Branka Pešovski* | Vesna Krstić | Silvana Dimitrijević
© 2022 IIETA. This article is published by IIETA and is licensed under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/).
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This article presents some research into titanium electrode anodizing in H2SO4 solutions in the potential range –1 to 5 V at different scan rates (10, 50, 100 mV s–1) using cyclic voltammetry measurements. Linear potentiodynamic measurements had used before and after the oxide layer formed on the titanium surface. The characterization and the exact determination of the titanium oxides species on the Ti substrate surface had performed by XRD and SEM-EDS analyses. Three characteristic oxide peaks appeared in the electrochemical measurements. These had observed at 0.2 V, 2.3 V, and 3.5 V on cyclic voltammetry curves at all scan rates for all H2SO4 concentrations. Even more, cyclic voltammetry measurements showed that all peaks observed were evident even at higher scan rates. That suggested that robust oxidation of the Ti surface had occurred. Voltammetric tests showed that the passivation of the titanium surface was effective. XRD and SEM-EDS measurements confirmed the existence of TiO2 species in a crystalline form on the titanium surface after anodizing in 3M H2SO4 and annealing the anodized Ti electrode at 400℃ in the airflow. A significant increase in the loading of TiO2 film annealed at 400℃ of the previous anodized Ti anode in H2SO4 was registered.
dimensionally stable electrode, titanium oxides, cyclic voltammetry, polarization, open circuit potential, XRD, SEM-EDS
The exact nature of formed protective oxide films on the titanium surface during the anodization process is still not well known, even though, in the study of surface modifications, many of the latest techniques for the investigating of properties of such films have been used [1–9]. For oxide films formed at high voltages (>50 V), the existence of both crystal phases rutile, and anatase is evidence. For the formed films on the lowervoltage (<10 V) there are more uncertainties in interpretations of one or more of three natural crystal modifications of oxides (rutile, anatase, and brucite). Therefore, a series of different oxides of low-valent titanium is present in the oxide film [10–14]. The thin anode films on titanium surfaces, predominantly amorphous structure have been found in previous studies [15].In the oxide film: TiO, Ti2O3, and Ti3O5 oxides can be present depending on whether the oxidation process is carried out chemically (in the airflow in H2SO4, HCl, HNO3, or NaCl solutions) or electrochemically (anode polarization at low or high potentials). Energy-dispersive X-ray spectroscopy (EDX) or transmission electron microscopy (TEM) of the anodized oxide films has shown that the nature of TiO2 film is also amorphous.
Therefore, the presence of the anatase crystals depends on the anode potential [16, 17]. The disagreements related to the morphology, increase in thickness, and crystallographic structure of the surface titanium oxides can result from differences in the surface preparation, type of solvent, methods of preparation, and conditions of growth of crystals [18–22].
Literature data show the dependence of oxide film structure on the rate of film thickness growth. It has also suggested that the degree of solubility of films increases significantly with a slight increase in the oxide film thickness [23]. It has also been shown that the films formed at a scan rate of 1 mVs–1 dissolve almost ten times more slowly than those at 1 Vs–1. The most likely explanation for this effect is that the aging of slowly growing films leads to a denser crystalline structure with an accompanying increase in a barrier height for ions migration.
Electrochemical dissolution of the passive film occurs at the boundary of the oxide-solution phase, according to equation (1), where the dissolution is not localized [24].
${TiO}_2+3 {H}^{+}+e^{-} \rightarrow {Ti}({OH})^{2+}+{H}_2 {O}$ (1)
The electrochemical parameters significantly influence the behavior and characteristics of titanium oxide growth. The growth of the oxide film thickness results in systematic changes in surface topography, especially in the pore surface configuration. Anionic incorporation into the oxide film will not change the chemical composition, crystals type, or the titanium oxide structure such as anatase, rutile, and even rare brucite [25, 26]. According to many researchers, a constant increase in the anode film thickness is a variable value because it changes over time [27, 28]. It significantly affects the surface properties of anodic oxide films and the forming of an intermediate layer of the TiO2-xsuboxide between a deep TiO layer of titanium substrate and a higher surface TiO2 layer [29]. The structure of anode film changes from the amorphous to the crystalline phase, which formes above the “critical” thickness of oxide film. Changes in the electrochemical parameters have quantitative and qualitative effects on electrochemical behavior. These changes also increase the current efficiency and the growth rate of the formed titanium oxide films. Of course, this behavior is significantly dependent on the nature of working electrolytes, as has been demonstrated by the experimental investigation [29, 11].
