In vitro corrosion performance of PEO coated Ti and Ti6Al4V used for dental and orthopaedic implants

SCT-21450; No of Pages 10 Surface & Coatings Technology xxx (2016) xxx–xxx

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In vitro corrosion performance of PEO coated Ti and Ti6Al4V used for dental and orthopaedic implants E. Matykina ⁎, R. Arrabal, B. Mingo, M. Mohedano, A. Pardo, M.C. Merino Departamento de Ciencia de Materiales, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 27 May 2016 Revised 29 July 2016 Accepted in revised form 7 August 2016 Available online xxxx Keywords: Plasma electrolytic oxidation Titanium alloys Simulated body fluid Hydrogen peroxide Corrosion

a b s t r a c t The present work describes the characteristics of 5–10 μm-thick plasma electrolytic oxidation (PEO) coatings with graded Ca/P ratio generated on titanium of commercial grades I (c.p. Ti) and V (Ti6Al4V) and some of the aspects of their bioactivity and corrosion resistance in vitro in long-term (up to 8 weeks) normal and shortterm (1 week) inflammatory conditions. Simulated body fluid (SBF) and artificial saliva at 37 °C were used as corrosive media; inflammatory conditions were simulated by controlling the pH and introducing hydrogen peroxide (usual metabolism product of inflammation-inducing bacteria). Additionally, the saliva was modified with fluoride ions. DC and AC electrochemical tests were used to characterize the corrosion protection mechanism of the coatings. Metal ion release (Ti4+, Al3+ and V5+) from PEO coated materials during their in vitro immersion in normal and inflammatory SBF was evaluated by ICP-MS. PEO coatings on Ti6Al4V alloy inhibited the liberation of titanium compared with non-coated alloy both at short-term inflammatory and long-term normal immersion conditions; but the liberation of aluminium and vanadium was greater from the coated than non-coated alloy at both conditions, due to the presence of these ions in the coatings. The PEO-coated c.p. Ti exhibited considerably higher stability with respect to Ti4+ ion release in SBF at all conditions compared with the PEO-coated Ti6Al4V. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Surface modification of Ti alloys by plasma electrolytic oxidation (PEO) [1–3] promotes accelerated osteointegration of the implant with newly formed bone due to the specifically tailored composition and morphology of the PEO coatings and is regarded as particularly suitable for immediate loading procedures [4,5]. The clinical follow-up of PEO-coated dental implants indicate favourable survival rates, however, the latter do not appear any better than those of machined implants within first 5 years [6,7]; 97.1% survival rate is reported for TiUnite® implants (comprising amorphous TiO2, anatase and 7–10 at.% P [8]) after 12 years [9]. Besides different clinical risk factors that may result in implant failure, corrosion and resultant titanium ions release may lead to adverse tissue reactions. Titanium is usually found in the interfacial bone, fibrous tissue around the implants and blood cells in the connective tissue [10], body fluids [11] and organs, such as liver and kidneys [12]. Zaffe et al. reported that titanium in hard and soft tissue around the implant does not extend beyond 1 mm from the device

⁎ Corresponding author. E-mail address: [email protected] (E. Matykina).

but its content in lymphocytes is considerably higher than in bone and fibrous tissue [10]; the authors suggest that this may be part of the mechanism how Ti is drained from around the implant an eventually accumulates in organs. Wachi et al. showed that Ti content in gingival tissues of a rat around a Ti implant exposed to 1000 ppm NaF solution at pH 4.2 was much greater (50 μg/g) compared to the absence of fluoride (15 μg/g). This is significant, since nearly all oral hygiene products contain about 0.1% fluoride ions. The authors further demonstrated that 9 ppm of Ti in gingival epithelium is sufficient to significantly increase the expression of cytokines and receptors for microbes related to inflammation and bone resorption, i.e. Ti increases the sensitivity of the gingival epithelium cells to microorganisms. This suggests that Ti ions may participate in development of peri-implant mucositis, and consequently, peri-implantitis and alveolar bone resorption [13]. Inflammatory conditions in human tissues that inevitably accompany the surgical procedure are also characterized by generation of H2O 2 as a result of enhanced release of respiratory burst product, O− 2 species. It has long been pointed out that in the presence of H2O2 Ti undergoes increased oxidation and dissolution, due to the formation of Ti(III) and Ti(IV) complexes with peroxide, which has been related to increased Ti ion release from the implants in-vivo [14]. TiO2 catalyses the decomposition of hydrogen peroxide, during

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which process a simultaneous intermediate formation and reformation of the oxide takes place:

