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Corrosion behavior of HA containing ceramic coated magnesium alloy in Hank's solution
Journal of Alloys and Compounds 698 (2017) 643e653
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Corrosion behavior of HA containing ceramic coated magnesium alloy in Hank's solution Hui Tang a, b, c, *, Tao Wu a, Hong Wang a, Xian Jian a, **, Yunfeng Wu a, *** a
School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu, 611731, China Center for Information in Biomedicine, University of Electronic Science and Technology of China, Chengdu, 610054, China c School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, 150001, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 August 2016 Received in revised form 30 November 2016 Accepted 14 December 2016 Available online 22 December 2016
As a novel alternative biodegradable metal, magnesium possesses great potential and feasibility for bone fracture fixation due to its suitable mechanical properties, high degradability and biocompatibility. However, poor corrosion resistance in physiological fluids restricts their practical applications. In this study, HA contained coating on AZ31 Mg alloy substrate is prepared by MAO technique under various treatment times. The effect of treatment time on microstructure, composition, mechanical and corrosion properties is systematically investigated. The results indicate that MAO treatment is an effective route to improve the corrosion resistance. The enhanced corrosion is attributed to the barrier effect of the coatings. The failure mechanism of the MAO samples in Hank's solution is studied by long time immersion. A model for corrosion mechanism and corrosion process of the MAO coating on AZ31 Mg alloy in Hank's solution is proposed. © 2016 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy Hydroxyapatite Mechanical properties Corrosion failure mechanism
1. Introduction In recent years, magnesium and its based alloys have attracted growing interest as a promising candidate material for orthopedic implants, owing to their advantageous mechanical and biological properties [1e3]. Moreover, magnesium based alloys experience a natural phenomenon to biodegrade in aqueous solutions. Benefiting from their in vivo degradation and absorption, additional surgeries may be avoided [4]. Unfortunately, magnesium often corrodes rapidly in chloride containing solutions including human body fluid or plasma [5]. If the implants made from magnesium alloys are used to repair the diseased bone tissue, they are possible to lose the mechanical property before the healing of bone tissue. To reduce the corrosion rate of magnesium and its alloys, surface modification is an effective approach to address related issue during the application [6,7]. Since magnesium alloys are considered biodegradable, calcium and phosphorus related coatings especially hydroxyapatite (HA) contained coatings would be preferred because their similarity in chemistry to natural bone and
outstanding biological responses in the physiological environment [8,9]. Various surface modifications have been developed to fabricate HA coating on magnesium, such as chemical conversion, electrophoretic, hydrothermal treatment and micro-arc oxidation (MAO) [10e14]. Among them, MAO is regarded as one of the most prospective surface modification methods because it can introduce a ceramic coating on magnesium alloy [15]. MAO treatment evolved from traditional anodic oxidation, has been successfully employed for the surface modification of magnesium alloys in recent years [16,17]. MAO technique could greatly improve both the corrosion and wear resistances by fabricating thick, hard, and well-adhered ceramic coatings on the surface of magnesium alloys [18]. In addition, MAO process have the ability to incorporating Ca and P ions into the coating, by controlling the composition and concentration of the electrolyte. Many research groups have recently been involved with forming HA crystal layer on magnesium alloys by MAO technique. The results show that the HA contained coatings have biological properties and could reduce the initial degradation
* Corresponding author. No. 2006, Xiyuan Ave, West Hi-Tech Zone, Chengdu, Sichuan, 611731, China. ** Corresponding author. No. 2006, Xiyuan Ave, West Hi-Tech Zone, Chengdu, Sichuan, 611731, China. *** Corresponding author. No. 2006, Xiyuan Ave, West Hi-Tech Zone, Chengdu, Sichuan, 611731, China. E-mail addresses: [email protected] (H. Tang), [email protected] (X. Jian), [email protected] (Y. Wu). http://dx.doi.org/10.1016/j.jallcom.2016.12.168 0925-8388/© 2016 Elsevier B.V. All rights reserved.
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rate in simulated body fluid [19,20]. Although the excellent behavior of HA containing MAO coatings on magnesium-base substrates has been described in the medical field, it has normally been tested for only short immersion times in simulated body fluid [21]. Up to now, the degradation behavior of HA containing MAO coated magnesium alloys, especially over a long period of time as required for biomedical applications, has never been discovered and reported. In order to develop magnesiumbased biomaterials, it is essential to understand the long-term degradation behavior and mechanism of HA containing MAO coated magnesium alloys in simulated body fluid. This paper presents a study on the corrosion mechanism of HA containing MAO coated AZ31 magnesium alloys in Hank's solution, in order to reveal its long-term corrosion behavior and to estimate the longevity of Mg-based implants in the human body. This study could provide valuable insight for predicting the MAO coated Mg alloys in vivo behavior. In addition, the effects of oxidation times on the micro-structures and properties of HA containing MAO coatings are also discussed. 2. Experiments 2.1. Preparation of MAO coating Magnesium alloy AZ31 (mass fraction: 3.0e3.4% Al, 0.8% Zn, 0.4% Mn, balance Mg) was employed in this study. The specimens with a dimension of10 10 1 mm, were cut from a AZ31 magnesium sheet. Prior to the MAO treatment, the samples were polished with various grades SiC abrasive papers (180, 280, 360, 600 up to 1000 grit). After polishing the samples, they were degreased ultrasonically in a metal cleaning agent for 2 min, rinsed in deionized water for 1 min, dehydrated in ethyl alcohol for 2 min, and then dried immediately in flowing air. The electrolyte used in this investigation consist of 4 g/L sodium hydroxide and 6.3 g/L calcium glycerophosphate (C3H7CaO6P). Analytical grade reagent and distilled water were used for preparation of all electrolytes used in the investigation. The MAO treatments were carried out using a HIT-4C power supplier. The samples and stainless steel plates were used as anodes and cathode, respectively. Samples were treated under a constant voltage mode. The anodic voltage was kept constantly at 400 V and maintained the cathodic voltage at 0 V, the frequency was 1000 Hz. The MAO treatments were conducted for 5, 10, 20, 40, and 60 min, respectively. Once coated, the samples were removed from the electrolyte, washed in water and dried in air. 2.2. Characterization of MAO coatings Microstructure and surface morphology were characterized by scanning electron microscopy (SEM, HITACHI S4800). For microstructural and morphological examinations, some specimens were sectioned, mounted in resin and polished according to standard metallographic preparation techniques. Coatings for SEM inspection were sputtered with a thin conductive film of gold. The SEM is also fitted with an energy dispersive X-ray spectrometer (EDX) attachment, allowing quantitative analysis of the elemental composition to be performed on a micron scale across sections of the coatings. At a number of randomly selected locations, the scanned regions were photographed and these micrographs were subjected to analysis of micropores and their sizes. There different regions on each sample surface were analyzed. Phase composition of the coatings was analyzed by X-ray diffraction (XRD) (Philips, X'Pert, Netherlands) using Cu Ka radiation. The coating thickness and the surface roughness values were matched simultaneously for the coatings. At 5 randomly selected locations, the thickness of the
coatings was measured using an eddy-current coating thickness measurement gauge (CTG-10, Time Company). 2.3. Electrochemical test A PARSTAT 2273 Model electrochemical potentiostat was used to evaluate the electrochemical properties of the coatings. All the electrochemical measurements were conducted in a conventional three-electrode electrochemical cell with the sample as the working electrode, a platinum plate as the auxiliary electrode and a saturated calomel electrode as the reference. Both electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curve analyses (Tafel plot) were performed in Hank's solution (composed of NaCl 8 g/L, KCl 0.4 g/L, CaCl2 0.14 g/L, NaHCO3 0.35 g/ L, C6H6O6 1.0 g/L, MgCl2$6H2O 0.1 g/L, MgSO4$7H2O 0.06 g/L, KH2PO4 0.06 g/L, Na2HPO4$12H2O 0.06 g/L) 37 ± 0.5 C. The pH of Hank's solution was adjusted to 7.4 prior to the electrochemical test. A working area of 1 cm2 was exposed to the electrolyte. Before measurements, the working electrode was immersed in the electrolyte for 30 min until a steady open-circuit potential value was reached. The electrochemical impedance spectroscopy measurements were carried out at the open circuit potential (OCP) in a frequency range from 100 kHz to 0.01 Hz. The amplitude of the sinusoidal potential was 10 mV with respect to the OCP. After this first measurement, the sample impedance was measured at different exposure times to investigate the long-term stability of the coatings. Fitting of the impedance spectra were performed using the software Zview follow variations in the coating capacitance with exposure time. The potentiodynamic polarization curves (Tafel plot) were measured by polarizing the specimen to 250 mV cathodically and þ250 mV anodically relative to the OCP at a scan rate of 0.5 mV/s. The corrosion potential (Ecorr) and corrosion current density (Icorr) were determined from the polarization curves by Tafel extrapolation method. In all the above measurements, at least three similar results were considered valid and their average values were reported. 3. Results 3.1. Surface morphologies and elemental ratio of MAO coatings Fig. 1 shows the SEM images of the oxide coatings on Mg alloy substrate obtained at various oxidation time. All the MAO coatings have porous microstructures and some volcano top-like pores. The pores distributed disorderly on the coating surface. A mass of melting products around the volcano pores. As can be seen, the diameter of the micropores in the MAO coating surface increases with increasing oxidation time. As shown in Fig. 1a, after treatment for 5 min, micropores with diameter size of about 0.7 mm are fine and uniform, dispersing on the surface of the coatings. But the film was just so thin that the grinding scratches formed during sample preparation can also be observed. As shown in Fig. 1b, after treatment for 10 min, the micropores sized of approximately 1 mm, are homogenously distributed on the MAO coating surface. The surface morphology of the samples for 20 min and 40 min anodization has no noticeable difference, as can be seen from Fig. 1 (c and d). These photos show that the diameter of the micropores increases, and the number of the micropores deceases. These pores are homogenously dispersed on the coating surface. It should be noticed that two distinct characteristics are clearly identified on the surface, when the treatment time is up to 60 min. One is sintered particles accumulation around with large-sized micropores (~5 mm), and the other is the surface around numerous smaller micropores (~1 mm). The evolution of the number and size of micropores produced on the surface with increasing treatment time is presented in Fig. 2.