The quantities of non-stoichiometric oxides obtained within the electrochemically formed oxide film depend on the potential and pH solution. This anodization can be represented according to the reaction (2) [30]:
${TiO}_n+{H}_3 {O}^{+}+e \rightleftarrows {TiO}_{n-1}({OH})\left({H}_2 {O}\right)$
for $1.5 \leq n \geq 2$ (2)
In sulphuric acid solutions at potentials above 3 V (vs. SCE), Kudelka et al. found quite different degrees of oxide growth on a variety of single grains of titanium oxide polycrystalline substrate [31]. Marino et al. [32] have published evidence that the anode current increases in passive titanium to 2.3 V (vs. SCE). That is probably due to a phase transformation of oxides in the older potentiodynamic formed oxide films, at potentials more positive than 2.3 V (vs. SCE). The X-ray diffraction obtained data indicate the presence of Ti3O5 in a dominant TiO2 matrix. The following reactions of TiO2 film dissolution in sulphuric acid solutions occur [33]:
${TiO}_2+4 {H}^{+} \rightarrow {Ti}^{4+}+2 {H}_2 {O}$ (3)
Different soluble Ti(IV) ions thus formed. TiO2, which had formed as a protective layer, has been completely removed from some parts of the electrode surface by the formation of the Ti3+ ions. Hydrogen evaluation occurs according to the following reaction [33]:
$T i+3 H^{+} \rightarrow T i^{3+}+3 H_{a d s}$ (4)
In the aerated solutions, Ti3+ is oxidized according to the following equation [33]:
$2 {H}^{+}+2 {Ti}^{3+}+1 / 2 {O}_2 \rightarrow 2 {Ti}^{4+}+{H}_2 {O}$ (5)
The amorphous TiO2 oxide or a chemically and mechanically intense anatase-type TiO2 stable base permanently covers the titanium surface. On the other hand, the stability of the oxide film at a pH range from neutral to a low alkaline environment in the deaerated aqueous solution, could not be confirmed with certainty. So the stability of the titanium oxide film and changes in its properties should study in more detail [34].
In many areas of the titanium application, as the production of dimensionally stable titanium anodes (DSAs), the presence of a small amount of TiO2 in the surface oxide layers for better adhesion of mixed metal oxide films (MMOs) on the Ti surface is necessary. On the other hand, the annealing at temperatures above 400℃ uses for preparing most MMO coatings on titanium DSAs, and for that should therefore be studied the formation of such titanium oxide films that can passivate the titanium surface and disable the operation of DSAs.
The main goal of this study is the electrochemical formation of titanium oxides films on the titanium metal substrate. That means using anodic oxidation treatments and an investigation of structural and electrochemical characteristics of obtained oxide films. We did experimental work with great care, and we have performed the chemical and electrochemical procedures so that the results obtained could interpret with a full guarantee. Linear potentiodynamic measurements of the Ti-electrode and (Ti+TiO2)-electrode were performed in sulfuric acid solutions with the scan rate of 1 mV s–1. The kinetic parameters of electrochemical processes of (Ti+TiO2)-electrode in the investigated H2SO4 solutions were determined. The coverage degree by the oxide films on the titanium surface at the potentials 0.5 V, 1.0 V, and 1.5 V was calculated. XRD and EDS analyses of anodized and then annealed Ti samples showed an increase in the loading of oxide anodic film on the titanium electrode.
We performed the experimental tests of electrochemical parameters for the formation and characterization of the titanium oxide layers in sulphuric acid solutions. The following concentrations of 1.5 M, 1 M, 0.5 M, 0.1 M, and 0.05 M H2SO4 were used. The investigation included the following measurements into the electrochemical behavior of an anodized titanium electrode in the above-mentioned H2SO4 solutions:
– Scanning of the titanium electrode cyclic voltammograms in sulphuric acid solutions at different scan rates (10 mV s–1, 50 mV s–1, and 100 mV s–1);
– Scans were carried out per five cycles for any preindicated acid concentration and all of the above-speed recording, in the potential range of –1 to 5 V;
– Linear potentiodynamic measurements had taken after measuring the open circuit potential in various concentrations of H2SO4 solutions;
– By using a cyclic voltammetry method, a passivizing TiO2 oxide layer is applied to the surface of titanium electrode in sulphuric acid solutions at a scan rate of 50 mV s–1;
– Linear potentiodynamic measurements were performed with a titanium electrode on which a passive TiO2 film has previously applied according to the procedure described, at a scan rate of 1 mV s–1.
– Anodic oxidation of a titanium electrode has performed in a 3 M H2SO4 solution before XRD and SEM-EDS measurements in a cyclic voltammetry range (–1 to 5 V) at a scan rate of 100 mV s–1 during five cycles.
All experimental recordings had performed at room temperature in a three-electrode cell. The working electrode has made of titanium, the reference electrode was a saturated calomel electrode (vs. SCE), and the auxiliary electrode was a platinum electrode. After each recording batch, the working electrode was polished with grinding paper from 280 to 2000 grit and washed with distilled water. The IVIUMSTAT.xRe instrument provided hardware and software support.
X-ray analysis had performed on the instrument "Rigaku MiniFlex 600" with a "D/teX Ultra 250" high-speed detector and a copper anode X-ray tube. Recording conditions were: a range of angles 3-90°, step 0.02°, recording speed 10°/min. The X-ray tube voltage was 40 kV, and the current intensity was 15 mA. The identification of the minerals had made by the PDXL 2 Version 2.4.2.0. of software, and the resulting diffractograms compared with the data from the ICDD database (PDF-2 Release 2015 RDB). The detection limit of the XRD analysis is about 1%.
The surface appearance and mineralogical composition investigation of the samples SEM (JEOL, JSM IT 300LV) and EDS (OXFORD Instruments) analyses were applied.
