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Microstructure of anodic barrier films on aluminium
METALLOGRAPHY
4, 403-414 (1971)
403
Microstructure of Anodic Barrier Films on Aluminium
I. E. K L E I N AND A. E. YANIV
Armament Development Authority, Israel Ministry of Defence, Tel Aviv, Israel
Optical and electron microscope examinations have been carried out on anodic oxide films formed on aluminium in H3BO3/Na~B407 solutions under various temperature and formation voltage conditions. Whereas at low temperatures and voltages a barrier type of film was produced, an increase in either parameter encouraged crystallization and porosity. The crystalline phase appeared as agglomerates in an amorphous matrix. Electron and x-ray diffraction showed that the crystalline phase was not homogeneous, consisting of y'-AI203, 7'-A120~, and at least one of the compounds boehmite or ~-A1203. A model is proposed for the interrelation between the amorphous and the crystalline phases in the film. This model is aimed to explain discrepancies in the literature.
GeJiige von anodischen Deckschischen auf Aluminium Auf Aluminium anodisch in HnBOs/Na2B4OT-L6sungen bei verschiedenen Temperaturen und Abscheidungsspannungen erzeugte Oxydschichten wurden optisch und elektronenmikroskopisch untersucht. W~ihrend bei tiefen Temperaturen und kleinen Spannungen ein dichter Film enstand, begfinstigte eine Zunahme der Temperatur oder der Spannung die Kristallisation und damit die Porosit~it. Die kristalline Phase lag in Form yon Agglomeraten in einer amorphen Matrix vor. Elektronen- und R6ntgenbeugungsaufnahmen zeigten, dab die kristalline Phase nicht homogen war, sondern aus ~,'-AI~O~, y-AlzO3, und mindestens einer der beiden Verbindungen B6hmit oder ~.-AI,Oz bestand. Ein Modell tiber die Beziehungen zwischen der amorpben und den kristallinen Phasen des Films wird vorgeschlagen. Dieses Modell wird beniJtzt, um Widersprtiche in der Literatur zu erkl~iren.
Microstructures des films de barriOre anodique dans l' aluminium Des observations par microscopie optique et ~lectronique furent entreprises sur des films d'oxyde anodique qui se forment sur de l'aluminium plong~ dans des solutions H~BOa/Na2B~O7 sous diff~rentes conditions de temperature et de voltage. Alors qu'~ basses temperatures et qu'h bas voltages une barri~re typique se d6veloppe, une augmentation de l'un ou l'autre de ces param~tres favorise la cristallisation et la porosit& La phase cristalline appara]t comme un agglom6rat dans la matrice amorphe. La diffraction des rayons-X et 61ectronique montrent que la phase cristalline n'est pas homog~ne et qu'elle se compose de ~,'-A1203, de y-A1203 et au moins d'un des compos~s de la bo~hmite ou de ~-A12On. On propose un mod61e pour expliquer la relation mutuelle entre les phases amorphes et cristallines dans le film afin de rationaliser les divergences que l'on relive dans la litt~rature. Copyright © 1971 by American Elsevier Publishing Company, Inc.
404
L E. Klein and -4. E. Yaniv
Introduction Barrier anodic films on aluminium are usually formed in neutral or slightly acidic solutions, the most common being H3BO3/Na2B4OT, NH4-tartrate , (NH4)zB4Ov, NH4B5Os, monophosphates, and certain dicarboxylic acids--for example, succinic and glutaric. The structure of a barrier film may be considered a particular case of a porous film. Setoh and Miyata 1 showed in 1932 that the anodic film on aluminium consists of two layers, the porous thick outer layer growing on an inner thin barrier or dielectric layer. The porous film is of a fibrous nature, each fiber having a hexagonal cross section with a pore in its center. At present two models for the formation of a porous anodic film on aluminium exist--the geometric model, proposed by Keller et al., ~ and the colloidal model of Murphy and Michelson. 3-5 Although the hexagonal structure of the film was attributed to the mechanism of pore formation in both models, it has been shown ~ that porefree barrier films have a hexagonal cross section too. Cell diameters of a barrier film formed at high voltages and of a porous film formed at low voltages but extrapolated to the same values were found to be equal ;7 anodizing under barrier conditions but for prolonged periods of time (10 minutes) or at higher temperatures leads to pore formation, s-l° suggesting that the same basic mechanism exists for barrier and porous film formation. The crystallographic structure of the barrier films was recently reviewed by Diggle et al. xl A few works that have been omitted in this review are worth mentioning. Brandenberger and H~ifeliTM correlated the structure of the barrier film with its formation voltage and found that boehmite was formed at low voltages (0.5 to 20 volts), crystalline y-AI~O3 and y'-AI~Oa at high voltages (580 to 600 volts), and all three structures in the intermediate range. Beck et al., TM on the other hand, reported that the barrier film formed in a solution of NH4-tartrate in ethanol was amorphous even when anodizing was carried out at 1200 volts. Van Geel and Schelen 14 and later Franklin 15 found a dependence of the structure on the temperature of the electrolyte; an amorphous film was formed at room temperature, but at 100°C )/-AI~Oa was formed. Dorsey, TM using ultramicrotomy and investigating the sections under an electron microscope, has shown that the barrier film consists of two layers, a compact, thick, impermeable lower layer and a thin layer permeable to the electrolyte, the latter layer thickening at elevated temperatures. Unfortunately, he did not perform diffraction analysis, and, although the physical properties of the layers were evaluated, they lack a final identification. Beck et al. TM also found two layers and reported a similar relative increase of one of the layers with voltage and temperature increase. Although there is an extensive literature on barrier anodic films on aluminium, no definite data are available on the influence of the formation voltage and electro-
Anodie Barrier Films
405
lyte temperature changes on their structure. There are scarcely any data available on the distinction of the crystallographic phases by x-ray diffraction studies. In the present investigation we have tried to elucidate these points and to add some information on the controversial crystallographic composition of barrier films on aluminium.
