Zwilling

Electrochimica Acta 45 (1999) 921 – 929
www.elsevier.nl/locate/electacta

Anodic oxidation of titanium and TA6V alloy in chromic
media. An electrochemical approach
V. Zwilling a, M. Aucouturier b, E. Darque-Ceretti a,*
a
Ecole des Mines de Paris, CEMEF (UMR CNRS 7635), BP 207, F-06904 Sophia-Antipolis, France
b
Lab. Metallurgie (URA CNRS 1107), Uni6ersite Paris-Sud, Bat. 410, F-91405 Orsay Cedex, France
´ ´ ˆ

Received 1 March 1999; received in revised form 28 June 1999

Abstract

In order to optimise anodisation surface treatment of titanium alloys, the electrochemical conditions of anodic
oxidation of pure titanium and of a TA6V (Ti-6% Al-4% V) alloy were investigated. Both voltametric (potentio-dy-
namic) and chrono-amperometric (constant voltage, either applied directly or progressively increased during the first
steps of the treatment) experiments are conducted. The electrolyte is a chromic acid (CA) (0.5 mol l − 1 Cr2O3)
solution with and without hydrofluoric acid (HF) addition (9.5 × 10 − 2 mol l − 1). A thin oxide compact film is formed
in CA electrolyte and a duplex film composed of a compact layer surmounted by a columnar porous layer grows from
the fluorinated electrolyte. The voltametric results indicate a breakdown of the compact film (in non-fluorinated
medium) for potential around 3 V/SCE, and a strong influence of the alloying elements of TA6V on the formation
of porous films in fluorinated medium. The chrono-amperometric measurements reveal a complex growth process of
the porous film, in which the residual current participates in the thickening of both compact and porous layers of the
film. The overall electrochemical efficiency is small and decreases with the treatment time. A growth mechanism
involving a poisoning (by CrVI-containing ions) and antidote (by F-containing species) competition is proposed.
© 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Anodisation; Titanium alloy; Porous film; Voltametry; Chronoamperometry

1. Introduction tion of titanium alloys is more recent and has been
proposed in order to improve their wear resistance or
Anodic oxidation is a commonly used surface treat- their adhesive properties against organic adhesives or
ment, especially on aluminium alloys for structural composites [3], and even for improvement of the bio-
applications. Its aim is to improve the corrosion or compatibility [4].
wear resistance, the external aspect, and the ability for In a previous paper, the structure and physico-chem-
adhesive bonding. The formation and structure of the istry of anodic oxide films grown on titanium and
anodic films on those alloys has been extensively stud- TA6V (Ti-6% A1-4% V) alloy in chromic media were
ied [1,2], and even if the detailed mechanisms are still described [5], and the morphological and structural
under discussion, the different steps involved during differences with the well-known anodic films on alu-
their electrochemical construction are well defined. The minium alloys were underlined. Those similarities and
application of anodic oxidation to the surface prepara- differences may be summarised by the following points.
As for anodic films grown on aluminium alloys, the
* Corresponding author. Fax: +33-493-654-304. films grown on titanium and TA6V can be either
E-mail address: ceretti@cemef.cma.fr (E. Darque-Ceretti) compact and thin (less than 20 nm) or porous and thick