2.1 Materials and solutions
The working electrode was made of a titanium plate, like a spatula shape in the lower part, and dimensions of 1.5 × 1.5 × 2.0 cm, with a handle-length of 15.0 cm. Grade II indicates the degree of titanium purity with a chemical composition shown in Table 1.
Table 1. The degree of Grade II titanium commercial purity
|
Content |
Purity degree Grade II (wt. %) |
|
N (max.) |
0.03 |
|
C (max.) |
0.10 |
|
H (max.) |
0.0125 |
|
Fe (max.) |
0.30 |
|
O (max.) |
0.25 |
|
Ti |
The rest |
The working solution was sulphuric acid, 96 % H2SO4 p.a. quality, manufactured at the “Zorka Factory” in Šabac, Serbia.
3.1 Cyclic voltammetry
Figure 1. shows, respectively, the CV curves recorded in sulphuric acid solutions of different concentrations at different scan rates in the potentials range of –1.0 to 5.0 V. All processes essential for the investigation of this system are in the selected potential range.
The way that section titles and other headings are displayed in these instructions, is meant to be followed in your paper [36]. An important parameter is the change in the scan rate because it influences the surface oxidation of the Ti-anode. The occurrence of the characteristic peaks that define the specific oxides assigned for the given test parameters was also supervised. The three apparent peaks at potentials 0.2, 2.2, and 3.5 V vs. SCE had detected at all scan rates. These current peaks formation are limited by the diffusion-controlled process.
Based on the CV curves shown, it is evident that after the titanium region presented in the active state, titanium oxidation took place, and the growth of the oxide film was usually in the form of TiO2 structure. It is suggested in recent data in literature that the first peak in the cycle may occur due to the formation of the titanium oxides with the lower Tivalence. These would probably be Ti2+ and Ti3+ in the form of TiO and Ti2O3 oxides, while the second and the third peaks correspond to the forming of TiO2 films [25 ]. Moreover, the layer that is formed mostly of TiO2 oxide was seen to have protective characteristics. The oxidation of a titanium substrate is shown by the following equation [37, 38]:
$T i+2 {H}_2 {O} \rightarrow {TiO}_2+4 {H}^{+}+4 e^{-}$ (6)
In addition to TiO2, Ti2O3 also can be formed as an unstable compound. Therefore Ti2O3 oxide was rapidly oxidized to TiO2 in contact with water according to the reaction [32, 39]:
${Ti}_2 {O}_3+{H}_2 {O} \rightarrow 2 {TiO}_2+2 {H}^{+}+2 e^{-}$ (7)
In all concentrations of sulphuric acid solutions tested at higher scan rates, the intensity of current peaks increases and indicates forceful titanium oxidation, which causes difficulty of a stable and compact film formation on the titanium surface. Blackwood et al. also showed that the stability of a potentiodynamic formed oxide film depends on the scan rate [33].
Figure 1. Titanium cyclic voltammograms in: A) 1.5 M, B) 1.0 M, C) 0.5 M of H2SO4 solution, respectively. The scan rate: 10, 50 and 100 mV s–1
Figure 1. Titanium cyclic voltammograms in: D) 0.1 M, E) 0.05 M of H2SO4 solution, respectively. The scan rate: 10, 50 and 100 mV s–1
Moreover, an increase in the intensity of current peaks in the concentration range examined indicates the diffusioncontrolled process [40].
The so-called "low-voltage oxide films" characteristic of voltages below 20 V (usually up to 5 V) can have an amorphous structure and defective stoichiometry, while crystalline TiO2 occurs at higher potentials [38,42]. The intermediate TiOx and TiO2-x layers are inserted between the TiO layer on the titanium surface and the outer TiO2 oxide layer, which is why the composition of such films is not uniform [29, 11]. Besides, the composition of electrolytes also has a strong influence on the crystallinity of formed TiO2 [43].
Furthermore, the crystalline TiO2 with a compact structure arises in the electrolyte with a higher water amount [42]. Nevertheless, the mechanism of the formation of the crystalline oxide form is still unclear. Probably solution impurities affect the conversion of formed amorphous film to the crystalline type in the coating [44]. The most common type of crystalline TiO2 is in the form of anatase in the anodization. The anatase type of TiO2 oxide gives a characteristic yellow color to the oxide film [27], obtained in our experiments. The disagreement in the literature data may be due to the different surface preparation and the conditions in which the growth of the oxide occurred, shown by the instant current density, chemical composition (solution concentration), and electrolyte temperature [4].
Furthermore, according to curves presented in Figure 1. the current density increases at all recording speeds at the potentials above 2 V, probably due to the start of intense oxygen evolution and the formation of unstable titanium peroxide TiO3.
Moreover, the sharp decrease in the current density in the reverse scan is because of the passivation with an oxide film, consisting of oxides formed on the titanium surface. The stoichiometry of these oxides had not entirely determined in experimental studies, but the following oxides: TiO, TiOx, Ti2O3 [30, 45, 46, 47], and the most stable TiO2 oxide most probably formed. The current peak at a potential around –0.23 V vs. SCE indicates, as well, a reduction of the unstable species on the metal surface [32]. Nevertheless, in the sulphuric acid solutions, Ti4+ ions are relatively facilely reduced to Ti3+ and Ti2+ species.
