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The incorporation and mobility of chromium species in anodic alumina films
CorrosionScience,Vol. 39, No. 4, pp. 71%730,1997 0 1997Ekvier Scitoa Ltd
Printed in Great Britain. All rights mserval mm-938X/97 s17.00+0.00
THE INCORPORATION AND MOBILITY OF CHROMIUM SPECIES IN ANODIC ALUMINA FILMS H. HABAZAKI,*
K. SHIMIZU,t P. SKELDON,$ G. E. TI-IOMPSON,$ X. ZHOU,S J. DE LAETS and G. C. WOODS
* Institute for Materials Research, Tohoku University, Sendai 980-77, Japan t University Chemical Laboratory, Keio University, 4-l-l Hiyoshi, Yokohama 223, Japan $ Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 lQD, U.K. Abetract-By
anodizing of a thin layer of Al-O.5 at.% Cr alloy, cu. 54 nm thick, superimposed on electropolished superpure aluminium, chromium species are incorporated into the anodic alumina as a thin band cu. 3 mn thick. This allows the relative mobility of chromium species, with respect to aluminium ions, to be determined, providing key information to assist development of insight into ionic transport processes in amorphous anodic films under high electric fields. During anodizing of the thin alloy Slm, chromium atoms are. not oxidized and are accumulated in a layer of alloy, cu. 2 nm thick, immediately beneath the anodic film as a consequence of prior oxidation of aluminium. When the alloy film is totally consumed by anodizing, all the chromium atoms in the enriched layer are oxidized and incorporated immediately into the anodic film, forming a thin chromium species-containing band of cu. 3 mn thickness. With further anodizing, the incorporated chromium species band migrates outwards without widening. From chemical sectioning of the anodic film and KPS surface analysis, the chromium species are present as Cr3 + ions in the anodic film. The relative migration rate of Cr’ + ions, with respect to Al3 + ions, determined from the precise location of the chromium species-containing layer, using transmission electron microscopy and Rutherford backscattering spectroscopy, is 0.74.0 1997 Elsevier Science Ltd. All rights reserved Keywords: A. aluminium, B. galvanostatic, B. RBS, B. TEM, B. KPS, C. anodic films.
INTRODUCTION Barrier-type anodic films on relatively pure aluminium develop by migration of A13+ ions outwards and migration of 02-/OHions inwards under a relatively high field. For anodizing at high current efficiency, about 40% of the film thickness is formed at the film/ electrolyte interface, with the remainder generated at the metal/film interface,’ over a wide range of current density and hence electric field strength.2 These results suggest that the transport processes of anions and cations are in some way linked,3 i.e. co-operative, although the precise mechanism has not been discussed fully. During anodic film growth, it is well known that electrolyte species, derived from borate, phosphate and tungstate anions, are incorporated into the respective films at the film/ electrolyte interface. These incorporated species show a range of mobilities; tungsten species are mobile outwards, boron species are immobile and phosphorus species are mobile inwards.4 Thus, these species (boron, molybdenum, tungsten) can be used as tracers (tungsten and phosphorus species) or markers (boron species). Clearly immobile species are
Manuscript received 28 August 1996. 719
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H. Habazakiet al
incorporated as neutral species into the films under the high electric field, whereas inwardly mobile and outwardly mobile species are present as cation and anion species, respectively. Since the available electrolytes for anodic oxidation of aluminium at high current efficiency are limited, not all ions of interest may be incorporated from this route. Anodic oxidation of ion-implanted aluminium and aluminium alloys offers an additional approach to incorporation of tracers or markers. The mobilities of various implanted elements have been examined previously using Rutherford backscattering spectroscopy,5 although the precise migration rates of most species, migrating faster than A13+ ions, could not be determined due to their direct ejection into the electrolyte at the film/electrolyte interface during anodizing. From the anodic oxidation of sputter-deposited single phase aluminium in thickening alumina films alloys, the relative migration rates of Zr4+,6 and Ta’+,’ ions . have been determined. Recently, a novel approach, utilizing sputter deposition of a thin layer of an appropriate dilute aluminium alloy on to electropolished aluminium, with subsequent interfacial enrichment of alloying element at the alloy/film interface during anodic oxidation of the alloy, has been developed to determine the migration rates of alloying element ions in anodic alumina films.8 The anodic oxidation of many dilute aluminium alloys results in the accumulation of alloying element atoms in a thin layer of alloy, cu. 2 nm thick, immediately beneath the anodic film, as a consequence of prior oxidation of aluminium.’ When the thin alloy layer is consumed totally by anodic oxidation, the alloying element atoms in the enriched layer are abruptly oxidized due to the presence of an air-modified electropolishing film sandwiched between the alloy layer and aluminium. The oxidized elements are incorporated into the anodic film, forming a thin band, which does not widen further during anodizing. From the positions of the alloying element-containing layer, determined by Rutherford backscattering spectroscopy and direct observation using transmission electron microscopy, the precise migration rates can then be determined. Using this approach, the migration rate of Zn* + ions has been successfully determined to be about 2.3 times that of Al3 + ions.’ From the presently known mobilities of several ions, Shimizu and Kobayashi have found generally good correlations between their migration rates in anodic alumina and the corresponding metal-oxygen bond energies, ionic radii and valencies.” The mobilities of other ions do not show such good agreement, suggesting that further factors control the ionic transport processes. Consequently, it is important to determine the mobilities ofa wide range of ions in amorphous alumina in order to develop understanding of factors controlling the ionic transport processes. When chromium species are incorporated into the anodic alumina film from chromate electrolytes, they are not distributed uniformly in the outer part of the film, in contrast with the uniform distribution of molybdate and tungstate species incorporated from their electrolytes. This is considered to be due to the transformation of Cr6+ ions to Cr3+ ions within the film under the high electric field. Since the mechanism of the transformation and . the mobilities of Cr3+ and Cr6+ ions are not yet known, the unusual distribution of chromium species incorporated from an electrolyte is not well understood. In the present study, the mobility and the chemical state of chromium species incorporated from the substrate as a thin band have been determined using transmission electron microscopy (TEM), Rutherford backscattering spectroscopy (RBS) and X-ray photo-electron spectroscopy (XPS).
