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Self-colour anodizing of titanium
Surface Technology, 16 (1982) 153 - 162
153
SELF-COLOUR ANODIZING OF TITANIUM
J.-L. D E L P L A N C K E , M. D E G R E Z , A. F O N T A N A
and R. W I N A N D
Unwersitg Libre de Bruxelles -- C P 165, Department of Metallurgy and Electrochemistry,
50 avenue F. D. Roosevelt, B 1050 Brussels (Belgium) (Received December 2, 1981)
Summary Relations between the colours and the electrochemical parameters (current density, quantity of electricity and temperature of the electrolyte) during the anodization of titanium sheets were studied in a 1 M H2SO4 solution. Mathematical theories were developed for the kinetics of growth of the anodic film and for the colours of this film. These theories show that the relation between the growth and the colour of the film is complicated. Nevertheless, it was possible to find experimental conditions that led to uniform and well-defined colours.
1. Introduction Titanium is a light metal with remarkable mechanical properties. Moreover, this metal is covered by a natural film of oxide that gives it an excellent resistance to corrosion. This is the reason why this metal is frequently used in aeronautics and in the chemical industry. Its production is growing continually. Unfortunately, this metal shows sensitivity to severe corrosion under specific circumstances. This sensitivity originates mainly from defects in the surface of the metal. In order to eliminate the defects, anodizing may be considered. Papers about the anodization o f titanium are numerous [1]. In some o f them, the appearance of colours on the surface of the anodized sheets has been remarked [2 - 10]. In addition, self-colour anodizing of titanium has been patented [ 11 - 17 ]. Nevertheless, we have n o t found in any of these papers either a systematic study of the colours obtained after anodizing or a theoretical explanation of their appearance. The purpose of this study is therefore twofold: on the one hand, to find the conditions that produce, after anodizing, uniform colours on large area samples and, on the other hand, to derive theories to explain the phenomena that are responsible for these colours. 0376-4583/82/0000-0000/$02.75
© Elsevier Sequoia/Printed in The Netherlands
154
2. Experimental details 2.1. Electrodes Rectangular anodes were prepared by sectioning a sheet o f IMI 115 titanium. The dimensions were 45 m m X 43 mm X 0.9 mm. The anodes were degreased in trichlorethylene, rinsed with ethanol, dried, and pickled in a m i x t ur e o f 75 vol.% nitric acid solution and 25 vol.% hydrofl uori c acid solution as r e c o m m e n d e d in ASTM Standards [18] until red fumes appeared. Th en the m i xt ur e was quickly diluted with distilled water. T he o t h e r electrode was an aluminium cathode.
2.2. Electrolyte Th e electrolyte was a diluted solution of pure sulphuric acid. This electrolyte was chosen among o t h e r possible solutions because sulphuric acid is readily available and n o t expensive. The concent rat i on of the solution was 1.02 M. T he electrolyte was d e o x y g e n a t e d using pure nitrogen. Experiments were p e r f o r m e d at constant t e m p e r a t u r e by means o f a Haake thermostat. Three temperatures were studied: 25, 40 and 60 °C.
2.3. Electrolytic cell and electrical supply The electrolytic glass cell with a heating jacket was supplied at constant current. This galvanostatic m e t h o d was chosen for two reasons. (1) When the voltage is applied in a potentiostatic experiment, the initial growth o f th e film occurs at a high and u n k n o w n transient current. (2) During a potentiostatic experiment, a continual decrease in the current is observed as a result o f the growth o f the film. The kinetics of the electrochemical growth thus c a n n o t be kept constant t h r o u g h o u t a potentiostatic experiment. Accordingly kinetic study o f the f o r m a t i o n of t he anodic film is very difficult to carry out. During o u r e x p e r i m e n t the voltage d r o p between cat hode and anode and the current intensity in the cell were recorded continuously by means o f a Servogor 120 BBC Goerz Metrawatt recorder. The a m o u n t o f electricity was also recorded by means of a Tacussel IG6-N integrator. The electrolysis was carried o u t at various c ur r e nt densities over periods from 1 s to 1.5 h.
2.4. Colour analysis In order to describe accurately the colours o f our samples, an objective criterion was chosen: t he colour o f each sample was characterized as a d o t in the X - Y diagram o f the International Commission of Lighting according to the m e t h o d described in t he IES Lighting Handbook [ 1 9 ] . Knowledge o f the distribution o f the luminous intensity reflected by the sample for wavelengths in the range 380 - 760 nm for 5 nm increments and o f the same distribution for the light source is needed. These data are obtained b y means o f a Zeiss RA3 s p e c t r o m e t e r coupled with the reflective device M40. The results are analysed by means of an HP 21 MX com put er.
