ISSN 20702051, Protection of Metals and Physical Chemistry of Surfaces, 2013, Vol. 49, No. 7, pp. 874–879. © Pleiades Publishing, Ltd., 2013. Original Russian Text © S.V. Gnedenkov, S.L. Sinebryukhov, O.A. Khrisanfova, A.G. Zavidnaya, V.S. Egorkin, A.V. Puz’, V.I. Sergienko, 2012, published in Korroziya: Materialy, Zashchita, 2012, No. 10, pp. 38–43.
PROTECTIVE COATINGS
Formation of Bioactive Anticorrosion Coatings on Resorbable Implants by Plasma Electrolytic Oxidation S. V. Gnedenkov, S. L. Sinebryukhov, O. A. Khrisanfova, A. G. Zavidnaya, V. S. Egorkin, A. V. Puz’, and V. I. Sergienko Institute of Chemistry, Far East Branch, Russian Academy of Sciences, Vladivostok, Russia email:
[email protected],
[email protected] Received July 3, 2012
Abstract—Calcium phosphate coatings (Ca/P = 1.61) containing magnesium oxide MgO and hydroxyapa tite Ca10(PO4)6(OH)2 accelerating the growth of bone tissue have been prepared by the method of plasma electrolytic oxidation (PEO) on MA8 magnesium alloy. The phase and element compositions, morphology, and anticorrosion properties of coatings were investigated. Such PEO layers were found to essentially reduce the corrosion rate of magnesium alloy (polarization resistance being increased by two orders). This makes it possible to consider the formed PEO coatings as likely anticorrosion layers for medical bioresorbable implants. Keywords: magnesium alloys, bioresorbable implants, plasma electrolytic oxidation DOI: 10.1134/S2070205113070071
INTRODUCTION Prospects for creation of biodegradable implants for medical treatment of complicated fractures are attract the attention to this highpriority field of material sci ence for the needs of implant surgery. The final aim is the development of an implant that degrades at a con trolled rate in the human organism and functions prop erly during the time needed for damaged bone to heal (12–14 weeks). Such implants should dissolve at a spe cific rate in the chloridecontaining medium of the human organism, thereby obviating the necessity of removing it via surgery. Magnesium alloys that can be used as biodegradable implants have received rather great attention. The main advantage of these materials is their biocompatibility, as well as acceptable mechanical properties (the density and Young modulus are comparable with the values of these parameters for cortical bone) [1, 2]. The dissolu tion products (magnesium cations) are not toxic to the organism and, correspondingly, do not provoke unde sirable negative consequences (toxicosis, allergic reac tions, tumors, etc.). The main factor limiting the use of magnesium alloys as biodegradable material, however, is their extraordinary high corrosion activity in chlo ridecontaining media, which results in premature loss in the implant’s mechanical strength before bone tissue has healed. One way to reduce magnesium’s corrosion rate is to form anticorrosion protective coatings on its sur face. It is also necessary that the growth of bone tissue did not prevail the rate of dissolution of the magne
sium alloy [3]. An anticorrosion and bioactive layer, by slowing down corrosion, accelerates the formation pro cess of new bone and so will be gradually replaced by osteogenesis products, i.e., natural bone tissue. Consequently, the development of methods of for mation of a coating that is biologically active (accelerat ing osteogenesis and osteointegration of bone), on the one hand, and possess anticorrosion protective proper ties (decreasing the dissolution rate of a magnesium implant in a physiological medium), on the other hand, is an important scientific and practical problem, the solution to which should significantly speed up the progress of “magnesium” implant surgery. Bioactive surface layers containing compounds of calcium phosphates that are similar to the natural bone tissues are the object of great interest. In this connec tion, the synthesis of calciumphosphate compounds including hydroxyapatite in the composition of anticor rosion coatings with a developed morphological struc ture on the surface of magnesium alloys would be appropriate. This will permit one to secure the optimal biocompatibility of an implant with bone tissue. Currently, items made from hydroxyapatite are already in use in traumatology, stomatology, orthopedy, and cosmetology as a bioactive material for regenera tion of bone tissue, being identical to its mineral com ponent in chemical composition. The physical and chemical properties of hydroxyapatite ensure ideal bio compatibility, stimulating active osteogenesis and reduction of bone tissue.