Experimental Procedure FI3BO3 and Na2B407 were of Analar grade; aluminium specimens were 99.99 pure; the H20 used was double-distilled. The specimens were in the shape of a flag, the stem being enclosed in a glass tube sealed with RTV silicone rubber. The exposed surface was 0.5 to 2 cm 2 for small specimens and 30 cm z for large ones, the latter being used for stripping the oxide. The reaction vessel was heated by means of a heating mantle, and the temperature was controlled by a Variac autotransformer. Prior to anodizing, the specimens were degreased, alkali-cleaned in a proprietary cleaning solution (Rokleen N O N 40-, a product of R. O. Hull & Co., Cleveland, Ohio), and dipped in 60% w/w H N O s for 1 minute. After each step the specimens were rinsed in distilled water for 2 minutes. Anodizing was carried out under constant-voltage conditions at various potentials up to 500 volts and under constant-current conditions of 1 mA/cm 2. For constant-voltage experiments a Lambda regulated power supply Model 50, 500 volts, 500 mA, was used. Constant current was provided by a Lambda dual regulated power supply L P D 425FMV, consisting of two units, each 250 volts, 130 mA, connected in series or in parallel. On switching on the current, the voltage rose up to a predetermined value, then remained constant while the current decreased. A 3~o H3BO3+0.05~o Na2B~O7 anodizing solution was used throughout. The temperature range was 20 ° to 95°C, increasing in increments of 15°±1°C. The surface of the anodized specimen was examined with a Reichert MeF microscope under natural and polarized light to distinguish between the various phases of the film. After dissolution of the oxide in a CrO3-H3PO 4 solution, the metal surface was examined under 2000 x magnification, using oil immersion. A J.E.M. 7 electron microscope was used for electron microscopy and electron diffraction studies. Since maximum thickness of the film allowable for transmission is ~1000/~, the anodized specimens were produced at maximum 70 volts. X-ray diffraction patterns were obtained by the Debye-Scherrer method with exposures up to 24 hours. Anodizing was carried out at a constant current density of 1 mA/cm 2 up to 500 volts; the oxide was then stripped, using the HgC12 method, dried in a vacuum oven, ground, and enclosed in a capillary tube.
406
I.E. Klein and A. E. Yaniv
Results
Optical Microscopy The optical micrographs of the upper surfaces of films produced under constant-voltage conditions are given in Figs. 1 and 2 and show the following tendencies: (1) At constant temperature, an increase in formation voltage led to an increase of the crystalline-phase component. (2) At a constant formation voltage, an increase in temperature likewise caused an increase of the crystalline phase. (3) Films formed at 500 volts were crystalline at all temperatures.
FIG. 1. Polarized light micrograph of the surface of a film formed at 20°C under constant voltage of (a) 100 volts, (b) 500 volts. The crystalline phase, which appeared dark in natural light micrographs and bright in polarized light micrographs, took the form of agglomerates in the amorphous phase matrix. It should be emphasized that the crystalline agglomerates detected may exist below the film surface, since it has been proved, using ellipsometric measurements, that the amorphous oxide is transparent. 17' 18 The micrograph of the metal surface reveals hexagonal cells of diameters given in Table I. A representative cell structure micrograph is given in Fig. 3.
