Fourier transform IR spectroscopical investigation of the anodic oxide films on aluminum

Materials Chemistry 5 ( 1 9 8 0 ) 199 - 2 1 2 © CENFOR S.R.L. - Printed in Italy

FOURIER TRANSFORM IR SPECTROSCOPICAL INVESTIGATION OF THE ANODIC OXIDE FILMS ON ALUMINUM Part. 1. Structural features during oxide films formation

M. HANDKE*, C. PALUSZKIEWICZ*, W. WYRWA** *

I n s t i t u t e o f Materials S c i e n c e A G H , C R A C O W - Poland.

**

I n s t i t u t e o f N o n f e r r o u s Metals, G L I W I C E . Poland.

Received 8 November 1979; accepted 20 February 1980 S u m m a r y - Rapid scanning Fourier Transform Infrared Spectroscopy has been applied to obtain spectra from aluminium anodic coatings. The spectra were recorded with different measuring techniques e.g. transmission, absorption-reflection, and ATR in the far and middle IR regions. Infra red light polarization has been also used. From the experimental data vibrational band assignments have been made. Based on these assignments a coating structure and coating formation mechanism has been proposed.

INTRODUCTION The structural problems of the anodic coating on aluminum surface are very important for anodized aluminum practical application and therefore many papers have been published in this subject for the last 40 years. Nevertheless molecular structure of the coating is not unequivocally explained. General structure of-~nodic coating is well known l"s . It consists of two main layers: a thin, compact called barrier layer close to metal and thick-porous layer. The porous layer is characterized by porous micro-channels, perpendicular to the metal surface. During aluminum oxidation by the air or during anodic oxidation in some electrolytes (eg.

200 boric acid) only an amorphous barrier layer is formed. For the industry purposes anodic oxidation is carried out with the use of sulphuric acid as an electrolyte. In such a case a barrier layer is obtained only at the beginning of the process and then progressively a porous layer is formed. According to the previous papers the mechanism of coating formation and its structure depend on many factors such as: type and electrolyte concentration, current density and anodizing time. The most widely used method for anodic coating investigation is infrared spectroscopy. Due to results of this method several models of its molecular structure have been proposed 6"9. However, those models resulting from different spectra interpretation were not consistent, also the oxide films spectra were dependent on measurements technique. Maeland et al. 9 noticed that according to the measuring method (transmission, absorption-reflection or ATR) different interaction between electromagnetic wave and a thin film is recorded. The use of different tech. rfiques and particularly absorption-reflection measurements with reflected beam polarization gives full spectral informations as was shown by Maeland et al. for barrier layer spectra. The aim of the present paper was a comprehensive spectroscopic investigation of anodic coating on aluminum in a wide spectral range (100-' 4000 cm "~) and with different measuring techniques. All measurements were performed on the rapid scanning Fourier Transform Spectrometer. Owing to this type of spectrometer spectra with high signal to noise ratio were obtained and thus measurements Were better than those obtained with the help of classical dispersive spectrometers. This paper presents results which concern the structure of anodic coating and the way of its forming. The second part of our work will describe results refering to the anodizing condition on the coating structure and its formation.

EXPERIMENTAL Anodizing procedure In anodic oxidation of aluminum surface the cathode was a lead sheet metal o f cylindrical shape whose diameter was 300 mm. A c~angeablc anode was a rectangular aluminum plate (99.99% Al) 100x25x 1 mm. Aluminum impurities were: Si -0.0022%, Cu -0.0008%, Ti -0.001%, Mn -0.0013 %, Fe - 0 . 0 0 2 ~ Zn - 0 . 0 0 1 ~ Mg -0.0017%. The aluminum sheet used for anodizing has been obtained by cold rolling

201 with a compression above 80% which secured homogenity of the oxide coating and its independence of the background metal structure. The anodizing area was each time 25 cm 2. The temperature of the process was constant - 293.15 -+ 0.5 K and its stability was secured by connecting a contact thermometer and a mercury relay with electromagnetic valve which adjusted the water flow in water jacket. An electric system was applied which enabled the regulation of anodic current intensity with stability + 1.5%. Voltage changes during anodic process were recorded by an automatic Micrograph BD5 type potentiometer of Kipp and Zonnen production. Before oxidation aluminum samples were degreased in acetone and then in alkaline bath. The time of degreasing was 5 minutes and temperature 313, 15 K. Anodic oxidation of the samples was performed in 243, 15 K with different current density and electrolyte concentration (H2 SO4) but with constant concentration of aluminum ions in the solution (10 g/l). Thickness of oxide films was measured optically. Infrared measurements

All spectra measurements were performed on the FTS-14V Digilab spectrometer. The parameter were the same disregarding the technique in use and they were: resolution 4 cm "1 , sampling interval 1 cm "1 and spectra were calculated after collecting 100 interferograms with double precision (32 bits word was used). Three measuring methods were applied: the main one was absorption-reflection (A-R) with reflected beam polarization. In" those measurements Harrick's reflectance attachment was applied 1°. Also measurements by means of ATR technique were carried out'for the coatings previously removed from metal and after their powdering transmission spectra were recorded with the use of the KBr pellet technique.

