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An X-ray photo-emission spectroscopy study of iron-oxide, iron-benzoic acid inhibitor films
Corrosion
Science, 1978, Vol. 18, pp. 315 to 321. Pergamon Press. Printed in Great Britain
AN X-RAY PHOTO-EMISSION SPECTROSCOPY STUDY OF IRON-OXIDE, IRON-BENZOIC ACID INHIBITOR FILMS* J. C. WOOD and N.-G. VANNERBERG Department of Inorganic Chemistry, Chalmers University of Technology, Gothenburg, Sweden Abstract--Surface oxide and benzoic acid films on pure iron have been examined by X-ray photoemission spectroscopy (XPS). The passivation and inhibitor properties of these films is correlated with chemical features arising in the spectra. Surface hydroxyl groups are observed on surfaces having good passivation properties. Benzoic acid forms an oriented salt film. INTRODUCTION
IN AN attempt to understand the passivating and inhibiting action of certain films, samples of pure iron have been exposed to oxidizing environments, and to solutions of benzoic acid. The resulting films have been examined by X-ray photo-emission spectroscopy (XPS). The present work reports principally on the varying nature of this oxide film before giving some preliminary results on its interaction with a known inhibitor, benzoic acid. Considerable work has been done on the nature of the film formed on oxidation of steel surfaces as determined by electron spectroscopic methods. The preferential reaction of one of the constituents of the steel, for example chromium, leading to concentration gradients at the steel/oxide interface and in the oxide film have been related to the known beneficial corrosion properties, l'a Similarly the nature of the metal surface and oxide film formed on pure iron at low coverages has been extensively investigated.a-6 Although there is agreement as to the chemical composition of the film formed under high temperature conditions,7 there is still no consensus as to the nature of the film formed on room temperature oxidation or in aqueous conditions? Thermodynamic considerations imply that the Fe s+ state is the stable one both under normal oxygen pressures and under UHV conditions, and that the film probably comprises at least two phases. In addition because the XPS (ESCA) lines corresponding to the Fe2p electrons in the iron 1"[and III oxidation states are relatively broad, it is difficult to get the fullest benefit from the ability of XPS to measure chemical shifts in the case of the iron oxide film. Attempts have been made to resolve this problem by curve subtraction procedures? Standard peak positions for the principal components expected to be present, Fe, FeaO4, Fe2Os, have previously been reported. 1° EXPERIMENTAL METHOD All XPS analyses reported here were made using a Hewlett Packard (HP 5950A) ESCA instrument, to which a separate UHV specimen handling system had been added, u Spectra were recorded using monochromatized AIKct (1486.6 eV) radiation. Electrostatic charging of the sample was investigated and found to be negligible. On practical iron surfaces where inhibition protection is required an oxide film will always preexist. Therefore although experiments were carried out in a UHV environment in the ESCA spectre*Manuscript received 20 April 1977; received for publication 28 July 1977. 315
316
J.C. WOODand N,-G. VANNERBERG
meter, no attempt was made to investigate the properties of a clean oxide-free metal surface. The only precautions taken in preparing the samples were those of careful handling to prevent unknown contamination. Polycrystalline samples of pure iron (C~ 40 ppm, 0 < 10 ppm, other < 5 ppm) were cut. The surface was abraded to 600 SiC grit, polished on silk, and finally ultrasonically cleaned in ethyl alcohol (99.5 %). Surface films were formed either by exposure to a dry, dust-free atmosphere, immersion in doubly distilled water, or by immersion in NaNO~ and NH~NO8solutions (A.R. Grade). Samples were treated for periods ranging from a few minutes to approx. I00 h. Individual samples