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Volume 3 Paper 7


Electrochemical and Corrosion Behaviour of Commercially and Analytically Pure Titanium in Alkaline Solutions

J. Pjescic, S. Mentus*, V. Komnenic and N. Blagojevic
Faculty of Metallurgy and Technology, Podgorica University, 81000 Podgorica, Montenegro Yugoslavia, and *Faculty of Physical Chemistry, Belgrade University, 11000 Belgrade Yugoslavia

Abstract

The electrochemical and corrosion behaviour of commercially and analytically (99.99%) pure titanium in 0.1 M; 0.5 M and 1.0 M NaOH (pH=12.73-13.49) were investigated and were found to depend on electrode purity and solution concentration. The corrosion, passive and transpassive regions were also analysed. The corrosion and polarization resistances as well as temperature influence on corrosion behaviour were determined.

Key words: alkaline solutions, corrosion rate, oxide film, titanium.

Introduction

Thanks to its low density and high mechanical and corrosion resistance, titanium is widely used as construction material in many fields of technology. Its corrosion resistance is due to a passivating oxide layer which is formed either spontaneously in air and/or in water solutions or using anodic polarization of titanium electrodes in both acid and alkaline solutions. According to the properties of the oxide film, titanium belongs to the valve metals. Recently titanium dioxide has attracted particular attention as one of the most promising semiconductor materials for solar energy conversion1-5. There are only few works in literature related with corrosion behaviour of titanium in alkaline solutions6-7.

In the work of� Braner and Nann6 the corrosion behaviour of titanium in 1.0 M and 6.0 M NaOH solutions was investigated using current-voltage dependence. The transpassive region was shifted toward more negative potentials compared to one in acid solutions. The anodic passive film was found to be thicker in 6.0 M than in 1.0 M NaOH, for the same current densities as well as in case of open circuit.

Wilhelmsen and Hurlen7 have investigated the behaviour of titanium using potentiostatic and galvanostatic methods. The electrodes were previously stabilized in alkaline solutions, pH=7-13. The results of capacity measurements of passive film TiO2 in potential region between �0.75 and 2.5 V vs SCE were also presented. They concluded that titanium passes from active to passive state at potentials more positive than �0.75 V. The polarization current does not depend on pH in passive region.

In several works which deal with titanium corrosion behaviour in acid solutions, some preliminary examinations were carried out in alkaline solutions. Blackwood et al8. have investigated deactivation of titanium electrodes in 0.1 M and 3.0 M H2SO4 using cyclo voltammetric (CV) method.

The electrodes were previously passivated in the same solutions under conditions of open circuit. The stability of oxide film was found to depend on its formation rate, temperature and acid concentration. The deactivation was attributed to the nucleation of particles during TiO2 formation as well as to two dimensional vacancies formation inside the oxide. This structure enables disproportionation reaction to occur, Ti+TiO2+2H2O4TiOOH, creating conductive form TiOOH. Toressi et al.9-10 have derived an expression for calculation of anodic passive film thickness,

The mechanism of hydrogen evolution reaction on titanium electrodes was also proposed

S + H+ (aq) + e SH         (Volmer reaction)
SH + H+(aq) H (Heyrowski reaction)
SH + SH H2 + 2S (Tafel reaction)

where S indicates the active site on the metal surface. The rate determining step was found to be the chemical recombination reaction of hydrogen atoms (the Tafel reaction).

Baba11 has shown that passive oxide films on titanium electrodes are formed in accordance with Faraday's Law, the film thickness increase can be calculated using equation

where d and do are the final and original film thicknesses, z is the number of electrons, F is Faraday's constant, r is density of the oxide, j is the current density, t is the time and M is the molar weight.

The resistance of the anodic oxide layer can be calculated using expression

and its values were found in the range between 107 and 109Ohm.

Dyer et al.12 have investigated in detail the causes of electrical breakthrough on titanium electrodes in passive and� transpassive� region� for� different� current densities (1-100mA/cm2).

