J. Phys. Chem. B 2001, 105, 3957-3964
3957
Study of the Corrosion Behavior of Electroplated Iron-Zinc Alloys Using X-ray Photoelectron Spectroscopy† Holly G. Hixson and Peter M. A. Sherwood* Department of Chemistry, 111 Willard Hall, Kansas State UniVersity, Manhattan, Kansas 66506-3701 ReceiVed: September 19, 2000; In Final Form: NoVember 15, 2000
The corrosive behavior of iron-zinc alloys that have been electroplated on mild steel in both aerated and deaerated quadruply distilled water has been studied using core and valence band X-ray photoelectron spectroscopy. Three alloys were electroplated for comparative purposes with a 17%, 41%, and 83% iron composition. Three alloys of each type were either left untreated or exposed to either detreated or aerated water. No oxidation above that for the untreated samples was found for the samples immersed in deaerated water. As expected, the 83% iron alloy was oxidized in aerated water. However, the 17% iron alloy showed decreased oxidation in aerated water with respect to the untreated alloy. The 41% iron alloy in deaerated water showed no increased oxidation above that of the untreated sample, but it did show increased oxidation in aerated water. The valence band region proved valuable for diagnostic purposes, and had features that could be understood by comparing the experimental data with spectra calculated from band structure calculations. The 17% iron alloy forms a protective zinc oxide surface, while the 41% iron alloy forms a much less protective iron-zinc spinel (Fe2ZnO4). The iron-rich alloy forms an unprotective FeOOH surface layer.
Introduction Iron-zinc alloy electrodeposition has been attempted successfully in the past and has been studied by various techniques.1-4 Zinc galvanization has been used for many years to provide a sacrificial anode for steel used in applications such as the protection of car bodies. Bearing in mind the widespread use of galvanization, and especially the economic impact of the automobile industry, it is clearly important to try to develop more effective methods of corrosion protection. One such method may be the production of an alloy that will protect against the corrosive effects of the environment (such as the effects of deicing salts and moist air) even when it becomes dented or scratched. Iron-zinc alloys have shown promise in corrosion protection in previous studies,1-4 and a study of these alloys is undertaken here to promote a better understanding of the corrosion-resistant properties afforded by the alloys. In this paper we report a study of iron-zinc alloys of various compositions that were prepared by an electrochemical method to provide representative examples for the investigation of the potential of these alloys for corrosion-resistant applications. We have chosen three compositions, one that is zinc-rich, one that is approximately of equal iron and zinc composition, and one that is iron-rich. We have done this to provide a basis for the investigation of the surface chemistry of these alloys before and after exposure to a corrosive environment. We recognize that there are various aspects that one might consider including alloy thickness and composition with depth into the alloy, but in this work we have focused on the surface chemistry and how this changes with alloy composition. We have used core and valence band X-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA) to probe the surface chemistry of the films, complemented by other analytical probes. †
Part of the special issue “John T. Yates, Jr. Festschrift”.
We report the surface chemistry associated with the three alloy compositions discussed above electroplated onto the surface of mild steel and a study of the corrosion behavior of these ironzinc alloys in both aerated and deaerated water. No previous studies have utilized the valence band region to analyze the alloy composition or the corrosion behavior of these alloys. Valence band data, interpreted by band structure calculations, can be used to complement core XPS data and extend the information that can be obtained beyond that available from core level spectra. We also report the use of sample biasing as a means of assisting in the investigation of surface films whose XPS data are effected by differential sample charging, leading to peak broadening and separation. Experimental Section Instrumentation. X-ray photoelectron spectra were obtained using an AEI ES200B spectrometer. The base pressure of the system is 10-9 Torr. All spectra were collected using achromic Mg KR radiation (1253.6 eV with a line width of ca. 0.8 eV), with a power of 240 W (12 kV, 20 mA). All data were recorded in the FRR (fixed retardation ratio) mode, using a retardation ratio of 23 for all the regions. All data were collected using at least 17 points/eV to ensure identification of any subtle features that might not appear at lower resolution. The spectrometer scale was calibrated using copper.5 The C 1s peak from residual hydrocarbon at 284.6 eV was used for spectral calibration. Argon gas of 99.99% purity obtained from the Oxygen Service Co. was used to deaerate the quadruply distilled water used in the experiments. All quadruply distilled water was made in our laboratory using a specially designed distillation apparatus. Sample biasing was performed by applying +25 V from an external power supply to the sample rod, which is in electrical contact with the metal sample. This biasing was performed to separate peaks affected by differential sample charging from
