that the internal part of the layer did not contain water in any important way. As the results we had obtained with M6ssbauer and with Auger did not give information about the depth profiling (indeed, Auger gave information only of the surface layer), we decided to proceed at a later date with other measurements. These are described below. XPS AND ISS In the United States at Texas A & M University, a great deal of extra equipment became available for operation by the author. In collaboration with Oliver Murphy and with T. Pou, 7 it was possible to carry out XPS measurements.* (Note the fundamental difference between the Auger and the XPS measurements.) Auger is a complex spectrum in which the originating electron beam causes energy changes in the atoms concerned, the final electron stream produced having an energy which is the difference of two energy changes occurring at the atom. On the other hand, the XPS is a much simpler spectroscopy in which an X-ray beam causes absorption and electron emission from electrons, the wavelength being clearly characteristic of each atom. By adjusting the angle of the X-ray beam, we could obtain results between 5/~, and 30/~, in the passive layer. The result was that, at the surface, and in conformity with the general idea that the surface is highly hydrated, the O : Fe ratio amounted to 2.6. As the depth was increased in the layer, we got several more readings over the distances 11-30/~, and the ratio O :Fe was between 2.03 and 1.96. From the earlier considerations, it is known that the ideal value for the hydrated oxide would be 2.00. The XPS tells us, therefore, that this hydrate (a polymer, amorphous) which is apparently characteristic of the passive film, exists from a position which we can call 'after the surface' well into the depth and up to the metal. The XPS measurements were continued by subjecting the film to chloride ions, and 1.51-1.52 was obtained for O : Fe in a broken-down (Cl--attacked) passive layer. Of course, this corresponds to the 1.50, which would correspond to the completely broken-down layer, the depassivated film; and it seems reasonable to associate this with 'the oxide' and perhaps Fe203. The XPS results can also be used to evaluate the ratio of water to iron, and this turned out to be 0.8 in the passive layer itself, which is not very different from the 1.0 * Dr V. Youngis to be thanked for her collaborationin these measurementsand for teachingthe author and his colleaguesa good deal about depth profilingand deconvolutionin XPS measurements.
Spectroscopic observations on the nature of passivity
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FIG. 6. O : Fe ratio as a function of depth, A, into the passive film on iron from ISS depth profiling: (a) filmformed in borate buffer solution at 0.3 V(NHE); and (b) passive filmafter breakdown [0.3 V(NHE) in borate-buffer solution containing 0.5 M NaCI]. expected for the ideal hydrate structure, and the amount was reduced to 0.1 (near to the expected 0.0, the value in the dry film). A n o t h e r technique we were able to use to advantage was ion-scattering spectroscopy in which ions are bounced off the surface of the material, which is sputtered to get various depths. This sputtering, of course, is a less desirable feature of such analyses because one has to ask to what extent it destroys the immediate surface layer, but the damage caused by the sputtering probably does not penetrate more than 5/~, and we were able to obtain the remarkable figure shown in Fig. 6 illustrating that 2.1 and 1.6 are the two values obtained for the passive layer and for the chloride-exposed layer for most of the depth of the passive film. Again, one sees that the area at the surface gives a higher value of O : Fe corresponding to the results of other measurements. It is seen, therefore, as a summary from the XPS and ISS values that the depth-dependent value of the O : F e is consistent with the hydrate formula once more, and the chloride-exposed films are, in their O : Fe values, consistent with the concept that the depassivation is associated with removal of the water. SIMS The Secondary Ion Mass Spectroscopy is a technique which involves the reflection from the surface of ions which have interchanged their kinetic energy with the surface in the sense that they have 'knocked bits out of' the latter, and these then enter a mass spectroscope. Mass ratios are measured, but the m a j o r factor to note with SIMS is that the results cannot be interpreted in a direct and simple way because there is a quantity which is analogous to the ionization cross-section concept of Auger. When struck by bombarding ions, each particle from the surface does not detach itself and move into the mass spectroscope with the same velocity, so that the ratio of masses measured may not give a reliable indication of the mass ratio present at the surface, or at a given depth (as obtained by argon ion b o m b a r d m e n t ) .
300
J. O'M. BocKlUS
On this basis, therefore, it was decided to look at the ratio of O H : O, the concept being that these particles are very similar, so that the difference in their 'evaporability' would be minimal. It was found that the ratio of O H : O was finite and constant throughout the film, 8'1° except for a small section near the surface where the hydrogen is particularly high. Even the value of this ratio is not far off what one would expect for the simplest interpretation of SIMS. If the formula of the passive material were Fe203H20, the ratio would be (in the ideal case) about 0.3, and about 0.2 was observed. Just as in the other cases, the value observed is much lower in the presence of chloride, and becomes increasingly low as the exposure to chloride is extended. However, a much more remarkable result is obtained when one looks at the SIMS value for CI:O. Here, one finds that the ratio---and here we are quoting the values at 20/~ depth----of C I : O increases up to breakdown time, whereafter the ratio decreases again. This, then, suggests that the diffusion of C1- increases, and interaction with water occurs, but that, after breakdown, the chloride diffuses out again (speculatively, coupled with Fe3+). One is reminded of some of the results of Heusler and Fischer, n who found that, during film breakdown, ferrous ions were rejected into the solution. ELLIPSOMETRY The use of ellipsometry in investigations of passive layers is by no means new. Indeed, a great deal of work has been done on this phase by many investigators, with the earlier results stretching back to 1965.12-18 However, the type of ellipsometry which was carried out in this earlier phase must not be confused with that being done at the present. Earlier, the two ellipsometric parameters which were clearly determinable were A and ~0. The trouble with these earlier experiments was that they always needed an auxiliary variable to be measured by non-ellipsometric means. Thus, the information which one desires to obtain from the film is not only its thickness but also its refractive index and the quantity, k, the so-called extinction coefficient. It was Paik 19 who showed in 1972 that the problem coud be solved by making simultaneous measurements of the reflectivity of the parallel component of light. Thus, the present ellipsometric experiments have been carried out with the Fourier Transform Spectrometer working with a rotating polarizer. This allows resolution of the message down to about 20 ms, and it also allows us to carry out measurements of wavelength dependence of the quantities, although at the present time this is only extended to the visible region and cannot be carried out in the infrared because of the difficulty of the absorption of infrared radiation by the prisms of the ellipsometer. The objective of the ellipsometric work in this instance was to obtain the spectroscopic ellipsometry on passive layers. At least one extra element of the investigation (which was carried out by V. Jovancicevic2°) should be stated. Thus, in Fig. 7 the data associated with the ellipsometric examination at2 = 4500 are shown. In Fig. 8, one sees the plot of thickness against potential for various sweep rates. This examination has been carried out with the exclusion of oxygen from the solution, and it shows a most interesting feature, namely that the thickness increases with potential to a certain potential region. Then there is a plateau where, over a potential range of about 0.1 V, there is no further increase in thickness. Later,
Spectroscopic observations on the nature of passivity
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the thickness grows, reaching finally a value of about 30--40/~ at high positive potentials. There are two interesting things to note about these results. The first is the plateau. The second is the dependence upon sweep rate. The interpretation of the plateau is that it represents the situation in which ferrous ions are undergoing conversion to ferric. There is evidence for this in the later spectroscopic information. It is noteworthy that such a plateau is not observed in the presence of oxygen, and this may be due to the adsorption of 02 ions on the ferrous ions, thus reducing the sharpness of potential at which the oxidation reaction occurs. In earlier publications, 15-18 it had been stressed that the proper way to obtain ellipsometric data on passive layers is always to go back to a fiducial position at a pure iron surface, unencumbered by any oxide, and then to transfer the surface to a certain potential at which the measurement will be made, followed by a reduction of the fiducial position and transfer to another potential, and so forth. Using a slow potential sweep seemed to have disadvantages in the fact that the oxide observed at one potential would be affected by the history of the oxide at another potential.
