surface characteristics and electrochemical impedance investigation of spark-anodized ti-6al-4v...

Surface Characteristics and Electrochemical ImpedanceInvestigation 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 oxidationtreatment 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 thatthe diameter of pores and porosity of oxide layer increase by increasing the anodizing voltage. Thethickness measurement of the oxide layers showed a linear increase of thickness with increasing theanodizing voltage. The EDS analysis of oxide films formed in H2SO4/H3PO4 at potentials higher thanbreakdown voltage demonstrated precipitation of sulfur and phosphor elements from electrolyte into theoxide layer. X-ray diffraction was employed to exhibit the effect of anodizing voltage on the oxide layerstructure. Roughness measurements of oxide layer showed that in spark anodizing, the Ra and Rzparameters would increase by increasing the anodizing voltage. The structure and Corrosion properties ofoxide 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 thecircuit 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 chargeregion

1. Introduction

Titanium and titanium alloys are widely used in severalapplications such as aerospace, marine, chemical, food, andbiomedical industries because of their suitable propertiesconsisting of high strength-to-weight ratio, excellent corrosionresistance, and good biocompatibility (Ref 1, 2). When thetitanium is exposed to the air, a very stable and protective oxidefilm is naturally formed on the metal surface, usually having avery low thickness no more than 7 lm (Ref 3, 4). By anodicoxidation or thermal oxidation treatments, it is possible to getan oxide film thicker, denser, and harder than the natural oxidefilm (Ref 5). A major advantage of the anodic oxidationcompared with other surface modifications is that the thickness,morphology, chemical composition, and degree of crystallinityof oxide film are easily controllable by selection of suitableelectrolyte and applying proper current density and voltage(Ref 6, 7).

The anodic oxidation of titanium alloys is classified into twotypes depending on the applied anodizing voltage. The first oneis the anodic oxidation at voltages lower than the dielectricbreakdown potential, producing a thin and smooth oxide filmwith amorphous or some crystalline structure. The second oneis anodizing at voltages higher than the breakdown potential,also referred to spark anodizing, producing a thick and roughoxide film with a high porosity and more degree of crystallinity(Ref 8). Studies have indicated that the micro-pores havingseveral micrometers diameter on the oxide film surface can beproduced by spark anodizing. The ions existed in the electrolyteare incorporated into these porous oxide films (Ref 7).However, Spark anodizing not only improves the corrosionand wear resistance of titanium surface, but also changes thechemical composition of oxide film depending on the selectedsolution (Ref 6, 9). For example, formation of micro-pores ontitanium surface and incorporation of phosphorus from elec-trolyte containing phosphoric acid into these pores during thespark anodizing would improve the osteoinductive properties ofimplant in the physiological fluids (Ref 5, 7).

The surface properties of oxide film formed by anodicoxidation are affected by anodizing parameters, particularly theapplied voltage. Kuromota et al. (Ref 1) pointed out that themorphology and thickness of anodic oxide films dependstrongly on the applied voltage. They indicated that theporosity, pore size, and film thickness increase by increasingthe anodizing voltage. Investigations have shown an increasinglinear dependency between the oxide thickness and theanodizing voltage. The growth constant of 1.5-6 nm/V hasbeen reported for oxide films formed at voltages lower than thebreakdown potential (Ref 4). However, depending on thethickness and surface properties required, a wide range ofanodizing voltages can be applied and it is possible to engineerthese properties by means of spark anodizing (Ref 9).

M.R. Garsivaz jazi, M.A. Golozar, K. Raeissi, and M. Fazel,Department ofMaterials Engineering, IsfahanUniversity of Technology,84156-83111 Isfahan, Iran. Contact e-mails: [email protected],[email protected], [email protected], and [email protected].

JMEPEG �ASM InternationalDOI: 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 ofthe oxide films and their interfaces. The EIS results indicatedthat the detailed impedance behavior of oxide film derives from

three regions of the interface: the space charge region, the innerbarrier layer that is a compact and good corrosion protectivelayer, 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 4.39160 232 0.72 4.84180 434 0.42 7.61200 832 0.21 21.8

Table 3 Thickness of oxide films

Anodizing voltage, V Film thickness, lm

140 2.1160 2.8180 3.5200 4.2

Table 4 Surface roughness data of oxide films

Anodizing voltage, V

Roughness, lm

Ra Rz

Untreated 0.09 0.68140 0.277 1.91160 0.339 2.34180 0.498 3.66200 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 ofanodizing voltage on the structural and corrosion character-istics of oxide films formed on Ti-6Al-4V alloy by sparkanodizing. The spark anodizing of samples was carried out atfour different voltages. The surface characteristics of anodicoxide films were investigated using scanning electron micros-copy (SEM), Energy dispersive spectroscopy (EDS), x-raydiffraction (XRD), and Electrochemical Impedance Spectros-copy (EIS).