Anodic Barrier Films
407
Fro. 2. Polarized light micrograph of the surface of a film formed at 95°C under constant voltage of (a) 100 volts, (b) 500 volts. I t can be seen f r o m the table that the cell d i a m e t e r increased with f o r m a t i o n voltage and bath t e m p e r a t u r e , except for films f o r m e d at 95°C. T h i s is in a g r e e m e n t w i t h Franklin, a5 b u t its reason is not at present known. TABLE I CELL DIAMETER (MICRONS) AS FUNCTION OF FORMATION VOLTAGE AND BATH TEMPERATURE
Formation voltage, V
20°C
35°C
Bath temperature 50°C 65°C 80°C
95°C
200 300 400 500
0.3 0.5 0,7 0.9
0.4 0.6 0,8 0.8
0.5 0.6 0,8 0.8
0.7 0.7 0.7 0.7
0.5 0.6 0.7 0.9
0.6 0.6 0.7 0.8
Electron Microscopy F i l m s p r o d u c e d at a constant voltage of 70 volts in the t e m p e r a t u r e range 20 ° to 95°C were e x a m i n e d by t r a n s m i s s i o n ; representative m i c r o g r a p h s are
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L E. Klein and A. E. Yaniv
given in Figs. 4 and 5. At 20°C there are a very few crystalline regions, which show up in the figures as dark dots (thicker or more absorbent regions). With temperature increase the crystalline domains increase, and at 95°C the whole film is crystalline. Magnification up to 100,000 x did not reveal the presence of pores, except for a few white spots in the crystalline phase of the 80°C and 95°C specimens. The appearance of pores at higher anodizing temperatures, already reported by Leach and Neufeld, x° explains some of the electrical properties of the film. 19 In these micrographs, lines of natural oxide can be seen in the anodic film, having been embedded in the metal during rolling.
FIG. 3. Cell structure of an anodic film. The resistance of the electrolyte decreased with temperature increase, resulting in higher initial current densities. As a result, heating up will occur and may be the cause of crystallization. To find out whether crystallization is due primarily to thermal or to electrochemical effects, two series of experiments were carried out. First, anodizing was carried out at 20°C in a 5% H~BOa+0.08 % Na2B4Or solution, which is saturated at this temperature and has a resistance lower than that of a 3% H3BO3+0.05 % NaoB40 7 solution at 20°C. Although anodizing was
FIG. 4.
Electron micrograph of a film formed under 70 volts at 35°C.
FIc. 5.
Electron micrograph of a film formed under 70 volts at 80°C.
FIG. 6. Electron micrograph of a film formed under 70 volts at 20°C in 5% H~BOa+ 0.08% NazB4OT.
FIo. 7.
Electron m i c r o g r a p h of a film f o r m e d u n d e r 1 m A / c m ~ u p to 60 volts at 35°C.
FIG. 8.
Electron m i c r o g r a p h of a film f o r m e d u n d e r 1 m A / c m z u p to 60 volts at 80°C.
FIa. 9. Electron diffraction p a t t e r n o f a crystalline region of a film f o r m e d u n d e r 70 volts at 50°C.
Anodic Barrier Films
411
carried out at a higher current density, the film produced was still completely amorphous, as shown in Fig. 6. In the second set of experiments, anodizing was carried out in the more dilute electrolyte at 1 mA/cm ~ up to 60 volts and the bath temperature was varied. It was found that at room temperature the anodic film was completely amorphous, and with rising temperature the amount of the crystalline phase increased, until at 95°C the film was completely crystalline (Figs. 7 and 8). It was therefore concluded that, under the prevailing conditions, crystallization is governed by the temperature of the electrolyte.
Electron Diffraction Typical electron diffraction patterns are given in Figs. 9 and 10. It can be seen from the discontinuity in the circumferential dotted line that the oxide crystals are large. The results are given in Table II.
Fro. 10. Electron diffraction pattern of an a m o r p h o u s region of a film formed u n d e r same conditions.
There is good agreement between these findings and the 7'-Al~Oz reported in the literature. The three lines for the largest interplanar distances do not belong to 7'-AlzO3; two of them belong to 7-A1203, and the other is unaccounted for.
X-Ray Diffraction The results of x-ray measurements are given in Table II. In addition to ),'-Al~O3 and ),-A1203, which undoubtedly are the major components of the crystalline phase, there are many lines corresponding to at least one of the phases boehmite [~-AI2(OOH)z], or ~-A1203.
