Influence of bicarbonate concentration on the conversion layer formation onto AZ31 magnesium alloy and its electrochemical corrosion behaviour in simulated body fluid

Abstract

The electrochemical corrosion behaviour of a magnesium carbonate conversion layer-coated AZ31 magnesium alloy was evaluated in simulated body fluid (SBF) solution. Bicarbonate solution was used to produce conversion layers and its concentration significantly influenced the conversion layer formation and electrochemical corrosion behaviour. Formation of a MgCO₃ conversion layer with Mg(OH)₂ onto AZ31 Mg alloy was identified by X-ray diffraction (XRD) and attenuated total reflectance infrared (ATR-IR) spectroscopy studies. The surface morphology and chemical composition of the conversion layer-coated AZ31 Mg alloys were analysed using scanning electron microscopy (SEM) attached to energy dispersive X-ray analysis (EDAX). Formation of a mud-crack pattern was ensured from the SEM morphologies. Potentiodynamic polarization, linear polarization, potentiostatic polarization and electrochemical impedance spectroscopy (EIS) techniques were used to study the electrochemical corrosion behaviour. An increase in the bicarbonate concentration up to 5 wt% resulted in about a 9 times reduction in the corrosion current density (i_corr) value. However, further increase of the bicarbonate concentration increased the i_corr values and thus the corrosion rate. EIS studies further supported the polarization results. The increase in the conversion layer and charge transfer resistance was evidenced when the concentration of bicarbonate is increased up to 5 wt%. A further increase in the bicarbonate concentration resulted in a decreasing resistance up to 9 wt%. These observations were confirmed from the equivalent circuit curve-fitting analysis. The surface morphology and compositional changes of the conversion layers altered the corrosion resistance of the AZ31 Mg alloy in SBF solution.

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1. Introduction

Development of magnesium (Mg) and its alloys as biodegradable implant materials has gained increased attention in recent years.^1–4 Due to their similarity to natural bone in terms of their mechanical properties, Mg alloys are preferred as degradable bone implant material.^5,6 However, the rapid degradation rate of Mg alloys is one of the major drawbacks for using these materials as successful implants.^7,8 Due to its more negative electrode potential, Mg is chemically very active and forms Mg ions when exposed to aqueous solutions. Several alloying elements are added to Mg for developing alloys that can stabilize the Mg phase and also control the rapid degradation rate.^9–11 The addition of small amounts of Al to develop the AZ31 Mg alloy ensures the presence of only one phase in the alloy, which increases the corrosion resistance of Mg by stabilizing the passive layer.^12,13 However, addition of large amounts of Al induces dementia by combining with inorganic phosphates, leading to a lack of phosphate in the human body.¹⁴ Alloying Mg with Zn significantly increases the corrosion resistance and retards the biodegradation rate of these alloys.

Although Mg alloys are known for their improved characteristics, careful selection of alloying elements is often important in order to avoid the adverse effects caused by them when implanted in the body.^15,16 Surface modification could be one of the possible methodologies to control the Mg degradation rate in physiological environments.^17,18 Sol–gel-synthesized ceramic coatings,^19,20 plasma electrolytic oxidation,²¹ polymer coatings,^22,23 nitriding and ion implantation,^24,25 chemical conversion coatings,^26–28 biomimetic coatings^29,30 etc., are commonly used surface modification methodologies for biomedical applications. Among them, chemical conversion coating is one of the easiest and most cost-effective methods. Besides, it does not require any sophisticated instrumentation facilities.³¹

A cerium conversion coating developed onto AZ31 Mg alloy resulted in the formation of compact coatings with tiny and thick cracks consisting of CeO₂, CeO, Ce₂O₃, MgO, Mg(OH)₂ and Al₂O₃ compounds which significantly improved the corrosion resistance in physiological solution.³² Formation of a Ca–P-based apatite layer was also detected for the samples soaked in the SBF solution for 14 days. Fluoride conversion coatings were developed onto pure Mg by immersing the samples in 48 wt% HF solution at room temperature for different periods from 6 to 24 h.³³ Formation of tetragonal MgF₂ with a crystallite size of several nanometres was confirmed from TF-XRD studies. Fluoride conversion coatings have recently been developed onto pure Mg by a different methodology which resulted in the variation of the F/O ratio.³⁴ As the F/O ratio is increased, the corrosion resistance of these coatings increased in Hank’s balanced salt solution. Formation of a MgCO₃ conversion layer onto pure Mg controlled its degradation rate in Hank’s solution.³⁵ A MgCO₃ conversion coating was formed using saturated NaHCO₃. Coatings prepared at the solution pH of 8.3 exhibited better corrosion resistance than those at pH 11.9. It was also stated that the fraction of carbonate (HCO₃⁻) ions is believed to be important to form protective coatings. The corrosion behaviour of alkaline heat-treated Mg–Ca alloys was reported by Gu et al.³⁶ Here, conversion coatings were produced by heating the samples in alkaline solution at 500 °C for 24 h. The corrosion rate of the Mg–Ca alloy was effectively decreased after alkaline heat treatment and did not induce any cytotoxicity to L-929 cells. NaHCO₃–MgCO₃ heat-treated pure Mg exhibited enhanced corrosion resistance in SBF solution.³⁷ No inhibitory effects on marrow cell growth and no cellular lysis was observed. The effect of NaHCO₃ treatment time on the corrosion behaviour of AZ61 and AZ31 in 0.6 M NaCl solution was studied by Feliu Jr et al.³⁸ It was reported that the formation of a uniform conversion coating with a significant increase in the Al oxide/hydroxide content for AZ61 alloy with a lower treatment time (10 and 60 min) could improve the corrosion resistance. In addition, the corrosion resistance varied in AZ31 and AZ61 alloys as a function of treatment and exposure time to 0.6 M NaCl solution. Lia et al. recently investigated the influence of bicarbonate concentration on the Mg degradation rate in SBF solution.³⁹ Significant variations in the electrochemical corrosion behaviour in SBF solution were noticed with varying the bicarbonate concentration. In particular, the degradation rate of Mg was less when the bicarbonate concentration did not exceed 27 mmol L⁻¹. These findings state that the bicarbonate present in SBF solution plays a crucial role in altering the Mg degradation rate. Therefore, deliberate conversion of Mg into its carbonates by soaking in bicarbonate solution prior to corrosion studies in SBF solution could alter the electrochemical corrosion behaviour. Systematic studies on the development of bicarbonate conversion coatings onto Mg alloys are limited and therefore need to be investigated. The present investigation is mainly focused on the development of magnesium carbonate conversion coatings onto AZ31 Mg alloy by varying the bicarbonate concentration. The influence of the bicarbonate concentration on the electrochemical corrosion behaviour was studied using potentiodynamic polarization, potentiostatic polarization and EIS studies. The obtained corrosion results substantiated the surface morphology and chemical compositional variation on the surface. A plausible corrosion mechanism to explain the variation of corrosion behaviour was proposed.