Figure 2. shows the CV curves obtained in sulphuric acid solutions. It can seem that an increase in the number of cycles leads to a significant decrease in the current density and the disappearance of the current peaks. That was easy to spot in the first cycle when the titanium oxide layers covered the surface. These layers had a significant variation in their stoichiometry and morphology, indicating oxidation of the unstable Ti2+ and Ti3+ type oxide species, like TiO, and Ti2O3, on the metal surface. It is also noticeable that after the first cycle at all concentrations of H2SO4, complete passivation of the titanium surface with stable TiO2 occurred.
3.2 Open circuit potential
Open circuit measurements of Ti and (Ti+TiO2)-electrodes had performed for 60 min in different concentrations of sulphuric acid solutions and the results obtained are presented in Figure 3., and Table 2. Based on the curves shown in Figure 3. it can see that in the sulphuric acid solutions, immediately after the electrode immersion, there is a sudden drop in the value of the open circuit potential, which most likely indicates titanium oxide dissolution on the electrode surface. Blackwood et al. found that the oxide film dissolution rate was the first-order reaction, given the proton concentration, and proposed the following dissolution reaction in an acidic solution [33].
On the OCP values of the potential for the (Ti+TiO2)- electrode at t=3600 s in 1.5 M and 1 M H2SO4, the formation of the oxide film occurs at more positive potentials in comparison to other H2SO4 concentrations. On the OCP potential for the Ti-electrode at t=3600 s in the same H2SO4 concentrations, the oxide film formation occurs at more negative potentials in comparison to 0.1 M and 0,05 M H2SO4 concentrations.
A deviation in the OCP potential value in 0.5 M H2SO4 at t=3600 s is observed. The oxide film formation on Ti-electrode occurs at a more positive potential, and on the (Ti+TiO2)- electrode occurs at a more negative potential value in comparison to all other H2SO4 concentrations.
Figure 2. Titanium cyclic voltammograms in different H2SO4 concentrations: A) 1.5 M, B) 1.0 M, C) 0.5 M, D) 0.1 M, E) 0.05 M. The scan rate is 50 mV s–1
$\mathrm{TiO}_2+\mathrm{H}_2 \mathrm{O}+\mathrm{H}^{+} \rightleftarrows \mathrm{Ti}(\mathrm{OH})_3^{+}$ (8)
However, the value of the open circuit potential (OCP) increases by the further increase in anodization time that is a consequence of the thicker layer formation of oxide films on the titanium surface [19, 35].
Table 2. The potential values for a Ti-electrode on OCP in different H2SO4 concentrations
|
Potential on |
H2SO4 concentration |
||||
|
t = 3600 s |
1.5 M |
1 M |
0.5 M |
0.1 M |
0.05 M |
|
E / VSCE |
–0.0636 |
–0.0926 |
0.0166 |
–0.0205 |
–0.0363 |
Based on data in Table 2., and Figure 3. it can conclude that because of the change in the H2SO4 concentration, in general, no significant deviation occurs in the oxidation process and formation of the oxide film on the electrode surface, both for bare Ti-electrode, as well as for (Ti+TiO2)- electrode.
Figure 3. The plot of the open circuit potential of (Ti+TiO2)- electrode in different concentrations of H2SO4 solutions
3.3 Linear potentiodynamic measurements
Figure 4. shows polarization curves in the various concentrations of sulphuric acid solutions of the titanium electrode and (Ti+TiO2)-electrode. A rapid formation of the oxide film observes from the polarization curves of the anode process at the beginning of the polarization process. By the further anodization process the passivation of the titanium surface causes increases in the potential because of the growth and the alignment of the oxide layers surface [48].
In the furthest course of the oxidation process, on potentials higher than 1.5 V, there is an increase in the current density that undoubtedly indicates further oxidation of titanium. The passivation in the whole range of concentration observes. In the concentration range from 0.05 M to 0.5 M, a passivation plateau starts at the potential of 0.4 V vs. SCE. With a further increase in the concentration, the beginning of the passivation plateau moves to lower potential values: 0.345 V vs. SCE in 1 M H2SO4 and 0.3 V vs. SCE in 1.5 M H2SO4. Moreover, the current density is constant in a wide range of potentials and increases at 1.54 V vs. SCE in 0.05 M H2SO4 up to 1.65 V vs. SCE in 1 M H2SO4 and 1.5 M H2SO4, suggesting that titanium oxides formed are more stable in a solution of a higher sulphuric acid concentration.
However, the current density increases at those potentials are due to the film dissolution on the titanium surface [19].The polarization curves of treated titanium electrodes indicate a lesser metals dissolution. In the pretreatment processes of titanium in H2SO4 solutions, the formation of a thicker oxide film on the titanium surface causes the lower current density anodization values. Furthermore, cathodic polarization curves detect a reduction of unstable titanium oxides with low- valence titanium, which has a good agreement with cyclic polarization measurements.
Based on the polarization curves presented in Figure 4. have determined kinetic parameters of electrochemical processes in the mediums investigated: the corrosion potential (Ecorr), corrosion current density (icorr), anodic (ba), and cathodic (bc) Tafel's slopes, which have presented in Table 3.