302
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The results obtained, shown in Fig. 8, show that the effect is there, but it is slight, i.e. the apparent variation of the thickness with sweep time is a minor factor. Figures 9 and 10 show the values of k at various 2's. The spectra show two peaks. One is at 5250 ~ and the other at 6000 A. The 5250 ~ peak is likely to represent Fe 3+ and d-d transitions therein. There is evidence for such transitions in a-Fe203. Here, the other d-d transition is in Fe 3+. Such spectra have also been observed by Plieth 21 and co-workers. The maximum at 6000 ~ is more difficult to interpret. It corresponds to an energy of 2.05 eV. One would expect this to represent Fe a+ because, when one goes to much more positive potentials, the strength of the peak fades to zero. However, there is no clear evidence for a peak at 2.05 eV in other iron compounds, although there are weak peaks at 2.0 and 2.3 eV in Fe304. Much more disquieting for clear explanation is the big decrease in intensity which occurs as one goes from more negative to more positive potentials. This would not be expected at first. Speculatively, one might think of an increase of hydration with potential, and that increased hydration perhaps decreases the intensity of the lines corresponding to the transitions which have been cited. The most disquieting feature of these spectra is the apparent presence of Fe 3+ spectra at - 0 . 4 V where, from other evidence, one would expect only Fe a+ ions to be present. There is a good deal of non-stoichiometry and excess 0 2- ions so that there would be some Fe 3+ ions needed in the ferrous oxide lattice.'? One of the needs of ellipsometry is for IR measurements. We need these because of our hoped-for ability to see water in the film. Doing IR measurements in ellipsometry is difficult because of the need to get rid of the glass prisms or supplanting them with those made of zinc selenide, which are very expensive to
Spectroscopic observations on the nature of passivity
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Refractive index spectra of iron for different potentials.
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304
J. O'M. BocI~.IS
make. Let us hope that the use of diffraction gratings may be a possible solution to the problem. This is the intent of the work in our laboratory at the present time. These ellipsometric measurements do not contribute greatly (in the phase reported here) to a knowledge of passive layers and their amorphous hydrated structure, but it is important to give this report at this stage because it shows that ellipsometric spectroscopy for passive layers is a reality, can be practiced now, and the extension to the IR range would then make the ellipsometer a very useful tool in the study of passive layers. RADIOTRACER MEASUREMENTS Although a report on these measurements is not consistent with the title of this report, not producing spectra, their results are relevant to the general picture of a passive layer which we are building up here, and therefore it would be beneficial to give a summary of them. They were carried out largely by T. Mizuno in 1984-1985, and partly by Jose Carbajal with the assistance of P. Zelenay. 22 Measurement of adsorption, and indeed absorption, by radiotracer techniques is not new. Indeed, the first in situ measurements were carried out by Blomgren and Bockris23in 1960. However, a new technique has been used here which originates in the work of Sadkowski et al. 24.25The new feature introduced by the Polish workers is the use of a glass composite into which has been injected an organic material which scintillates upon being activated by radioactive substances. Thus, in the apparatus shown in Fig. 11, the composite is held at the bottom of the cell, having had the desired metallic coating deposited upon it. It is particularly easy to evaporate such a coating on the surface of the membrane if it is iron, because of the relatively low melting point of iron compared with platinum. For platinum, however, the deposition takes several hours for a thickness of about 0.1/zm. Figure 12 shows a typical circuit associated with this apparatus. It can be seen that the scintillation which arises in the glass composite is taken by means of a light pipe to a nearby measuring apparatus, and the scintillations counted. From this, if the roughness of the metal deposit on the glass specimen is known, it is possible to calculate, exactly, the coverage. One of the difficulties of the measurement is the calculation of roughness. The result finally obtained is proportional to the roughness factor, the ratio of real to apparent surface area, and this measurement brings certain difficulties. Thus, the measurement of roughness factors on noble metals is easy because one can utilize hydrogen peaks. However, in the case of iron it is not possible to use such voltammogramic methods because of codissolution of the iron (although considerable efforts have been made to overcome this). In the present case, three methods have been used. (1) It was assumed that the normal peak in the I-V curve for passivation corresponds to a monolayer of FeO, and on this basis the roughness factor was calculated to be 6.5. (2) The second method is to put the potential into the highly passive region where the film has been built up, and then compare the coulombs to build-up after a certain time whilst the basic film has been formed upon the rough deposit made upon glass, and then compare the coulombs used there with the corresponding deposit made in a highly polished iron specimen, assuming this has a roughness factor of 1.15. From this, we obtain a roughness value of 7.6. Finally, we obtained a value for the decay of
Spectroscopic observations on the nature of passivity
305
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current from the potentiostatic transient, and this was found to be 10.7. We ended, therefore, with an average value of 8.5 + 2.5. This uncertainty is a difficulty of the radiotracer measurement. The determination of the roughness factor to the extent that we hve seen above is the principal (indeed, the only significant) error, although the method is restricted to concentration range of around 10-3-10 -1 M because at the lower concentration the signal is too weak, and at a higher concentration the background overwhelms the signal. In Fig. 13, the diagram of adsorption as a function of potential is given. There are two clear regions, one corresponding to the adsorption of chloride ion on iron, and the second, more positive, region corresponding to the adsorption of chloride ion on the passive layer. It is clear from the data that the dF/dV line has a lesser slope in the case of adsorption on the passive ion, and this hints at the possibility of chemi-bonding and reduction of the surface charge of the iron upon the oxide. Something similar was observed with the rate of reduction of oxygen on passive iron, where the rate is higher
306
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Spectroscopic observations on the nature of passivity
307
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than that on iron itself (higher bonding of the oxygen to the passive layer, than to iron). In Fig. 14 the isotherm on the bare iron is given. The so-called monolayer coverage changes considerably with potential, becoming higher with more positive potential. One of the elements which came out of these examinations was a distinction between adsorption and absorption. It was found that, when the aggregation of the iron on the surface is a matter of adsorption, coverage is far less than 1. The value of rise time was low (a few seconds), but when the final value of the adsorption corresponded to a much higher value (the rise time now being about 60 s or more), absorption was clearly taking place. A series of results which show this are the F-time relationship shown in Fig. 15. These values are shown for a concentration of 10 -3 M chloride, and obviously indicate a value of F which corresponds to a situation where absorption is taking place, but no significant penetration or rise above the value corresponding to 0 = 1 can be seen. The most important result obtained in these investigations was that shown in Fig. 16, carried out at 5.10 -3 M of chloride. Here, a critical difference in the absorption-time pattern occurs, because breakdown sets in. The breakdown is monitored by the current-time relationships, and these undergo an inflection upwards at a certain moment. Corresponding to this, one sees that, at about the time of breakdown the 0 - t relation takes a turn upwards. However, this relation has to be looked at in terms of the actual value of the 0, which becomes greater than 1. It seems reasonable to interpret this, therefore, as indicating an absorption of chloride into the passive layer. The apparent 0 increases above the limit of 1, and indicates absorption into the film.