on which a surface film had been preformed were further immersed in solutions of benzoic acid. Unless specified, concentrations were 10 g/l for all solutions, except for benzoic acid which formed a saturated solution in water (3.45 g/I, 20°C). Preliminary qualitative experiments were made to investigate the passivation properties of the various oxide films in order to relate them to the chemical structure of the film. Samples were placed in thermostated chambers and exposed to filtered air 22°C, 90~o r.h., containing 1 ppm SO~. The flow rate was 1 ml/s. EXPERIMENTAL RESULTS AND DISCUSSION Samples were polished to a mirror finish before formation of the surface films. After exposure to air, water or sodium nitrite solution, for periods up to 50 h the mirror finish of the surface was unaltered, implying that the oxide film was thin and that its rate of formation was slow. For such cases, the kinetics of oxide film formation have been quantified using XPS peak intensities. 2 In the case of a m m o n i u m nitrate, the surface quickly dulled (4 h) and as the reaction proceeded a black, poorly adhered film was formed. X-ray diffraction of the film formed after 100 h exposure showed magnetite to be the only component. Removal of this layer showed the underlying surface to be etched. Optical microscopic examination revealed no evidence of faceting. This suggests that this oxidation method is not sensitive to crystailite orientation, as has been observed in high temperature oxidations, 7 but tends to confirm the opinion of Tikkanen et al. that passivation is achieved by promoting the dissolution of carbon 1~ although in this case it is not thought to be in the form of carbide. XPS spectra of the Fe2p and O 1s peaks for the four samples recorded under conditions where the film thickness is 4--5 monolayers (as evidenced by the relative magnitude of the metallic Fe2p component in the XPS spectrum) are shown in Fig. 1. In separate scans at higher sensitivity it was confirmed that none of the oxide films, once formed, contained nitrogen in any form, demonstrating that for the nitrate and nitrite ions, the reaction was one solely of oxidation with evolution of gaseous nitrogen-containing products rather than surface nitration. The form of the O l s spectra in Fig. 1 shows that the nature of the film produced under solution conditions is sensitive to the method of production. This contrasts with high temperature oxidation where, as stated above, it has been suggested that the oxidation products are particularly sensitive to the various crystallite faces present at the polycrystalline metal surfaces. 7 For the formation of a film free from flaws, at a molecular level, some idea of the chemical order of the film can be gained from the spectral resolution apparent in the various chemical environments of oxygen (Fig. 1). For constant instrumental parameters, and as comparison is being made at similar film thicknesses, the varying resolution must principally reflect the perfection of the oxide film. Thus, in the presence of nitrite ions, the film is chemically more specific, with evidence of order after only four or five adlayers. In oxygenated water, the ordering takes longer to become apparent, both in time and film thickness. This suggests some stereochemical specificity
An X-ray photo-emission spectroscopy study of iron-oxide
0 Is
e 2p
317
M
7.- /a/
•
I
I
711
706
L
1
533
528
Binding energy, eV
FIG. 1. O1s XPS peaks from oxide films at comparable film thickness (a) water, (b) air, (c) NaNO2 soln., (d) NH4NOs soln. in the case of the nitrite ion. In the presence of NH4NOa, similar chemical order is not apparent. However, some confirmation for the earlier suggestion that NH4NO8 promotes dissolution of carbide precipitates can be seen in the corresponding Cls peaks in Fig. 2. In this case,
Cls
F L IK'¢/~
b
1
J
285
28~ Binding energy
FIG. 2.
Cls XPS peaks from oxide films prepared (a) ion-bombarded metal, (b) water, (c) NAN02 soln., (d) NH~NOs soln.
318
J.C. WOODand N.-G.
