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Oxide film formation and thickening on titanium in water
Thin Solid Films, 167 (1988) 269-280
269
GENERAL FILM BEHAVIOUR
OXIDE FILM FORMATION WATER
AND THICKENING
ON TITANIUM
IN
A. M. SHAMS EL DIN AND A. A. HAMMOUD
Materials Testing Laboratory, Water and Electricity Department, P.O. Box 219. Abu Dhabi (UnitedArab Emirates) (Received April 22,1987; revised November 13,1987; accepted May 16,1988)
Active titanium electrodes were immersed in distilled water at various temperatures and their potentials were followed as a function of time. At temperatures of 40°C or less, the potentials changed continuously towards nobler values. At higher temperatures, the initial potential rise was followed by a gradual decrease, suggesting a change in the structure of the passivating film. Plots of potential rise as a function of the logarithm of the immersion time were linear, allowing the computation of the rates of oxide thickening. This followed a direct logarithmic growth law. Following treatment in water, the electrodes were reacted in a standard acid solution that attacked the oxide. The potential-time curves exhibited a definite arrest corresponding to the Flade step of titanium. The length z of the arrest changed with the time of oxide formation at first according to z = kt”, where k and n are constants. When the oxide attained a definite thickness, r became independent oft. The Flade arrest of titanium was subjected to analysis according to the theory of chronopotentiometry of irreversible electrode processes. The results obtained strongly supported the conclusion that TiO, undergoes a physical transformation when it is prepared in water above 40 “C.
1. INTRODUCTION
The attributed this film kcal mol-
oustanding resistance of titanium to various types of corrosion is to the presence of a thin compact film of TiO, ‘. The ready formation of in air is related to the large decrease in free energy AG = -203.8 ’ 2, associated with the reaction
Ti+02
= TiO,
(1)
However, the development of the oxide in aqueous media is explained on the basis that the Ti/Ti02 potential is more negative than the potential of the hydrogen electrode in the same medium3. Accordingly, water acts as the oxidizing agent in the reaction Ti + 2H20 = Ti02 + 2H, oo40-6090/88/$3.50
(21 0 Elsevier Sequoia/Printed in The Netherlands
270
A. M. SHAM!4 EL DIN, A. A. HAMMOUD
The understanding of reaction (2) is of importance not only from the academic point of view, but also for technical reasons. Condensers incorporating titanium tubes occasionally undergo cleaning to remove scales and deposits. This is carried out either mechanically with steel brushes or chemically by subjecting the tubes to an acid wash. This might lead to the partial destruction or the total removal of the protective oxide film. Thus it is of value to establish the rate of subsequent tim growth and the properties of the new film. The bulk of our knowledge on the thickening of TiOz in aqueous solutions is obtained from polarization experiments 4- ‘. The characteristics of the oxide are decided, however, by the type of electroyte under investigation. Further, phase transformation might also occur as the oxide increases in thickness”*“. The same can also be said about oxides grown under open-circuit conditions, where incorporation of solution anions can influence the rate of thickening”. The aim of the present investigation is to study the kinetics of reaction (2) in the absence of any interfering factor. In order to eliminate the effect of anions on the rate and morphology of oxide growth, work was carried out in pure (electrolyte-free) water, commencing with an active metal surface. Further, a simple, yet reliable, technique is necessary to evaluate the thickness of TiOz after increasing time intervals of the reaction. All three requirements are satisfied in the present study. To our knowledge no work on titanium from the present standpoint has been hitherto published. 2.
EXPEFUbfENTAL DETAILS
Titanium electrodes measuring 4.0 cm x 2.6 cm and having a side arm for electrical connection were used. The electrodes had the following impurities: C, 0.10%; Fe, 0.30%; Nz, 0.03%; Hz, 0.015%; 02, 0.25%. The initial surface treatment of the electrodes involved their abrasion with fine emery paper, degreasing with acetone, followed by washing with running distilled water. To obtain an oxide-free surface, advantage is made of the fact that TiO, on the surface of titanium undergoes reductive dissolution in moderately concentrated HCl or HzS04 13: TiOz+4H++e-
= Ti3++2Hz0
(3)
or TiOz +4H+ + 2e- = Ti2+ + 2H20
(4)
However, as a result of the simultaneous occurrence of reaction (2), it is unlikely that a surface completely free of the oxide could be obtained. At the steady state the thickness of the surface oxide will be decided by the rate of the two opposing reactions. These are influenced by the prevailing experimental conditions, i.e. the nature and concentration of the attacking acid and by the temperature of the solution. By keeping these variables constant, the preparation of electrodes of comparable surface activity is ensured. In the present study activation of the titanium electrodes was achieved in 3.5 M HCl at 35 “C. The electrode potential was followed with time until a value from - 650 to - 700 mV (measured with respect to a
OXIDE FILM FORMATION AND THICKENING ON TITANEJM IN WATER
271
saturated calomel electrode) (SCE) was measured. This potential indicated the free dissolution of the bare metal in the acid solution, accompanied by gaseous hydrogen. The electrode was then withdrawn from the acid, rapidly washed with running distilled water and introduced into the passivation cell which contained distilled water thermostated at a preset value. The variation in the potential of the electrode with time during the process of passivation was followed relative to an SCE half-cell using a Fluke (Holland) type 8050A digital multimeter. The time of passivation was counted from the moment the electrode was withdrawn from the acid solution. After passivation in water for increasing periods, the thickness of the TiO, film was estimated in terms of the times needed for the reactivation of the electrodes in 3.5 M HCl at 35 “C. The potential of the electrodes was followed until the inflection denoting complete removal of the oxide was recorded. Experiments under identical conditions were repeated at least in duplicates and deviation in activation times between identical runs was within 3-5 min. Passivation experiments were carried out at 28,40,50,60,70 and 80 “C. 3.