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At present, calciumphosphate coatings on magne sium alloys are prepared by the anodizing method. It was reported in [4] that coatings containing MgO and MgF2 were obtained by this method on AM60D mag nesium alloy, as well as meshlike coatings containing CaHPO4 ⋅ 2H2O on pure magnesium. Treating such coatings with a base upon heating may result in biomi metic precipitation of calcium phosphate on the implant surface. Coatings consisting of βCa3(PO4)2 were prepared by the authors of work [4] on the surface of a magne sium alloy containing manganese and zinc. Tricalcium phosphate Ca3(PO4)2, however, cannot be synthesized in pure form because of its phase instability. Tricalcium phosphate can be stabilized in substitution of Ca by Mg and, in addition, is favorable for its conversion in hydroxyapatite [5]. Direct synthesis of hydroxyapatite from a solution containing calcium salt of ethylenedi aminetetraacetic acid (EDTA) and potassium dihydro phosphate KH2PO4 has been described in [6]. The growth of crystals of hydroxyapatite was found to increase with the growth of treatment temperature. As this takes place, a layer of magnesium hydroxide Mg(OH)2 is formed between the substrate and the hydroxyapatite layer. Its formation, however, can be avoided by correcting the concentrations of the Ca2+ 3– and PO 4 ions in solution, because magnesium cor rodes in solutions at pH < 11. Mg2+ ions prevent the crystallization of hydroxyapatite on magnesium sub strate, substituting Ca2+ ions into the hydroxyapatite structure. A method for the fabrication of a coating on magnesium using the solgel process was proposed in [7]. The layer of magnesium oxide was preliminarily deposited on magnesium, and then the biocompatible coating having low toxicity (consisting of titanium dioxide and hydroxyapatite) was formed. A quite technologically advanced and popular mod ern method of surface modification of metals and alloys is the plasma electrolytic oxidation (PEO). The authors of [8] studied the composition and morphology of coat ings obtained on the AZ91D alloy by the PEO method in two compositions of electrolytes (I and II) containing sodium hydroxide, sodium hexametaphosphate, and calcium hypophosphite (electrolyte I); and sodium metasilicate, sodium hexametaphosphate, and calcium hypophosphite (electrolyte II). The morphology studies revealed numerous pores on the surface. In the coating composition, only MgO (when used electrolyte I) and MgO, Mg2SiO4 (when used electrolyte II) were found. The authors of this work assert that Ca and P are present in coatings, but they suggest that these elements are in the composition of the Xray amorphous phase formed as result of fast cooling of the melt by electrolyte after completion of the plasma discharge. Data of corrosion tests [8] show that the corrosion current density of samples with coatings is about two orders lower than that of samples without a coating. The
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Ca/P ratio was found to be regulated by the concentra tion of Ca2+ ions in electrolyte composition and the parameters of the PEO process. The weakness of this process is that it uses complicated multicomponent electrolytes. The authors of [9] have proposed a method of depositing anticorrosion calciumcontaining coatings on biodegradable magnesium alloy by the PEO method in an anodic galvanostatic pulse regime at a current density of 30 mA/cm2 and pulse duration of 2 ms and with an interval between pulses of 18 ms for 15 min in basic phosphate electrolyte containing calcium hydrox ide Ca(OH)2 and sodium phosphate Na3PO4. The drawback of this method, however, is the low corrosion resistance of the coatings formed with its help, which is a result of the friability, porosity, and defects of the polycrystalline surface layer, as well as the insufficient density of the layer adjoining the substrate containing a sufficient amount of magnesium phos phate Mg3(PO4)2 along with magnesium oxide. During the maintenance of these coatings in a corrosionactive medium, in particular, containing chloride ions, the lat ter penetrate into the pores and defects of the coating and destroy it, interacting with the substrate. In addi tion, these coatings do not contain hydroxyapatite with a high level of biological activity. Paper [10] evaluates the modern level of develop ment of science and engineering in the field of creating biologically active coatings on resorbable metallic materials. The authors claim that these materials repre sent a new class of highly active biomaterials that sup posedly have a positive influence on the process of heal ing damaged tissues or organs and are thereafter dis solved gradually. Alloying magnesium with such elements as Al, Zn, Mn, Zr, and Y is reported to decrease the rate of metal dissolution. This makes it possible for an organism to regulate a better medium pH around the implant and decrease the amount of hydrogen that evolves as a result of corrosion. Along with this, the presence of some of the above alloy ing elements in the composition of a magnesium alloy can result in their permissible concentration in the organism being exceeded because of degradation of the implant, this giving rise to various negative phenomena: allergies, tumors, amnesia, and Parkinson’s disease. On the other hand, it cannot be used as an implant mate rial, due to the low strength parameters of pure magne sium [11], especially in the case of implants subject to loads. It should be noted that Mn is not extraneous ele ment in the human organism and, along with Ca and Zn, can be used up to a definite level of concentration as the alloying component of magnesium alloys used in implant surgery [11]. Thus, it is effective to use coatings on owalloy magnesium alloys with protective coatings containing calciumphosphate compounds that not only promote osteogenesis, but also have high anticor rosion properties.
