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Effect of pH on the degradation kinetics of a Mg–0.8Ca alloy for orthopedic implants

  • Aya Mohamed EMAIL logo , Hans-Georg Breitinger and Ahmed M. El-Aziz
Published/Copyright: October 20, 2020
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Corrosion Reviews
From the journal Corrosion Reviews Volume 38 Issue 6

Abstract

One of the promising applications of magnesium and magnesium alloys is their use as biodegradable implants in biomedical applications. The pH around an orthopedic implant greatly affects the degradation kinetics of biodegradable Mg–Ca alloys. At the location of a fracture, local pH changes, and this has to be considered in the optimization of implant materials. In this study, the effect of the pH of a physiological buffer on degradation of a Mg–0.8Ca alloy was studied. The pH of Hank’s balanced salt solution (HBSS) was adjusted to 1.8, 5.3 and 8.1. Degradation of a Mg–0.8Ca implant was tested using immersion test and electrochemical techniques. Immersion tests revealed an initial weight gain for all samples followed by weight loss at extended immersion time. Weight gain was highest at acidic pH (1.8) and lowest at alkaline pH (8.1). This was in agreement with results from electrochemical polarization tests where the degradation rate was highest (7.29 ± 2.2 mm/year) at pH 1.8 and lowest (0.31 ± 0.06 mm/year) in alkaline medium of pH 8.1. The pH of all HBSS buffers except the most acidic (pH 1.8) reached a steady state of ∼pH 10 at the end of the two-month immersion period, independent of the initial pH of the solution. Corrosion products formed on the sample surfaces were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX) and X-ray diffractometry (XRD), revealing the formation of magnesium and calcium phosphates with distinct morphologies that were different for each of the pH conditions. Thus, pH of physiological buffers has a significant effect on the degradation and corrosion of Mg–Ca alloys used for biomedical applications.

Keywords: biodegradable; electrochemical polarization means; immersion test; orthopedics; pH; simulated body fluid

1 Introduction

Biodegradable implants have great potential for orthopedic applications where natural tissue regeneration is possible, such as screws and pins for temporary bone fixations (Prasad et al. 2017). Such implants should only generate nontoxic degradation products, and would ideally stimulate cell attachment, supporting close bone–to–bone integration of the implant after short implantation periods (Coelho and Jimbo 2014). This effect has been demonstrated for magnesium–calcium (Mg–Ca)-based biodegradable implants (Mohamed et al. 2019; Pogorielov et al. 2017). Mechanical properties of Mg are similar to those of human bone, and both Mg and Ca ions are main constituents of the human bone, thus the simultaneous release of those ions during the implant’s degradation is no source of toxicity and may improve the bone healing process (Radha and Sreekanth 2017). Furthermore, calcium is a major component in the hydroxyapatite complex (HAp, [Ca10(PO4)6(OH)2]), the mineral that provides rigidity and stability of bone (Guyton and Hall 2011).

Recently, biodegradable Mg–Ca alloys have demonstrated great potential in in vivo tests. Implantation of Mg–Ca screws in rabbit hind legs supported bone formation and tissue integration within six weeks after surgery (Erdmann et al. 2011). A long-term clinical study using implants made of a Mg–Ca–Zn alloy revealed the formation of a biomimicking calcification matrix at the degrading interface (Lee et al. 2016), showing complete replacement of the implant by new bone within one year of implantation (Lee et al. 2016). This was also in agreement with a study reporting the applicability of a Mg–0.5Ca alloy as a temporary biodegradable orthopedic implant showing controlled in vivo degradation, reduced inflammation and high bone-formation capability (Makkar et al. 2018).

Despite the excellent biocompatibility of Mg–Ca alloys, there is the challenge of controlling the degradation rate in the physiological environment. Rapid degradation may lead to premature loss of mechanical stability of the implant before the healing process is completed (Johnson and Liu 2013). Indeed, selecting a low percentage of calcium (<2 wt. %) for the alloy improved the corrosion resistance of pure Mg (Harandi et al. 2013). This was confirmed in other studies, and an alloy of Mg–0.6Ca (0.6 wt. % Ca) was proposed to possess optimum corrosion resistance in simulated body fluid (Wan et al. 2008). Furthermore, Mg–0.8Ca was shown to be a promising candidate alloy for biodegradable implants due to its degradation behavior in simulated body fluid (Bita et al. 2016). Formation of carbonated hydroxyapatite [Ca10(PO4)3(CO3)3(OH)2] having the same morphology as biological apatite in human bone was shown for Mg–0.8Ca implants (Mohamed et al. 2019).