RESULTS AND DISCUSSION
In the present investigation the potential of the titanium electrode was followed with time during the process of passivation in distilled water at various temperatures and during activation in 3.5 M HCl at 35 “C. Each set of measurements yields a certain type of information corresponding to oxide formation and oxide breakdown respectively. For clarity in presentation, each set is dealt with separately. 3.1. Pamivation The curves of Fig. l(a) represent the variation with time of the potential of active oxide-free titanium electrodes during passivation for increasing periods in distilled water at 28 “C. It is evident that the potential of the titanium electrode progressively shifts towards less negative values, first rapidly, and later at a reduced rate. According to Evans’ rule 14,this behaviour denotes the building and thickening of the oxide film on the metal surface. Oxidation occurs by virtue of the decrease in the free energy of reaction (2), which amounts to 82.92 kcal mol- ’ ‘. The way by which the oxide develops on the metal is best understood by presenting the results on semi-logarithmic plots, with the approximately parallel straight lines of Fig. l(b) obtained. The curves satisfy the general relationship E = a+blogt
(5)
where a and b are constants. Relation (5) describes the behaviour of metals’2*‘5 or alloys16*17 covered with thin semi-conducting oxide films, thickening according to a logarithmic growth law. In electrolyte solutions, film thickening is explained on the basis of adsorption of anions on the oxide surface, whereby they create electric fields suiIiciently high to allow the migration of metal (oxide) ions through the film to the oxide-electrolyte (oxide-metal) interface’ 5. The idea of anion adsorption is not, however, explicitly clear in the present case, where the metal is subjected to the action of distilled water
A. M. SHAM3 EL DIN,
The
(a)
0 0))
A. HAMMDUD
, min
,,,
0.5
A.
1.0
1.5
I
,
2.0
2.5
log t
Fig. 1. Variation in the titanium electrode potential with time in distilled water at 28 “C:(a) E-t plots;(b) E-log t plots.
only. However, as is evident from reaction (2), the evolution of hydrogen gives rise to an increase in the interfacial pH. The resulting OH- ions are assumed to be specifically and strongly absorbed on the surface, where they induce the electric field necessary for the promotion of ion migration through the oxide. According to the theory of oxide growth under open-circuit conditionsr5, the constant b in eqn. (5) amounts to 2.303 8/B where 8 is the rate of thickening of the oxide film and B is identified as 1’ B=sb
(6)
The term a’ is a transference coefficientrg and 6’ is the width of the energy barrier surmounted by the ion during migration through the oxide. The mode of thickening of TiOz under open-circuit conditions is not known. Recently, however, radioactive marker studies on anodically formed glassy TiO, revealed that thickening occurs by way of diffusion of both 02- and Ti4+ ions 20. The metal transport number Tm varied between 0.35 and 0.39, depending on the
OXIDE FILM FORMATION AND THICKENING ON TITANIUM IN WATER
273
formation rate. Assuming the same to apply to the case und+ consideration, and using the lower value for Tm, an average of 2.7 is calculated for n in eqn. (6). The slope of the lines of Fig. 1 (b) amounts to 0.165 V per log t unit. Taking the value TV’ as 0.5 and of 6’ as 1 nm, the rate of TiOZ thickening in distilled water at 28 “C can readily be evaluated as 3.74 nm per logt unit. The variation in the open-circuit potential of the titanium electrode in distilled water at higher temperatures is of particular interest. The curves of Fig. 2 represent the behaviour at temperatures between 40 and 80°C. For short immersion times, the potential drifts continously towards positive values. For longer times the potential turns back to more negative values, after passing through a flat maximum. The change in potential direction occurs earlier, and the drop in potential is larger the higher the temperature. Plots of the variation in potential until the times of development of maxima as a function of the logarithm of immersion times gave families of parallel lines whose slope depended on the ambient temperature. The slopes were 0.143,0.143,0.165,0.186 and 0.240 V per log time unit at 40,50,60,70 and 80 “C respectively. The corresponding rates of oxide thickening are 3.12, 3.02, 3.39, 3.70 and 4.64 nm per log t unit. These figures reveal that 5 is a minimum between 40 and 50 “C. Inspection of the curves of Fig. 2 reveals that at 40 “C the E-t curves ultimately attain constant values. At 50°C the curves exhibit the first development of the flat maxima. As indicated previously, the maxima increase in magnitude with rise in temperature. Apparently the development of maxima and the variation in 8 are interrelated and are brought about by a change in the structure and/or properties of the passivating TiO, film. It is commonly agreed that electrolytically produced TiOz at room temperature has a non-crystalline glassy structure6*9s21. There is convincing evidence, however, that during its growth, crystallization sets ini”*“. A rise in temperature is expected to enhance this process. The technique of open-circuit potential measurement does not allow sufficient information about the process of crystallization of TiO,. The results at hand indicate, however, that the rate of thickening of the glassy form is somewhat higher than that of the crystalline modification. With a rise in temperature above 50 “C, 5 increases again, apparently as a result of an increase in the mobility of the migrating ions. Later, evidence is presented which supports the existence of two types of TiO?. 3.2. Activation Following their passivation in distilled water for increasing time intervals, the titanium electrodes were self-activated in 3.5 M HCl at 35 “C. The variation in the potential with time was followed until a constant potential was reached. The curves of Fig. 3 represent a family of activation curves obtained after passivation for various times at 40 “C. A well-defined arrest is evident with a sharp potential drop, signifying the complete removal of the oxide from the metal surface. Similar sets of curves were also obtained when passivation was carried out at other temperatures. The step of Fig. 3 (and the like) has been identified as the Flade arrest of titanium, corresponding to the reductive dissolution of TiOz ’ 3. The activation curves exhibit a number of interesting features which deserve mention. Thus, for example, the time r of activation counted from the moment of
274
A. M. SHAMS EL DIN, A. A. HAMMOUD
-0
-200 40
If s
D
30
co‘c ‘I
rm ,w
-6oo-
I
20
u)
60
I
a0
loo
la
l40
160
la0
la,
220
240
60 'C
m'c
20
40
60
80
la3
120
140
160
la0
200
220
2bO
-2co-
80-c
:?[~(Iyy+y
s n ia
0
0 40
60
80
loo
Time
120
,
,
,
,
,
*
140
160
lea
200
220
240
( min 1
Fig. 2. Variation in the titanium electrode potential with time in distilled water at elevated tempexatures.