The incorporation and mobility of chromium species
EXPERIMENTAL
721
METHOD
Al-O.5 at.% Cr alloy films, about 54 nm thick, were prepared by DC magnetron sputtering using a Microvac 350 system. Dual targets of 99.999% aluminium and 99.9% chromium discs of 38 mm diameter were used to prepare the alloy films. The alloy films were deposited on to 99.99% pure aluminium substrates which had been electropolished at 20 V for 5 min in ethanol-perchloric acid electrolyte at less than 283 K. The composition of the alloy films was confirmed by RBS to an accuracy of about 5%. Specimens were anodized at a constant current density of 50 Am-* in stirred 0.01 M ammonium pentaborate electrolyte at 293 K to selected voltages of 50, 100, 150 and 200 V. An aluminium sheet was used as the counter electrode. Electron transparent sections of the anodized specimens were obtained using a Reichert-Jung ultramicrotome. The encapsulated specimens were trimmed initially with glass knives and suitably thin sections, about 10 nm thick, were prepared using a diamond knife. The sections were observed in a JEOL TEM 2000FX II transmission electron microscope operating at 120 kV. The specimens, as-deposited and anodized, were analyzed by RBS using a 2.0 MeV alpha particle beam supplied by the Van de Graaff accelerator of the University of Paris. The beam current and diameter were about 60 nA and 0.5 mm, respectively. Scattered particles were detected at 150” to the incident beam direction. The data were analyzed using the RUMP program,” with scaling of the stopping power for oxygen in accord with recent RBS analyses. ‘* Chemical sectioning of anodic alumina was performed in 2 M H2S04 solution at 333 K. After washing in distilled water and drying, XPS analysis was carried out by Shimazu ESCA850 electron spectrometer using Mg Kcl excitation (1253.6 eV). Binding energies of electrons were calibrated by a method described elsewhere. l3 Briefly, the binding energies of Au 4f7,* and 4f5,* electrons of gold metal, and Cu 2p 3,2and 2p1,2 electrons of copper metal were taken at 84.07, 87.74, 932.53 and 952.35 eV, respectively. The kinetic energy of Cu L3M4,5M4,5Auger electrons of copper metal was taken at 918.65 eV. Further, the shift of apparent binding energy through charging effects was calibrated using C 1s electrons of contaminant hydrocarbons (285.0 eV).
EXPERIMENTAL
RESULTS
Voltage-time response
The voltage-time response of the Al-O.5 at.% Cr alloy, superimposed on the electropolished aluminium, during anodizing at a constant current density of 50 A m-* in 0.01 M ammonium pentaborate electrolyte at 293 K is shown in Fig. 1. Two voltage surges of about 2-3 V, at the beginning of anodizing and at about 73 V, are observed. The first voltage surge is due to the presence of an air-formed film on the alloy surface and the second surge confirms the presence of an air-modified electropolishing film,* about 2-3 nm thick, sandwiched between the alloy film and the aluminium substrate. The detection of the second voltage surge reveals that the alloy film is oxidized totally by anodizing to 73 V. After the first and second voltage surges, the voltage increases linearly with time. The slopes before and after the second voltage surge are similar, about 2.3 V s- ‘, typical of that for anodizing high purity aluminium at high current efficiency. Thus, the growth of the anodic film on this specimen proceeds at high current efficiency during oxidation of the alloy and the
H. Habazaki et al.
122
200 _
I
Al-O.6
I
I
I
et96 Cr Alloy
I
I
I
I
I
/ Aluminium
1 0
20
40 Time
I 8
60
60
Fig. 1. Voltage-time response of the Ala.5 at.% Cr alloy film sputter-deposited on to 99.99% pure aluminium during anodizing at 50 A m-’ to 200 V in 0.01 M ammonium pentaborate electrolyte at 293 K.
underlying aluminium substrate. Assuming a nm V- ’ of 1.214and a Pilling-Bedworth ratio of 1.61 I5 for anodic alumina formed on the Al-0 alloy, the average thickness of the original alloy film is estimated to be 54nm, although the original alloy surface was probably relatively rough,’ due to development of a columnar, crystallographic structure during sputtering. In the voltage-time response of sputter-deposited Al-Cr alloys containing about 1.5 at.% chromium, the linear relationship between the voltage and anodizing time is not observed and the slope gradually increases with time. Unpublished work of the authors suggests that the increasing slope is associated with the formation of flaws in the anodic film. Such gradual change in the slope is not detected in the voltage-time response of the present alloy superimposed on aluminium, suggesting relatively uniform film formation over the macroscopic surface of the dilute alloy. Transmission electron microscopy (TEM) A transmission electron micrograph of an ultramicrotomed section of the specimen anodized to 100 V is shown in Fig. 2. The anodic film is amorphous and has relatively flat and parallel metal/film and film/electrolyte interfaces. The film is present on aluminium, indicating that the thin alloy layer has already been consumed. The thickness of the film is 118 f 3 nm, corresponding to a nm V- ’ ratio of 1.18 & 0.03, which is closely similar to that for films on high purity aluminium (nm V- ’ ratio of 1.2).14 In the anodic film of generally featureless and uniform appearance, thin light and dark bands are present, located about 15% and 25% of the film thickness, respectively, measured from the metal/film interface. The light band composed of voids, probably contains argon bubbles, originally incorporated at, or near, the interface between the alloy film and the underlying aluminium substrate.* In agreement with a previous finding that argon bubbles are immobile in growing anodic alumina films,’ the bubbles in the present film are also immobile during film growth. The dark band, about 3 nm thickness, is considered to be the chromium species-containing band. The presence of chromium species in the band is
123
The incorporation and mobility of chromium species
‘CrBand Voids Band
Fig. 2. Transmission electron micrograph of an ultramicrotomed section of the Al-O.5 at.% Cr alloy film sputter-deposited on to 99.99% pure electropolished ahmdnium after anodizing at 50 A me2 to 100 V in 0.01 M ammonium pentaborate electrolyte at 293 K.
confirmed by later RBS. The relatively dark appearance of the band is due to enhanced electron scattering from chromium species, compared with aluminium ions of reduced atomic number. The formation of this narrow band indicates that during anodic oxidation of the Al-G alloy almost no chromium species are incorporated into the anodic film and chromium atoms are accumulated in a thin layer, about 2 mu thick, just beneath the anodic film, as a consequence of prior oxidation of aluminium. The accumulated chromium atoms are oxidized immediately when the alloy film is consumed totally by anodizing.* From the positions of the chromium species-containing band and the immobile light band or marker, it is clear that chromium species migrate outwards in the growing alumina film. In the film formed to 150 V (Fig. 3), the dark band, of similar thickness, is present at about 46% of the film thickness, measured from the metal/film interface, although the light band is not revealed so clearly and knife marks, generated during ultramicrotomy, run almost parallel to the metal/film interface. Rutherford backscattering
spectroscopy
(RBS)
The experimental and simulated RBS spectra of as-deposited specimens and specimens anodized to several voltages are shown in Fig. 4. A region of the experimental spectra for elastic scattering from chromium is revealed in greater detail in Fig. 5. The peak energy of the yield for chromium shifts to lower energies with anodizing voltage. In particular, the shift is larger during anodizing of the alloy up to about 70 V (Fig. 6). The change in the energy shift at about 70 V indicates that chromium species are incorporated into the anodic film at this voltage. This voltage is closely similar to that of the second voltage surge observed in Fig. 1. The shift of the peak energy to lower energy with increasing anodizing voltage from 50 to 200 V reveals the slower migration rate of chromium species, compared with A13+ ions. From the simulated spectra (Fig. 4), which fit well the corresponding experimental spectra, the precise position and composition of the chromium-containing
H. Habazaki et al.
724
Cr Band Voids Band
Fig. 3. Transmission electron micrograph of an ultramicrotomed section of the Al-O.5 at.% Cr alloy film sputter-deposited on to 99.99% pure electropolished aluminium after anodizing at 50 A me2 to 150 V in 0.01 M ammonium pentaborate electrolyte at 293 K.
