Effect of anodic oxidation parameters on the titanium oxides formation

Corrosion Science 49 (2007) 939–948 www.elsevier.com/locate/corsci

EVect of anodic oxidation parameters on the titanium oxides formation M.V. Diamanti, M.P. Pedeferri

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Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Via Mancinelli 7, 20131 Milano – Italia Received 13 March 2006; accepted 26 April 2006 Available online 16 June 2006

Abstract The aim of this paper is to evaluate the eVects of titanium anodic oxidation in a sulphuric acid electrolyte on the crystallinity of the oxide layer and to adjust the process parameters, in order to maximize the TiO2 crystalline phase, specially for what concerns the anatase form. In particular, the relationship between anatase formation and anodization parameters, such as current density and applied potential, will be evidenced. From XRD analysis two opposite trends emerged: oxide conversion to anatase is promoted either by an increase in current density or by a decrease in sulphuric acid concentration. © 2006 Elsevier Ltd. All rights reserved. Keywords: Anodic Wlms (C); Galvanostatic polarization (B); X-ray diVraction (B); Titanium (A)

1. Introduction Every metal, with the exception of gold, when exposed to the atmosphere undergoes a process of corrosion that leads to the build up of a thin layer on its surface, made of the oxides and hydroxides coming from the reaction of the metal itself with the aqueous vapour present in the air. These reaction products can protect the surface from further alterations in some circumstances; yet the behaviour of the protective layers can be modiWed, *

Corresponding author. Tel.: +390223993110; fax: +390223993180. E-mail address: [email protected] (M.P. Pedeferri).

0010-938X/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2006.04.002

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in order to meet some speciWc requirements or simply to lower the surface reactivity. The electrochemical process of anodic oxidation of metals allows the obtaining of an oxide layer much thicker and denser than that formed naturally in the atmosphere. In particular, the so generated oxide will have determined characteristics, such as thickness, colour, density, homogeneity and insulating properties, as a function of the process parameters imposed. Titanium anodizing technology is not as much consolidated as for aluminium, which has been largely studied for the last 20 years [1]. In fact, few data are available and experimental procedures are still to be developed; in most cases, TiO2 Wlms generated this way are thin and porous [2–4]. The parameters that most aVect the oxide characteristics are the electrolyte solution (concentration of reagents, pH and temperature), the electrical potential diVerence imposed between cathode and anode and the current density imposed to reach that value of potential diVerence [5,6]. For what concerns the electrolyte, an important requirement is that it should not be aggressive towards the growing oxide, to avoid its dissolution during the process, or at least it should be ensured that the oxide growth rate is higher than the dissolution one. The most commonly used electrolytes in titanium anodizing are phosphoric and sulphuric acids, at diVerent degrees of dilution [7], ammonium sulphate or sodium bicarbonate solutions (1–15 wt %), solutions containing Xuoride ions and more. Potential diVerence and current density applied can vary within a wide range of values, being the polarization potential approximately between few volts and 250 V depending on the oxide characteristics required. In fact, low potentials (1–130 V) allow the obtaining of a smooth, amorphous oxide, about 3–100 nm thick, whose colour changes as a function of thickness and, consequently, of applied voltage (interference colour) [3,8]. On the contrary high potentials (100–250 V), combined with high current densities, are the parameters used in anodic spark deposition (ASD) processes, which lead to an oxide that can range from few tens to hundreds micrometers thick [1,9]; its surface features are a glassy appearance and the presence of holes, both due to the establishment of electric arcs. These arcs start at the weak sites of the pre-existent oxide and move over the whole surface, leaving holes where each arc is generated and melting the oxide because of the high energy density they carry (current density of 104 A/cm2, reaching temperatures near to 8000 K): local melting of the oxide allows ions present in the electrolyte to enter the oxide in formation, thus modifying its chemical and structural composition [10–13]. In the last years, a new class of anodization techniques has been developed, consisting of the anodic oxidation of titanium in electrolytes containing Xuoride ions: particular surface structures can be achieved with this method. In fact, these ions tend to perforate locally the TiO2 barrier layer, which causes the oxide to thicken around the small holes while the ions consume the oxide forming on their bottom: the result is a nanotubular morphology, height and diameter of the nanotubes depending on the Xuoride ions concentration and solution pH. These structures arise only when speciWc polarization parameters are applied, namely, low potentials and long anodization times [14–16]. Titanium dioxide may present both amorphous and crystalline structures, depending on process parameters. Crystalline oxides, that is, anatase and rutile, present several peculiar features, such as photocatalytic behaviour, superhydrophilicity and biocompatible properties [17–21]. This work focuses on the evaluation of the eVects of titanium anodic oxidation in a sulphuric acid electrolyte on the formation of TiO2 crystal phases in the oxide layer; the