immersion in acid to the sharp inflection in potential, at first varies with the time t of treatment in water. However, after a certain definite passivation time, dependent on temperature, r becomes independent of t. As will be shown shortly, z is a measure of the oxide thickness on the metal surface. Accordingly, it is concluded that TiQ, increases in thickness with the time of treatment in water to attain a constant value. The relation between z and t is best represented by plotting the two variables on a double-
OXIDE FILM FORMATION AND THICKENING ON TITANIUM IN WATER
275
Time lminl
Fig. 3. Variation in the titanium electrode potential with time in 3.5 M HCl, following passivation in distilled water at 40 “C for various time intervals.
logarithmic scale (Fig. 4(a)). The curves are formed of two distinct segments. Along the initial lower portion, log r varies linearly with log t, suggesting that the two parameters are related as T = kt”
(7)
where k and n are constants; k represents the time of activation when the electrodes are passivated in water for 1 min. The value amounts to 0.43,3.63,8.71,15.85,25.70 and 50.12 min when passivation is carried out at 28, 40, 50, 60, 70 and 80°C respectively. In the domain of applicability of eqn. (7), passivation for the same time produces thicker oxides at higher temperatures. A plot of log k versus l/T is given in Fig. 4(b). For temperatures above 40 “C, the data fall on a straight line, from the slope of which the activation energy for formation of the oxide is calculated as 14.4 kcal mol- ‘. The Arrhenius plot does not extend to cover the result at lower temperatures. This behaviour appears to be related to a change in the course of the oxidation E-t curves (Fig. 2), which takes place within the same temperature range. The change in course of the oxidation E-t curves has been attributed to the start of crystallization of the glassy oxide. As is further seen from the curves of Fig. 4(a), log r attains constancy after sufficiently long passivation by treatment in distilled water. The higher the treatment temperature the faster t reaches its limiting value. The constant thickness of TiOz appears to be only slightly dependent on its preparation temperature. We have recently presented a theoretical model for the analysis of the Flade arrest of titanium in acid solutions I3 . The model involved the application of the theory of electrolysis with constant current density (chronopotentiometry)22, as applied to irreversible electrode processes 23P24.The constant cathodic current operating on the surface of the electrode was assumed to originate from a constant corrosion cell voltage and a constant solution resistance13. This line of thought led
276
A. M. SHAMS EL DIN, A. A. HAMMOUD
OXIDE FILM FORMATION AND THICKENING
ON TITANIUM
IN WATER
277
to an equation relating the variation in the electrode potential E with time along the activation step. The relation reads E = const -2.303~log(r1~z-t”‘)
In eqn. (8) r (min) is the time from the start of the activation experiment to the abrupt change in the electrode potential (Fig. 3), and t is any time between 0 and r. a is the transference coefficient of the irreversible electrode reaction involving the exchange of n electrons. According to eqn. (8) the plot of the electrode potential E as a function of log (? - PI*) should yield a straight line with a slope of O.O61/an(at 35 “C). In Fig. 5 curves are given for the analysis of the activation step of titanium electrodes which have been passivated in water at 28 “C. The approximately parallel straight lines of the figure support the validity of the analysis procedure. The lines have a slope of 0.116 V per log unit, strongly suggesting an irreversible one-electron process whose a value amounts to 0.53. The Flade arrest of titanium can therefore be justifiably ascribed to the electrochemical reaction Ti02 +4H+ + e- = Ti3+ + 2H20
(9)
In this respect the results conform with those of other workers25*26, who consider that the corrosion of titanium in acid solution produces Ti3+ ions. The analysis of the activation curves of electrodes passivated at higher temperatures is of particular interest. In Fig. 6 the curves for 70 “C are depicted as an example. When passivation was conducted for short times (t < 10 min), analysis gave rise to single straight lines covering the whole span of the step. The slopes of the lines compare with those measured at lower temperatures, and correspond to an irreversible electrode reaction governed by the transfer of a single electron. However, when passivation was carried out for longer periods, the analysis curves were formed of two segments, each with a definite slope. The slope of the lower segment (corresponding to advanced times on the Flade arrest) is the same for all curves and compares with that determined at lower temperatures. It indicates an irreversible one-electron cathodic reaction with an a value of around 0.45. The slope of the upper part of the analysis curves progressively increases with time of passivation to reach a limiting value of0.34 V per log unit. Before an attempt is made to explain the development of two slopes in the analysis curves at high temperatures, attention is drawn to the fact that the double slopes are only recorded when, during the previous oxidation experiment, the potential exhibited a dome-shaped maximum. Experiments in which the potential continously drifted towards positive values produced single lines in the analysis of the activation curves. It must therefore be concluded that the two phenomena are interconnected. Further eleboration on the same idea would suggest that, since the maxima of Fig. 2 develop at advanced times during oxidation, their effects are recorded first on subsequent activation (reduction). In other words, the segments with higher slopes in the activation curves are to be tied with changes in the oxide film developing at later times. That Ti02 reduces during one and the same experiment according to two
A. M. SHAMS EL DIN, A. A. HAMMOUD
278
I.
-03
*
-06
-0.5
-0.1
,
4.3
”
-0.2
-0.1 log,(Tb-
c
0
“, 0.1
0.2
0.3
. 0.6
* 0.5
* 0.6
. O.?
a
0.8
4
h9
tb 1
Fig. 5. Theoretical analysis of the activation curves of titanium electrodes previously passivated in distilled water at 28 “C.
Fig. 6. Theoretical analysis of the activation curves of titanium electrodes previously passivated in distilled water at 70 “C.