412
I . E . Klein and A. E. Yaniv
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Anodic Barrier Films
413
Discussion Optical and electron microscopy have shown t h a t films produced at low temperatures and voltages are amorphous and compact, while high temperatures and voltages encourage crystallization and porosity. Crystallization is governed mainly by the temperature of the electrolyte. It is quite obvious that these transformations are not caused by a uniform heating up of the specimens to a few hundred degrees (where thermal crystallization will occur), because an increase of a few tens of degrees has a drastic effect on the crystallographic structure. This strong influence of higher temperatures is probably due to an increase in the solubility of the oxide, 1° leading to formation of a porous structure, with A13+ ions moving into solution. Crystallization can be caused as follows: (1) High local current densities in the pores give rise to high temperature and hence crystallinity. This was postulated by Zahavi *° when anodizing in Na,SO 4 solutions. (2)A1 a+ moving into solution may act as a nucleator for the transformation from an amorphous to a more thermodynamically stable crystalline phase. Anodizing at room temperature in an electrolyte contaminated by minute quantities (1 to 2 ppm) of AI3+ was found to produce a film with some crystalline domains but further work is needed to distinguish between the two mechanisms. Electron diffraction of films produced at low voltages has shown that the crystalline phase is y+7/-Al2Oa with an unexplained line of interplanar distance 3.28, which could be from boehmite. X-ray diffraction of films produced at 500 volts shows that, in addition to y+9/-Al,Oa, at least one of the compounds boehmite or :~-A1,Oa exists in films formed both at 20°C and 95°C. Brandenberger and H~ifeliTM have found boehmite particularly at low formation voltages. Although ~-Al,O a is the most stable of the aluminium oxides, no data could be found in the literature on its appearance in anodic films. Nevertheless, the presence of at least one of these phases is indicated in the present work. The crystalline phase appears as agglomerates in an amorphous matrix, which is in agreement with Franklin .1 and Stirland and Bicknell, 7 but the two-layer structure suggested by Altenpoh122 and Dorsey TM was not revealed.
Summary On the basis of these results and results obtained from electrical measurements of anodic barrier filmsfl we propose a model consisting of two layers, in which the bottom layer of the barrier film is amorphous, and the upper layer is composed of crystalline agglomerates in an amorphous matrix. With increase of temperature of the electrolyte or of formation voltage, the upper layer becomes completely crystalline and may penetrate into the bottom layer. Since Dorsey's work was carried out at 60°C in a 2 M H3BOa solution, ultramicrotomic section
414
I . E . Klein and A . E. Yaniv
did not show agglomerates. On the other hand, Franklin and Stirland et al. worked at room temperature and examined their films by surface micrography; they detected agglomerates, b u t could not have revealed a two-layer structure.
Acknowledgment We wish to thank Professor M. Folman of the Technion Israel Institute of Technology, Haifa, for many helpful discussions.
References 1. S. Setoh and A. Miyata, Sci. Papers lnst. Phys. Chem. Res. Tokyo, 17 (1932) 189. 2. F. Keller, M. S. Hunter, and D. L. Robinson, ft. Electrochem. Soc., 100 (1953) 411. 3. J. F. Murphy and C. E. Michelson, A D A Conference on Anodizing AI, Nottingham, 1961. The Aluminium Development Association, London (1962). 4. J. F. Murphy, Plating, 54 (1967) 1241. 5. C. E. Michelson, jr. Electrochem. Soc., 115 (1968) 213. 6. J. C. Grosskreutz and G. G. Shaw, ft. Appl. Phys., 35 (1964) 2195. 7. D. J. Stirland and R. W. Bicknell, ft. Electrochem. Soc., 106 (1959) 481. 8. T. P. Hoar and J. Yahalom, .7. Electrochem. Soc., 110 (1963) 614. 9. A. J. Brock and G. C. Wood, Electrochim. Acta, 12 (1967) 395. 10. J. S. L. Leach and P. Neufeld, Corrosion Sci., 9 (1969) 413. 11. J. W. Diggle, T. C. Downie, and C. W. Goulding, Chem. Rev., 69 (1969) 365. 12. E. Brandenberger and R. J. H/ifeli, Helv. Chim. Acta, 31 (1948) 1168. 13. T. R. Beck, D. W. Hamilton, and R. B. Gillette, Electrochem. Soc. Meeting, Dallas, Texas (1967). 14. W. Ch. Van Geel and B. J. J. Schelen, Philips Res. Repts., 12 (1957) 240. 15. R. W. Franklin, Nature, 180 (1957) 1470. 16. G. A. Dorsey, Jr., .7. Electrochem. Soc., 116 (1969) 466. 17. M. A. Barrett and A. B. Winterbottom, First. Intern. Congr. Met. Corr., London, 1961. L. Kenworthy, Editor. Butterworths, London (1962). 18. R. M. Goldstein, R. J. Lederich, and F. W. Leonhard, ft. Electrochem. Soc., 117 (1970) 503. 19. I. E. Klein, M.Sc. Thesis, Technion I.I.T. (1970). 20. J. Zahavi, D.Sc. Thesis, Technion I.I.T. (1970). 21. R.W. Franklin, A D A Conference on Anodizing AI, Nottingham, 1961. The Aluminium Development Association, London, 1962. 22. D. Altenpohl, Convention Record IRE, 3 (1954) 35. Accepted March 16, 1971
4, 403-414 (1971)
403
Microstructure of Anodic Barrier Films on Aluminium
I. E. K L E I N AND A. E. YANIV
Armament Development Authority, Israel Ministry of Defence, Tel Aviv, Israel
Optical and electron microscope examinations have been carried out on anodic oxide films formed on aluminium in H3BO3/Na~B407 solutions under various temperature and formation voltage conditions. Whereas at low temperatures and voltages a barrier type of film was produced, an increase in either parameter encouraged crystallization and porosity. The crystalline phase appeared as agglomerates in an amorphous matrix. Electron and x-ray diffraction showed that the crystalline phase was not homogeneous, consisting of y'-AI203, 7'-A120~, and at least one of the compounds boehmite or ~-A1203. A model is proposed for the interrelation between the amorphous and the crystalline phases in the film. This model is aimed to explain discrepancies in the literature.