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60
Intensity, pulses
50 40 30 20 10 0 10
20
30
40 50 Angle, deg
60
70
80
Fig. 1. Phase composition of PEO coating on MA8 mag nesium alloy.
(а)
(b)
(c)
10 µm
5 µm
5 µm
Fig. 2. SEM image of PEO coating surface obtained on magnesium: (a) general view of coating, (b) dark part of coating, and (c) bright part of coating.
According to the experimental data [12, 13], PEO coatings on titanium containing hydroxyapatite possess bioactivity and can be used in medicine on an implant surface. It was shown in [14] that PEO coatings formed on magnesium in silicate fluoride electrolyte exhibit high anticorrosion properties. In this work, a method of plasma electrolytic synthe sis of calciumphosphate compounds on the surface of magnesium alloy of the Mg–Mn–Ce system contain ing a minimum amount of alloying elements was pro posed. The phase and element compositions of coatings were determined, and their morphologies and anticor rosion properties were investigated. Such PEO layers containing hydroxyapatite will ensure implant bioactiv ity with bone tissue and decrease the rate of their corro sion damage as compared with metallic implant with out coating and respectively accelerate the solution of problem of using magnesium implants in medicine. EXPERIMENTAL METHOD To obtain the coatings by the PEO method, rectan gular samples with a size of 5 mm ×30 mm × 1 mm from deformable MA8 magnesium alloy (1.5–2.5 wt % Mn, 0.15–0.35 wt % Ce, Mg is balance) were used. Before oxidation, the samples were mechanically treated up to obtaining roughness parameter Ra = 0.12 μm, washed in distilled water and ethanol. Electrolyte was prepared through sequential dissolving of their constituent com ponents in distillate water based on 25 g/L of calcium glycerophosphate (C3H7O6P)Ca ⋅ 2H2O and 5 g/L of sodium fluoride NaF with mixing. Then electrolyte pH was adjusted to 10.9–11.3 by adding 20% NaOH solu tion. The magnesiumalloy substrate was put into an elec trolytic bath filled with the electrolyte, in which the processed sample was a working electrode and a hollow cooler built from steel as a spiral cooled by flowing water was used as a counter electrode. Electrolyte was allowed to keep for 30 min before the beginning of oxidation. In the PEO process, the electrolyte temperature was main tained at a level of 25°C. The treatment process was performed in the bipolar (anode–cathode) regime. Control of electric parame ters was carried out through automatic system of man agement and control connected with a computer with appropriate software. A reverse thyristor rectifier was used as the current source [15]. The phase composition of coatings was studied on an automatic D8 Advance Xray diffractometer (CuKα radiation) produced by Bruker. Identification of obtained Xray patterns was performed using the EVA program with the PDF2 powder database. The ele ment composition of coatings was determined by the method of Energydispersive Xray spectroscopy (EDS) on a Hitachi S5500 electron microscope. Electrochemical properties were investigated with potentiodynamic polarization methods and electro
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Table 1. Elemental and phase composition of coatings obtained on MA8 magnesium alloy Studied area
Elemental composition, at % Phase composition Ca
P
Mg
Na
O
C
F
Ca/P
Bright
14.20
8.84
2.39
4.56
38.56
19.60
11.85
1.61
Dark
6.60
8.70
8.31
0.83
33.00
35.56
7.00
0.76
MgO Ca10(PO4)6(OH)2
chemical impedance spectroscopy on a 12558WB elec trochemical system (Solarton Analytical, United King dom). Measurements were performed in a threeelec trode cell in 3% NaCl solution at room temperature. The niobium net covered by platinum was used as a counterelectrode. A silverchloride electrode filled with saturated KCl solution (the potential versus a normal hydrogen electrode is equal to 0.201 V) was the refer ence electrode.