2. Experimental methods 2.1. Materials

−

2TiO2 þ H2 O2 þ 2e →Ti2 O3 þ H2 O þ O2

ð1Þ

Ti2 O3 þ H2 O2 →2TiO2 þ H2 O þ 2e−

ð2Þ

Tengvall et al. [14] suggest that these reactions are indeed more complex, i.e. they are accompanied with H+ and water uptake so that TiOOH(H2O) complexes are formed, which polymerize and form a matrix with time. On one hand, this matrix is suggested to be a good ionexchanger, which presents a good environment for cells and proteins where eventually biological integration occurs [15]. On the other, a reaction of titanium dioxide with hydrogen peroxide forms Ti(IV)O22 − or Ti(IV)O− 2 with redox potentials from 0.5 V up to 1.7 V at pH 7, therefore a danger of crevice corrosion exists for Ti/TiO2 systems in inflammatory conditions. A number of works have investigated the electrochemical behaviour of titanium in physiological media (PBS, SBF, MEM, artificial saliva) modified with H2O2 and pointed out the increased corrosion activity of Ti in such solutions [16,17]. Mabilleau et al. observed that the surface roughness of titanium considerably increases after immersion in artificial saliva modified with fluoride ions, H2O2 and lactic acid (common product of oral bacteria activity which reduces the oral pH) [18]. Höhn and Virtanen studied Ti, A, and V ion release and calcium phosphate formation of anodized and as-received Ti6Al4V in DMEM (Dulbecco's Modified Eagle Medium) under normal (pH 7.4) and inflammatory conditions (pH 5 and presence of H2O2), reporting marked increase of the ion liberation from both materials at pH ≤ 5 [19]. Corrosion resistance of PEO-coated Ti and its alloys in different physiological media (SBF, MEM, Ringer's, Hank's, PBS solutions) is frequently studied using electrochemical DC and AC methods and shows marked improvement compared with non-treated alloys in terms of reduced passive current density, increased total impedance at high frequency domain and a resistance to diffusion of species through the coating [20–28]. Metal ion release into the media have been studied to a lesser extent; most of the published works concern with slow release of a specific element (Ca, P, Zn, Ag, Mn) responsible for an added functional property, such as an antibacterial or osteogenic activity [29–32]. In other cases the reduced release of unwanted (from the biocompatibility point of view) substrate alloying elements is sought [20,31]; for instance, PEO coatings on memory shape NiTi alloys are reported to reduce the Ni release by ~ 3 times and by up to ~ 10 times when posttreated by SPTFE (superdispersed polytetrafluoroethylene) [20]. The authors of the present work have previously demonstrated reduced Ti release from PEO-coated c.p. Ti in SBF [33]. To date, only Messer et al. have evaluated the corrosion resistance of commercial TiUnite® implants under inflammation conditions; the latter were simulated by monocytic cells and lipopolysaccharide in cell culture media. The electrochemical measurements performed for up to 26 h of exposure suggested that corrosion of oxidized implants was lower compared to machined implant [34]. The authors of the present work have previously evaluated the long-term (up to 8 weeks of exposure) electrochemical behaviour of PEO-treated c.p. Ti and associated ion release in fluoride modified artificial saliva [35] and SBF under non-inflammatory conditions [33] and have shown that in both cases PEO treatment have increased the corrosion resistance of Ti and reduced the Ti4+ liberation. This study explores the role of PEO coatings on short-term (7 days) corrosion of c.p. Ti and Ti6Al4V in inflammatory artificial saliva and SBF. Long-term (8 weeks) metal ion release into SBF from the coated Ti6Al4V alloy is also investigated.

Specimens of dimensions 30 × 20 × 0.5 mm were cut from commercially pure (c.p.) Ti (Grade I, max wt.%: 0.2 Fe, 0.18 O, 0.03 N, 0.015H, and 0.18C), and Ti6Al4V (Grade V, max wt.%: 0.25 Fe, 0.02 O, 0.05 N, 0.015H, 0.08C, 3.5–4.5 V, 5.5–6.76 Al, bal. Ti) titanium foils, degreased in isopropanol and rinsed in distilled water. Following subsequent pickling for ~ 20 s in a mixture containing 12 mL HF (40 wt%), 40 mL HNO3 (70 wt%) and 48 mL H2O at room temperature, the specimens were rinsed in distilled water and dried. A working area of 3 cm2 was isolated using Lacquer 45 resin (McDermid plc.). 2.2. Surface treatment PEO treatment was carried out for 90 and 600 s using a 2 kW regulated AC power supply (EAC-S2000, ET Systems electronic). A square waveform voltage signal was applied with a positive-to-negative pulse ratio of 490 V/60 V at 50 Hz frequency and initial ramp of 60 s to achieve the voltage amplitude. The root mean square (rms) current density limit was set at 400 mA cm−2, while the voltage peak-to-peak value was maintained constant. The rms voltage and current responses were acquired electronically, with a sampling time of 0.1 s, employing a Keithley KUSB-3116 data acquisition card (16 bit, 500 kS/s) and Labview program (National Instruments). Table 1 provides the compositions of the electrolytes used for coating generation on c.p. Ti and Ti6Al4V alloy, treatment times and resultant coating thicknesses. All electrolytes were suspensions with pH ~7.0, formed due to a reaction between the electrolyte components. Although calcium acetate as well as either of the phosphate sources have high enough solubility in water, they react with formation of either 0.01 M of Ca3(PO3)6 or a 0.025 M of Ca(H2PO4)2, both of which are practically insoluble. Different sources of phosphate were used with the aim of achieving a relatively high Ca/P ratio and crystalline Ca\\P compounds in the coatings. Triethanolamine, a common surfactant agent, was added into the electrolyte used for anodizing of Ti6Al4V for extended time in order to stabilize the suspension, i.e. to prevent the coalescence of precipitated particles and study its effect on the coating composition. The treatment was performed in a 1 L double-walled cell with re-circulating cooling system that maintained the temperature of the electrolyte at 20 °C. The counter electrode was made of AISI 316 stainless steel and had dimensions of 7.5 × 15 cm. After PEO, the specimens were rinsed in distilled water and dried in warm air. 2.3. Surface characterisation Coating thicknesses were measured by the eddy current method, using a Fischer ISOSCOPE FMP10 portable instrument, taking the average of ten measurements with a standard deviation of ~ 0.5 μm and later confirmed with cross-sectional scanning electron microscopy (SEM). Plan views and cross-sections of coatings were examined by SEM, using a JEOL JSM-6400 microscope equipped with Oxford Link energy dispersive X-ray (EDX) microanalysis hardware. EDX surface area analysis results are cited as an average of three measurements performed at different locations. Cross-sections were prepared by grinding through successive grades of silicon carbide paper, with final polishing to a 1 μm diamond finish. Phase composition was examined by X-ray diffraction (XRD), using a Philips X'Pert diffractometer (Cu Kα = 1.54056 Å) at a scanning speed of 0.01o per second for a scan range of 2θ from 10 to 80°. Surface hardness was measured on polished coating cross-sections applying a load of 0.025 kg for 15 s using an AKASHI MVK-E3 Vickers microhardness machine. The cited values are the average of ten measurements. Roughness parameter Ra (arithmetic average of the absolute profile deviations within the scanning path) was obtained using a Surtronic 25 roughness tester (Taylor Hobson) and