10.1021/jp003388t CCC: $20.00 © 2001 American Chemical Society Published on Web 01/26/2001
3958 J. Phys. Chem. B, Vol. 105, No. 18, 2001
Hixson and Sherwood
TABLE 1: Alloy Electrodeposition Parameters sample treatment set 1: aerated water
set 2: deaerated water
set 3: untreated
iron-rich (83% Fe) zinc-rich (17% Fe) similar iron-zinc comp (41.5% Fe) iron-rich (83% Fe) zinc-rich (17% Fe) similar iron-zinc comp (41.7% Fe) iron-rich (83% Fe) zinc-rich (17% Fe) similar iron-zinc comp (41.5% Fe)
cathode size (cm2)
iFe anode (A)
iZn anode (A)
icathode (A)
[Fe2+] (M)
[Zn2+] (M)
5.14 5.20 5.40
1.182 0.273 0.663
0.206 1.131 0.795
1.388 1.404 1.458
0.05 0.01 0.05
0.01 0.05 0.05
5.60 5.27 5.52
1.288 0.276 0.677
0.224 1.147 0.813
1.512 1.423 1.480
0.05 0.01 0.05
0.01 0.05 0.05
5.53 5.27 5.46
1.272 0.276 0.670
0.221 1.147 0.804
1.493 1.423 1.474
0.05 0.01 0.05
0.01 0.05 0.05
those that were not subject to charging. The use of differential sample charging as a means for identification of chemically different surface species is an approach that we have found effective in other systems.6 In this study the peaks that were not affected by differential sample charging moved +25 V by the +25 V bias, thus separating them from peaks arising from species that were not in good electrical contact with the sample holder, and thus subject to differential sample charging. Where we reported biased sample spectra, we have adjusted the binding energy scale to reflect the bias (i.e., for a +25 V bias, the binding energies were shifted by +25 V). Sample Preparation. Commercial mild steel was cut into approximately 0.6 mm × 5 mm sections to be used as the cathode (substrate) in the alloy depositions. The exact measurement of the cathode was obtained prior to electrodeposition of all alloys to calculate the current needed, using the method described below. All samples were polished with alumina and degreased with acetone, followed by rinsing in deionized water before the electrodepositions were performed. Samples of iron metal foil (of 99.9975% purity) and zinc metal foil (of 99.998% purity) were obtained from Alfa ÆSAR. The samples were cut into approximately 0.5 mm × 5 mm sections to be used as anodes in the electrodeposition process. The measurements of these anodes was not critical for obtaining the value of the necessary current, as can be seen from the equations below. The anodes were degreased with acetone and rinsed with deionized water before the electrodeposition process. The cathode was held in the center of the bath surrounded by the four anodes. A dual-anode system was used to prepare the alloys. Two strips of iron metal and two strips of zinc metal were placed opposite one another (one iron and one zinc strip on either side of the cell) in an electrochemical cell. The anodes were connected using gold contacts and ordinary connectors. The anodes were connected to each other (iron to iron and zinc to zinc) to ensure that current supplied from the power source would flow through both anodes of a specific type. Each set of anodes was then connected to a separate power supply that could provide separate amounts of current through the anodes. The amount of current set to flow through the iron anodes was calculated from an equation derived by Salt1 using the dimensions of the cathode (both sides) as follows:
iFe anode )
icathode(% Fe comp of alloy) 85 + 0.15(% Fe comp of alloy)
where iFe anode is the current necessary to pass through the iron anodes to produce the desired alloy composition, icathode is the current necessary to pass through the cathode to produce the desired alloy composition, and % Fe comp of alloy is simply the desired composition of iron in the alloy (e.g., for 83% Fe,
the number is simply 83). The current necessary to pass through the cathode, icathode, is calculated as follows:
icathode ) jA where j is the current density (A/cm2) and A is the area of the cathode (cm2). The current density was chosen to be 0.27 A/cm2 for the basis of these calculations following the recommendations of Salt,1,2 who recommended that the current density should be kept between 0.22 and 0.32 A/cm2. The current necessary to pass through the zinc anodes is simply calculated as follows after the above calculations:
iZn anode ) icathode - iFe anode The electrolyte used followed the recommendations of Salt.1,2 The electrolyte bath that was used in these experiments was composed using the same relative concentrations for the iron and zinc as were used for the current calculations. For example, if 17% of the current flowed through the iron anodes, then 17% of the solution would be predicted to be composed of iron (and 83% of the current and solution composition would be present for zinc). The molarities of Fe2+ and Zn2+ in the electrolyte bath are given in Table 1, which shows the final alloy composition based upon the equations above. It will be seen that three alloy compositions, one an iron-rich alloy with 83% iron, one a zinc-rich alloy with 17% iron, and one an alloy of approximately equal iron-zinc composition with about 41% iron. Fresh samples were prepared for three different corrosion studies