surface characteristics and electrochemical impedance investigation of spark-anodized Ti-6Al-4V alloy

Surface Characteristics and Electrochemical Impedance
Investigation of Spark-Anodized Ti-6Al-4V Alloy
M.R. Garsivaz jazi, M.A. Golozar, K. Raeissi, and M. Fazel
(Submitted May 29, 2013; in revised form October 20, 2013)
In this study, the surface characteristic of oxide films on Ti-6Al-4V alloy formed by an anodic oxidation
treatment in H2SO4/H3PO4 electrolyte at potentials higher than the breakdown voltage was evaluated.
Morphology of the surface layers was studied by scanning electron microscope. The results indicated that
the diameter of pores and porosity of oxide layer increase by increasing the anodizing voltage. The
thickness measurement of the oxide layers showed a linear increase of thickness with increasing the
anodizing voltage. The EDS analysis of oxide films formed in H2SO4/H3PO4 at potentials higher than
breakdown voltage demonstrated precipitation of sulfur and phosphor elements from electrolyte into the
oxide layer. X-ray diffraction was employed to exhibit the effect of anodizing voltage on the oxide layer
structure. Roughness measurements of oxide layer showed that in spark anodizing, the Ra and Rz
parameters would increase by increasing the anodizing voltage. The structure and Corrosion properties of
oxide layers were studied using electrochemical impedance spectroscopy (EIS) techniques, in 0.9 wt.%
NaCl solution. The obtained EIS spectra and their interpretation in terms of an equivalent circuit with the
circuit elements indicated that the detailed impedance behavior is affected by three regions of the interface:
the space charge region, the inner compact layer, and outer porous layer.
Keywords anodizing, breakdown voltage, EIS, space charge
region
1. Introduction
Titanium and titanium alloys are widely used in several
applications such as aerospace, marine, chemical, food, and
biomedical industries because of their suitable properties
consisting of high strength-to-weight ratio, excellent corrosion
resistance, and good biocompatibility (Ref 1, 2). When the
titanium is exposed to the air, a very stable and protective oxide
film is naturally formed on the metal surface, usually having a
very low thickness no more than 7 lm (Ref 3, 4). By anodic
oxidation or thermal oxidation treatments, it is possible to get
an oxide film thicker, denser, and harder than the natural oxide
film (Ref 5). A major advantage of the anodic oxidation
compared with other surface modifications is that the thickness,
morphology, chemical composition, and degree of crystallinity
of oxide film are easily controllable by selection of suitable
electrolyte and applying proper current density and voltage
(Ref 6, 7).
The anodic oxidation of titanium alloys is classified into two
types depending on the applied anodizing voltage. The first one
is the anodic oxidation at voltages lower than the dielectric
breakdown potential, producing a thin and smooth oxide film
with amorphous or some crystalline structure. The second one
is anodizing at voltages higher than the breakdown potential,
also referred to spark anodizing, producing a thick and rough
oxide film with a high porosity and more degree of crystallinity
(Ref 8). Studies have indicated that the micro-pores having
several micrometers diameter on the oxide film surface can be
produced by spark anodizing. The ions existed in the electrolyte
are incorporated into these porous oxide films (Ref 7).
However, Spark anodizing not only improves the corrosion
and wear resistance of titanium surface, but also changes the
chemical composition of oxide film depending on the selected
solution (Ref 6, 9). For example, formation of micro-pores on
titanium surface and incorporation of phosphorus from elec-
trolyte containing phosphoric acid into these pores during the
spark anodizing would improve the osteoinductive properties of
implant in the physiological fluids (Ref 5, 7).
The surface properties of oxide film formed by anodic
oxidation are affected by anodizing parameters, particularly the
applied voltage. Kuromota et al. (Ref 1) pointed out that the
morphology and thickness of anodic oxide films depend
strongly on the applied voltage. They indicated that the
porosity, pore size, and film thickness increase by increasing
the anodizing voltage. Investigations have shown an increasing
linear dependency between the oxide thickness and the
anodizing voltage. The growth constant of 1.5-6 nm/V has
been reported for oxide films formed at voltages lower than the
breakdown potential (Ref 4). However, depending on the
thickness and surface properties required, a wide range of
anodizing voltages can be applied and it is possible to engineer
these properties by means of spark anodizing (Ref 9).
M.R. Garsivaz jazi, M.A. Golozar, K. Raeissi, and M. Fazel,
Department of Materials Engineering, Isfahan University of Technology,
84156-83111 Isfahan, Iran. Contact e-mails: fardin_garsivaz@yahoo.com,
m.golozar@cc.iut.ac.ir, k_raeissi@cc.iut.ac.ir, and Mohammad.fazel@ma.
iut.ac.ir.
JMEPEG ÓASM International
DOI: 10.1007/s11665-014-0911-1 1059-9495/$19.00
Journal of Materials Engineering and Performance