2. Experimental

2.1 Development of conversion coatings

AZ31 Mg alloy (2.83 wt% Al, 0.8 wt% Zn, 0.37 wt% Mn, and balance Mg) was used as substrate material in the present investigation. Samples were abraded with silicon carbide (SiC) emery sheets up to 2000# prior to carrying out the conversion layer. The abraded samples were then ultrasonicated in acetone for about 15 min, thoroughly washed with double-distilled (DD) water and air-dried. Conversion layers onto AZ31 Mg alloy were prepared by immersing the samples in aqueous NaHCO₃ solution (1, 3, 5, 7 and 9 wt/vol%) for 6 h in a polypropylene tube at 30 °C. After 6 h, the samples were thoroughly washed with DD water to wash away the unreacted NaHCO₃ adsorbed onto the surface. The conversion layer-coated samples were then ultrasonicated in DD water until the loosely bound reaction products existing on the surface were removed. The samples were then air-dried, stored in a vacuum chamber and used for further characterizations. The details of the NaHCO₃ concentration and the sample codes are given in Table 1. All the chemicals used in the present investigation are of AR grade.

Table 1 Bicarbonate concentration used to produce conversion layers onto AZ31 Mg alloy and sample codes

S. no.	Concentration of NaHCO3 (wt%)	Sample code
1	1.0	BCTM-1
2	3.0	BCTM-3
3	5.0	BCTM-5
4	7.0	BCTM-7
5	9.0	BCTM-9

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2.2 Surface characterizations

XRD studies of the conversion layer-coated samples were carried out using a Bruker D8 Advance instrument with Cu Kα radiation, 40 kV, 40 mA and a 2° min⁻¹ scan rate. Attenuated total reflectance infrared (ATR-IR) spectral analysis was performed with a Perkin Elmer spectrum-two spectrometer in the frequency range of 400–4000 cm⁻¹. JEOL-JSM 6360 scanning electron microscopy (SEM) attached with an INCA X-Sight Oxford Instruments EDAX analyser was used to study the surface morphology and obtain the surface chemical composition respectively. The samples were sputter-coated with Pt in order to avoid the charging effect during SEM and EDAX. SEM and EDAX was then carried out at an accelerating voltage of 20 kV with the working distance of 10 mm. EDAX spot analysis was done at three different places to ensure the composition and an average of three values was presented. Cross-sectional SEM images were obtained to study the cross-sectional coating morphology. For these studies, the samples were mounted in to the cold setting resin and cured at ambient temperature for about 12 h in vacuum desiccators. The samples were then successively abraded up to 2000# SiC emery sheets followed by mirror polishing using 0.05 μm Al₂O₃ powder, thoroughly washed with ethanol, air-dried and stored in desiccators. Prior to the cross-sectional SEM morphological observation, the samples were sputter-coated with Pt. The cross-sectional surface morphology of the samples was observed using SEM with back-scattered electron mode at an accelerating voltage of 20 kV.

2.3 Electrochemical characterizations

Electrochemical studies were carried out using a conventional three-electrode flat cell. Conversion layer-coated AZ31 Mg was used as the working electrode (1 cm²), while a saturated calomel electrode (SCE) and Pt sheet were used as the reference and counter electrodes, respectively. SBF solution was used as a physiological medium (pH ∼ 7.4 at 37 °C) and its composition can be found elsewhere.⁴⁰ The solution was renewed for each experiment and all the experiments were carried out at similar conditions. Auto lab PGSTAT 12 Potentiostat/Galvanostat, The Netherlands was used for linear, potentiodynamic and potentiostatic polarization and also for EIS studies. Linear polarization studies were carried out in the potential range open-circuit potential (OCP) ± 10 mV to find out the polarization resistance (R_p) values. A potentiodynamic polarization study was carried out from OCP − 0.050 V to several mV after the breakdown potential and until attainment of the maximum anodic current density of about 1 mA at a scan rate of 1 mV s⁻¹. Potentiostatic polarization studies were carried out in the potential range from −1.65 to −1.62 V_SCE, and each potential was incremented by 30 mV for 600 s. The change in current response was represented as a function of time. EIS measurements were performed at OCP over the frequency range from 10⁵ Hz to 10⁻² Hz with a sinusoidal perturbation of ±10 mV. ZSimpwin v3.40 software was used for curve-fitting analysis of the obtained impedance results. Duplicate experiments were also performed to obtain consistent results.

3. Results and discussion

3.1 X-ray diffraction studies

XRD patterns of the formed conversion layers onto AZ31 Mg are shown in Fig. 1. Existence of high intense peaks is attributed to the substrate Mg. Whereas the presence of peaks at 22.5°, 24.25°, 32.29°, 34.5° and 57.57° are attributed to the 002, 211, 212, 210, 004 and 314 planes of MgCO₃·3H₂O. These are in good agreement with the reported data (JCPDS 20-0669, JCPDS-70-1433) and were also reported in our previous report.^41–43 The peak at 42.8° corresponds to the 200 plane of MgO (JCPDS 87-0651). Low intensity peaks corresponding to MgCO₃ indicate the formation of a thin conversion layer onto the surface.³⁵ In addition, no significant change was observed in the peak intensities of MgCO₃ in the XRD pattern with varying bicarbonate concentration, which further confirmed the existence of a thin film and does not produce any significant difference in the phase composition.

Fig. 1 XRD patterns of the conversion layer formed onto AZ31 Mg alloy with varying bicarbonate concentrations. Symbols denote the peaks of () Mg, () MgCO₃·3H₂O, () Mg(OH)₂ and () MgO.

3.2 Attenuated total reflectance infrared (ATR-IR) spectroscopy studies

Fig. 2 shows the ATR-IR spectra of the formed conversion layers. Four major bands appearing around 1490–1380, 1060–1000, 865–860 and 620 cm⁻¹ are mainly attributed to the carbonate (–CO₃²⁻) group.^44,45 These peaks further indicate the normal to ν₁–ν₄ vibrations of carbonate ions. Peaks appearing around 1490–1380 cm⁻¹ and 1060–1000 cm⁻¹ pertain to ν₃, CO asymmetric stretching, and ν₁, CO symmetric stretching, respectively. The bands appearing around 865–860 and 620 cm⁻¹ are attributed to ν₂, –CO₃ out-of-plane deformation, and ν₄, OCO in-plane deformation, respectively.⁴⁶ In addition to these peaks, characteristic absorption stretching and bending vibrations for the –O–H group of M–OH/adsorbed water appear around 3260 and 1646 cm⁻¹ respectively. A band appearing around 450 cm⁻¹ is mainly due to the stretching bands of Mg–O.⁴⁷ ATR-IR studies further confirm the formation of a MgCO₃ conversion layer onto AZ31 Mg treated with bicarbonate solution.