discussed below. The composition of (NH4)2SO4 in each solution was approximately 0.1 M, the composition of KCl in each solution was approximately 0.02 M, and the composition of citric acid was approximately 5 × 10-4 M. The compositions of the baths and relative current proportions were varied to change the compositions of the alloys. The pH’s of the solutions were adjusted using dilute sulfuric acid (diluted using quadruply distilled water) to between 1 and 2. All electrodepositions were performed in a heated water bath (the temperatures of the baths were kept at about 70 °C). The lengths of time of the depositions were dependent upon the lifetimes of the anodes, which often dissolve after an extended period of time (usually around 20 min). After electrodeposition of the alloys, the samples were placed in a heating cabinet at 100 °C for 15 min and were then left in air to dry overnight. X-ray Powder Diffraction Studies. X-ray diffraction (XRD) powder studies were carried out using a Scintag XDS 2000 instrument. The X-ray wavelength of Cu KR1 (1800 W) is 0.154 059 nm. The data were collected under the 2θ step scanning mode with a step size of 0.01°. Corrosion Studies. Samples in set 1 were placed in separate beakers of aerated, quadruply distilled water for a period of 13
Corrosion Behavior of Iron-Zinc Alloys days. Samples in set 2 were placed in separate specially designed airtight cells used to deaerate the quadruply distilled water and seal the samples from air contamination for a period of 13 days. Samples in set 3 were placed in a desiccator for a period of 13 days. After the period of 13 days, the wet samples were placed in dry beakers in the desiccator. After being dried in the desiccator, the samples were analyzed using XPS. No detectable sulfur was found on the sample surfaces. Data Analysis. Nonlinear background removal was achieved using an iterative method.7 The O 1s spectra were fitted using a nonlinear least-squares method with a Gaussian-Lorentzian peak shape,8-10 including the effect of X-ray satellites. The Gaussian-Lorentzian ratio was 0.5 for all oxide peaks. All the fitting parameters for particular compounds, such as peak width and peak separation, were chosen on the basis of curve-fitting of the spectra of samples of these compounds. Calculations. The band structure calculations were carried out using an extended version of the program CRYSTAL,11,12 which allowed these low-symmetry materials to be studied. Calculations were performed for the R3c structure of R-Fe2O3,13 the Pbnm structure of R-FeOOH,14,15 the C6mc structure of ZnO,16 and the Fd3m structure of Fe2ZnO4.17 The basis set of Wachters18 was used for iron and zinc, and the 8-51G basis set of Causa` et al.19 was used for oxygen except in the case of R-Fe2O3 and R-FeOOH, where the STO-3G basis set was used. All the calculations were restricted Hartree-Fock (RHF) calculations. The density of states obtained from this calculation was modified to separate out contributions to the density of states of particular types of atomic contribution (in this case Fe and Zn 3d and 4s and O 2s and 2p). These density of state contributions were then adjusted by their photoelectron cross sections and combined together to give a photoelectron cross section density of states. Scofield atomic photoelectron cross sections were used.20 This photoelectron cross section adjusted density of states was then convoluted with a GaussianLorentzian product function8 corresponding to the line width of the X radiation used. Results and Discussion XRD. X-ray diffraction patterns were recorded for a mild steel sample, the pure iron metal, the pure zinc metal, the ironrich alloy, the zinc-rich alloy, and the alloy of similar iron and zinc composition, as well as on the rhodium metal and the 17% and 83% iron alloys plated on rhodium metal (to eliminate interference from underlying iron). These data were collected to ensure that alloys, rather than metal mixtures, had been prepared. The diffractograms were compared with those in the Powder Diffraction File (PDF). The iron diffractogram agreed with PDF#060696, the zinc with PDF#040831. The alloy diffractograms were different from those of the metals. The 17% iron alloy gave a diffractogram that was similar to that of the 20% iron alloy (Fe3Zn10), PDF#710399. Other iron-zinc alloys in the PDF database have substantial differences for different iron composition, and iron compositions greater than 28% are not reported. We have thus not been able to compare our 41% and 83% iron alloys with diffractograms in the PDF database. Use of Sample Biasing To Improve Spectral Quality. In this study we encountered some spectra that appeared to show features that arose from differential sample charging, leading to spectral broadening and in some cases peak splitting. This phenomenon was especially noticeable in the spectra arising from zinc-rich samples. To save space, we have not shown all the spectral differences observed, but Figure 1 illustrates one of the more pronounced cases. Figure 1 shows a number of
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3959
Figure 1. XPS spectra of zinc-rich alloy treated in aerated water and biased to +25 V: (A) O 1s region, (B) Zn 2p region, (C) Fe 2p region, (D) valence band region, (a) unbiased sample, (b) sample biased to +25 V.