bands are obtained as well as the total thickness of the films formed to different voltages. These data, which are consistent with those obtained by TEM observation, are summarized in Table I. The distance, r, of the chromium-containing band, normalized to the total film thickness, measured from the metal/film interface at a particular voltage, P’, can be expressed as follows:’ r = (0.6 + 0.4 x I,“:)(1 - Vo/V) in which Ve is the voltage at which chromium species are incorporated into the film, that is about 73 V from Fig. 1, and Zp is the relative mobility of chromium species with respect to A13+ ions; the linear relationship between r and V-’ is shown in Fig. 7. From the slope of the linear region, the relative migration rate of chromium species, with respect to A13+ ions, is 0.74 f 0.05.
Table 1. Film thicknesses, positions and compositions of the chromium-containing bands in the anodic films developed by anodizing at 50 A me2 in 0.0 1M ammonium pentaborate electrolyte at 293 K to various voltages Anodizing voltage (VI 50 100 150 200
Film thickness (nm)
Normalized position of the chromium band, r
Atomic composition of the chromium band [Cr]/[Cr + Al]
63+3 118+3 183+3 242&3
0 0.25 +0.02 0.46 + 0.02 0.57+0.02
0.15_+0.01 0.15+0.01 0.15~0.01
125
The incorporation and mobility of chromium species 6000
‘-WV(b)
Al-O.5
at%
Anodized
r-
Cr Alloy
to 50
V
‘i
Measured
-
-
Simulated
Simulated 200 Channel
300 Channel
Number
300 Number
I_I_II/T_
(
ON
Al-O.5
Anodized
-
Simulated
-
Channel
’
at% to
Cr Alloy 150
J
V
Simulated
Channel
Number
Number
6000
(e)
Al-O.5
3
at%
Anodized
/-+I
Cr Alloy
to 200
V
“‘----I
5 4000 s
$ $ 2000 E
d x 30
8
200 Channel
‘< .~
.
300 Number
Fig. 4. Experimental and simulated Rutherford backscattering spectra of the Al-O.5 at.% Cr alloy film sputter-deposited on to 99.99% pure electropolished aluminium; (a) as-deposited; (b) after anodizing at 50 A m-’ to 50V in 0.01 M ammonium pentaborate electrolyte at 293 K; (c) 1OOV; (d) 150 V; (e) 200 V.
.X-ray photo-electron spectroscopy (XPS)
In order to determine the chemical state of chromium species incorporated into anodic alumina, chemical sectioning of the anodic film and subsequent XPS surface analysis have been performed. On the basis of the previously reportedI dissolution rate of anodic alumina in 2 M HzS04 at 333 K, the specimen anodized to 100 V was immersed for 5.4 x IO3s, so that the film material above the chromium species containing band was dissolved and the band was located close to the surface of the film. The XPS spectrum of Cr 2p electrons obtained from the specimen sectioned chemically in this manner is shown in Fig. 8. The peak binding energy of Cr 2p3,Zelectrons is 577.0 eV, which corresponds to Cr3+ ions. 17
H. Habazaki et al.
126
f 7- ~, --r---j1--200 -
AI-O.5 at%
Cr Alloy-
ohzj 280
‘-300
320 Channel
340
360
380
Number
Fig. 5. Experimental Rutherford backscattering spectra of the Ala.5 at.% Cr alloy film sputterdeposited on to 99.99% pure electropolished aluminium as-deposited and anodized at 50 A m-* to 50, 100, 150 and 200 V in 0.01 M ammonium pentaborate electrolyte at 293 K.
DISCUSSION Interfacial
enrichment
of chromium
atoms
Anodic oxidation of many dilute aluminium alloys proceeds in two stages. In the initial stage, prior oxidation of aluminium atoms occurs and the alloying element atoms accumulate in a thin alloy layer, about l-2 nm thick, immediately beneath the anodic
Al-O.5
0
100 Anodizing Voltage
at % Cr Alloy
200 I V
Fig. 6. Change in the energy of leading edge of the yield from chromium nuclei in the Ala.5 at.% Cr alloy film sputter-deposited on to 99.99% pure electropolished aluminium as a function of voltage for anodizing at 50 A m-2 in 0.01 M ammonium pentaborate electrolyte at 293 K.
121
The incorporation and motility of chromium species
0.006
0.008
0.01
0.012
0.014
Fig. 7. Variation of the distance, r, of the chromium-containing band, normalized with respect to the total film thickness, from the metal/film interface with the reciprocal of the 6lm formation voltage, for anodizing of the Al-O.5 at.% Cr alloy superimposed on aluminium at high current efficiency in 0.01 M ammonium pentaborate electrolyte.
film.’ After critical amounts of alloying element atoms are accumulated in the thin alloy layer, aluminium and alloying element atoms are oxidized and their ions are incorporated into anodic films in their approximate alloy proportions in the presence of a steady-state composition of the enriched layer.‘8 The anodic oxidation of the present Al-O.5 at.% Cr alloy also results in prior oxidation of aluminium and accumulation of chromium atoms in an alloy layer just beneath the anodic film, without incorporation of chromium species at the available resolution up to the total consumption of the alloy layer. The total atoms in the original alloy, determined by RBS, is 1.6 x lOI atoms cmm2, and almost all the chromium atoms are incorporated into the anodic film after the development of the thin band of enrichment, which is observed directly in Figs 2 and 3. From the correlation between the Gibbs free energy per equivalent for formation of the alloying element oxides and the amounts of alloying element atoms in the steady-state enriched layer formed in dilute aluminium alloys,’ the steady-state composition of the enriched layer in dilute Al-Cr alloys is expected to be about 2.0 x lOI atoms cme2 . This is in agreement with the present finding that chromium atoms in the Al-O.5 at.% Cr alloy, about 54 nm thick, are not oxidized during anodizing of the alloy, since the chromium content in the enriched alloy layer has not reached the critical value due to the initial alloy layer being of limited thickness. Valency and mobility of chromium species in anodic alumina
It is known that anodic alumina formed on high purity aluminium in chromate electrolyte contains both Cr6+ and Cr3+ ions.” For example, in the film formed to 100 V, a relatively narrow band containing chromium species lies at a relative distance of 0.15 of the total film thickness, measured from the film/electrolyte interface (Fig. 9). The film regions above the band, extending to the film/electrolyte interface, also contain chromium species.” Since electrolyte-derived species, such as molybdate and tungstate species, are generally
H. Habazaki et al.
728
-r-
/
I
I
Cr 2p
,+.