Table 3. Electrochemical parameters of (Ti+TiO2)-electrode
|
H2SO4 |
Ecorr /mV |
icorr/mA cm–2 |
ba |
bc |
|
1.5 M |
–0.44 ∙ 10–2 |
1.74 ∙ 10–4 |
545.26 |
–283.53 |
|
1 M |
7.84 ∙ 10–2 |
1.57 ∙ 10–4 |
359.59 |
–106.90 |
|
0.5 M |
–8.86 ∙ 10–2 |
1.18 ∙ 10–4 |
597.20 |
–332.40 |
|
0.1 M |
–1.44 ∙ 10–2 |
3.29 ∙ 10–5 |
111.98 |
–671.43 |
|
0.05 M |
–7.32 ∙ 10–2 |
2.70 ∙ 10–5 |
357.21 |
–428.33 |
Table 4. also shows the coverage degree of the titanium surface at different concentrations of H2SO4 at the potentials 0.5 V, 1.0 V, and 1.5 V, to obtain an image as complete as possible of the influence of the H2SO4 concentration on the coverage degree of the titanium surface with Ti-oxide.
Table 4. Coverage degree of titanium electrode surfaces with oxide in various concentrations of H2SO4 solutions on constant potentials
|
H2SO4 concentration |
Coverage degree, $\theta$(%) |
||
|
E=0.5 V |
E=1.0 V |
E=1.5 V |
|
|
1.5 M |
98.24 |
97.62 |
96.78 |
|
1 M |
98.32 |
97.43 |
96.32 |
|
0.5 M |
98.32 |
97.04 |
94.95 |
|
0.1 M |
98.04 |
96.54 |
94.58 |
|
0.05 M |
88.60 |
86.11 |
83.44 |
Based on these results, it can saw that as the potential increases, there is a decrease in the surface coverage degree, indicating the occurrence of the dissolution of the titanium oxide films. It can also saw that with the lowering of acid concentration, the degree of oxide surface coverage falls, which indicates the fact that with the increase of H2SO4 concentration, the oxide formation on titanium surface unfolds faster.
XRD and SEM-EDS recordings show the indisputable presence of Ti-oxides in the anodized oxide layer. This oxide layer forms on the surface of the titanium electrode in 3M H2SO4.
Figure 4. The potentiodynamic polarization curves in H2SO4 solutions: A) Ti electrodes, and B) (Ti+TiO2)-electrode. The scan rate is 1 mV s–1
The light yellow color of anodizing anatase crystals TiO2, from acidic solutions, is the most common crystalline TiO2, as said before [16, 17, 27, 40]. The anodization of Ti and getting of thick TiO2 films before modification of titanium surface with mixed platinum metal oxides is not a good recommendation because of possible anode passivation. For some titanium applications, such as DSAs (Dimensionally Stable Anodes) preparation, thick layers lead to electrode passivation. However, the presence of TiO2 on the surface is necessary for better adhesion and stabilization of the electrocatalytic oxides layer. The TiO2 protecting films are permanently rebuilding when the electrodes are polarized anodically. The different deactivation mechanisms of DSAs can be the passivation of titanium substrate, coating detachment, and mechanical damages, which should consider in the production and application of these anodes [41, 42].
Different ways of making DSAs with mixed metal oxide (MMOs) layers often involve using a thermal treatment, and it is the electrode annealing at different temperatures. For this reason, it has been necessary to study the behavior of the anodized Ti substrate, followed by annealing at 400℃ (characteristic annealing temperature for TiO2 crystals of the anatase type) [43].
Figure 5. shows the XRD image of the Ti substrate after anodization in 3 M H2SO4 and after annealing sample at 400℃ in the airflow, previously prepared in the same way.
According to the results present in Figure 5. crystalline TiO2 has formed by anodization of titanium in various concentrations of H2SO4. The intensity of peaks for the anatase type of crystals also suggests a thin TiO2 film formation. The height of the same peaks increases after the annealing titanium electrode at 400℃, which indicates that the oxide film of anatase becomes thicker after heat treatment.
SEM-EDS analyses (Figure 6.) confirm the results obtained by XRD measurements. It can also observe that after anodization on the titanium surface Ti and O2 species exist. This suggests the formation of an oxide layer on the electrode surface. According to chemical analysis, after annealing of titanium, the content of O2 species increase indicates thicker TiO2 oxide films (Table 5.).
Table 5. Chemical composition of titanium surface after anodization in 3 M H2SO4 and after annealing at 400℃ at a time of 1 h
|
Atomic species |
O % |
Ti % |
Total % |
|
after anodization |
17.30 |
82.70 |
100.00 |
|
after thermal treat |
28.40 |
71.60 |
100.00 |
Figure 5. XRD patterns of Ti electrode: A) after anodizing in 3M H2SO4, B) after annealing the anodized Ti electrode, at 400℃ in the airflow
Figure 6. SEM-EDS images of А) Ti electrode after anodizing in 3M H2SO4, B) аnnealed Ti electrode at 400℃ 1 h after anodizing in 3M H2SO4
The formation of oxides on the surface of titanium in H2SO4 solutions carried out over a wide range of potentials. According to the results obtained, and characterization of formed oxide films observed several conclusions. TiO2 is the most stable formed oxide, but TiO and Ti2O3 oxides are also formed by anodization from solutions investigated. The results obtained during CV measurements also indicate the rapid formation of an oxide layer with variations of the stoichiometry and morphology on the titanium surface.