GeJiige von anodischen Deckschischen auf Aluminium Auf Aluminium anodisch in HnBOs/Na2B4OT-L6sungen bei verschiedenen Temperaturen und Abscheidungsspannungen erzeugte Oxydschichten wurden optisch und elektronenmikroskopisch untersucht. W~ihrend bei tiefen Temperaturen und kleinen Spannungen ein dichter Film enstand, begfinstigte eine Zunahme der Temperatur oder der Spannung die Kristallisation und damit die Porosit~it. Die kristalline Phase lag in Form yon Agglomeraten in einer amorphen Matrix vor. Elektronen- und R6ntgenbeugungsaufnahmen zeigten, dab die kristalline Phase nicht homogen war, sondern aus ~,'-AI~O~, y-AlzO3, und mindestens einer der beiden Verbindungen B6hmit oder ~.-AI,Oz bestand. Ein Modell tiber die Beziehungen zwischen der amorpben und den kristallinen Phasen des Films wird vorgeschlagen. Dieses Modell wird beniJtzt, um Widersprtiche in der Literatur zu erkl~iren.
Microstructures des films de barriOre anodique dans l' aluminium Des observations par microscopie optique et ~lectronique furent entreprises sur des films d'oxyde anodique qui se forment sur de l'aluminium plong~ dans des solutions H~BOa/Na2B~O7 sous diff~rentes conditions de temperature et de voltage. Alors qu'~ basses temperatures et qu'h bas voltages une barri~re typique se d6veloppe, une augmentation de l'un ou l'autre de ces param~tres favorise la cristallisation et la porosit& La phase cristalline appara]t comme un agglom6rat dans la matrice amorphe. La diffraction des rayons-X et 61ectronique montrent que la phase cristalline n'est pas homog~ne et qu'elle se compose de ~,'-A1203, de y-A1203 et au moins d'un des compos~s de la bo~hmite ou de ~-A12On. On propose un mod61e pour expliquer la relation mutuelle entre les phases amorphes et cristallines dans le film afin de rationaliser les divergences que l'on relive dans la litt~rature. Copyright © 1971 by American Elsevier Publishing Company, Inc.
404
L E. Klein and -4. E. Yaniv
Introduction Barrier anodic films on aluminium are usually formed in neutral or slightly acidic solutions, the most common being H3BO3/Na2B4OT, NH4-tartrate , (NH4)zB4Ov, NH4B5Os, monophosphates, and certain dicarboxylic acids--for example, succinic and glutaric. The structure of a barrier film may be considered a particular case of a porous film. Setoh and Miyata 1 showed in 1932 that the anodic film on aluminium consists of two layers, the porous thick outer layer growing on an inner thin barrier or dielectric layer. The porous film is of a fibrous nature, each fiber having a hexagonal cross section with a pore in its center. At present two models for the formation of a porous anodic film on aluminium exist--the geometric model, proposed by Keller et al., ~ and the colloidal model of Murphy and Michelson. 3-5 Although the hexagonal structure of the film was attributed to the mechanism of pore formation in both models, it has been shown ~ that porefree barrier films have a hexagonal cross section too. Cell diameters of a barrier film formed at high voltages and of a porous film formed at low voltages but extrapolated to the same values were found to be equal ;7 anodizing under barrier conditions but for prolonged periods of time (10 minutes) or at higher temperatures leads to pore formation, s-l° suggesting that the same basic mechanism exists for barrier and porous film formation. The crystallographic structure of the barrier films was recently reviewed by Diggle et al. xl A few works that have been omitted in this review are worth mentioning. Brandenberger and H~ifeliTM correlated the structure of the barrier film with its formation voltage and found that boehmite was formed at low voltages (0.5 to 20 volts), crystalline y-AI~O3 and y'-AI~Oa at high voltages (580 to 600 volts), and all three structures in the intermediate range. Beck et al., TM on the other hand, reported that the barrier film formed in a solution of NH4-tartrate in ethanol was amorphous even when anodizing was carried out at 1200 volts. Van Geel and Schelen 14 and later Franklin 15 found a dependence of the structure on the temperature of the electrolyte; an amorphous film was formed at room temperature, but at 100°C )/-AI~Oa was formed. Dorsey, TM using ultramicrotomy and investigating the sections under an electron microscope, has shown that the barrier film consists of two layers, a compact, thick, impermeable lower layer and a thin layer permeable to the electrolyte, the latter layer thickening at elevated temperatures. Unfortunately, he did not perform diffraction analysis, and, although the physical properties of the layers were evaluated, they lack a final identification. Beck et al. TM also found two layers and reported a similar relative increase of one of the layers with voltage and temperature increase. Although there is an extensive literature on barrier anodic films on aluminium, no definite data are available on the influence of the formation voltage and electro-
Anodie Barrier Films
405
lyte temperature changes on their structure. There are scarcely any data available on the distinction of the crystallographic phases by x-ray diffraction studies. In the present investigation we have tried to elucidate these points and to add some information on the controversial crystallographic composition of barrier films on aluminium.