VANNERBERO
one of the three peaks apparent in the spectra, corresponding to that observed from the pure metal and all other oxide films, is considerably reduced. This dissolution of carbon is not observed in oxide films produced by the other methods, including NO2-. It was simultaneously observed that samples exposed to NH4NOa for > 24 h, where surface etching had occurred beneath a growing film of magnetite, showed better inhibition properties when exposed to an SOs environment than did the other oxide films. The corrosion inhibiting nature of a surface film will result from a cumulative effect of such properties as the reactivity or solubility of the film towards its environment, its thickness and the freedom of the film from flaws. A measure of the relative thickness of films can be gained from the contributions of metallic and oxide Fe compounds to the XPS spectrum. For identical instrumental conditions, and because it is expected that similar chemical species are being compared in all oxide films, this comparison can be made on the basis of relative peak intensities. The rate of formation of the oxide films is shown in Fig. 3. The ordinate is plotted
'.oI
o
4-
Time
50 ,
h
Fro. 3. The rate of oxide film growth under varying conditions (a) NH~NOs soln., (b) NaNO2, soln., (c) water, (d) air. as the ratio FemeJFemct+Feox, determined from the integrated intensities of the relevant Fe2p peaks, and is thus related to the film thickness. For the first three environments, the rate of oxide formation decreases with time of exposure. In ammonium nitrate, the reaction continues at a rate fast enough to confirm the formation of pure magnetite by X-ray diffraction after 36 h immersion, and demonstrates that NH4NO8 is corrosive towards iron. However, on removing the sample from the
An X-ray photo-emission spectroscopy study of iron-oxide
319
solution, the remaining nitrate and ammonium ions are rapidly consumed, leaving pure oxide film. The rate of film growth can be seen to increase in the environmental order air < H20(O2) < NO2- < NOs- reflecting the expected relative oxidizing ability. The asymptotic behaviour of the curves for the first two environments, dry air and H20, while in part arising from the surface sensitive nature of the XPS experiment, shows that the rate of film growth slows to a very small value, while such behaviour is not apparent for the films produced in the nitrate and nitrite solutions. The magnetite film produced in NH4NOa solution was readily removed from the surface to reveal the etched metal surface reported above. On the basis of depth profile studies, it is generally assumed that the two Ols signals commonly observed, correspond to F e - O - F e and Fe-O-H, the latter being at higher binding energy. Ageing of the films characterized in Fig. l, in humid conditions for 3 days produces an increase in film thickness and a broadening of the Ols peaks such that at least three distinct components are apparent (Fig. 4). Exposure tests show samples with the largest high binding energy content to have the best passivation characteristics. This is similar to the behaviour observed for stainless steels. 2 In exposure tests, the etched surface obtained after removal of the film produced
SK c/s
I
I 528
533
Binding energy
FIG. 4. The effect of ageing on NH4NOa prepared films. (a) Thin film sample as in Fig. l(d). (b) Aged thin film sample after 3 days. (c) Etched surface after removal of magnetite film. (d) Aged etched surface after 3 days.
320
J.C. Wood and N.-G. VANNERBERG
in NH4NO8 solution showed the best passivation properties. However, the increased - O H content of this surface is also accompanied by reduction in carbon content (Fig. 3), and it requires further study to determine whether the passivity arises (a) because of reduced carbon content, (b) because of increased -OH film content, or (c) because of (b) having been promoted by (a). Figure 5 shows spectra from iron/benzoic acid combinations in the Cls and Fe2p regions. The spectrum of Cls from benzoic acid in the solid state is shown in Fig. 5(a). The splitting between the extreme peaks is 3.8 eV and corresponds well with the expected spectrum for benzene ring and earboxyl carbon atoms. 13 Figure 5(b) shows the corresponding spectrum when a surface film is produced from benzoic acid/ethanol solution. The close correspondence between Figs. 5(a) and 5(b) shows that no salt formation has occurred. Examination of Fe2p peak intensities shows that a thick film ( > 2.0 rim) has been formed on top of the existing oxide. By contrast, the spectrum resulting from film formation in aqueous solution shows salt formation to have occurred. The splitting Cls (carbonyl)--Cls (aromatic) is reduced to 3.0 eV. The change in Cls line shape, in particular the broadening, arises because of interactions of the aromatic nucleus, the carbonyl peak being unchanged. The much thinner film, evidenced by the relative Fe2p metal/oxide peak heights, leads to the expectation that salt formation has occurred predominantly with the metal surface. This would mean that the hydrophobic aromatic nucleus becomes the external surface of the solid phase. Acknowledgements--The authors thank R. Pompe for X-ray diffractiondata and U. Elvestr6m for technical assistance with the XPS (ESCA) analyses.A grant from the Swedish Board for Technical Development (STU) is gratefullyacknowledged.
Fe 2p
CIs
J/
/
711
706
288
284
Binding energy
FIG. 5. Benzoic acid films(a) solid Call5 COOH, (b) benzoicacid film from alcohol solution, (c) benzoicacid filmfrom aqueous solution.
