Journal of New Materials for Electrochemical Systems 9, 221-232 (2006)
© J. New. Mat. Electrochem. Systems
Behaviour of Titanium in Sulphuric Acid
— Application to DSAs —
∗
D. Devilliers, M.T. Dinh, E. Mahé, D. Krulic, N. Larabi and N. Fatouros
Université Pierre et Marie Curie—Paris 6, Laboratoire LI2C-Electrochimie, CNRS UMR 7612
4 Place Jussieu - 75252 Paris Cedex 05 - France
Received: January 04, 2006, Accepted: March 02, 2006
Abstract: Titanium-based Dimensionally Stable Anodes (DSAs) are commonly used in industrial cells, even in corrosive solutions. It is necessary to control the corrosion of their titanium substrates. In this paper, the behaviour of titanium electrodes in
sulphuric acid solutions is studied by means of potential decay curves at open circuit potential and impedance spectroscopy measurements. In addition, an electroanalytical determination of soluble Ti(IV) species in the acid electrolytic solutions is proposed, giving
a convenient tool for detecting in situ the dissolution of TiO2 and the corrosion of Ti, which can lead to the deactivation of DSAs.
Keywords: Titanium, Dimensionally Stable Anodes, deactivation process, titanium corrosion, titanium electroanalysis
acid [4-7]. At room temperature, titanium is rapidly corroded in concentrated acid solutions: H2 SO4 , HCl, H3 PO4
and some organic acids (oxalic acid) [8,9]. The corrosion
rate depends on the nature of the acid, its concentration,
the temperature of the solution, and the presence of a passivating layer on the metal. It is reported that the corrosion current intensity decreases if a thick oxide layer is
present. The anodic transfer coefficient also decreases with
layers of increasing thickness. In H2 SO4 , HCl and H3 PO4 ,
generalised corrosion occurs. Robin et al. have compared
the behaviour of titanium and titanium alloys in sulphuric
acid [10].
The metal dissolution is slow in oxidant-containing aqueous solutions without complexing agents, due to the presence of the native TiO2 film on the substrate [11-13]. The
presence of oxidizing species such as oxygen and chromic
ions is reported to improve the performance of titanium
[14]. In hot concentrated HCl or H2 SO4 , general corrosion of titanium occurs, but, in that kind of environments,
the presence of oxidizing agents and certain multivalent
metal ions have the ability to passivate Ti. For that reason, the experiments must be carried out with high purity
H2 SO4 in order to get reproducible results [15]. Titanium
is severely corroded in fluorhydric acid, due to the formation of TiF6 2− complexes [16].
Many authors have studied the corrosion of titanium by
different techniques; most of them are rapidly reviewed
1. INTRODUCTION
The great technological interest in titanium is due to its
versatility, leading to many different applications in spacial, aeronautic, naval and electrochemical industries. Indeed, titanium is used as a substrate material for Dimensionally Stable Anodes (DSAs). They constitute particular
modified electrodes with remarkable electrochemical properties [1].Before deposition of metallic mixed oxides on the
surface of the substrate, the latter must be etched in acid
medium; concentrated HCl or H2 SO4 are commonly used
for that purpose [2].
In order to be able to predict the service life of DSAs, the
behaviour of titanium in concentrated acid solutions must
be studied, especially in sulphuric acid solutions which are
commonly used in industry. As other valve metals such
as tantalum, titanium is thermodynamically unstable in
aqueous media and is oxidized by oxygen present in the surrounding environment. Electrogenerated oxide films may
also be formed on that metals [3]. The metal is covered
by a stable oxide film of TiO2 when the metal is exposed
to air or anodised. Its corrosion resistance is excellent in
many media, but its corrosion rate may be significant in
concentrated acid solutions. For example, the protecting
oxide film is dissolved gradually in concentrated sulfuric
∗ To
whom corresponding to: E-mail: [email protected], Phone:
33-1 44 27 36 77, Fax: 33-1 44 27 38 56
221
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D. Devilliers et al. / J. New Mat. Electrochem. Systems
below. At last, the determination of the amount of Ti(IV)
in solution by electroanalytical techniques is described.
1.1. Open Circuit Potential (OCP) studies and polarisation curves
As a titanium oxide layer is always formed in air, freshly
polished titanium dipped in a corrosive medium takes at
least 24h to attain a reproducible value of the stationary
state current and OCP. The stable value expected i.e —
0.66 V is obtained in strong acids after dissolution of the
air-formed oxide layer [17].
The evolution of the rest potential of titanium in acidic
media has been studied by several authors [18-20]. Its
value strongly depends on the concentration of the acid.
In concentrated solutions, titanium rapidly attains the active state and corrosion occurs rapidly.
K. Azumi et al. [21] have studied the behaviour of titanium electrodes during long-term immersion experiments.
The corrosion resistance was evaluated by monitoring the
OCP of the electrodes, some samples being previously polarized at 10 V in order to form an anodic oxide film. A
potential shift in the positive direction usually means the
growth of the oxide film. Such a phenomenon was observed
even in deaerated neutral solutions.
Thomas and Nobe [22] have studied the corrosion of Ti
in 0.5 M H2 SO4 . They have found that the OCP of the
metal was in the passive range (400-700 mV vs SCE). However, if the electrode was first pretreated in 1M HF, a rapid
decrease of the OCP was observed and the titanium dissolution occurred as the active state was reached (corrosion
potential: Ecor = -680 mV vs SCE). That rapid decrease
of the OCP is called “self-activation”. The same behaviour
was observed by Brauer and Nann [23] who noticed that
the time necessary for the appearance of self-activation was
decreasing with increasing molar concentration of H2 SO4 .