To date, studies on low-calcium content Mg implants were concerned with the degradation under neutral physiological pH levels (∼pH 7.4). It should be noted that the pH of the body is not neutral at the location of a bone fracture. The pH at and around the injury may drop to 5.5 during the initial period of the implantation, and will then be shifted to more alkaline values during degradation of the implant (Ng et al. 2010). It was indeed reported that the corrosion resistance of AZ31 magnesium alloy was higher in alkaline solutions than in neutral and acidic ones (Thirumalaikumarasamy et al. 2014).

Given the fact that the pH of the body fluid strongly affects the degradation kinetics of Mg–Ca implants, the main objective of this study was to investigate the effect of different pH values of simulated body fluid on the degradation behavior of Mg–0.8Ca implants. Three pH levels were studied, a strongly acidic pH of 1.8, a moderately acidic pH of 5.3 and an alkaline pH of 8.1. Degradation behavior was assessed by immersion tests and electrochemical polarization methods. Microstructural analysis by scanning electron microscopy (SEM) was performed to identify the morphology of the corrosion products. Chemical composition of the corrosion products was investigated by energy dispersive spectroscopy (EDX) and X–ray diffraction (XRD) techniques.

2 Materials and methods

A cast Mg–0.8Ca alloy (Helmholtz–Zentrum Geesthacht, Germany) was the material investigated in this study.

2.1 Material preparation

Small cubic samples (1 × 1 × 1 cm3) were cut from the cylindrical Mg–0.8Ca alloy ingots using a water-cooled electric saw. These test pieces were used for the assessments of the in vitro degradation behavior, as well as the microstructural analysis. The cut samples were then mechanically wet ground with SiC papers starting from 180 grit, and up to 1200 grit. This was directly followed by polishing with diamond suspension of 6 μm, and finally up to 1 μm to achieve a mirror-like surface. Finally, the samples were rinsed with distilled water and dried with a stream of air prior to experiments.

For the simulation of the body environment, Hank’s balanced salt solution (HBSS) of neutral pH (∼7) was prepared, as described in literature studies (Ng et al. 2010).

2.2 Altering the pH of the body fluid

pH adjustment of HBSS was achieved by adding phosphate buffers to HBSS prepared in 250 ml distilled water (Table 1). Three solutions of different pH values of HBSS were obtained.

Table 1:

Altering the pH of Hank’s balanced salt solution (HBSS).

Buffers added to HBSS prepared in 250 ml distilled water Final pH of modified HBSS
1.75 ml of H3PO4 1.8
3.445 g of NaH2PO4 5.3
3.545 g of Na2HPO4 8.1

2.3 Immersion test

Four specimens of Mg–0.8Ca alloy with polished surfaces were initially weighed on an analytical balance (sensitivity range 0.0001 g), then statically immersed in four beakers of the different pH media of HBSS at area/volume ratio of 1 cm2/10 ml. All samples were incubated at 37.5 °C for two months.

At regular intervals during the immersion period, the pH of the solution was recorded. In addition, the samples were removed from solution, washed carefully with distilled water, dried with a stream of air and weighed.

2.4 Electrochemical tests

The corrosion behavior of the samples was investigated by potentiodynamic polarization method. A saturated calomel electrode (SCE) was used as reference electrode and platinum as counter electrode. HBSS of different pH values was used as electrolyte and the temperature was adjusted to 37.5 °C. First, the open-circuit potential (OCP) test was done for 3 h. Second, a potential cyclic voltammetry (PCV) test was performed directly after the OCP test. One complete cycle was run from −2000 to −1500 mV and then back to −2000 mV with a scan rate of 0.2 mV/s. The corrosion rate was calculated through Tafel’s Extrapolation method (McCafferty 2005) using VoltaMaster 4 software. All tests were performed in triplicates to ensure the reliability of the results.

2.5 Characterization of corrosion products

The morphology of corrosion products formed on the surface of the Mg–0.8Ca alloy test pieces after immersion in HBSS of different pH values was studied using a field emission scanning electron microscope (FESEM, Quanta 250 FEG, Netherlands) equipped with energy dispersive spectroscopy (EDX) attachments for elemental analysis of the phases formed during immersion. Tracking of nucleation and growth of corrosion products was accomplished by studying the same test pieces after different immersion periods of 1, 7, 21 and 60 days.

Phase identification of constituents of the formed corrosion products on the surface of the tested Mg–0.8Ca alloys surface was done by X-ray diffractometry (XRD, X’Pert PRO, PANalytical, Netherlands) using CuKα radiation (λ = 0.1541 nm) in the angular region of 2θ = 10°–60°.