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Fig. 1. Surface morphologies of MAO coated samples produced at different oxidation durations (a) 5 min, (b) 10 min, (c) 20 min, (d) 40 min, and (e) 60 min.
Fig. 2. Changes in the micropores number and size of the MAO coatings measuredas a functional of treatment time. Fig. 3. Changes in the Ca, P, O and Mg content of the MAO coatings measuredas a function of treatment time.
It is obvious that micropore numbers of the MAO coating decrease with increasing coating time, while the average micropore size increases as the treatment time increased from 5 to 60 min. The results indicate that the coating time has a significant impact on the surface morphology of the MAO coating in the present electrolyte. The formation of micropores is due to the emission of the gas bubbles and molten oxide through the microarc discharge channels, which are trapped in growing coating [22,23]. The size of the pores increases with prolonging oxidation time, this result is good agreement with other publications [24,25]. The reason for the change of the pores can be ascribed to the increasing of energy density during spark discharge as the increasing oxidation time. Higher energy density induced larger discharge sparks, and form large pores [26,27]. The similar observations also have reported by Husseinet al. [28]. The EDS results indicate that the major surface constituents of the MAO coatings are Mg, Ca, P, and O elements. Fig. 3 shows the effects of coating time on the relative contents of the elements on the surface of the coatings. It can be observed that the Ca and P concentrations of the MAO coatings increase with coating time. This is in accordance with the reported fact that high energy processing employed during MAO process result in large discharges,
which can introduce more elements arising from the electrolyte into the coating [29,30]. On the contrary, the relative concentration of Mg decreases with increasing coating time. O content increases initially up to 10 min, and then decreases with increasing coating time. The presence of Ca and P in MAO coatings suggests that electrolyte species penetrate into the oxide coating during MAO process. The derivation process of O is complicated. It might come from the anions and the oxygen dissolved in the electrolyte. Mg is considered to be incorporated into the coatings from metal substrate. It might be owing to the diffusion and electrophoresis that manifested both the substrate and the electrolyte components involved in the coating formation during the MAO process [31]. 3.2. Cross-section morphologies and element distribution The cross-section SEM micrograph of the coatings processed at different coating time is shown in Fig. 4. From these figures, it is clear that the average thickness of the coatings increases with coating time. In the first 5 min of treatment, a thin uniform compact layer is formed. The coating with 10 min anodization is relatively
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Fig. 4. Cross-section morphologies of the coatings formed at different oxidation durations: (a) 5 min, (b) 10 min, (c) 20 min, (d) 40 min, and (e) 60min.
thick, and also uniform and compact. The sample coated at 20 min shows relatively smooth and homogeneous microstructure with few micropores. As treatment goes on, the coating becomes nonuniform and rough with some micropores and cracks appearing near the coating surface. Many micropores could be seen on the cross-section micrograph of the coating treated with 40 min, but these micropores and cracks are not connected to each other or perforated through the whole oxide coatings. The size of these micropores increase with increasing coating time, and many larger size cavities can be found when the sample coated at 60 min. It should be noted that there is no evident discontinuity at the interface of the coating/substrate, which implies the good bonding between the coating and the substrate. The coating thickness can also be measured from the crosssection morphologies and the thickness variation with the oxidation time is presented in Fig. 5. It can be noted that the thickness of
Fig. 5. Variation of thickness of MAO coatings measuredas a function of treatment time.
the coatings increases with coating time, but the growth rate of the coatings decreases with coating time. The coating treated for 5 min shows the formation of a 5 mm thin coating, the thickest coating (22 mm) is formed in the group treated for 60 min. In the first period (before 5 min), coatings grow linearly and quickly. The duration of the linear regime decreases as the increasing treatment time. Such decreasing growth kinetics for the coatings formation was reported by other researchers [32,33]. Indeed, according to these researchers, coatings growth results from molten magnesium which is oxidized flowing out through the discharge channels. In this way, magnesium oxide is formed, which contributes to coatings growth when being ejected from the channels and rapidly deposited at surface. The growth rate remains at a constant value as long as the spark channels reach the metal surface. Then when the coatings become thick, it is notoriously difficult to be broken down, so the growth rate decreases rapidly [34,35]. It is reported that MAO coatings can be divided into two parts based on the structure of the cross-section micrograph [36]. One part is porous outer layer, and the other part is dense inner layer. In contrast, some researchers believe that the polish process will destroy the structure of the coating. And the coating could not be separated based on the structure of the cross-section micrograph [37]. In this study, the interface between the outer part and the inner part is not certain. In order to check the structure of the coatings, the samples were bended, and then immersed into liquid nitrogen. This process is repeated three times for each sample. And the surface morphologies were observed by SEM. Fig. 6 shows the typical morphology of samples after this process. After the bending and immersing, some part of the coating was stripped. Porous and dense regions can be identified as marked in Fig. 6. Area A is part of outer layer, shows typical porous structure. While area B is located in the inner layer, displays dense and compact. This clearly demonstrates that the MAO coating can be divided into two parts, the outer porous layer and the inner dense layer. This observation is in good agreement with other publications based on the cross-section micrograph [38]. The reason for the two-layer structure can be explained as different transient temperature field in the outer layer and inner layer [39].
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Fig. 6. Typical morphology of MAO coatings after bended and immersion process.
EDS analysis of the cross-section of the MAO sample formed by 40 min is shown in Fig. 7. The coating can be roughly divided into two regions, the interface region and the outer region. Oxygen extremely increases in the interface region, and magnesium rapidly decreases. Ca and P increase gradually along the coating thickness. The highest value appears near the surface of the coating, while the composition of aluminium changes slightly in the coating. It should be noticed that calcium and phosphorus is also detected in the interface region, indicating that electrolyte components can reach the inner part of the coating, and finally be absorbed in to the coating. 3.3. Phase and chemical composition of the coatings Surface XRD patterns of MAO coatings produced at different
Fig. 7. Element line analysis of MAO coating formed at 40 min.