Fig. 2 ATR-IR spectra of AZ31 Mg alloy treated with varying bicarbonate concentration.

3.3 Surface morphology and chemical composition analysis

The surface morphologies with varying bicarbonate concentrations of the formed conversion layers are shown in Fig. 3. From Fig. 3, it is seen that the variation in bicarbonate concentration has produced considerable change in the surface morphology. After 6 h of treatment time, the surface was covered with cracked dry mud layers invariably with all bicarbonate concentrations. Formation of these cracks could be due to the release of H₂ during the reaction of bicarbonate with Mg to form the MgCO₃ conversion layer or the dehydration of the surface layer after treatment.³⁸ However, when the bicarbonate concentration is increased up to 5%, the coating was relatively compact and covered almost the entire surface. A further increase in the bicarbonate concentration resulted in the detachment of the conversion layer from the surface and the grain boundaries were well distinguished. This could be due to the increase of local stress in the grain boundary region thereby producing defective coatings. Cross-sectional morphologies of all the conversion layer-coated samples are given as insets in Fig. 3 and the average conversion coating layer thickness values have been included for better understanding. The conversion layers formed in the case of BCTM-3 and BCTM-5 are uniform with a thickness of ∼4 to 5 μm and consist of micro cracks. Furthermore, no visual detachment of the conversion layer from the substrate was noticed. In the case of BCTM-1, the thickness was about 1 μm and exhibited several micro cracks. BCTM-7 and BCTM-9 exhibited a relatively thick layer with visible cracks compared to BCTM-1. Detachment of the coating layer is also visible from the cross-sectional observations.

Fig. 3 Surface and cross-sectional morphologies of AZ31 Mg alloy treated with varying bicarbonate concentration. (a) BCTM-1, (b) BCTM-3, (c) BCTM-5, (d) BCTM-7 and (e) BCTM-9.

The surface chemical composition of the conversion layer-coated alloy was analyzed over the cracked outer as well as at the inner surface. The average chemical compositions of all the conversion coating layers are compared (Fig. 4a and b). Al enrichment was identified from the composition analysis of the outer conversion surface. The amount of Al also increased when the bicarbonate concentration increased up to 5 wt%. However, the increasing trend did not continue for a further increase in the bicarbonate concentration. Interestingly, with the increase in the concentration of C in BCTM-5, a marginal decrease in the O content was recorded. These results indicated the increase of MgCO₃ content in the conversion layer. Careful analysis of the surface composition at the inner surface also revealed an increase of the C and O content and a decrease of Mg. This further substantiates that the conversion of Mg into MgCO₃ occurred even at the inner surface of the coatings (Fig. 4b). Line scanning was performed across the conversion layer formed onto BCTM-5 and the amount of Mg was significantly reduced while that of O increased in the cracked conversion layer. Enrichment of Al was also seen in the cracked layer. Furthermore, the amount of Na and C was also high on the conversion layer. These observations further confirmed the formation of a conversion layer consisting of MgCO₃. The presence of a higher amount of Mg at the inner layer is mainly due to substrate Mg.

Fig. 4 EDAX average chemical composition of AZ31 Mg treated with varying bicarbonate concentration (a) on the cracked conversion layer (sites 1 and 2 at SEM) and (b) on the inner layer (sites 3 and 4 at SEM).

3.4 Electrochemical studies

3.4.1 Potentiodynamic, linear and potentiostatic polarization studies. Potentiodynamic polarization curves of the conversion layer formed onto AZ31 Mg alloy in SBF solution are shown in Fig. 5. The electrochemical parameters viz. corrosion current density (i_corr), corrosion potential (E_corr) and polarization resistance (R_p) values derived from the polarization curves are plotted in Fig. 6a–c. The polarization curves significantly varied as a function of the bicarbonate concentration. During the anodic potential sweep, the current density was reduced from BCTM-1 to BCTM-5, and further increased for BCTM-7 and BCTM-9 (Fig. 5). A rapid increase in current density at the anodic region is attributed to the breakdown of the passive film and further attack by aggressive ions present in the solution. The potential at which the current density rapidly increased at the anodic region is known as the breakdown potential (E_b). The E_b value was shifted to a more positive potential for BCTM-5 and shifted to a negative potential for BCTM-7 and BCTM-9. Breakdown of the conversion layer was altered by the concentration of bicarbonate used for producing the conversion layer. A decrease in the anodic current density and shift of E_b to a positive potential support the slow kinetics of Mg dissolution and enhanced corrosion resistance of the conversion layer-coated AZ31 Mg. It has been previously reported that a hydrothermally-formed hexagonal Mg(OH)₂ conversion coating onto pure Mg exhibited a positive shift of E_corr and lower i_corr values in phosphate buffered saline (PBS) solution.⁴⁸ These findings further support the results obtained in the present investigation.

Fig. 5 Potentiodynamic polarization curves of the conversion layer formed onto AZ31 Mg alloy in SBF solution.

Fig. 6 Comparison of electrochemical parameters derived from potentiodynamic polarization curves.

The corrosion current density (i_corr) values were significantly reduced by increasing the bicarbonate concentration to 5% and a further increase resulted in an increase of the i_corr values (Fig. 6a). BCTM-1 to BCTM-9 exhibited lower i_corr values compared to uncoated AZ31 Mg alloy.²³ In particular, the i_corr value of BCTM-5 (8 μA cm⁻²) was about 9 times lower than that of the uncoated AZ31 Mg (70 μA cm⁻²) in SBF solution. These results clearly indicated the effective corrosion protection provided by the MgCO₃ layer formed on the surface, which can be explained based on the surface morphologies. As seen in Fig. 3c, BCTM-5 exhibited a relatively uniform layer and almost covered the entire surface, whereas BCTM-7 and BCTM-9 had defects in the conversion layer and these defects facilitated direct contact of the electrolyte with the substrate and accelerated Mg dissolution. Yan et al. observed the reduction in i_corr values of fluoride conversion layer-coated AZ31 B alloy in SBF and the reduction in i_corr values was attributed to the coating thickness. Furthermore, longer treatment time in hydrofluoric acid solution could produce a thicker film with fewer pores, resulting in better corrosion protection.⁴⁹

Polarization resistance (R_p) values of BCTM-1 to BCTM-9 were obtained from linear polarization and are also compared in Fig. 6b. The R_p value of BCTM-5 was about 4.7 kΩ cm², which is about 2 and 3.5 times higher than that of BCTM-3 (2.3 kΩ cm²) and BCTM-1 (1.4 kΩ cm²) respectively. BCTM-7 and BCTM-9 showed R_p values of 1.9 and 1.7 kΩ cm² respectively. Corrosion rates were calculated according to ASTM G 102-89 (ref. 50) and the formula used to calculate the corrosion rate is given below:

corrosion rate (CR) = K₁ × i_corr/ρ × EW mm per year

where K₁ = 3.27 × 10⁻³ mm g per μA per cm per year, i_corr = corrosion current density (μA cm⁻²), ρ = density in g cm⁻³, and EW = equivalent weight.