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Table 1 Electrolyte compositions. Coating

PEO time (s)

Coating thickness (μm)

Electrolyte

c.p.Ti/PEO Ti6Al4V/PEO1 Ti6Al4V/PEO2

90 90 600

5 5 10

C4H6CaO4 - 0.125 M, NaH2PO4 ⋅2H2O - 0.025 M, KOH - 0.018 M C4H6CaO4 - 0.12 M, (NaPO3)6 - 0.01 M, KOH - 0.018 M C4H6CaO4 - 0.12 M, (NaPO3)6 - 0.01 M, KOH - 0.018 M, C6H15NO3 - 5 mL/L

TalyProfile software. The presented values are the average of 5 measurements performed over a distance of 4 mm. The coating adhesion was measured using a portable pull-off adhesion tester PosiTest (DeFelsko®) at a loading rate of 1.0 MPa/s. ∅ 10 mm dollies were glued to the specimens using an epoxy adhesive LOCTITE® 907™ Hysol®. The pore population density and pore size of the coatings have been estimated using ImageJ software. 2.4. Immersion tests Coated and non-treated alloys were exposed to a simulated body fluid solution (m-SBF, Table 2) as in [36] with a pH 7.4 for up to 4 and 8 weeks for c.p. Ti and Ti6Al4V alloy, respectively. Each specimen was immersed in 25 mL of the solution, placed in a tightly-sealed container, and thermostated at 37 °C. Two specimens for each type of material were used for reproducibility. Immersion tests in m-SBF modified with hydrogen peroxide simulating inflammation were carried out during 7 days. Fusayama-Meyer formulation [37] was used to prepare artificial saliva (Table 2). The normal saliva (pH 5.3) was modified with 100 ppm NaF (0.1 g L−1) in addition to 100 mM of H2O2 simulating inflammatory conditions and acidified to pH 4.0 using lactic acid. The level of fluoride ions in saliva was chosen in accordance with the fact that it varies from 100 to 350 ppm immediately after use of an oral hygiene product, depending on the product or combination of thereof (dentrifice, mouthwash, etc.), and decreases to 7–10 ppm in 24 h (if products have not been used again within this period) [38]. 2.5. Electrochemical tests DC and AC electrochemical tests were carried out after various immersion periods (from 1 d to 56 d, as needed) using an AUTOLAB PGSTAT30 (Eco Chemie) potentiostat in naturally aerated media (as Table 2 Compositions of the physiological media. Media

Reagents

m-SBF

NaCl NaHCO3 Na2CO3 KCl K2HPO4·3H2O MgCl2·6H2O 0.2 M – NaOHb HEPESb CaCl2 Na2SO4 1.0 M - NaOH H2O2 KCl NaCl CaCl2·2H2O NaH2PO4·2H2O Na2S·9H2O CO(NH2)2 H2O2 NaF

Artificial saliva

a

2.6. Ion release analysis The immersion test solutions were acidified with 200 μL of nitric acid (65 wt.%) to dissolve precipitated titanium hydroxide. After filtering through a nylon filter with 22 μm pore size and 1:6 dilution in Millipore water, the samples were analysed by inductively coupled plasma mass spectroscopy (ICP-MS) using an Agilent 7700 ICP-MS instrument equipped with concentric quartz nebulizer and Peltier-cooled nebulising camera. The instrument was operated in standard mode, without using the reaction cell. Argon was used as plasma maintaining carrier gas. ICP-MS operating conditions were as follows: forward power 1550 W, argon flow 0.99 L min−1, nebulizer pump 0.3 rps. The following isotopes were measured: 47Ti, 27Al, 51V. Calibrations were made using both deionised water and 1:6 diluted SBF-blank solutions in the range from 0 to 50 μg L− 1. The calibration curves were corrected with respect to a background level of elements detected in blank SBF. The results are cited as an average of two measurements with the errors representing the deviation of the maximum and minimum from the mean.

pH 5.403 g L−1 0.504 g L−1 0.426 g L−1 0.225 g L−1 0.230 g L−1 0.311 g L−1 100 mL L−1 17.892 g L−1 0.293 g L−1 0.072 g L−1 15 mL L−1 100 mM 0.4 g L−1 0.4 g L−1 0.906 g L−1 0.69 g L−1 0.005 g L−1 1 g L−1 100 mM 0.1 g L−1

3. Results and discussion 3.1. Coating morphology and microstructure 7.4a

6.8 5.3

5.2; 4.0c

Buffered at pH 7.4 at 36.5 °C with HEPES and 1.0 M-NaOH aqueous solution. HEPES (2-(4-(2-hydroxyethyl)-1-piperaziny) ethanesulfonic acid) previously was dissolved in 100 mL of 0.2 M-NaOH aqueous solution. c Acidified with lactic acid. b

per Table 2) at 37 °C. All measurements were reproduced at least twice. Potentials were measured with respect to a Ag/AgCl reference electrode. Solution concentration inside the reference electrode compartment was 3 M KCl, providing a potential of 0.210 V with respect to the standard hydrogen electrode. A platinum foil (~ 1 cm2) was used as the auxiliary electrode. The specimens were polarized at a rate of 0.3 mV s−1 from −250 mV to +3500 mV relative to the open circuit potential (OCP). Electrochemical impedance spectroscopy (EIS) or, AC measurements, were carried out applying a sinusoidal perturbation of 10 mV amplitude and a frequency sweep from 105 Hz to 10−2 Hz. The frequency response was analysed using ZView software, the goodness of fit of the simulated spectra corresponded to chi-squared (square of the standard deviation between the original data and the calculated spectrum) values b0.01. The errors for the individual parameters of the equivalent electrical circuits (such as CPE and R) were b 5%.