Experimental Procedure FI3BO3 and Na2B407 were of Analar grade; aluminium specimens were 99.99 pure; the H20 used was double-distilled. The specimens were in the shape of a flag, the stem being enclosed in a glass tube sealed with RTV silicone rubber. The exposed surface was 0.5 to 2 cm 2 for small specimens and 30 cm z for large ones, the latter being used for stripping the oxide. The reaction vessel was heated by means of a heating mantle, and the temperature was controlled by a Variac autotransformer. Prior to anodizing, the specimens were degreased, alkali-cleaned in a proprietary cleaning solution (Rokleen N O N 40-, a product of R. O. Hull & Co., Cleveland, Ohio), and dipped in 60% w/w H N O s for 1 minute. After each step the specimens were rinsed in distilled water for 2 minutes. Anodizing was carried out under constant-voltage conditions at various potentials up to 500 volts and under constant-current conditions of 1 mA/cm 2. For constant-voltage experiments a Lambda regulated power supply Model 50, 500 volts, 500 mA, was used. Constant current was provided by a Lambda dual regulated power supply L P D 425FMV, consisting of two units, each 250 volts, 130 mA, connected in series or in parallel. On switching on the current, the voltage rose up to a predetermined value, then remained constant while the current decreased. A 3~o H3BO3+0.05~o Na2B~O7 anodizing solution was used throughout. The temperature range was 20 ° to 95°C, increasing in increments of 15°±1°C. The surface of the anodized specimen was examined with a Reichert MeF microscope under natural and polarized light to distinguish between the various phases of the film. After dissolution of the oxide in a CrO3-H3PO 4 solution, the metal surface was examined under 2000 x magnification, using oil immersion. A J.E.M. 7 electron microscope was used for electron microscopy and electron diffraction studies. Since maximum thickness of the film allowable for transmission is ~1000/~, the anodized specimens were produced at maximum 70 volts. X-ray diffraction patterns were obtained by the Debye-Scherrer method with exposures up to 24 hours. Anodizing was carried out at a constant current density of 1 mA/cm 2 up to 500 volts; the oxide was then stripped, using the HgC12 method, dried in a vacuum oven, ground, and enclosed in a capillary tube.
406
I.E. Klein and A. E. Yaniv
Results
Optical Microscopy The optical micrographs of the upper surfaces of films produced under constant-voltage conditions are given in Figs. 1 and 2 and show the following tendencies: (1) At constant temperature, an increase in formation voltage led to an increase of the crystalline-phase component. (2) At a constant formation voltage, an increase in temperature likewise caused an increase of the crystalline phase. (3) Films formed at 500 volts were crystalline at all temperatures.
FIG. 1. Polarized light micrograph of the surface of a film formed at 20°C under constant voltage of (a) 100 volts, (b) 500 volts. The crystalline phase, which appeared dark in natural light micrographs and bright in polarized light micrographs, took the form of agglomerates in the amorphous phase matrix. It should be emphasized that the crystalline agglomerates detected may exist below the film surface, since it has been proved, using ellipsometric measurements, that the amorphous oxide is transparent. 17' 18 The micrograph of the metal surface reveals hexagonal cells of diameters given in Table I. A representative cell structure micrograph is given in Fig. 3.