An X-ray photo-emission spectroscopy study of iron-oxide
321
REFERENCES I. J. P. GOAD and J. G. CUNNINGHAM, J. electron Spectrosc. 3, 435 (1974). 2. I. OLEFJORD, Scand. J. Metall. 3, 129 (1974); I. OtEFJORD and N.-G. VANNERBERG, Proc. 7th Scand. Corrosion Congress, Trondhei m, p. 434 (1975); |. OLEFJO~D, Corros. Sci. 15, 687, 697 (1975). 3. M. SULEMAN and E. B. PA'r'rINSON, Surface Sci. 35, 75 (1973). 4. M. SEO, J. B. LUMSOEN and R. W. STAEHLE, Surface Sci. 50, 541 (197.5). 5. K. VEDA and R. SHIMXZU,Surface Sci. 43, 77 (1974). 6. G. ERTL and K. WANDELT, Surface Sci. 50, 479 (1975). 7. W. E. BOGGS, R. H. KACHIK and G. E. PELLISSlER, J. electrochem Soc. 116, 424 (1969). 8. R. W. REVlE, J. O'M. BOCKRIS and B. G. BAKER, Surface Sci. 52, 664 (1975). 9. K. ASAMI, K. HASmMOTO and S. SHIMODAIRA, Corros. Sci. 16, 35 (1976). 10. J. E. CASTLE and M. J. DURBIN, Carbon 13, 23 (1975). 11. I. OLEF~ORD, Metal Sci. 9, 263 (197.5). 12. M. H. TIKKANeN and P. PURANEN, Proc. 7th Scand. Corrosion Congress, Trondheim, p. 245 (1975). 13. K. SEIGnAHN, C. NORDUNQ, A. FAHLMAN, R. NORDnERG, K. HAMRIN, T. BERGMARK:, S.-E. KARLSSON, I. LINDGREN and B. LINDaERG, ESCA. Atom, Molecular and Solid State Structure by means o f Electron Spectroscopy. AImquist & Wiksell, Uppsala (1967).
Science, 1978, Vol. 18, pp. 315 to 321. Pergamon Press. Printed in Great Britain
AN X-RAY PHOTO-EMISSION SPECTROSCOPY STUDY OF IRON-OXIDE, IRON-BENZOIC ACID INHIBITOR FILMS* J. C. WOOD and N.-G. VANNERBERG Department of Inorganic Chemistry, Chalmers University of Technology, Gothenburg, Sweden Abstract--Surface oxide and benzoic acid films on pure iron have been examined by X-ray photoemission spectroscopy (XPS). The passivation and inhibitor properties of these films is correlated with chemical features arising in the spectra. Surface hydroxyl groups are observed on surfaces having good passivation properties. Benzoic acid forms an oriented salt film. INTRODUCTION
IN AN attempt to understand the passivating and inhibiting action of certain films, samples of pure iron have been exposed to oxidizing environments, and to solutions of benzoic acid. The resulting films have been examined by X-ray photo-emission spectroscopy (XPS). The present work reports principally on the varying nature of this oxide film before giving some preliminary results on its interaction with a known inhibitor, benzoic acid. Considerable work has been done on the nature of the film formed on oxidation of steel surfaces as determined by electron spectroscopic methods. The preferential reaction of one of the constituents of the steel, for example chromium, leading to concentration gradients at the steel/oxide interface and in the oxide film have been related to the known beneficial corrosion properties, l'a Similarly the nature of the metal surface and oxide film formed on pure iron at low coverages has been extensively investigated.a-6 Although there is agreement as to the chemical composition of the film formed under high temperature conditions,7 there is still no consensus as to the nature of the film formed on room temperature oxidation or in aqueous conditions? Thermodynamic considerations imply that the Fe s+ state is the stable one both under normal oxygen pressures and under UHV conditions, and that the film probably comprises at least two phases. In addition because the XPS (ESCA) lines corresponding to the Fe2p electrons in the iron 1"[and III oxidation states are relatively broad, it is difficult to get the fullest benefit from the ability of XPS to measure chemical shifts in the case of the iron oxide film. Attempts have been made to resolve this problem by curve subtraction procedures? Standard peak positions for the principal components expected to be present, Fe, FeaO4, Fe2Os, have previously been reported. 1° EXPERIMENTAL METHOD All XPS analyses reported here were made using a Hewlett Packard (HP 5950A) ESCA instrument, to which a separate UHV specimen handling system had been added, u Spectra were recorded using monochromatized AIKct (1486.6 eV) radiation. Electrostatic charging of the sample was investigated and found to be negligible. On practical iron surfaces where inhibition protection is required an oxide film will always preexist. Therefore although experiments were carried out in a UHV environment in the ESCA spectre*Manuscript received 20 April 1977; received for publication 28 July 1977. 315
316
J.C. WOODand N,-G. VANNERBERG