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duration are given in Fig. 8. Magnesium, magnesium oxide and hydroxyapatite phases are achieved in the MAO coatings by XRD analysis. Similar broad range patterns are generated for all the samples. The spectra showed only reflections associated to hydroxyapatite. The coating composition consists of magnesium oxide as a major phase, and hydroxyapatite as a minor phase. The intensity of magnesium significantly decreases with increasing duration time. The intensity of magnesium oxide and hydroxyapatite increases with increasing duration time. Up to 60 min, hydroxyapatite is found in the coating structure. The crystallinity of the coating during MAO is also correlated to the intensity of discharge. For longer growth time, higher energy required for breakdown of the coating, and higher intensity discharge can be observed on the surface of magnesium during MAO process. This causes a higher temperature in the MAO process, which favors a better crystal of hydroxyapatite. The potentiodynamic polarization curves of the MAO coatings are shown in Fig. 9, and the curve for uncoated Mg alloy is included as a reference. Parameters of the corrosion test such as corrosion potential (Ecorr) and corrosion current density (Icorr) of the specimens have been evaluated by Tafel extrapolation at the linear stage of the anodic and cathodic curves and are summarized in Table 1. It can be seen from Table 1 that all the MAO coated samples exhibit smaller Ecorr in magnitude than the uncoated AZ31 alloy. A smaller Ecorr magnitude in all coated samples suggests that they are less reactive than the AZ31 magnesium alloy. The corrosion current density of the samples with MAO coatings are 1e2 orders lower than that of the magnesium substrate, which indicates that the corrosion resistance of magnesium alloy has been greatly improved by MAO treatment. The corrosion current density (Icorr) decreases with MAO time up to 40 min and then it increases with increasing coating treatment time. The MAO specimen treated for 40 min exhibits the lowest Icorr, which suggests the best corrosion resistance among all the coatings. It is interesting to note that thicker layer of the 60 min treatment shows less protection than the layer after 40 min treatment. This indicates that thicker coating does not definitely mean better corrosion resistance. Such a phenomenon may be due to the variation of structure of the coatings. Increasing treatment time leads to a loose structure [40,41]. Based on microstructure observation, electrochemical tests are usually used to investigate the relationship between the coating microstructure and the corrosion behavior of the MAO coatings. Among the electrochemical methods, electrochemical impedance spectroscopy (EIS) as a non-destructive testing, does not affect the surface state of the samples and is commonly used to provide information about corrosion mechanism. Numerous studies have been made to evaluate the corrosion behavior of the MAO coatings by EIS [42e44]. However, the long-term degradation mechanism of the MAO coated Mg alloys in Hank's solution is not yet understood. In order to investigate the corrosion mechanism of the MAO coatings, the sample coated with 40 min was immersed in Hank's solution for 48 days. Fig. 10 shows the results of the MAO sample obtained after 0.5, 3, 6, 12, 24 and 36 h of exposures. Bode-phase plots in 36 h immersion displayed two distinct parts. One capacitive loop in the mediate frequency range, and one straight line in the low frequency range, represents the electrochemical response of inner layer and outer layer in the coating, respectively. According to the EIS plots, an obvious changes can be observed due to prolonging immersion time. The maximum phase in the plots decreases with increasing immersion time, and the corresponding frequency shifts to higher with prolonging immersion time, which indicates the deterioration of the MAO coating [45,46]. As shown in Fig. 10b, it can be seen that the value of impedance in low frequency and mediate frequency range show a prominent and well-regulated evolution with immersion time, which illustrates clearly the
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Fig. 8. XRD patterns of the MAO coatings: (a) 5 min, (b) 10 min, (c) 20 min, (d) 40 min, and (e) 60 min.
Fig. 9. Polarization curves of bare magnesium and the MAO coatings.
Table 1 Electrochemical data from the polarization test of different samples in Hank's solution. Samples
Ecorr(V)
Icorr (A/cm2)
Sub 5 min 10 min 20 min 40 min 60 min
1.45 0.98 0.88 0.79 0.76 0.72
1.05 5.67 1.71 9.48 6.93 8.75
105 106 106 107 107 107
deterioration process of the MAO coating in immersion process [47]. The impedance response indicates that the MAO coating is gradually penetrated by the corrosive solution during immersion. Fig. 10c shows that the diameter of the circles decreases with increasing immersion time, which is in good agreement with the Bode-impedance plots. It should be noted that the EIS spectra has similar shape in immersion 36 h, indicating that they have same corrosion mechanism [48]. Owing to this characteristic, immersion in 36 h is determined as early immersion stage. Taking into account the microstructural characteristics and the
EIS behaviors of the MAO coating sample, the corresponding equivalent circuits for fitting the EIS data is proposed in Fig. 11. The equivalent circuit is established based on a reasonable fitting of experimental values and the special structure of this MAO coating. Rs is solution resistance of bulk electrolyte between reference and working electrodes. Rs has nothing to do with electrode process and its value depends primarily on the testing medium and the geometry of employ cell. A parallel R and CPE network, accounting for the global barrier properties of the MAO coating, which is accounting for the resistance of the corrosive solution inside the pores and cracks of the coating. It can be seen from Fig. 6 that the MAO coating is composed by outer porous layer and inner dense layer. The corrosive ion is easy to penetrate into the outer porous layer through the pores. So the corrosion resistance of the whole MAO coating would mostly be determined by the inner barrier layer [49,50]. An additional Warburg element (Zw) at low frequency region, associated with the diffusion of corrosive ions across the inner layer because of the presence of concentration gradient in the inner layer during immersion process. This equivalent circuit model reveals the electrochemical corrosion behavior of the coated specimen in an early stage of immersion, which is depended on the combination of kinetic and diffusion processes [51,52]. Fig. 12 shows variation of the resistance with time in the early immersion stage. It can be seen that the resistance decreases with increasing immersion time, which is consistent with Bodeimpedance results. It should be noted that the impedance decline occurred mainly at low frequency and mediate frequency ranges, corresponding to the degradation of inner layer itself. When the MAO coated specimen was immersed in Hank's solution, the corrosive solution would penetrate from the outer porous layer, and reach the interface of porous layer and inner barrier layer. The infiltration process of electrolyte into barrier layer was a slow diffusion reaction for its much compact structure during the early immersion stage. This behavior evoked the diffusion characteristic in EIS spectra. In the early stage of immersion stage (0.5e36 h), the equivalent circuit included one pair of resistance and one pair of diffusion. This means that the corrosive medium is not contact with the substrate Mg alloy because of the shorter period of immersion. As time increases, the corrosive ions dissolves into the inner layer, enlarges the structural imperfection of inner layer, which increases the effective surface area immersed in corrosive medium. Consequently, the corrosion resistance of the sample with MAO coating decreases.
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Fig. 10. Changes of the EIS plots as a functional of immersion time in early immersion time: (a) Bode-phase, (b) Bode-resistance, and (c) Nyquist.
Fig. 11. Electrical equivalent circuit is used for fitting the experimental EIS spectra in early immersion time.
Fig. 12. Corrosion resistance of the coating during early immersion time.
The EIS spectra of the MAO coating immersed for 2 and 24 days is shown in Fig. 13. EIS make changes after immersion for 2 days. It can be seen that two maximum phase lags appear at medium frequency and low frequency. Which correspond to two time constant. Interestingly, it is observed that not only maximum phases in all spectra decrease with increasing immersion time, but also they are always shifting to higher frequency. The impedance modulus Bode diagrams reveal three segments, one arc in low frequency region, another arc in high-medium frequency, and one linear in high frequency. The resistance in low frequency and mediate frequency decreases with increasing immersion time. Two circles can be observed in Nyquist plots, and the diameter of the circles decrease with increasing immersion time. Due to this characteristic, immersion for 2 and 24 days, is determines as a middle immersion stage. To enable an accurate analysis of the impedance diagram, the equivalent circuit model is performed, as shown in Fig. 13d. The
equivalent circuit consists of two time constants namely CPEf with a resistor Rf in parallel, corresponding to the medium frequency characteristics of the coating, and CPEct with a resistor Rct in parallel, corresponding to the low frequency characteristics of charge transfer resistance and double-layer capacitance of the electrode. A Warburg element (Zw) was also included to accurately fit the data. This Warburg element suggested partial control of corrosion by diffusion electrolyte within the MAO coating. As illustrated in Fig. 14, the coating resistance Rf and charge transfer resistance Rct decrease with increasing immersion time. This kind of tendency is linked up with the intrusion of corrosion media and indicates that the perfection of MAO coating and its corrosion protection deteriorate while exposure duration prolonging. The charge transfer resistance Rct is higher than the corresponding value of coating resistance Rf. This indicates that the interface layer between the barrier layer and the substrate is promising for protecting the Mg alloy from corrosion during the immersion. As for immersion in two days, contact area is small although corrosive solution passes through the barrier layer and reaches the substrate. So the value of charge transfer resistance Rct is much higher than that of coating resistance. Once corrosion media contact with substrate, the protecting layer starts to break down. Consequently, the fresh reactive surface is exposed to the corrosion medium and corrosion accelerates, which results in a smaller of Rct value. The equivalent circuit model and change of the corrosion resistance reveal that the electrochemical corrosion behavior in middle stage of immersion is controlled by kinetic process [53]. Representative EIS of MAO coated Mg alloy exposed to Hank's solution after immersion over 30 days are shown in Fig. 15. It is clearly shown that the Bode-phase plots reveal two segments. In the low frequency region, a linear relationship can be observed, corresponding to the diffusion process during the reaction with the substrate. One phase angle extreme in bode plot means that the corresponding electrode process contains one time constant. With increasing immersion time, the maximum phase of the plots decreases, and the corresponding frequency shifts to higher frequency. Which illustrates that the corrosion resistance of the coated sample decreases with prolonging immersion time. The Bode-impedance reveals two segments in the high-medium frequency region, a plateau can be observed between the absolute impedance. This frequency region corresponds to a pure resistor behavior of the MAO coating. As immersion time increased (from 30 to 48 days, Fig. 15c), the radii of the semicircular in Nyquist diagrams decreases with prolonging immersion time. Appearance of new EIS shape implies the sub-process has been changed. In that case, it is necessary to put forward new equivalent circuit to explain what happened. An equivalent circuit shown in Fig. 15d is used to fit the EIS plots. In this equivalent circuit, Rs is the solution resistance,
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Fig. 13. EIS plots and electrical equivalent circuit for immersed during middle immersion stage: (a) Bode-phase, (b) Bode-resistance, (c) Nyquist, and (d) equivalent circuit model.