308
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Spectroscopicobservationson the nature of passivity
309
Does this critical increase in absorption occur before, at, or after breakdown? It seems--but the distinction is not a clear one--that the inflection on absorption upwards occurs before the breakdown. It is tempting, therefore, to regard the breakdown as occurring as a consequence of this absorption. CONCLUSIONS (1) The M6ssbauer results indicate that the passive film differs from the corresponding oxide by having an amorphous character. An experiment by Cahan and co-workers has verified this .26 They have shown by an electron microscopic technique that they can turn on the amorphous character by introducing water vapor, whereupon the patterns become disordered, i.e. amorphousness is indicated. (2) The passivity of the material (one is tempted to say its amorphousness) is destroyed by heating the material mildly (i.e. drying it, removing the water), or bringing it into contact with chloride ions. It seems reasonable, therefore, to suggest that these chloride ions remove the water, and destroy the passivity by removing its amorphousness. (3) The O : F e ratio is a monitor of the character of the layer. If passivity is present, then five different techniques indicate that the ratio is high (and near to 2.0) when the film is passive, but lower (near to 1.5) when the film has been made unpassive by contacting it with chloride ion or heating it. The evidence is, therefore, very clear that a change occurs in the film on these processes, and that this change is associated with a reduction of O : F e from a value which is consistent with the idea that passivity is associated with Fe20 3 • H20, and the 'oxide character'--non-passive--is associated with a structure that is some form of Fe203 (or, sometimes, Fe304). These results, and the effect found by heating the layer, are reminiscent of much else that has been published on passive layers, particularly from the electron diffractional point of view.27 Thus, if the film is immediately converted from a passive film to a normal oxide film by heat, it is reasonable to suspect that the electron beam used in the L E E D work would result in this conversion. These L E E D results would confirm the oxide, Fe~O3, and only results which were obtained in situ (or ex situ with careful control of the temperature, keeping it low so that the vacuum does not remove the water) will actually lead to results relevant to passive layer considerations. (4) There are several evidences (all with spectroscopy, together with the radiotracer work) which suggest that chloride ion penetrates the oxide, becomes absorbed therein, and in some way removes the water. The removal mechanism for water can only be speculatively identified as a place exchange mechanism in which the water is replaced by chloride, which then accelerates the dissolution of iron by its contact with the iron base at the film and now, with the crystalline film (the amorphousness having been destroyed), removes iron out through the film to the solution at a greater rate than was possible with the properly passive layer. Finally, it may be said that some of these ideas have been suggested in parallel by other workers. Thus, T. P. Hoar 2v3° pointed out that passive films sometimes have an amorphous character. Sato 31 and co-workers carried out tritium experiments in which they claim to have proved that two water molecules are associated with iron in the passive layer. (However, re-evaluation of their results by Revie et al. 1 suggests that the value actually associated with the experiments is one water molecule per iron atom.) More recently, Okamoto and Hashimoto 32'33 both suggested that water is an essential part of passivity.
310
J. O'M. BocKms
Thus, to verify the essential conclusion of these studies, that amorphousness, caused by the introduction of water, is the essential part of passivity, the measurements should be repeated by others under the proper conditions. It must be again stressed that these are continued wetness, i.e. in situ investigations, or vacuum situations in which the proper conditions are taken, to be quite sure that the water is not removed by drying or by electron bombardment. GENERAL DISCUSSION A comparison of the mechanism for the passivation of iron which has appeared under the name of the present author with the work of others was published by Pou etal. in 1984. 7 O'Grady and Bockris were the first4 to introduce the concept that the amorphous character of passive layers was caused by the presence of water, and this relation between the presence and absence of coordinated water is the most important feature in the forming of passive layer, as far as iron is concerned. By comparison with some of the well-known suggestions in the recent past, one finds that S a t o 34 c o m e s to the conclusion that there are two layers which make up the passive film, one being a barrier layer next to the metal consisting of ferric ions, and the other being a ferric oxide layer with a small amount of borate over it. Such conclusions came largely from ellipsometry and coulombic measurements.34 This model would not be consistent with the line spectroscopic work of Pou et al., 7,8 who made several probes of the constitution of the passive layer as a function of depth and found that, although there was some heterogeneity within 5 ,~ of the surface, the composition then remained constant back to the metal. Ord made several ellipsometric studies 35'36and concluded that the inner layer was Fe304, and on the outer layer, Fe203. This work would be subject to the same statement of inconsistency with the results of Pou and the other evidence summarized here as that of Sato. A good deal of M6ssbauer work has subsequently been published by Hoffmann and his colleagues, and now Scherson. 37,38These workers do not always come out with explicit statement of structural conclusions, but their results are qualitatively similar to those of O'Grady, though they differ numerically in some respects. There is no statement of disagreement of their results with the O'Grady-based model. EXAFS is obviously a very important technique to apply to passive layers, and this has been done recently by Kruger and his co-workers in several rather detailed studies. 39-41 The essential difference between the approach of Kruger and that of O'Grady is the measurement of the internuclear distance, and it is here that the EXAFS results shine because they show quite clearly that the introduction of water into the structure does cause changes which make the film more vitreous. This amorphousness (which had been suggested earlier by Revesz and Kruger) 42 is then similar to that which was suggested by the interpretation of the O'Grady-M6ssbauer work: Very detailed work by Lumsden and his colleagues using numerous ex situ methods to examine the film, have come to the conclusion that the film is always affected in its structure by the presence of water, and that if heat removes this water, it again becomes ordered. 43,44 Although it is not possible as yet to say there is a consensus about the mechanism of iron passivity and its amorphousness caused by water (this is what O'Grady
Spectroscopic observations on the nature of passivity
311
suggested), there are many lines of reasoning, in spectroscopic work, which seem to go in the same direction. Indeed, when the spectroscopic methods are applied, something to do with a vitreous nature, connection to water, and changes upon heating, always seems to come out of the results. Acknowledgements--These works have been supported over the years by a number of agencies, but particular thanks are to be given to the Office of Naval Research for the support of work under Contract N0001480Co113, and for discussion with Frank Herr.