0013-4686/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 2 8 3 - 2

922 V. Zwilling et al. / Electrochimica Acta 45 (1999) 921–929

(more than 50 nm). In CA electrolytes, addition of The pure Ti (Ti 40) has an equiaxial structure (30 mm
fluoride ionic species to the electrolyte is necessary to grain).
obtain a porous structure on Ti and TA6V, but that The specimens (1.5 ×1.5 cm2) are mechanically pol-
condition is not necessary for Al [6]. ished until 1/4 mm diamond paste and rinsed ultra-
In both cases, the porous anodic films are made up sonically before anodising. This surface preparation
of a compact layer near the substrate surmounted by was chosen as electrolytic polishing proved to lead to
a columnar porous layer. The respective thickness of non-homogeneous films [7]. This is probably due to
both layers is different for the two materials: the surface chemical modification by the polishing proce-
compact layer is very thin on aluminium alloys (a few dure.
nm), but may reach more than 50 nm on alloyed The electrolyte compositions are given in Table 2,
titanium. together with the indication of the most probable spe-
The distribution of the pores in the porous films is cies present in the solution, obtained from the con-
in both cases regular on a columnar honeycomb-like centration – pH diagrams computed by Pourbaix [8].
lattice with an average distance of 10–50 nm, and the As previously indicated, anodisation in CA without
film is apparently surmounted by shallow protrusions. HF addition (CA) leads to the formation of a com-
The film on Al alloys is made up of more or less pact film, whereas porous films are grown in the CA/
hydrated aluminium oxide (Al2O3·nH2O), containing HF electrolyte.
various proportions of alloying elements of the sub- The voltametric characterisations (potentio-dynamic
strate and contaminating species from the anodising I – V curves) are conducted in a classical three-elec-
electrolyte [2]. On TA6V alloy, its composition is trode cell with a saturated calomel reference electrode
TiO2 +Al2O3 (and a small amount of vanadium), its (SCE), and a platinum counter-electrode, in non-aer-
structure may be partially crystalline (rutile structure), ated non-stirred conditions. The working electrode
depending on the elaboration conditions [5], and it is potential is continuously increased from 0 mV up to
contaminated by fluorine from the electrolyte. 4000 mV at a rate of 2000 mV per hour.
The aim of the present paper is to describe the Chrono-amperometric measurements (I – t curves)
electrochemical investigation conducted in order to are obtained in a two-electrode cell (electrode dis-
understand the different mechanisms involved in the tance= 4 cm) with a titanium counter-electrode
growth of anodic oxide films on titanium and TA6V (cathode). This arrangement and the procedure are
alloy in CA with or without fluoride additions. A chosen in order to conform to the industrial ano-
growth model is proposed. dising procedure: the voltage between the specimen
and a titanium cathode is increased by five equal
2. Experimental steps of 1 min each, up to the final voltage of 5 or
10 V. The specimen is then maintained at the final
The composition, thermal treatment and metallurgi- voltage for variable times between 1 and 55 min (i.e.
cal structure of the studied materials are summarised 6 – 60 min total time). The current density is continu-
in Table 1. The microstructure of the mainly studied ously measured as a function of time and is inte-
TA6V alloy is biphased and made up of Al-enriched grated to obtain the electric charge exchange during
a (HCP) elongated grains (10 mm or more) together the different steps. Experiments are also performed by
with small V-enriched b (BCC) grains (2 or 3 mm). directly reaching (without intermediary steps) the final

Table 1
Substrate description

Substrate Composition Thermomechanical history Microstructure (phases) Thickness (mm)

Ti 40 Ti Fully annealed Equiaxial a (HCP) 0.8
TA6V Ti, 6 wt.% Al, 4 wt.% V 750°C, 1 h, under Ar a (HCP)+b (BCC) 2

Table 2
Electrolyte composition

Name CrO3 content (mol 1−1) HF content (mol l−1) pH Major species in solution

CA 0.5 0 2 Cr2O7−
CA/HF 0.5 9.5×10−2 2 Cr2O7−+HF

V. Zwilling et al. / Electrochimica Acta 45 (1999) 921–929 923

Fig. 1. Anodic polarisation curves of Ti 40; (A) in chromic acid (CA); (B) in fluorinated chromic acid (CA/HF) electrolyte.