1000
-.
8
.rz, .+
n
:_. ;
E E s +
v._
.
500
ic .I
575
.+
-.
..
-..-..,/A,.~.-.
i77.0 eV
Binding
580
Energy / eV
585
Fig. 8. X-ray photo-electron spectrum of Cr 2p electrons for the Ala.5 at.% Cr alloy film sputterdeposited on to 99.99% pure electropolished aluminium, after anodizing at 50A mm2 to 100 V in 0.01 M ammonium pentaborate electrolyte at 293 K and subsequent chemical sectioning in 2 M HzS04 at 333 K for 5.4 x IO3 s.
incorporated uniformly in the outer parts of the films, the formation of such a band is unusual and may be associated with the transformation of Cr6+ ions to Cr3+ ions in anodic alumina under the high electric field.” Using a novel tracer technique,20 the migration rate of the chromium containing band has been determined to be 0.4 of that of A13+ ions. However, the valency of chromium species in the band has not determined precisely and
Critical voltage
( for transformation
>
(100 v)
Chromate Layer containg Cr6+ ions
. . . . . . . . . . . . . .
Band containing
Cr3’
* IOIlS
Aluminium
Aluminium Fig. 9. Schematic illustration showing the distribution of chromium species in anodic alumina formed in chromate electrolyte at high current efficiency. Cr 6+ ions, assumed to be immobile, are predominantly present in the initial stages of anodizing. Further anodizing results in the transformation of Cr6 + tons, which reach to a particular depth from the film/electrolyte interface, to Cr3’ ions, which migrate outwards at a rate of 0.74 of that of A13+ ions.
The incorporation and mobility of chromium species
729
cracks and voidage were developed in the bands, possibly associated with crystalline alumina formation. In the present case, chromium species are incorporated into anodic alumina, forming a thin enriched band, at the metal/film interface; further, no chromium species are detected in the anodic film. Subsequently, the band migrates outwards, without spreading, suggesting that chromium species of only one valency are present in the band. Clearly, from XPS analysis, the chromium species are Cr3+ ions (Fig. 8). The migration rate of the Cr3+ ions, Z;:, determined from the position of the band at each anodizing voltage, is about 0.74 of that of A13+ ions in anodic alumina under the high electric field. This migration rate is larger than that for Mo6+ (ZgO= 0.5) and W6+ (I$? = 0.25) ions 4 incorporated from the respective anodizing electrolytes. Recently, Shimizu and Kobayashi” have found good correlation between the relative mobilities of various cations incorporated into anodic alumina and the corresponding single metal-oxygen bond energies. Based on their approach, the single metal-oxygen bond energies of Cr3+-0, Mo6+-0 and W6+-0 are 53.4, 85.7 and 97.2 kcal mol-‘, respectively. Thus, the faster migration of chromium species, compared with molybdenum and tungsten species, can be correlated with a weaker Cr3+-0 bond than Mo6+-0 and W6+-0 bonds. However, there is a discrepancy between the mobilities of A13+ and Cr3+ ions and their single-metal oxygen bond strengths. Although the A13+-0 bond is stronger than the Cr3+ -0 bond, the migration rate of chromium is slower than that of A13+ ions. This discrepancy may arise partly from the use of thermodynamic data for the various crystalline oxides to calculate the single metal-oxygen bonds due to the lack of relevant data for amorphous oxides. The presence of only Cr3 + ions (from the alloy) in anodic alumina suggests that the stable chromium species under the field is Cr3+ ions, although both Cr3+ and Cr6+ ions are present in anodic films formed in chromate electrolytes.‘g In the latter case, the transformation reaction of Cr6 + ions to Cr3 + ions under the field is not fast, and hence Cr6+ ions are detected in the films. From the Cr6+-0 bond energy,” Cr6+ ions are expected to migrate outwards more slowly than Cr3+ ions or to be immobile. Thus, one possibility for the formation of the chromium-enriched band in anodic alumina developed in chromate electrolytes is that a Cr6+ ion is transformed at a particular time after incorporation, that is, at a particular depth from the film/electrolyte interface. As shown schematically in Fig. 9, up to a critical voltage only Cr6+ ions, which are assumed to be immobile, are present in the film. Above a critical voltage, the transformation of Cr6+ to Cr3+ ions occurs always at a particular depth from the film/electrolyte interface. Due to the relatively faster migration of Cr3+ ions, the transformed Cr3+ ions are enriched in a thin band. From an X-ray absorption study,lg it has been confirmed that the predominant species in anodic alumina formed at relatively low voltages are Cr6+ ions. Since the precise mechanism of the transformation is not yet understood, further studies to define the unusual behaviour of chromium incorporation from electrolytes are needed.
CONCLUSIONS (1) By anodic oxidation of a sputter-deposited Al-O.5 at.% Cr alloy film, about 54 nm thick, superimposed on electropolished aluminium, a thin chromium species enriched band, which can be used as a tracer, is successfully developed in anodic alumina. This band is formed as a consequence of prior oxidation of aluminium, with chromium atoms
730
H. Habazaki er al.
accumulating in a thin alloy layer just beneath the anodic film, during anodizing of the alloy. (2) The chromium species in the anodic alumina film are predominantly Cr3+ ions, in contrast to the presence of both Cr3+ and Cr6+ ions in anodic alumina formed in chromate electrolytes. This suggests that Cr3+ species are the most stable species in the anodic film under the field. (3) The Cr3 + ions, present in a thin band of film material, migrate outwards at a rate of 0.74 of that of A13+ ions. Acknowledgements-The authors thank the Engineering and Physical Sciences Research Council for financial support and the European Commission for the provision of an Industrial Fellowship to JL.
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P. Skeldon, G.E. Thompson and G.C. Wood, Phil. Trans. R. Sot. Land. A (1996) in press. H. Habazaki, X. Zhou, K. Shimizu, P. Skeldon, G.E. Thompson and G.C. Wood, Thin Solid Films (1996) in press. H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson and G.C. Wood, submitted to Corros. Sci., in press (1997). K. Shimizu and K. Kobayashi, J. Suj. Fin& Sot. Jpn. 46, 402 (1995). L.R. Doolittle, Nucl. Instr. and Meth. B9, 344 (1985). J.C. Cheang Wang, J. Li, C. Ortega, J. Siejka, G. Vizkelethy and L. Lemaitre, Nucl. Ins@. and Merh. B64,
169 (1992). 13. K. Asami, J. Electron Specirosc. Relat. Phenom. 9, 469 (1977). 14. A.C. Harkness and L. Young, Cutr. J. Chem. 44, 2409 (1966). 15. J.P.S. Pringle, Electrochim. Actu 25, 1420 (1980). 16. H. Konno, S. Kobayashi, H. Takahashi and M. Nagayama, Electrochim. 17. K. Asami and K. Hashimoto, Corros. Sci. 17, 559 (1977).