A comparison was made between the corrosion rate and i_corr values and is given in Fig. 6c. It is observed that the corrosion rate of BCTM-5 is about 0.18 mm per year and was lower among all the coatings. The corrosion rate of BCTM-7 and BCTM-9 increased compared to that of BCTM-5 mainly due to the existence of defects in the conversion layer that could allow the corrosive ions through the conversion layer. Increasing the bicarbonate concentration above 5 wt% to form the conversion layer significantly affected the corrosion phenomena as the conversion layer was formed with defects. It is expected that the change in solution pH near the surface could produce local stress at the grain boundaries. Therefore, the conversion layers were formed with defects when the concentration exceeded 5 wt% resulting in lower corrosion resistance. However, it has been observed that these conversion layers exhibited a lower corrosion rate compared to untreated AZ31 Mg. It is important to mention that the i_corr values measured from the polarization studies may not be accurate to correlate with the calculated corrosion rates due to the negative different effect (NDE).^51,52 Usage of DC polarization is quite different for Mg from other metallic materials due to its NDE. Dissolution of Mg in corrosive environments is distinctive from other metals as Mg can support cathodic hydrogen evolution on its surface during anodic polarization. Cathodic hydrogen evolution upon anodically polarized Mg is characterized by the rate of the hydrogen evolution reaction (HER) increasing with anodic polarization, a phenomenon called NDE. Several theories and models, viz. (1) impurity element-based models, (2) film-based models, (3) metal spalling contributing to NDE and (4) univalent Mg-based models and magnesium hydride intermediate models, have been proposed to explain the NDE of Mg.⁵² Formation of a partial film on the corroding surface could act as cathodic sites and this helps the hydrogen evolution reaction to take place on these sites. Hence, during this situation the current measured by the potentiostat is not accurate due to some electrons by-passing the electronic pathway to the potentiostat. Therefore, the corrosion rates of Mg measured based on the current flow through the potentiostat are not the actual values. Recently King et al. reported combined impedance, mass loss and hydrogen collection studies to accurately measure the corrosion rate of Mg alloys.⁵³ Therefore, in the present investigation, the measured i_corr could be used to qualitatively assess the coating properties.

Fig. 7 shows the photographic images and SEM micrographs of BCTM-1 to BCTM-9 subjected to potentiodynamic polarization studies. Among all the coatings, BCTM-1 showed a severely corroded surface after polarization, confirming its poor corrosion resistance. In the case of BCTM-3 and BCTM-5 the isolated corrosion damages were noticed. Moreover, the number of pits formed onto BCTM-5 is relatively less compared to others, indicating the protection against corrosive ion attack. SEM surface morphology revealed the existence of a relatively compact layer after a corrosion test for BCTM-5 further supported its corrosion resistance. Furthermore, the pits propagate as the bicarbonate concentration exceeds 5%, providing ample reasons for the poor corrosion resistance of BCTM-7 and BCTM-9 compared to that of BCTM-3 and BCTM-5.

Fig. 7 Photographic images and SEM surface morphologies of the conversion layer formed onto AZ31 Mg alloy after the potentiodynamic polarization test.

Potentiostatic polarization curves of BCTM-1 to BCTM-9 are depicted in Fig. 8 as a function of applied potential from −1.65 to −1.20 V and the final current values are also compared. The resulting current was increased with the increase of potential towards the anodic direction for BCTM-1 to BCTM-9. In our previous report, we observed a similar behaviour for the uncoated AZ31 alloy as well.⁵⁴ Comparison of these results with that of the uncoated alloy reveals that the resulting current is lower in all potential values, confirming the existence of a stable conversion layer on the surface. BCTM-1 exhibited fluctuation in the current response from −1.38 to −1.32 V_SCE indicating the breakdown of the conversion layer. Similar behaviour was not observed in the case of BCTM-3 and BCTM-5 indicating the passive behaviour of the conversion layer. The resulting current was less for BCTM-5 compared to others and is consistent with that of the i_corr values in SBF solution. It is interesting to note that the resulting current increased for BCTM-7 and BCTM-9 revealing their poor corrosion resistance.

Fig. 8 Potentiostatic polarization curves of the conversion layer formed onto AZ31 Mg alloy as a function of potential.

3.4.2 Electrochemical impedance spectroscopy (EIS) studies. EIS studies were carried out after stabilizing the OCP for about 1 h in SBF solution and the obtained results are represented as Bode phase angle and magnitude plots in Fig. 9a and b respectively. Two distinct peaks appeared in the phase angle plots along with an inductive behaviour (Fig. 9a). Brooks et al. also observed a similar behaviour for AZ91 Mg alloy in simulated inflammatory test electrolyte solution.⁵⁵ A peak appearing in the frequency range of 10 kHz to 10 Hz is attributed to the charge transfer between the conversion layer and the electrolyte.⁵⁶ It is interesting to note that the phase angle maximum was found to gradually increase from −40 to −55°, and the increasing trend in peak area was noticed from BCTM-1 to BCTM-5, indicating capacitive behaviour of the conversion layer. An increase in the peak area from BCTM-1 to BCTM-5 is attributed to the existence of a relatively compact conversion layer which could hinder the diffusion of the electrolyte to the surface.⁵⁷ It is worth mentioning that the increase of the phase angle maxima in the high frequency region is an indication of the existence of a conversion layer onto the Mg surface. A second peak appearing between 10 Hz to 0.1 Hz pertains to the charge transfer between the conversion layer and the substrate. Phase angle values at a low frequency (0.01 Hz) reached −10° irrespective of the conversion layers and are attributed to the surface relaxation process of adsorbed species on the corroding surface.^58,59

Fig. 9 Electrochemical impedance spectra of the conversion layer formed onto AZ31 Mg alloy. (a) Bode phase angle and (b) Bode magnitude plots.