Fig. 14. Changes of the coating resistance Rf and charge transfer resistance during middle immersion stage.
CPEct and Rt is the double-layer capacitance and charge transfer resistance. Warburg element (Zw) indicates the diffusion processes occur on the surface of the electrochemically active site. The element (CPEct (Rc Zw) is used to describe the diffusion process of water molecules and electrochemical reactions at the substrate/ MAO coating interface. Owing to this different characteristic of EIS spectra immersed between 30 and 48 days, this immersion duration is noted as late immersion stage. The equivalent circuit model reveals that the electrochemical corrosion behavior is controlled by the combination of kinetic and diffusion process in late immersion stage. The corrosion resistance values of the coated sample as a function of immersion time are presented in Fig. 16. As
demonstrated in the bar chart, the charge transfer resistance of the coated sample decreases with increasing immersion time during late immersion stage, which indicates that the reactive area of the substrate increases with prolonging immersion time. Based on the above description of corrosion process, the corrosion mechanism of HA containing ceramic coated on Mg alloy can be schematically illustrated in Fig. 17. The HA containing ceramic coating is composed of two layers: the outer porous layer and the inner dense layer. The outer porous layer has many deep pores. In contrast, the inner barrier layer is dense and there is no pore inside. Once the MAO coated sample was immersed into Hank's solution (within 0.5 h), the corrosive electrolyte was absorbed into the outer layer through the pores. The corrosive
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Fig. 15. EIS plots and electrical equivalent circuit for late immersion stage: (a) Bode-phase, (b) Bode-resistance, (c) Nyquist, and (d) equivalent circuit model.
Fig. 16. Changes of corrosion resistance during late immersion stage.
electrolyte passed through the outer layer, reached the interface between the outer layer and inner barrier layer. With prolonged immersion time (0.5e36 h), corrosive solution gradually penetrated into the inner compact layer until corrosive solution passed through the inner barrier layer, and reached inner layer/substrate interface. At this immersion stage, the corrosion resistance was found to decrease with increasing immersion time, which was ascribed to the degradation of the inner barrier layer. Because MgO phase in this MAO coating is not stable in Hank's solution and is prone to form Mg(OH)2 [54,55]. The degradation of MgO in the coating, could lead to the formation of defects in the compact layer. So this stage could be characterized as the deterioration of barrier
layer stage. With the extension of immersion time (2e24 days), the inner barrier layer was saturated by the corrosive solution, and the weak area in inner barrier layer was broken. Substrate started to be exposed in the corrosive solution. Once the corrosive solution contacted with Mg substrate, the substrate would be corroded quickly, due to the fact that magnesium is highly chemical active in Hank's solution. At the same time, due to the protection of the MAO coating, the undamaged area of the coating could still effectively protect the substrate. Meanwhile, owing to the aggression of corrosive solution, the defects in the coating was enlarged. So the protection of the coating became deteriorated further with prolonging immersion time. This stage could be characterized as the deterioration of the coating and the substrate. With the immersion time over 30 days, the resistance of the coating was found to decrease obviously. The corrosion resistance of the coating was far smaller than that of the charge transfer resistance. The segment corresponding to electrochemistry of the coating was completely overwhelmed by the characteristics of charge transfer, so only one time constant could be observed in EIS plots. In this stage, the compact structure of the coating was destroyed and many throughgoing channels were formed, and the coating lost the protective properties. So this stage could be characterized as the deterioration of the substrate. 4. Conclusions In this study, HA containing coating are fabricated by MAO technique on AZ31 Mg alloy at various treatment time. The coating is composing of outer porous layer and inner barrier layer, can improve corrosion resistance of the substrate. The treatment time has a significant influence on the microstructure, composition, mechanical and corrosion properties. The potentiodynamic
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Fig. 17. Schematic diagram of the corrosion process and mechanism of MAO coated AZ31 Mg alloy upon immersion in the Hank's solution: (a) early immersion stage, (b) middle immersion stage, and (c) late immersion stage.
polarization shows that the MAO coating treated at 40 min has the lowest corrosion current density and the best corrosion resistance. A model for corrosion mechanism of HA containing MAO coating in Hank's solution is suggested. The main characteristic in early immersion stage is barrier layer deterioration, which is controlled by the combination of kinetic and diffusion processes. The main characteristic in middle immersion stage is the coating and the substrate deterioration, which is controlled by kinetic process. The main characteristic in late immersion stage is substrate deterioration, which is controlled by the combination of kinetic and diffusion process. Conflict of interest There is no conflict of interest. Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant No.51402045) and the Fundamental Research Funds for the Central Universities (No.ZYGX2014J089). References [1] J. Hofstetter, E. Martinelli, A.M. Weinberg, M. Becker, B. Mingler, € ffler, Assessing the degradation performance of P.J. Uggowitzer, J.F. Lo ultrahigh-purity magnesium in vitro and in vivo, Corros. Sci. 91 (2015) 29e36. [2] H. Hornberger, S. Virtanen, A.R. Boccaccini, Biomedical coatings on magnesium alloys - a review, Acta Biomater. 8 (2012) 2442e2455. [3] J.W. Lee, H.S. Han, K.J. Han, J. Park, H. Jeon, M.R. Ok, H.K. Seok, J.P. Ahn, K.E. Lee, D.H. Lee, S.J. Yang, S.Y. Cho, P.R. Cha, H. Kwon, T.H. Nam, J.H.L. Han, H.J. Rho, K.S. Lee, Y.C. Kim, D. Mantovani, Long-term clinical study and multi scale analysis of in vivo biodegradation mechanism of Mg alloy, Pnas 113 (2016) 717e721. [4] R. Rojaee, M. Fathi, K. Raeissi, A. Sharifnabi, Biodegradation assessment of nanostructured fluoridated hydroxyapatite coatings on biomedical grade magnesium alloy, Ceram. Int. 40 (2014) 15149e15158. [5] S. Virtanen, Biodegradable Mg and Mg alloys: corrosion and biocompatibility, Mater. Sci. Eng. B 176 (2011) 1600e1680. [6] G.L. Song, Control of biodegradation of biocompatible magnesium alloys, Corros. Sci. 49 (2007) 1696e1701. [7] S.V. Dorozhkin, Calcium orthophosphate coatings on magnesium and its biodegradable alloys, Acta Biomater. 10 (2014) 2919e2934. [8] S.V. Dorozhkin, Calcium orthophosphate bioceramics, Ceram. Int. 41 (2015) 13913e13966. [9] S.V. Dorozhkin, Multiphasic calcium orthophosphate (CaPO4) bioceramics and their biomedical applications, Ceram. Int. 42 (2016) 6529e6554. [10] J. Hu, C. Wang, W.C. Ren, S. Zhang, F. Liu, Microstructure evolution and corrosion mechanism of dicalcium phosphate dihydrate coating on magnesium alloy in simulated body fluid, Mater. Chem. Phys. 119 (2010) 294e298. [11] R. Rojaee, M. Fathi, K. Raeissi, M. Taherian, Electrophoretic deposition of bioactive glass nanopowders on magnesium based alloy for biomedical applications, Ceram. Int. 40 (2014) 7879e7888. [12] X. Zhang, X.W. Li, J.G. Li, X.D. Sun, Preparation and characterizations of bioglass ceramic Cement/Ca-P coating on pure magnesium for biomedical applications, Appl. Mater. Interfaces 6 (2014) 513e525.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Corrosion behavior of HA containing ceramic coated magnesium alloy in Hank's solution Hui Tang a, b, c, *, Tao Wu a, Hong Wang a, Xian Jian a, **, Yunfeng Wu a, *** a
School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu, 611731, China Center for Information in Biomedicine, University of Electronic Science and Technology of China, Chengdu, 610054, China c School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, 150001, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 August 2016 Received in revised form 30 November 2016 Accepted 14 December 2016 Available online 22 December 2016
As a novel alternative biodegradable metal, magnesium possesses great potential and feasibility for bone fracture fixation due to its suitable mechanical properties, high degradability and biocompatibility. However, poor corrosion resistance in physiological fluids restricts their practical applications. In this study, HA contained coating on AZ31 Mg alloy substrate is prepared by MAO technique under various treatment times. The effect of treatment time on microstructure, composition, mechanical and corrosion properties is systematically investigated. The results indicate that MAO treatment is an effective route to improve the corrosion resistance. The enhanced corrosion is attributed to the barrier effect of the coatings. The failure mechanism of the MAO samples in Hank's solution is studied by long time immersion. A model for corrosion mechanism and corrosion process of the MAO coating on AZ31 Mg alloy in Hank's solution is proposed. © 2016 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy Hydroxyapatite Mechanical properties Corrosion failure mechanism