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Printed in Great Britain. All rights mserval mm-938X/97 s17.00+0.00
THE INCORPORATION AND MOBILITY OF CHROMIUM SPECIES IN ANODIC ALUMINA FILMS H. HABAZAKI,*
K. SHIMIZU,t P. SKELDON,$ G. E. TI-IOMPSON,$ X. ZHOU,S J. DE LAETS and G. C. WOODS
* Institute for Materials Research, Tohoku University, Sendai 980-77, Japan t University Chemical Laboratory, Keio University, 4-l-l Hiyoshi, Yokohama 223, Japan $ Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 lQD, U.K. Abetract-By
anodizing of a thin layer of Al-O.5 at.% Cr alloy, cu. 54 nm thick, superimposed on electropolished superpure aluminium, chromium species are incorporated into the anodic alumina as a thin band cu. 3 mn thick. This allows the relative mobility of chromium species, with respect to aluminium ions, to be determined, providing key information to assist development of insight into ionic transport processes in amorphous anodic films under high electric fields. During anodizing of the thin alloy Slm, chromium atoms are. not oxidized and are accumulated in a layer of alloy, cu. 2 nm thick, immediately beneath the anodic film as a consequence of prior oxidation of aluminium. When the alloy film is totally consumed by anodizing, all the chromium atoms in the enriched layer are oxidized and incorporated immediately into the anodic film, forming a thin chromium species-containing band of cu. 3 mn thickness. With further anodizing, the incorporated chromium species band migrates outwards without widening. From chemical sectioning of the anodic film and KPS surface analysis, the chromium species are present as Cr3 + ions in the anodic film. The relative migration rate of Cr’ + ions, with respect to Al3 + ions, determined from the precise location of the chromium species-containing layer, using transmission electron microscopy and Rutherford backscattering spectroscopy, is 0.74.0 1997 Elsevier Science Ltd. All rights reserved Keywords: A. aluminium, B. galvanostatic, B. RBS, B. TEM, B. KPS, C. anodic films.
INTRODUCTION Barrier-type anodic films on relatively pure aluminium develop by migration of A13+ ions outwards and migration of 02-/OHions inwards under a relatively high field. For anodizing at high current efficiency, about 40% of the film thickness is formed at the film/ electrolyte interface, with the remainder generated at the metal/film interface,’ over a wide range of current density and hence electric field strength.2 These results suggest that the transport processes of anions and cations are in some way linked,3 i.e. co-operative, although the precise mechanism has not been discussed fully. During anodic film growth, it is well known that electrolyte species, derived from borate, phosphate and tungstate anions, are incorporated into the respective films at the film/ electrolyte interface. These incorporated species show a range of mobilities; tungsten species are mobile outwards, boron species are immobile and phosphorus species are mobile inwards.4 Thus, these species (boron, molybdenum, tungsten) can be used as tracers (tungsten and phosphorus species) or markers (boron species). Clearly immobile species are
Manuscript received 28 August 1996. 719
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incorporated as neutral species into the films under the high electric field, whereas inwardly mobile and outwardly mobile species are present as cation and anion species, respectively. Since the available electrolytes for anodic oxidation of aluminium at high current efficiency are limited, not all ions of interest may be incorporated from this route. Anodic oxidation of ion-implanted aluminium and aluminium alloys offers an additional approach to incorporation of tracers or markers. The mobilities of various implanted elements have been examined previously using Rutherford backscattering spectroscopy,5 although the precise migration rates of most species, migrating faster than A13+ ions, could not be determined due to their direct ejection into the electrolyte at the film/electrolyte interface during anodizing. From the anodic oxidation of sputter-deposited single phase aluminium in thickening alumina films alloys, the relative migration rates of Zr4+,6 and Ta’+,’ ions . have been determined. Recently, a novel approach, utilizing sputter deposition of a thin layer of an appropriate dilute aluminium alloy on to electropolished aluminium, with subsequent interfacial enrichment of alloying element at the alloy/film interface during anodic oxidation of the alloy, has been developed to determine the migration rates of alloying element ions in anodic alumina films.8 The anodic oxidation of many dilute aluminium alloys results in the accumulation of alloying element atoms in a thin layer of alloy, cu. 2 nm thick, immediately beneath the anodic film, as a consequence of prior oxidation of aluminium.’ When the thin alloy layer is consumed totally by anodic oxidation, the alloying element atoms in the enriched layer are abruptly oxidized due to the presence of an air-modified electropolishing film sandwiched between the alloy layer and aluminium. The oxidized elements are incorporated into the anodic film, forming a thin band, which does not widen further during anodizing. From the positions of the alloying element-containing layer, determined by Rutherford backscattering spectroscopy and direct observation using transmission electron microscopy, the precise migration rates can then be determined. Using this approach, the migration rate of Zn* + ions has been successfully determined to be about 2.3 times that of Al3 + ions.’ From the presently known mobilities of several ions, Shimizu and Kobayashi have found generally good correlations between their migration rates in anodic alumina and the corresponding metal-oxygen bond energies, ionic radii and valencies.” The mobilities of other ions do not show such good agreement, suggesting that further factors control the ionic transport processes. Consequently, it is important to determine the mobilities ofa wide range of ions in amorphous alumina in order to develop understanding of factors controlling the ionic transport processes. When chromium species are incorporated into the anodic alumina film from chromate electrolytes, they are not distributed uniformly in the outer part of the film, in contrast with the uniform distribution of molybdate and tungstate species incorporated from their electrolytes. This is considered to be due to the transformation of Cr6+ ions to Cr3+ ions within the film under the high electric field. Since the mechanism of the transformation and . the mobilities of Cr3+ and Cr6+ ions are not yet known, the unusual distribution of chromium species incorporated from an electrolyte is not well understood. In the present study, the mobility and the chemical state of chromium species incorporated from the substrate as a thin band have been determined using transmission electron microscopy (TEM), Rutherford backscattering spectroscopy (RBS) and X-ray photo-electron spectroscopy (XPS).