155
3. Results 3.1. E l e c t r o c h e m i c a l results
Figures 1 - 5 s h o w the anode-to-cathode voltage drop versus the a m o u n t of electricit:~ for five current densities. At a given current density no difference between the curves should be observed provided that the p h e n o m e n o n 70'
70 V[V)
aO
50
50
~0
/.0
3O
20
20
10
10
I0
20
3b
~0
Fig. 1. A n o d e - t o - c a t h o d e volta.~e d r o p V as a f u n c t i o n o f t h e a m o u n t Q o f e l e c t r i c i t y f o r a c u r r e n t d e n s i t y o f 4 0 . 5 A m - ( t e m p e r a t u r e , 25 °C). Fig. 2. A n o d e - t o - c a t h o d e v o l t a g e d r o p V as a f u n c t i o n o f t h e a m o u n t Q o f e l e c t r i c i t y f o r a c u r r e n t d e n s i t y o f 32.4 A m - ~ ( t e m p e r a t u r e , 25 ° C ) .
70" v{v) 60 50 V{V) 40
z,O
30
30 20
10
10 ~Q
2p
~
&o
s9 o(¢)
Fig. 3. A n o d e - t o - c a t h o d e voltage d r o p V as a f u n c t i o n o f t h e a m o u n t Q o f e l e c t r i c i t y f o r a c u r r e n t d e n s i t y o f 24.3 A m - 2 ( t e m p e r a t u r e , 25 °C). Fig. 4. A n o d e - t o - c a t h o d e voltage d r o p V as a f u n c t i o n o f t h e a m o u n t Q o f e l e c t r i c i t y for a c u r r e n t d e n s i t y o f 16.2 A m - 2 ( t e m p e r a t u r e , 25 °C).
156 (V3
30 f
~
20
10
q~
2; ____?o_
4o
~o
6;) ~lc)
Fig. 5. Anode-to~athode voltage drop V as a function of the amount Q of electricity for a current density of 8.1 A m - 2 (temperature, 25 °C).
is highly reproducible. This is approximately true at 40.5 and 8.1 A m-2. For the intermediate values, the curves, similar at the beginning, diverge rapidly. The voltage drop measured at the end of the electrolysis for each sample m a y be plotted v e r s u s the amount of electricity, whatever the current density is. This plot is shown in Fig. 6. Two types of behaviour, one called "high" current density, the other one called " l o w " current density, may be distinguished. They break up at a b o u t 9 V. Nevertheless, the aspect of all the curves is the same: an initial fast increase in the voltage drop followed b y a region of much slower increase. This aspect has already been observed b y Ammar and Kammal [20] and by Jouve e t al. [21] respectively for current densitites lower and higher than ours. Figures 1 - 5 show also that each curve, whatever the current density, presents an inflexion point at a b o u t 9 V. A potentiostatic study shows that gas evolution is observed at this voltage at the surface of the anode. Figures 7 and 8 show the influence of the temperature of the electrolyte on the overall voltage drop for current densities of respectively 40.5 A
FI o.i x
~
10
4o
~o
6o
~ Ic)
Fig. 6. Anode-to-cathode voltage drop Ve at t h e end o f e l e c t r o l y s i s as a f u n c t i o n o f t h e a m o u n t Q of electrmity for various current d e n s i t i e s (temperature, 25 °C): +, 40.5 A m-2; o, 32.4 A m - 2 ; x, 24.3 A m-2;m, 16.2 A m - 2 ; e , 8.1 A m -2.
157 60:
SO.
20
6O Ve (V}
Ve• /
50 ~0
J
-
'IO
10
O(F_)
10
2o
30
O(C) 10
20
30
Fig. 7. Anode-to-cathode voltage drop Ve at the end of the electrolysis as a function of the amount Q of electricity for various temperatures (current density, 40.5 A m-2): - - , 20 °C;- - -, 40 °C;-- - --, 60 °C. Fig. 8. Anode-to-cathode voltage drop Ve at the end of the electrolysis as a function of the amount Q of electricity for various temperatures (current density, 16.2 A m-2): - - , 20 °C;- - - , 40 °C;-- - --, 60 °C.
m - 2 and 16.2 A m - 2 . A n increase in t e m p e r a t u r e greatly decreases t h e voltage at t h e e n d o f t h e electrolysis at high c u r r e n t d e n s i t y and o n l y slightly at l o w c u r r e n t d e n s i t y . H o w e v e r , in this case at high t e m p e r a t u r e a s t e a d y value o f t h e voltage d r o p m a y be o b t a i n e d .
3.2. Results o f the study o f the eolours T h e t i t a n i u m a n o d e s s h o w e d c o l o u r s t h a t c h a n g e d d u r i n g t h e process o f electrolysis. In o r d e r t o establish a relation b e t w e e n these c o l o u r s and t h e e l e c t r o c h e m i c a l d a t a , seven samples, each o f w h i c h was representative o f o n e o f t h e seven d i f f e r e n t c o l o u r s o f t h e visible s p e c t r u m , were selected. Figure 9 shows t h e d i s t r i b u t i o n o f t h e l u m i n o u s intensities o f these samples versus t h e wavelength. bSO'D~l~
08
06
Fig. 9. Absorption of the light reflected by samples as a function of the wavelength X: curve 1, yellow; curve 2, light brown; curve 3, brown; curve 4, violet; curve 5, dark blue; curve 6, sky blue; curve 7, green.