Electrochemical impedance spectroscopy (EIS) is a power-
ful analyzing technique to study the structure and properties of
the oxide films and their interfaces. The EIS results indicated
that the detailed impedance behavior of oxide film derives from
three regions of the interface: the space charge region, the inner
barrier layer that is a compact and good corrosion protective
layer, and the outer layer that is a porous and penetrable layer
(Ref 10).
Table 1 Chemical composition of Ti-6Al-4V alloy
Alloying element Al V H (max) O (max) Fe (max) N (max) C (max) Y (max) Ti
Percentage, wt.% 5.5-6.75 3.5-4.5 0.015 0.2 0.3 0.05 0.08 0.005 Remain
Table 2 Image analysis extracted from Fig. 1
Anodizing voltage, V Pore dia. mean, nm Pore density, pore/lm2
Porosity, %
140 209 0.86 4.39
160 232 0.72 4.84
180 434 0.42 7.61
200 832 0.21 21.8
Table 3 Thickness of oxide films
Anodizing voltage, V Film thickness, lm
140 2.1
160 2.8
180 3.5
200 4.2
Table 4 Surface roughness data of oxide films
Anodizing voltage, V
Roughness, lm
Ra Rz
Untreated 0.09 0.68
140 0.277 1.91
160 0.339 2.34
180 0.498 3.66
200 0.611 3.94
Fig. 1 SEM images of samples anodized at (a) 140 V, (b) 160 V, (c) 180 V, and (d) 200 V

The main purpose of this paper is to study the effect of
anodizing voltage on the structural and corrosion character-
istics of oxide films formed on Ti-6Al-4V alloy by spark
anodizing. The spark anodizing of samples was carried out at
four different voltages. The surface characteristics of anodic
oxide films were investigated using scanning electron micros-
copy (SEM), Energy dispersive spectroscopy (EDS), x-ray
diffraction (XRD), and Electrochemical Impedance Spectros-
copy (EIS).
2. Experimental Procedure
2.1 Materials and Preparation
Ti-6Al-4V samples having dimensions of 150 9 150 9
2 mm3
were used as substrates. Chemical composition of the
material used is shown in Table 1. Specimens were grounded
using 400-1200 SiC emery paper and mechanically polished
with Al2O3 suspension to a mirror finish. Polished samples
Fig. 2 Relationship between the mean pore diameter and the porosity of oxide layers with anodizing voltage
Fig. 3 SEM cross section images of the oxide films obtained at (a) 140 V, (b) 160 V, (c) 180 V, and (d) 200 V