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impact of process parameters modiWcation will be studied, in order to maximize the achievement of these structures, specially for what concerns the anatase form. In particular, the relationship between anatase formation and process parameters will be evidenced. 2. Experimental Anodizing was carried out on commercially pure titanium samples, namely, grade 2 in ASTM classiWcation, which is the most available and workable titanium grade and whose nominal composition, in terms of maximum impurities percentage allowed, is O < 0.25% – N < 0.03% – H < 0.015% – Fe < 0.3%. Samples of two diVerent dimensions (2 cm £ 5 cm and 2 cm £ 3 cm) were cut out of a 0.5 mm thick titanium sheet. Samples were anodized as received after being degreased with acetone. Galvanostatic polarization of the samples was obtained by using an amperostat (model Eutron Rivoli, Italy). Test conditions applied vary in terms of sulphuric acid concentration, current density and polarization potential. Investigations with respect to the electrolyte inXuence on the oxide in formation were made using Wve diVerent solutions, lying in the concentration range 0.25–2 M, obtained from analytical grade sulphuric acid and deionized water; all reagents were purchased from Carlo Erba reagents. Samples were anodized by imposing a current density ranging from 100 to 1080 A/m2 and a cell potential from 10 to 150 V. All experiments were carried out at room temperature. For each set of anodizing parameters adopted, oxidation kinetics will be observed, together with the growth of crystal TiO2 phases. Since applied potential is proportional to oxide thickness, information about oxidation kinetics can be supplied from the growth rate of potential in time, which is proportional to the oxide growth rate if dissipative contributes are neglected; the presence of crystal phases (speciWcally, anatase and rutile) in the oxide is examined through X-ray diVraction and the morphology is investigated with Scanning Electron Microscope technique. 3. Results DiVerent series of experiments have been performed by changing the operative parameters, as reported in Table 1. At Wrst, a series of tests at constant current density was made in all of the H2SO4 solutions chosen, applying a current density equal to 200 A/m2. For every sample anodized in a Table 1 List of sulphuric acid concentrations and current densities applied in tests

Series 1 Series 2 Series 3 Series 4 Series 5 Series 6 Series 7 Series 8 Series 9

Electrolyte molarity

Current density (A/m2)

H2SO4 0.25 M H2SO4 0.5 M H2SO4 0.7 M H2SO4 1 M H2SO4 2 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M

200 200 200 200 200 100 400 800 1080

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Table 2 Maximum value of cell potential achieved as a function of solution Maximum potential attained (V) H2SO4 0.25 M H2SO4 0.5 M H2SO4 0.7 M H2SO4 1 M H2SO4 2 M

120 110 100 90 70

given solution, the cell potential reached a diVerent value: the Wrst sample was polarized to 10 V, then a potential increase of 10 V with respect to the previous test value was applied to each of the following samples (e.g., sample 1 anodized to 10 V, sample 2–20 V, sample 3– 30 V, etc.). Each series of experiments with increasing potential was limited to the potential at which oxide growth slowed down signiWcantly, that is, when the slope of the potential to time curve changed remarkably from that recorded in the Wrst test (Table 2). This sequence of experiments at increasing potential values was performed for every diVerent solution or current density analyzed. 4. EVect of electrolyte Fig. 1 presents the potential growth rate for the samples anodized in 0.25 M H2SO4 at 200 A/m2. The highest potential reached in these conditions was 120 V. To illustrate the oxide growth kinetics, a potential versus time curve was derived by plotting the data collected in the 12 tests: cell potential values were acquired every 10⬙ during polarization for each of the 12 samples processed. Only data regarding the sample polarized to 120 V are shown entirely; additionally, points surrounding the curve represent the total time the other samples spent to reach the potential value imposed (Fig. 1). The curve shows an average growth rate, in the Wrst 10⬙ polarization, of 3.37 V/s and the single points provide evidence of the good repeatability of polarization rate for each sample anodized. 150

Cell potential [V]

120

90

60

30

0 0

60

120

180

240

300 Time [s]

360

420

480

540

600

Fig. 1. Cell potential versus time plot for samples anodized in sulphuric acid 0.25 M, with potential values ranging from 10 to 120 V and current density 200 A/m2.

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500

Intensity [count/s]

450 400 350 300 250 200 150 100 24,01

24,61

25,21

120V 110V 100V 90V

25,81

26,41 27,01 Angle [2 theta] 27,61

28,21

Fig. 2. XRD analysis of samples anodized in 0.25 M H2SO4 (cell potentials 10–120 V, current density 200 A/m2).