Fig. 9b shows the corresponding Bode magnitude plots of BCTM-1 to BCTM-9. Capacitive behaviour can be identified from the change in impedance modulus at different frequencies of Bode magnitude plots.⁶⁰ Two distinct plateaus were identified in the magnitude plots. The impedance value at the beginning of the frequency (100 kHz) corresponds to solution resistance (R_s). No significant change was observed in R_s for the conversion layer obtained from various bicarbonate concentrations. The impedance value of the coatings varied appreciably at 100 Hz, the value for BCTM-5 being about 1 kΩ cm² and high compared to that of BCTM-1 and AZ31 Mg.²³ An increase in the impedance value is mainly attributed to the existence of a conversion layer on the surface of AZ31 Mg. The higher the impedance value, the higher the resistance will be to the corrosive ion penetration to the substrate.⁵⁶ The protecting ability of the conversion layer is characterized by the impedance values at the low frequency region (0.01 Hz). The low frequency region provides valuable information on the kinetics of Mg dissolution. The time taken to acquire information in the low frequency region is relatively higher compared to that in the high frequency region, which shows that the interaction between the substrate and the electrolyte in the low frequency region is relatively high. Hence, Mg undergoes oxidation readily in the low frequency region to its ions. The impedance values increased up to BCTM-5 in the present investigation, thus confirming the protection extended by the conversion layer. A decreasing trend in impedance values for BCTM-7 and BCTM-9 is due to the adsorption and accumulation of aggressive ions onto the conversion layer, thereby accelerating the Mg dissolution.

Impedance results of the formed conversion layers onto AZ31 with cracks were further subjected to curve-fitting analysis using a suitable equivalent circuit (EC) model. The proposed EC model is depicted in Fig. 10, where R_s is the solution resistance, CPE is the constant phase element of the electrolyte/conversion layer (CPE₁) and conversion layer/substrate (CPE₂) interfaces, R₁ is the conversion layer and R₂ is the charge transfer resistance, and R_L and L are the inductive resistance and inductance relating to adsorbed intermediates during corrosion. The EC model was selected based on the literature available for the corrosion of Mg alloys.^61–65 The curve-fitting was performed using ZSimpwin software and the non-linear least square fitting error was less than 5%. The CPE is generally used to indicate the deviation from the pure capacitive behaviour and to compensate for the non-homogeneity of the systems. The impedance of the CPE is represented as Z_CPE = 1/C(jω)_n, where C is pseudocapacitance, j is an imaginary coefficient, ω is angular frequency, and n is closeness to the ideal capacitive behaviour.⁶⁶ If n = 1, the CPE can simulate a pure capacitor (C), n = 0.5 is a Warburg diffusion element and n = 0 is a resistor.⁵⁹ Equivalent circuit parameters have been compared and are given in Fig. 11a–d. It is seen from Fig. 11a that the R₁ and R₂ values increase from BCTM-1 to BCTM-5 and decrease further. The R₁ and R₂ values of BCTM-5 are about 2.97 and 3.41 kΩ cm² respectively. The CPE₁ and CPE₂ values are about 100 and 4.2 μS sⁿ cm⁻² respectively. These results clearly indicate the enhanced corrosion protection offered by BCTM-5 compared to that of the others.

Fig. 10 Equivalent circuit model used for curve-fitting of EIS results.

Fig. 11 Comparison of EC parameters derived from the curve-fitting of EIS results.

The increase in the R₁ and R₂ values and the decrease in the CPE₁ and CPE₂ values of BCTM-5 are attributed to the surface morphology and composition of the conversion layer. The formed conversion layer is relatively uniform for BCTM-5 compared to the others, thus controlling the penetration of aggressive ions to attack the substrate surface. Reports are available on the formation of carbonate layers on the Mg alloy surfaces providing better passivation and slowing down the Cl⁻ ion-induced corrosion.^67–69 A decrease in the R₁ and R₂ values and an increase of the CPE₁ and CPE₂ values for BCTM-7 and BCTM-9 is attributed to the defects in the formed conversion layer. Besides, the surface coverage by the conversion layer is not uniform as in the case of BCTM-5, hence the diffusion of aggressive ions into the conversion layer and further initiation of the attack of the substrate. Adsorption of aggressive ions was confirmed by the appearance of the inductive loops in the extreme low frequency region and the obtained R_L and L values are given in Fig. 11d. The value of R_L increased from BCTM-1 to BCTM-5 and further declined for BCTM-7 and BCTM-9. An increase in the R_L and L values for BCTM-5 indicates adsorption of a relatively lower amount of corrosive ions on the surface compared to the other coatings.^63,64,70 On the other hand, a decrease in the R_L and L values for BCTM-7 and BCTM-9 further confirms the corrosive ion attack, resulting in degradation of the conversion layer. EC parameters are also consistent with the R_p values obtained from the linear polarization studies. The above results confirm that the produced conversion layers are effective in controlling the corrosive ion attack and reducing the corrosion rate of the AZ31 alloy in SBF solution. In particular, the performance of BCTM-5 was found to be very effective. Table 2 compares the electrochemical parameters derived from potentiodynamic polarization and EIS studies of BCTM-1 to BCTM-9 in SBF solution. It is apparent from the table that the obtained results are in good agreement with each other.

Table 2 Comparison of electrochemical parameters derived from potentiodynamic polarization and EIS studies of the conversion layer formed onto AZ31 Mg alloy

Parameters	BCTM-1	BCTM-3	BCTM-5	BCTM-7	BCTM-9
Potentiodynamic polarization studies
Ecorr (VSCE)	−1.573	−1.568	−1.548	−1.643	−1.615
icorr (μA cm^−2)	50.0 ± 5.0	13.2 ± 2.2	8.00 ± 1.0	19.5 ± 2.0	25.0 ± 4.5
Rp (kΩ cm^2)	1.408 ± 0.10	2.314 ± 0.17	4.716 ± 0.22	1.923 ± 0.18	1.694 ± 0.16
Corrosion rate (mm per year)	1.128 ± 0.12	0.298 ± 0.05	0.180 ± 0.02	0.439 ± 0.045	0.564 ± 0.10
EIS studies
Q1 (μS s^n cm^−2)	480 ± 50	140 ± 15	100 ± 12	240 ± 24	270 ± 29
n1	0.877	0.878	0.88	0.869	0.864
R1 (kΩ cm^2)	0.55 ± 0.05	1.85 ± 0.19	2.97 ± 0.26	1.26 ± 0.13	0.69 ± 0.08
Q2 (μS s^n cm^−2)	7.70 ± 0.75	6.26 ± 0.63	4.20 ± 0.45	4.80 ± 0.43	5.20 ± 0.54
n2	0.840	0.856	0.862	0.835	0.843
R2 (kΩ cm^2)	0.625 ± 0.06	2.201 ± 0.21	3.415 ± 0.35	1.968 ± 0.21	0.861 ± 0.07
RL (kΩ cm^2)	0.235 ± 0.02	0.987 ± 0.10	1.685 ± 0.15	0.496 ± 0.05	0.341 ± 0.03
L (kH cm^−2)	5.72 ± 0.05	10.6 ± 1.2	13.7 ± 1.3	8.40 ± 0.9	6.60 ± 0.7