The surface morphology of the alloys following the surface preparation prior to PEO treatment is presented in Fig. 1. HF/HNO3 pickling revealed equiaxed grains of c.p. Ti with fine surface texture which depended on the grain orientation and the associated chemical dissolution rate of Ti. The Ti6Al4V alloy following the same pickling procedure revealed equiaxed alpha grains (at%: 86.7 Ti, 9.1 Al, 4.2 V) and integranular beta grains (at%: 82.6 Ti, 6.4 Al, 11.0 V). Both microstructures are characteristic of cold-rolled and annealed Ti alloys. Fig. 2 presents the rms voltage-time transitions acquired during generation of PEO coatings in c.p. Ti and the alloy. The microdischarges could not be observed in suspension electrolytes, but their initiation voltage is indicated by the acoustic noise and electric signals at ~ 50 s and ~ 200 Vrms. The addition of TEA in the electrolyte for PEO of Ti6Al4V alloy lowered the forming voltage and facilitated sustainable sparking and uniform coating morphology during extended treatment time.

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a)

b) 2

1

50 μm

50 μm Fig. 1. Secondary electron micrographs of (a) non-coated c.p. Ti and (b) Ti6Al4V alloy.

The coatings formed during 90 s of PEO treatment on both materials were about 5 μm-thick and comprised a sub-micrometric inner dense layer and an outer layer permeated by relatively large discharge channels (Fig. 3(a, c)). The coating formed on the alloy disclosed bigger pores size and lower pore population density than the coating formed on c.p. Ti (Table 4, Fig. 3(b, d)). Both coatings were composed of anatase and rutile; additionally, the surface of the coated Ti6Al4V alloy was covered with nanoscaled particles, possibly hydroxyapatite, as suggested by XRD analysis (Fig. 4) and the Ca/P ratio of 1.67, determined by EDX area analysis (Table 3). Nevertheless, a substantial part of Ca and P containing compounds is also likely to be amorphous. The outer layer of the 10 μm-thick coating formed on Ti6Al4V during 600 s of treatment was highly porous (Fig. 3(e)) and the pores on the surface were almost entirely clogged by precipitates, including Ca3(PO4)2 (Fig. 3(f) and Fig. 4). Both 5 and 10 μm-thick coatings formed on Ti6Al4V alloy contained appreciable amounts of V (1.2–1.3 at.%) and Al (1.9–2.4 at.%, Table 3), incorporated into the coating from the substrate. All three coatings exhibited similar adhesion strength of ~4.4 MPa. As for the microhardness, only that of the 10 μm-thick coating on Ti6Al4V could be measured (Table 4); its relatively low value (HV0.025 = 205) is attributed to the coating porosity. The thicknesses of the 5 μm-thick coatings were comparable with the size of the microindentation and, therefore, rendered the measurements unreliable.

RMS voltage (V)

300

Ti6Al4V/PEO1 Ti6Al4V/PEO2 c.p.Ti/PEO 200

100

0 0

100

200

300

400

500

600

Time (s) Fig. 2. RMS Voltage-time curves for the PEO treatment of Ti alloys.

700

3.2. Electrochemical behaviour of PEO-coated c.p. Ti in artificial saliva Fig. 5(a) presents the comparison of the DC polarisation tests of PEOcoated and non-coated c.p. Ti carried out after 7 days of immersion in normal and inflammatory artificial saliva at pH ~5.2–5.3 (the addition of H2O2 and NaF insignificantly altered the pH). The effect of 100 mM of H2O2 and 0.1 g L−1 of NaF on c.p. Ti mainly manifests itself through a negative shift of the corrosion potential, Ecorr, from − 50 mV to − 202 mV and narrowing of the width of the passivity region from 1.67 V to 0.76 V; the passive current density, however remains unaffected at ~ 1.5 μA/cm−2. PEO-coated c.p. Ti demonstrates slightly more noble value of the Ecorr in the inflammatory conditions (54 mV vs. −50 mV) and has no well-defined passive current region. In either of the solutions the passive current densities keep changing with the potential and are slightly higher in inflammatory saliva than in normal saliva in the 0…750 mV region of potentials, e.g. ~40 nA/cm−2 vs. ~20 nA/ cm−2 at 250 mV. Evidently, the PEO-coated c.p. Ti remains more than an order of magnitude more passive than c.p. Ti in both conditions. Bode plots of EIS spectra of c.p. Ti (Fig. 6(a)) demonstrate that capacitive behaviour of the material extends to lower frequencies with immersion time in inflammatory saliva at both pH values. Initially (1 d of immersion), the acidification causes a reduction of the total impedance |Z| from 247 kΩ cm2 to 42.7 kΩ cm2, i.e. a reduced corrosion resistance. However, after a week of immersion it recovers to 232 kΩ cm2 due to a thickening of the passive film and becomes comparable with the impedance of c.p. Ti in non-acidified inflammatory conditions (517 kΩ cm2). The spectra were fitted using a Randles equivalent electrical circuit (Fig. 6(a), inset), where Rel is the ohmic drop in the electrolyte, Rb and CPEb are the resistance and capacitance of the barrier oxide film, respectively. Here and further in the work all the capacitances were represented with constant phase elements (CPE) that reflect the nonhomogeneity of the materials (i.e., presence of pores, voids, nanocrystals etc. in the films), via an exponential factor 0 b n b 1 in the formula for an impedance Z of a capacitor: Z = 1/[CPE (jω)n], pffiffiffiffiffiffiffiffi where ω is radial frequency, and j ¼ −1 is the imaginary number. CPE, therefore, corresponds to a numerical value of admittance of the system, 1/Z, at ω = 1 rad s−1, and at n = 1, CPE becomes an ideal capacitor. The fitting parameters are gathered in Table 5. It is evident that the Rb value is drastically reduced after 7 d of immersion at pH 4.0 (0.34 MΩ cm2) compared with pH 5.3 (88 MΩ cm2), and is accompanied by an increase in the CPEb. This is in agreement with the EIS behaviour of Ti observed in H2O2-containing PBS solutions [16,17]: the oxides formed in the presence of H2O2 were reported to be thicker, rougher (hence the higher CPEb) and exhibit higher ionic conductivities (hence the lower Rb) than those formed in peroxide-free solutions. Ti is also known to recover its resistance with time while still exposed to