1. Introduction In recent years, magnesium and its based alloys have attracted growing interest as a promising candidate material for orthopedic implants, owing to their advantageous mechanical and biological properties [1e3]. Moreover, magnesium based alloys experience a natural phenomenon to biodegrade in aqueous solutions. Benefiting from their in vivo degradation and absorption, additional surgeries may be avoided [4]. Unfortunately, magnesium often corrodes rapidly in chloride containing solutions including human body fluid or plasma [5]. If the implants made from magnesium alloys are used to repair the diseased bone tissue, they are possible to lose the mechanical property before the healing of bone tissue. To reduce the corrosion rate of magnesium and its alloys, surface modification is an effective approach to address related issue during the application [6,7]. Since magnesium alloys are considered biodegradable, calcium and phosphorus related coatings especially hydroxyapatite (HA) contained coatings would be preferred because their similarity in chemistry to natural bone and
outstanding biological responses in the physiological environment [8,9]. Various surface modifications have been developed to fabricate HA coating on magnesium, such as chemical conversion, electrophoretic, hydrothermal treatment and micro-arc oxidation (MAO) [10e14]. Among them, MAO is regarded as one of the most prospective surface modification methods because it can introduce a ceramic coating on magnesium alloy [15]. MAO treatment evolved from traditional anodic oxidation, has been successfully employed for the surface modification of magnesium alloys in recent years [16,17]. MAO technique could greatly improve both the corrosion and wear resistances by fabricating thick, hard, and well-adhered ceramic coatings on the surface of magnesium alloys [18]. In addition, MAO process have the ability to incorporating Ca and P ions into the coating, by controlling the composition and concentration of the electrolyte. Many research groups have recently been involved with forming HA crystal layer on magnesium alloys by MAO technique. The results show that the HA contained coatings have biological properties and could reduce the initial degradation
* Corresponding author. No. 2006, Xiyuan Ave, West Hi-Tech Zone, Chengdu, Sichuan, 611731, China. ** Corresponding author. No. 2006, Xiyuan Ave, West Hi-Tech Zone, Chengdu, Sichuan, 611731, China. *** Corresponding author. No. 2006, Xiyuan Ave, West Hi-Tech Zone, Chengdu, Sichuan, 611731, China. E-mail addresses: [email protected] (H. Tang), [email protected] (X. Jian), [email protected] (Y. Wu). http://dx.doi.org/10.1016/j.jallcom.2016.12.168 0925-8388/© 2016 Elsevier B.V. All rights reserved.
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rate in simulated body fluid [19,20]. Although the excellent behavior of HA containing MAO coatings on magnesium-base substrates has been described in the medical field, it has normally been tested for only short immersion times in simulated body fluid [21]. Up to now, the degradation behavior of HA containing MAO coated magnesium alloys, especially over a long period of time as required for biomedical applications, has never been discovered and reported. In order to develop magnesiumbased biomaterials, it is essential to understand the long-term degradation behavior and mechanism of HA containing MAO coated magnesium alloys in simulated body fluid. This paper presents a study on the corrosion mechanism of HA containing MAO coated AZ31 magnesium alloys in Hank's solution, in order to reveal its long-term corrosion behavior and to estimate the longevity of Mg-based implants in the human body. This study could provide valuable insight for predicting the MAO coated Mg alloys in vivo behavior. In addition, the effects of oxidation times on the micro-structures and properties of HA containing MAO coatings are also discussed. 2. Experiments 2.1. Preparation of MAO coating Magnesium alloy AZ31 (mass fraction: 3.0e3.4% Al, 0.8% Zn, 0.4% Mn, balance Mg) was employed in this study. The specimens with a dimension of10 10 1 mm, were cut from a AZ31 magnesium sheet. Prior to the MAO treatment, the samples were polished with various grades SiC abrasive papers (180, 280, 360, 600 up to 1000 grit). After polishing the samples, they were degreased ultrasonically in a metal cleaning agent for 2 min, rinsed in deionized water for 1 min, dehydrated in ethyl alcohol for 2 min, and then dried immediately in flowing air. The electrolyte used in this investigation consist of 4 g/L sodium hydroxide and 6.3 g/L calcium glycerophosphate (C3H7CaO6P). Analytical grade reagent and distilled water were used for preparation of all electrolytes used in the investigation. The MAO treatments were carried out using a HIT-4C power supplier. The samples and stainless steel plates were used as anodes and cathode, respectively. Samples were treated under a constant voltage mode. The anodic voltage was kept constantly at 400 V and maintained the cathodic voltage at 0 V, the frequency was 1000 Hz. The MAO treatments were conducted for 5, 10, 20, 40, and 60 min, respectively. Once coated, the samples were removed from the electrolyte, washed in water and dried in air. 2.2. Characterization of MAO coatings Microstructure and surface morphology were characterized by scanning electron microscopy (SEM, HITACHI S4800). For microstructural and morphological examinations, some specimens were sectioned, mounted in resin and polished according to standard metallographic preparation techniques. Coatings for SEM inspection were sputtered with a thin conductive film of gold. The SEM is also fitted with an energy dispersive X-ray spectrometer (EDX) attachment, allowing quantitative analysis of the elemental composition to be performed on a micron scale across sections of the coatings. At a number of randomly selected locations, the scanned regions were photographed and these micrographs were subjected to analysis of micropores and their sizes. There different regions on each sample surface were analyzed. Phase composition of the coatings was analyzed by X-ray diffraction (XRD) (Philips, X'Pert, Netherlands) using Cu Ka radiation. The coating thickness and the surface roughness values were matched simultaneously for the coatings. At 5 randomly selected locations, the thickness of the
coatings was measured using an eddy-current coating thickness measurement gauge (CTG-10, Time Company). 2.3. Electrochemical test A PARSTAT 2273 Model electrochemical potentiostat was used to evaluate the electrochemical properties of the coatings. All the electrochemical measurements were conducted in a conventional three-electrode electrochemical cell with the sample as the working electrode, a platinum plate as the auxiliary electrode and a saturated calomel electrode as the reference. Both electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curve analyses (Tafel plot) were performed in Hank's solution (composed of NaCl 8 g/L, KCl 0.4 g/L, CaCl2 0.14 g/L, NaHCO3 0.35 g/ L, C6H6O6 1.0 g/L, MgCl2$6H2O 0.1 g/L, MgSO4$7H2O 0.06 g/L, KH2PO4 0.06 g/L, Na2HPO4$12H2O 0.06 g/L) 37 ± 0.5 C. The pH of Hank's solution was adjusted to 7.4 prior to the electrochemical test. A working area of 1 cm2 was exposed to the electrolyte. Before measurements, the working electrode was immersed in the electrolyte for 30 min until a steady open-circuit potential value was reached. The electrochemical impedance spectroscopy measurements were carried out at the open circuit potential (OCP) in a frequency range from 100 kHz to 0.01 Hz. The amplitude of the sinusoidal potential was 10 mV with respect to the OCP. After this first measurement, the sample impedance was measured at different exposure times to investigate the long-term stability of the coatings. Fitting of the impedance spectra were performed using the software Zview follow variations in the coating capacitance with exposure time. The potentiodynamic polarization curves (Tafel plot) were measured by polarizing the specimen to 250 mV cathodically and þ250 mV anodically relative to the OCP at a scan rate of 0.5 mV/s. The corrosion potential (Ecorr) and corrosion current density (Icorr) were determined from the polarization curves by Tafel extrapolation method. In all the above measurements, at least three similar results were considered valid and their average values were reported. 3. Results 3.1. Surface morphologies and elemental ratio of MAO coatings Fig. 1 shows the SEM images of the oxide coatings on Mg alloy substrate obtained at various oxidation time. All the MAO coatings have porous microstructures and some volcano top-like pores. The pores distributed disorderly on the coating surface. A mass of melting products around the volcano pores. As can be seen, the diameter of the micropores in the MAO coating surface increases with increasing oxidation time. As shown in Fig. 1a, after treatment for 5 min, micropores with diameter size of about 0.7 mm are fine and uniform, dispersing on the surface of the coatings. But the film was just so thin that the grinding scratches formed during sample preparation can also be observed. As shown in Fig. 1b, after treatment for 10 min, the micropores sized of approximately 1 mm, are homogenously distributed on the MAO coating surface. The surface morphology of the samples for 20 min and 40 min anodization has no noticeable difference, as can be seen from Fig. 1 (c and d). These photos show that the diameter of the micropores increases, and the number of the micropores deceases. These pores are homogenously dispersed on the coating surface. It should be noticed that two distinct characteristics are clearly identified on the surface, when the treatment time is up to 60 min. One is sintered particles accumulation around with large-sized micropores (~5 mm), and the other is the surface around numerous smaller micropores (~1 mm). The evolution of the number and size of micropores produced on the surface with increasing treatment time is presented in Fig. 2.