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A schematic representation depicting the corrosion process of the conversion layer-coated AZ31 in SBF solution is given in Fig. 12. Several possible corrosive ions present in the SBF solution are taken into consideration to explain the corrosion mechanism. Attack of chloride ions on AZ31 Mg alloy is more vigorous due to the absence of any kind of barrier. The conversion layer formed onto BCTM-1 and BCTM-3 has increased the amount of cracked regions thereby exposing more area for the corrosive attacks resulting in the formation of corrosion product layers on the surface. However, in the case of BCTM-5, the conversion layer is relatively compact which is evident from the surface morphological studies (Fig. 3c). Existence of a relatively compact layer effectively reduces the direct contact between the substrate and the electrolyte. Therefore, the attack by aggressive ions is significantly reduced, controlling the corrosion rate. A further increase in the bicarbonate concentration (BCTM-7 and BCTM-9) resulted in the increased corrosion rates due to the increased contact area of the substrate with electrolyte.

Fig. 12 Schematic representation of corrosive attack in the conversion layer formed onto AZ31 Mg alloys in SBF solution.

4. Conclusions

(1) Magnesium carbonate conversion layers have successfully been developed onto AZ31 Mg alloy to improve its corrosion resistance in SBF solution.

(2) Variation of bicarbonate concentration to produce a conversion layer significantly affected the corrosion behaviour of AZ31 Mg. The formation of a MgCO₃ conversion layer was confirmed by XRD and ATR-IR studies.

(3) The conversion layer had a mud-crack pattern and was relatively uniform onto BCTM-5. The EDAX composition and line scanning across the surface revealed a higher amount of Mg, O and C onto the cracked surface than that of the uncracked region.

(4) The i_corr values of the conversion layer-coated samples were significantly less. In particular, BCTM-5 exhibited about a 9 times lower i_corr value compared to the others. Potentiostatic polarization results clearly indicated the existence of a conversion layer and its breakdown with the increase of potential.

(5) EIS studies and the EC parameters derived from the impedance results further confirmed the enhanced charge transfer resistance of the conversion layer. Based on the results it is concluded that BCTM-5 showed higher charge transfer resistance values and lower capacitance values confirming the optimum concentration to produce the conversion layers to improve the corrosion resistance of AZ31 Mg in SBF solution.

Acknowledgements

Authors A. Srinivasan and N. Rajendran acknowledge the financial assistance received from the Department of Science and Technology-Science and Engineering Research Board (DST-SERB), New Delhi (SR-SP-01-14, Dt.08.02.2011). The facilities provided by DST-FIST and UGC-DRS are gratefully acknowledged. Author A. Srinivasan also acknowledges the BK21 Plus project, SNU materials division fellowship. This work was financially supported by the World Premier Materials Program funded by the Ministry of Trade, Industry and Energy and the National Research Foundation grant (NRF-2015R1A2A1A01006795) of Korea through the Research Institute of Advanced Materials and Magnesium Technology Innovation Center.