If the titanium electrode is passivated by anodic polarization before being in contact with the corrosive solution of H2 SO4 , the self activation does not appear rapidly.
Blackwood et al. [24,25] concluded that that phenomenon
was related to the regular dissolution of the oxide film. It
was shown that thinning of the oxide film in 3 M H2 SO4 at
OCP occurs by uniform dissolution at constant rate rather
than by localized attacks [25]. These results were obtained
by in situ ellipsometry.
J.Krysa et al. have studied the corrosion rate of titanium in 1-4 M sulphuric acid at temperatures of 25-60◦ C
[19,26]. The OCP of titanium suddenly decreases; thus the
metal is in the active state. The duration after which that
decrease is obtained depends strongly on the pre-treatment
of the sample: for a given concentration of H2 SO4 an electrode pre-treated in HF gives rise to an immediate sharp
decrease; electrodes without any pretreatment give rise to
a long potential plateau before the sharp decrease of potential.
The rest potential of Ti in 10 N H2 SO4 is reported to be
-1.14V vs Hg, Hg2 SO4 , 10 N H2 SO4 reference electrode,
i.e. in the active corrosion region of the metal [15].
The thickness of protecting oxide films formed in air at
room temperature is about 5nm. Blackwood et al. [25]
have studied the formation of anodic TiO2 films potentiodynamically grown in 3 M H2 SO4 . After several hours
at OCP in the same solution, partial dissolution has occurred; the amount of dissolved material was deduced from
the amount of electricity used during the re-growth of the
film. It was found that the dissolution is uniform and slow,
but the stability of the film depends on its mode of preparation. Müller et al. [27] found that anodisation of titanium
in acid solutions is accompanied by a potential independent chemical dissolution.
Frayret et al. [28,29] proposed a model to explain dissolution and passivation of titanium. In nitric acid, the
OCP shifts to higher values for long exposure times, in relation to thickening of the TiO2 film formed at the surface
of the electrode (oxidizing character of HNO3 ). The corrosion potential Ecor is stable when the rate of film formation
equals the rate of film dissolution [10].
Titanium is used in desalination plants in Middle East
countries (heat exchanger tubes) due to its excellent
erosion-corrosion resistance in sea water. However, in these
plants, descaling is a common maintenance operation (removing of CaCO3 and Mg(OH)2 ). This operation is carried out using an acid with or without inhibitors [30]. The
authors have studied the behaviour of titanium in 0.5%
H2 SO4 solution (a sufficient concentration for clearing Ti
tubes in desalination plants). The corrosion rate at 50◦ C
is very low: 0.0025 mm/y in aerated solution and 0.0020
mm/y in deaerated-inhibited solution. Other authors have
reported highest rates in more concentrated sulphuric acid:
1.9 mm/y in 4% H2 SO4 at 50◦ C [31].
The corrosion current density is more important with
concentrated HNO3 solutions (about 2μA/cm2 corresponding to 25 μm/year)
In H2 SO4 (20-80 % solutions), the OCP shifts towards
lower values.
The stabilized values are in the active region of the Pourbaix diagram [10] (-680 to -750 mV/SCE) and the corrosion
current density increases with acid concentration. (0.17
mA cm−2 for 20% to 6.86 mA cm−2 for 80%).
1.2. EQCM
Recently, a new technique for the study of corrosion of
titanium in sulphuric acid has been proposed [32]. The titanium foils were glued onto the crystal of an electrochemical quartz crystal microbalance (EQCM); the dissolution
of the active metal and the growth of the passive film at
higher anodic potentials were studied from the measurement of mass changes. Experiments were made in 5M
H2 SO4 . The air-formed oxide layer was removed by applying a cathodic constant potential (-500 mV). Thus, an
active OCP of -673 ± 5 mV was obtained and dissolution of
the metal occurred, at a rate of 0.8 μg min−1 cm−2 . When
applying increasing potentials, the anodic current, related
to dissolution of Ti, gave a decrease in mass in the EQCM
experiments. After the passivation potential Ep (-470mV
vs SCE), the current decreased (oxide formation); the mass
continued to decrease in the active-passive region because
Behaviour of Titanium in Sulphuric Acid
-Application to DSAs- / J. New Mat. Electrochem. Systems
the passive layer was not intact. Above Ep , Ti4+ started
to form, in addition to Ti3+ .
In 0.1 M H2 SO4 solutions, the oxygen concentration was
sufficient to keep the surface passive. The OCP measured
was in the range -50 to 150 mV/ SCE, due to different
oxide layer thicknesses [32].
1.3. Impedance spectroscopy
Azumi and Seo have studied the impedance response of
anodic TiO2 films during a potential sweep in neutral solutions [33]. Impedance was measured at a fixed frequency
of 10 Hz in order to determine the capacity of the interface,
related to the thickness of the film and to its composition.
Usually, the authors prefer to plot the impedance diagrams in a large frequency domain. According to different
authors, the Nyquist diagrams of a Ti electrode at the rest
potential exhibit one or two capacitive loops.
The impedance diagram at the rest potential exhibits
one semicircle at “low frequencies”: the maximum of the
imaginary part Z” is obtained for f=2.5 Hz. No semicircle
is obtained at high frequencies. The corrosion resistance is
estimated to be ca. 100 Ω cm2 [15].