OXIDE FILM FORMATION AND THICKENING ON TITANIUM IN WATER
different patterns is of particular interest. Two mechanisms can be account for this behaviour. The first is based on the idea that the oxide two different mechanisms. Reduction according to an irreversible process has already been suggested. A reduction reaction involving can also be considered13, namely
279
thought of to is reduced via one-electron two electrons
TiO, + 4H+ + 2e- = Ti’+ + 2H,O However, it is not clear why the reduction of the oxide should follow a dual mechanism, particularly when the oxide had been produced at higher temperatures. Further, if reaction (10) was actually operating, it would be governed by an unreasonably small transfer coefficient, amounting to 0.09. The alternative to the dual slope of the analysis curves assumes the same mechanism of reduction (same number of electrons) and a change in the nature of the reducible species. The kinetics of reduction will vary, and this will affect inter alia the value of CLThus, on the assumption that n remains equal to one electron, the slope of 0.34 would correspond to an a value of 0.18. Compared with that of the lower section, this value indicates that reduction has become “more irreversible”, i.e. the reducing oxide is less reactive (more stable) than before. Since the reactivity of a solid increases with the extent of its structural disorder, it is reasonable to assume that the less-reactive kinetically more-difficult-to-reduce oxide is the crystalline modification. As the segments of the analysis curves with higher slopes are recorded at the initial stages of activation, it can be concluded that crystalline TiOz constitutes the outer layers of the passivating film and that reduction starts at the oxide-acid interphase. With the removal of the crystalline layers, dissolution of the inner glassy film sets in, and the slope of the analysis curve decreases correspondingly. To our knowledge the effect of temperature on the crystallization of TiOZ has not previously been examined. Leach and Sidgwick l1 , however, studied the nature of the oxide produced electrolytically in 1.88 M solutions of H2S04 and H3P04 in relation to the rate of formation. When anodization was carried out at low current densities, glassy TiO, was obtained. However, with currents sufficiently high to shift the potential above a certain critical value (Ec,n) crystalline TiO, developed at the metal-oxide interface. Ion migration through crystalline TiOZ required higher electric fields than that through the glassy oxide modification. At high current intensities (fields) the arrangement of the anodically formed oxide was thus metalcrystalline oxide-amorphous oxide-electrolyte. This layering differs from that suggested above for the oxide grown under open-circuit conditions at elevated temperatures, i.e. metal-amorphous oxide-crystalline oxide-water. This difference is not, however, to be unexpected. The open-circuit potentials are much lower than E,eritneeded to effect crystallization. This transformation in the present case does not occur through an increase in the field. The necessary energy required to cause transformation is supplied in the form of heat. With this visualization in mind, it can readily be understood why the analysis curves of electrodes filmed at low temperatures for any time, or at high temperatures for short times, are always formed of single-sloped straight lines. In neither case does the passivating film incorporate crystalline TiOz.
280
A. M. SHAMS EL DIN, A. A. HAMMOUD
APPENDIX A:
n t E F
NOMENCLATURE
number of electrons time (min) electrode potential (V) Faraday (96500 coulombs) Gibbs free energy of formation gas constant (1.98 cal deg - ‘) absolute temperature
AG R T
Greek symbols a, a’ transferrence coefficients
6 C? z
width of energy barrier (nm) rate of oxide thickening (nm (log t)- ‘) length of activation step (min)
REFERENCES 1 2 3 4 5 6 7 8
9 10 11 12 13
L. L. Shreir, Corrosion, Vol. 1, Newnes-Butterworth, London, 1979, pp. 5-34. W. M. Latimer, Oxidarion Pofentials, Prentice Hall, Englewood Cliff, NJ, 1964, p. 226. K. J. Vetter, Efekbochemische Kinetik, Springer, Berlin, 1961, p. 631. C. D. Hall and N. Hackerman, J. Phys. Chem., 57(1953) 262. H. Bohm, MetalloberJache, II (1957) 197. H. Johnsen, G. Adams and P. van Rysselberghe, J. Electrochem. Sot., 104 (1957) 339. H. Chiba, J. Electrochem. Sot. Jpn., 27 (1959) JX M. E. Sibert, J. blectrochem. Sot., 120 (1963) 65. I. A. Ammar and I. Kamel, Electrochim. Acta, I6(1971) 1539,1555. J. Yahalom and J. Zahavi, Electrochim. Acta, 15 (1970) 1429. J. S. L. Leach and D. H. Sidgwick, Proc. 7th Int. Congr. on Metallic Corrosion, Mainz. 1981, Deehema, Frankfurt-am-Main, 1981, p. 82. J. M. Abd El Kader, F. M. Abd El Wahab, H. A. El Shayeb and M. G. A. Khedr, Br. Corr. J., I6 (1981) 111. A. M. Shams El Din, H. A. El Shayeb and F. M. Abd El Wahab, J. Electroanal. Chem., 214 (1986) 567.