References

1. M. P. Staiger, A. M. Pietak, J. Huadmai and G. Dias, Magnesium and its alloys as orthopedic biomaterials: a review, Biomaterials, 2006, 27(9), 1728–1734.
2. Y. F. Zheng, X. N. Gu and F. Witte, Biodegradable metals, Mater. Sci. Eng., R, 2014, 77, 1–34.
3. Y. Chen, Z. Xu, C. Smith and J. Sankar, Recent advances on the development of magnesium alloys for biodegradable implants, Acta Biomater., 2014, 10, 4561–4573.
4. 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 and D. Mantovani, Long-term clinical study and multiscale analysis of in vivo biodegradation mechanism of Mg alloy, Proc. Natl. Acad. Sci. U. S. A., 2016, 113(3), 716–721.
5. F. Witte, V. Kaese, H. Haferkamp, E. Switzer, A. Meyer-Lindenberg, C. J. Wirth and H. Windhagen, In vivo corrosion of four magnesium alloys and the associated bone response, Biomaterials, 2005, 26, 3557–3563.
6. G. Song and S. Song, A possible biodegradable magnesium implant materials, Adv. Eng. Mater., 2007, 9(4), 298–302.
7. N. T. Kirkland, J. Lespagnol, N. Birbilis and M. P. Staiger, A survey of bio-corrosion rates of magnesium alloys, Corros. Sci., 2010, 52(2), 287–291.
8. A. H. M. Sanchez, B. J. C. Luthringer, F. Feyerabend and R. Willumeit, Mg and Mg alloys: how comparable are in vitro and in vivo corrosion rates? A review, Acta Biomater., 2015, 13, 16–31.
9. Y. Ding, C. Wen, P. Hodgson and Y. Li, Effects of alloying elements on the corrosion behavior and biocompatibility of biodegradable magnesium alloys: a review, J. Mater. Chem. B, 2014, 2, 1912–1933.
10. N. Hort, Y. Huang, D. Fechner, M. Stormer, C. Blawert, F. Witte, C. Vogt, H. Drucker, R. Willumeit, K. U. Kainer and F. Feyerabend, Magnesium alloys as implant materials – principles of property design for Mg–RE alloys, Acta Biomater., 2010, 6, 1714–1725.
11. D. Mareci, G. Bolat, J. Izquierdo, C. Crimu, C. Munteanu, I. Antoniac and R. M. Souto, Electrochemical characteristics of bioresorbable binary MgCa alloys in Ringer’s solution: revealing the impact of local pH distributions during in vitro dissolution, Mater. Sci. Eng., C, 2016, 60, 402–410.
12. J. H. Nordlien, K. Nisancioglu, S. Ono and N. Masuko, Morphology and structure of water-formed oxides on ternary MgAl alloys, J. Electrochem. Soc., 1997, 144, 461–466.
13. G. Baril, C. Blanc and N. Pebere, AC impedance spectroscopy in characterizing time-dependent corrosion of AZ91 and AM50 magnesium alloys, J. Electrochem. Soc., 2001, 148, B489–B496.
14. G. Song, Control of biodegradation of biocompatible magnesium alloys, Corros. Sci., 2007, 49, 1696–1701.
15. J. Wang, J. Tang, P. Zhang, Y. Li, J. Wang, Y. Lai and L. Qin, Surface modification of magnesium alloys developed for bioabsorbable orthopaedic implants: a general review, J. Biomed. Mater. Res., Part B, 2012, 100(6), 1691–1701.
16. H. Hornberger, S. Virtanen and A. R. Boccaccini, Biomedical coatings on magnesium alloys-a review, Acta Biomater., 2012, 8, 2442–2455.
17. M. Carboneras, L. A. Hernandez-Alvarado, Y. E. Mireles, L. S. Hernandez, M. C. Garcia-Alonso and M. L. Escudero, Chemical conversion treatments to protect biodegradable magnesium in applications as temporary implants for bone repair, Rev. Metall., 2010, 46, 86–92.
18. J. E. Gray-Munro, C. Seguin and M. Strong, Influence of surface modification on the in vitro corrosion rate of magnesium alloy AZ31, J. Biomed. Mater. Res., Part A, 2008, 221–230.
19. P. Amaravathy, C. Rose, S. Sathiyanarayanan and N. Rajendran, Evaluation of in vitro bioactivity and MG63 oesteoblast cell response for TiO₂ coated magnesium alloys, J. Sol-Gel Sci. Technol., 2012, 64, 694–703.
20. X. Wang, S. Cai, G. Xu, X. Ye, M. Ren and K. Huang, Surface characteristics and corrosion resistance of sol gel derived CaO–P₂O₅–SrO–Na₂O bioglass–ceramic coated Mg alloy by different heat-treatment temperatures, J. Sol-Gel Sci. Technol., 2013, 67, 629–638.
21. T. S. N. Sankara Narayanan and M. Ho Lee, A simple strategy to modify the porous structure of plasma electrolytic oxidation coatings on magnesium, RSC Adv., 2016, 6, 16100–16114.
22. J. N. Li, P. Cao, X. N. Zhang and Y. H. He, In vitro degradation and cell attachment of a PLGA coated biodegradable Mg–6Zn based alloys, J. Mater. Sci., 2010, 45, 6038–6045.
23. A. Srinivasan, P. Ranjani and N. Rajendran, Electropolymerization of pyrrole over AZ31 Mg for biomedical applications, Electrochim. Acta, 2013, 88, 310–321.
24. R. Xu, X. Yang, P. Li, K. W. Suen, G. Wu and P. K. Chu, Electrochemical properties and corrosion resistance of carbon ion implanted magnesium, Corros. Sci., 2014, 82, 173–179.
25. H. M. Wong, Y. Zhao, V. Tam, S. Wu, P. K. Chu, Y. Zheng, M. K. Tsun, F. K. L. Leung, K. D. K. Luk, K. M. C. Cheung and K. W. K. Yeung, In vivo simulation of bone formation by aluminum and oxygen plasma surface-modified magnesium implants, Biomaterials, 2013, 34(38), 9863–9876.
26. X. B. Chen, D. R. Nisbet, R. W. Li, P. N. Smith, T. B. Abbott, M. A. Easton, D. H. Zhang and N. Birbilis, Controlling initial biodegradation of magnesium by a biocompatible strontium phosphate conversion coatings, Acta Biomater., 2014, 10(3), 1463–1474.
27. X. B. Chen, N. Birbilis and T. B. Abbott, Review of corrosion-resistant conversion coatings for magnesium and its alloys, Corrosion, 2011, 67(3), 1–16.
28. T. Yan, L. Tan, D. Xiong, X. Liu, B. Zhang and K. Yang, Fluoride treatment and in vitro corrosion behavior of an AZ31B magnesium alloy, Mater. Sci. Eng., C, 2010, 30(5), 740–748.
29. S. Shadanbaz and G. J. Dias, Calcium phosphate coatings on magnesium alloys for biomedical applications: a review, Acta Biomater., 2012, 8, 20–30.
30. J. E. Gray-Munro, C. Seguin and M. Strong, Influence of surface modification on the in vitro corrosion rate of magnesium alloy AZ31, J. Biomed. Mater. Res., Part A, 2009, 91(1), 221–230.
31. A. Drynda, T. Hassel, R. Hoehn, A. Perz, F. Wilhelm Bach and M. Peuster, Development and biocompatibility of a novel corrodible fluoride coated magnesium–calcium alloy with improved degradation kinetics and adequate mechanical properties for cardiovascular applications, J. Biomed. Mater. Res., Part A, 2010, 93(2), 763–775.
32. X. Cui, Y. Yang, E. Liu, G. Jin, J. Zhong and Q. Li, Corrosion behaviors in physiological solution of cerium conversion coatings on AZ31 magnesium alloys, Appl. Surf. Sci., 2011, 257, 9703–9709.
33. K. Y. Chiu, M. H. Wong, F. T. Cheng and H. C. Man, Characterization and corrosion studies of fluoride conversion coatings on degradable Mg implants, Surf. Coat. Technol., 2007, 202, 590–598.
34. T. S. N. Sankaranarayanan, I. S. Park and M. H. Lee, Tailoring the composition of fluoride conversion coatings to achieve better corrosion protection of magnesium for biomedical applications, J Mater. Chem. B, 2014, 2, 3356–3382.