Mc Aleer et al.13 carried out the same investigation confirming the previous results, with an additional conclusion that conductive hydroxyl bridges of TiO2 (H2O)1.4 or TiO0.6(OH)2.8 type were formed at potential of electrical breakthrough.

Experimental

Commercial titanium according to DIN 37025; Fe-0.20%, O2-0.10%, N2-0.05%, C-0.008%, H2-0.13%, Pd-0.15%, active surface area 0.2cm2 and analytical grade titanium (99.99%), active surface area 0.3cm2 in a form of rod was protected by PTFE in such a manner that only its cross section was exposed to electrolyte. The electrode preparation consisted only in polishing with 1200 grit emery paper immediately before dipping into cell solution.� The testing solutions were�

0.1 M NaOH + 1.9 M NaClO4� pH = 12.73,

0.5 M NaOH + 1.5 M NaClO4, pH = 13.13

1 M NaOH + 1.0 M NaClO4, pH = 13.49

in order to maintain the ionic strength constant. By an argon stream the solution was kept of exposure to air. Saturated calomel electrode was used as reference electrode and wide Pt foil was used as counter electrode. The measurements were carried out by an original PKS-system with built in software for electrochemical and corrosion investigation as well as by a PAR EG&G potentiostat/galvanostat Model 273. A scanning electron microscope (JEOL Model JCXA-733) was also used. The working temperature of the solutions was 25C, and the temperature influence was investigated in the range 25 and 55C.

Results and Discussion

a)   b)
fig1left.jpg (24923 bytes) Fig1RIGHT.jpg (24041 bytes)

Fig. 1. The voltammograms obtained in a 0.1 M NaOH+1.9 M NaClO4, pH=12.73; commercial pure titanium, v =50 mV/s, t =25�C; a) freshly prepared electrode; b) third repeated cycle (note that for this and other voltammograms the x-axis is potential vs SCE, while the y axis is current density in μA/cm2 - click on the figure for a full-size view)

a)  b)
Fig2 LEFT.jpg (24609 bytes) Fig2 RIGHT.jpg (24890 bytes)

Fig. 2. The voltammograms obtained in a 0.5 M NaOH+1.5 M NaClO4, pH=12.73; commercial pure titanium, v =50 mV/s, t =25�C; a) freshly prepared electrode; b) third repeated cycle

a) b)

Fig. 3. The voltammograms obtained in a 1.0 M NaOH+1.0 M NaClO4, pH=12.73; commercial pure titanium, v =50 mV/s, t =25�C; a) freshly prepared electrode ;b) third repeated cycle

a) b)

Fig. 4. The voltammograms obtained in a 0.1 M NaOH+1.9 M NaClO4, pH=12.73; analytical pure titanium, v =50 mV/s, t =25�C; a) freshly prepared electrode; b) third repeated cycle

a) b)

Fig. 5. The voltammograms obtained in a 0.5 M NaOH+1.5 M NaClO4, pH=12.73; analytical pure titanium, v =50 mV/s, t =25�C; a) freshly prepared electrode; b) third repeated cycle

a) b)

Fig. 6. The voltammograms obtained in a 1.0 M NaOH+1.0 M NaClO4, pH=12.73; analytical pure titanium, v =50 mV/s, t =25�C; a) freshly prepared electrode; b) third repeated cycle

For all cyclovoltammetric investigations the initial potential was -1.1V vs SCE, which is close to Ti/TiO2 standard potential (-0.86V).

The currents which were noticed at this potential were attributed to hydrogen evolution on thin oxide films, formed during short exposure of the electrodes to air.

At more positive potentials, in the vicinity of the corrosion potential, titanium anodically dissolves to Ti+ and Ti2+. In the potential region between �0.5 and 0.5 or 0.75 V (depending on titanium purity and NaOH concentration) vs corrosion potential, corrosion reactions proceed forming passive oxide layer

Ti + 2OH- TiO2 + H2 + 4e

Ti + 2H2O TiO2 + 2H2

This passive region is characterised by the dominant presence of TiO2 on the titanium surface. The TiO2 layer is formed only partially during the first cycle, while it is completed after second cycle (Figures "b").