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Fig. 1. Surface morphologies of MAO coated samples produced at different oxidation durations (a) 5 min, (b) 10 min, (c) 20 min, (d) 40 min, and (e) 60 min.
Fig. 2. Changes in the micropores number and size of the MAO coatings measuredas a functional of treatment time. Fig. 3. Changes in the Ca, P, O and Mg content of the MAO coatings measuredas a function of treatment time.
It is obvious that micropore numbers of the MAO coating decrease with increasing coating time, while the average micropore size increases as the treatment time increased from 5 to 60 min. The results indicate that the coating time has a significant impact on the surface morphology of the MAO coating in the present electrolyte. The formation of micropores is due to the emission of the gas bubbles and molten oxide through the microarc discharge channels, which are trapped in growing coating [22,23]. The size of the pores increases with prolonging oxidation time, this result is good agreement with other publications [24,25]. The reason for the change of the pores can be ascribed to the increasing of energy density during spark discharge as the increasing oxidation time. Higher energy density induced larger discharge sparks, and form large pores [26,27]. The similar observations also have reported by Husseinet al. [28]. The EDS results indicate that the major surface constituents of the MAO coatings are Mg, Ca, P, and O elements. Fig. 3 shows the effects of coating time on the relative contents of the elements on the surface of the coatings. It can be observed that the Ca and P concentrations of the MAO coatings increase with coating time. This is in accordance with the reported fact that high energy processing employed during MAO process result in large discharges,
which can introduce more elements arising from the electrolyte into the coating [29,30]. On the contrary, the relative concentration of Mg decreases with increasing coating time. O content increases initially up to 10 min, and then decreases with increasing coating time. The presence of Ca and P in MAO coatings suggests that electrolyte species penetrate into the oxide coating during MAO process. The derivation process of O is complicated. It might come from the anions and the oxygen dissolved in the electrolyte. Mg is considered to be incorporated into the coatings from metal substrate. It might be owing to the diffusion and electrophoresis that manifested both the substrate and the electrolyte components involved in the coating formation during the MAO process [31]. 3.2. Cross-section morphologies and element distribution The cross-section SEM micrograph of the coatings processed at different coating time is shown in Fig. 4. From these figures, it is clear that the average thickness of the coatings increases with coating time. In the first 5 min of treatment, a thin uniform compact layer is formed. The coating with 10 min anodization is relatively
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Fig. 4. Cross-section morphologies of the coatings formed at different oxidation durations: (a) 5 min, (b) 10 min, (c) 20 min, (d) 40 min, and (e) 60min.
thick, and also uniform and compact. The sample coated at 20 min shows relatively smooth and homogeneous microstructure with few micropores. As treatment goes on, the coating becomes nonuniform and rough with some micropores and cracks appearing near the coating surface. Many micropores could be seen on the cross-section micrograph of the coating treated with 40 min, but these micropores and cracks are not connected to each other or perforated through the whole oxide coatings. The size of these micropores increase with increasing coating time, and many larger size cavities can be found when the sample coated at 60 min. It should be noted that there is no evident discontinuity at the interface of the coating/substrate, which implies the good bonding between the coating and the substrate. The coating thickness can also be measured from the crosssection morphologies and the thickness variation with the oxidation time is presented in Fig. 5. It can be noted that the thickness of
Fig. 5. Variation of thickness of MAO coatings measuredas a function of treatment time.
the coatings increases with coating time, but the growth rate of the coatings decreases with coating time. The coating treated for 5 min shows the formation of a 5 mm thin coating, the thickest coating (22 mm) is formed in the group treated for 60 min. In the first period (before 5 min), coatings grow linearly and quickly. The duration of the linear regime decreases as the increasing treatment time. Such decreasing growth kinetics for the coatings formation was reported by other researchers [32,33]. Indeed, according to these researchers, coatings growth results from molten magnesium which is oxidized flowing out through the discharge channels. In this way, magnesium oxide is formed, which contributes to coatings growth when being ejected from the channels and rapidly deposited at surface. The growth rate remains at a constant value as long as the spark channels reach the metal surface. Then when the coatings become thick, it is notoriously difficult to be broken down, so the growth rate decreases rapidly [34,35]. It is reported that MAO coatings can be divided into two parts based on the structure of the cross-section micrograph [36]. One part is porous outer layer, and the other part is dense inner layer. In contrast, some researchers believe that the polish process will destroy the structure of the coating. And the coating could not be separated based on the structure of the cross-section micrograph [37]. In this study, the interface between the outer part and the inner part is not certain. In order to check the structure of the coatings, the samples were bended, and then immersed into liquid nitrogen. This process is repeated three times for each sample. And the surface morphologies were observed by SEM. Fig. 6 shows the typical morphology of samples after this process. After the bending and immersing, some part of the coating was stripped. Porous and dense regions can be identified as marked in Fig. 6. Area A is part of outer layer, shows typical porous structure. While area B is located in the inner layer, displays dense and compact. This clearly demonstrates that the MAO coating can be divided into two parts, the outer porous layer and the inner dense layer. This observation is in good agreement with other publications based on the cross-section micrograph [38]. The reason for the two-layer structure can be explained as different transient temperature field in the outer layer and inner layer [39].
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Fig. 6. Typical morphology of MAO coatings after bended and immersion process.
EDS analysis of the cross-section of the MAO sample formed by 40 min is shown in Fig. 7. The coating can be roughly divided into two regions, the interface region and the outer region. Oxygen extremely increases in the interface region, and magnesium rapidly decreases. Ca and P increase gradually along the coating thickness. The highest value appears near the surface of the coating, while the composition of aluminium changes slightly in the coating. It should be noticed that calcium and phosphorus is also detected in the interface region, indicating that electrolyte components can reach the inner part of the coating, and finally be absorbed in to the coating. 3.3. Phase and chemical composition of the coatings Surface XRD patterns of MAO coatings produced at different
Fig. 7. Element line analysis of MAO coating formed at 40 min.