The Surface Charge Approach (SCA) may be used for
giving interpretation of the impedance spectra. In that
model, a capacitive behaviour is observed at high frequency. That loop is the parallel combination of the barrier
film capacitance Cb and the resistance of charge carriers
migration Rb . The thickness of the film varies linearly with
the applied potential E; consequently, 1/Cb varies linearly
with E [34]. Below the passivation potential, the loop is attributed to the double layer capacitance and charge transfer resistance.
For other authors, the impedance diagrams at OCP exhibit two capacitive loops. The first loop (CL , RL ) in the
low frequency region is attributed to the space charge layer
formed in the oxide, and to the charge transfer resistance
through the oxide film [21]. The second loop (CH , RH ),
observed in the high frequency region is attributed to absorption / desorption of solution species [21], although such
phenomena are usually observed at low frequencies.
RL and CL −1 increased with immersion time, corresponding to the thickening of the oxide in the neutral solution at 298 K [21].
In 2 M sulphuric acid, the Nyquist plot of a titanium
electrode recorded at the OCP exhibits two capacitive
loops [35]; the high frequency loop is attributed to charge
transfer; the diameter of the first semi-circle gives the value
of the charge transfer resistance, Rct .
Evaluation of the corrosion resistance of several titanium materials is performed, comparing the values of Rct :
Higher values indicate a higher corrosion resistance [35].
Baszkiewicz et al. [36] have improved the corrosion resistance of titanium with the modification of the surface by
plasma electrolytic oxidation and subsequent hydrothermal treatment. As a very large voltage was applied to
the electrode (400 V), the breakdown of the native TiO2
films occurred, accompanied by sparks. Thus a porous oxide coating was formed. After that operation, the sample
223
was treated at 220◦ C in an autoclave in presence of water. The authors have studied the corrosion resistance of
their samples in simulated body fluid by impedance spectroscopy. For oxidized samples, an (RQ)Q equivalent circuit is correct; however, for the oxidized and hydrothermally heated samples, an R(RQ)(RQ) circuit is necessary
because the Nyquist diagrams exhibit two loops. For the
oxidized samples, after several hours of exposure in the
“corrosive” medium, the parameter “n” of the CPE increases, suggesting that the layers become tighter. The
hydrothermal treatment leads to a modification of the oxide film: it is supposed to be composed of a compact inner
layer and a porous outer layer. During exposure in the
corrosive medium, the inner layer is rebuilt (the values of
R and n decrease) whereas for the outer layer R and n
increase, suggesting that the outer layer becomes tighter.
Nevertheless, the real surface of the samples is not known
and it is difficult to deduce precise information from impedance measurements. The authors suggested that more
precise results should be obtained by measuring the concentration of titanium ions in the corrosive medium after
a long time of exposure.
1.4. Reactions
Among the studies reported above, some of them have
shown the evidence of coupling between film growth and
metal dissolution in the passivation process of titanium.
That phenomenon is also observed for other metals such
as Nb and Mo [34]. One must take into account several
reactions, including dissolution of the metal oxide and oxidation of the metal.
The anodic dissolution of titanium in sulphuric acid
leads to Ti3+ ions, according to Armstrong et al. [37].
In sulphuric acid, the dissolution of the TiO2 layer leads
to colourless Ti(IV), but when the bared metal is reached,
the oxidation of the Ti substrate leads to Ti(III), as violet
Ti3+ ions, which can be further oxidised into Ti(IV) by
dissolved oxygen.
The following reactions occur:
Dissolution of the TiO2 films in acid media:
TiO2 +4H+ → Ti4+ +2H2 O
Colorless soluble Ti(IV) species are formed.
When the protecting TiO2 layer is completely removed
from some parts of the electrode, titanium is in the active
state and Ti3+ ions are formed:
Ti+3H+ → Ti3+ +3Hads
Evolution of hydrogen occurs.
In aerated solutions, Ti3+ is oxidised:
1
2H+ +2Ti3+ + O2 → 2Ti4+ +H2 O
2
1.5. Determination of the concentration of Ti(IV)
At the mercury drop electrode, the reduction of titanic
salts in acid solutions proceeds to the titanous state. The
kinetics of the reduction of Ti(IV) in 0.4-9 M H2 SO4 was
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D. Devilliers et al. / J. New Mat. Electrochem. Systems
systematically investigated. Experiments in differential
pulse voltammetry (DPV) show a sequence of electrochemical reactions which occur in succession or even simultaneously [38].
In DC polarography, whatever the concentration of
H2 SO4 , the limiting current is diffusion-controlled and directly proportional to Ti(IV) concentration [38-40]. The
determination of the diffusion coefficient of Ti(IV) species
at 25◦ C was performed as a function of H2 SO4 concentration up to 9 M. From the limiting current, concentrations
up to about 10−4 M can then be directly determined [38].
In H2 SO4 with C > 3.5 M, a well-defined peak appears in
DPV. The peak current is directly proportional to the analytical concentration of Ti(IV) and depends on the H2 SO4
concentration. For practical analytical purposes, the ratio of the peak current density to Ti(IV) concentration,
∆Ip /S[Ti(IV)], where S is the electrode area, was tabulated
in the concentration range from 3 to 9 M in H2 SO4 .[41].
As was said above, titanium is commonly used as a substrate material for DSAs in concentrated acid solution.
In addition, recently, Ti/TiO2 /PbO2 electrodes have been
proposed for the oxidation of Cr(III) to Cr(VI) in sulphuric
acid solutions [42]. It is important to evaluate the corrosion
resistance of that material in order to predict the service
life of industrial electrodes.