158
Figure 10 shows the reduced trichromatic coordinates of these samples in the diagram o f the International Commission of Lighting. For each current density, samples with the same reduced trichromatic coordinates were selected and plotted as functions of the electrochemical observations (Fig. 11).
G8.
,~-.q,~o '5~o ~o
07.
x~.~o
06,
"~o $7o
05, s00 y 0~
~5~0 . .7.zx%sgo ~ .6" ~ I~;3"~600
02 01
~80~
J
0136o02
03xOL~
05
06
07
Fig. I0. Reduced trichromatic coordinates: point 0, light source, point I, yellow; point 2, lightbrown; point 3, brown; point 4, violet;point 5, dark blue; point 6, sky blue; point 7, green.
'704Ve (V)
a
~
60
.
.
.
.
.
.
.
.
.
.
.
b
,
4O
30
..
2O
10
lc,
2.0
a9
L..o
s.o
60
Q{()
Fig. 11. Relation between the colours of the samples (- - -~ and the electrochemical parameters (--) (temperature, 25 °C): curve a, 40.5 A m-2; curve b, 32.4 A m-2; curve c, 24.3 A m-2; curve d, 16.2 A m-2; curve e, 8.1 A m-2; curve I, yellow; curve 3, brown; curve 4, violet,curve 5, dark blue, curve 6, sky blue; curve 7, green.
159 4. Discussion
4.1. Electrochemical aspects Our results have been used as a basis for an a t t e m p t to explain theoretically the kinetics of the growth of the anodic film on titanium. The different kinetic steps o f growth of an anodic film in general have been presented by Vermileya [22] as follows: nucleation of the anodic film; growth of the crystals;possible formation of a continuous film;thickening of the film; limitation of the film thickness. As a result of our experiments, these steps have to be changed in the case of titanium. The first three steps may be withdrawn because there is, before anodization, a natural continuous oxide film. The last two steps must be changed as follows: thickening of the natural oxide film; oxygen evolution; dissolution of the film in sulphuric acid. The discharge of oxygen, observed during our experiments, has also been observed by Jouve et al. [21] but has not been observed by Ammar and Kammal [ 2 0 ] , probably because they did n o t reach a high enough value of the voltage drop between the electrodes and thus of the anodic potential. The dissolution of the titanium oxide film in sulphuric acid is thermodynamically possible [23] and was experimentally observed by Sinigaglia et al. [24]. From a theoretical point of view, there are only two phenomena, diffusion and migration, t h a t may be responsible for the thickening of the film. A quick calculation shows that diffusion is not of great importance because the speed o f the growth" by diffusion at 25 °C in the solid state is at most about 0.1 A s-1 . We considered, as some workers do [2 - 4], t h a t Ti 4+ ion migration in a strong electric field in an ionic conductor that may eventually show some electronic conduction (this allows oxygen evolution to occur) is responsible for the film growth. Mott and Cabrera's theory [25] of growth by migration, already developed for anodizing aluminium, can be used in our case. If there is no discharge of oxygen at the surface of the anode and if the dissolution o f the film in sulphuric acid may be neglected, the variation in the voltage V as a function o f the current density J, the time t and the initial thickness 60 will be as follows (in standard units): V = A(5.4 X 1 0 - n J t + 60) ln(BJ) where A and B are constants. This formula was used for the linear part of the curves in Fig. 3 between 0 and about 10 V and for a current density of 24.3 A m -2. We obtained for ~0 a value of about 30 A, which is in agreem e n t with literature values [26]. If there is oxygen evolution at the anode at a partial current density J+ and if A5 = kt" is the law of dissolution of the film in sulphuric acid, where A5 represents the variation in the thickness during dissolution, the above formula will become
V = A{5.4 × 1 0 - n ( J - J+)t + 5o -- kt n} In
15.4 × l O - 1 1 ( j - - J ÷ ) - - k n t " - i 5.4 X 10-11/B
1
160 The values of each parameter in this formula would have to be experimentally determined by further studies. 4.2. Coloration
of the films
Two theories have been proposed to explain the colour of the anodized titanium sheets.
o
,,b
A,A,b
Fig. 12.
Multiple-beam interference.
(1) The colours could be due to stoichiometric defects in the composition of the film [10]. (2) Interference phenomena may cause the coloration [ 12, 13, 15, 17 ]. Figure 12 illustrates the second theory, which is called the "multiple beam interference" theory. If the incident beam 1 is a beam of white light, the reflected beam, which is formed by the interfering beams 2, 5, 8, 11, ..., will be coloured. This colour will be the one reinforced by the interferences and will thus be the complementary colour of the extinguished colour. The value of the extinguished wavelength will be dependent on the thickness of the oxide film. This theory has been described in several books [27 - 29]. Table 1 shows the conditions for the extinction and reinforcement of wavelengths in ideal conditions, i.e. a transparent oxide film (index of refraction, n2) with a constant thickness on an even and perfectly reflecting surface of titanium (index of refraction, n3). When the value of the thickness d introduced in this formula is increased, a succession of colours is obtained. Th~s theoretical succession is exactly the same as the succession that we have observed experimentally, i . e . yellow, brown, dark blue, sky blue, green, yellow. Figure 10 shows that a range of wavelengths is absorbed instead of only one wavelength. This can be attributed to non-uniformity in the thickness of the oxide layer. Nevertheless, the value of the most strongly absorbed TABLE 1 C o n d i t i o n s for t h e r e i n f o r c e m e n t and e x t i n c t i o n o f w a v e l e n g t h s under ideal c o n d i t i o n s Reinforcement
Extinction
47/
4~/ -k
n2 < n3
--
n2 >n3
--
n 2 d c o s 12 = 2k~"
4y
k
n2d
cos
t2 = ( 2 k
+
1)~"
n2d
c o s 12 = ( 2 k + 1)71
n2d
cos z2 = 2kTr
4~ --
k
161
wavelength increases with the thickness of the oxide film, i.e. with the duration of anodization at constant current. We therefore think that the p h e n o m e n a o f interference are responsible for the colours of the films. It may, however, be possible that for thick crystallized films these colours are a consequence o f stoichiometric defects.