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duration are given in Fig. 8. Magnesium, magnesium oxide and hydroxyapatite phases are achieved in the MAO coatings by XRD analysis. Similar broad range patterns are generated for all the samples. The spectra showed only reflections associated to hydroxyapatite. The coating composition consists of magnesium oxide as a major phase, and hydroxyapatite as a minor phase. The intensity of magnesium significantly decreases with increasing duration time. The intensity of magnesium oxide and hydroxyapatite increases with increasing duration time. Up to 60 min, hydroxyapatite is found in the coating structure. The crystallinity of the coating during MAO is also correlated to the intensity of discharge. For longer growth time, higher energy required for breakdown of the coating, and higher intensity discharge can be observed on the surface of magnesium during MAO process. This causes a higher temperature in the MAO process, which favors a better crystal of hydroxyapatite. The potentiodynamic polarization curves of the MAO coatings are shown in Fig. 9, and the curve for uncoated Mg alloy is included as a reference. Parameters of the corrosion test such as corrosion potential (Ecorr) and corrosion current density (Icorr) of the specimens have been evaluated by Tafel extrapolation at the linear stage of the anodic and cathodic curves and are summarized in Table 1. It can be seen from Table 1 that all the MAO coated samples exhibit smaller Ecorr in magnitude than the uncoated AZ31 alloy. A smaller Ecorr magnitude in all coated samples suggests that they are less reactive than the AZ31 magnesium alloy. The corrosion current density of the samples with MAO coatings are 1e2 orders lower than that of the magnesium substrate, which indicates that the corrosion resistance of magnesium alloy has been greatly improved by MAO treatment. The corrosion current density (Icorr) decreases with MAO time up to 40 min and then it increases with increasing coating treatment time. The MAO specimen treated for 40 min exhibits the lowest Icorr, which suggests the best corrosion resistance among all the coatings. It is interesting to note that thicker layer of the 60 min treatment shows less protection than the layer after 40 min treatment. This indicates that thicker coating does not definitely mean better corrosion resistance. Such a phenomenon may be due to the variation of structure of the coatings. Increasing treatment time leads to a loose structure [40,41]. Based on microstructure observation, electrochemical tests are usually used to investigate the relationship between the coating microstructure and the corrosion behavior of the MAO coatings. Among the electrochemical methods, electrochemical impedance spectroscopy (EIS) as a non-destructive testing, does not affect the surface state of the samples and is commonly used to provide information about corrosion mechanism. Numerous studies have been made to evaluate the corrosion behavior of the MAO coatings by EIS [42e44]. However, the long-term degradation mechanism of the MAO coated Mg alloys in Hank's solution is not yet understood. In order to investigate the corrosion mechanism of the MAO coatings, the sample coated with 40 min was immersed in Hank's solution for 48 days. Fig. 10 shows the results of the MAO sample obtained after 0.5, 3, 6, 12, 24 and 36 h of exposures. Bode-phase plots in 36 h immersion displayed two distinct parts. One capacitive loop in the mediate frequency range, and one straight line in the low frequency range, represents the electrochemical response of inner layer and outer layer in the coating, respectively. According to the EIS plots, an obvious changes can be observed due to prolonging immersion time. The maximum phase in the plots decreases with increasing immersion time, and the corresponding frequency shifts to higher with prolonging immersion time, which indicates the deterioration of the MAO coating [45,46]. As shown in Fig. 10b, it can be seen that the value of impedance in low frequency and mediate frequency range show a prominent and well-regulated evolution with immersion time, which illustrates clearly the
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Fig. 8. XRD patterns of the MAO coatings: (a) 5 min, (b) 10 min, (c) 20 min, (d) 40 min, and (e) 60 min.
Fig. 9. Polarization curves of bare magnesium and the MAO coatings.
Table 1 Electrochemical data from the polarization test of different samples in Hank's solution. Samples
Ecorr(V)
Icorr (A/cm2)
Sub 5 min 10 min 20 min 40 min 60 min
1.45 0.98 0.88 0.79 0.76 0.72
1.05 5.67 1.71 9.48 6.93 8.75
105 106 106 107 107 107
deterioration process of the MAO coating in immersion process [47]. The impedance response indicates that the MAO coating is gradually penetrated by the corrosive solution during immersion. Fig. 10c shows that the diameter of the circles decreases with increasing immersion time, which is in good agreement with the Bode-impedance plots. It should be noted that the EIS spectra has similar shape in immersion 36 h, indicating that they have same corrosion mechanism [48]. Owing to this characteristic, immersion in 36 h is determined as early immersion stage. Taking into account the microstructural characteristics and the
EIS behaviors of the MAO coating sample, the corresponding equivalent circuits for fitting the EIS data is proposed in Fig. 11. The equivalent circuit is established based on a reasonable fitting of experimental values and the special structure of this MAO coating. Rs is solution resistance of bulk electrolyte between reference and working electrodes. Rs has nothing to do with electrode process and its value depends primarily on the testing medium and the geometry of employ cell. A parallel R and CPE network, accounting for the global barrier properties of the MAO coating, which is accounting for the resistance of the corrosive solution inside the pores and cracks of the coating. It can be seen from Fig. 6 that the MAO coating is composed by outer porous layer and inner dense layer. The corrosive ion is easy to penetrate into the outer porous layer through the pores. So the corrosion resistance of the whole MAO coating would mostly be determined by the inner barrier layer [49,50]. An additional Warburg element (Zw) at low frequency region, associated with the diffusion of corrosive ions across the inner layer because of the presence of concentration gradient in the inner layer during immersion process. This equivalent circuit model reveals the electrochemical corrosion behavior of the coated specimen in an early stage of immersion, which is depended on the combination of kinetic and diffusion processes [51,52]. Fig. 12 shows variation of the resistance with time in the early immersion stage. It can be seen that the resistance decreases with increasing immersion time, which is consistent with Bodeimpedance results. It should be noted that the impedance decline occurred mainly at low frequency and mediate frequency ranges, corresponding to the degradation of inner layer itself. When the MAO coated specimen was immersed in Hank's solution, the corrosive solution would penetrate from the outer porous layer, and reach the interface of porous layer and inner barrier layer. The infiltration process of electrolyte into barrier layer was a slow diffusion reaction for its much compact structure during the early immersion stage. This behavior evoked the diffusion characteristic in EIS spectra. In the early stage of immersion stage (0.5e36 h), the equivalent circuit included one pair of resistance and one pair of diffusion. This means that the corrosive medium is not contact with the substrate Mg alloy because of the shorter period of immersion. As time increases, the corrosive ions dissolves into the inner layer, enlarges the structural imperfection of inner layer, which increases the effective surface area immersed in corrosive medium. Consequently, the corrosion resistance of the sample with MAO coating decreases.
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Fig. 10. Changes of the EIS plots as a functional of immersion time in early immersion time: (a) Bode-phase, (b) Bode-resistance, and (c) Nyquist.
Fig. 11. Electrical equivalent circuit is used for fitting the experimental EIS spectra in early immersion time.
Fig. 12. Corrosion resistance of the coating during early immersion time.
The EIS spectra of the MAO coating immersed for 2 and 24 days is shown in Fig. 13. EIS make changes after immersion for 2 days. It can be seen that two maximum phase lags appear at medium frequency and low frequency. Which correspond to two time constant. Interestingly, it is observed that not only maximum phases in all spectra decrease with increasing immersion time, but also they are always shifting to higher frequency. The impedance modulus Bode diagrams reveal three segments, one arc in low frequency region, another arc in high-medium frequency, and one linear in high frequency. The resistance in low frequency and mediate frequency decreases with increasing immersion time. Two circles can be observed in Nyquist plots, and the diameter of the circles decrease with increasing immersion time. Due to this characteristic, immersion for 2 and 24 days, is determines as a middle immersion stage. To enable an accurate analysis of the impedance diagram, the equivalent circuit model is performed, as shown in Fig. 13d. The
equivalent circuit consists of two time constants namely CPEf with a resistor Rf in parallel, corresponding to the medium frequency characteristics of the coating, and CPEct with a resistor Rct in parallel, corresponding to the low frequency characteristics of charge transfer resistance and double-layer capacitance of the electrode. A Warburg element (Zw) was also included to accurately fit the data. This Warburg element suggested partial control of corrosion by diffusion electrolyte within the MAO coating. As illustrated in Fig. 14, the coating resistance Rf and charge transfer resistance Rct decrease with increasing immersion time. This kind of tendency is linked up with the intrusion of corrosion media and indicates that the perfection of MAO coating and its corrosion protection deteriorate while exposure duration prolonging. The charge transfer resistance Rct is higher than the corresponding value of coating resistance Rf. This indicates that the interface layer between the barrier layer and the substrate is promising for protecting the Mg alloy from corrosion during the immersion. As for immersion in two days, contact area is small although corrosive solution passes through the barrier layer and reaches the substrate. So the value of charge transfer resistance Rct is much higher than that of coating resistance. Once corrosion media contact with substrate, the protecting layer starts to break down. Consequently, the fresh reactive surface is exposed to the corrosion medium and corrosion accelerates, which results in a smaller of Rct value. The equivalent circuit model and change of the corrosion resistance reveal that the electrochemical corrosion behavior in middle stage of immersion is controlled by kinetic process [53]. Representative EIS of MAO coated Mg alloy exposed to Hank's solution after immersion over 30 days are shown in Fig. 15. It is clearly shown that the Bode-phase plots reveal two segments. In the low frequency region, a linear relationship can be observed, corresponding to the diffusion process during the reaction with the substrate. One phase angle extreme in bode plot means that the corresponding electrode process contains one time constant. With increasing immersion time, the maximum phase of the plots decreases, and the corresponding frequency shifts to higher frequency. Which illustrates that the corrosion resistance of the coated sample decreases with prolonging immersion time. The Bode-impedance reveals two segments in the high-medium frequency region, a plateau can be observed between the absolute impedance. This frequency region corresponds to a pure resistor behavior of the MAO coating. As immersion time increased (from 30 to 48 days, Fig. 15c), the radii of the semicircular in Nyquist diagrams decreases with prolonging immersion time. Appearance of new EIS shape implies the sub-process has been changed. In that case, it is necessary to put forward new equivalent circuit to explain what happened. An equivalent circuit shown in Fig. 15d is used to fit the EIS plots. In this equivalent circuit, Rs is the solution resistance,
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Fig. 13. EIS plots and electrical equivalent circuit for immersed during middle immersion stage: (a) Bode-phase, (b) Bode-resistance, (c) Nyquist, and (d) equivalent circuit model.