In this paper, the behaviour of titanium is studied in
sulphuric acid solutions by the mean of potential decay
curves at open circuit, and impedance measurements. The
influence of the purity of the metal and the presence of a
TiO2 film, anodically built, are evidenced.
In addition, the determination of the amount of Ti(IV)
in solution was performed by a pulse voltammetric technique, allowing to attain the direct quantitative evaluation
of the dissolution of the protecting TiO2 films and the corrosion of the metal.
2. EXPERIMENTAL
2.1. Materials
Pure titanium disks (diameter: 10 mm, Goodfellow)
were used as working electrodes. Two kinds of purity were
tested: (i) catalogue number 410; purity = 99.6 %; main
impurities (in ppm): Al = 300; Fe = 1500; O = 2000; Sn
= 200; (ii) catalogue number 432; purity = 99.99 %; main
impurities (in ppm): Al = 1.3; Cr = 1.3; Fe = 3.65; Ta <
5; V = 0.38; Zr = 0.76.
All the samples were mechanically polished with
Buehler-Met grinder paper up to grid size 1200, cleaned
in methanol in an ultrasonic bath, and rinsed with distilled water. This procedure results in the formation of a
native oxide layer on the titanium substrates. Its thickness
and composition have been evaluated [43].
In addition, some samples were electrochemically polished in non aqueous solvent, according to a procedure
described elsewhere [44]. The disks were mounted on a
special holder, adapted on a Tacussel rotating disk electrode system. Thus, they were anodised in a 0.5 M H2 SO4
solution according to a galvanostatic procedure (j = 5 mA
cm−2 ) until the final potential was 10 V vs SCE. With such
conditions, the thickness of the oxide layer (35 nm) is very
reproducible [43]. For verifying the reproducibility, up to
5 samples were tested for one kind of experiments.
2.2. Electrochemical devices
For long-time experiments (measurement of OCP), several sealed electrochemical cells were built, in order to
avoid evaporation. In these cells, a reference electrode was
immersed periodically for the measurement of the OCP of
the working electrode. For short time experiments (measurement of o.c.p. in 5 M H2 SO4 ), the reference electrode
was immersed continuously in the electrolytic solution.
For long-time experiments in sulphuric acid solutions,
the best choice of reference electrode is a mercury sulphate
electrode in order to prevent the diffusion of Cl− ions from
SCE electrodes.
However, the use of commercial sulphate electrodes
Hg/Hg2 SO4 /K2 SO4 is no more correct because K+ cations
may diffuse from the reference electrode compartment to
the electrochemical cell and H+ cations from the electrochemical cell to the reference electrode compartment; these
variations of composition of the filling solution and the
electrolyte of the cell may alter the results of the experiments. For that reason we have prepared two reference
electrodes without liquid junction potential and adapted
to our media:
i. Hg/Hg2 SO4 /0.5 M H2 SO4 electrode for experiments
performed with 0.5 M H2 SO4 solutions in the cell.
It is denoted ESAM (for Electrode with Sulphuric
Acid). Its potential is 0.669 V vs SHE.
ii. Hg/Hg2 SO4 /5 M H2 SO4 electrode for experiments in
the cell containing 5 M H2 SO4 ; it is denoted ESA5M.
Its potential is 0.564 V vs SHE.
All the potentials mentioned in this paper are given with
respect to those two electrodes.
The auxiliary electrodes were platinum wires.
An Autolab PGSTAT 30 potentiostat was used for the
polarization resistance determination. The polarization
curves were plotted, applying a slow potential scan (v =
1 mV s−1 ) at the working electrode, in the vicinity of the
corrosion potential Ecor (Ecor ± 20mV).
Some impedance measurements were carried out using a
frequency response analyzer SOLARTRON 1255 coupled
to a EG&G 273 A potentiostat; other experiments were
performed with the Autolab PGSTAT 30 equipped with
the FRA2 frequency response analyzer module. The amplitude of the applied sinusoidal signal was 10mV; the impedance experiments were performed in the range 100 kHz
— 2.6 mHz. Fitting and simulations were performed with
the “Equivalent Circuit” software (Boukamp) or the FRA2
Autolab software.
Behaviour of Titanium in Sulphuric Acid
-Application to DSAs- / J. New Mat. Electrochem. Systems
225
Table 1. Variation of the OCP and values of the impedance parameters for a non-anodised titanium electrode immersed
in 0.5 M H2 SO4 .
Immersion time (h)
0.25
2
3
5
72
168
336
504
672
1008
1344
2352
Er (mV/ESAM)
−378
−411
−454
−510
−514
−518
−521
−524
−530
−541
−545
−550
Rs (Ωcm2 )
3.8
3.8
3.8
6.2
3.1
3.0
3.3
3.1
3.4
3.6
3.2
3.0
Rox (kΩcm2 )
3950
2210
1750
1180
856
511
498
434
413
331
276
29
Ctot (μFcm−2 )
21.7
24.1
26.2
30.4
55.5
65.9
70.9
78.4
89.8
102
105.4
113
n
0.966
0.967
0.967
0.963
0.965
0.968
0.966
0.972
0.973
0.974
0.974
0.94
-100
-200
-300
-400
-500
-600
0
500
1000
1500
2000
2500
Time (h)
Figure 1. Variation of the OCP of an anodised titanium
electrode (purity 99.6 %) in 0.5 M H2 SO4 . Initial thickness
of the anodic oxide layer: 35 nm.
Figure 2. Nyquist diagram for an anodised titanium electrode at the OCP (E = -520 mV/ESAM) after t = 624 h
of immersion in 0.5 M H2 SO4 .