Open circuit potential and potentiodynamic measurements indicate that there is generally no significant deviation in the oxidation process and the forming of the oxide film on the Ti and (Ti+TiO2)-electrode surface. Based on the results of the potentiodynamic measurements, it can saw that the formation of the oxide films is initiated at the very beginning of the polarization process. Furthermore, passivation of the titanium surface occurs because of the growth of the surface oxide layer and surface leveling.
XRD and SEM-EDS analyses confirmed the presence of titanium oxide films on the titanium anode. After thermal treatment of the electrode, XRD recordings showed a higher intensity of anatase peaks in the anodized oxide films. SEM- EDS recordings also confirmed the higher presence of oxides on the thermally treated titanium electrode. The appearance of the anatase structure of crystals in the oxide film was also certified.
This work was financially supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, Grant No. 451-03-68/2022-14/200052. The authors are also grateful to Prof. Nicola Bolton and Dr. Jim Fleming for their overall help, support, and generosity.
[1] Li, Y., Ma, Q., Han, J., Ji, L., Wang, J., Chen, J., Wang, Y. (2014). Controllable preparation, growth mechanism and the properties research of TiO2 nanotube arrays. Applied Surface Science, 297: 103-108. https://doi.org/10.1016/j.apsusc.2014.01.086
[2] Regonini, D., Satka, A., Jaroenworaluck, A., Allsopp, D.W.E., Bowen, C.R., Stevens, R. (2012). Factors influencing surface morphology of anodized TiO2 nanotubes. Electrochimica Acta, 74: 244-253. https://doi.org/10.1016/j.electacta.2012.04.076
[3] Yin, L., Ji, S., Liu, G., Xu, G., Ye, C. (2011). Understanding the growth behavior of titania nanotubes. Electrochemistry Communications, 13(5): 454-457. https://doi.org/10.1016/j.elecom.2011.02.019
[4] Leitner, K., W., Schultze, J., Stimming, U. (1986). Photoelectrochemical Investigations of Passive Films on Titanium Electrodes. Journal of The Electrochemical Society, 133(8): 1561. https://iopscience.iop.org/issue/1945-7111/133/8
[5] Regonini, D., Bowen, C.R., Jaroenworaluck, A., Stevens, R. (2013). A review of growth mechanism, structure and crystallinity of anodized TiO2 nanotube. Materials Science and Engineering: R: Reports, 74(12): 377-406. https://doi.org/10.1016/j.mser.2013.10.001
[6] Ozkan, S., Mazare, A., Schmuki, P. (2018). Critical parameters and factors in the formation of spaced TiO2 nanotubes by self-organizing anodization. Electrochimica Acta, 268(1): 435-447. https://doi.org/10.1016/j.electacta.2018.02.120
[7] Maoying, L., Hongtu, M., Dongliang, Y., Hua, H., Xuran, X., Xufei, Z. (2017). Formation mechanism of anodic titanium oxide in mixed electrolytes. Materials Research Bulletin, 95: 539-545. https://doi.org/10.1016/j.materresbull.2017.08.041
[8] Shiyi, C., Qun, C., Mingqi, G., Shuo, Y., Rong, J., Xufei, Z. (2017). Morphology evolution of TiO2 nanotubes by a slow anodization in mixed electrolytes. Surface and Coatings Technology, 321: 257-264. https://doi.org/10.1016/j.surfcoat.2017.04.067
[9] Munirathinam, B., Pydimukkala, H., Ramaswamy, N., Neelakantan, L. (2015). Influence of crystallite size and surface morphology on electrochemical properties of annealed TiO2 nanotubes. Applied Surface Science, 355: 1245-1253. https://doi.org/10.1016/j.apsusc.2015.08.017
[10] Özçiçek Pekmez, N., Ugur, M., Karaca, E., Ertekin, Z., Pekmez, K. (2021). Room temperature electrosynthesis of TinO2n-1 film and its bilayer with PNMPy on mild steel for corrosion protection in sulphuric acid. Electrochimica Acta, 376: 137996. https://doi.org/10.1016/j.electacta.2021.137996
[11] Blackwood, D.J., Petter, L . M . (1989). The influence of growth rate on the properties of anodic oxide films on titanium. Electrochimica Acta, 34(11): 1505-1511. https://doi.org/10.1016/0013-4686(89)87033-1
[12] Hurlen, T., Hornkjol, S. (1991). Anodic growth of passive films on titanium. Electrochimica Acta, 6(1): 189-195. https://doi.org/10.1016/0013-4686(91)85200-Q
[13] Jiang, Z., Dai, X., Middleton, H. (2011). Investigation on passivity of titanium under steady-state conditions in acidic solutions. Materials Chemistry and Physics, 126(3): 859-865. https://doi.org/10.1016/j.matchemphys.2010.12.028