Fig. 2. Anodic polarisation curves of TA6V; (A) in chromic acid (CA); (B) in fluorinated chromic acid (CA/HF) electrolyte.

voltage. The elaboration is sometimes interrupted at 3. Results
the beginning of the procedure (short time and/or low
voltage) in order to follow the evolution of the film 3.1. Potentio-dynamic experiments
structure [5].
In order to monitor the geometrical evolution of The potentio-dynamic curves of Figs. 1 and 2 illus-
the specimens, constant voltage experiments are car- trate the electrochemical behaviour of Ti 40 and
ried out on specimens whose surface is partly isolated TA6V in the two types of media. The curves exhibit
from the solution by an inert varnish, and the step the same general shape: an abrupt augmentation of
between the non-oxidised and the oxidised surfaces the anodic current for potentials between 0 and 100
is measured by profilometry. The anodic film thick- mV/SCE, attributed to a stabilisation of the surface
ness itself is measured by profilometric measurement and cell system, and a constant current plateau be-
of sputtering craters obtained by SIMS analysis, fol- tween 1000 and 2500 mV/SCE.
lowing a procedure detailed in another publication In non-fluorinated electrolyte CA (formation of a
[5]. compact film), the behaviour of the two materials is


identical: a constant current is immediately reached and
remains very small (less than 0.01 mA cm − 2) until 2600
mV (3000 mV for TA6V). For higher potentials, a wave
appears and the current stabilises its value to 0.1 mA
cm − 2 on Ti 40 or 0.3 mA cm − 2 on TA6V.
The addition of HF in the solution (CA/HF elec-
trolyte) drastically increases the anodic current density.
Complementary experiments with electrolytes of vary-
ing HF content showed that the plateau current density
on TA6V is increased by 33 mA cm − 2 per HF mol l − 1.
That value is smaller than the value of 183 mA cm − 2
per HF mol l − 1 reported by Caprani and Epelboin [9].
The discrepancy may be explained by differences in the
experimental procedure: electrolyte agitation, tempera-
ture, etc. as the growth and dissolution mechanism
discussed in the next section should be highly depen-
Fig. 4. Chrono-amperometric record on TA6V in CA elec-
dent on those factors. trolyte under different voltages.
Between 100 and 700 mV/SCE in CA/HF electrolyte,
the behaviour of pure Ti 40 and TA6V alloy are
different. The current density smoothly reaches the high and probably controlled by the performance of the
plateau for Ti 40, but the curve exhibits a shoulder and constant voltage source used. After a sharp and rapid
a maximum for TA6V. That difference will be discussed drop during the first seconds the current density-versus-
in Section 4. time curves exhibit various shapes which depend
strongly on the applied voltage. For low applied
3.2. Chrono-amperometry voltage (1 and 2 V), the current falls rapidly to a
negligible value ( B 0.1 mA cm − 2). For intermediate
The variation of the current density as a function of voltage, the current drops down to a nonnegligible
time has been recorded either under direct application plateau ($ 0.5 mA cm − 2) and then decreases slowly
of the voltage, for both Ti 40 and TA6V substrates and (TA6V under 3 and 5 V) or remains constant until the
both CA and CA/HF electrolytes, or under application end of the experiment (Ti 40 under 3 V). For high
of voltage by successive steps, only for the TA6V alloy. voltage (Ti 40 under 5 and 10 V, TA6V under 10 V),
the curves exhibit a more or less pronounced minimum,
3.2.1. Direct 6oltage application in chromic medium and the current increases as a function of time.
(CA)
Figs. 3 and 4 show the variation of the current 3.2.2. Direct 6oltage application influorinated chromic
density for pure titanium (Ti 40) and TA6V alloy medium (CA/HF)
anodised in a CrO3 solution without fluoride addition When HF is added to the electrolyte, the curves
(formation of a compact film [5]) respectively, for dif- (Figs. 5 and 6) are very different from the classical
ferent constant voltages. The initial current density is behaviour: for low voltage (1 and 2 V), the current

Fig. 3. Chrono-amperometric record on Ti 40 in CA elec- Fig. 5. Chrono-amperometric record on Ti 40 in CA/HF
trolyte under different voltages. electrolyte under different voltages.


either remains nearly zero or increases slowly to a
non-negligible value (0.5 – 0.8 mA cm − 2). For all other
voltages (3, 5 and 10 V) on TA6V and for intermediate
voltage (3 and 5 V) on Ti 40, the current drops rapidly
during the first seconds to reach a plateau situated
between 0.8 and 1.5 mA cm − 2. For the highest voltage
(10 V) on Ti 40, the curve exhibits a minimum and an
important maximum to finally reach a constant plateau
at 1.5 mA cm − 2.