3. RESULT AND DISCUSSION
3.1. Behaviour of titanium in 0.5 M H2 SO4
For this section, only 99.6 % purity titanium has been
tested.
3.1.1. Anodised titanium
The OCP of the samples decreases sharply from -190 to
-480 mV vs ESAM during the first twenty four hours of
immersion. After that period, the variation is not so important. After 2200 hours, the values of the rest potential
is about -520 mV and does not correspond to a corrosion
situation (see Figure 1)
The impedance diagram, plotted at the OCP in the
Nyquist representation, exhibits only one infinite branch,
even after t = 624 h of immersion (see Figure 2). The an-
gle of that branch, relative to the imaginary axis is linked
to the dimensionless parameter, n, of the constant phase
element (CPE), representing the behaviour of the interface. For all these experiments, the equivalent circuit is
only composed of a resistance Rs (resistance of the electrolyte) in series with a CPE. As the value of n lies between
0.94 and 0.97 during the first 800 hours of immersion, the
CPE is almost a pure capacitance. Its value continually increases from 6.7 μF cm−2 after 0.25 hour, to 51 μF cm−2
after 768 hours. For very long durations, the value of n
decreases to 0.9. This capacitive term is composed of two
terms in series: the double layer capacitance and the capacitance of the protecting oxide layer.
The increase of C is related to the decrease of the thick-
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D. Devilliers et al. / J. New Mat. Electrochem. Systems
Table 2. Variation of the OCP and values of the impedance parameters for a anodised titanium electrode (purity 99.6
%) immersed in 5 M H2 SO4 .
Immersion time (h) Er (mV/ESA5M) Rs (Ωcm2 ) Rox (kΩcm2 ) Cox
0
40
8.92
24
−230
5.66
48
−360
6.32
120
−982
6.61
197
144
−935
7.36
65
168
−947
5.76
46
Figure 3. Evolution of the Nyquist diagrams for a nonanodised titanium electrode in 0.5 M H2 SO4 at the OCP,
for three durations of immersion.
ness of the protecting TiO2 layer. The interface does not
behave as a pure capacitance for long time experiments;
this phenomenon could be attributed to the increase of
roughness.
3.1.2. Non-anodised titanium
For these experiments, the samples are only covered by a
native oxide layer after wet polishing with abrasive paper.
Nevertheless, as for the anodised samples the OCP values
continuously decrease and a value of -550mV vs ESAM is
attained after 2350 hours of immersion.
The impedance diagrams do not exhibit one infinite
branch, but an arc of a semi-circle (see Figure 3). The
equivalent circuit is now Rs (Ct , Rox ).Where Rs is the resistance of the electrolyte, Rox the resistance of the oxide
layer, and Ct the capacitance of the oxide layer). A CPE
may be used instead of a pure capacitance, for a better fitting of the curve. Thus, a value of n∼0.97 is obtained, even
for very long time experiments (t = 1350 h). As previously,
the value of Ct increases for long duration of immersion,
although Rox decreases. All the data are reported in Table
(μFcm-2)
5.33
11.3
17.8
258
509
616
n1 Rtc (Ωcm2 ) Cdl (Fcm−2 ) n2
0.96
0.97
0.97
0.90
0.92
36.2
0.18
1.0
0.92
26.8
0.244
1.0
Figure 4. Evolution of the Nyquist diagrams for an anodised titanium (purity 99.6 %) electrode at the OCP in
5 M H2 SO4 . The OCP and the duration of immersion are
indicated in the inset.
1.
These experiments show that the oxide layer (anodic oxide or native oxide) initially present on the titanium substrate is not completely removed, even after 2350 h of immersion i.e. corrosion is not observed, although the thickness of that protecting layer decreases with time.
The situation is different in concentrated sulphuric acid
solutions, as reported below.
3.2. Behaviour of titanium in 5 M H2 SO4
3.2.1. Anodised titanium
In this section, two kinds of samples were studied, with
different purity: 99.6 or 99.99 %.
For the 99.6 % sample, the OCP decreases rapidly when
it is immersed in 5 M H2 SO4 . After 120 hours, Er = -982
mV vs ESA5M. This very low value indicates that titanium
is in the active state and that corrosion occurs. The initial
yellow colour of TiO2 disappears rapidly; in addition, the
solution becomes violet, indicating the presence of Ti3+
Behaviour of Titanium in Sulphuric Acid
-Application to DSAs- / J. New Mat. Electrochem. Systems
227
Figure 5. SEM image of a titanium sample (purity 99.6
%) electropolished and anodised, after 120 h of immersion
in 5 M H2 SO4 . General view, showing the grains at the
surface
Figure 6. Details of a corroded grain of the same titanium
sample.
ions.
The evolution of the Nyquist diagram obtained with
those samples is presented in Figure 4. At the beginning
of the experiment (t = 0; Er = 40 mV vs ESA5M) the diagram is limited to one infinite branch, as in the case of the
0.5 M H2 SO4 solution (see previous section). However, for
more than 100 h of immersion, the diagrams exhibit one
capacitive loop, which diameter decreases. After 144 h, a
second capacitive loop is also present.