core regions, and illustrates how the spectra can be “improved” by the application of a 25 V positive bias (Figure 1b). In most of the cases below we present spectra biased to a 25 V positive bias to eliminate the effects of differential sample charging. We have presented intensity ratios for both biased and unbiased samples because the biasing approach will change the absolute values of these ratios, though it will be seen below that the same trends are found for both the biased and unbiased samples. It will be seen below that the valence band region exhibits the same general features with some broadening in the unbiased sample case. A possible explanation for the differences in atomic ratios for the biased and unbiased samples is that the iron and zinc species present on the heterogeneous sample surface do not respond to an applied bias to the same extent, with the iron peaks shifting to a lesser extent than the zinc peaks. This would be consistent with the greater resistivity of Fe2O3 as compared to ZnO. This observation leads to the assumption that the broadened features in the unbiased samples (peaks arising from differential charging) occur as a result of the presence of differentially charged zinc-oxygen surface species and not iron-oxygen surface species for the zinc-rich alloys. Surface Analysis of 83%, 41%, and 17% Iron Content Iron-Zinc Alloys. Overall, C 1s, O 1s, Fe 2p, Zn 2p, and valence band spectra were recorded for all samples. Only the O 1s, Fe 2p, Zn 2p, and valence band spectra will be discussed in detail. Table 2 gives a rough approximation of the relative amounts of iron, zinc, and oxygen on the surface of the samples. All of these atomic ratios assume that the data come from a homogeneous mixed Fe/O, Zn/O, or Fe/Zn region, which is a very crude model. The atomic ratios were adjusted for photoelectron cross section20 and analyzer transmission function. The ratios were calculated using values of inelastic mean free path calculated using the Seah and Dench equations.21 The O 1s spectra were fitted to two or three peaks, which are shown in Figure 2. Figures 3-5 show the results of the O 1s, Zn 2p, and Fe 2p spectra for the untreated and treated samples. O 1s Region. Most of the area ratios listed in Table 2 for Fe 2p/O 1s and Zn 2p/O1s are small, indicating that considerable oxygen is present on the surface of the samples. The O 1s region (Figures 3-5) was fitted to three components, namely, chemisorbed water, hydroxide, and oxide occurring at approximately 533.5, 531.9, and 529.9 eV. The relative amount of iron and zinc species present in the surface region can be easily tracked by noting the changes in
3960 J. Phys. Chem. B, Vol. 105, No. 18, 2001
Hixson and Sherwood
TABLE 2: Atomic Ratios Based upon the Zn 2p, Fe 2p, and O 1s Core Regions sample treatment
Figure
83% iron alloy, untreateda 83% iron alloy, untreated 83% iron alloy, deaerated 83% iron alloy, aerated 17% iron alloy, untreateda 17% iron alloy, deaerateda 17% iron alloy, aerateda 17% iron alloy, untreated 17% iron alloy, deaerated 17% iron alloy, aerated 41% iron alloy, untreateda 41% iron alloy, deaerateda 41% iron alloy, aerateda 41% iron alloy, untreated 41% iron alloy, deaerated 41% iron alloy, aerated
3a
a
3b 3c 4a 4b 4c
5a 5b 5c
Zn 2p/ O 1s
Fe 2p/ O 1s
Zn 2p/ Fe2p
0.03 0.05 0.02 0.00 0.05 0.22 0.16 0.12 0.27 0.23 0.26 0.26 0.08 0.32 0.24 0.12
0.13 0.13 0.14 0.13 0.02 0.02 0.04 0.01 0.02 0.02 0.13 0.14 0.10 0.06 0.06 0.10
0.15 0.52 0.21 0.00 1.54 9.14 2.76 6.77 12.89 8.81 1.50 1.37 0.63 3.58 2.90 0.95
Samples biased to +25 V.
Figure 3. XPS spectra of 83% iron alloy samples: (A) O 1s region, (B) Zn 2p region, (C) Fe 2p region, (a) untreated sample, (b) sample treated in deaerated water, (c) sample treated in aerated water.
Figure 2. O 1s spectra: (A) 83% iron alloy spectra, (B) 17% iron alloy spectra, (C) 41% iron alloy spectra, (a) untreated alloy spectra, (b) deaerated water spectra, (c) aerated water spectra.
the Fe 2p/Zn 2p atomic ratio. The 83% iron alloy samples show a significant fall in the amount of zinc present at the surface after treatment in aerated water. Treatment in aerated water removes zinc species from the surface, leaving a surface consisting of oxidized iron. Treatment of the 17% iron alloy samples in either deaerated or aerated water shows an increase in the relative amount of surface zinc species, with the greatest increase occurring after treatment with deaerated water. The 41% iron alloy shows a decrease in the amount of zinc present with respect to oxygen for the alloy treated in deaerated and aerated water, with the greatest decrease in the latter case. The same trends are observed for the biased and the unbiased samples, though there are substantive quantitative differences which are expected because the biasing process discriminates between sample species in most cases showing smaller zinc concentrations on biasing. The Fe 2p/O 1s area ratios show insignificant change for the samples treated in either deaerated water or aerated water in the amount of oxygen present on the surfaces of the 83% iron alloy samples. The lack of significant change is probably due
Figure 4. XPS spectra of 17% iron alloy samples: (A) O 1s region, (B) Zn 2p region, (C) Fe 2p region, (a) biased, untreated sample, (b) biased sample treated in deaerated water, (c) biased sample treated in aerated water.