were ultrasonically cleaned with acetone followed by etching in
300 g/L HNO3-30 g/L HF at room temperature for 60 s. After
that, specimens were rinsed in distilled water and transferred
into the anodizing bath. Spark oxidation was carried out
potentiostatically in 1.5 M H2SO4-0.3 M H3PO4 at 25 °C using
a commercially pure titanium plate as a cathode. Four different
voltages of 140, 160, 180, and 200 V were applied for 20 min
in the anodizing process. Finally, the anodized sample was
rinsed in distilled water and immediately dried.
2.2 Testing Methods
Morphology of the oxide layer was investigated using
scanning electron microscope (SEM) model Hitachi S4160.
The size and density of pores were determined from the
FE-SEM images using the manual microstructure distance
measurement software (developed by Ferdowsi university, Iran)
(Table 2). The energy dispersive spectroscopy (EDS) technique
was used to determine the chemistry of anodized layers. The
thickness of anodized layers was calculated from the SEM
images of oxide layer cross sections (Table 3). The crystalline
structure of the oxide films was identified using x-ray
diffraction (XRD) model Bruker D8 ADVANCE. The surface
roughness of oxides produced was measured using a mechan-
ical stylus profilometry, model Mahr (Table 4).
Electrochemical Impedance Spectroscopy (EIS) carried out
using a three-electrode cell consisted of platinum electrode as
the counter electrode, saturated calomel electrode as the
reference electrode, and anodized samples as working elec-
trode. The voltage perturbation amplitude was ±10 mV in the
frequency range from 100 kHz to 10 mHz. All of the
electrochemical tests were carried out in 0.9 wt.% NaCl at
25 °C after 60 min immersion in order to reach the steady-state
open circuit potential (OCP). The surface area exposed to the
dissolution of all the measured samples was 0.65 cm2
. All tests
are repeated at least three times to check for repeatability. The
EIS spectra were fitted with a suitable equivalent circuit model
using the Z-view software.
3. Results and Discussion
3.1 Surface Morphology
Figure 1 shows the surface morphology of the oxide layers
obtained at different anodizing voltages. As it is seen, the
surface of oxide layers has a porous cell structure. These pores
form on the anodized films produced at voltages higher than the
breakdown voltage (Ref 5, 11). The breakdown voltage of
titanium in 1.5 M H2SO4-0.3 M H3PO4 is $90 V (Ref 11).
Therefore, sparking would start at this voltage and anodic
oxidation at higher voltages can produce an oxide layer with
pore morphology. Figure 2 shows the variation of pore
diameter and porosity of oxide layers versus anodizing voltage.
As it is seen, the pore size and porosity of oxide layer increase
by increasing the anodizing voltage. Similar results have also
been reported for samples anodized in other electrolytes
(Ref 6, 7, 11-13).
3.2 Thickness Measurement
The SEM cross section images of the oxide films obtained at
different anodizing voltages are shown in Fig. 3. It is seen that
the film thickness increases by increasing the anodizing
Fig. 4 Variation of the oxide film thickness as a function of anod-
izing voltage
Fig. 5 EDS analysis of sample anodized at 200 V

voltage. Similar results have also been reported for samples
anodized at different ranges of voltages (Ref 14-16). Figure 4
shows the variation of the oxide film thickness as a function of
anodizing voltage, indicating an approximate linear depen-
dency between the film thickness and voltage with a growth
constant of about 35 nm/V.
3.3 Chemical Composition
Figure 5 shows the EDS analysis of the anodized layer
obtained at 200 V. The analysis indicates pick up of phos-
phorous and sulfur elements by the oxide layer from the
anodizing electrolyte (H2SO4/H3PO4). The incorporation of
elements into the oxide layers during the spark anodizing has
also been reported by other authors in different solutions
(Ref 1, 11, 17).
3.4 Crystalline Structure
Figure 6 shows the XRD patterns of samples anodized at
different voltages. The XRD patterns show the peaks for anatase
(titanium oxide) and titanium (substrate). Similar results have
also been observed for samples anodized in H2SO4/H3PO4
electrolyte (Ref 6, 11). It is seen that the intensity of anatase peaks
increases by increasing the anodizing voltage, indicating a higher
percentage of anatase in anodized layer produced at higher
voltages. Therelationship between anatase formation and applied
voltage has also been reported for samples anodized in sulfuric
acid solution (Ref 5).
3.5 Surface Roughness
Figure 7 shows variation of the surface roughness param-
eters (Ra and Rz) of oxide layers as a function of anodizing
voltage. It can be seen that these parameters increase as the
anodizing voltage increases. Similar results have also been
reported by others (Ref 6, 13, 18). It has been mentioned that
the surface roughness increases because the anodic oxide layers
produced at voltages above the breakdown voltage have
numerous pores with projections surrounding them (Ref 6).
3.6 Corrosion Evaluation by Electrochemical Impedance
Spectroscopy (EIS)
The electrochemical impedance data recorded in 0.9 wt.%
NaCl at 25 °C for specimens anodized at different voltages are
presented in Nyquist, Bode Z, and Bode phase formats as seen
in Fig. 8. In Fig. 8, all the EIS spectra exhibit minimum two
time constants: the first in the high-frequency domain related to
the outer porous layer and the second in the low-frequency
domain related to the inner barrier layer. The porous layer
responds probably due to the natural self-sealing in the NaCl
solution as reported by some authors (Ref 10). A confirmation
for this claim is the observation of a spectrum with one time
constant for the specimen anodized at 180 V, immersed in
10 vol.% H2SO4 (25 °C) as seen in Fig. 9. As shown in the
Bode plots obtained for this sample (Fig. 9b), a highly
capacitive behavior of the passive film over a wide frequency
Fig. 6 XRD patterns of samples anodized at (a) 140 V, (b) 160 V,
(c) 180 V, and (d) 200 V
Fig. 7 Variation of the surface roughness parameters (Ra and Rz)
of oxide layers as a function of anodizing voltage