The incorporation and mobility of chromium species
EXPERIMENTAL
721
METHOD
Al-O.5 at.% Cr alloy films, about 54 nm thick, were prepared by DC magnetron sputtering using a Microvac 350 system. Dual targets of 99.999% aluminium and 99.9% chromium discs of 38 mm diameter were used to prepare the alloy films. The alloy films were deposited on to 99.99% pure aluminium substrates which had been electropolished at 20 V for 5 min in ethanol-perchloric acid electrolyte at less than 283 K. The composition of the alloy films was confirmed by RBS to an accuracy of about 5%. Specimens were anodized at a constant current density of 50 Am-* in stirred 0.01 M ammonium pentaborate electrolyte at 293 K to selected voltages of 50, 100, 150 and 200 V. An aluminium sheet was used as the counter electrode. Electron transparent sections of the anodized specimens were obtained using a Reichert-Jung ultramicrotome. The encapsulated specimens were trimmed initially with glass knives and suitably thin sections, about 10 nm thick, were prepared using a diamond knife. The sections were observed in a JEOL TEM 2000FX II transmission electron microscope operating at 120 kV. The specimens, as-deposited and anodized, were analyzed by RBS using a 2.0 MeV alpha particle beam supplied by the Van de Graaff accelerator of the University of Paris. The beam current and diameter were about 60 nA and 0.5 mm, respectively. Scattered particles were detected at 150” to the incident beam direction. The data were analyzed using the RUMP program,” with scaling of the stopping power for oxygen in accord with recent RBS analyses. ‘* Chemical sectioning of anodic alumina was performed in 2 M H2S04 solution at 333 K. After washing in distilled water and drying, XPS analysis was carried out by Shimazu ESCA850 electron spectrometer using Mg Kcl excitation (1253.6 eV). Binding energies of electrons were calibrated by a method described elsewhere. l3 Briefly, the binding energies of Au 4f7,* and 4f5,* electrons of gold metal, and Cu 2p 3,2and 2p1,2 electrons of copper metal were taken at 84.07, 87.74, 932.53 and 952.35 eV, respectively. The kinetic energy of Cu L3M4,5M4,5Auger electrons of copper metal was taken at 918.65 eV. Further, the shift of apparent binding energy through charging effects was calibrated using C 1s electrons of contaminant hydrocarbons (285.0 eV).
EXPERIMENTAL
RESULTS
Voltage-time response
The voltage-time response of the Al-O.5 at.% Cr alloy, superimposed on the electropolished aluminium, during anodizing at a constant current density of 50 A m-* in 0.01 M ammonium pentaborate electrolyte at 293 K is shown in Fig. 1. Two voltage surges of about 2-3 V, at the beginning of anodizing and at about 73 V, are observed. The first voltage surge is due to the presence of an air-formed film on the alloy surface and the second surge confirms the presence of an air-modified electropolishing film,* about 2-3 nm thick, sandwiched between the alloy film and the aluminium substrate. The detection of the second voltage surge reveals that the alloy film is oxidized totally by anodizing to 73 V. After the first and second voltage surges, the voltage increases linearly with time. The slopes before and after the second voltage surge are similar, about 2.3 V s- ‘, typical of that for anodizing high purity aluminium at high current efficiency. Thus, the growth of the anodic film on this specimen proceeds at high current efficiency during oxidation of the alloy and the
H. Habazaki et al.
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200 _
I
Al-O.6
I
I
I
et96 Cr Alloy
I
I
I
I
I
/ Aluminium
1 0
20
40 Time
I 8
60
60
Fig. 1. Voltage-time response of the Ala.5 at.% Cr alloy film sputter-deposited on to 99.99% pure aluminium during anodizing at 50 A m-’ to 200 V in 0.01 M ammonium pentaborate electrolyte at 293 K.
underlying aluminium substrate. Assuming a nm V- ’ of 1.214and a Pilling-Bedworth ratio of 1.61 I5 for anodic alumina formed on the Al-0 alloy, the average thickness of the original alloy film is estimated to be 54nm, although the original alloy surface was probably relatively rough,’ due to development of a columnar, crystallographic structure during sputtering. In the voltage-time response of sputter-deposited Al-Cr alloys containing about 1.5 at.% chromium, the linear relationship between the voltage and anodizing time is not observed and the slope gradually increases with time. Unpublished work of the authors suggests that the increasing slope is associated with the formation of flaws in the anodic film. Such gradual change in the slope is not detected in the voltage-time response of the present alloy superimposed on aluminium, suggesting relatively uniform film formation over the macroscopic surface of the dilute alloy. Transmission electron microscopy (TEM) A transmission electron micrograph of an ultramicrotomed section of the specimen anodized to 100 V is shown in Fig. 2. The anodic film is amorphous and has relatively flat and parallel metal/film and film/electrolyte interfaces. The film is present on aluminium, indicating that the thin alloy layer has already been consumed. The thickness of the film is 118 f 3 nm, corresponding to a nm V- ’ ratio of 1.18 & 0.03, which is closely similar to that for films on high purity aluminium (nm V- ’ ratio of 1.2).14 In the anodic film of generally featureless and uniform appearance, thin light and dark bands are present, located about 15% and 25% of the film thickness, respectively, measured from the metal/film interface. The light band composed of voids, probably contains argon bubbles, originally incorporated at, or near, the interface between the alloy film and the underlying aluminium substrate.* In agreement with a previous finding that argon bubbles are immobile in growing anodic alumina films,’ the bubbles in the present film are also immobile during film growth. The dark band, about 3 nm thickness, is considered to be the chromium species-containing band. The presence of chromium species in the band is
123
The incorporation and mobility of chromium species
‘CrBand Voids Band
Fig. 2. Transmission electron micrograph of an ultramicrotomed section of the Al-O.5 at.% Cr alloy film sputter-deposited on to 99.99% pure electropolished ahmdnium after anodizing at 50 A me2 to 100 V in 0.01 M ammonium pentaborate electrolyte at 293 K.
confirmed by later RBS. The relatively dark appearance of the band is due to enhanced electron scattering from chromium species, compared with aluminium ions of reduced atomic number. The formation of this narrow band indicates that during anodic oxidation of the Al-G alloy almost no chromium species are incorporated into the anodic film and chromium atoms are accumulated in a thin layer, about 2 mu thick, just beneath the anodic film, as a consequence of prior oxidation of aluminium. The accumulated chromium atoms are oxidized immediately when the alloy film is consumed totally by anodizing.* From the positions of the chromium species-containing band and the immobile light band or marker, it is clear that chromium species migrate outwards in the growing alumina film. In the film formed to 150 V (Fig. 3), the dark band, of similar thickness, is present at about 46% of the film thickness, measured from the metal/film interface, although the light band is not revealed so clearly and knife marks, generated during ultramicrotomy, run almost parallel to the metal/film interface. Rutherford backscattering
spectroscopy
(RBS)
The experimental and simulated RBS spectra of as-deposited specimens and specimens anodized to several voltages are shown in Fig. 4. A region of the experimental spectra for elastic scattering from chromium is revealed in greater detail in Fig. 5. The peak energy of the yield for chromium shifts to lower energies with anodizing voltage. In particular, the shift is larger during anodizing of the alloy up to about 70 V (Fig. 6). The change in the energy shift at about 70 V indicates that chromium species are incorporated into the anodic film at this voltage. This voltage is closely similar to that of the second voltage surge observed in Fig. 1. The shift of the peak energy to lower energy with increasing anodizing voltage from 50 to 200 V reveals the slower migration rate of chromium species, compared with A13+ ions. From the simulated spectra (Fig. 4), which fit well the corresponding experimental spectra, the precise position and composition of the chromium-containing
H. Habazaki et al.
724
Cr Band Voids Band
Fig. 3. Transmission electron micrograph of an ultramicrotomed section of the Al-O.5 at.% Cr alloy film sputter-deposited on to 99.99% pure electropolished aluminium after anodizing at 50 A me2 to 150 V in 0.01 M ammonium pentaborate electrolyte at 293 K.
bands are obtained as well as the total thickness of the films formed to different voltages. These data, which are consistent with those obtained by TEM observation, are summarized in Table I. The distance, r, of the chromium-containing band, normalized to the total film thickness, measured from the metal/film interface at a particular voltage, P’, can be expressed as follows:’ r = (0.6 + 0.4 x I,“:)(1 - Vo/V) in which Ve is the voltage at which chromium species are incorporated into the film, that is about 73 V from Fig. 1, and Zp is the relative mobility of chromium species with respect to A13+ ions; the linear relationship between r and V-’ is shown in Fig. 7. From the slope of the linear region, the relative migration rate of chromium species, with respect to A13+ ions, is 0.74 f 0.05.