meter, no attempt was made to investigate the properties of a clean oxide-free metal surface. The only precautions taken in preparing the samples were those of careful handling to prevent unknown contamination. Polycrystalline samples of pure iron (C~ 40 ppm, 0 < 10 ppm, other < 5 ppm) were cut. The surface was abraded to 600 SiC grit, polished on silk, and finally ultrasonically cleaned in ethyl alcohol (99.5 %). Surface films were formed either by exposure to a dry, dust-free atmosphere, immersion in doubly distilled water, or by immersion in NaNO~ and NH~NO8solutions (A.R. Grade). Samples were treated for periods ranging from a few minutes to approx. I00 h. Individual samples on which a surface film had been preformed were further immersed in solutions of benzoic acid. Unless specified, concentrations were 10 g/l for all solutions, except for benzoic acid which formed a saturated solution in water (3.45 g/I, 20°C). Preliminary qualitative experiments were made to investigate the passivation properties of the various oxide films in order to relate them to the chemical structure of the film. Samples were placed in thermostated chambers and exposed to filtered air 22°C, 90~o r.h., containing 1 ppm SO~. The flow rate was 1 ml/s. EXPERIMENTAL RESULTS AND DISCUSSION Samples were polished to a mirror finish before formation of the surface films. After exposure to air, water or sodium nitrite solution, for periods up to 50 h the mirror finish of the surface was unaltered, implying that the oxide film was thin and that its rate of formation was slow. For such cases, the kinetics of oxide film formation have been quantified using XPS peak intensities. 2 In the case of a m m o n i u m nitrate, the surface quickly dulled (4 h) and as the reaction proceeded a black, poorly adhered film was formed. X-ray diffraction of the film formed after 100 h exposure showed magnetite to be the only component. Removal of this layer showed the underlying surface to be etched. Optical microscopic examination revealed no evidence of faceting. This suggests that this oxidation method is not sensitive to crystailite orientation, as has been observed in high temperature oxidations, 7 but tends to confirm the opinion of Tikkanen et al. that passivation is achieved by promoting the dissolution of carbon 1~ although in this case it is not thought to be in the form of carbide. XPS spectra of the Fe2p and O 1s peaks for the four samples recorded under conditions where the film thickness is 4--5 monolayers (as evidenced by the relative magnitude of the metallic Fe2p component in the XPS spectrum) are shown in Fig. 1. In separate scans at higher sensitivity it was confirmed that none of the oxide films, once formed, contained nitrogen in any form, demonstrating that for the nitrate and nitrite ions, the reaction was one solely of oxidation with evolution of gaseous nitrogen-containing products rather than surface nitration. The form of the O l s spectra in Fig. 1 shows that the nature of the film produced under solution conditions is sensitive to the method of production. This contrasts with high temperature oxidation where, as stated above, it has been suggested that the oxidation products are particularly sensitive to the various crystallite faces present at the polycrystalline metal surfaces. 7 For the formation of a film free from flaws, at a molecular level, some idea of the chemical order of the film can be gained from the spectral resolution apparent in the various chemical environments of oxygen (Fig. 1). For constant instrumental parameters, and as comparison is being made at similar film thicknesses, the varying resolution must principally reflect the perfection of the oxide film. Thus, in the presence of nitrite ions, the film is chemically more specific, with evidence of order after only four or five adlayers. In oxygenated water, the ordering takes longer to become apparent, both in time and film thickness. This suggests some stereochemical specificity
An X-ray photo-emission spectroscopy study of iron-oxide
0 Is
e 2p
317
M
7.- /a/
•
I
I
711
706
L
1
533
528
Binding energy, eV
FIG. 1. O1s XPS peaks from oxide films at comparable film thickness (a) water, (b) air, (c) NaNO2 soln., (d) NH4NOs soln. in the case of the nitrite ion. In the presence of NH4NOa, similar chemical order is not apparent. However, some confirmation for the earlier suggestion that NH4NO8 promotes dissolution of carbide precipitates can be seen in the corresponding Cls peaks in Fig. 2. In this case,
Cls
F L IK'¢/~
b
1
J
285
28~ Binding energy
FIG. 2.