The potential in this passive region is stable and only slightly  changes with time due to inhibition of corrosion reactions that is caused by growing TiO2 passive layer. The electrode enters transpassive region at potentials more positive than 0.5 or 0.75 V vs corrosion one. This region is characterized by an electrical breakthrough and subsequent slow rise of current. The rise is attributed to the evolution of oxygen that is enabled by transformation of TiO2 passive layer to conductive TiOOH form.

TiOOH is formed through reaction of disproportionation which proceed due to particle nucleation and simultaneous formation at vacancies. This explanation of Ti behaviour in transpassive region, together with possible formation of hydroxyl bridges is accepted in available literature8-13.

In our experiments, using CV method, the applied sweep rate of 50 mV/s was high enough to provide rapid nucleation and simultaneous formation at two dimensional vacancies during TiO2 formation. The intensity of titanium passivation is found to be determined by purity of Ti electrodes and NaOH concentration. Both commercial and analytical Ti undergo faster passivation in more concentrated solutions. The analysis Pourbaix diagram for Ti confirms this conclusion14.

The TiO2 passive layer is found to be significantly thicker for analytical than for commercially pure titanium. This is due to presence of impurities (Fe, H2, N2 ...) in commercial Ti which also form their oxides, decreasing the rate of the basic reaction. The voltammograms (Fig.7a, 7b, and 8a, 8b) were used for determination of kinetics and mechanism of titanium dissolution reaction as well as of hydrogen evolution reaction9-10. Using the values of current and potential from the voltammograms the Tafel lines were plotted. The calculated values of the slopes are shown in Tables 5. On the basis of these values. the Tafel reaction (recombination of hydrogen adatoms to molecule) is found to be the rate determining step in� NaOH� concentration� range� between� 0.1 and 0.5 M. The mechanism of titanium anodic dissolution to Ti+ and Ti2+ up to 50 mV vs corrosion potential is determined in a similar manner, and the following reactions are proposed

Ti Ti+ + e

Ti+ Ti2+ + e

The Tafel slopes observed (Table 7) suggest that the first step is rate determining.

a) b)

Fig. 7. a) the segment of voltammogram �b� from fig. 1. in the potential range between �0.65 and �0.25 V vs SCE (data available as Table 1)
           b) the segment of voltammogram �b� from fig. 2. in the potential range between �0.71 and �0.35 V vs SCE (data available as Table 2)

a) b)

Fig. 8.  a) the segment of voltammogram �b� from fig. 4. in the potential range between �0.86 and�� �0.35 V vs SCE (data available as Table 3)
����������� b) the segment of voltammogram �b� from fig. 5. in the potential range between �0.77 and� �0.38 V vs SCE (data available as Table 4)

[mV]

 

log j [mA/cm2]

Fig. 9. E=f(log j), The potential-current density dependence
        +      values from table 1
        *      values from table 2

[mV]

 

log j[mA/cm2]

Fig. 10. E=f(log j), The potential-current density dependence
        +      values from table 3
        *      values from table 4

Table 5 The values of� Tafel slopes from fig. 7a, 7b, 8a and 8b

Electrodes

Solutions

Anodic slope
ba(mV/dec)

Cathodic slope
bc(mV/dec)

Commercially pure titanium

0.1MNaOH+1.9MNaClO4
0.5MNaOH+1.5MNaClO4

158
131

34
51

Analytical pure titanium

0.1MNaOH+1.9MNaClO4
0.5MNaOH+1.5MNaClO4

160
160

31
34

The corrosion resistance in the corrosion region as well as polarization resistance in both passive and transpassive regions were calculated (Tables 6 and 7) using obtained voltammograms Fig.1,2,3,4,5 and 6. It is obvious that the polarization resistances are higher than corrosion ones for both analytical and commercial Ti electrode in the whole range of investigated concentrations. This can be explained by the presence at conductive Ti+ and Ti2+ ions in the corrosion region. The highest values of polarization resistances were found in the passive region, while the resistances in the transpassive one were lower, but still much higher than in corrosion region14.