3.2.3. Voltage application by steps in chromic medium
(CA) on TA6V
As explained in Section 2, that procedure is the
industrial procedure. The voltage is increased by 1
min-steps of 1 or 2 V, depending on the final voltage of
Fig. 6. Chrono-amperometric record on TA6V in CA/HF
5 or 10 V, and maintained at the final voltage for 15
electrolyte under different voltages.
min. Fig. 7 shows the obtained curves in the CA
electrolyte (formation of a compact thin anodic oxide).
With the exception of the first two or three steps (0, 1
and 2 V, or 0 and 2 V), the system has difficulties in
reaching a steady-state during the stepping procedure.
As for the constant voltage experiments described
above, the 3 V value seems to be a critical threshold
between two different regimes. For a final voltage of 10
V, the current density does not even reach a steady-
state and increases continuously.

3.2.4. Voltage application by steps influorinated
chromic medium(CA/HF) on TA6V substrate
The same procedure applied in CA/HF electrolyte
leads to the curves displayed in Fig. 8, far more regular
than the preceding ones. A steady-state is reached for
every step. The final current density plateau (1.5 mA
cm − 2) is, as for constant voltage experiments, higher
than in the non-fluorinated electrolyte, and independent
of the final voltage (5 or 10 V). That final value is
already reached as soon as the applied voltage is larger
or equal to 3 or 4 V.

3.3. Profilometry of surface steps, thickness
measurements

All profilometric measurements were performed for
TA6V substrates anodised in CA/HF medium (porous
film), as underlined in a previous publication [5] the
film grown in the CA medium is too thin to allow valid
profilometric estimations. Two kinds of measurements
were carried out.
One concerns specimens partly covered by an inert
varnish before anodisation. The measured step corre-
sponds to the distance between the initial polished
surface (covered with a very thin native oxide film) and
Fig. 7. Chrono-amperometric record on TA6V in CA elec-
the top of the anodic film after treatment (following the
trolyte for the voltage increase procedure by steps. The num- industrial step procedure). The measured steps are neg-
bers show the applied voltage at each step. (a) record on 20 ative. In other words, the final result of the process is a
min; (b) zoom on the first 6 min. dissolution of the substrate, not compensated by the


are given in last column of Table 3 for the purpose of
interpretation.

4. Discussion

From the previously published results on the struc-
ture and physico-chemistry of the films [5], one may
summarise their main features as follows.
The film composition is always TiO2 on Ti 40 sub-
strate, and TiO2 + Al2O3 (with Ti/A1$5) on TA6V
substrate. Fluorine is incorporated into the films grown
from a fluorinated electrolyte, and small quantities of
vanadium are present on the film grown on TA6V
substrates. Chromium is never incorporated, except as
an extreme surface adsorption during the very first
minutes of the treatment.
The films grown from CA are compact and thin and
their thickness increases linearly with the applied
voltage ($7 nm V − 1 for Ti, $5 nm V − 1 for TA6V).
The films grown from CA/HF are made up of a
compact underlayer surmounted by a columnar nano-
porous layer surmounted itself by small protrusions.
The total estimated thickness is shown in Table 3 for
TA6V substrates. It increases with the applied voltage
for the explored conditions (V 510 V, t 560 min.).
Observations of replicas by transmission electron mi-
croscopy [5,6] revealed the porous layer thickness to be
about one half of the total thickness.
The curves of Figs. 1 and 2 exhibit, apart from the
transition between 0 and 100 mV/SCE, two perturbed
Fig. 8. Chrono-amperometric record on TA6V in CA/HF regions, one around 2500 – 3000 mV/SCE in the CA
electrolyte for the voltage increase procedure by steps. The electrolyte, and one between 200 and 700 mV/SCE,
numbers show the applied voltage at each step. (a) record on only for the TA6V substrate in CA/HF electrolyte. The
20 min; (b) zoom on the first 6 min. current density wave observed at 2500 – 3000 mV/SCE
in CA electrolyte cannot be related to any of the
electrochemical reactions that could be expected be-
tween the alloy components and the electrolyte [8]. It is
growth of the anodic oxide film. Table 3 gives the attributed to a modification of the structure of the
values of the measured steps for various conditions. compact anodic film, and more precisely to a break-
The second kind of measurement was done on sput- down phenomenon, confirmed by the microscopic pits
tering craters after SIMS profiling of the anodic films. observed on the surface for those voltages (Fig. 9).
The procedure and discussion of such measurements The anomaly appearing at 200 – 700 mV/SCE is
are given in another publication [5], and only the results clearly a consequence of the addition elements present
Table 3
Step height (measured by profilometry) between the initial surface and the top of the anodic film and film thickness (measured by
profilometry of SIMS crater)a