Cls XPS peaks from oxide films prepared (a) ion-bombarded metal, (b) water, (c) NAN02 soln., (d) NH~NOs soln.
318
J.C. WOODand N.-G.
VANNERBERO
one of the three peaks apparent in the spectra, corresponding to that observed from the pure metal and all other oxide films, is considerably reduced. This dissolution of carbon is not observed in oxide films produced by the other methods, including NO2-. It was simultaneously observed that samples exposed to NH4NOa for > 24 h, where surface etching had occurred beneath a growing film of magnetite, showed better inhibition properties when exposed to an SOs environment than did the other oxide films. The corrosion inhibiting nature of a surface film will result from a cumulative effect of such properties as the reactivity or solubility of the film towards its environment, its thickness and the freedom of the film from flaws. A measure of the relative thickness of films can be gained from the contributions of metallic and oxide Fe compounds to the XPS spectrum. For identical instrumental conditions, and because it is expected that similar chemical species are being compared in all oxide films, this comparison can be made on the basis of relative peak intensities. The rate of formation of the oxide films is shown in Fig. 3. The ordinate is plotted
'.oI
o
4-
Time
50 ,
h
Fro. 3. The rate of oxide film growth under varying conditions (a) NH~NOs soln., (b) NaNO2, soln., (c) water, (d) air. as the ratio FemeJFemct+Feox, determined from the integrated intensities of the relevant Fe2p peaks, and is thus related to the film thickness. For the first three environments, the rate of oxide formation decreases with time of exposure. In ammonium nitrate, the reaction continues at a rate fast enough to confirm the formation of pure magnetite by X-ray diffraction after 36 h immersion, and demonstrates that NH4NO8 is corrosive towards iron. However, on removing the sample from the
An X-ray photo-emission spectroscopy study of iron-oxide
319
solution, the remaining nitrate and ammonium ions are rapidly consumed, leaving pure oxide film. The rate of film growth can be seen to increase in the environmental order air < H20(O2) < NO2- < NOs- reflecting the expected relative oxidizing ability. The asymptotic behaviour of the curves for the first two environments, dry air and H20, while in part arising from the surface sensitive nature of the XPS experiment, shows that the rate of film growth slows to a very small value, while such behaviour is not apparent for the films produced in the nitrate and nitrite solutions. The magnetite film produced in NH4NOa solution was readily removed from the surface to reveal the etched metal surface reported above. On the basis of depth profile studies, it is generally assumed that the two Ols signals commonly observed, correspond to F e - O - F e and Fe-O-H, the latter being at higher binding energy. Ageing of the films characterized in Fig. l, in humid conditions for 3 days produces an increase in film thickness and a broadening of the Ols peaks such that at least three distinct components are apparent (Fig. 4). Exposure tests show samples with the largest high binding energy content to have the best passivation characteristics. This is similar to the behaviour observed for stainless steels. 2 In exposure tests, the etched surface obtained after removal of the film produced
SK c/s
I
I 528
533
Binding energy
FIG. 4. The effect of ageing on NH4NOa prepared films. (a) Thin film sample as in Fig. l(d). (b) Aged thin film sample after 3 days. (c) Etched surface after removal of magnetite film. (d) Aged etched surface after 3 days.