RESULTS Infrared spectroscopy is a widely used research method in the anodic oxide coating investigation. Many authors 6, 8, 9 which published their results gave inconsistent bands assignments in IR spectra. A main aim of this paper was to fred out experimentally a reliable band assignment and from that to consider the mol-

202 ecular structure and mechanism o f anodic oxide coating formation. Authors o f previous papers did not agree as to the band assignments due to A1-O-A1 and AI-O-H vibrations. Eg. Dorsey 6 attributes the band at 960 cm "1 to

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Fig. 1 - Absorption-reflection IR spectra o f the anodic coating obtained in 20% H2S04 with 1 A/dm 2 current density but in different anodising time. a. Spectra plotted in uniform absorbance scale.

203 A1-O-H bending vibrations whereas the band about 650 cm "~ to A1-O stretching vibrations. Vedder and Vermileya a Maeland, Rittenhouse and Bird 9 noticed a possibility of observing in thin films (d < X) A-R spectra longitudal (LO) and trans-

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Fig. 1 - Absorption-reflection IR spectra o f the anodic coating obtained in 20% ti2S04 with 1 A/dm 2 current density but in different anodising time. b. Spectra plotted with automatic scale expansion.

204 verse (TO) optical modes splitting. The authors suggested that the band at 960 cm "I is due to LO modes of AI-O-A1 whereas the band of about 650 cm "1 in transmission spectra to TO modes of this bonds. As the subject matter of those investigations was only anhydrous barrier layer they did not propose A1-O-H bands assignments in the porous layer spectra. The correctness of Maeland et al. bands assignments was practically proved owing to the use of reflected beam polarization. Recently these assignment has been confirmed by Kember, Chenery, Sheppard and Fell 11 with the help of IR emission spectra measurements. On the Fig. la and lb A-R spectra of anodic oxide coatings on aluminum are shown. Coating thickness varied from below 0.5 pm to a 20 pm which was due to different time of anodizing process. Fig. la and lb differ only in a selection of units on the absorbanc~ scale. In Fig. la they are uniform, whereas in lb for each spectrum an automatic scale expansion was applied owing to which band are more easily observable but the intensity of bands cannot be compared directly. Fig. 2 is a comparison of the same samples spectra in the far infrared region. Spectra o f the samples whose time of oxidation was below 300 seconds were omitted because they were identical and did not have characteristic absorption bands. From this comparison it is seen that there occurs an evolution of bands shape and its position which denotes that during formation the anodic coating changes structurally. At first, with small thickness 920, 1170 cm'! bands are observed as well as a shoulder at about 1320 cm q ; no bands in far infrared region can be seen under this condition. When coating thickness increases a spectrum of coatings is changed below 1100 cm "~ and above this value the intensity and position is only changed. This observation becomes more complicated when the same spectra are recorded with reflected beam polarization. Spectra of several selected samples in different polarization are shown on Fig. 3. Due to light polarization it was possible to select TO and LO optical modes. Absence of this selection in A-R spectra of oxide coating caused difficulties in bands assignments. In case of very thin coatings (below 0.5/~m) a polarized 920 cm "1 band is observed which is due to LO modes of A1-O-A1 bond. It seems that all: bands recorded in this case, according to optical properties of thin films (Berreman 12), are only due to LO modes, this fact explain the absence of bands in perpendicular polarization. As the coating thickness increases its optical properties change, and in A-R spectra without polarization appear for the first time TO modes. The apparent 920 cm "~ band shift could be explained by the presence of A1-OH TO modes

205 in thicker coatings. Its position causes coincidence. The LO vibration of the same bond is recorded in the opposite polarization at 1060 cm "1 . This assignment has been cheked by an additional experiment, namely, a sample of oxide coating on aluminum has been hydrated by means of boiling it in H20 and D20. 45O

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Fig. 2 - Absorption-reflection far IR spectra o f anodic coatings obtained in different anodising time; spectra are plotted with automatic scale expansion.