The equivalent circuit passes progressively from the simple one Rs ,Cox , to Rs (Cox Rox ) and at least Rs (Cox Rox )
(Cdl Rct ). The low-frequency loop is attributed to the
contribution of the double layer capacitance Cdl in parallel with the charge transfer resistance Rct due to corrosion of the metal. The same equivalent circuit is proposed
by other authors [21,36]. However, Baszkiewicz et al. do
not present the Nyquist diagrams; they only give a table
of data. These data indicate that the time constants of
the two loops are not very different; in such case, the two
loops are probably not clearly distinguished on the diagrams; in addition, very low values of the dimensionless
parameter, n, of the CPE are sometimes obtained by the
authors (0.68). In our case, the lowest value of n was only
0.9 (see Table 2). The non-ideal capacitive behaviour in
the experiments of Baszkiewicz et al. is probably due to
the different method of preparation of their samples. In
our experiments, electropolishing followed by careful anodisation leads to compact oxide layers.
The values of Rox are not given in Table 2 for the three
first experiments because the equivalent circuit is just composed of a resistance, Rs , and Cox in series.
The value of the capacitance of the interface increases
as previously observed with 0.5 M H2 SO4 solutions. The
value of n is near 0.97, i.e. the capacitive term is almost
ideal. After 120 h of immersion, the first loop (high frequency loop) leads to the value of the capacitive term Cox
which does not behave as an ideal term (the value of n is
0.9 or 0.92).
The charge transfer resistance is extracted from the second loop (low frequency loop); it is estimated to be Rct ∼
30 Ω cm2 , while a very large capacitive term is obtained,
which is usually attributed to the double layer capacitance.
However, the evaluation of these two terms is not easy because the area of the sample is strongly modified during
the corrosion process. As can be seen with a microscope,
the roughness is very important after long time immersion experiments. The surface of the sample after 120 h
of immersion in 5 M H2 SO4 is presented in Figure 5. On
this SEM micrograph, it can be seen that the corrosion
rate is not uniform on all the grains of the samples. This
observation has two important consequences:
i. in the impedance diagrams, two loops are observed.
One of them (the high frequency loop) may be attributed to the part of the electrode which is not
attacked so strongly.
ii. the increase of the capacitive terms is related to the
bare part of the electrode, characterised by a large
area, due to a considerable increase of the roughness
(see Figure 6).
From these experiments, it is concluded that corrosion of
the titanium samples occurs relatively rapidly. As shown
below, the corrosion rate may decrease considerably if a
high purity metal is used instead of a 99.6 % sample.
228
D. Devilliers et al. / J. New Mat. Electrochem. Systems
E0 (mV/ESA5M)
0
-500
-1000
-1500
0
20
40
60
80
100
Immersion time (h)
Figure 7. Variation of the OCP of an anodised titanium
electrode (purity 99.99 %) in 5 M H2 SO4 . Initial thickness
of the anodic oxide layer: 35 nm.
Figure 8. Nyquist diagram for an anodised titanium (purity 99.99 %) electrode at the OCP (E = -300 mV vs MSE
5M) after t = 5280 h of immersion in 5 M H2 SO4 .
Figure 9. Variation of the OCP of a non-anodised titanium
electrode (purity 99.6 %) in 5 M H2 SO4 .
Figure 10. Evolution of the Nyquist diagrams for a nonanodised titanium electrode (purity 99.6 %) in 5 M H2 SO4 ,
at the OCP. Short durations of immersion.
Behaviour of Titanium in Sulphuric Acid
-Application to DSAs- / J. New Mat. Electrochem. Systems
Figure 11. Evolution of the Nyquist diagrams for a nonanodised titanium electrode (purity 99.6 %) in 5 M H2 SO4 ,
at the OCP. Long durations of immersion.
Another set of experiments was performed with high
purity titanium(99.99 %). With an anodised sample, the
OCP tends to a constant value of ∼ -300 mV vs ESA5M,
instead of -960 mV as seen previously (see Figure 7).
After 400 h, we have observed the sample by AFM and by
optical microscopy. No alteration of the surface was visible.
Impedance diagrams were plotted after a very long time
of immersion (5280 h, corresponding to 7 months). The
Nyquist diagram, presented in Figure 8, indicates that the
sample is not yet corroded.
From these experiments, we deduce that the rate of corrosion strongly depends on the purity of the metal. However, the cost of high purity titanium is too high and that
quality of metal cannot be used in industrial Electrochemistry for preparing DSAs. For that reason, the behaviour
of non- anodised samples in 5 M H2 SO4 hereafter was only
performed with 99.6 % purity samples.
3.2.2. Non—anodised titanium
These experiments were performed only with 99.6 % purity samples, after wet polishing with abrasive paper. As
it was expected, the OCP decreases rapidly from its initial
value (-364 mV vs ESA5M) to about — 1200 mV after 14
hours of immersion (see Figure 9). During these “short
time” experiments, the potential of the working electrode
was measured continuously vs a ESA5M reference electrode. The evolution of hydrogen bubbles was visible at
the end of the experiments.
After the minimum at -1200 mV, the OCP increases very
slowly and tends to ∼ -1100 mV after 100 hours of immersion. This value is in perfect agreement with the rest potential of titanium in 10 N H2 SO4 (-1.14 V vs Hg/Hg2 SO4 ,
10 N H2 SO4 reference electrode) found by other authors
[15].
The solution becomes violet, due to the formation of
229
Ti(III) as Ti2 (SO4 )3 , according to the literature [45]. If
that solution is in contact with air, the violet colour vanishes, due to the oxidation of Ti(III) into Ti(IV) species
by oxygen.
The impedance diagrams are presented in Figure 10 for
short time of immersion, only one capacitive loop is observed. Its diameter decreases rapidly. When corrosion
arises, a second capacitive loop appears (Figure 11). The
equivalent circuit has already been described in the previous section. All the data are reported in Table 3.