5. Conclusions It may be concluded that the t w o theories that we have developed, the one a b o u t the kinetics of growth of the oxide film and the other about the coloration o f these films, have a c o m m o n point, the thickness of the film. Accordingly, the colours of the samples and the electrochemical parameters can be related to one another. Further experiments should be performed in order to characterize more precisely the films formed by anodizing and to determine the limits of validity of our theories. From a practical point of view, we have found a surface treatment that leads to uniform colours and the experimental conditions necessary for good reproducibility of a given colour. These data may be employed for the following industrial uses: ornamental effects in jewelry; the colouring of titanium sheets for architectural purposes; local "printing" b y anodizing instead of b y painting, which does n o t adhere very well to the normal titanium surface; automatic choice of a titanium piece from among others according to its colour instead of its dimensions.
Acknowledgments The authors want to express their gratitude to a number of colleagues in the university who helped to achieve this work, either b y fruitful discussions or b y allowing them the use o f the facilities o f their laboratories: in particular, t h e y are grateful to Professors R. Poncelet, O. Goche and F. Bouillon.
References 1 J.-L. Delplancke, Anodisation autocolor~e du titane, Travail de Fin d'Etudes en Vue de l'Obtention du Grade d 'ingdnieur Civil Chimiste, Universit~ Libre de Bruxelles, 1981 2 F. Kover and M. Musselin, Rev. Gen. Elect., 76 (1967) 793. 3 F. Kover and M. Musselin, Thin Solid Films, 2 (1968) 211. 4 F. Kover, Thdse de Doctorat d'Etat, Facult6 des Sciences de Paris, 1967, enregistrement CNRS A.O.1088. 5 A. Aladjem, J. Mater. Sci., 8 (1973) 688. 6 G. Darnault, MetaUurgie (Mons), 12 (1972) 146. 7 H. Richaud, Rev. Mdtall. (Paris), 54 (1957) 787. 8 P. Pedeferri and R. Pionteili, Electrochim. Met., 3 (1968) 211.
162 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
N. P. Peksheva, Zashch. Met., 14 (1978) 74. I. Rozenfel'd, A. Akimov and E. Oshe, Zashch. Met., 5 (1969) 217. E.W. Kendall, U.S. Patent 3,616,279, 1971. P. Derouwaux and E. Spycher, Swiss Patent 513,012, 1971. P. Derouwaux and E. Spycher, Swiss Patent 522,516, 1972. J. B. Cotton and P. C. S. Hayfield, Br Patent 1,175,355, 1969. F . W . Lewis and J. H. Clayton, Br. Patent 1,319,542, 1973. Hi-Shear Corporation, Br. Patent 1,100,912, 1968. J. Weigel, Ft. Patent 1,398,453, 1965. A S T M Stand. B481-68, Part 7, 1971. J. E. Kaufmann and J. F. Christensen (eds.), IES Lighting Handbook, Illuminating Engineering Society, New York, 5th edn., 1972. I. A. A m m a r and I. Kamal, Electrochim. Acta, 16 (1971) 1539. G~ Jouve, A. Politi, P. Lacombe and G. Vuye, J. Less-Common Met., 59 (1978) 175. D. M. Vermileya, Adv. Electrochem. Electrochem. Eng., 3 (1963) 211. M. Pourbaix, Atlas d'Equilibres Electrochimiques d 25 °C, Gauthier-Villars, Paris, 1963. D. Sinigaglia, G. Taccani and B. Vicentini, Werkst. Korros., 24 (1973) 1027. J. Bernard, L'Oxydation des Mdtaux, Vol. 1, Gauthier-Villars, Paris, 1962. V. Andreeva and V. Kazarin,Proc. 3rd Int. Conf on Metallic Corrosion, 1966, Mir, Moscow, 1969, p. 464. M. Born and E. Wolf, Principles o f Optics, Pergamon, Oxford, 1965. J. P. Mathieu, Optiq ue, Vol. 1, Optiq ue Electro magndtiq ue, Soci~t~ d'Edition d 'Enseignement Sup~rieur, Paris, 1965. S. Tolansky, Multiple-beam Interference Microscopy o f Metals, Academic Press, New York, 1970.