2. Experimental Procedure

2.1 Materials and Preparation

Ti-6Al-4V samples having dimensions of 1509 15092 mm3 were used as substrates. Chemical composition of thematerial used is shown in Table 1. Specimens were groundedusing 400-1200 SiC emery paper and mechanically polishedwith 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 in300 g/L HNO3-30 g/L HF at room temperature for 60 s. Afterthat, specimens were rinsed in distilled water and transferredinto the anodizing bath. Spark oxidation was carried outpotentiostatically in 1.5 M H2SO4-0.3 M H3PO4 at 25 �C usinga commercially pure titanium plate as a cathode. Four differentvoltages of 140, 160, 180, and 200 V were applied for 20 minin the anodizing process. Finally, the anodized sample wasrinsed in distilled water and immediately dried.

2.2 Testing Methods

Morphology of the oxide layer was investigated usingscanning electron microscope (SEM) model Hitachi S4160.The size and density of pores were determined from theFE-SEM images using the manual microstructure distancemeasurement software (developed by Ferdowsi university, Iran)(Table 2). The energy dispersive spectroscopy (EDS) techniquewas used to determine the chemistry of anodized layers. Thethickness of anodized layers was calculated from the SEMimages of oxide layer cross sections (Table 3). The crystallinestructure of the oxide films was identified using x-raydiffraction (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 outusing a three-electrode cell consisted of platinum electrode asthe counter electrode, saturated calomel electrode as thereference electrode, and anodized samples as working elec-trode. The voltage perturbation amplitude was ±10 mV in thefrequency range from 100 kHz to 10 mHz. All of theelectrochemical tests were carried out in 0.9 wt.% NaCl at25 �C after 60 min immersion in order to reach the steady-stateopen circuit potential (OCP). The surface area exposed to thedissolution of all the measured samples was 0.65 cm2. All testsare repeated at least three times to check for repeatability. TheEIS spectra were fitted with a suitable equivalent circuit modelusing the Z-view software.

3. Results and Discussion

3.1 Surface Morphology

Figure 1 shows the surface morphology of the oxide layersobtained at different anodizing voltages. As it is seen, thesurface of oxide layers has a porous cell structure. These poresform on the anodized films produced at voltages higher than thebreakdown voltage (Ref 5, 11). The breakdown voltage oftitanium in 1.5 M H2SO4-0.3 M H3PO4 is �90 V (Ref 11).Therefore, sparking would start at this voltage and anodicoxidation at higher voltages can produce an oxide layer withpore morphology. Figure 2 shows the variation of porediameter and porosity of oxide layers versus anodizing voltage.As it is seen, the pore size and porosity of oxide layer increaseby increasing the anodizing voltage. Similar results have alsobeen 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 atdifferent anodizing voltages are shown in Fig. 3. It is seen thatthe 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 samplesanodized at different ranges of voltages (Ref 14-16). Figure 4shows the variation of the oxide film thickness as a function ofanodizing voltage, indicating an approximate linear depen-dency between the film thickness and voltage with a growthconstant of about 35 nm/V.

3.3 Chemical Composition

Figure 5 shows the EDS analysis of the anodized layerobtained at 200 V. The analysis indicates pick up of phos-phorous and sulfur elements by the oxide layer from theanodizing electrolyte (H2SO4/H3PO4). The incorporation of

elements into the oxide layers during the spark anodizing hasalso 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 atdifferent voltages. The XRD patterns show the peaks for anatase(titanium oxide) and titanium (substrate). Similar results havealso been observed for samples anodized in H2SO4/H3PO4

electrolyte (Ref 6, 11). It is seen that the intensity of anatase peaksincreases by increasing the anodizing voltage, indicating a higherpercentage of anatase in anodized layer produced at highervoltages. The relationship between anatase formation and appliedvoltage has also been reported for samples anodized in sulfuricacid 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 anodizingvoltage. It can be seen that these parameters increase as theanodizing voltage increases. Similar results have also beenreported by others (Ref 6, 13, 18). It has been mentioned thatthe surface roughness increases because the anodic oxide layersproduced at voltages above the breakdown voltage havenumerous pores with projections surrounding them (Ref 6).

3.6 Corrosion Evaluation by Electrochemical ImpedanceSpectroscopy (EIS)

The electrochemical impedance data recorded in 0.9 wt.%NaCl at 25 �C for specimens anodized at different voltages arepresented in Nyquist, Bode Z, and Bode phase formats as seenin Fig. 8. In Fig. 8, all the EIS spectra exhibit minimum twotime constants: the first in the high-frequency domain related tothe outer porous layer and the second in the low-frequencydomain related to the inner barrier layer. The porous layerresponds probably due to the natural self-sealing in the NaClsolution as reported by some authors (Ref 10). A confirmationfor this claim is the observation of a spectrum with one timeconstant for the specimen anodized at 180 V, immersed in10 vol.% H2SO4 (25 �C) as seen in Fig. 9. As shown in theBode plots obtained for this sample (Fig. 9b), a highlycapacitive 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 �90�.This spectrum just exhibits a response from the barrier layerand shows no response to the porous layer, whereas the Bodespectra for specimens anodized at various voltages immersed in0.9 wt.% NaCl (Fig. 8b) show transformation into the mini-mum two time constants.