range is observed with the minimum phase angle close to À90°.
This spectrum just exhibits a response from the barrier layer
and shows no response to the porous layer, whereas the Bode
spectra for specimens anodized at various voltages immersed in
0.9 wt.% NaCl (Fig. 8b) show transformation into the mini-
mum two time constants.
In the Bode plots (Fig. 8b), for the sample anodized at
200 V, a diffusion process was observed with the minimum of
phase angle of about À45° at medium frequencies. Further-
more, the lower impedance values at low frequencies
(f < 1 Hz) for samples anodized at higher voltage clearly point
to the decreasing of the barrier layer resistance with increasing
the anodizing voltage.
A comparison of the Nyquist spectra shown in Fig. 8a
indicated that the samples anodized at higher voltage have
lower impedance at low frequencies (specially lower value of
real part of impedance), which indicated that oxide film has
higher porosity and subsequently lower corrosion resistance,
which can be attributed to the diffusion of solution ions into the
metal surface.
The electrical-equivalent circuit model used for impedance
data fitting of oxide films is shown in Fig. 10. Electrical
element used in this model consists of Re (X/cm2
): the
resistance of the electrolyte, Cp (F/cm2
): the capacitance of the
porous layer, Rp (X/cm2
): the resistance of the porous layer, Cb
(F/cm2
): the capacitance of the barrier layer, and Rb (X/cm2
):
the resistance of the barrier layer. Investigations have indicated
that the capacitive behavior of the metal/oxide/electrolyte
interfaces is influenced by the space charge region. This space
charge region is produced by the valence and conduction
bands bending on the surface of the semiconductor material
(Ref 19). It is highly recommended to recognize the space
charge region effect on the interpretation of EIS results
specially for pre-formed TiO2 growth by the high-field
mechanism (Ref 10). This factor is jointed into the equivalent
circuit as a parallel RC consists of Rsc (X/cm2
): the resistance
of the space charge region and Csc (F/cm2
): the capacitance of
the space charge region. Additionally, the electrical elements
to represent possible diffusion of ions across the oxide layer
can also incorporate into the equivalent circuit by adding a
Fig. 8 Impedance spectrum of anodized samples immersed in 0.9% NaCl solution (a) Nyquist plot, (b) Bode phase angle and Bode impedance
plot