to the presence of a thick iron hydroxide/iron oxide layer on the surface of the samples. Visual inspection of the deaerated water sample showed little change with respect to the untreated sample, but the aerated water sample revealed a thick orangered powdery coating that was noticeably different from the untreated and deaerated water samples. Details of the curve-fitting process for the O 1s core region are given in Table 3. The biased, untreated 83% iron alloy sample shows hydroxide and oxide oxygen on the sample surface. The 83% iron alloy samples treated in both deaerated and aerated water show hydroxide and oxide oxygen present on the sample surface. The hydroxide intensity for the 83% iron alloy samples increases for both the deaerated and aerated water samples. The biased, untreated 17% iron alloy shows only oxide
Corrosion Behavior of Iron-Zinc Alloys
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3961
TABLE 3: Curve-Fitting Results for the O 1s Core Regiona H2O sample treatment 83% iron alloy, untreatedc
Figure 3A,a
fwhm 83% iron alloy, untreated
OH
peak 0.00 533.5
peak
%
peak
%
0.00 (0.00)
531.1
26.2 (3.61)
529.4
73.8 (2.90)
0.98 531.9
13.5 (10.4)
fwhm 83% iron alloy, deaerated
3A,b
0.88 0.00
fwhm 83% iron alloy, aerated
3A,c
0.00
0.00 (0.00)
fwhm 17% iron alloy, untreatedc
4A,a
0.00
0.00 (0.00)
fwhm 17% iron alloy, deaeratedc
4A,b
0.00
0.00 (0.00)
fwhm 17% iron alloy, aeratedc
4A,c
0.00
0.00 (0.00)
0.00
0.00 (0.00)
fwhm 17% iron alloy, untreated fwhm 17% iron alloy, deaerated
533.5
fwhm 17% iron alloy, aerated
0.91 533.5
fwhm 41% iron alloy, untreatedc
5A,a
1.09 0.00
fwhm 41% iron alloy, deaeratedc
5A,b
0.00
fwhm 41% iron alloy, aeratedc
5A,c
532.3
fwhm 41% iron alloy, untreated
1.16 533.5
fwhm 41% iron alloy, deaerated
0.93 533.5
fwhm 41% iron alloy, aerated
1.09 533.5
fwhm
O2
%
0.00 (0.00)
18.1 (23.4)
0.87 0.00 531.7b 1.04 531.9b 1.01 0.00
0.91 531.7
0.00 (0.00) 0.00 (0.00)
1.09 0.00 531.4b 0.91 530.4
18.5 (14.3) 34.6 (6.18) 29.5 (9.06)
0.93
0.86 531.1
532.1
39.9 (10.2)
15.8 (17.0)
0.88 531.6
1.16 531.8 0.93 531.8 1.09 531.9 0.93
41.5 (4.22) 38.0 (4.32) 44.2 (6.80) 0.00 (0.00) 63.4 (6.20) 52.9 (11.0) 0.00 (0.00) 32.1 (13.5) 32.7 (14.6) 0.00 (0.00) 47.5 (9.30) 42.3 (9.38) 46.9 (4.71) 44.2 (7.49) 43.8 (7.14)
0.98 530.2 0.88 530.0 0.86 529.7 0.87 531.1 1.07 530.0b 1.04 530.4b 529.9 1.19 530.4 0.91 529.5 1.09 530.0
45.0 (3.18) 62.0 (2.45) 55.8 (5.48) 100.0 (2.37) 36.6 (12.6) 47.1 (12.7) 1.01 100.0 (1.98) 49.8 (6.08) 27.4 (9.09) 100.0 (5.62)
1.48 529.7b 0.91 528.9 1.16 529.6 0.93 530.1 1.09 530.2
39.2 (12.3) 18.5 (6.91) 26.3 (10.1) 40.4 (7.42)
0.93
Peak ) peak center. % ) % area of the O 1s core XPS region. Values of binding energy have been corrected for comparison purposes. Samples biased to +25 V. Values in parentheses represent 95% confidence limits for peak area shown as a percentage uncertainty in the peak area. a
b
c
oxygen present on the sample surface. Both the 17% iron alloy treated in deaerated water and the biased, 17% iron alloy treated in aerated water show the presence of mainly oxygen in the hydroxide region together with oxide oxygen. The biased, 17% iron alloy sample treated in aerated water shows an increase in oxide oxygen above that for the biased, deaerated water sample. The biased 41% iron alloy shows only oxide oxygen present on its surface. The biased 41% iron alloy treated in deaerated water shows similar amounts of hydroxide and oxide oxygen intensity. The 41% iron alloy treated in aerated water also shows hydroxide intensity similar to that of the dearated water sample, with a decrease in oxide intensity with respect to the untreated and deaerated water samples, and it also shows an adsorbed water peak. The hydroxide oxygen in water-treated samples may correspond to significant water adsorption in the surface region after water exposure. Zn 2p and Fe 2p Regions. The Zn 2p region was not curve
fitted because Zn 2p shifts are known to be negligible between metal and oxidized zinc. All the Zn 2p peaks had a binding energy of about 1022 eV. The Fe 2p region was not curve fitted because shifts between different Fe(III) species are also negligible. All the Fe 2p peaks had a binding energy of about 711 eV. Therefore, only the area ratios (relative intensities) are discussed here. The Zn 2p/Fe 2p area ratios for the 83% iron alloy samples show a large increase in the amount of iron present, probably as iron hydroxide, for the aerated water sample, with respect to the biased, untreated 83% iron alloy. The 17% iron alloy shows a large increase in the amount of zinc present for both the biased, deaerated water and biased, aerated water samples, with the largest increase occurring for the deaerated water sample, with respect to the biased, untreated sample. The 41% iron alloy shows an increase in the amount of iron present for both the biased, deaerated water and aerated water samples, with the largest increase occurring for the aerated
3962 J. Phys. Chem. B, Vol. 105, No. 18, 2001
Figure 5. XPS spectra of 41% iron alloy samples: (A) O 1s region, (B) Zn 2p region, (C) Fe 2p region, (a) biased, untreated sample, (b) biased sample treated in deaerated water, (c) biased sample treated in aerated water.