REFERENCES 1. B. G. BAKER,J. O'M. BOCKRISand R. W. REVIE,J. electrochem. Soc. 122, 1460 (1975); R. W. REVIE, B. G. BAKERand J. O'M. BOCKRIS,Surf. Sci. 52,664 (1975). 2. W. E. O'GRADY, Ph.D. Thesis, University of Pennsylvania (1973). 3. R. PRADOSand M. L. GOOD, J. inorg. Nucl. Chem. 33, 3733 (1971). 4. W. E. O'GRADY and J. O'M. BOCKRIS, Chem. Phys. Lett. No. 2, 5,116 (1970); W. E. O'G~DY and J. O'M. BOCKRIS, Surf. Sci. 38,249 (1973). 5. W. E. O'GRADY, J. electrochem. Soc. 127,555 (1980). 6. A . T . HUBBARD,Crit. Rev. Anal. Chem. 3,201 (1973); A. T. HUBBARD,International Symposium of Characterization of Adsorbed Species in Catalytic Reactions, Ottawa (1974). 7. T. E. Pou, O. J. MURPHY, V. YOUNG,J. O'M. BOCKRISand L. L. TONGSEN, J. electrochem. Soc. 131, 1243 (1984). 8. O.J. MURPHY,T. E. POU, J. O'M. BOCKRISand L. L. TONGSEN,J. electrochem. Soc. 131,2785 (1984). 9. O. J. MURPHY, J. O'M. BOCKRIS,T. E. Pou, D. L. COCKEand G. SPARROW,J. electrochem. Soc. 129, 2149 (1982). 10. J. O'M. BOCKRIS, O. J. MURPHYand D. L. COCKE, J. electrochem. Soc. 129, 1276 (1982). 11. K. E. HEUSLER and L. FISCHER, Werkst. Korros. 27, 551 (1976); K. E. HEUSLER and L. FISCHER, Werkst. Korros. 27,697 (1976); K. E. HEUSLERand L. FISCHER, Werkst. Korros. 27,788 (1976). 12. A. K. N. REDDY, M. G. B. RAO and J. O'M. BOCKRIS,J. Chem. Phys. 42, 2246 (1965). 13. J. KRUGERand J. P. CALVERT,J. electrochem. Soc. 114, 43 (1967). 14. N. SATO, K. KUDO and T. NODA, Corros. Sci. 10,785 (1970). 15. J. O'M. BOCKRIS,M. GENSHAWand V. BRUSlC, Symp. Farad. Soc., 4, 177 (1970). 16. M. GENSHAW,V. BRUSlC,H. WROBLOWAand J. O'M. BOCKRlS, Proceedings of the Conference on the Stability of Metals, Mexico City (June 1970). 17. J. O'M. BOCKRIS,M. A. GENSHAW,V. BRUSlCand H. WROBLOWA,Electrochim. Acta. 16,1859 (1971). 18. H. WROBLOWA,V. BRUSlCand J. O'M. BOCKRIS,J. Phys. Chem. 75, 2823 (1971). 19. W. K. PAIKand J. O'M. BOCKRIS,Surf. Sci. 33,617 (1972). 20. V. JOVANClCEVIC,R. C. KAINTHLA,Z. TANG, B. YANG and J. O'M. BOCKRIS,Langmuir 3, 388 (1987). 21. N. PLIETHetal., private communication (September 1987). 22. V. JOVANCICEV1C,J. O'M. BOCKRIS,J. L. CARBAJAL,P. ZELENAYand T. MIZUNO,J. electrochem. Soc. 133, 2219 (1986). 23. E. BLOMGRENand J. O'M. BOCKRIS,Nature 186,305 (1960). 24. A. WIECKOWSKI,J. electrochem. Soc. 122,252 (1975). 25. A. WIECKOWSKI,M. SZKLARCZYKand J. SOBKOWSKI,J. electroanal. Chem., 113, 79 (1980). 26. K. KURODA, B. D. CAHAN, G. NAZRI, E. YEAGERand T. E. MITCHELL,J. electrochem. Soc. 129, 2163 (1982). 27. M. NAGAYAMAand M. COHEN, J. electrochem. Soc. 109,781 (1962). 28. T. P. HOAR, J. electrochem. Soc. 117, 17c (1970). 29. C. L. McBEE and J. KRUGER, Electrochim. Acta 17, 1337 (1972). 30. G. OKAMOTO,Corros. Sci. 13,471 (1973). 31. N. SATO,K. KUDO and R. NISHIMURA,J. electrochem. Soc. 123, 1419 (1976). 32. G. OKAMOTO,private communication. 33. K. HASHIMOTO,private communication. 34. N. SATO, in Comprehensive Treatise of Electrochemistry (eds J. BOCKRIS, B. E. CONWAY, E. YEAGER and R. E. WHITE), Vol. 4, p. 193. Plenum, New York (1981). 35. J. L. ORb and D. J. DESMET, J. electrochem. Soc. 123, 1876 (1976). 36. Z. Q. HUANGand J. L. ORD, J. electrochem. Soc. 132, 24 (1985).
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37. J. ELDRIDGE, M. E. KOV.DESCHand R. W. HOFEMAN,J. Vac. Sci. Technol. 20, 934 (1981). 38. M. E. KORDESCH, D. SCHERSONand R. W. HO~V,AN, J. electroanal. Chem. 164,383 (1984). 39. J. KRUGER, in Surface, Inhibition and Passivation (eds E. MCCAFFERTYand R. J. BROI)D), p. 120. Electrochemical Society, Pennington, New Jersey (1986). 40. G. G. LONG, J. KRUGER, D. R. BLACKand M. KURIYAMA,J. electrochem. Soc. 130,240 (1983). 41. G. G. LONG, J. KRUGER, D. R. BLACKand M. KURIYAMA,J. electroanal. Chem. 150,603 (1983). 42. A. G. REVESZand J. KRUGER,in Passivity o f Metals (eds R. P. FRANKENTHALand J. KRUGER), p. 137. Electrochemical Society, Princeton, New Jersey (1978). 43. D. A. HARRINGTON,A. WIECKOWSKI,S. D. ROSASCO,B. C. SCHARDT,G. N. SALAITA,A. J. HUBBARD and J. B. LUMSDEN, Corros. Sci. 25,849 (1985). 44. J. B. LUMSDENand P. J. STOCKER,J. electrochem. Soc. 133, 1978 (1986).
SPECTROSCOPIC
OBSERVATIONS PASSIVITY*
0010-938X/89 $3.00 + 0.00 Pergamon Press plc
ON THE NATURE
OF
J. O ' M . BOCKRIS Surface Electrochemistry Laboratory, Texas A&M University, College Station, TX 77843, U.S.A. Abstract--Spectroscopic characterizations of passive films on iron are reviewed. Some results obtained from Mossbauer, Auger, XPS, ISS, SIMS, ellipsometry and radiotracer measurements are discussed in the light of understanding the nature of passive films. It is evident from the spectroscopic investigations that the amorphous character due to the presence of water is an essential part of iron passivity. INTRODUCTION
THE THEMEof the present paper is that spectroscopic investigations of passive layers may lead to information which is difficult to obtain from purely electrochemical (current-voltage or impedance) examinations. The work which is reviewed in this paper started in 1973, but has been mainly carried out in the last three or four years. It includes a description of the first e x situ high-vacuum measurements made in electrochemistry (Revie et al., 1975). 1 One of the themes which comes out of the present work is the importance of the time elapsing between breakdown and the taking of measurements on the degraded film; and the conditions under which such measurements are made (e.g. vacuum, but under conditions of very low temperature, etc.). MOSSBAUER
It is best to start by saying a few words about the use made of the M6ssbauer approach, the use of which was suggested by graduate student W. O'Grady. 2 The apparatus that was used in our work is shown in Fig. 1. It consists of an emitter and absorber. The emitter consists of a piece of iron which is enriched to about 2% with iron 57. The absorber is the specimen on which passive film is produced, and it has the same isotopic composition (with respect to iron) as the emitter. In M6ssbauer spectroscopy, we deal with an isomer shift and a quadrupole shift. The 'isomer' shift refers to the two nuclei in different states. In the one state, the nucleus has a certain environment (e.g. iron in iron), and in the other, the iron nucleus is surrounded by, for example, oxygen atoms corresponding to the structure of a certain oxide. The nuclear resonance is shifted from the original value to a new value, and the major point of the M6ssbauer approach is that the high nuclear energies result in an extremely narrow line width, so that the tuning of the spectra is extremely sensitive, and very small changes in the environment, namely in the energy of the nucleus, can be detected.
* This paper was given at the German-American Colloquium on Electrochemical Passivation, which was held in Niimbrecht in the Federal Republic of Germany from 8-13 September 1987. Manuscript received 20 June 1988. 291
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Spectroscopic observations on the nature of passivity
293
The way one retunes the spectrum into resonance, and thus measures the isomer shift, is to impart kinetic energy to the absorber so that the absorber is shifted up and down with velocities reaching a few mm s -~. At some velocity, between 0 and, say, 20 mm per second, resonance will be reestablished, absorption will occur, and a minimum will be obtained on the intensity vs velocity graph. Isomer shifts are given in terms of millimeters per second. Correspondingly, the quadrupole shift refers to the fact that the nucleus itself may be a quadrupole, and the quadrupole interaction with the surroundings also gives rise to a measurable quantity which is characteristic of the splitting which occurs in the frequencies associated with the nuclei as a result of the interaction of the nuclear quadrupole with the surrounding ligand field (actually, it is the ligand field gradient which is involved). In Fig. 2, there are some raw data which involve measurements at the liquid helium temperature. In Table 1, the basic data on the isomer shift and the quadrupole shift are given. It is important to look closely at this table because it was interpretation of the data given here which first led us to a suggestion which became the basis of a lot of other work we have done in the area of passive film formation. In the research carried out by W. O ' G r a d y at the University of Pennsylvania, we first compared the MOssbauer frequencies and patterns we had obtained from the passive film measured in situ with the results of corresponding measurements made
TABLE 1.
M~SSBAUER
Compound Fe(OH)2 FeO Fe(OH)3 a-Fe~O3 Fe203" nH20 Fe203- 2H20 F-Fe~O3 a-FeOOH fl-FeOOH y-FeOOH 6FeOOH 6FeOOH Fe304
Amorphous Fe(OH)3- 0.9H20 Amorphous iron (III) oxide (thin film) Amorphous iron (III) oxide (thick film) Passive film (insitu) * Isomer shift relative to SNP.