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a)

5

b)

10 μm

20 μm c)

d)

10 μm

20 μm

e)

f)

20 μm

10 μm

Fig. 3. Cross-sectional backscattered electron (left) and plan view secondary electron (right) micrographs of Ca-P-containing PEO coatings on c.p. Ti (a, b) and Ti6Al4V alloy (c–f); (c, d) after 90 s treatment; (e, f) after 600 s treatment.

1500

Ti A Ti Ti

Ti

Intensity (a.u.)

1000

R c.p.Ti/PEO

500

A R Ti A R

Ti Ti

HA

A A

R

HA

Ti6Al4V/PEO1

Ti A Ti CaTiO3

Table 3 EDS analysis of the plan view area of the coatings as per Fig. 2 (b, d, f).

Ca3(PO4 )2 CaTiO3 Ti6Al4V/PEO2

Coating

0 20

peroxide or once the peroxide is removed from the media, i.e. when the inflammatory process is over. In the same conditions (pH 4.0, 1 d) the PEO-coated c.p. Ti coating exhibits a superior corrosion resistance compared with c.p. Ti. For short immersion times, two time constants are observed in phase angle-frequency dependencies at both pH values (Fig. 6(b)): the two peaks at high and low frequencies correspond, respectively, to the behaviour of the outer porous part of the coating and the inner barrier part (CPEpor/Rpor and CPEb/Rb, respectively, in (Fig. 6(b) inset). With time (7 d), as the outer part of the coating becomes fully permeated by the electrolyte, the high frequency peak disappears and only the

40

60

2θ Fig. 4. X-ray difractograms of the coated alloys.

80

c.p. Ti/PEO Ti6Al4V/PEO1 Ti6Al4V/PEO2

at. %

Ratio Ca/P

O

P

Ca

Ti

V

Al

59.9 52.2 56.4

2.3 5.5 6.9

4.6 9.2 12.2

33.2 29.4 21.4

– 1.3 1.2

– 2.4 1.9

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2.0 1.67 1.77

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Table 4 Adhesion strength, roughness and microhardness of the coatings. Material

Adhesion strength (MPa)

Roughness (μm)

– – 4.5 ± 0.1 4.4 ± 0.1 4.3 ± 0.1

Ti6Al4V c.p. Ti c.p. Ti/PEO Ti6Al4V/PEO1 Ti6Al4V/PEO2

Ra

Rz

0.74 ± 0.02 0.29 ± 0.02 0.58 ± 0.11 1.46 ± 0.02 1.50 ± 0.02

4.01 ± 0.02 1.90 ± 0.20 3.55 ± 0.71 8.42 ± 0.02 8.55 ± 0.02

Microhardness (HV0.025)

Pore size (μm)

Pore population density (mm−2)

368 ± 21 140 ± 7 – – 205 ± 17

– – 0.4–2.0 0.4–3.6 0.1–2.2

– – ~36 × 103 ~20 × 103 *

* The porosity was not estimated, due to the technical difficulty of the image analysis, as almost all the pores are blocked by Ca-P-containing precipitates.

electrolyte in the pores offers resistance to the applied potential perturbation, hence the drop in Rpor value from 480 Ω cm2 to 90 Ω cm2. The interconnected pores of PEO coatings impede the mass transport during corrosion [39]. An indefinite length Warburg diffusion, represented by CPEdiff, had to be included into the equivalent circuit, in order to fit adequately the phase angle slope observed at 1…10 Hz (Fig. 6(c)). Its contribution was notable: for instance, for 7 d of immersion at pH 5.3, where the corresponding exponential factor n is in fairly good proximity (0.47) to the theoretical value of 0.5 (Table 5), the diffusion coefficient σ calculated as σ = 1/[20.5 CPEdiff] yields ~583 Ω s−0.5 cm2. In general, the coating behaviour is largely similar in both normal and inflammatory saliva conditions after 1 d of immersion (Rb = 1270…1293 kΩ cm2), but exhibits an increase of the inner layer resistance from 935 Ω cm2 to 2046 Ω cm2 after 7 d in the presence of H2O2. This may be related

c.p.Ti Saliva norm. c.p.Ti Saliva inflam. c.p.Ti/PEO Saliva norm. c.p.Ti/PEO Saliva inflam.