Figure 6. XPS valence band spectra of (A) 83% iron alloy, (B) 17% iron alloy, and (C) 41% iron alloy. (a) shows the untreated sample, (b) shows the sample treated in deaerated water, and (c) shows the sample treated in aerated water.
water sample, with respect to the biased, untreated sample. Visual inspection of the aerated water sample showed an increase in areas of orange-red color, probably indicating an increase in FeOOH on the sample surface. Valence-Band Region. Figures 6 and 7 show the valence band regions, the latter figure showing the data for a 25 V positive bias in the case of the 41% and 17% iron alloys and the untreated 83% iron alloy. The basic characteristics of the ironzinc alloy valence bands are, in general, the O 2s peak at 2223 eV, the Zn 3d peak at about 10 eV, and the outer valence band region near the Fermi edge at around 5 eV. This outer valence band region shows the presence of Zn 3d KR3,4 satellites for the ZnO and Zn(OH)2 valence bands as a result of the high intensity of the Zn 3d region, but features due to Fe2O3 and
Hixson and Sherwood
Figure 7. XPS valence band spectra of the samples in Figure 6 for a 25 V bias. (A,b) and (A,c) have no bias applied.
FeOOH can also be distinguished in this region (Fe2O3 gives three peaks, and FeOOH gives two peaks22). The O 2s region, which usually shows features similar to those of the O1s region in oxidized metals, in oxidized iron samples displays greater width in FeOOH than in Fe2O3 due to the presence of both oxide and hydroxide oxygen.22 The O 2s region in ZnO and Zn(OH)2 is markedly different from the same region in oxidized iron, being broadened by satellite features. The satellite features are of such high intensity that the O 2s region is much less distinct than is usually found in oxide systems. In addition oxidized zinc is marked by the much higher intensity found for the Zn 3d region than the O 2s region. A number of workers have provided a detailed analysis of the O 2s satellite features in ZnO, e.g., see ref 23 and references therein, and ZnO XPS valence band spectra have been widely studied, e.g., see refs 24-30. The O 2s region is of comparable intensity to the Fe 3d region in the iron oxides. The valence band spectra can be understood by comparing the experimental data with spectra generated from band structure calculations. This is important to provide confidence that the valence band spectra were obtained from a surface region with the same composition as the bulk standard compounds examined. Figure 8 compares the valence band spectra of Fe2O3, FeOOH, ZnO, Zn(OH)2, and Fe2ZnO4 with spectra that are additions of the zinc and iron oxides and hydroxides. A nonlinear iterative method was used to remove the background from the spectra,7 which were then modified to have the same width in the region of the spectrum before the center of the O 2s peak and in the region of the spectrum following the center of the O 2s peak. These spectra were normalized on the basis of the O 2s intensity (at about 22 eV). These addition spectra clearly show that the spectrum of the spinel, Fe2ZnO4, is different from that of mixtures of Fe2O3 and FeOOH with ZnO and Zn(OH)2. In particular the mixture spectra in Figure 8B have a broader feature to lower binding energy of the Zn 3d region. The experimental spectra are compared with spectra generated from calculations in Figures 9 and 10. It was noted in the beginning of this valence band section that Fe2O3 shows three peaks and FeOOH gives two peaks in the outer valence band region. The spectra of R-Fe2O3 and
Corrosion Behavior of Iron-Zinc Alloys
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3963
Figure 10. XPS valence band spectra of (A) ZnO and (B) Fe2ZnO4 showing the background-subtracted experimental data (a) after removal of the KR3,4 X-ray satellites compared with the spectra (b) generated from band structure calculations. The band structure calculations convoluted the photoelectron cross section adjusted density of states with a photon function of fwhm ) 1 eV for ZnO and Fe2ZnO4. The spectrum of Fe2ZnO4 is also shown generated following convolution with a photon function of fwhm ) 2 eV.