Table 1. Film thicknesses, positions and compositions of the chromium-containing bands in the anodic films developed by anodizing at 50 A me2 in 0.0 1M ammonium pentaborate electrolyte at 293 K to various voltages Anodizing voltage (VI 50 100 150 200
Film thickness (nm)
Normalized position of the chromium band, r
Atomic composition of the chromium band [Cr]/[Cr + Al]
63+3 118+3 183+3 242&3
0 0.25 +0.02 0.46 + 0.02 0.57+0.02
0.15_+0.01 0.15+0.01 0.15~0.01
125
The incorporation and mobility of chromium species 6000
‘-WV(b)
Al-O.5
at%
Anodized
r-
Cr Alloy
to 50
V
‘i
Measured
-
-
Simulated
Simulated 200 Channel
300 Channel
Number
300 Number
I_I_II/T_
(
ON
Al-O.5
Anodized
-
Simulated
-
Channel
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at% to
Cr Alloy 150
J
V
Simulated
Channel
Number
Number
6000
(e)
Al-O.5
3
at%
Anodized
/-+I
Cr Alloy
to 200
V
“‘----I
5 4000 s
$ $ 2000 E
d x 30
8
200 Channel
‘< .~
.
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Fig. 4. Experimental and simulated Rutherford backscattering spectra of the Al-O.5 at.% Cr alloy film sputter-deposited on to 99.99% pure electropolished aluminium; (a) as-deposited; (b) after anodizing at 50 A m-’ to 50V in 0.01 M ammonium pentaborate electrolyte at 293 K; (c) 1OOV; (d) 150 V; (e) 200 V.
.X-ray photo-electron spectroscopy (XPS)
In order to determine the chemical state of chromium species incorporated into anodic alumina, chemical sectioning of the anodic film and subsequent XPS surface analysis have been performed. On the basis of the previously reportedI dissolution rate of anodic alumina in 2 M HzS04 at 333 K, the specimen anodized to 100 V was immersed for 5.4 x IO3s, so that the film material above the chromium species containing band was dissolved and the band was located close to the surface of the film. The XPS spectrum of Cr 2p electrons obtained from the specimen sectioned chemically in this manner is shown in Fig. 8. The peak binding energy of Cr 2p3,Zelectrons is 577.0 eV, which corresponds to Cr3+ ions. 17
H. Habazaki et al.
126
f 7- ~, --r---j1--200 -
AI-O.5 at%
Cr Alloy-
ohzj 280
‘-300
320 Channel
340
360
380
Number
Fig. 5. Experimental Rutherford backscattering spectra of the Ala.5 at.% Cr alloy film sputterdeposited on to 99.99% pure electropolished aluminium as-deposited and anodized at 50 A m-* to 50, 100, 150 and 200 V in 0.01 M ammonium pentaborate electrolyte at 293 K.
DISCUSSION Interfacial
enrichment
of chromium
atoms
Anodic oxidation of many dilute aluminium alloys proceeds in two stages. In the initial stage, prior oxidation of aluminium atoms occurs and the alloying element atoms accumulate in a thin alloy layer, about l-2 nm thick, immediately beneath the anodic
Al-O.5
0
100 Anodizing Voltage
at % Cr Alloy
200 I V
Fig. 6. Change in the energy of leading edge of the yield from chromium nuclei in the Ala.5 at.% Cr alloy film sputter-deposited on to 99.99% pure electropolished aluminium as a function of voltage for anodizing at 50 A m-2 in 0.01 M ammonium pentaborate electrolyte at 293 K.
121
The incorporation and motility of chromium species
0.006
0.008
0.01
0.012
0.014
Fig. 7. Variation of the distance, r, of the chromium-containing band, normalized with respect to the total film thickness, from the metal/film interface with the reciprocal of the 6lm formation voltage, for anodizing of the Al-O.5 at.% Cr alloy superimposed on aluminium at high current efficiency in 0.01 M ammonium pentaborate electrolyte.
film.’ After critical amounts of alloying element atoms are accumulated in the thin alloy layer, aluminium and alloying element atoms are oxidized and their ions are incorporated into anodic films in their approximate alloy proportions in the presence of a steady-state composition of the enriched layer.‘8 The anodic oxidation of the present Al-O.5 at.% Cr alloy also results in prior oxidation of aluminium and accumulation of chromium atoms in an alloy layer just beneath the anodic film, without incorporation of chromium species at the available resolution up to the total consumption of the alloy layer. The total atoms in the original alloy, determined by RBS, is 1.6 x lOI atoms cmm2, and almost all the chromium atoms are incorporated into the anodic film after the development of the thin band of enrichment, which is observed directly in Figs 2 and 3. From the correlation between the Gibbs free energy per equivalent for formation of the alloying element oxides and the amounts of alloying element atoms in the steady-state enriched layer formed in dilute aluminium alloys,’ the steady-state composition of the enriched layer in dilute Al-Cr alloys is expected to be about 2.0 x lOI atoms cme2 . This is in agreement with the present finding that chromium atoms in the Al-O.5 at.% Cr alloy, about 54 nm thick, are not oxidized during anodizing of the alloy, since the chromium content in the enriched alloy layer has not reached the critical value due to the initial alloy layer being of limited thickness. Valency and mobility of chromium species in anodic alumina
It is known that anodic alumina formed on high purity aluminium in chromate electrolyte contains both Cr6+ and Cr3+ ions.” For example, in the film formed to 100 V, a relatively narrow band containing chromium species lies at a relative distance of 0.15 of the total film thickness, measured from the film/electrolyte interface (Fig. 9). The film regions above the band, extending to the film/electrolyte interface, also contain chromium species.” Since electrolyte-derived species, such as molybdate and tungstate species, are generally
H. Habazaki et al.
728
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/
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Cr 2p
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E E s +
v._
.
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ic .I
575
.+
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..