EXPERIMENTAL RESULTS AND DISCUSSION
Before commencement of electrochemical mea surements, samples were held in electrolyte for 15 min to establish stationary potential Ec. Potentiodynamic polarization curves were recorded at a potential sweep rate 1 mV/s. The samples were polarized in the anode direction beginning from potential E = Ec – 30 mV. The values of free corrosion current Ic and polarized resis tance Rp were calculated using SternGeary equation. In impedance measurements we used a signal with a sinusoidal form having an amplitude of 10 mV and fre quency from 0.005 Hz to 1 MHz with a logarithmic sweep 10 points per decade as the perturbation signal. The experiment and analysis of experimental depen dences were carried out using the CorrWare, ZPlot, ZView, and CorrView software (Scribner, United States).
It was found in a morphological study of the coating which was obtained (Fig. 2) that its surface is developed and inhomogeneous: along with flat dense dark areas (see Fig. 2b), scaly bright formations (see Fig 2c), excrescences, and hollows, as well as pores, are found.
Figure 1 shows data of Xray diffraction analysis of a coating formed on MA8 magnesium alloy in glycero phosphatefluoride electrolyte. The composition of the surface layer was found to include magnesium oxide and hydroxyapatite. According to the data of optical microscopy obtained on a cross section, the thickness of the coating is about 60 μm.
Data on the elemental composition testify to the presence of such elements as Ca, P, Mg, Na, O, C, and F in the coating (Table 1). On the bright parts of the PEO coating, the calcium content is higher than on dark parts, while the magnesium concentration on bright parts is lower, which is evidence of a lower con tent of magnesium oxide on a given coating part. The Ca/P ratio on bright parts of coatings is higher, coming to 1.61 (see Table 1). This value is close to the Ca/P ratio for bone tissue (1.67). 105 |Z|, Ω cm2
–1.25 2
1
104
1 2
103 102 101
–1.50
10–2 10–1 100 101 102 103 104 105 106
–60 –1.75 10–9 10–8 10–7 10–6 10–5 10–4 10–3 10–2 10–1 I, A/cm2
θ, deg
E, V (vs Ag/AgCl)
–1.00
–35 –10 10–2 10–1 100 101 102 103 104 105 106 f, Hz
Fig. 3. Potentiodynamic polarization curves taken in 3% NaCl solution for the samples from MA8 magnesium alloy: (1) without coating; (2) with PEO coating.
Fig. 4. Electrochemical impedance spectra obtained in 3% NaCl solution for samples from MA8 magnesium alloy: (1) without coating; (2) with PEO coating.
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Table 2. Corrosion properties of samples from MA8 magnesium alloy in 3% NaCl solution Sample
Ec, V (vs Ag/AgCl)
Ic, A/cm2
Rp, Ω cm2
|Z|f → 0 Hz, Ω cm2
Without coating
–1.564
5.10 × 10–5
4.89 × 102
8.09 × 102
With coating
–1.519
1.15 × 10–6
2.68 × 104
5.07 × 104
On the dark parts of the coating, the calcium content is half as great and the magnesium content is twice higher than on the bright part. This area is depleted in calcium and comprised mainly of MgO. According to the data presented in work [16], the formation of a binary compound involving sodium and magnesium fluorides 60 μm is also possible on dark parts. The Ca/P ratio on the dark part is much smaller than on the bright one, coming to 0.76. The presence of carbon in the coating composition is the result of absorbed and/or unconsumed organic electrolyte components, namely, calcium glycerophos phate in the channels of plasma microdischarges during PEO. Organic radicals in the PEO process contribute to longer burning of hightemperature plasma discharge, which results in the synthesis of crystalline compounds in the composition of the coating [17] notably hydroxyapatite in this case. This is the reason that an higher content of C10(PO4)6(OH)2 is observed on bright parts where the carbon concentration is lower than one on dark parts. Similar PEO coatings containing hydroxyapatite formed on titanium alloys according to the experimen tal data obtained both in vitro and in vivo [13] possess a high level of biological activity, sufficiently accelerating the growth of bone tissue on the implant surface. Potentiodynamic polarization curves of studied samples are presented in Fig. 3. Data of electrochemical impedance measurements (Fig. 4) are