153
SELF-COLOUR ANODIZING OF TITANIUM
J.-L. D E L P L A N C K E , M. D E G R E Z , A. F O N T A N A
and R. W I N A N D
Unwersitg Libre de Bruxelles -- C P 165, Department of Metallurgy and Electrochemistry,
50 avenue F. D. Roosevelt, B 1050 Brussels (Belgium) (Received December 2, 1981)
Summary Relations between the colours and the electrochemical parameters (current density, quantity of electricity and temperature of the electrolyte) during the anodization of titanium sheets were studied in a 1 M H2SO4 solution. Mathematical theories were developed for the kinetics of growth of the anodic film and for the colours of this film. These theories show that the relation between the growth and the colour of the film is complicated. Nevertheless, it was possible to find experimental conditions that led to uniform and well-defined colours.
1. Introduction Titanium is a light metal with remarkable mechanical properties. Moreover, this metal is covered by a natural film of oxide that gives it an excellent resistance to corrosion. This is the reason why this metal is frequently used in aeronautics and in the chemical industry. Its production is growing continually. Unfortunately, this metal shows sensitivity to severe corrosion under specific circumstances. This sensitivity originates mainly from defects in the surface of the metal. In order to eliminate the defects, anodizing may be considered. Papers about the anodization o f titanium are numerous [1]. In some o f them, the appearance of colours on the surface of the anodized sheets has been remarked [2 - 10]. In addition, self-colour anodizing of titanium has been patented [ 11 - 17 ]. Nevertheless, we have n o t found in any of these papers either a systematic study of the colours obtained after anodizing or a theoretical explanation of their appearance. The purpose of this study is therefore twofold: on the one hand, to find the conditions that produce, after anodizing, uniform colours on large area samples and, on the other hand, to derive theories to explain the phenomena that are responsible for these colours. 0376-4583/82/0000-0000/$02.75
© Elsevier Sequoia/Printed in The Netherlands
154
2. Experimental details 2.1. Electrodes Rectangular anodes were prepared by sectioning a sheet o f IMI 115 titanium. The dimensions were 45 m m X 43 mm X 0.9 mm. The anodes were degreased in trichlorethylene, rinsed with ethanol, dried, and pickled in a m i x t ur e o f 75 vol.% nitric acid solution and 25 vol.% hydrofl uori c acid solution as r e c o m m e n d e d in ASTM Standards [18] until red fumes appeared. Th en the m i xt ur e was quickly diluted with distilled water. T he o t h e r electrode was an aluminium cathode.
2.2. Electrolyte Th e electrolyte was a diluted solution of pure sulphuric acid. This electrolyte was chosen among o t h e r possible solutions because sulphuric acid is readily available and n o t expensive. The concent rat i on of the solution was 1.02 M. T he electrolyte was d e o x y g e n a t e d using pure nitrogen. Experiments were p e r f o r m e d at constant t e m p e r a t u r e by means o f a Haake thermostat. Three temperatures were studied: 25, 40 and 60 °C.
2.3. Electrolytic cell and electrical supply The electrolytic glass cell with a heating jacket was supplied at constant current. This galvanostatic m e t h o d was chosen for two reasons. (1) When the voltage is applied in a potentiostatic experiment, the initial growth o f th e film occurs at a high and u n k n o w n transient current. (2) During a potentiostatic experiment, a continual decrease in the current is observed as a result o f the growth o f the film. The kinetics of the electrochemical growth thus c a n n o t be kept constant t h r o u g h o u t a potentiostatic experiment. Accordingly kinetic study o f the f o r m a t i o n of t he anodic film is very difficult to carry out. During o u r e x p e r i m e n t the voltage d r o p between cat hode and anode and the current intensity in the cell were recorded continuously by means o f a Servogor 120 BBC Goerz Metrawatt recorder. The a m o u n t o f electricity was also recorded by means of a Tacussel IG6-N integrator. The electrolysis was carried o u t at various c ur r e nt densities over periods from 1 s to 1.5 h.
2.4. Colour analysis In order to describe accurately the colours o f our samples, an objective criterion was chosen: t he colour o f each sample was characterized as a d o t in the X - Y diagram o f the International Commission of Lighting according to the m e t h o d described in t he IES Lighting Handbook [ 1 9 ] . Knowledge o f the distribution o f the luminous intensity reflected by the sample for wavelengths in the range 380 - 760 nm for 5 nm increments and o f the same distribution for the light source is needed. These data are obtained b y means o f a Zeiss RA3 s p e c t r o m e t e r coupled with the reflective device M40. The results are analysed by means of an HP 21 MX com put er.