PARAMETERS OF THE
Isomer shift* (ram/s) 1.44 1.18 0.59 0.61 0.64 0.62 Td 0.535 Oh 0.675 0.70 0.640 + 0.006 0.648 + 0.006 0.64 + 0.06 0.76 + 0.2
WELL-CHARACTERIZEDOXIDES Quadrupole split (ram/s) 2.92 0.8 0.65 0.42 0.62 0.64 0.84 0.68 0 0.700 -4- 0.006 0.594 _+ 0.006 0.48 + 0.06
0 _+0.01
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(300°K) (300°K) (77°K)
(77°K) (72°K) (83°K)
Td 525 (80°K) Oh 505 (80°K) Td 500 (300°K) Oh 482 (300°K) 450 (300°K) 460 (4°K) 460--490 (4.2°K)
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294
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Proposed representation polymeric film of hydrated iron oxide.
in situ for all the oxides which were available to us. We found that the M6ssbauer patterns for the passive layer were substantially different, both in the isomer shift and the quadrupole shift, from those we were obtaining for any of the oxides. For example, passive iron gives a value of 0.71 for the isomer shift and 1.00 for the quadrupole shift. When the Fe203 is dry (or after drying the passive layer for a full hour at about 150°C), then there is a distinct change in the M6ssbauer spectrum corresponding to the passive layer, and the value of the isomer shift becomes 0.66 (the accuracy of these shifts is about 0.01, so that the difference is very significant). Having made these detailed comparisons, we were disturbed by the fact that we could not get agreement of the results for passive layers with those of any of the iron oxide with the passive layer. Then, O'Grady happened to come across a paper by Prados and Good 3 which was on the M6ssbauer spectrum of amorphous hydrate polymers of iron in the solid state containing dioxy and trioxy bonds, and in this paper some of the values, both of the isomer shift and of the polymer hydrate, were very similar to those which O'Grady had obtained with the passive layers. This was the origin, then, of the original suggestion made in 1973 in a brief note, but elaborated on in 1980 in a paper by O'Grady, 4'5 in which the structure of the passive layer as a polymer hydrate was put forward for the first time. This structure is given in Fig. 3. AUGER
SPECTRA
The M6ssbauer measurements were made in the Chemistry Department at the University of Pennsylvania in 1971,2 but the Auger measurements were made at the Flinders University in Adelaide, Australia, in 1973,* and published in 1975. At this time, Auger techniques were new, and had not been applied to electrochemistry at all, largely because, of course, at first the requirements of a vacuum and conditions appeared to put electrochemical situations at a disadvantage. One of the reasons the author had chosen to go to the Flinders University was because it contained a strong team in high-vacuum surface chemistry led by Bruce Baker. Winston Revie, a former co-worker of Herb Uhlig, came to work at Flinders * The apparatus devised by Revie e t al. ~ was different from that devised by Hubbard, but the principle was the s a m e - - t r a n s f e r from the electrochemical situation to that of the high v a c u u m without exposure to outside air. In the Hubbard apparatus, however, the specimen was not frozen.
Spectroscopic observations on the nature of passivity
agnet Softiron
295
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Univeristy in Adelaide, collaborating with Baker and the author in the first Auger measurements made in an electrochemical situation. This point is worth emphasizing because the work of Hubbard in the area of ex situ measurements in electrochemistry has become so well known. In fact, unknown to the authors in Australia, Hubbard 6 was working in parallel with them and had published a summary of a talk which he had given and which was referred to in the first journal paper on ex situ measurement in electrochemistry which was published by Revie et al. in 1975.t The apparatus devised by Revie et al. is shown in Fig. 4. The essential objective, of course, is to carry out the electrochemical part of the measurements and then transfer of the specimen without contact with the atmosphere to the high vacuum situation, where it can be subjected to electron irradiation, and therefore the Auger measurements made. This is no small task because one has to extract the specimen from the electrolytic solution inside the compartment, and then obtain the removal of the surface water, without subjecting the specimen to a high vacuum which would be likely to eliminate the internal water, then raising the specimen into the high-vacuum chamber and bringing it under very strict and rigid control to avoid the destruction of the water (for it had been shown in the M6ssbauer work that if the specimen was allowed to dry, the passive structure was destroyed).
296
J. O'M. BOCKRIS
How this was achieved in the work by Revie et al. is shown in Fig. 4. The essence of the arrangement is the windlass, which is shown at the top. It is, in fact, a little crane which is operated by an external magnet. On winding the crane downward, a hook was arranged which could be brought by an external magnet to take hold of the iron specimen and draw this up to the first section (the one below the through-hole valve). In this section, the specimen could be subjected to a low vacuum, about 10-3 torr, which easily removed the surface water in about 1 min (later a washing train was involved). After the removal of the superficial water (at room temperatures), the specimen was moved up through the through-hole valve, which was then closed, and the specimen brought up into contact with the liquid air-containing vessel, freezing the specimen and putting it in a position to be subjected to the stream from the electron gun, and the Auger-measuring apparatus. In this way, we sought to prevent disruption of the water content of the passive layer which, in the Mfssbauer measurements, we had found vital, for when we removed the water by gently heating the specimen (i.e., drying it), we had found that the spectrum given by the Mrssbauer moved back from that characterizing a passive layer (and similar to those of Prados and Good in the hydrated polymer oxides) and became simply that of y-Fe203. There are two diagrams which I would like to put forward here for the results in the Auger work. In the first (Fig. 5a), one sees the diagram for pure iron, where the Auger energy peak is 46, and then when one exposes the iron to oxygen at 10-3 ton" for 1 min, two peaks arise, 42 and 50. The 50 turns out to the oxygen Auger line. The passive layer is shown in the third diagram (Fig. 5c), and here one sees that a peak at 50 is maintained, but the other character of the spectrum is changed. When one heats the passive layer, it goes back to be similar to the oxide (perhaps ~-Fe203, though this was not verified). The important thing to see here is that the passive layer has an Auger spectrum which is different from that of the oxide caused by a reacting iron with oxygen in the dry. The important part of the Auger work is from the spectra, which we examined at high energy. The high energy Auger line for oxygen is at 508 eV, and it is a ls O KLL transition. The iron lines under the high energy section are at 590, 645 and 695 eV. In order to obtain some information about what we were dealing with (which was our objective), one has to take into account the ionization cross-section, i.e. one cannot simply take the area of the peaks and compare them to obtain the total amount of iron and oxygen present. We first carried this exercise out for 'the oxide', that is to say the entity we had produced by exposing the evaporated iron film to oxygen. Here, we found that the area of the O-peak, multiplied by the ionization cross-section for this O, divided by the sum of the iron peaks multiplied by the ionization cross-section for iron, gave 1.5. The critical point was the corresponding value for the passive layer, and here we carried out six runs and got, by similar calculations to those which we had made for the oxide, the value 1.8 + 0.2. Obviously, this stretches up (at 2.0) to the ratio value which we would expect for Fe203 • I-I20 and, but only qualitatively, it agrees with the description we had originated on the basis of the Mrssbauer peaks and their comparison with Prados and Good's iron hydroxy polymers. Perhaps more significant than this suggestive numerical approximate agreement was the fact that when we heated the passive layers, and then re-examined the Auger
Spectroscopicobservationson the nature of passivity
297
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FI~. 5. Low energyAuger spectra of evaporated filmsof iron: (a) clean filmas evaporated; (b) after exposure to O2 at 10-3 torr for 1 min; (c) passivated in boric acid-potassium hydroxide buffer solution, pH = 8.1, at 0.300 V(NHE) for 10 min; and (d) passivated as in (c) and subsequently heated to 150°C.