presented in the form of Bode plots (dependences of impedance modu lus |Z| and phase angle theta on frequency f). Calculated corrosion parameters of studied samples are presented in Table 2. Analysis of obtained results allows the con clusion to be drawn that the PEO coating essentially decreases the rate of sample dissolution in the active area. The impedance spectrum in the frequency range between 102 and 106 Hz of the sample with PEO coating has a complicated nature connected with the morphol ogy of the surface layer (see Fig. 2). The value of the impedance modulus at low frequencies characterizing the protective coating properties is |Z|f → 0 Hz = 5.07 × 104 Ω cm2, which is more than 60 times greater than the corresponding parameter for the sample without a coat ing (||Z||f → 0 Hz = 8.09 × 102 Ω cm2). Thus, according to the obtained experimental data, the coatings formed on the surface of magnesium alloy on the one hand accelerate bonetissue growth on the implant surface because of hydroxyapatite content and,
on the other hand, greatly reduce magnesiumalloy corrosion. Thus, the unique method of surface treat ment that has been developed shows real promise for creation of biodegradable magnesium implants that may lead implant surgery to a qualitatively new level. CONCLUSIONS 1. It has been shown that protective coatings con taining hydroxyapatite at a Ca/P = 1.61 ratio, which is close to that of bone tissue, were obtained on MA8 magnesium alloy in an electrolyte solution containing calcium glycerophosphate and sodium fluoride in the PEO bipolar mode. 2. These coatings have a welldeveloped porous sur face and greatly decrease the rate of magnesium alloy corrosion, which allows one to consider them as prom ising coatings for bioresorbable medical implants. ACKNOWLEDGMENTS This work was financially supported by The Presid ium of FEB RAS (Grant No. 12IISO04013), RFBR (Grant No. 110398513_r_vostok_a), and The Ministry of Education and Science of Russian Federa tion (Contract No. 02.G25.31.0035). REFERENCES 1. Zeng, R.C., Dietzel, W., Witte, F., et al., Progress and challenge for magnesium alloys as biomaterials, Adv. Eng. Mater, 2008, vol. 10, no. 8, pp. B3–B14. 2. Carboneras, M., GarciaAlonso, M.C., and Escu dero, M.L., Biodegradation kinetics of modified magne siumbased materials in cell culture medium, Corros. Sci., 2011, vol. 53, pp. 1433–1439. 3. Hiromoto, S., Shishido, T., Yamamoto, A., et al., Pre cipitation control of calcium phosphate on pure magne sium by anodization, Corros. Sci., 2008, vol. 50, pp. 2906–2913. 4. Tan, L.L., Wang, Q., Geng, F., et al., Preparation and characterization of CaP coatings on AZ31 magnesium alloy, Trans. Nonferrous Met. Soc. China, 2010, vol. 20, pp. 648–654. 5. Lee, D., Sfeir, Ch., and Kuneta, P., Novel insitu synthe sis and characterization of nanostructured magnesium substituted [beta]tricalcium phosphate ([beta] TCMP), Mater. Sci. Eng., C, 2009, vol. 29, pp. 69–77. 6. Tomazawa, M., Hiromoto, S., and Yoshimoto, H., Microstructure of hydroxyapatitecoated magnesium
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FORMATION OF BIOACTIVE ANTICORROSION COATINGS prepared in aqueous solution, Surf. Coat. Technol, 2010, vol. 204, pp. 3243–3247. 7. Kuzukawa, K., Jpn Patent 202782 (A), 2007. 8. Yao, Zh., Li, L., and Jiang, Zh., Adjustment of the ratio of Ca/P in the ceramic coating on Mg alloy by plasma electrolytic oxidation, Appl. Surf. Sci., 2009, vol. 255, pp. 6724–6728. 9. Shrinivasan, P.B., Liang, J., Blawert, C., et al., Charac terization of calcium containing plasma electrolytic oxi dation coatings on AM50 magnesium alloy, Appl. Surf. Sci., 2010, vol. 256, pp. 4017–4022. 10. Purnama, A., Hermawan, H., Couet, J., and Mantovani, D., Assessing the biocompability of degradable metallic materials; stateofart and focus of the potential of genetic regulation, Acta Biomater., 2010, vol. 6, pp. 1800–1807. 11. Gu, X.N. and Zheng, Y.F., A review on magnesium alloys as biogradable materials, Front. Mater. Sci. China, 2010, vol. 4, no. 2, pp. 111–115. 12. Gnedenkov, S.V., Khrisanfova, O.A., khov, S.L., RF Patent 2348744, 2009.
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Sinebryu
Translated by E. Kapinus
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