155
3. Results 3.1. E l e c t r o c h e m i c a l results
Figures 1 - 5 s h o w the anode-to-cathode voltage drop versus the a m o u n t of electricit:~ for five current densities. At a given current density no difference between the curves should be observed provided that the p h e n o m e n o n 70'
70 V[V)
aO
50
50
~0
/.0
3O
20
20
10
10
I0
20
3b
~0
Fig. 1. A n o d e - t o - c a t h o d e volta.~e d r o p V as a f u n c t i o n o f t h e a m o u n t Q o f e l e c t r i c i t y f o r a c u r r e n t d e n s i t y o f 4 0 . 5 A m - ( t e m p e r a t u r e , 25 °C). Fig. 2. A n o d e - t o - c a t h o d e v o l t a g e d r o p V as a f u n c t i o n o f t h e a m o u n t Q o f e l e c t r i c i t y f o r a c u r r e n t d e n s i t y o f 32.4 A m - ~ ( t e m p e r a t u r e , 25 ° C ) .
70" v{v) 60 50 V{V) 40
z,O
30
30 20
10
10 ~Q
2p
~
&o
s9 o(¢)
Fig. 3. A n o d e - t o - c a t h o d e voltage d r o p V as a f u n c t i o n o f t h e a m o u n t Q o f e l e c t r i c i t y f o r a c u r r e n t d e n s i t y o f 24.3 A m - 2 ( t e m p e r a t u r e , 25 °C). Fig. 4. A n o d e - t o - c a t h o d e voltage d r o p V as a f u n c t i o n o f t h e a m o u n t Q o f e l e c t r i c i t y for a c u r r e n t d e n s i t y o f 16.2 A m - 2 ( t e m p e r a t u r e , 25 °C).
156 (V3
30 f
~
20
10
q~
2; ____?o_
4o
~o
6;) ~lc)
Fig. 5. Anode-to~athode voltage drop V as a function of the amount Q of electricity for a current density of 8.1 A m - 2 (temperature, 25 °C).
is highly reproducible. This is approximately true at 40.5 and 8.1 A m-2. For the intermediate values, the curves, similar at the beginning, diverge rapidly. The voltage drop measured at the end of the electrolysis for each sample m a y be plotted v e r s u s the amount of electricity, whatever the current density is. This plot is shown in Fig. 6. Two types of behaviour, one called "high" current density, the other one called " l o w " current density, may be distinguished. They break up at a b o u t 9 V. Nevertheless, the aspect of all the curves is the same: an initial fast increase in the voltage drop followed b y a region of much slower increase. This aspect has already been observed b y Ammar and Kammal [20] and by Jouve e t al. [21] respectively for current densitites lower and higher than ours. Figures 1 - 5 show also that each curve, whatever the current density, presents an inflexion point at a b o u t 9 V. A potentiostatic study shows that gas evolution is observed at this voltage at the surface of the anode. Figures 7 and 8 show the influence of the temperature of the electrolyte on the overall voltage drop for current densities of respectively 40.5 A
FI o.i x
~
10
4o
~o
6o
~ Ic)
Fig. 6. Anode-to-cathode voltage drop Ve at t h e end o f e l e c t r o l y s i s as a f u n c t i o n o f t h e a m o u n t Q of electrmity for various current d e n s i t i e s (temperature, 25 °C): +, 40.5 A m-2; o, 32.4 A m - 2 ; x, 24.3 A m-2;m, 16.2 A m - 2 ; e , 8.1 A m -2.
157 60:
SO.
20
6O Ve (V}
Ve• /
50 ~0
J
-
'IO
10
O(F_)
10
2o
30
O(C) 10
20
30
Fig. 7. Anode-to-cathode voltage drop Ve at the end of the electrolysis as a function of the amount Q of electricity for various temperatures (current density, 40.5 A m-2): - - , 20 °C;- - -, 40 °C;-- - --, 60 °C. Fig. 8. Anode-to-cathode voltage drop Ve at the end of the electrolysis as a function of the amount Q of electricity for various temperatures (current density, 16.2 A m-2): - - , 20 °C;- - - , 40 °C;-- - --, 60 °C.
m - 2 and 16.2 A m - 2 . A n increase in t e m p e r a t u r e greatly decreases t h e voltage at t h e e n d o f t h e electrolysis at high c u r r e n t d e n s i t y and o n l y slightly at l o w c u r r e n t d e n s i t y . H o w e v e r , in this case at high t e m p e r a t u r e a s t e a d y value o f t h e voltage d r o p m a y be o b t a i n e d .
3.2. Results o f the study o f the eolours T h e t i t a n i u m a n o d e s s h o w e d c o l o u r s t h a t c h a n g e d d u r i n g t h e process o f electrolysis. In o r d e r t o establish a relation b e t w e e n these c o l o u r s and t h e e l e c t r o c h e m i c a l d a t a , seven samples, each o f w h i c h was representative o f o n e o f t h e seven d i f f e r e n t c o l o u r s o f t h e visible s p e c t r u m , were selected. Figure 9 shows t h e d i s t r i b u t i o n o f t h e l u m i n o u s intensities o f these samples versus t h e wavelength. bSO'D~l~
08
06
Fig. 9. Absorption of the light reflected by samples as a function of the wavelength X: curve 1, yellow; curve 2, light brown; curve 3, brown; curve 4, violet; curve 5, dark blue; curve 6, sky blue; curve 7, green.
158
Figure 10 shows the reduced trichromatic coordinates of these samples in the diagram o f the International Commission of Lighting. For each current density, samples with the same reduced trichromatic coordinates were selected and plotted as functions of the electrochemical observations (Fig. 11).