U. R. Evans, The Corrosion and Oxidation of Metals, Arnold, London, 1960, p. 898. 15 J. M. Abd El Kader and A. M. Shams El Din, Br. Corros. J., 14 (1979) 40. 16 J. M. Abd El Kader, F. M. Abd El Wahab, M. G. A. Khedr and A. M. Shams El Din, Mater. Chem., 7(1982) 313. 17 A. M. Shams El Din, F. M. Abd El Wahab, J. M. Abd El Kader, T. M. H. Saber and A. T. Kuhn, Bull. Electrochem. (India), 2 (1986) 401. 18 K. J. Vetter, in H. Fischer, K. Hautfe and W. Wiederholt (eds.), Passiuierende Fibne and Dichschichlen, Springer, Berlin, 1956, p. 72. 19 K. J. Vetter, Z. Naturforsch, Teil A., 7 (1952) 328. 20 N. Khalil and J. S. L. Leach, Elecfrochim. Acta, 31(1986) 1279. 21 I. Sanghi and S. Visvanathan, Electrochim. Acfa, 7(1962) 567. 22 P. Delahay, New Instrumental Methodr in Electrochemistry, Wiley-Interscience, New York, 1954, p. 179. 23 P. Delahay and T. Berzins, J. Am. Chem. Sot., 75 (1953) 2486. 24 2. Galus, Fundamentals of Electrochemical Analysis, Ellis Honvood, London, 1976, p. 242. 25 E. Brauer and E. Nann, Proc. 5th Int. Congr. on Metallic Corrosion, Tokyo, 1972, National Association of Corrosion Engineers, Houston, TX, 1974, p. 503. 26 E. M. M. Sutter, A. Comet and J. Pagetti, Proc. 6th Eur. Symp. on corrosion Inhibitors, Ferrara, 1985, University of Ferrara, 1985, p. 485. 14
269
GENERAL FILM BEHAVIOUR
OXIDE FILM FORMATION WATER
AND THICKENING
ON TITANIUM
IN
A. M. SHAMS EL DIN AND A. A. HAMMOUD
Materials Testing Laboratory, Water and Electricity Department, P.O. Box 219. Abu Dhabi (UnitedArab Emirates) (Received April 22,1987; revised November 13,1987; accepted May 16,1988)
Active titanium electrodes were immersed in distilled water at various temperatures and their potentials were followed as a function of time. At temperatures of 40°C or less, the potentials changed continuously towards nobler values. At higher temperatures, the initial potential rise was followed by a gradual decrease, suggesting a change in the structure of the passivating film. Plots of potential rise as a function of the logarithm of the immersion time were linear, allowing the computation of the rates of oxide thickening. This followed a direct logarithmic growth law. Following treatment in water, the electrodes were reacted in a standard acid solution that attacked the oxide. The potential-time curves exhibited a definite arrest corresponding to the Flade step of titanium. The length z of the arrest changed with the time of oxide formation at first according to z = kt”, where k and n are constants. When the oxide attained a definite thickness, r became independent oft. The Flade arrest of titanium was subjected to analysis according to the theory of chronopotentiometry of irreversible electrode processes. The results obtained strongly supported the conclusion that TiO, undergoes a physical transformation when it is prepared in water above 40 “C.
1. INTRODUCTION
The attributed this film kcal mol-
oustanding resistance of titanium to various types of corrosion is to the presence of a thin compact film of TiO, ‘. The ready formation of in air is related to the large decrease in free energy AG = -203.8 ’ 2, associated with the reaction
Ti+02
= TiO,
(1)
However, the development of the oxide in aqueous media is explained on the basis that the Ti/Ti02 potential is more negative than the potential of the hydrogen electrode in the same medium3. Accordingly, water acts as the oxidizing agent in the reaction Ti + 2H20 = Ti02 + 2H, oo40-6090/88/$3.50
(21 0 Elsevier Sequoia/Printed in The Netherlands
270
A. M. SHAM!4 EL DIN, A. A. HAMMOUD
The understanding of reaction (2) is of importance not only from the academic point of view, but also for technical reasons. Condensers incorporating titanium tubes occasionally undergo cleaning to remove scales and deposits. This is carried out either mechanically with steel brushes or chemically by subjecting the tubes to an acid wash. This might lead to the partial destruction or the total removal of the protective oxide film. Thus it is of value to establish the rate of subsequent tim growth and the properties of the new film. The bulk of our knowledge on the thickening of TiOz in aqueous solutions is obtained from polarization experiments 4- ‘. The characteristics of the oxide are decided, however, by the type of electroyte under investigation. Further, phase transformation might also occur as the oxide increases in thickness”*“. The same can also be said about oxides grown under open-circuit conditions, where incorporation of solution anions can influence the rate of thickening”. The aim of the present investigation is to study the kinetics of reaction (2) in the absence of any interfering factor. In order to eliminate the effect of anions on the rate and morphology of oxide growth, work was carried out in pure (electrolyte-free) water, commencing with an active metal surface. Further, a simple, yet reliable, technique is necessary to evaluate the thickness of TiOz after increasing time intervals of the reaction. All three requirements are satisfied in the present study. To our knowledge no work on titanium from the present standpoint has been hitherto published. 2.
EXPEFUbfENTAL DETAILS
Titanium electrodes measuring 4.0 cm x 2.6 cm and having a side arm for electrical connection were used. The electrodes had the following impurities: C, 0.10%; Fe, 0.30%; Nz, 0.03%; Hz, 0.015%; 02, 0.25%. The initial surface treatment of the electrodes involved their abrasion with fine emery paper, degreasing with acetone, followed by washing with running distilled water. To obtain an oxide-free surface, advantage is made of the fact that TiO, on the surface of titanium undergoes reductive dissolution in moderately concentrated HCl or HzS04 13: TiOz+4H++e-
= Ti3++2Hz0
(3)
or TiOz +4H+ + 2e- = Ti2+ + 2H20
(4)
However, as a result of the simultaneous occurrence of reaction (2), it is unlikely that a surface completely free of the oxide could be obtained. At the steady state the thickness of the surface oxide will be decided by the rate of the two opposing reactions. These are influenced by the prevailing experimental conditions, i.e. the nature and concentration of the attacking acid and by the temperature of the solution. By keeping these variables constant, the preparation of electrodes of comparable surface activity is ensured. In the present study activation of the titanium electrodes was achieved in 3.5 M HCl at 35 “C. The electrode potential was followed with time until a value from - 650 to - 700 mV (measured with respect to a
OXIDE FILM FORMATION AND THICKENING ON TITANEJM IN WATER
271
saturated calomel electrode) (SCE) was measured. This potential indicated the free dissolution of the bare metal in the acid solution, accompanied by gaseous hydrogen. The electrode was then withdrawn from the acid, rapidly washed with running distilled water and introduced into the passivation cell which contained distilled water thermostated at a preset value. The variation in the potential of the electrode with time during the process of passivation was followed relative to an SCE half-cell using a Fluke (Holland) type 8050A digital multimeter. The time of passivation was counted from the moment the electrode was withdrawn from the acid solution. After passivation in water for increasing periods, the thickness of the TiO, film was estimated in terms of the times needed for the reactivation of the electrodes in 3.5 M HCl at 35 “C. The potential of the electrodes was followed until the inflection denoting complete removal of the oxide was recorded. Experiments under identical conditions were repeated at least in duplicates and deviation in activation times between identical runs was within 3-5 min. Passivation experiments were carried out at 28,40,50,60,70 and 80 “C. 3.