Anodic Barrier Films
407
Fro. 2. Polarized light micrograph of the surface of a film formed at 95°C under constant voltage of (a) 100 volts, (b) 500 volts. I t can be seen f r o m the table that the cell d i a m e t e r increased with f o r m a t i o n voltage and bath t e m p e r a t u r e , except for films f o r m e d at 95°C. T h i s is in a g r e e m e n t w i t h Franklin, a5 b u t its reason is not at present known. TABLE I CELL DIAMETER (MICRONS) AS FUNCTION OF FORMATION VOLTAGE AND BATH TEMPERATURE
Formation voltage, V
20°C
35°C
Bath temperature 50°C 65°C 80°C
95°C
200 300 400 500
0.3 0.5 0,7 0.9
0.4 0.6 0,8 0.8
0.5 0.6 0,8 0.8
0.7 0.7 0.7 0.7
0.5 0.6 0.7 0.9
0.6 0.6 0.7 0.8
Electron Microscopy F i l m s p r o d u c e d at a constant voltage of 70 volts in the t e m p e r a t u r e range 20 ° to 95°C were e x a m i n e d by t r a n s m i s s i o n ; representative m i c r o g r a p h s are
408
L E. Klein and A. E. Yaniv
given in Figs. 4 and 5. At 20°C there are a very few crystalline regions, which show up in the figures as dark dots (thicker or more absorbent regions). With temperature increase the crystalline domains increase, and at 95°C the whole film is crystalline. Magnification up to 100,000 x did not reveal the presence of pores, except for a few white spots in the crystalline phase of the 80°C and 95°C specimens. The appearance of pores at higher anodizing temperatures, already reported by Leach and Neufeld, x° explains some of the electrical properties of the film. 19 In these micrographs, lines of natural oxide can be seen in the anodic film, having been embedded in the metal during rolling.
FIG. 3. Cell structure of an anodic film. The resistance of the electrolyte decreased with temperature increase, resulting in higher initial current densities. As a result, heating up will occur and may be the cause of crystallization. To find out whether crystallization is due primarily to thermal or to electrochemical effects, two series of experiments were carried out. First, anodizing was carried out at 20°C in a 5% H~BOa+0.08 % Na2B4Or solution, which is saturated at this temperature and has a resistance lower than that of a 3% H3BO3+0.05 % NaoB40 7 solution at 20°C. Although anodizing was
FIG. 4.
Electron micrograph of a film formed under 70 volts at 35°C.
FIc. 5.
Electron micrograph of a film formed under 70 volts at 80°C.
FIG. 6. Electron micrograph of a film formed under 70 volts at 20°C in 5% H~BOa+ 0.08% NazB4OT.
FIo. 7.
Electron m i c r o g r a p h of a film f o r m e d u n d e r 1 m A / c m ~ u p to 60 volts at 35°C.
FIG. 8.
Electron m i c r o g r a p h of a film f o r m e d u n d e r 1 m A / c m z u p to 60 volts at 80°C.
FIa. 9. Electron diffraction p a t t e r n o f a crystalline region of a film f o r m e d u n d e r 70 volts at 50°C.
Anodic Barrier Films
411
carried out at a higher current density, the film produced was still completely amorphous, as shown in Fig. 6. In the second set of experiments, anodizing was carried out in the more dilute electrolyte at 1 mA/cm ~ up to 60 volts and the bath temperature was varied. It was found that at room temperature the anodic film was completely amorphous, and with rising temperature the amount of the crystalline phase increased, until at 95°C the film was completely crystalline (Figs. 7 and 8). It was therefore concluded that, under the prevailing conditions, crystallization is governed by the temperature of the electrolyte.
Electron Diffraction Typical electron diffraction patterns are given in Figs. 9 and 10. It can be seen from the discontinuity in the circumferential dotted line that the oxide crystals are large. The results are given in Table II.
Fro. 10. Electron diffraction pattern of an a m o r p h o u s region of a film formed u n d e r same conditions.
There is good agreement between these findings and the 7'-Al~Oz reported in the literature. The three lines for the largest interplanar distances do not belong to 7'-AlzO3; two of them belong to 7-A1203, and the other is unaccounted for.
X-Ray Diffraction The results of x-ray measurements are given in Table II. In addition to ),'-Al~O3 and ),-A1203, which undoubtedly are the major components of the crystalline phase, there are many lines corresponding to at least one of the phases boehmite [~-AI2(OOH)z], or ~-A1203.