3.5 3.0 2.5

E (VAg/AgCl)

2.0 1.5 1.0 0.5 0.0 -0.5 1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

-2

(a)

j (A cm ) Ti6Al4V SBF Ti6Al4V SBF+H2O2

3.5

Ti6Al4V/PEO1 SBF Ti6Al4V/PEO1 SBF+H2O2

3.0

E(VAg/AgCl)

2.5 2.0

Ti6Al4V/PEO2 SBF Ti6Al4V/PEO2 SBF+H2O2

1.5

c.p.Ti SBF c.p.Ti SBF+H2O2 c.p.Ti/PEO SBF c.p.Ti/PEO SBF+H2O2

1.0 0.5 0.0 -0.5 -10

10

-9

10

-8

10

-7

-6

10

10 2

i(A/cm )

-5

10

-4

10

(b)

Fig. 5. Potentiodynamic polarisation curves for coated and non-coated Ti alloys obtained after 7 days of immersion in (a) artificial saliva in normal and fluorinated inflammatory conditions, both at pH 5.3; (b) SBF in normal and inflammatory conditions.

to the fact that the so called barrier layer in PEO coatings on Ti, although compact, is not in fact an amorphous defect-free film [1], it contains submicron-size voids that may be clogged during increased dissolution and passivation processes in the presence of H2O2. PEO-coated c.p. Ti immersed in near-neutral media typically demonstrates capacitive behaviour in a wider range of low frequencies than non-coated c.p. Ti; this behaviour is not fully revealed here, since the lower frequency limit was 0.01 Hz (Fig. 6(c)). For that reason the maximum |Z |0.01Hz values for PEO-coated c.p. Ti are lower than those for c.p. Ti, but this does not signify an inferior corrosion resistance of the coated material. 3.3. Electrochemical behaviour of PEO coated alloys in SBF Previously we have studied the effect of the coating thickness on corrosion resistance of c.p. Ti in SBF and demonstrated greater stability of a 5 μm-range PEO coatings in terms of titanium release. In this work the effect of peroxide on DC polarisation behaviour of 5 μm-thick PEO coating on c.p. Ti is compared to that for 5 μm- and 10 μm-thick coatings on Ti6Al4V alloy after 7 d of immersion (Fig. 5(b)). The following can be inferred from analysis of the polarisation curves: (i) the presence of H2O2 does not significantly affect the passive current density of neither of the alloys, yielding ~15 μA cm−2; (ii) PEO-coated c.p. Ti is almost ten times more passive than PEO-coated Ti6Al4V alloy in both normal and inflammatory conditions; (iii) PEO-coated c.p. Ti becomes slightly more passive in inflammatory conditions compared with normal conditions in terms of Ecorr (20 mV positive shift) and passive current density (~20 nA cm-2 lower); (iv) 5 and 10 μm-thick coatings improve the corrosion resistance of Ti6Al4V alloy by 2.5 and 3 times; (v) both coatings on the alloy show increased passive current densities in inflammatory conditions by about 0.1 μA cm−2 and a positive Ecorr shift by up to ~ 100 mV which indicate their greater tendency to oxidize. The latter may result in incorporation of the Ti4 + ions into the passive film or their loss into the solution. Fig. 7 presents the Nyquist and Bode plots of the EIS spectra acquired for Ti6Al4V alloy with and without 10 μm-thick coating (Ti6Al4V/PEO2) for up to 56 d of immersion time in normal and inflammatory SBF and the corresponding equivalent electrical circuits. The 5 μm-thick coating (Ti6Al4V/PEO2) data were largely similar to that of the 10 μm-thick one. Ti6Al4V alloy displays a capacitive behaviour in a wide range of frequencies (Fig. 7(a)), typical for Ti alloys in neutral media and largely similar to that of c.p. Ti in SBF or artificial saliva. Exposure to H2O2-containing media during 7 days does not produce appreciable difference in total Z of the alloy at high frequencies (649 kΩ cm2 vs. 644 kΩ cm2), but the resultant passive film is apparently thinner and more compact, as the fitting data suggest (Table 5). Possibly, 7 days period is too short to cause a significant change in the alloy corrosion resistance; for instance, Höhn and Virtanen observed a notable deterioration of total impedance |Z| after 56 d of immersion of Ti6Al4V in H2O2-containing DMEM, and absence of the second time constant that is expected in non-inflammatory conditions due to formation of a porous layer on the top of the barrier layer [19]. In the present work the 56 days of exposure of Ti6Al4V in normal SBF results in a slight decrease of the total impedance at low (10 mHz) frequencies and increase at medium (10–100 Hz) frequencies.

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E. Matykina et al. / Surface & Coatings Technology xxx (2016) xxx–xxx

|Z| (Ω cm -2 )

106

pH 4.0 pH 4.0 pH 5.3 pH 5.3

105 104

1d 7d 1d 7d

103 102 10-2

10-1

100

101

102

103

104

Frequency (Hz)

-100 Rel

CPEb

theta

-75

Rb

-50 -25 0 10-2

10-1

100

101

102

103

104

(a)

Frequency (Hz)

|Z| (Ω cm-2 )

106

pH4.0 pH4.0 pH5.3 pH5.3

105 104

1d 7d 1d 7d

103 102 10-2

10-1

100

101

102

103

104

Frequency (Hz)

-100

Rel

CPEpor

Rpor

theta

-75

CPEdif

CPEb Rb

-50 -25 0 10-2

10-1

100

101

102

103

104

Frequency (Hz)

(b)

|Z| (Ω cm -2)