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Binding
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Energy / eV
585
Fig. 8. X-ray photo-electron spectrum of Cr 2p electrons for the Ala.5 at.% Cr alloy film sputterdeposited on to 99.99% pure electropolished aluminium, after anodizing at 50A mm2 to 100 V in 0.01 M ammonium pentaborate electrolyte at 293 K and subsequent chemical sectioning in 2 M HzS04 at 333 K for 5.4 x IO3 s.
incorporated uniformly in the outer parts of the films, the formation of such a band is unusual and may be associated with the transformation of Cr6+ ions to Cr3+ ions in anodic alumina under the high electric field.” Using a novel tracer technique,20 the migration rate of the chromium containing band has been determined to be 0.4 of that of A13+ ions. However, the valency of chromium species in the band has not determined precisely and
Critical voltage
( for transformation
>
(100 v)
Chromate Layer containg Cr6+ ions
. . . . . . . . . . . . . .
Band containing
Cr3’
* IOIlS
Aluminium
Aluminium Fig. 9. Schematic illustration showing the distribution of chromium species in anodic alumina formed in chromate electrolyte at high current efficiency. Cr 6+ ions, assumed to be immobile, are predominantly present in the initial stages of anodizing. Further anodizing results in the transformation of Cr6 + tons, which reach to a particular depth from the film/electrolyte interface, to Cr3’ ions, which migrate outwards at a rate of 0.74 of that of A13+ ions.
The incorporation and mobility of chromium species
729
cracks and voidage were developed in the bands, possibly associated with crystalline alumina formation. In the present case, chromium species are incorporated into anodic alumina, forming a thin enriched band, at the metal/film interface; further, no chromium species are detected in the anodic film. Subsequently, the band migrates outwards, without spreading, suggesting that chromium species of only one valency are present in the band. Clearly, from XPS analysis, the chromium species are Cr3+ ions (Fig. 8). The migration rate of the Cr3+ ions, Z;:, determined from the position of the band at each anodizing voltage, is about 0.74 of that of A13+ ions in anodic alumina under the high electric field. This migration rate is larger than that for Mo6+ (ZgO= 0.5) and W6+ (I$? = 0.25) ions 4 incorporated from the respective anodizing electrolytes. Recently, Shimizu and Kobayashi” have found good correlation between the relative mobilities of various cations incorporated into anodic alumina and the corresponding single metal-oxygen bond energies. Based on their approach, the single metal-oxygen bond energies of Cr3+-0, Mo6+-0 and W6+-0 are 53.4, 85.7 and 97.2 kcal mol-‘, respectively. Thus, the faster migration of chromium species, compared with molybdenum and tungsten species, can be correlated with a weaker Cr3+-0 bond than Mo6+-0 and W6+-0 bonds. However, there is a discrepancy between the mobilities of A13+ and Cr3+ ions and their single-metal oxygen bond strengths. Although the A13+-0 bond is stronger than the Cr3+ -0 bond, the migration rate of chromium is slower than that of A13+ ions. This discrepancy may arise partly from the use of thermodynamic data for the various crystalline oxides to calculate the single metal-oxygen bonds due to the lack of relevant data for amorphous oxides. The presence of only Cr3 + ions (from the alloy) in anodic alumina suggests that the stable chromium species under the field is Cr3+ ions, although both Cr3+ and Cr6+ ions are present in anodic films formed in chromate electrolytes.‘g In the latter case, the transformation reaction of Cr6 + ions to Cr3 + ions under the field is not fast, and hence Cr6+ ions are detected in the films. From the Cr6+-0 bond energy,” Cr6+ ions are expected to migrate outwards more slowly than Cr3+ ions or to be immobile. Thus, one possibility for the formation of the chromium-enriched band in anodic alumina developed in chromate electrolytes is that a Cr6+ ion is transformed at a particular time after incorporation, that is, at a particular depth from the film/electrolyte interface. As shown schematically in Fig. 9, up to a critical voltage only Cr6+ ions, which are assumed to be immobile, are present in the film. Above a critical voltage, the transformation of Cr6+ to Cr3+ ions occurs always at a particular depth from the film/electrolyte interface. Due to the relatively faster migration of Cr3+ ions, the transformed Cr3+ ions are enriched in a thin band. From an X-ray absorption study,lg it has been confirmed that the predominant species in anodic alumina formed at relatively low voltages are Cr6+ ions. Since the precise mechanism of the transformation is not yet understood, further studies to define the unusual behaviour of chromium incorporation from electrolytes are needed.
CONCLUSIONS (1) By anodic oxidation of a sputter-deposited Al-O.5 at.% Cr alloy film, about 54 nm thick, superimposed on electropolished aluminium, a thin chromium species enriched band, which can be used as a tracer, is successfully developed in anodic alumina. This band is formed as a consequence of prior oxidation of aluminium, with chromium atoms
730
H. Habazaki er al.
accumulating in a thin alloy layer just beneath the anodic film, during anodizing of the alloy. (2) The chromium species in the anodic alumina film are predominantly Cr3+ ions, in contrast to the presence of both Cr3+ and Cr6+ ions in anodic alumina formed in chromate electrolytes. This suggests that Cr3+ species are the most stable species in the anodic film under the field. (3) The Cr3 + ions, present in a thin band of film material, migrate outwards at a rate of 0.74 of that of A13+ ions. Acknowledgements-The authors thank the Engineering and Physical Sciences Research Council for financial support and the European Commission for the provision of an Industrial Fellowship to JL.
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P. Skeldon, G.E. Thompson and G.C. Wood, Phil. Trans. R. Sot. Land. A (1996) in press. H. Habazaki, X. Zhou, K. Shimizu, P. Skeldon, G.E. Thompson and G.C. Wood, Thin Solid Films (1996) in press. H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson and G.C. Wood, submitted to Corros. Sci., in press (1997). K. Shimizu and K. Kobayashi, J. Suj. Fin& Sot. Jpn. 46, 402 (1995). L.R. Doolittle, Nucl. Instr. and Meth. B9, 344 (1985). J.C. Cheang Wang, J. Li, C. Ortega, J. Siejka, G. Vizkelethy and L. Lemaitre, Nucl. Ins@. and Merh. B64,
169 (1992). 13. K. Asami, J. Electron Specirosc. Relat. Phenom. 9, 469 (1977). 14. A.C. Harkness and L. Young, Cutr. J. Chem. 44, 2409 (1966). 15. J.P.S. Pringle, Electrochim. Actu 25, 1420 (1980). 16. H. Konno, S. Kobayashi, H. Takahashi and M. Nagayama, Electrochim. 17. K. Asami and K. Hashimoto, Corros. Sci. 17, 559 (1977).
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18. H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson and G.C. Wood, Phil. Msg. B 73, 445 (1996). 19. S.W.M. Chung, J. Robinson, G.E. Thompson, G.C. Wood and H.S. Isaacs, Phi/. Msg. B 63, 557 (1991). 20. P. Skeldon, K. Shimizu, G.E. Thompson and G.C. Wood, Phil. Trans. R. Sot. Lond. A 348,295 (1994).