G8.
,~-.q,~o '5~o ~o
07.
x~.~o
06,
"~o $7o
05, s00 y 0~
~5~0 . .7.zx%sgo ~ .6" ~ I~;3"~600
02 01
~80~
J
0136o02
03xOL~
05
06
07
Fig. I0. Reduced trichromatic coordinates: point 0, light source, point I, yellow; point 2, lightbrown; point 3, brown; point 4, violet;point 5, dark blue; point 6, sky blue; point 7, green.
'704Ve (V)
a
~
60
.
.
.
.
.
.
.
.
.
.
.
b
,
4O
30
..
2O
10
lc,
2.0
a9
L..o
s.o
60
Q{()
Fig. 11. Relation between the colours of the samples (- - -~ and the electrochemical parameters (--) (temperature, 25 °C): curve a, 40.5 A m-2; curve b, 32.4 A m-2; curve c, 24.3 A m-2; curve d, 16.2 A m-2; curve e, 8.1 A m-2; curve I, yellow; curve 3, brown; curve 4, violet,curve 5, dark blue, curve 6, sky blue; curve 7, green.
159 4. Discussion
4.1. Electrochemical aspects Our results have been used as a basis for an a t t e m p t to explain theoretically the kinetics of the growth of the anodic film on titanium. The different kinetic steps o f growth of an anodic film in general have been presented by Vermileya [22] as follows: nucleation of the anodic film; growth of the crystals;possible formation of a continuous film;thickening of the film; limitation of the film thickness. As a result of our experiments, these steps have to be changed in the case of titanium. The first three steps may be withdrawn because there is, before anodization, a natural continuous oxide film. The last two steps must be changed as follows: thickening of the natural oxide film; oxygen evolution; dissolution of the film in sulphuric acid. The discharge of oxygen, observed during our experiments, has also been observed by Jouve et al. [21] but has not been observed by Ammar and Kammal [ 2 0 ] , probably because they did n o t reach a high enough value of the voltage drop between the electrodes and thus of the anodic potential. The dissolution of the titanium oxide film in sulphuric acid is thermodynamically possible [23] and was experimentally observed by Sinigaglia et al. [24]. From a theoretical point of view, there are only two phenomena, diffusion and migration, t h a t may be responsible for the thickening of the film. A quick calculation shows that diffusion is not of great importance because the speed o f the growth" by diffusion at 25 °C in the solid state is at most about 0.1 A s-1 . We considered, as some workers do [2 - 4], t h a t Ti 4+ ion migration in a strong electric field in an ionic conductor that may eventually show some electronic conduction (this allows oxygen evolution to occur) is responsible for the film growth. Mott and Cabrera's theory [25] of growth by migration, already developed for anodizing aluminium, can be used in our case. If there is no discharge of oxygen at the surface of the anode and if the dissolution o f the film in sulphuric acid may be neglected, the variation in the voltage V as a function o f the current density J, the time t and the initial thickness 60 will be as follows (in standard units): V = A(5.4 X 1 0 - n J t + 60) ln(BJ) where A and B are constants. This formula was used for the linear part of the curves in Fig. 3 between 0 and about 10 V and for a current density of 24.3 A m -2. We obtained for ~0 a value of about 30 A, which is in agreem e n t with literature values [26]. If there is oxygen evolution at the anode at a partial current density J+ and if A5 = kt" is the law of dissolution of the film in sulphuric acid, where A5 represents the variation in the thickness during dissolution, the above formula will become
V = A{5.4 × 1 0 - n ( J - J+)t + 5o -- kt n} In
15.4 × l O - 1 1 ( j - - J ÷ ) - - k n t " - i 5.4 X 10-11/B
1
160 The values of each parameter in this formula would have to be experimentally determined by further studies. 4.2. Coloration
of the films
Two theories have been proposed to explain the colour of the anodized titanium sheets.
o
,,b
A,A,b
Fig. 12.
Multiple-beam interference.
(1) The colours could be due to stoichiometric defects in the composition of the film [10]. (2) Interference phenomena may cause the coloration [ 12, 13, 15, 17 ]. Figure 12 illustrates the second theory, which is called the "multiple beam interference" theory. If the incident beam 1 is a beam of white light, the reflected beam, which is formed by the interfering beams 2, 5, 8, 11, ..., will be coloured. This colour will be the one reinforced by the interferences and will thus be the complementary colour of the extinguished colour. The value of the extinguished wavelength will be dependent on the thickness of the oxide film. This theory has been described in several books [27 - 29]. Table 1 shows the conditions for the extinction and reinforcement of wavelengths in ideal conditions, i.e. a transparent oxide film (index of refraction, n2) with a constant thickness on an even and perfectly reflecting surface of titanium (index of refraction, n3). When the value of the thickness d introduced in this formula is increased, a succession of colours is obtained. Th~s theoretical succession is exactly the same as the succession that we have observed experimentally, i . e . yellow, brown, dark blue, sky blue, green, yellow. Figure 10 shows that a range of wavelengths is absorbed instead of only one wavelength. This can be attributed to non-uniformity in the thickness of the oxide layer. Nevertheless, the value of the most strongly absorbed TABLE 1 C o n d i t i o n s for t h e r e i n f o r c e m e n t and e x t i n c t i o n o f w a v e l e n g t h s under ideal c o n d i t i o n s Reinforcement
Extinction
47/
4~/ -k
n2 < n3
--
n2 >n3
--
n 2 d c o s 12 = 2k~"
4y
k
n2d
cos
t2 = ( 2 k
+
1)~"
n2d
c o s 12 = ( 2 k + 1)71
n2d
cos z2 = 2kTr
4~ --
k
161
wavelength increases with the thickness of the oxide film, i.e. with the duration of anodization at constant current. We therefore think that the p h e n o m e n a o f interference are responsible for the colours of the films. It may, however, be possible that for thick crystallized films these colours are a consequence o f stoichiometric defects.