spectra after we had heated them, we found that the O : F e ratio had gone down to 1.2 + 0.2. This seemed in agreement with the indication from the M6ssbauer measurements that removal of the water (which we assumed was a reasonable interpretation from the effect of mild heating) gave an oxide back again, and we interpreted the value of 1.2 + 0.2 as being consistent with Fe30 4. In the A u g e r experiments, we did not bring the passive layer into contact with concentrated chloride and depassivate it--as we did later on with XPS and ISS---but we did subj ect the evaporated iron sheet to the passivation potential, in the presence of strong KCI. Of course, we did not expect to form a passive film under these conditions, and sure enough the A u g e r peaks which we now obtained had the ratio o f O : F e o f l . 2 + 0.1. Thus, it seemed that the Auger measurements gave rise to a picture in which the
298
J. O'M. BocKms
passive layer was different from the oxide which one forms by subjecting the iron to oxygen, that this difference corresponded to something which had a higher degree of oxygen in it than would correspond to Fe203, and that both heating and exposure to KC1 gave rise to films which have a smaller O : Fe ratio, and perhaps corresponded to the Fe304. Note there is a slight difference here from the M6ssbauer results, which seemed to indicate the dried sample corresponding to Fe203. Qualitatively, however, the agreement was good, and encouraged us in the concept that an essential feature associated with passive layer was hydration. These M6ssbauer and Auger measurements were lacking in one respect. As we contacted colleagues and told them our results, their general reaction was that it was agreed that water was a part of the passive layer structure, but only at the surface, and that the internal part of the layer did not contain water in any important way. As the results we had obtained with M6ssbauer and with Auger did not give information about the depth profiling (indeed, Auger gave information only of the surface layer), we decided to proceed at a later date with other measurements. These are described below. XPS AND ISS In the United States at Texas A & M University, a great deal of extra equipment became available for operation by the author. In collaboration with Oliver Murphy and with T. Pou, 7 it was possible to carry out XPS measurements.* (Note the fundamental difference between the Auger and the XPS measurements.) Auger is a complex spectrum in which the originating electron beam causes energy changes in the atoms concerned, the final electron stream produced having an energy which is the difference of two energy changes occurring at the atom. On the other hand, the XPS is a much simpler spectroscopy in which an X-ray beam causes absorption and electron emission from electrons, the wavelength being clearly characteristic of each atom. By adjusting the angle of the X-ray beam, we could obtain results between 5/~, and 30/~, in the passive layer. The result was that, at the surface, and in conformity with the general idea that the surface is highly hydrated, the O : Fe ratio amounted to 2.6. As the depth was increased in the layer, we got several more readings over the distances 11-30/~, and the ratio O :Fe was between 2.03 and 1.96. From the earlier considerations, it is known that the ideal value for the hydrated oxide would be 2.00. The XPS tells us, therefore, that this hydrate (a polymer, amorphous) which is apparently characteristic of the passive film, exists from a position which we can call 'after the surface' well into the depth and up to the metal. The XPS measurements were continued by subjecting the film to chloride ions, and 1.51-1.52 was obtained for O : Fe in a broken-down (Cl--attacked) passive layer. Of course, this corresponds to the 1.50, which would correspond to the completely broken-down layer, the depassivated film; and it seems reasonable to associate this with 'the oxide' and perhaps Fe203. The XPS results can also be used to evaluate the ratio of water to iron, and this turned out to be 0.8 in the passive layer itself, which is not very different from the 1.0 * Dr V. Youngis to be thanked for her collaborationin these measurementsand for teachingthe author and his colleaguesa good deal about depth profilingand deconvolutionin XPS measurements.
Spectroscopic observations on the nature of passivity
299
+
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.
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FIG. 6. O : Fe ratio as a function of depth, A, into the passive film on iron from ISS depth profiling: (a) filmformed in borate buffer solution at 0.3 V(NHE); and (b) passive filmafter breakdown [0.3 V(NHE) in borate-buffer solution containing 0.5 M NaCI]. expected for the ideal hydrate structure, and the amount was reduced to 0.1 (near to the expected 0.0, the value in the dry film). A n o t h e r technique we were able to use to advantage was ion-scattering spectroscopy in which ions are bounced off the surface of the material, which is sputtered to get various depths. This sputtering, of course, is a less desirable feature of such analyses because one has to ask to what extent it destroys the immediate surface layer, but the damage caused by the sputtering probably does not penetrate more than 5/~, and we were able to obtain the remarkable figure shown in Fig. 6 illustrating that 2.1 and 1.6 are the two values obtained for the passive layer and for the chloride-exposed layer for most of the depth of the passive film. Again, one sees that the area at the surface gives a higher value of O : Fe corresponding to the results of other measurements. It is seen, therefore, as a summary from the XPS and ISS values that the depth-dependent value of the O : F e is consistent with the hydrate formula once more, and the chloride-exposed films are, in their O : Fe values, consistent with the concept that the depassivation is associated with removal of the water. SIMS The Secondary Ion Mass Spectroscopy is a technique which involves the reflection from the surface of ions which have interchanged their kinetic energy with the surface in the sense that they have 'knocked bits out of' the latter, and these then enter a mass spectroscope. Mass ratios are measured, but the m a j o r factor to note with SIMS is that the results cannot be interpreted in a direct and simple way because there is a quantity which is analogous to the ionization cross-section concept of Auger. When struck by bombarding ions, each particle from the surface does not detach itself and move into the mass spectroscope with the same velocity, so that the ratio of masses measured may not give a reliable indication of the mass ratio present at the surface, or at a given depth (as obtained by argon ion b o m b a r d m e n t ) .
300
J. O'M. BocKlUS
On this basis, therefore, it was decided to look at the ratio of O H : O, the concept being that these particles are very similar, so that the difference in their 'evaporability' would be minimal. It was found that the ratio of O H : O was finite and constant throughout the film, 8'1° except for a small section near the surface where the hydrogen is particularly high. Even the value of this ratio is not far off what one would expect for the simplest interpretation of SIMS. If the formula of the passive material were Fe203H20, the ratio would be (in the ideal case) about 0.3, and about 0.2 was observed. Just as in the other cases, the value observed is much lower in the presence of chloride, and becomes increasingly low as the exposure to chloride is extended. However, a much more remarkable result is obtained when one looks at the SIMS value for CI:O. Here, one finds that the ratio---and here we are quoting the values at 20/~ depth----of C I : O increases up to breakdown time, whereafter the ratio decreases again. This, then, suggests that the diffusion of C1- increases, and interaction with water occurs, but that, after breakdown, the chloride diffuses out again (speculatively, coupled with Fe3+). One is reminded of some of the results of Heusler and Fischer, n who found that, during film breakdown, ferrous ions were rejected into the solution. ELLIPSOMETRY The use of ellipsometry in investigations of passive layers is by no means new. Indeed, a great deal of work has been done on this phase by many investigators, with the earlier results stretching back to 1965.12-18 However, the type of ellipsometry which was carried out in this earlier phase must not be confused with that being done at the present. Earlier, the two ellipsometric parameters which were clearly determinable were A and ~0. The trouble with these earlier experiments was that they always needed an auxiliary variable to be measured by non-ellipsometric means. Thus, the information which one desires to obtain from the film is not only its thickness but also its refractive index and the quantity, k, the so-called extinction coefficient. It was Paik 19 who showed in 1972 that the problem coud be solved by making simultaneous measurements of the reflectivity of the parallel component of light. Thus, the present ellipsometric experiments have been carried out with the Fourier Transform Spectrometer working with a rotating polarizer. This allows resolution of the message down to about 20 ms, and it also allows us to carry out measurements of wavelength dependence of the quantities, although at the present time this is only extended to the visible region and cannot be carried out in the infrared because of the difficulty of the absorption of infrared radiation by the prisms of the ellipsometer. The objective of the ellipsometric work in this instance was to obtain the spectroscopic ellipsometry on passive layers. At least one extra element of the investigation (which was carried out by V. Jovancicevic2°) should be stated. Thus, in Fig. 7 the data associated with the ellipsometric examination at2 = 4500 are shown. In Fig. 8, one sees the plot of thickness against potential for various sweep rates. This examination has been carried out with the exclusion of oxygen from the solution, and it shows a most interesting feature, namely that the thickness increases with potential to a certain potential region. Then there is a plateau where, over a potential range of about 0.1 V, there is no further increase in thickness. Later,
Spectroscopic observations on the nature of passivity
301
4500A R
0.509 0.507
R 0.505
o.5o3
108
/k 104 100 30.(3 30.2
29.8 29.4
-o.8
-o4
-o2
olo
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Reflectivity (R) and ellipsometric parameters (A, 9) of the iron electrode as a function of potential at 4500 ~ .