35. Y. Al-Abdullat, S. Tsutsumi, N. Nakajima, M. Ohta, H. Kuwahara and K. Ikeuchi, Surface modification of magnesium by NaHCO₃ and corrosion behavior in Hank’s solution for new biomedical applications, Mater. Trans., 2001, 42(8), 1777–1780.
36. X. N. Gu, W. Zheng, Y. Cheng and Y. F. Zheng, A study on alkaline heat treated Mg–Ca alloy for the control of the biocorrosion rate, Acta Biomater., 2009, 5, 2790–2799.
37. L. Li, J. Gao and Y. Wang, Evaluation of cyto-toxicity and corrosion behavior of alkali-heat-treated magnesium in simulated body fluid, Surf. Coat. Technol., 2004, 185, 92–98.
38. S. Feliu Jr, A. Samaniego, A. A. El-Hadad and I. Llorente, The effect of NaHCO₃ treatment time on the corrosion resistance of commercial magnesium alloys AZ31 and AZ61 in 0.6 M NaCl solution, Corros. Sci., 2013, 67, 204–216.
39. Z. Lia, G. L. Song and S. Song, Effect of bicarbonate on biodegradation behaviour of pure magnesium in a simulated body fluid, Electrochim. Acta, 2014, 115, 56–65.
40. T. Kokubo and H. Takadama, How useful is SBF in predicting in vivo bone bioactivity?, Biomaterials, 2006, 27, 2907–2915.
41. Z. Hao, J. Pan and F. Du, Synthesis of basic magnesium carbonate micro rods with a surface of “house of cards” structure, Mater. Lett., 2009, 63, 985–988.
42. C. Wenting, L. Zhibao and G. P. Demopoulos, Effects of temperature on the preparation of magnesium carbonate hydrates by reaction of MgCl₂ with Na₂CO₃, Chin. J. Chem. Eng., 2009, 17(4), 661–666.
43. A. Srinivasan, K. Seon Shin and N. Rajendran, Applications of dynamic electrochemical impedance spectroscopy (DEIS) to evaluate protective coatings formed on AZ31 magnesium alloy, RSC Adv., 2015, 5, 29589–29593.
44. S. Frykstrand, J. Forsgren, A. Mihranyan and M. Stromme, On the pore forming mechanism of upsalite, a micro- and mesoporous magnesium carbonate, Microporous Mesoporous Mater., 2014, 190, 99–104.
45. H. Kwon and D. G. Park, Infra-red study of surface carbonation on polycrystalline magnesium hydroxide, Bull. Korean Chem. Soc., 2009, 30(11), 2567–2573.
46. T. Kornprobst and J. Plank, Synthesis and properties of magnesium carbonate xerogels and aerogels, J. Non-Cryst. Solids, 2013, 361, 100–105.
47. J. K. Lin, K. L. Jeng and J. Y. Uan, Crystallization of a chemical conversion layer that forms on AZ91D magnesium alloy in carbonic acid, Corros. Sci., 2011, 53, 3832–3839.
48. R. K. Gupta, K. Mensah-Darkwa and D. Kumar, Corrosion protective conversion coatings on magnesium disks using a hydrothermal technique, J. Mater. Sci. Technol., 2014, 30(1), 47–53.
49. T. Yan, L. Tan, D. Xiong, X. Liu, B. Zhang and K. Yang, Fluoride treatment and in vitro corrosion behavior of an AZ31B magnesium alloy, Mater. Sci. Eng., C, 2010, 30, 740–748.
50. Standard practice for calculation of corrosion rates and related information from electrochemical measurements, ASTM Standards G 102–89.
51. N. Birbilis, A. D. King, S. Thomas, G. S. Frankel and J. R. Scully, Evidence for enhanced catalytic activity of magnesium arising from anodic dissolution, Electrochim. Acta, 2014, 132, 277–283.
52. S. Thomas, N. V. Medhekar, G. S. Frankel and N. Birbilis, Corrosion mechanism and hydrogen evolution on Mg, Curr. Opin. Solid State Mater. Sci., 2015, 19, 85–94.
53. A. D. King, N. Birbilis and J. R. Scully, Accurate electrochemical measurement of magnesium corrosion rates; a combined impedance, mass-loss and hydrogen collection study, Electrochim. Acta, 2014, 121, 394–406.
54. A. Srinivasan, K. Seon Shin and N. Rajendran, Dynamic electrochemical impedance spectroscopy (DEIS) studies of AZ31 magnesium alloy in simulated body fluid solution, RSC Adv., 2014, 4, 27791–27795.
55. E. K. Brooks, S. Der and M. T. Ehrensberger, Corrosion and mechanical performance of AZ91 exposed to simulated inflammatory conditions, Mater. Sci. Eng., C, 2016, 60, 427–436.
56. A. Zomorodian, M. P. Garcia, T. Moura e Silva, J. C. S. Fernandes, M. H. Fernandes and M. F. Montemor, Corrosion resistance of a composite polymeric coating applied on biodegradable AZ31 magnesium alloy, Acta Biomater., 2013, 9(10), 8660–8670.
57. M. Bethencourt, F. J. Botana, M. J. Cano, M. Marcos, J. M. Sanchez-Amaya and L. Gonzalez-Rovira, Using EIS to analyse samples of Al–Mg alloy AA5083 treated by thermal activation in cerium salt baths, Corros. Sci., 2008, 50, 1376–1384.
58. Y. Zhao, M. Jamesh, W. K. Li, G. Wu, C. Wang, Y. Zheng, K. W. K. Yeung and P. K. Chu, Enhanced antimicrobial properties, cytocompatibility and corrosion resistance of plasma-modified biodegradable magnesium alloys, Acta Biomater., 2014, 10, 544–556.
59. R. Pinto, M. G. S. Ferreira, M. J. Carmezim and M. F. Montemor, The corrosion behaviour of rare-earth containing magnesium alloys in borate buffer solution, Electrochim. Acta, 2011, 56, 1535–1545.
60. T. Yan, L. Tan, B. Zhang and K. Yang, Fluoride Conversion Coating on Biodegradable AZ31B Magnesium Alloy, J. Mater. Sci. Technol., 2014, 30(7), 1–9.
61. G. S. Duffo and E. Q. Castillo, Development of an artificial saliva solution for studying the corrosion behavior of dental alloys, Corrosion, 2004, 60, 594.
62. C. L. Liu, Y. J. Wang, R. C. Zeng, X. M. Zhang, W. J. Huang and P. K. Chu, In vitro corrosion degradation behavior of Mg–Ca alloy in the presence of albumin, Corros. Sci., 2010, 52, 3341–3347.
63. M. Jamesh, S. Kumar and T. S. N. Sankara Narayanan, Corrosion behavior of commercially pure Mg and ZM21 Mg alloy in Ringer’s solution-long term evaluation by EIS, Corros. Sci., 2011, 53, 645–654.
64. M. Ibrahim Jamesh, G. Wu, Y. Zhao, D. R. McKenzie, M. M. M. Bilek and P. K. Chu, Electrochemical corrosion behavior of biodegradable Mg–Y–RE and Mg–Zn–Zr alloys in Ringer’s solution and simulated body fluid, Corros. Sci., 2015, 91, 160–184.
65. Y. Song, D. Shan, R. Chen, F. Zhang and E.-H. Han, Biodegradable behaviors of AZ31magnesium alloy in simulated body fluid, Mater. Sci. Eng., C, 2009, 29, c1039–1045.
66. J. N. Balaraju, A. Srinivasan, G. Yoganandan, V. K. William Grips and K. S. Rajam, Effect of Mn/Mo incorporated oxide layer on the corrosion behavior of AA 2024 alloy, Corros. Sci., 2011, 53, 4084–4092.
67. S. Feliu Jr, A. Pardo, M. C. Merino, A. E. Coy, F. Viejo and R. Arrabal, Correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys, Appl. Surf. Sci., 2009, 255, 4102–4108.
68. S. Feliu Jr, C. Maffiotte, J. C. Galvan and V. Barranco, Atmospheric corrosion of magnesium alloys AZ31 and AZ61 under condensation conditions, Corros. Sci., 2011, 53, 1865–1872.
69. L. Wang, T. Shinohara and B. P. Zhang, XPS study of the surface chemistry on AZ31 and AZ91 magnesium alloys in dilute NaCl solution, Appl. Surf. Sci., 2010, 256, 5807–5812.
70. X. Li, Z. Weng, W. Yuan, X. Luo, H. M. Wong, X. Liu, S. Wua, K. W. K. Yeung, Y. Zheng and P. K. Chu, Corrosion resistance of dicalcium phosphate dihydrate/poly(lactic-co-glycolic acid) hybrid coating on AZ31 magnesium alloy, Corros. Sci., 2016, 102, 209–221.

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