capacitance (Qd) series to the barrier layer resistance (as
shown in Fig. 10). Constant phase elements (CPE) were used
as opposed to using pure capacitors of barrier layer and
diffusion element in the fitting routine. The impedance of a
phase element is defined as Zcpe = [C(jx)n
]À1
, where À1 £ n
£ 1. The value of n is associated with non-uniform distri-
bution of current as a result of roughness and surface defects
(Ref 20).
Figure 8 shows the experimental data (by the symbols) and
the simulated data obtained using the equivalent circuit model
shown in Fig. 10. The good agreement between experimental
and simulated data indicates that this model is capable of
interpreting the electrochemical behavior of the bi-layered
oxide film.
The electrochemical parameters extracted by fitting the
model for specimens anodized at different voltages are
presented in Table 5. Variations of Cp, Rp, Cb, and Rb versus
the anodizing voltage are given in Fig. 11. The oxide layer
thickness (d) is reciprocal proportional to the capacitance (C)
using Eq. 1. (Ref 10, 21):
d ¼
e0erA
C
ðEq 1Þ
where d is the thickness of layer, er is the dielectric constant
of the oxide film, e0 is the vacuum permittivity
(8.854 9 10À14
F/cm), and C is the capacitance.
As shown in Fig. 8, the Cb increases and Rb decreases by
increasing the anodizing voltage, which can be associated with
Fig. 9 Impedance spectrum of sample anodized at 180 V, immersed in 10 vol.% H2SO4 solution (a) Nyquist plot, (b) Bode phase angle and
Bode impedance
Fig. 10 Equivalent circuit model used for impedance data fitting of
oxide films

decreasing of barrier layer thickness (Eq. 1) due to the
dissolving of this layer exposed to the aggressive solution.
Conversely, the Cp decreases by increasing the anodizing
voltage, indicating an increase of the porous layer thickness
(Eq. 1). It seems that decreasing the corrosion resistance of
specimens anodized at higher voltages is concerned with
decreasing the resistance of the barrier layer. These results are
in good conformability to the EIS spectra (Fig. 8).
Table 5 Electrochemical impedance parameters extracted from the EIS spectra
Anodizing voltage, V 140 V 160 V 180 V 200 V
Re, X/cm2
0.1 0.1 0.1 2.4
Rsc, X/cm2
71470 3.7E-7 3.05E-7 2.73E-7
Csc, F/cm2
1.76E-5 7.80E-6 6.70E-6 8.25E-6
Rp, X/cm2
280 310 360 375
CP, F/cm2
4.29E-8 3.21E-8 2.58E-8 2.15E-8
Rb, X/cm2
51200 39500 25000 18500
CPEb, F/cm2
1.3E-6 2.0E-6 2.6E-6 3.2E-6
Nb 0.633 0.565 0.543 0.543
CPEd, F/cm2
2.5E-7 3.7E-7 7.55E-7 1.99E-7
nd 0.9 0.892 0.811 0.678
v2
7E-3 3E-3 4E-4 5E-4
Fig. 11 Variations of Rb, Cb, Rp, and Cp versus the anodizing voltage

4. Conclusions
1. The surface morphology of oxide films formed on Ti-6Al-
4V by spark anodizing in H2SO4/H3PO4 electrolyte exhib-
ited a porous cell structure. The pore size and porosity of
oxide layer increased by increasing the anodizing voltage.
2. The thickness of oxide films formed by spark anodizing
increased by increasing the anodizing voltage on a linear
basis with a growth constant of about 35 nm/V.
3. The EDS analysis of oxide films produced in H2SO4/
H3PO4 electrolyte indicated that the phosphorous and sul-
fur elements are incorporated on the anodic oxide surface
during spark anodizing process from the electrolyte.
4. The XRD studies showed that the major structure of anodic
films is anatase. Moreover, the intensity of the anatase
peaks was increased by increasing the anodizing voltage.
5. The roughness tests indicated that the Ra and Rz parameters
of oxide film increase as the anodizing voltage increases.
6. The EIS results showed that the oxide films have two
layers consisting of inner barrier layer and outer porous
layer. Furthermore, the impedance behavior of oxide film
is affected by the space charge region.
7. The resistance of the barrier layer decreases and the resis-
tance of the porous layer increases by increasing the
anodizing voltage. Moreover, the capacitance of the bar-
rier layer increases and the capacitance of the porous
layer decreases by increasing the anodizing voltage.
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