It can be thought that during the corrosion process, the
high frequency loop (Rox Cox ) may be attributed to the
contribution of the part of the electrode which is still covered by a thin layer of oxide, or by insoluble corrosion
products. We have also measured the polarization resistance of the titanium sample in 5 M H2 SO4 after 5 hours
of immersion i.e. when the metal is still covered by a protecting oxide layer. The value of Rp was found to be 4.5
× 105 Ω cm2 . This value is very similar to the value of
Rox extracted from the impedance diagrams (see Table 3).
After 70 h of immersion, the Rp value is 80 Ω cm2 ; it is the
same order of magnitude that the value extracted from the
impedance diagram: Rox + Rct ∼ 110 Ω cm2 . This small
value of Rp means that titanium is strongly corroded: jcor
= 2μA cm−2 , corresponding to vcor = 0.7 mm/y.
It must be noticed that the determination of Rp from polarisation curve at the vicinity of Er is not accurate if that
value is not constant. That determination is only correct
when Er remains almost constant, i.e. for the experiment
after 70 h of immersion in our case.
The measurement of the OCP allows to know if titanium
is in the active state or not; the impedance diagrams also
give information on the behaviour of the protecting oxide
layer present on the surface of the electrodes. However, it
is useful to have an independent method of investigation in
order to know if there is a permanent competition between
formation and dissolution of titanium oxide.
For that reason, we have coupled our experiments with
an electroanalytical method which allows the in situ determination of the Ti(IV) content in the H2 SO4 solution.
The DPV technique has been carried out for the direct
determination of soluble Ti(IV) in sulphuric acid. First,
we give hereafter the characteristic results obtained with 3
samples:
• Sample 1: High purity titanium (99.99 %), electropolished, and anodised immersed in 5 M H2 SO4 during
8760 h. At the end of immersion time, the rest potential was -191 mV vs ESA5M.
• Sample 2: Titanium (purity 99.6 %) electropolished and anodised, after 6 days of immersion in 5
M H2 SO4 . The rest potential was: -1023 mV vs
ESA5M.
• Sample 3: Titanium (purity 99.6 %) mechanically
polished; non anodised, immersed 14 h in 5 M
H2 SO4 . Its rest potential was -1027 mV vs ESA5M.
230
D. Devilliers et al. / J. New Mat. Electrochem. Systems
Table 3. Variation of the OCP and values of the impedance parameters for a non-anodised titanium electrode (purity
99.6 %) immersed in 5 M H2 SO4 .
Immersion time (h) Er (mV/ESA5M) Rs (Ωcm2 ) Rox (kΩcm2 ) Cox
0.25
−200
11.21
3660
3
−360
11.96
1460
5
−490
11.42
427
7
−553
11.13
131
23
−1110
9.77
0.156
70
−1006
10.85
0.063
(μFcm−2 )
16.7
24.3
30.7
37.8
86.2
160
n1 Rtc
0.96
0.96
0.94
0.93
0.92
0.92
(Ωcm2 ) Cdl (Fcm−2 ) n2
92
0.05
0.88
45
0.04
0.8
Table 4. Determination of the soluble Ti(IV) species in concentrated sulphuric acid
after several hours of immersion, by DPV.
Experimental conditions: ∆E = 40 mV; ∆t = 40ms; S=2 mm2
In column 5, S is expressed in mm2 and ∆Ip in mA.
Sample
[H2 SO4 ]/M
1
2
3
5.95
5.3
5.05
Immersion
Time/h
8760
144
72
Rest potential
(mV vs MSE5M)
−191
−1023
−1027
The results obtained with these three samples are given
in Table 4. For each determination, it was necessary to
check the actual concentration of sulphuric acid because,
for long-time experiments, some evaporation of water may
occur.
Another result will also be discussed below. It was obtained with a diluted H2 SO4 solution (0.5 M). Experiments
were performed in 0.5 M H2 SO4 in the presence of 0.2 M
H2 C2 O4 . In this medium, a well-defined diffusion peak is
obtained in DPV. From this peak, Ti(IV) concentrations
about 10−6 M can be accurately determined [46].
- Sample 4: Titanium (purity 99.6 %) electropolished
and anodised, immersed in 0.5 M H2 SO4 during 4080 h.
The rest potential was -410 mV vs ESAM. The Ti(IV)
concentration was found to be: 2.2 × 10−5 M.
3.3. Discussion
In concentrated sulphuric acid, the “high” value of the
rest potential of the ultra pure titanium sample (sample 1)
indicates that it is not in the active state, even after a very
long duration of immersion. This means that a protecting
TiO2 film is still present on the electrode. However, the
large amount of Ti(IV) species present in solution mean
that dissolution of the protecting TiO2 film did occur during the immersion, but not enough rapidly for giving a
completely bared metal.
The two “low” purity samples (2 and 3) were both in
the active state when the immersion experiments were
stopped. It is thus correct to find large amounts of Ti(IV)
in solution.
Low amounts of Ti(IV) were found for the titanium electrode immersed in 0.5 M H2 SO4 (Sample 4). Even after
4080 h of immersion, the active state was not reached. The
∆I p
S[Ti(IV)]
0.85
0.68
0.61
∆Ip
/μA
2.9
9.1
12.2
[Ti(IV)]/M
1.7 10−3
6.7 10−3
1.0 10−2
protecting TiO2 layer is scarcely dissolved in 0.5 M H2 SO4 .
These results are in agreement with those of J. Krysa et al.