Changes in the crystalline structure of the oxides can be observed as the Wnal potential applied increases. The XRD patterns obtained for each sample anodized in these conditions indicate that anatase formation begins in the sample polarized to 90 V, and the crystal phase appears to increase with the increasing potential (Fig. 2). Concerning the diVraction patterns, the data exposed in this paragraph in relation to each analysis are limited to the angular range 24–29°, as this range includes distinct peaks of both anatase and rutile crystal phases, as proof of the actual presence of these phases in the oxide analysed. In fact, the peak visible at an angle of 24.6° indicates the presence of anatase, the one at 27.4° corresponds to rutile phase. Tests carried out in 0.5 M H2SO4 show analogous trends for what concerns kinetics as well as oxide tendency to convert to anatase: yet, this electrolyte granted the best results in terms of both these features. In fact, oxides formed in this solution present a slightly higher growth rate (approximately, 3.54 V/s in the Wrst 10⬙ polarization) and a higher intensity of the anatase peaks; moreover, anatase formation can be observed for 70 V polarization potential or more. Further increases in sulphuric acid concentration (namely, the use of 0.7 M, 1 M and 2 M H2SO4) led invariably to an earlier appearance of anatase, with respect to the anodizing potential, and to a substantial decrease in the oxide formation rate and in the intensity of the anatase XRD peak. Kinetics and anatase formation trends as a function of applied potential are equivalent to those exposed before. Data regarding the average potential increments in the Wrst 10⬙ polarization and time required to reach 70 V in the diVerent solutions are reassumed in Table 3; to evaluate the kinetic behaviour at prolonged process times, samples anodized to 70 V were chosen as representative for that potential value was the highest one reached in every solution. Table 3 Potential growth rate in the Wrst 10⬙ of polarization (dV/dt) and anodization time required to reach 70 V (T70 V) as a function of solution

dV/dt [V/s] (Wrst 10⬙) T70 V [s]

H2SO4 0.25 M

H2SO4 0.5 M

H2SO4 0.7 M

H2 SO4 1 M

H2SO4 2 M

3.37 58

3.54 45

2.97 44

2.90 60

1.58 510

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Table 4 Maximum value of cell potential achieved as a function of current density Maximum potential attained (V) 100 A/m2 200 A/m2 400 A/m2 800 A/m2 1080 A/m2

70 110 120 140 150

In the light of these data, additional studies were carried out in 0.5 M H2SO4 as in this electrolyte the best results, in terms of crystal phase formation and potential increases, were accomplished. 4.1. EVect of current density The second parameter of interest investigated was current density: starting from the previous experiments value (200 A/m2), tests were performed applying both lower and higher values. In particular, current densities equal to 100 A/m2, 400 A/m2, 800 A/m2 and 1080 A/m2 were used (Table 4). Samples processed at low current density, that is, 100 A/m2, showed a slow oxidation rate, which decreases with increasing potential much faster than in the previous tests, thus allowing lower potentials to be achieved. XRD patterns show a decrease in the anatase peak intensities, too. An opposite trend can be identiWed in experiments carried out at higher current densities: the more the applied current increased, the more the oxide seemed to convert to anatase and the higher potential was achieved; also potential growth rate raised signiWcantly. Moreover, XRD patterns of samples anodized at high potential and current density values demonstrate the presence of both TiO2 crystal phases, anatase and rutile: results concerning samples anodized at 1080 A/m2 are given to exemplify these trends, as these tests showed both the best kinetics and degree of crystal phases formation in the oxide (Figs. 3 and 4). As for the experiments carried out in diVerent solutions, the average potential growth rates in the Wrst 10⬙ polarization are presented, as well as the time required to reach 70 V, in the diVerent conditions tested (Table 5). 5. Discussion A correlation between anodization parameters and either kinetics or anatase formation can be recognized. Actually, current density and sulphuric acid concentration have opposite inXuences on those features: in fact, an increase in the former leads to faster polarization processes and higher intensities of anatase XRD peaks, as well as higher potential values reached (Fig. 5), while identical eVects can be ascribed to a decrease in the acid molarity (Fig. 6). An exception is represented by samples anodized in 0.25 M H2SO4: in this solution, tests are a little slower and anatase formation seems to be inhibited. A crucial role, regarding the oxide tendency to form crystal phases, is played by the cell potential. It shows clearly the presence of a sort of activation threshold: once this value is overlaid, anatase begins to form, plausibly increasing in quantity with the increase of

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150

Cell potential [V]

120

90

60

30

0 0

10

20

30

40

50 Time [s]

60

70

80

90

100

Fig. 3. Cell potential versus time plot for samples anodized in sulphuric acid 0.5 M, with potential values ranging from 10 to 150 V and current density 1080 A/m2.