Final voltage (V) Time (min) Step height (nm) Film thickness (nm)

5 6 – 95920
5 20 −8009 75 135 910
5 60 −8009 75 –
10 6 – 100910
10 20 −21009 100 155 920

a
TA6V in CA/HF electrolyte


Table 4
Correlation between the working electrode (WE)/Ti counter-
electrode (CE) and the working electrode/SCE reference elec-
trode potential in a constant voltage experiment

WE/CE voltage (V) WE/SCE potential (V)

1 0.360
2 1.360
3 2.360
5 4.360
10 9.360

of the substrate occurs during the experiment. The
current-versus-time curves show that electrochemical
reactions progress continuously. The formation of the
Fig. 9. Scanning electron micrograph of TA6V anodised in CA
anodic film appears as a competition between a solid
electrolyte under 5 V for 20 min. Breakdown pit.
oxide layer growth and a dissolution of that layer at its
interface with the electrolyte. The results of the
physico-chemical investigation [5] have shown that the
in the TA6V alloy. Amongst the possible electrochemi-
chromate ions, detected on the film surface only during
cal reactions [8], only the reactions of vanadium re-oxi-
the first anodisation minutes, and fluoride ions, always
dation (e.g. 2VO++ +3H2OUV2O5 +6H+ +2e−,
incorporated into the whole thickness of the films, play
E0 =3439100 mV/SCE) may be invoked to explain
an important role in the growth mechanism. It is pro-
that anomaly, as chromium-containing ion reactions posed that the growing and pore formation process
should occur in both media. Such a contribution of obey a growth-dissolution mechanism monitored by a
vanadium from the substrate to the growth mechanism poison-antidote competition, as recently proposed in
is confirmed by the detection of small amounts of that the literature for another system (randomly porous
element in the film [5]. It is responsible for the differ- structure on Li in SiOCl2) [11].
ence in dissolution rate of the anodic film between the In a first step, a non-porous layer is formed by a
a an b phases of the alloy. Mott – Cabrera mechanism. The CrVI species would
In order to propose valuable interpretation of the play a poisoning role and, in absence of an antidote,
chrono-amperometric curves, it is necessary to correlate the growth would stop rapidly.
the values of the applied voltage in that kind of experi- When the solution contains fluorine ions these would
ment to the corresponding actual values of the elec- play an antidote role, and the local competition be-
trode potential of the substrate. This was done by tween CrVI- and F-containing species leads to a contin-
adding a reference calomel electrode to the two-elec- uous growth of the film with a randomly porous
trode cell. The result of that correlation is given in structure, due to the competition between oxide forma-
Table 4. tion and dissolution.
The critical behaviour observed around 3 V in the It has not been possible to relate, as done in a
constant voltage experiments (Figs. 3, 4 and 7) may published work on aluminium [12,13], the minimum
then be related to the breakdown detected through the observed on the current-versus-time curves (Figs. 5 and
potentio-dynamic experiments. For compact anodisa- 6) with the transition between the compact and the
porous film formation.
tion in CA electrolyte, a breakdown of the film pro-
From the chrono-amperometric experiments (Figs. 5,
vokes a definitive perturbation of the growth process
6 and 8) it is possible to calculate the quantity of
above 3 V, and explains why the current density does
electricity consumed during the successive stages of the
not fall to zero, or even increases for the highest process. The comparison with the actual thickness mea-
voltages. One may consider that, for low voltages, the sured for instance by profilometry is difficult, as as-
growth of the compact film is controlled by a Mott– sumptions have to be made on the nature and
Cabrera mechanism [10], and that, above the break- crystalline structure (a mixture of amorphous oxide and
down voltage, a great part of the current is used to rutile TiO2 containing small amounts of oxidised Al),
repair the continuously reinitiated breakdown damage. and on the density (pore amount) of the films. A rough
The growth process of the porous film in CA/HF estimation of the growth efficiency (defined as the ratio
electrolyte is more complex. The step height measure- between the actual thickness of the film measured by
ments, confirmed by chemical analysis of the electrolyte profilometry of SIMS crater and the calculated thick-
after anodisation, prove that an important dissolution ness supposing that all the current has been used for