320
J.C. Wood and N.-G. VANNERBERG
in NH4NO8 solution showed the best passivation properties. However, the increased - O H content of this surface is also accompanied by reduction in carbon content (Fig. 3), and it requires further study to determine whether the passivity arises (a) because of reduced carbon content, (b) because of increased -OH film content, or (c) because of (b) having been promoted by (a). Figure 5 shows spectra from iron/benzoic acid combinations in the Cls and Fe2p regions. The spectrum of Cls from benzoic acid in the solid state is shown in Fig. 5(a). The splitting between the extreme peaks is 3.8 eV and corresponds well with the expected spectrum for benzene ring and earboxyl carbon atoms. 13 Figure 5(b) shows the corresponding spectrum when a surface film is produced from benzoic acid/ethanol solution. The close correspondence between Figs. 5(a) and 5(b) shows that no salt formation has occurred. Examination of Fe2p peak intensities shows that a thick film ( > 2.0 rim) has been formed on top of the existing oxide. By contrast, the spectrum resulting from film formation in aqueous solution shows salt formation to have occurred. The splitting Cls (carbonyl)--Cls (aromatic) is reduced to 3.0 eV. The change in Cls line shape, in particular the broadening, arises because of interactions of the aromatic nucleus, the carbonyl peak being unchanged. The much thinner film, evidenced by the relative Fe2p metal/oxide peak heights, leads to the expectation that salt formation has occurred predominantly with the metal surface. This would mean that the hydrophobic aromatic nucleus becomes the external surface of the solid phase. Acknowledgements--The authors thank R. Pompe for X-ray diffractiondata and U. Elvestr6m for technical assistance with the XPS (ESCA) analyses.A grant from the Swedish Board for Technical Development (STU) is gratefullyacknowledged.
Fe 2p
CIs
J/
/
711
706
288
284
Binding energy
FIG. 5. Benzoic acid films(a) solid Call5 COOH, (b) benzoicacid film from alcohol solution, (c) benzoicacid filmfrom aqueous solution.
An X-ray photo-emission spectroscopy study of iron-oxide
321
REFERENCES I. J. P. GOAD and J. G. CUNNINGHAM, J. electron Spectrosc. 3, 435 (1974). 2. I. OLEFJORD, Scand. J. Metall. 3, 129 (1974); I. OtEFJORD and N.-G. VANNERBERG, Proc. 7th Scand. Corrosion Congress, Trondhei m, p. 434 (1975); |. OLEFJO~D, Corros. Sci. 15, 687, 697 (1975). 3. M. SULEMAN and E. B. PA'r'rINSON, Surface Sci. 35, 75 (1973). 4. M. SEO, J. B. LUMSOEN and R. W. STAEHLE, Surface Sci. 50, 541 (197.5). 5. K. VEDA and R. SHIMXZU,Surface Sci. 43, 77 (1974). 6. G. ERTL and K. WANDELT, Surface Sci. 50, 479 (1975). 7. W. E. BOGGS, R. H. KACHIK and G. E. PELLISSlER, J. electrochem Soc. 116, 424 (1969). 8. R. W. REVlE, J. O'M. BOCKRIS and B. G. BAKER, Surface Sci. 52, 664 (1975). 9. K. ASAMI, K. HASmMOTO and S. SHIMODAIRA, Corros. Sci. 16, 35 (1976). 10. J. E. CASTLE and M. J. DURBIN, Carbon 13, 23 (1975). 11. I. OLEF~ORD, Metal Sci. 9, 263 (197.5). 12. M. H. TIKKANeN and P. PURANEN, Proc. 7th Scand. Corrosion Congress, Trondheim, p. 245 (1975). 13. K. SEIGnAHN, C. NORDUNQ, A. FAHLMAN, R. NORDnERG, K. HAMRIN, T. BERGMARK:, S.-E. KARLSSON, I. LINDGREN and B. LINDaERG, ESCA. Atom, Molecular and Solid State Structure by means o f Electron Spectroscopy. AImquist & Wiksell, Uppsala (1967).