Table 6. The corrosion and polarization resistances for commercially pure titanium

Solutions

Corrosion and polarization resistances [Ohm]

Corrosion resistances

Polarization resistances in passive region, the mean values

Polarization resistances in transpassive region, the mean values

0.1M NaOH + 1.9M NaClO4

Rcor.=667
E=-0.49V

Rp=6777.5
E=+0.2V

Rp=6300
E=+0.75V

0.5M NaOH + 1.5M NaClO4

Rcor.=714
E=-0.49V

Rp=6916.2
E=+0.2V

Rp=6350
E=+0.75V

1.0M NaOH + 1.0M NaClO4

Rcor.=1333.3
E=-0.49V

Rp=18625.1
E=+0.2V

Rp=6400
E=-0.8V

Table 7. The corrosion and polarization resistances for analytical pure titanium

Solutions

Corrosion and polarization resistances [Ohm]

Corrosion resistances

Polarization resistances in passive region, the mean values

Polarization resistances in transpassive region, the mean values

0.1M NaOH + 1.9M NaClO4

Rcor.=1000.5
E=-0.69V

Rp=22700.8
E=+0.5V

Rp=6066.7
E=+1.2V

0.5M NaOH + 1.5M NaClO4

Rcor.=1150
E = -0.69V

Rp= 23660.1
E = +0.5V

Rp= 6066.7
E = +1.2V

1.0M NaOH + 1.0M NaClO4

Rcor.=2005
E=-0.69V

Rp=40750.3
E=+0.5V

Rp=8000
E=+0.8V

Both, corrosion and polarization resistances have higher values for analytical than for commercial titanium electrodes (Tables 6 and 7) in all investigated solutions. The curves "b" in voltammograms show that the anodic current densities- in passive regions are lower for analytical than for commercial Ti electrodes.

The temperature influence on electrochemical and corrosion behaviour of analytically pure titanium in 1.0 M NaOH solution was also investigated. The voltammograms (Fig 9 and 10) obtained at 35 and 550C show that in case of repeated cycles (curves "b") the electrodes were completely passivated at both temperatures. The current slowly� rises in the potential range between 0.3 and 0.6 V with subsequent rapid rise between 0.6 and 0.8 V.

a) b)

Fig. 11. The voltammograms obtained a 1.0 M NaOH+1.0 M NaClO4, analytical pure titanium,  v = 50 mV/s, t = 35�C, a) freshly prepared electrode; b) third repeated cycle

a) b)

Fig. 12. The voltammograms obtained a 1.0 M NaOH+1.0 M NaClO4, analytical pure titanium, v = 50 mV/s, t = 55�C, a) freshly prepared electrode; b) third repeated cycle

Conclusions

The electrochemical and corrosion behaviour of analytically and commercially pure titanium electrodes in alkaline solutions was investigated in order to determine the influence of the state of electrode surface, concentration of NaOH and temperature.

In order to get the reproductive results, the preparation of the electrode surface with 1200 grit emery paper immediately before dipping the electrode into solution is needed.

The electrode purity does not influence neither the mechanism of hydrogen evolution reaction nor of the titanium anodic dissolution, but does affect the values of corrosion and polarization resistances. The lower resistances were found for the electrodes of commercial purity. The NaOH concentration has an essential influence on corrosion and polarization resistances causing their increase in more concentrated solutions. The rate of TiO�2 growth increases with increasing temperature up to 35�C. The further rise of temperature (up to 55�C) does not show any influence.

References

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14. Pourbaix diagrams, Atlas of Electrochemical Equilibria in Aqueous Solution (1966)