412
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Anodic Barrier Films
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Discussion Optical and electron microscopy have shown t h a t films produced at low temperatures and voltages are amorphous and compact, while high temperatures and voltages encourage crystallization and porosity. Crystallization is governed mainly by the temperature of the electrolyte. It is quite obvious that these transformations are not caused by a uniform heating up of the specimens to a few hundred degrees (where thermal crystallization will occur), because an increase of a few tens of degrees has a drastic effect on the crystallographic structure. This strong influence of higher temperatures is probably due to an increase in the solubility of the oxide, 1° leading to formation of a porous structure, with A13+ ions moving into solution. Crystallization can be caused as follows: (1) High local current densities in the pores give rise to high temperature and hence crystallinity. This was postulated by Zahavi *° when anodizing in Na,SO 4 solutions. (2)A1 a+ moving into solution may act as a nucleator for the transformation from an amorphous to a more thermodynamically stable crystalline phase. Anodizing at room temperature in an electrolyte contaminated by minute quantities (1 to 2 ppm) of AI3+ was found to produce a film with some crystalline domains but further work is needed to distinguish between the two mechanisms. Electron diffraction of films produced at low voltages has shown that the crystalline phase is y+7/-Al2Oa with an unexplained line of interplanar distance 3.28, which could be from boehmite. X-ray diffraction of films produced at 500 volts shows that, in addition to y+9/-Al,Oa, at least one of the compounds boehmite or :~-A1,Oa exists in films formed both at 20°C and 95°C. Brandenberger and H~ifeliTM have found boehmite particularly at low formation voltages. Although ~-Al,O a is the most stable of the aluminium oxides, no data could be found in the literature on its appearance in anodic films. Nevertheless, the presence of at least one of these phases is indicated in the present work. The crystalline phase appears as agglomerates in an amorphous matrix, which is in agreement with Franklin .1 and Stirland and Bicknell, 7 but the two-layer structure suggested by Altenpoh122 and Dorsey TM was not revealed.
Summary On the basis of these results and results obtained from electrical measurements of anodic barrier filmsfl we propose a model consisting of two layers, in which the bottom layer of the barrier film is amorphous, and the upper layer is composed of crystalline agglomerates in an amorphous matrix. With increase of temperature of the electrolyte or of formation voltage, the upper layer becomes completely crystalline and may penetrate into the bottom layer. Since Dorsey's work was carried out at 60°C in a 2 M H3BOa solution, ultramicrotomic section
414
I . E . Klein and A . E. Yaniv
did not show agglomerates. On the other hand, Franklin and Stirland et al. worked at room temperature and examined their films by surface micrography; they detected agglomerates, b u t could not have revealed a two-layer structure.
Acknowledgment We wish to thank Professor M. Folman of the Technion Israel Institute of Technology, Haifa, for many helpful discussions.
References 1. S. Setoh and A. Miyata, Sci. Papers lnst. Phys. Chem. Res. Tokyo, 17 (1932) 189. 2. F. Keller, M. S. Hunter, and D. L. Robinson, ft. Electrochem. Soc., 100 (1953) 411. 3. J. F. Murphy and C. E. Michelson, A D A Conference on Anodizing AI, Nottingham, 1961. The Aluminium Development Association, London (1962). 4. J. F. Murphy, Plating, 54 (1967) 1241. 5. C. E. Michelson, jr. Electrochem. Soc., 115 (1968) 213. 6. J. C. Grosskreutz and G. G. Shaw, ft. Appl. Phys., 35 (1964) 2195. 7. D. J. Stirland and R. W. Bicknell, ft. Electrochem. Soc., 106 (1959) 481. 8. T. P. Hoar and J. Yahalom, .7. Electrochem. Soc., 110 (1963) 614. 9. A. J. Brock and G. C. Wood, Electrochim. Acta, 12 (1967) 395. 10. J. S. L. Leach and P. Neufeld, Corrosion Sci., 9 (1969) 413. 11. J. W. Diggle, T. C. Downie, and C. W. Goulding, Chem. Rev., 69 (1969) 365. 12. E. Brandenberger and R. J. H/ifeli, Helv. Chim. Acta, 31 (1948) 1168. 13. T. R. Beck, D. W. Hamilton, and R. B. Gillette, Electrochem. Soc. Meeting, Dallas, Texas (1967). 14. W. Ch. Van Geel and B. J. J. Schelen, Philips Res. Repts., 12 (1957) 240. 15. R. W. Franklin, Nature, 180 (1957) 1470. 16. G. A. Dorsey, Jr., .7. Electrochem. Soc., 116 (1969) 466. 17. M. A. Barrett and A. B. Winterbottom, First. Intern. Congr. Met. Corr., London, 1961. L. Kenworthy, Editor. Butterworths, London (1962). 18. R. M. Goldstein, R. J. Lederich, and F. W. Leonhard, ft. Electrochem. Soc., 117 (1970) 503. 19. I. E. Klein, M.Sc. Thesis, Technion I.I.T. (1970). 20. J. Zahavi, D.Sc. Thesis, Technion I.I.T. (1970). 21. R.W. Franklin, A D A Conference on Anodizing AI, Nottingham, 1961. The Aluminium Development Association, London, 1962. 22. D. Altenpohl, Convention Record IRE, 3 (1954) 35. Accepted March 16, 1971