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207 Spectra of the samples obtained in this way are shown on Fig. 4. After hydration there appears in a spectrum an additional band at 1070 cm "l whereas after the process in D 2 0 a band appears at 810 cm "l (Fig. 4b). A value of the iso-

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Fig. 4 - AbsorlJtion-reflection spectra of barrier layer before (broken line) and after (solid line) hydration in boiling. H20 (a) or D2Q (b).

208 topic shift fulfills the expections for 6 OH ~ ~ OD. When the coating is sufficiently thick (about 20/~m) both band i.e. 980 and 1070 cm "1 are observed in unpolarized spectra because in this case properties of thin films are not valid any more. Infrared transmission spectra of anodic oxide coatings are completely different. In this work many such measurements have been carried out for the coatings chemically removed from metal. They were direct measurements of removed film as well as after their powdering in KBr pellets. There were also measurements performed by means of ATR technique with a KRS-5 crystal. All these measurements gave identical results. Fig. 5 presents examples of transmission spectra of the coating powdered in

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Fig. 5 - Transmission IR spectra o f thick layers o f anodic coatings removed from aluminum surface and pressed in KBr pellets.

209 KBr pellets. In those spectra a very broad complex band in the region 4 0 0 - 1000 cm "1 is observed. This band probably consists of a few single bands due to A1-O-AI and A1-O-H vibrations. The band at 1130 cm "1 comes undoubtedly from the S-O stretching vibration o f SO4 anion built" into the coating obtained in sulphuric acid. The bands o f 1630 cm "1 and about 3500 cm "1 are due to bending and stretching vibrations of OH groups. Considering the optical properties o f thin coatings, absorption-reflection measurements are more useful in this type of research than transmission technique. On the basis of the above discussion and other authors' data, the following bands assignments can be made in the spectra o f anodic coatings on aluminum. Table 1. Band (cm_ 1 )

Assignment

References

3 4 0 0 - 3600 2 9 0 0 - 3100

stretching vibration OH groups

well-known region of OH vibrations

1620 - 1640

bending vibration OH groups

1130 - 1190

S-O stretching vibration

1050 - 1080 955 - 980 9 2 0 - 940 about 650 below 350

A1-OH LO modes A1-OH TO modes A1-O LO modes A1-O TO modes A1-O-A1bending vibration

typical frequencies of SO4 anions this work ref. (8, 9); this work ref. (6); this work

The above assignment allows to explain a molecular structure and mechanism of coating formation. As it can be seen from the spectra shown on Fig. 1 and 2, at the beginning of the anodic oxidation a anhydrous, amorphous aluminum layer is formed, called a barrier layer. The characteristic band o f this layer is the peak at 920 cm "1 which is due to AI-O longitudinal optical modes. This conclusion is a confirmation of Maeland's et al. work. On the other hand Dorsey's suggestion referring to the complexity o f that layer and a great amount o f OH grups in its structure does not seem right. If Dorsey's suggestion is followed, attributing the 1340 cm "1 shoulder to the

210 stretching AI=O vibration (similar band appears in the boehmite spectrum) the presence of A I = O bonds in barrier layer would have to be accepted. With the coating thickness increasing a cand at 375 cm "1 appears whereas in the middle IR region appears a bands at 9~5 cm "1 due to AI-OH vibrations. At tLe same time the intensity of OH group bands is increased. Those means that the layer formed has structure similar to boehmite and precisely to pseudoboehmite (because the typical structure of 3000 " 3 4 0 0 cm "1 band is not observed).With further coating growth, A1-OH bands are moved towards lower wavenumbers, 375 cm"1 band is also shifted and thus, the OH bands intensity are increased. Those facts denote further hydration of the coating and its structure becomes close to bayerite (/3- AI(OH)3). Similarity of this structure to the bayerite structure is only an approximation of this layer structure. Contrary to the bayerite this coating is not exactly crystalline but rather amorphous. This is indicated by the absence of lattice vibrations bands as well as by crystal field splitting, i.e. OH vibrations. The final conclusion can be drawn, the.coating obtained on aluminum during its anodic oxidation consists of three essential layers: 1. 2. 3.

amorphous, rather anhydrous aluminum oxide layer called barrier layer whose thickness is less than 0.5 lam; an oxyhydroxide whose structure is dose to pseudoboehmite structure with thickness of 0.5 to about 5/am; hydroxide layer which can be considerably thick whose structure is similar to the bayeritestructure.

Oxy-hydroxide and hydroxide layers compose a so called porous layer. Separate problems to be studied are: the structural role of electrolyte ions and the influence of different anodic oxidation parameters on coating structure. These problems will be considered in the second part of the paper.

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