106

c.p.Ti 1 d c.p.Ti 7 d c.p.Ti/PEO 1 d c.p.Ti/PEO 7 d

105 104 103 102 10-2

10-1

100

101

102

103

104

103

104

Frequency (Hz) -100

theta

-75 -50 -25 0 10-2

10-1

100

101

102

Frequency (Hz)

(c)

7

The EIS response of the coated alloy reveals a clear influence of a diffusion process at frequencies b 1 Hz (Nyquist plot, Fig. 7(b)). After 7 d of immersion in normal conditions, Randles circuit with CPEcoat/Rcoat element in series with CPEdiff fitted the data well, suggesting that at this time point there is no separate response of the barrier and porous part of the coating. This circuit was successfully used in [27] to describe the PEO coatings obtained in electrolytes with dispersed nano-HA particles. After prolonged exposure in normal SBF, as the electrolyte permeated the pores, two well defined time constants of the outer and inner layers were observed at 3 kHz and 10 Hz (Bode plot, Fig. 7b), respectively. This required the incorporation of a CPEb/Rb unit, describing the capacitance and resistance of the barrier layer, in order to fit the data. This circuit also described the data obtained after 7 d immersion in the presence of H2O2. The pronounced contribution from the barrier layer in the latter case may be related to its increased heterogeneity (CPE b = 8.63 μS s − 0.68 cm − 2 ). After 56 d of immersion in normal SBF the Rb + Rpor (21 kΩ cm2) is greater than Rcoat (8.93 kΩ cm2) after 7 d which may be attributed to clogging of the defects in the film with hydrolysed corrosion product species (such as Ti(OH)4 and Al(H2O)3(OH)3)). 3.4. Ion liberation in SBF Previously, we have shown that long-term Ti4 + ion release from PEO-coated c.p. Ti into SBF and artificial saliva is considerably lower than that from non-coated c.p. Ti [33,35]. The present work evaluates the metal ion release from c.p. Ti with 5 μm-thick coating and Ti6Al4V with 5 and 10 μm-thick coatings over 4 or 8 weeks of immersion in normal SBF and short-term immersion (1 week) in inflammatory SBF compared to non-coated materials (Fig. 8). It is evident that after PEO coatings reduce Ti4+ ion release by 5–6 times after long term exposure, compared with that for both non-coated alloys. However, the release of Al3+ and V5+ ions during 8 weeks of immersion is ~10 times and ~3 times, respectively, higher than from the non-treated substrate. The concentration of both ions in SBF was considerably higher than their respective normal levels in blood serum [40–42]. For instance, the normal level of V in blood serum is 30 ppb, which would be equivalent to a cumulative liberation of 250 ng cm−2; 100 ppb of Al is considered as neurologically toxic level, which would be equivalent to 833 ng cm− 2 of cumulative liberation. Data in Fig. 8(c, d) suggest that Al3 + liberation from Ti6Al4V with 5 and 10 μmthick coatings after 1 week of simulated inflammatory conditions exceeds the toxicity level by 17% and 35%, respectively, provided that Al3+ ions accumulate. Further, after 8 weeks of immersion, the accumulated release of Al3+ is two to three times above the toxicity level. As for V5+ ion release, it is insignificantly affected per se by the presence of H2O2, but in 1 week of immersion of Ti6Al4V/PEO2 in either of the solutions reaches 360 ng/cm2, which is 44% above the normal blood level. In case of the Ti6Al4V/PEO1, V5+ liberation remains within the normal limit in inflammatory conditions, but increases with immersion time in normal conditions, exceeding the normal level by four times. The above considerations are made without taking into the account the excretion of the ions in vivo through kidneys which prevents to a certain extent the accumulation of ions. They serve to demonstrate that while PEO coatings studied here provide an efficient barrier for Ti4+ electrochemical dissolution, they do not inhibit the release of Al and V ions from Ti6Al4V alloy. The excessive amounts of Al3+ and V5+ (compared to those for the non-coated alloy) must be attributed to their lixiviation from the coatings (Table 3); these elements may form amorphous oxides or simply dope the TiO2 lattice. Electrochemically, it Fig. 6. EIS spectra for (a, b) non-coated and PEO-coated c.p. Ti, respectively, after immersion in artificial saliva at inflammatory conditions at different pH for 1 and 7 d; (c) comparison of the two materials at pH 5.3 for 1 and 7 d of immersion. Respective equivalent electrical circuits used to fit the spectra are provided in the insets.

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8

E. Matykina et al. / Surface & Coatings Technology xxx (2016) xxx–xxx

Table 5 Fitted electrical parameters of the EIS spectra. Materials/conditions c.p.Ti in art. saliva with H2O2

pH 5.3 pH 4.0 − +H2O2 −

Ti6Al4V in SBF

CPEb (μS s−n cm−2)

n

Rb (MΩ cm2)

−

−

−

−

−

1d 7d 1d 7d 7d 7d 56 d

0.82 0.94 0.91 0.96 0.88 0.86 0.82

0.56 88.33 0.05 0.34 7.30 17.80 8.54

− − − − − − −

− − − − − − −

− − − − − − −

− − − − − − −

− − − − − − −

31.2 27.2 71.1 37.8 17.9 17.5 21.2

CPEcoat (μS s−n cm−2)

Materials/conditions –

Ti6Al4V/PEO2 in SBF

7d

3.60 CPEpor (μS s

Materials/conditions c.p.Ti/PEO in art. saliva with H2O2

pH 5.3 pH 4.0

Ti6Al4V/PEO2 in SBF

+H2O2 −

1d 7d 1d 7d 7d 56 d

−n

cm

−2

)

13.22 13.29 5.33 6.43 1.72 12.0

n

Rcoat (kΩ cm2)