Intensity [count/s]

4600 3700 2800 1900 1000

150V 140V 130V 120V 110V 100V 90V 80V 70V

100 24,01 24,61 25,21 25,81

26,41

Angle [2 theta]

27,01

27,61

28,21

Fig. 4. XRD analysis of samples anodized in 0.5 M H2SO4 (cell potentials 10–150 V, current density 1080 A/m2). Table 5 Potential growth rate in the Wrst 10⬙ of polarization (dV/dt) and anodization time required to reach 70 V (T70 V) as a function of current density

dV/dt [V/s] (Wrst 10⬙) T70V [s]

100 A/m2

200 A/m2

400 A/m2

800 A/m2

1080 A/m2

1.64 418

3.54 45

5.36 24

13.79 5.5

18.97 3.6

polarization potential. The value of this threshold is strongly aVected by the concentration of sulphuric acid and by the applied current density. Moreover, for high potentials and current densities, there is the concurrent growth of rutile: this causes a gradual reduction in the intensity of anatase peaks from anodization potentials 20–30 V greater than rutile formation threshold. An example of this behaviour is illustrated in Fig. 7, which represents the correlation between potential and anatase formation for samples anodized in 0.5 M H2SO4

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M.V. Diamanti, M.P. Pedeferri / Corrosion Science 49 (2007) 939–948 4500 1080 A/m2 800 A/m2 400 A/m2 200 A/m2 100 A/m2

Intensity [counts]

3600 2700 1800 900 0 50

60

70

80

90

100

110

120

130

140

150

Cell potential [V]

Fig. 5. Relationship between polarization potential and anatase peak intensity for samples anodized in 0.5 M H2SO4 at diVerent current densities. 1000

0.25 M 0.5 M 0.7 M 1M 2M

900

Intensity [counts/s]

800 700 600 500 400 300 200 100 0 50

60

70

80

90 100 110 Cell potential [V]

120

130

140

150

Fig. 6. Relationship between polarization potential and anatase peak intensity for samples anodized with current density 200 A/m2 at diVerent H2SO4 molarities.

at 1080 A/m2, underlying the threshold for crystal phase appearing, the increase in the intensity of anatase peak for high potentials and the beginning of rutile formation. The trends in anatase and rutile formation should be attributed to the slightly higher thermodynamic stability of the rutile phase with respect to the anatase phase, which makes rutile more likely to form, even though anodization conditions should promote the conversion of the amorphous oxide to anatase, thanks to the non-equilibrium solidiWcation conditions that establish in the oxide during ASD. It is worth noticing that the ASD phenomena appeared to activate on the surfaces of a few samples, namely, those anodized at potentials higher than 100 V and current densities from 400 A/m2 up. Evidence of the setting up of local electric sparks is given by the SEM images of samples surface: craters are clearly detectable on the surface, left by the establishment of electric arcs, while the remaining surface is particularly smooth, which is typical of a molten and rapidly quenched material (Fig. 8). Indeed, it is the ASD establishment

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160

Anatase (4396 counts/s)

140

Rutile (822 counts/s)

Cell potential [V]

120 100 80

Anatase (3102 counts/s)

60

Anatase (242 counts/s)

40

Rutile (844 counts/s)

20 0 0

60

120

180

240

300 Time [s]

360

420

480

540

600

Fig. 7. Trend in oxide crystallization with respect to cell potential during titanium anodic oxidation (formation thresholds of anatase and rutile are represented).

Fig. 8. SEM image of sample anodized in 0.5 M H2SO4 with current density 1080 A/m2 and cell potential 140 V: oxide surface.

that makes anatase growth possible, due to its particular melting and quenching conditions of non-equilibrium that exceptionally change the oxide characteristics. 6. Conclusions This paper presented the generation of a titanium dioxide Wlm and the modiWcations its thickness and crystal phase can undergo through a simple anodizing process, which enables to convert to some extent the amorphous oxide into an anatase or a rutile phase; it

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also examined how polarization parameters can aVect the resulting oxide structure and morphology. Sulphuric acid is an optimal electrolyte when the ultimate aim of anodization is the obtaining of anatase. In fact, it requires neither a preliminary surface treatment, such as chemical polishing, nor subsequent thermal treatments of conversion of an amorphous oxide which is likely to grow in other solutions: therefore, sulphuric acid allows a straightforward formation of anatase, much more easily than any other electrolytes. From the XRD analysis two opposite trends emerged: oxide conversion to anatase is promoted either by an increase in current density or by a decrease in sulphuric acid concentration, with an optimum molarity value of 0.5. Cell potential controls anatase and rutile formation since no crystal phase appears to grow for potentials lower than a threshold value, which in turn changes with current density and acid concentration; once this value is exceeded, oxide conversion increases with potential. Finally, sample morphology is greatly aVected by high potential anodizing, for ASD phenomena occur, giving rise to small craters on the surface. Still many features of the anodizing process must be analyzed: a complete understanding of the process, and of the structures it can produce as a function of the parameters imposed, can lead to an easy obtaining of the desired oxide, in terms of thickness and crystal phases formation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

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