RESULTS AND DISCUSSION
In the present investigation the potential of the titanium electrode was followed with time during the process of passivation in distilled water at various temperatures and during activation in 3.5 M HCl at 35 “C. Each set of measurements yields a certain type of information corresponding to oxide formation and oxide breakdown respectively. For clarity in presentation, each set is dealt with separately. 3.1. Pamivation The curves of Fig. l(a) represent the variation with time of the potential of active oxide-free titanium electrodes during passivation for increasing periods in distilled water at 28 “C. It is evident that the potential of the titanium electrode progressively shifts towards less negative values, first rapidly, and later at a reduced rate. According to Evans’ rule 14,this behaviour denotes the building and thickening of the oxide film on the metal surface. Oxidation occurs by virtue of the decrease in the free energy of reaction (2), which amounts to 82.92 kcal mol- ’ ‘. The way by which the oxide develops on the metal is best understood by presenting the results on semi-logarithmic plots, with the approximately parallel straight lines of Fig. l(b) obtained. The curves satisfy the general relationship E = a+blogt
(5)
where a and b are constants. Relation (5) describes the behaviour of metals’2*‘5 or alloys16*17 covered with thin semi-conducting oxide films, thickening according to a logarithmic growth law. In electrolyte solutions, film thickening is explained on the basis of adsorption of anions on the oxide surface, whereby they create electric fields suiIiciently high to allow the migration of metal (oxide) ions through the film to the oxide-electrolyte (oxide-metal) interface’ 5. The idea of anion adsorption is not, however, explicitly clear in the present case, where the metal is subjected to the action of distilled water
A. M. SHAM3 EL DIN,
The
(a)
0 0))
A. HAMMDUD
, min
,,,
0.5
A.
1.0
1.5
I
,
2.0
2.5
log t
Fig. 1. Variation in the titanium electrode potential with time in distilled water at 28 “C:(a) E-t plots;(b) E-log t plots.
only. However, as is evident from reaction (2), the evolution of hydrogen gives rise to an increase in the interfacial pH. The resulting OH- ions are assumed to be specifically and strongly absorbed on the surface, where they induce the electric field necessary for the promotion of ion migration through the oxide. According to the theory of oxide growth under open-circuit conditionsr5, the constant b in eqn. (5) amounts to 2.303 8/B where 8 is the rate of thickening of the oxide film and B is identified as 1’ B=sb
(6)
The term a’ is a transference coefficientrg and 6’ is the width of the energy barrier surmounted by the ion during migration through the oxide. The mode of thickening of TiOz under open-circuit conditions is not known. Recently, however, radioactive marker studies on anodically formed glassy TiO, revealed that thickening occurs by way of diffusion of both 02- and Ti4+ ions 20. The metal transport number Tm varied between 0.35 and 0.39, depending on the
OXIDE FILM FORMATION AND THICKENING ON TITANIUM IN WATER
273
formation rate. Assuming the same to apply to the case und+ consideration, and using the lower value for Tm, an average of 2.7 is calculated for n in eqn. (6). The slope of the lines of Fig. 1 (b) amounts to 0.165 V per log t unit. Taking the value TV’ as 0.5 and of 6’ as 1 nm, the rate of TiOZ thickening in distilled water at 28 “C can readily be evaluated as 3.74 nm per logt unit. The variation in the open-circuit potential of the titanium electrode in distilled water at higher temperatures is of particular interest. The curves of Fig. 2 represent the behaviour at temperatures between 40 and 80°C. For short immersion times, the potential drifts continously towards positive values. For longer times the potential turns back to more negative values, after passing through a flat maximum. The change in potential direction occurs earlier, and the drop in potential is larger the higher the temperature. Plots of the variation in potential until the times of development of maxima as a function of the logarithm of immersion times gave families of parallel lines whose slope depended on the ambient temperature. The slopes were 0.143,0.143,0.165,0.186 and 0.240 V per log time unit at 40,50,60,70 and 80 “C respectively. The corresponding rates of oxide thickening are 3.12, 3.02, 3.39, 3.70 and 4.64 nm per log t unit. These figures reveal that 5 is a minimum between 40 and 50 “C. Inspection of the curves of Fig. 2 reveals that at 40 “C the E-t curves ultimately attain constant values. At 50°C the curves exhibit the first development of the flat maxima. As indicated previously, the maxima increase in magnitude with rise in temperature. Apparently the development of maxima and the variation in 8 are interrelated and are brought about by a change in the structure and/or properties of the passivating TiO, film. It is commonly agreed that electrolytically produced TiOz at room temperature has a non-crystalline glassy structure6*9s21. There is convincing evidence, however, that during its growth, crystallization sets ini”*“. A rise in temperature is expected to enhance this process. The technique of open-circuit potential measurement does not allow sufficient information about the process of crystallization of TiO,. The results at hand indicate, however, that the rate of thickening of the glassy form is somewhat higher than that of the crystalline modification. With a rise in temperature above 50 “C, 5 increases again, apparently as a result of an increase in the mobility of the migrating ions. Later, evidence is presented which supports the existence of two types of TiO?. 3.2. Activation Following their passivation in distilled water for increasing time intervals, the titanium electrodes were self-activated in 3.5 M HCl at 35 “C. The variation in the potential with time was followed until a constant potential was reached. The curves of Fig. 3 represent a family of activation curves obtained after passivation for various times at 40 “C. A well-defined arrest is evident with a sharp potential drop, signifying the complete removal of the oxide from the metal surface. Similar sets of curves were also obtained when passivation was carried out at other temperatures. The step of Fig. 3 (and the like) has been identified as the Flade arrest of titanium, corresponding to the reductive dissolution of TiOz ’ 3. The activation curves exhibit a number of interesting features which deserve mention. Thus, for example, the time r of activation counted from the moment of
274
A. M. SHAMS EL DIN, A. A. HAMMOUD
-0
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220
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Fig. 2. Variation in the titanium electrode potential with time in distilled water at elevated tempexatures.