the thickness grows, reaching finally a value of about 30--40/~ at high positive potentials. There are two interesting things to note about these results. The first is the plateau. The second is the dependence upon sweep rate. The interpretation of the plateau is that it represents the situation in which ferrous ions are undergoing conversion to ferric. There is evidence for this in the later spectroscopic information. It is noteworthy that such a plateau is not observed in the presence of oxygen, and this may be due to the adsorption of 02 ions on the ferrous ions, thus reducing the sharpness of potential at which the oxidation reaction occurs. In earlier publications, 15-18 it had been stressed that the proper way to obtain ellipsometric data on passive layers is always to go back to a fiducial position at a pure iron surface, unencumbered by any oxide, and then to transfer the surface to a certain potential at which the measurement will be made, followed by a reduction of the fiducial position and transfer to another potential, and so forth. Using a slow potential sweep seemed to have disadvantages in the fact that the oxide observed at one potential would be affected by the history of the oxide at another potential.
302
J. O'M. BocKms I
60
I
I
I
Borafe Buffer IBH 8.4]
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FIG. 8.
Thickness-potential relationship of iron for different sweep rates.
The results obtained, shown in Fig. 8, show that the effect is there, but it is slight, i.e. the apparent variation of the thickness with sweep time is a minor factor. Figures 9 and 10 show the values of k at various 2's. The spectra show two peaks. One is at 5250 ~ and the other at 6000 A. The 5250 ~ peak is likely to represent Fe 3+ and d-d transitions therein. There is evidence for such transitions in a-Fe203. Here, the other d-d transition is in Fe 3+. Such spectra have also been observed by Plieth 21 and co-workers. The maximum at 6000 ~ is more difficult to interpret. It corresponds to an energy of 2.05 eV. One would expect this to represent Fe a+ because, when one goes to much more positive potentials, the strength of the peak fades to zero. However, there is no clear evidence for a peak at 2.05 eV in other iron compounds, although there are weak peaks at 2.0 and 2.3 eV in Fe304. Much more disquieting for clear explanation is the big decrease in intensity which occurs as one goes from more negative to more positive potentials. This would not be expected at first. Speculatively, one might think of an increase of hydration with potential, and that increased hydration perhaps decreases the intensity of the lines corresponding to the transitions which have been cited. The most disquieting feature of these spectra is the apparent presence of Fe 3+ spectra at - 0 . 4 V where, from other evidence, one would expect only Fe a+ ions to be present. There is a good deal of non-stoichiometry and excess 0 2- ions so that there would be some Fe 3+ ions needed in the ferrous oxide lattice.'? One of the needs of ellipsometry is for IR measurements. We need these because of our hoped-for ability to see water in the film. Doing IR measurements in ellipsometry is difficult because of the need to get rid of the glass prisms or supplanting them with those made of zinc selenide, which are very expensive to
Spectroscopic observations on the nature of passivity
o71
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i
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0,5
0.4
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5000
6000
7000
Extinction coefficient spectra of iron or different potentials.
!
1
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3.2
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2.6
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Refractive index spectra of iron for different potentials.
303
304
J. O'M. BocI~.IS
make. Let us hope that the use of diffraction gratings may be a possible solution to the problem. This is the intent of the work in our laboratory at the present time. These ellipsometric measurements do not contribute greatly (in the phase reported here) to a knowledge of passive layers and their amorphous hydrated structure, but it is important to give this report at this stage because it shows that ellipsometric spectroscopy for passive layers is a reality, can be practiced now, and the extension to the IR range would then make the ellipsometer a very useful tool in the study of passive layers. RADIOTRACER MEASUREMENTS Although a report on these measurements is not consistent with the title of this report, not producing spectra, their results are relevant to the general picture of a passive layer which we are building up here, and therefore it would be beneficial to give a summary of them. They were carried out largely by T. Mizuno in 1984-1985, and partly by Jose Carbajal with the assistance of P. Zelenay. 22 Measurement of adsorption, and indeed absorption, by radiotracer techniques is not new. Indeed, the first in situ measurements were carried out by Blomgren and Bockris23in 1960. However, a new technique has been used here which originates in the work of Sadkowski et al. 24.25The new feature introduced by the Polish workers is the use of a glass composite into which has been injected an organic material which scintillates upon being activated by radioactive substances. Thus, in the apparatus shown in Fig. 11, the composite is held at the bottom of the cell, having had the desired metallic coating deposited upon it. It is particularly easy to evaporate such a coating on the surface of the membrane if it is iron, because of the relatively low melting point of iron compared with platinum. For platinum, however, the deposition takes several hours for a thickness of about 0.1/zm. Figure 12 shows a typical circuit associated with this apparatus. It can be seen that the scintillation which arises in the glass composite is taken by means of a light pipe to a nearby measuring apparatus, and the scintillations counted. From this, if the roughness of the metal deposit on the glass specimen is known, it is possible to calculate, exactly, the coverage. One of the difficulties of the measurement is the calculation of roughness. The result finally obtained is proportional to the roughness factor, the ratio of real to apparent surface area, and this measurement brings certain difficulties. Thus, the measurement of roughness factors on noble metals is easy because one can utilize hydrogen peaks. However, in the case of iron it is not possible to use such voltammogramic methods because of codissolution of the iron (although considerable efforts have been made to overcome this). In the present case, three methods have been used. (1) It was assumed that the normal peak in the I-V curve for passivation corresponds to a monolayer of FeO, and on this basis the roughness factor was calculated to be 6.5. (2) The second method is to put the potential into the highly passive region where the film has been built up, and then compare the coulombs to build-up after a certain time whilst the basic film has been formed upon the rough deposit made upon glass, and then compare the coulombs used there with the corresponding deposit made in a highly polished iron specimen, assuming this has a roughness factor of 1.15. From this, we obtain a roughness value of 7.6. Finally, we obtained a value for the decay of
Spectroscopic observations on the nature of passivity
305
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current from the potentiostatic transient, and this was found to be 10.7. We ended, therefore, with an average value of 8.5 + 2.5. This uncertainty is a difficulty of the radiotracer measurement. The determination of the roughness factor to the extent that we hve seen above is the principal (indeed, the only significant) error, although the method is restricted to concentration range of around 10-3-10 -1 M because at the lower concentration the signal is too weak, and at a higher concentration the background overwhelms the signal. In Fig. 13, the diagram of adsorption as a function of potential is given. There are two clear regions, one corresponding to the adsorption of chloride ion on iron, and the second, more positive, region corresponding to the adsorption of chloride ion on the passive layer. It is clear from the data that the dF/dV line has a lesser slope in the case of adsorption on the passive ion, and this hints at the possibility of chemi-bonding and reduction of the surface charge of the iron upon the oxide. Something similar was observed with the rate of reduction of oxygen on passive iron, where the rate is higher
306
J. O'M. BocK~s
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1.0
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PassiveLayer 5 ' 10-2 M CI"
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FIG. 13. Potential dependence of chloride adsorption on bare iron and passive layer as a function of concentration of chloride in the solution.