Figure 8. XPS valence band spectra of pure compounds and added spectra: (A) pure compound XPS spectra, (a) ZnO spectrum, (b) Zn(OH)2 spectrum, (c) Fe2ZnO4 spectrum, (d) Fe2O3 spectrum, (e) FeOOH spectrum, (B) added valence band spectra, (a) ZnO + Fe2O3, (b) ZnO + FeOOH, (c) Zn(OH)2 + Fe2O3, (d) Zn(OH)2 + FeOOH. The Fe2O3 and FeOOH spectra where obtained with monochromatic X radiation. The ZnO, Zn(OH)2, and Fe2ZnO4 spectra had the KR3,4 X-ray satellites removed. Figure 11. (a) Enlarged area of the valence band spectrum of ironrich iron-zinc alloy treated in aerated water. (b) Full valence band spectrum of iron-rich iron-zinc alloy treated in aerated water.
Figure 9. XPS valence band spectra of (A) R-Fe2O3 and (B) R-FeOOH showing the background-subtracted experimental data (a) obtained with monochromatic X radiation compared with spectra (b) generated from band structure calculations. The band structure calculations convoluted the photoelectron cross section adjusted density of states with a photon function of fwhm ) 1 eV.
FeOOH are compared with spectra generated from band structure calculations in Figure 9, which indicates these three (a, b, and c) and two (shown as b and c) peak structures, which we previously interpreted22 by cluster calculations. This reflects the fact that the bands are relatively flat in these compounds. The two peaks marked c and d for the FeOOH are in a region of high O 2s character and consist of largely hydroxide oxygen (peak c) and oxide oxygen (peak d). Figure 10 shows the spectra of ZnO and Fe2ZnO4 compared with spectra generated from band structure calculations. The differences between ZnO and Fe2ZnO4 are clear, with the former having a distinct peak (shown as a) about 6 eV to lower binding energy of the intense Zn 3d region, and the latter a distinct peak (shown as a) about 4 eV to
lower binding energy of the intense Zn 3d region. It should be noted that the most intense feature in this region is predicted to be very close in energy to the Zn 3d region, consistent with the smaller separation for the spinel. The two-peak broad structure to low binding energy of the Zn3d region seen in Fe2ZnO4 is predicted in the calculation. ZnO and Zn(OH)2 have similar spectra (Figure 8), though the former displays a distinct twopeak feature to low binding energy of the Zn 3d region. In examining the zinc-containing spectra, it is important to remember how the spectra are dominated by the Zn 3d region, and so the apparently “low-intensity” features provide most of the diagnostic differences. Examination of the 83% iron alloy samples indicates that the surface oxide in the 83% iron alloy sample (Figure 6/7A,a) appears to be largely composed of the spinel, Fe2ZnO4. Exposure of this alloy to aerated water appears to lead to decomposition to form a surface layer of FeOOH. The relative intensity of the O 2s feature increases upon going from the untreated to the deaerated water to the aerated sample. Oxidation by aerated water causes a marked fall in the intensity of the Zn 3d peak at 10 eV. Figure 11 shows in an enlarged spectrum that there is still some Zn 3d intensity present in Figure 6/7A,c. This feature is too narrow to be the third peak that is characteristic of Fe2O3. It is apparent that the amount of FeOOH increases greatly for the 83% iron alloy treated in aerated water, with respect to the biased, untreated sample. The 17% iron alloy shows a surface that appears to be a mixture of ZnO and Fe2ZnO4 which converts into a pure ZnO
3964 J. Phys. Chem. B, Vol. 105, No. 18, 2001 surface after exposure to aerated water. The formation of pure ZnO is characterized by a significant decrease in the relative intensity of the O 2s peak for both water-exposed samples. The changes in the valence band region are also reflected by decreasing amounts of oxygen in the core regions (Table 2). The distinct doublet feature around 2 eV in Figure 6/7C,c is the KR3,4 X-ray satellite from the Zn 3d region. It has been removed in the spectra shown in Figure 8 to facilitate comparison between the spectra. The 41% iron alloy has a surface that is largely composed of a mixture of ZnO and Fe2ZnO4. Exposure to water leads to a significant difference in surface composition from the 17% iron alloy case. In this case the exposure leads to an increase in the amount of spinel on the surface, leading to a spectrum largely composed of Fe2ZnO4 after exposure to aerated water. The changes in the valence band region are also reflected by increasing amounts of oxygen in the core regions (Table 2). Conclusions The valence band data reveal information complementary to the core region data. The region allows one to distinguish species present on the sample surfaces that give identical core binding energy shifts. Analysis of the valence band data gives strong supporting evidence for the presence of a spinel of iron and zinc, Fe2ZnO4, whose valence band spectrum was different from that of spectra generated by addition of the