[14] Xu, W., Haarberg, G.M., Seland, F., Sunde, S., Ratvik, A.P., Holmin, S., Gustavsson, J., Afvander, Å., Zimmerman, E., Åkre. T. (2019). The durability of the thermally decomposed IrO2-Ta2O5 coated titanium anode in a sulfate solution. Corrosion Science, 150: 76- 90. https://doi.org/10.1016/j.corsci.2019.01.018
[15] Sul, Y.-T., Johansson, C.B., Jeong, Y., Albrektsson, T. (2001). The electrochemical oxide growth behaviour on titanium in acid and alkaline electrolytes. Medical Engineering & Physics, 23(5): 329-346. https://doi.org/10.1016/S1350-4533(01)00050-9
[16] Vercesi, G.P., Rolewicz, J., Comninellis, C. (1991). Characterization of dsa-type oxygen evolving electrodes. Choice of base metal. Thermochimica Acta, 176: 31-47. https://doi.org/10.1016/0040-6031(91)80257-J
[17] Diamanti, M.V., Pedeferri, M.P. (2007). Effect of anodic oxidation parameters on the titanium oxides formation. Corrosion Science, 49(2): 939-948. https://doi.org/10.1016/j.corsci.2006.04.002
[18] Kang, K.W., Limandri, S., Castellano, G., Suárez, S., Trincavelli, J. (2017). Thickness determination of anodic titanium oxide films by electron probe microanalysis. Materials Characterization, 130: 50-55. http://dx.doi.org/10.1016/j.matchar.2017.05.027
[19] Alves, A.C., Wenger, F., Ponthiaux, P., Celis, J.-P., Pinto, A.M., Fernandes, J.C.S., Rocha, L.A. (2017). Corrosion mechanisms in titanium oxide-based films produced by anodic treatment. Electrochimica Acta, 234: 16-27. https://doi.org/10.1016/j.electacta.2017.03.011
[20] Motola, M., Satrapinskyy, L., Roch, T., Subrt, J., Kupcík, J., Klementova, M., Jakubickova, M., Peterka, F., Plesch, G. (2017). Anatase TiO2 nanotube arrays and titania films on titanium mesh for photocatalytic NOX removal and water cleaning. Catalysis Today, 287: 59- 64. https://doi.org/10.1016/j.cattod.2016.10.011
[21] Manjaiah, M., Laubscher, R.F. (2017). Effect of anodizing on surface integrity of Grade 4 titanium for biomedical applications. Surface and Coatings Technology, 310: 263-272. https://doi.org/10.1016/j.surfcoat.2016.12.038
[22] Munirathinam, B., Narayanan, R., Neelakantan, L. (2016). Electrochemical and semiconducting properties of thin passive film formed on titanium in chloride medium at various pH conditions. Thin Solid Films, 598: 260-270. https://doi.org/10.1016/j.tsf.2015.12.025
[23] McAIeer, J.F., Peter, L.M. (1982). Instability of Anodic Oxide Films on Titanium. Journal of The Electrochemical Society, 129(6): 1252. https://iopscience.iop.org/article/10.1149/1.2124097/
[24] Ohtsuka, T., Masuda, M., Sato, N. (1987). Cathodic Reduction of Anodic Oxide Films Formed on Titanium. Journal of The Electrochemical Society, 134(10): 2406-2410. http://hdl.handle.net/2115/62191
[25] Blondeau, G., Froelicher, M., Froment, M., Hugot Le Goff, A. ( 1977). Structure and growth of anodic oxide films on titanium and TA6V alloy. Journal of the Less Common Metals, 5(2): 215-222. https://doi.org/10.1016/0022-5088(77)90043-1
[26] Fonseca, C., Traverse, A., Tadjeddine, A., Belo, M. C. (1995). A characterization of titanium anodic oxides by X-ray absorption spectroscopy and grazing X-ray diffraction. Journal of Electroanalytical Chemistry, 388(1-2): 115-122. https://doi.org/10.1016/0022-0728(94)03806-E
[27] Delplancke, J.L., Winand, R . ( 1 9 8 8 ) Galvanostatic anodization of titanium—I. Structures and compositions of the anodic films. Electrochimica Acta, 33(11): 1539- 1549. https://doi.org/10.1016/0013-4686(88)80223-8
[28] Kuromoto, N.K., Simao, R.A., Soares, G.A. (2007). Titanium oxide films produced on commercially pure titanium by anodic oxidation with different voltages. Materials Characterization, 58(2): 114-121. https://doi.org/10.1016/j.matchar.2006.03.020
[29] Serruys, Y., Sakout, T . , Gorse, D . (1993). Anodic oxidation of titanium in 1M H2SO4, studied by Rutherford backscattering. Surface Science, 282(3): 279-287. https://doi.org/10.1016/0039-6028(93)90934-C
[30] Camara, O.R., De Pauli, C.P., Giordano, M.C. (1984). Potentiodynamic behaviour of mechanically polished titanium electrodes. Electrochimica Acta, 29(8): 1111-1117. https://doi.org/10.1016/0013-4686(84)87163-7