Table 5
Comparison of measured and calculated (chrono-amperometry) thickness TA6V in CA/HF electrolyte

Voltage (V) Time (min) Measured thickness (nm) Calculated thickness (nm) Growth efficiency (%)

5 6 959 20 190 50
5 20 135 9 20 870 16
10 6 1009 20 220 45
10 20 1559 20 915 17

building a compact TiO2 film) is proposed in Table 5 TA6V alloy in CA electrolyte with or without fluorine
for films grown on TA6V in CA/HF electrolyte. species addition, in combination with the results of a
Due to the influence of the porosity, the proposed structural and physico-chemical study [5], has allowed a
efficiencies must be considered as overestimated, but it precise description of the growth process of the anodic
is interesting to note that they are always smaller than films. It is proposed that the growth mechanism in
50%, and decrease with time: an increasing fraction of fluorinated electrolyte involves a competition between
the current is consumed for dissolution. Evidently, a dissolution and oxide formation, in which the CrVI ions
part of the current could also be consumed for other and fluorine species play locally a respective poisoning
electrochemical reactions (e.g. oxygen evolution). The and antidote role. The result of that competition is the
calculated values are compatible with the measured step growth of porous film with a regular columnar struc-
heights between the initial surface and the film surface ture. This process does not stop for long periods, and a
(Table 3). steady state is reached, with a continuous consumption
The chrono-amperometric curves (Fig. 7) exhibit, for of current. The efficiency of the anodic film formation
each voltage step, a logarithmic decrease followed by a is estimated to be less than 50% and decreases during
plateau, and a separate integration of these two parts the treatment, in association with a modification of the
have been performed. The result of those calculations is porous part of the film thickness and structure during
that both parts of the current contribute to the growth the first 20 min.
of both the compact and porous layers. The efficiency The compact film formed in CA electrolyte grows by
of the residual current, only present after 6 min. is itself a more classical Mott – Cabrera mechanism at a rate of
decreasing for treatment times longer than 10 min. $7 nm V − 1 for Ti, and $ 5 nm V − 1 for TA6V. Its
The particular shape of the oxidised titanium SIMS breakdown voltage is estimated to be of the order of 3
profiles reported in the previously quoted physico- V, and the process (current efficiency) is highly per-
chemical study have shown that the thickness of the turbed at higher voltage.
compact layer of the film is already stabilised after a 6 The present electrochemical study is useful for the
min treatment ($65 nm at 5 V, $80 nm at 10 V), interpretation of structural investigations [5,14] on tita-
whereas the porous thickness increases between 6 and nium alloy anodisation.
20 min from $30 to $ 65 nm (for both 5 and 10 V
voltages) when the residual current remains constant.
Evaluation of the surface porosity ratio (pore surface/
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