spectra of iron and zinc oxides and hydroxides. O 1s data also give insight into the possible oxygen species present on the sample surface. For example, the predominant species of oxygen present on the untreated 17% iron alloy surface can be seen to be a mixture of ZnO and Fe2ZnO4 and not Zn(OH)2 on the basis of the chemically shifted features in the O 1s region. In cases where both ZnO and Fe2ZnO4 are present, it is not possible to accurately quantify the relative amounts of ZnO and Fe2ZnO4 because of an overlap of many of their features. It is possible to get an approximate measure of the relative amounts of these two compounds by comparing the valence band spectra of the alloys with a sum of experimental data for ZnO and Fe2ZnO4; however, bearing in mind the errors associated with differential charging, and the possible dependence of the biased spectra upon only part of the surface region, such an analysis has not been performed. The 83% iron alloy gives poor resistance to corrosion. This lower resistance is exhibited by the presence of a surface high in FeOOH intensity (an unprotective layer) and the observation of a thick rustlike layer on the sample surface after treatment in aerated water. The 41% iron alloy shows an intermediate level of corrosive behavior. Visual inspection revealed formation of orange-red (rustlike) areas on the sample surface in aerated water. Comparison with the untreated, biased alloy sample revealed no increase in oxygen intensity for the biased, deaerated water sample, but a significant increase in oxygen intensity was shown for the aerated water sample. This increase in oxygen results from the formation of the spinel, Fe2ZnO4, which appears to
Hixson and Sherwood have substantially reduced corrosion protection compared with ZnO (formed on the surface of the 17% iron alloy). The 17% iron alloy shows considerable promise as a corrosion-resistant material. Visual inspection of this alloy showed no orange-red formations that are characteristic of oxidized iron on any of the sample surfaces. Biasing and treatment in deaerated water caused no oxidation, with the surface being protected by a surface film that was almost entirely ZnO. Acknowledgment. This material was based upon work supported by the National Science Foundation under Grant No. CHE-9421068. References and Notes (1) Salt, F. W. Electroplat. Met. Finish. 1956, 3. (2) Jepson, A.; Meecham, S.; Salt, F. W. Trans. Inst. Met. Finish. 1955, 32, 160. (3) Sagiyama, M.; Hiraya, A. Corros. Eng. 1993, 42, 889. (4) Nagoshi, M.; Kondo, T. Hyomen Gijutsu (J. Surf. Finish. Soc. Jpn.) 1994, 45, 239. (5) Annual Book of ASTM Standards. Surf. Interface Anal. 1998, 26, 642. (6) Havercroft, N. J.; Sherwood, P. M. A. Surf. Interface Anal. 2000, 29, 265. (7) Proctor, A.; Sherwood, P. M. A. Anal. Chem. 1982, 54, 13. (8) Ansell, R. O.; Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Electroanal. Chem. Interfacial Electrochem. 1979, 98, 79. (9) Sherwood, P. M. A. In Practical Surface Analysis by Auger and Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; Wiley: London, 1983; p 445. (10) Sherwood, P. M. A. In Practical Surface Analysis by Auger and Photoelectron Spectroscopy, 2nd ed.; Briggs, D., Seah, M. P., Eds.; Wiley: London, 1990; Vol. 1, p 555. (11) Pisani, C.; Dovesi, R.; Roetti, C. Hartree-Fock Ab Initio Treatment of Crystalline systems; Lecture Notes in Chemistry, 48; Springer: Berlin, 1988; QCPE 577. (12) Saunders, V. R.; Dovesi, R.; Roetti, C.; Causa`, M.; Harrison, N. M.; Orlando, R.; Zicovich-Wilson, C. M. Crystal 98 User’s Manual; University of Torino: Torino, 1998. (13) Blake, R. L.; Zoltai, T.; Hessevick, R. E.; Finger, U. W. U.S., Bur. Mines Rep. InVest. 1970, No. 7384. (14) Szytula, A.; Burewicz, A.; Dimitrijevi_, Z.; Kra_nicki, S.; R_any, H.; Todorovi_, Wanic, A.; Wolski, W. Phys. Status Solidi 1968, 26, 429. (15) Sampson, C. F. Acta Crystallogr. 1969, B25, 1683. (16) Wyckoff, R. W. G. Crystal Structures, 2nd ed.; Wiley: New York, 1963; Vol. 1, p 111. (17) Wyckoff, R. W. G. Crystal Structures, 2nd ed.; Wiley: New York, 1965; Vol. 3, p 75. (18) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. (19) Causa`, M.; Dovesi, R.; Pisani, C.; Roetti, C. Phys. ReV. B 1986, 33, 1308. (20) Scofeld, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129. (21) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2. (22) Welsh, I. D.; Sherwood, P. M. A. Phys. ReV. B 1989, 40, 6386. (23) Scrocco, M. Phys. Status Solidi B 1984, 116, 137. (24) Ley, L.; Pollak, R. A.; McFeely, F. R.; Kowalczyk, S. P.; Shirley, D. A. Phys. ReV. B 1974, 9, 600. (25) Zwicker, G.; Jacobi, K. Solid State Commun. 1985, 54, 701. (26) Chelikowsky, J. R. Solid State Commun. 1977, 22, 351. (27) Tsukada, M.; Miyazaki, E.; Adachi, H. J. Phys. Soc. Jpn. 1981, 50, 3032. (28) Scrocco, M. Phys. ReV. B 1982, 27, 3406. (29) Tossell, J. A. Chem. Phys. 1976, 15, 303. (30) Didziulis, S. V.; Cohen, S. L.; Butcher, K. D.; Solomon, E. I. Inorg. Chem. 1988, 27, 2238.