immersion in acid to the sharp inflection in potential, at first varies with the time t of treatment in water. However, after a certain definite passivation time, dependent on temperature, r becomes independent of t. As will be shown shortly, z is a measure of the oxide thickness on the metal surface. Accordingly, it is concluded that TiQ, increases in thickness with the time of treatment in water to attain a constant value. The relation between z and t is best represented by plotting the two variables on a double-
OXIDE FILM FORMATION AND THICKENING ON TITANIUM IN WATER
275
Time lminl
Fig. 3. Variation in the titanium electrode potential with time in 3.5 M HCl, following passivation in distilled water at 40 “C for various time intervals.
logarithmic scale (Fig. 4(a)). The curves are formed of two distinct segments. Along the initial lower portion, log r varies linearly with log t, suggesting that the two parameters are related as T = kt”
(7)
where k and n are constants; k represents the time of activation when the electrodes are passivated in water for 1 min. The value amounts to 0.43,3.63,8.71,15.85,25.70 and 50.12 min when passivation is carried out at 28, 40, 50, 60, 70 and 80°C respectively. In the domain of applicability of eqn. (7), passivation for the same time produces thicker oxides at higher temperatures. A plot of log k versus l/T is given in Fig. 4(b). For temperatures above 40 “C, the data fall on a straight line, from the slope of which the activation energy for formation of the oxide is calculated as 14.4 kcal mol- ‘. The Arrhenius plot does not extend to cover the result at lower temperatures. This behaviour appears to be related to a change in the course of the oxidation E-t curves (Fig. 2), which takes place within the same temperature range. The change in course of the oxidation E-t curves has been attributed to the start of crystallization of the glassy oxide. As is further seen from the curves of Fig. 4(a), log r attains constancy after sufficiently long passivation by treatment in distilled water. The higher the treatment temperature the faster t reaches its limiting value. The constant thickness of TiOz appears to be only slightly dependent on its preparation temperature. We have recently presented a theoretical model for the analysis of the Flade arrest of titanium in acid solutions I3 . The model involved the application of the theory of electrolysis with constant current density (chronopotentiometry)22, as applied to irreversible electrode processes 23P24.The constant cathodic current operating on the surface of the electrode was assumed to originate from a constant corrosion cell voltage and a constant solution resistance13. This line of thought led
276
A. M. SHAMS EL DIN, A. A. HAMMOUD
OXIDE FILM FORMATION AND THICKENING
ON TITANIUM
IN WATER
277
to an equation relating the variation in the electrode potential E with time along the activation step. The relation reads E = const -2.303~log(r1~z-t”‘)
In eqn. (8) r (min) is the time from the start of the activation experiment to the abrupt change in the electrode potential (Fig. 3), and t is any time between 0 and r. a is the transference coefficient of the irreversible electrode reaction involving the exchange of n electrons. According to eqn. (8) the plot of the electrode potential E as a function of log (? - PI*) should yield a straight line with a slope of O.O61/an(at 35 “C). In Fig. 5 curves are given for the analysis of the activation step of titanium electrodes which have been passivated in water at 28 “C. The approximately parallel straight lines of the figure support the validity of the analysis procedure. The lines have a slope of 0.116 V per log unit, strongly suggesting an irreversible one-electron process whose a value amounts to 0.53. The Flade arrest of titanium can therefore be justifiably ascribed to the electrochemical reaction Ti02 +4H+ + e- = Ti3+ + 2H20
(9)
In this respect the results conform with those of other workers25*26, who consider that the corrosion of titanium in acid solution produces Ti3+ ions. The analysis of the activation curves of electrodes passivated at higher temperatures is of particular interest. In Fig. 6 the curves for 70 “C are depicted as an example. When passivation was conducted for short times (t < 10 min), analysis gave rise to single straight lines covering the whole span of the step. The slopes of the lines compare with those measured at lower temperatures, and correspond to an irreversible electrode reaction governed by the transfer of a single electron. However, when passivation was carried out for longer periods, the analysis curves were formed of two segments, each with a definite slope. The slope of the lower segment (corresponding to advanced times on the Flade arrest) is the same for all curves and compares with that determined at lower temperatures. It indicates an irreversible one-electron cathodic reaction with an a value of around 0.45. The slope of the upper part of the analysis curves progressively increases with time of passivation to reach a limiting value of0.34 V per log unit. Before an attempt is made to explain the development of two slopes in the analysis curves at high temperatures, attention is drawn to the fact that the double slopes are only recorded when, during the previous oxidation experiment, the potential exhibited a dome-shaped maximum. Experiments in which the potential continously drifted towards positive values produced single lines in the analysis of the activation curves. It must therefore be concluded that the two phenomena are interconnected. Further eleboration on the same idea would suggest that, since the maxima of Fig. 2 develop at advanced times during oxidation, their effects are recorded first on subsequent activation (reduction). In other words, the segments with higher slopes in the activation curves are to be tied with changes in the oxide film developing at later times. That Ti02 reduces during one and the same experiment according to two
A. M. SHAMS EL DIN, A. A. HAMMOUD
278
I.
-03
*
-06
-0.5
-0.1
,
4.3
”
-0.2
-0.1 log,(Tb-
c
0
“, 0.1
0.2
0.3
. 0.6
* 0.5
* 0.6
. O.?
a
0.8
4
h9
tb 1
Fig. 5. Theoretical analysis of the activation curves of titanium electrodes previously passivated in distilled water at 28 “C.
Fig. 6. Theoretical analysis of the activation curves of titanium electrodes previously passivated in distilled water at 70 “C.