5. Conclusions It may be concluded that the t w o theories that we have developed, the one a b o u t the kinetics of growth of the oxide film and the other about the coloration o f these films, have a c o m m o n point, the thickness of the film. Accordingly, the colours of the samples and the electrochemical parameters can be related to one another. Further experiments should be performed in order to characterize more precisely the films formed by anodizing and to determine the limits of validity of our theories. From a practical point of view, we have found a surface treatment that leads to uniform colours and the experimental conditions necessary for good reproducibility of a given colour. These data may be employed for the following industrial uses: ornamental effects in jewelry; the colouring of titanium sheets for architectural purposes; local "printing" b y anodizing instead of b y painting, which does n o t adhere very well to the normal titanium surface; automatic choice of a titanium piece from among others according to its colour instead of its dimensions.
Acknowledgments The authors want to express their gratitude to a number of colleagues in the university who helped to achieve this work, either b y fruitful discussions or b y allowing them the use o f the facilities o f their laboratories: in particular, t h e y are grateful to Professors R. Poncelet, O. Goche and F. Bouillon.
References 1 J.-L. Delplancke, Anodisation autocolor~e du titane, Travail de Fin d'Etudes en Vue de l'Obtention du Grade d 'ingdnieur Civil Chimiste, Universit~ Libre de Bruxelles, 1981 2 F. Kover and M. Musselin, Rev. Gen. Elect., 76 (1967) 793. 3 F. Kover and M. Musselin, Thin Solid Films, 2 (1968) 211. 4 F. Kover, Thdse de Doctorat d'Etat, Facult6 des Sciences de Paris, 1967, enregistrement CNRS A.O.1088. 5 A. Aladjem, J. Mater. Sci., 8 (1973) 688. 6 G. Darnault, MetaUurgie (Mons), 12 (1972) 146. 7 H. Richaud, Rev. Mdtall. (Paris), 54 (1957) 787. 8 P. Pedeferri and R. Pionteili, Electrochim. Met., 3 (1968) 211.
162 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
N. P. Peksheva, Zashch. Met., 14 (1978) 74. I. Rozenfel'd, A. Akimov and E. Oshe, Zashch. Met., 5 (1969) 217. E.W. Kendall, U.S. Patent 3,616,279, 1971. P. Derouwaux and E. Spycher, Swiss Patent 513,012, 1971. P. Derouwaux and E. Spycher, Swiss Patent 522,516, 1972. J. B. Cotton and P. C. S. Hayfield, Br Patent 1,175,355, 1969. F . W . Lewis and J. H. Clayton, Br. Patent 1,319,542, 1973. Hi-Shear Corporation, Br. Patent 1,100,912, 1968. J. Weigel, Ft. Patent 1,398,453, 1965. A S T M Stand. B481-68, Part 7, 1971. J. E. Kaufmann and J. F. Christensen (eds.), IES Lighting Handbook, Illuminating Engineering Society, New York, 5th edn., 1972. I. A. A m m a r and I. Kamal, Electrochim. Acta, 16 (1971) 1539. G~ Jouve, A. Politi, P. Lacombe and G. Vuye, J. Less-Common Met., 59 (1978) 175. D. M. Vermileya, Adv. Electrochem. Electrochem. Eng., 3 (1963) 211. M. Pourbaix, Atlas d'Equilibres Electrochimiques d 25 °C, Gauthier-Villars, Paris, 1963. D. Sinigaglia, G. Taccani and B. Vicentini, Werkst. Korros., 24 (1973) 1027. J. Bernard, L'Oxydation des Mdtaux, Vol. 1, Gauthier-Villars, Paris, 1962. V. Andreeva and V. Kazarin,Proc. 3rd Int. Conf on Metallic Corrosion, 1966, Mir, Moscow, 1969, p. 464. M. Born and E. Wolf, Principles o f Optics, Pergamon, Oxford, 1965. J. P. Mathieu, Optiq ue, Vol. 1, Optiq ue Electro magndtiq ue, Soci~t~ d'Edition d 'Enseignement Sup~rieur, Paris, 1965. S. Tolansky, Multiple-beam Interference Microscopy o f Metals, Academic Press, New York, 1970.