Fig. 14. Changes of the coating resistance Rf and charge transfer resistance during middle immersion stage.
CPEct and Rt is the double-layer capacitance and charge transfer resistance. Warburg element (Zw) indicates the diffusion processes occur on the surface of the electrochemically active site. The element (CPEct (Rc Zw) is used to describe the diffusion process of water molecules and electrochemical reactions at the substrate/ MAO coating interface. Owing to this different characteristic of EIS spectra immersed between 30 and 48 days, this immersion duration is noted as late immersion stage. The equivalent circuit model reveals that the electrochemical corrosion behavior is controlled by the combination of kinetic and diffusion process in late immersion stage. The corrosion resistance values of the coated sample as a function of immersion time are presented in Fig. 16. As
demonstrated in the bar chart, the charge transfer resistance of the coated sample decreases with increasing immersion time during late immersion stage, which indicates that the reactive area of the substrate increases with prolonging immersion time. Based on the above description of corrosion process, the corrosion mechanism of HA containing ceramic coated on Mg alloy can be schematically illustrated in Fig. 17. The HA containing ceramic coating is composed of two layers: the outer porous layer and the inner dense layer. The outer porous layer has many deep pores. In contrast, the inner barrier layer is dense and there is no pore inside. Once the MAO coated sample was immersed into Hank's solution (within 0.5 h), the corrosive electrolyte was absorbed into the outer layer through the pores. The corrosive
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Fig. 15. EIS plots and electrical equivalent circuit for late immersion stage: (a) Bode-phase, (b) Bode-resistance, (c) Nyquist, and (d) equivalent circuit model.
Fig. 16. Changes of corrosion resistance during late immersion stage.
electrolyte passed through the outer layer, reached the interface between the outer layer and inner barrier layer. With prolonged immersion time (0.5e36 h), corrosive solution gradually penetrated into the inner compact layer until corrosive solution passed through the inner barrier layer, and reached inner layer/substrate interface. At this immersion stage, the corrosion resistance was found to decrease with increasing immersion time, which was ascribed to the degradation of the inner barrier layer. Because MgO phase in this MAO coating is not stable in Hank's solution and is prone to form Mg(OH)2 [54,55]. The degradation of MgO in the coating, could lead to the formation of defects in the compact layer. So this stage could be characterized as the deterioration of barrier
layer stage. With the extension of immersion time (2e24 days), the inner barrier layer was saturated by the corrosive solution, and the weak area in inner barrier layer was broken. Substrate started to be exposed in the corrosive solution. Once the corrosive solution contacted with Mg substrate, the substrate would be corroded quickly, due to the fact that magnesium is highly chemical active in Hank's solution. At the same time, due to the protection of the MAO coating, the undamaged area of the coating could still effectively protect the substrate. Meanwhile, owing to the aggression of corrosive solution, the defects in the coating was enlarged. So the protection of the coating became deteriorated further with prolonging immersion time. This stage could be characterized as the deterioration of the coating and the substrate. With the immersion time over 30 days, the resistance of the coating was found to decrease obviously. The corrosion resistance of the coating was far smaller than that of the charge transfer resistance. The segment corresponding to electrochemistry of the coating was completely overwhelmed by the characteristics of charge transfer, so only one time constant could be observed in EIS plots. In this stage, the compact structure of the coating was destroyed and many throughgoing channels were formed, and the coating lost the protective properties. So this stage could be characterized as the deterioration of the substrate. 4. Conclusions In this study, HA containing coating are fabricated by MAO technique on AZ31 Mg alloy at various treatment time. The coating is composing of outer porous layer and inner barrier layer, can improve corrosion resistance of the substrate. The treatment time has a significant influence on the microstructure, composition, mechanical and corrosion properties. The potentiodynamic
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Fig. 17. Schematic diagram of the corrosion process and mechanism of MAO coated AZ31 Mg alloy upon immersion in the Hank's solution: (a) early immersion stage, (b) middle immersion stage, and (c) late immersion stage.
polarization shows that the MAO coating treated at 40 min has the lowest corrosion current density and the best corrosion resistance. A model for corrosion mechanism of HA containing MAO coating in Hank's solution is suggested. The main characteristic in early immersion stage is barrier layer deterioration, which is controlled by the combination of kinetic and diffusion processes. The main characteristic in middle immersion stage is the coating and the substrate deterioration, which is controlled by kinetic process. The main characteristic in late immersion stage is substrate deterioration, which is controlled by the combination of kinetic and diffusion process. Conflict of interest There is no conflict of interest. Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant No.51402045) and the Fundamental Research Funds for the Central Universities (No.ZYGX2014J089). References [1] J. Hofstetter, E. Martinelli, A.M. Weinberg, M. Becker, B. Mingler, € ffler, Assessing the degradation performance of P.J. Uggowitzer, J.F. Lo ultrahigh-purity magnesium in vitro and in vivo, Corros. Sci. 91 (2015) 29e36. [2] H. Hornberger, S. Virtanen, A.R. Boccaccini, Biomedical coatings on magnesium alloys - a review, Acta Biomater. 8 (2012) 2442e2455. [3] J.W. Lee, H.S. Han, K.J. Han, J. Park, H. Jeon, M.R. Ok, H.K. Seok, J.P. Ahn, K.E. Lee, D.H. Lee, S.J. Yang, S.Y. Cho, P.R. Cha, H. Kwon, T.H. Nam, J.H.L. Han, H.J. Rho, K.S. Lee, Y.C. Kim, D. Mantovani, Long-term clinical study and multi scale analysis of in vivo biodegradation mechanism of Mg alloy, Pnas 113 (2016) 717e721. [4] R. Rojaee, M. Fathi, K. Raeissi, A. Sharifnabi, Biodegradation assessment of nanostructured fluoridated hydroxyapatite coatings on biomedical grade magnesium alloy, Ceram. Int. 40 (2014) 15149e15158. [5] S. Virtanen, Biodegradable Mg and Mg alloys: corrosion and biocompatibility, Mater. Sci. Eng. B 176 (2011) 1600e1680. [6] G.L. Song, Control of biodegradation of biocompatible magnesium alloys, Corros. Sci. 49 (2007) 1696e1701. [7] S.V. Dorozhkin, Calcium orthophosphate coatings on magnesium and its biodegradable alloys, Acta Biomater. 10 (2014) 2919e2934. [8] S.V. Dorozhkin, Calcium orthophosphate bioceramics, Ceram. Int. 41 (2015) 13913e13966. [9] S.V. Dorozhkin, Multiphasic calcium orthophosphate (CaPO4) bioceramics and their biomedical applications, Ceram. Int. 42 (2016) 6529e6554. [10] J. Hu, C. Wang, W.C. Ren, S. Zhang, F. Liu, Microstructure evolution and corrosion mechanism of dicalcium phosphate dihydrate coating on magnesium alloy in simulated body fluid, Mater. Chem. Phys. 119 (2010) 294e298. [11] R. Rojaee, M. Fathi, K. Raeissi, M. Taherian, Electrophoretic deposition of bioactive glass nanopowders on magnesium based alloy for biomedical applications, Ceram. Int. 40 (2014) 7879e7888. [12] X. Zhang, X.W. Li, J.G. Li, X.D. Sun, Preparation and characterizations of bioglass ceramic Cement/Ca-P coating on pure magnesium for biomedical applications, Appl. Mater. Interfaces 6 (2014) 513e525.
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