[26] who found that the self-activation time (time necessary
for obtaining the steady state corrosion rate) depends on
the thickness of the TiO2 layer present on the electrode
before its immersion in 4 M H2 SO4 . These authors give an
expression of the steady state corrosion rate of the metal,
rcor : rcor increases linearly with the cube of the H2 SO4 concentration. Our results give an additional information: the
rate of dissolution of the TiO2 layer initially present on the
electrodes also increases with the H2 SO4 concentration.
“Low” purity titanium (99.6%) is strongly corroded in
5 M H2 SO4 and the active state may be reached rapidly,
especially if the metal is not anodised; on the contrary,
the active state was not reached with high purity titanium (99.99 %) even after 7 months of immersion in 5
M H2 SO4 . However, it does not mean that the dissolution
of the TiO2 protecting layer did not occur for that material, since Ti(IV) soluble species were evidenced in the
resulting solution. It only means that the dissolution rate
is very slow, so that the active state is not reached even
after 7 months. Indeed, when a Ti sample is immersed in
sulphuric acid, two competing reactions occur: formation
of TiO2 (oxidation of titanium by water or by dissolved
oxygen if the solution is not deaerated) and dissolution of
TiO2 , leading to Ti(IV) soluble species.
Anodisation of a titanium electrode delays the appearance of the active state (and thus corrosion of the sample);
however for applications to DSA, it is not recommended
to build a thick TiO2 before modifying the surface by a
RuO2 -TiO2 electrocatalytic layer since thick layers lead to
the passivation of the electrodes. Another solution may
consist in choosing high purity titanium substrates; how-
Behaviour of Titanium in Sulphuric Acid
-Application to DSAs- / J. New Mat. Electrochem. Systems
231
is not in direct contact with the corrosive medium if the
PbO2 is sufficiently compact.
4. CONCLUSION
Figure 12. Schematic deactivation of a DSA in acid
medium. The electrocatalytic XO2 particle (X = Ir or
Ru) is progressively isolated and is finally detached from
the electrode surface.
ever, their cost is too high. Our results show that DSAs
operating in concentrated acid solutions may suffer from
TiO2 dissolution continuously. Of course, when the electrodes are polarized anodically, the TiO2 protecting film is
rebuilt permanently, but the dissolution of TiO2 may induce a detachment of the XO2 electrocatalytic oxide particles (IrO2 or RuO2 ), and contribute to the loss of performance. The composition of the layer is about 30 % of
precious metal oxide and 70 % of TiO2 . The presence of
TiO2 is necessary for stabilization of the layer. Among
the different deactivation mechanisms of DSAs, one finds:
substrate passivation, coating consumption or detachment,
and mechanical damages [47,48]. The macroscopic deactivation of electrodes by formation of a passivating layer may
be understood by a phenomenological model [49], which allows to understand the hysteresis in the j-U characteristics;
that model may probably be applied to the deactivation of
DSAs resulting from the growth of a passive TiO2 layer
under the electrocatalytic layer.
Here, we consider the second cause of deactivation: coating consumption or detachment. In concentrated sulphuric
acid, the dissolution of TiO2 is a purely chemical reaction,
which occurs independently of the applied potential. When
a DSA is used in such a medium, TiO2 will be dissolved;
the anodic potential applied to the electrode allows TiO2
to be formed again on the surface, but this phenomenon
may probably be responsible for the detachment of electrocatalytic particles from the surface of the electrode, as
shown in Figure 12. Consequently, industrial DSA must
not be used in too much acidic media. On the contrary,
“low cost DSAs” Ti/TiO2 /PbO2 electrodes, which have
been proposed for the oxidation of Cr(III) into Cr(VI) in
concentrated sulphuric acid [42] may be used because TiO2
In this paper, confident values of OCP were obtained because reliable reference electrodes without junction were
used for the experiments. These results allow to understand the deactivation process of DSAs. Indeed, it is
sometimes observed that the electrocatalytic layer onto
the surface of the titanium substrate progressively disappear, falling down at the bottom of the electrolysis cell.
In acid solutions, TiO2 is dissolved at the interface surface/electrolyte. The electrocatalytic XO2 particles (X=
Ru or Ir) are progressively isolated and are no more firmly
attached to the surface of the electrode. That phenomenon occurs, even if the metallic substrate is never in direct contact with the acid medium because titanium oxide
is progressively reformed from the reaction of Ti with water
(passivation reaction).
It is not always possible to evidence the dissolution of
TiO2 from usual electrochemical methods (polarisation resistance determination; impedance measurements) because
that reaction is counter-balanced by the passivation of the
electrode (formation of a TiO2 layer).
When the dissolution rate is greater than the formation rate in sulphuric acid, some parts of the electrode are
naked. This situation is easy to detect by different techniques:
1. from the OCP curves: sharp decrease of the potential, leading to very negative values.
2. from impedance diagrams: decrease of the real part
of impedance, especially in the high frequency loop.
3. from the measurement of the polarisation resistance
Rp : its value is very small when the electrode is no
more covered by a TiO2 layer.
However, if the concentration of the acid is not very important (i.e. 0.5 M H2 SO4 ), the dissolution rate is smaller
than the passivation rate; thus, the metal is never naked.
Ageing in diluted sulphuric acid solutions do not lead to
dramatic variations of the OCP, the impedance or the Rp
values.
The dissolution of TiO2 may lead to the deactivation of
DSAs; however, the phenomenon is not easy to prove by
classical electrochemical techniques, except by the determination of the concentration of Ti(IV) in the solution,
for example by the electroanalytical technique described
above. It is a convenient tool for detecting in situ the dissolution of TiO2 and the corrosion of Ti, which can lead
to the deactivation of DSAs.
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