[31] Kudelka, S., Michaelis, A., Schultze, J.W. (1996). Effect of texture and formation rate on ionic and electronic properties of passive layers on Ti single crystals. Electrochimica Acta, 41(6): 863-870. https://doi.org/10.1016/0013-4686(95)00375-4
[32] Marino, C.E.B., de Oliveira, E.M., Rocha-Filho, R.C., Biaggio, S.R. (2001). On the stability of thin-anodic- oxide films of titanium in acid phosphoric media. Corrosion Science, 43(8): 1465-1476. https://doi.org/10.1016/S0010-938X(00)00162-1
[33] Blackwood, D.J., Peter, L.M., Williams, D.E. (1988). Stability and open circuit breakdown of the passive oxide film on titanium. Electrochimica Acta, 33(8): 1143- 1149. https://doi.org/10.1016/0013-4686(88)80206-8
[34] Butler, M.A. (1983). Localized Photoelectrochemical Measurements of Passive Films on Titanium. Journal of The Electrochemical Society, 130(12): 2358. https://doi.org/10.1149/1.2119586
[35] Fazel, M., Salimijazi, H.R., Golozar, M.A., Garsivazjazi, M.R. (2015). A comparison of corrosion, tribocorrosion and electrochemical impedance properties of pure Ti and Ti6Al4V alloy treated by micro-arc oxidation process. Applied Surface Science, 324: 751-756. https://doi.org/10.1016/j.apsusc.2014.11.030
[36] Devilliers, D., Dinh, M.T., Mahe, E., Krulic, D., Larabi, N., Fatouros, N. (2006). Behaviour of Titanium in Sulphuric Acid — Application to DSAs —. Journal of New Materials for Electrochemical Systems, 9: 221-232. https://www.yumpu.com/en/document/view/15351418
[37] Torresi, R.M., Cámara, O.R., De Pauli, C.P. (1987). Influence of the hydrogen evolution reaction on the anodic titanium oxide film properties. Electrochimica Acta, 32(9): 1357-1363. https://doi.org/10.1016/0013-4686(87)85067-3
[38] Ohtsuka, T., Otsuki, T. (2003). The aging of the anodic oxide of titanium during potentiostatic condition by ellipsometry. Corrosion Science, 45(8): 1793-1801. https://doi.org/10.1016/S0010-938X(02)00256-1
[39] Neupane, M.P., Park, I.S., Lee, S.J., Kim, K.A.,, Lee, M.H., Bae, T.S. (2009). Study of Anodic Oxide Films of Titanium Fabricated by Voltammetric Technique in Phosphate Buffer Media. International Journal of Electrochemical Science, 4(2): 197-207. https://www.researchgate.net/publication/234115305
[40] Heyrovsky, M., Jirkovsky, J., Bartackova, M.S. (1995). Polarography and Voltammetry of Aqueous Colloidal TiO2 Solutions. Langmuir, 11(11): 4300-4308. https://doi.org/10.1021/la00011a022
[41] Zwilling, V., Darque-Ceretti, E., Boutry-Forveille, A., David, D., Perrin, M.Y., Aucouturier, M. (1999). Structure and physicochemistry of anodic oxide films on titanium and TA6V alloy. Surface and Interface Analysis, 27(7): 629-637. http://doi.wiley.com/10.1002/%28SICI%291096- 9918%28199907%2927%3A7%3C629%3A%3AAID-SIA551%3E3.0.CO%3B2-0
[42] Su, Z., Zhang, L., Jiang, F., Hong, M. (2013). Formation of crystalline TiO2 by anodic oxidation of titanium. Progress in Natural Science: Materials International, 23(3): 294-301. https://doi.org/10.1016/j.pnsc.2013.04.004
[43] Allam, N.K., Grimes, C.A. (2009). Room Temperature One-Step Polyol Synthesis of Anatase TiO2 Nanotube Arrays: Photoelectrochemical Properties. Langmuir, 25(13): 7234–7240. https://doi.org/10.1021/la9012747
[44] Fadl-allah, S.A., Mohsen, Q. (2010). Characterization of native and anodic oxide films formed on commercial pure titanium using electrochemical properties and morphology techniques. Applied Surface Science, 256(20): 5849. https://doi.org/10.1016/j.apsusc.2010.03.058
[45] Ginley, D.S., Knotek, M.L. (1979). Hydrogen in TiO2 Photoanodes. Journal of The Electrochemical Society, 126(12): 2163. https://doi.org/10.1149/1.2128890
[46] Kelsall, G.H., Robbins, D.J. (1990). Thermodynamics of Ti-H2O-F(-Fe) systems at 298 K. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 283(1-2): 135-157. https://doi.org/10.1016/0022-0728(90)87385-W
[47] Vasilyeva, M.S., Lukiyanchuk, I.V., Ustinov, A.Yu., Shchitovskaya, E.V., Marinina, G.I. (2020). Anodic- cathodic formation of pH-sensitive TiO2-MoOx films on titanium. Journal of Electroanalytical Chemistry, 873: 114388. http://dx.doi.org/10.1016/j.jelechem.2020.114388
[48] Schmidt, H., Stechemesser, G., Witte, J., Soltani-farshi, M. (1998). Depth distributions and anodic polarization behaviour of ion implanted Ti6Al4V. Corrosion Science, 40(9): 7234–7240. https://doi.org/10.1016/S0010-938X(98)00062-6