OXIDE FILM FORMATION AND THICKENING ON TITANIUM IN WATER
different patterns is of particular interest. Two mechanisms can be account for this behaviour. The first is based on the idea that the oxide two different mechanisms. Reduction according to an irreversible process has already been suggested. A reduction reaction involving can also be considered13, namely
279
thought of to is reduced via one-electron two electrons
TiO, + 4H+ + 2e- = Ti’+ + 2H,O However, it is not clear why the reduction of the oxide should follow a dual mechanism, particularly when the oxide had been produced at higher temperatures. Further, if reaction (10) was actually operating, it would be governed by an unreasonably small transfer coefficient, amounting to 0.09. The alternative to the dual slope of the analysis curves assumes the same mechanism of reduction (same number of electrons) and a change in the nature of the reducible species. The kinetics of reduction will vary, and this will affect inter alia the value of CLThus, on the assumption that n remains equal to one electron, the slope of 0.34 would correspond to an a value of 0.18. Compared with that of the lower section, this value indicates that reduction has become “more irreversible”, i.e. the reducing oxide is less reactive (more stable) than before. Since the reactivity of a solid increases with the extent of its structural disorder, it is reasonable to assume that the less-reactive kinetically more-difficult-to-reduce oxide is the crystalline modification. As the segments of the analysis curves with higher slopes are recorded at the initial stages of activation, it can be concluded that crystalline TiOz constitutes the outer layers of the passivating film and that reduction starts at the oxide-acid interphase. With the removal of the crystalline layers, dissolution of the inner glassy film sets in, and the slope of the analysis curve decreases correspondingly. To our knowledge the effect of temperature on the crystallization of TiOZ has not previously been examined. Leach and Sidgwick l1 , however, studied the nature of the oxide produced electrolytically in 1.88 M solutions of H2S04 and H3P04 in relation to the rate of formation. When anodization was carried out at low current densities, glassy TiO, was obtained. However, with currents sufficiently high to shift the potential above a certain critical value (Ec,n) crystalline TiO, developed at the metal-oxide interface. Ion migration through crystalline TiOZ required higher electric fields than that through the glassy oxide modification. At high current intensities (fields) the arrangement of the anodically formed oxide was thus metalcrystalline oxide-amorphous oxide-electrolyte. This layering differs from that suggested above for the oxide grown under open-circuit conditions at elevated temperatures, i.e. metal-amorphous oxide-crystalline oxide-water. This difference is not, however, to be unexpected. The open-circuit potentials are much lower than E,eritneeded to effect crystallization. This transformation in the present case does not occur through an increase in the field. The necessary energy required to cause transformation is supplied in the form of heat. With this visualization in mind, it can readily be understood why the analysis curves of electrodes filmed at low temperatures for any time, or at high temperatures for short times, are always formed of single-sloped straight lines. In neither case does the passivating film incorporate crystalline TiOz.
280
A. M. SHAMS EL DIN, A. A. HAMMOUD
APPENDIX A:
n t E F
NOMENCLATURE
number of electrons time (min) electrode potential (V) Faraday (96500 coulombs) Gibbs free energy of formation gas constant (1.98 cal deg - ‘) absolute temperature
AG R T
Greek symbols a, a’ transferrence coefficients
6 C? z
width of energy barrier (nm) rate of oxide thickening (nm (log t)- ‘) length of activation step (min)
REFERENCES 1 2 3 4 5 6 7 8
9 10 11 12 13
L. L. Shreir, Corrosion, Vol. 1, Newnes-Butterworth, London, 1979, pp. 5-34. W. M. Latimer, Oxidarion Pofentials, Prentice Hall, Englewood Cliff, NJ, 1964, p. 226. K. J. Vetter, Efekbochemische Kinetik, Springer, Berlin, 1961, p. 631. C. D. Hall and N. Hackerman, J. Phys. Chem., 57(1953) 262. H. Bohm, MetalloberJache, II (1957) 197. H. Johnsen, G. Adams and P. van Rysselberghe, J. Electrochem. Sot., 104 (1957) 339. H. Chiba, J. Electrochem. Sot. Jpn., 27 (1959) JX M. E. Sibert, J. blectrochem. Sot., 120 (1963) 65. I. A. Ammar and I. Kamel, Electrochim. Acta, I6(1971) 1539,1555. J. Yahalom and J. Zahavi, Electrochim. Acta, 15 (1970) 1429. J. S. L. Leach and D. H. Sidgwick, Proc. 7th Int. Congr. on Metallic Corrosion, Mainz. 1981, Deehema, Frankfurt-am-Main, 1981, p. 82. J. M. Abd El Kader, F. M. Abd El Wahab, H. A. El Shayeb and M. G. A. Khedr, Br. Corr. J., I6 (1981) 111. A. M. Shams El Din, H. A. El Shayeb and F. M. Abd El Wahab, J. Electroanal. Chem., 214 (1986) 567.
U. R. Evans, The Corrosion and Oxidation of Metals, Arnold, London, 1960, p. 898. 15 J. M. Abd El Kader and A. M. Shams El Din, Br. Corros. J., 14 (1979) 40. 16 J. M. Abd El Kader, F. M. Abd El Wahab, M. G. A. Khedr and A. M. Shams El Din, Mater. Chem., 7(1982) 313. 17 A. M. Shams El Din, F. M. Abd El Wahab, J. M. Abd El Kader, T. M. H. Saber and A. T. Kuhn, Bull. Electrochem. (India), 2 (1986) 401. 18 K. J. Vetter, in H. Fischer, K. Hautfe and W. Wiederholt (eds.), Passiuierende Fibne and Dichschichlen, Springer, Berlin, 1956, p. 72. 19 K. J. Vetter, Z. Naturforsch, Teil A., 7 (1952) 328. 20 N. Khalil and J. S. L. Leach, Elecfrochim. Acta, 31(1986) 1279. 21 I. Sanghi and S. Visvanathan, Electrochim. Acfa, 7(1962) 567. 22 P. Delahay, New Instrumental Methodr in Electrochemistry, Wiley-Interscience, New York, 1954, p. 179. 23 P. Delahay and T. Berzins, J. Am. Chem. Sot., 75 (1953) 2486. 24 2. Galus, Fundamentals of Electrochemical Analysis, Ellis Honvood, London, 1976, p. 242. 25 E. Brauer and E. Nann, Proc. 5th Int. Congr. on Metallic Corrosion, Tokyo, 1972, National Association of Corrosion Engineers, Houston, TX, 1974, p. 503. 26 E. M. M. Sutter, A. Comet and J. Pagetti, Proc. 6th Eur. Symp. on corrosion Inhibitors, Ferrara, 1985, University of Ferrara, 1985, p. 485. 14