0.72

8.93

CPEdif (μS s−n cm−2)

n

0.43 2

−n

n

Rpor (kΩ cm )

CPEdif (μS s

0.72 0.62 0.80 0.55 0.8 0.62

0.48 0.09 0.09 0.12 2.18 3.00

808.5 857.1 493.6 1204.9 0.36 48.91

cm

0.32 −2

)

−n

n

CPEb (μS s

0.41 0.47 0.34 0.40 0.35 0.38

169.6 219.5 126.8 127.1 8.63 1.70

−2

cm

)

–

–

–

–

|Z| (Ω cm -2)

Ti6Al4V-SBF-1semana-AC2.z Ti6Al4V-SBF+H2O2-7dias-AC2.z Rel CPEb Ti6Al4V-SBF-8semanas-AC2.z fit_ti6al4v-sbf-1semana-ac2.z Rb fit_ti6al4v-sbf+h2o2-7dias-ac2.z fit_ti6al4v-sbf-8semanas-ac2.z

n

Rb (kΩ cm )

0.99 0.99 0.99 0.99 0.68 0.91

1270.2 935.6 1293.4 2046.7 9.70 18.06

-500000

4

10

103 102 101 10-2

Z (Ω cm-2)

Ti6Al4V SBF 7 d Ti6Al4V SBF+H2O2 7 d Ti6Al4V SBF 56 d

105

10-1

100

101

102

103

104

103

104

Frequency (Hz)

-90

-250000

theta

-65 -40 -15 0 0

250000

500000

Z (Ω

cm -2

10-2

750000

10-1

100

)

101

102

Frequency (Hz)

(a)

105

-75000

Rel

-50000

CPEcoat

|Z|(Ω cm-2)

Rel

Ti6Al4V-53+TA-SBF-7dias-AC2.z Rcoat CPEdif Ti6Al4V-53+TA-SBF-8semanas-AC2.z fit_ti6al4v-53+ta-sbf-7dias-ac2.z CPEpor fit_ti6al4v-sbf+h2o2-53+ta-1semana-ac1.z fit_ti6al4v-53+ta-sbf-8semanas-ac2.z Rpor CPEdif CPEb

102 101 10-2

Rb

Z (Ω cm-2)

104 103

Ti6Al4V/PEO2 SBF 7 d Ti6Al4V/PEO2 SBF+H2O2 7 d Ti6Al4V/PEO2 SBF 56 d 10-1

100

101

102

103

104

103

104

Frequency (Hz)

-65

-25000

theta

-55 -45 -35 -25

0 0

25000

50000 -2

Z (Ω cm )

75000

10-2

10-1

100

101

102

Frequency (Hz)

(b)

Fig. 7. EIS spectra for Ti6Al4Valloy (a) non-coated and (b) with 10 μm-thick PEO coating after immersion in normal SBF for 7 and 56 days and in inflammatory SBF for 7 days.

Please cite this article as: E. Matykina, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.08.018

– 2

106

-750000

–

E. Matykina et al. / Surface & Coatings Technology xxx (2016) xxx–xxx

2500

2500 SBF 1 week SBF+H2O2 1 week SBF 4 weeks

2000

SBF 1 week SBF+H2O2 1 week SBF 8 weeks

2000

-2

Ion liberation (ng cm )

9

1500

1500

1000

1000

500

500

0

0 c.p.Ti

Ti

c.p.Ti/PEO

Al

V

Ti6Al4V

c.p. Ti

(a)

-2

Ion liberation (ng cm )

2500

2000

(b) 2500

SBF 1 week SBF+H2O2 1 week SBF 8 weeks

2000

1500

1500

1000

1000

500

500

SBF 1 week SBF+H2O2 1 week SBF 8 weeks

0

0 Ti

Al

Ti

V

Al

V

Ti6Al4V/PEO2

Ti6Al4V/PEO1

(c)

(d)

Fig. 8. Ion release following 1, 4 and 8 weeks of immersion of c.p. Ti and Ti6Al4V alloys with and without PEO coatings in SBF in normal and inflammatory conditions.

is not possible to avoid incorporation of Al and V in the oxide material, as Al3+ and V5+ ions, as well as Ti4+, migrate outwards under the electric field. Decrease of their liberation during exposure in vitro and in vivo may be possible through optimization of the PEO process conditions (electrolyte composition and/or electrical input signal) for Ti6Al4V alloy in order to promote formation of insoluble crystalline compounds, such as titanium aluminate, titanium vanadate and vanadium oxide, aluminium and vanadium phosphates. 4. Conclusions PEO coating with overstoichiometric Ca/P ratio have been generated on c.p. Ti and coatings containing hydroxyapatite or apatite were designed for Ti6Al4V alloy. The electrochemical behaviour of PEO-coated c.p. Ti is largely similar in both normal and inflammatory artificial saliva. All coatings reduce the long-term liberation of Ti4+ ions into the SBF by five to six times. PEO-coatings on Ti6Al4V alloy contain Al3+ and V5+ ions, which are chemically lixiviated into SBF during long- and shortterm exposure so that the cumulative Al3+ and V5+ ions release from PEO-coated Ti6Al4V alloy exceeds that from the non-coated alloy. The PEO process conditions for Ti6Al4V alloy need to be further optimised in order to ensure the promotion of insoluble crystalline compounds in the coatings in order to reduce the chemical dissolution of Al and V. Acknowledgements The authors are grateful to Regional Government of Madrid and EU Structural Funds for support via Multimat Challenge Programme (S2013/MIT-2862-CM). E. Matykina is grateful to the MICINN (Spain)

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Please cite this article as: E. Matykina, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.08.018