Spectroscopic observations on the nature of passivity
307
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Steady-state values of adsorption of chloride as a function of concentration at different potentials.
than that on iron itself (higher bonding of the oxygen to the passive layer, than to iron). In Fig. 14 the isotherm on the bare iron is given. The so-called monolayer coverage changes considerably with potential, becoming higher with more positive potential. One of the elements which came out of these examinations was a distinction between adsorption and absorption. It was found that, when the aggregation of the iron on the surface is a matter of adsorption, coverage is far less than 1. The value of rise time was low (a few seconds), but when the final value of the adsorption corresponded to a much higher value (the rise time now being about 60 s or more), absorption was clearly taking place. A series of results which show this are the F-time relationship shown in Fig. 15. These values are shown for a concentration of 10 -3 M chloride, and obviously indicate a value of F which corresponds to a situation where absorption is taking place, but no significant penetration or rise above the value corresponding to 0 = 1 can be seen. The most important result obtained in these investigations was that shown in Fig. 16, carried out at 5.10 -3 M of chloride. Here, a critical difference in the absorption-time pattern occurs, because breakdown sets in. The breakdown is monitored by the current-time relationships, and these undergo an inflection upwards at a certain moment. Corresponding to this, one sees that, at about the time of breakdown the 0 - t relation takes a turn upwards. However, this relation has to be looked at in terms of the actual value of the 0, which becomes greater than 1. It seems reasonable to interpret this, therefore, as indicating an absorption of chloride into the passive layer. The apparent 0 increases above the limit of 1, and indicates absorption into the film.
308
J. O'M. Bocraus
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Time dependence o f 'surface' concentration o f chloride for 10 -3 M C I - in the solution at different potentials.
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F[~. ]6. Total 'surface' concentration o f chloride for 5 x 10 -3 M e l - in the solution at different potentials as a function of time. F - t and 1-t curves for 0.80 V ( N H E ) (insert).
Spectroscopicobservationson the nature of passivity
309
Does this critical increase in absorption occur before, at, or after breakdown? It seems--but the distinction is not a clear one--that the inflection on absorption upwards occurs before the breakdown. It is tempting, therefore, to regard the breakdown as occurring as a consequence of this absorption. CONCLUSIONS (1) The M6ssbauer results indicate that the passive film differs from the corresponding oxide by having an amorphous character. An experiment by Cahan and co-workers has verified this .26 They have shown by an electron microscopic technique that they can turn on the amorphous character by introducing water vapor, whereupon the patterns become disordered, i.e. amorphousness is indicated. (2) The passivity of the material (one is tempted to say its amorphousness) is destroyed by heating the material mildly (i.e. drying it, removing the water), or bringing it into contact with chloride ions. It seems reasonable, therefore, to suggest that these chloride ions remove the water, and destroy the passivity by removing its amorphousness. (3) The O : F e ratio is a monitor of the character of the layer. If passivity is present, then five different techniques indicate that the ratio is high (and near to 2.0) when the film is passive, but lower (near to 1.5) when the film has been made unpassive by contacting it with chloride ion or heating it. The evidence is, therefore, very clear that a change occurs in the film on these processes, and that this change is associated with a reduction of O : F e from a value which is consistent with the idea that passivity is associated with Fe20 3 • H20, and the 'oxide character'--non-passive--is associated with a structure that is some form of Fe203 (or, sometimes, Fe304). These results, and the effect found by heating the layer, are reminiscent of much else that has been published on passive layers, particularly from the electron diffractional point of view.27 Thus, if the film is immediately converted from a passive film to a normal oxide film by heat, it is reasonable to suspect that the electron beam used in the L E E D work would result in this conversion. These L E E D results would confirm the oxide, Fe~O3, and only results which were obtained in situ (or ex situ with careful control of the temperature, keeping it low so that the vacuum does not remove the water) will actually lead to results relevant to passive layer considerations. (4) There are several evidences (all with spectroscopy, together with the radiotracer work) which suggest that chloride ion penetrates the oxide, becomes absorbed therein, and in some way removes the water. The removal mechanism for water can only be speculatively identified as a place exchange mechanism in which the water is replaced by chloride, which then accelerates the dissolution of iron by its contact with the iron base at the film and now, with the crystalline film (the amorphousness having been destroyed), removes iron out through the film to the solution at a greater rate than was possible with the properly passive layer. Finally, it may be said that some of these ideas have been suggested in parallel by other workers. Thus, T. P. Hoar 2v3° pointed out that passive films sometimes have an amorphous character. Sato 31 and co-workers carried out tritium experiments in which they claim to have proved that two water molecules are associated with iron in the passive layer. (However, re-evaluation of their results by Revie et al. 1 suggests that the value actually associated with the experiments is one water molecule per iron atom.) More recently, Okamoto and Hashimoto 32'33 both suggested that water is an essential part of passivity.
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Thus, to verify the essential conclusion of these studies, that amorphousness, caused by the introduction of water, is the essential part of passivity, the measurements should be repeated by others under the proper conditions. It must be again stressed that these are continued wetness, i.e. in situ investigations, or vacuum situations in which the proper conditions are taken, to be quite sure that the water is not removed by drying or by electron bombardment. GENERAL DISCUSSION A comparison of the mechanism for the passivation of iron which has appeared under the name of the present author with the work of others was published by Pou etal. in 1984. 7 O'Grady and Bockris were the first4 to introduce the concept that the amorphous character of passive layers was caused by the presence of water, and this relation between the presence and absence of coordinated water is the most important feature in the forming of passive layer, as far as iron is concerned. By comparison with some of the well-known suggestions in the recent past, one finds that S a t o 34 c o m e s to the conclusion that there are two layers which make up the passive film, one being a barrier layer next to the metal consisting of ferric ions, and the other being a ferric oxide layer with a small amount of borate over it. Such conclusions came largely from ellipsometry and coulombic measurements.34 This model would not be consistent with the line spectroscopic work of Pou et al., 7,8 who made several probes of the constitution of the passive layer as a function of depth and found that, although there was some heterogeneity within 5 ,~ of the surface, the composition then remained constant back to the metal. Ord made several ellipsometric studies 35'36and concluded that the inner layer was Fe304, and on the outer layer, Fe203. This work would be subject to the same statement of inconsistency with the results of Pou and the other evidence summarized here as that of Sato. A good deal of M6ssbauer work has subsequently been published by Hoffmann and his colleagues, and now Scherson. 37,38These workers do not always come out with explicit statement of structural conclusions, but their results are qualitatively similar to those of O'Grady, though they differ numerically in some respects. There is no statement of disagreement of their results with the O'Grady-based model. EXAFS is obviously a very important technique to apply to passive layers, and this has been done recently by Kruger and his co-workers in several rather detailed studies. 39-41 The essential difference between the approach of Kruger and that of O'Grady is the measurement of the internuclear distance, and it is here that the EXAFS results shine because they show quite clearly that the introduction of water into the structure does cause changes which make the film more vitreous. This amorphousness (which had been suggested earlier by Revesz and Kruger) 42 is then similar to that which was suggested by the interpretation of the O'Grady-M6ssbauer work: Very detailed work by Lumsden and his colleagues using numerous ex situ methods to examine the film, have come to the conclusion that the film is always affected in its structure by the presence of water, and that if heat removes this water, it again becomes ordered. 43,44 Although it is not possible as yet to say there is a consensus about the mechanism of iron passivity and its amorphousness caused by water (this is what O'Grady
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suggested), there are many lines of reasoning, in spectroscopic work, which seem to go in the same direction. Indeed, when the spectroscopic methods are applied, something to do with a vitreous nature, connection to water, and changes upon heating, always seems to come out of the results. Acknowledgements--These works have been supported over the years by a number of agencies, but particular thanks are to be given to the Office of Naval Research for the support of work under Contract N0001480Co113, and for discussion with Frank Herr.
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