In the Bode plots (Fig. 8b), for the sample anodized at200 V, a diffusion process was observed with the minimum ofphase angle of about �45� at medium frequencies. Further-more, the lower impedance values at low frequencies(f< 1 Hz) for samples anodized at higher voltage clearly pointto the decreasing of the barrier layer resistance with increasingthe anodizing voltage.

A comparison of the Nyquist spectra shown in Fig. 8aindicated that the samples anodized at higher voltage havelower impedance at low frequencies (specially lower value ofreal part of impedance), which indicated that oxide film hashigher porosity and subsequently lower corrosion resistance,which can be attributed to the diffusion of solution ions into themetal surface.

The electrical-equivalent circuit model used for impedancedata fitting of oxide films is shown in Fig. 10. Electricalelement used in this model consists of Re (X/cm2): theresistance of the electrolyte, Cp (F/cm

2): the capacitance of theporous 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 indicatedthat the capacitive behavior of the metal/oxide/electrolyteinterfaces is influenced by the space charge region. This spacecharge region is produced by the valence and conductionbands bending on the surface of the semiconductor material(Ref 19). It is highly recommended to recognize the spacecharge region effect on the interpretation of EIS resultsspecially for pre-formed TiO2 growth by the high-fieldmechanism (Ref 10). This factor is jointed into the equivalentcircuit as a parallel RC consists of Rsc (X/cm2): the resistanceof the space charge region and Csc (F/cm

2): the capacitance ofthe space charge region. Additionally, the electrical elementsto represent possible diffusion of ions across the oxide layercan 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 impedanceplot


capacitance (Qd) series to the barrier layer resistance (asshown in Fig. 10). Constant phase elements (CPE) were usedas opposed to using pure capacitors of barrier layer anddiffusion element in the fitting routine. The impedance of aphase element is defined as Zcpe = [C(jx)n]�1, where �1 £ 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) andthe simulated data obtained using the equivalent circuit modelshown in Fig. 10. The good agreement between experimentaland simulated data indicates that this model is capable ofinterpreting the electrochemical behavior of the bi-layeredoxide film.

The electrochemical parameters extracted by fitting themodel for specimens anodized at different voltages arepresented in Table 5. Variations of Cp, Rp, Cb, and Rb versusthe anodizing voltage are given in Fig. 11. The oxide layerthickness (d) is reciprocal proportional to the capacitance (C)using Eq. 1. (Ref 10, 21):

d ¼ e0erAC

ðEq 1Þ

where d is the thickness of layer, er is the dielectric constantof the oxide film, e0 is the vacuum permittivity(8.8549 10�14 F/cm), and C is the capacitance.

As shown in Fig. 8, the Cb increases and Rb decreases byincreasing 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 andBode impedance

Fig. 10 Equivalent circuit model used for impedance data fitting ofoxide films


decreasing of barrier layer thickness (Eq. 1) due to thedissolving of this layer exposed to the aggressive solution.Conversely, the Cp decreases by increasing the anodizingvoltage, indicating an increase of the porous layer thickness

(Eq. 1). It seems that decreasing the corrosion resistance ofspecimens anodized at higher voltages is concerned withdecreasing the resistance of the barrier layer. These results arein 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 2.4Rsc, X/cm2 71470 3.7E-7 3.05E-7 2.73E-7Csc, F/cm

2 1.76E-5 7.80E-6 6.70E-6 8.25E-6Rp, X/cm2 280 310 360 375CP, F/cm

2 4.29E-8 3.21E-8 2.58E-8 2.15E-8Rb, X/cm2 51200 39500 25000 18500CPEb, F/cm

2 1.3E-6 2.0E-6 2.6E-6 3.2E-6Nb 0.633 0.565 0.543 0.543CPEd, F/cm

2 2.5E-7 3.7E-7 7.55E-7 1.99E-7nd 0.9 0.892 0.811 0.678v2 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 ofoxide layer increased by increasing the anodizing voltage.

2. The thickness of oxide films formed by spark anodizingincreased by increasing the anodizing voltage on a linearbasis 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 surfaceduring spark anodizing process from the electrolyte.

4. The XRD studies showed that the major structure of anodicfilms is anatase. Moreover, the intensity of the anatasepeaks was increased by increasing the anodizing voltage.

5. The roughness tests indicated that the Ra and Rz parametersof oxide film increase as the anodizing voltage increases.

6. The EIS results showed that the oxide films have twolayers consisting of inner barrier layer and outer porouslayer. Furthermore, the impedance behavior of oxide filmis 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 theanodizing voltage. Moreover, the capacitance of the bar-rier layer increases and the capacitance of the porouslayer decreases by increasing the anodizing voltage.

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http://dx.doi.org/10.1016/j.electacta.2011.07.129