3 Results and discussion

3.1 Immersion test

The immersion test results (Figure 1) showed that all tested samples experienced weight gain over the entire immersion period in all tested media (acidic, neutral and alkaline). Weight gain was due to the deposition of corrosion products on the samples’ surface. High degradation rate will result in the accumulation of corrosion products on the sample’s surface, rendering greater weight gain. It is noted that removal/falling off of the formed deposits will result in weight loss, in the same trend shown in Figure 1.

Figure 1: Weight gain (mg/cm2) versus immersion time for Mg–0.8Ca alloy at different pH values of Hank’s balanced salt solution (HBSS). Squares: pH 1.8. Closed circles: pH 5.3. Triangles: pH 7. Open circles: pH 8.1.
Figure 1:

Weight gain (mg/cm2) versus immersion time for Mg–0.8Ca alloy at different pH values of Hank’s balanced salt solution (HBSS). Squares: pH 1.8. Closed circles: pH 5.3. Triangles: pH 7. Open circles: pH 8.1.

Both, strongly acidic pH 1.8 and slightly acidic pH 5.3 exhibited the highest weight gain, while the least weight gain was observed at alkaline pH of 8.1. This agrees with a higher degradation rate at acidic pH, and such a corrosion behavior would be expected from the pourbaix diagram of pure Mg (Shaw 2003), showing that the dissolution of magnesium into Mg2+ ions is more favorable at low pH values. Faster corrosion at acidic pH can be attributed to the difficulty in formation of a passive layer (Mg(OH)2). Mg(OH)2 was found to be more stable at higher pH, which reduces the rate of degradation of Mg–0.8Ca in neutral and alkaline solutions.

The following reaction mechanism was proposed for corrosion of Mg in acidic conditions:

Under neutral and alkaline conditions, the cathodic reaction generates OH−:

Under acidic condition, the concentration of H+ is high, and upon reduction at the cathodic surface the H atoms combine to produce H2 gas (Song et al. 2007). In neutral and alkaline media, the energy from the anodic corrosion of Mg is sufficient to reduce water molecules, which will generate hydrogen gas and hydroxyl (OH−) ions (Heakal and Bakry 2019).

The pH values of the immersion solutions were greatly affected by the degradation of the Mg–0.8Ca alloy (shown below in Figure 2). Over the two-month immersion period, pH value shifted toward a strongly alkaline value of ∼pH 10. This was observed for all the tested media except the most acidic (pH 1.8) whose pH value also increased by 4 units.

Figure 2: pH changes of HBSS of different pH values over the 60-day immersion period. Squares: pH 1.8. Closed circles: pH 5.3. Triangles: pH 7. Open circles: pH 8.1.
Figure 2:

pH changes of HBSS of different pH values over the 60-day immersion period. Squares: pH 1.8. Closed circles: pH 5.3. Triangles: pH 7. Open circles: pH 8.1.

The alkalization of the solution favors the formation of Mg(OH)2 on the alloy surface and saturation of the Mg2+ ions. Indeed, this behavior has been observed before (Zeng et al. 2007).

3.2 Electrochemical tests

3.2.1 Open-circuit potential (OCP) test

OCP tests were run over a period of 3 h. Open-circuit potentials were shifted to more negative values, approaching steady-state values (Est.). Est. values in acidic solution (pH 1.8 and pH 5.3 denoted by letters (a) and (b), respectively, Figure 3) were more negative compared with those in neutral and alkaline solutions (pH 7 and 8.1, letters (c) and (d), respectively). This was likely due to the tendency of passive layer formation to decrease at low pH values (Shaw 2003).

Figure 3: Open-circuit potential (OCP) test for Mg–0.8Ca alloy immersed in HBSS of different pH values. (a) pH 1.8. (b) pH 5.3. (c) pH 7. (d) pH 8.1.
Figure 3:

Open-circuit potential (OCP) test for Mg–0.8Ca alloy immersed in HBSS of different pH values.

(a) pH 1.8. (b) pH 5.3. (c) pH 7. (d) pH 8.1.

3.2.2 Potential cyclic voltammetry (PCV) test

PCV tests were performed as a complete cycle from −2000 to −1500 mV and back to −2000 mV at a scan rate of 0.2 mV/s to explore the reaction mechanism of alloy corrosion in different pH media. At the lowest pH of 1.8 (letter (a) in Figure 4), PCV indicated rapid anodic dissolution of Mg–0.8Ca sample according to Mg → Mg2+ + 2é. Current density showed a linear increase with applied potential in the forward scan, and a similar decrease during the reverse scan. The strong linear relation between potential and current density indicated complete absence of passive layer formation, which explains the high dissolution rate that was observed. This is also evident from the pourbaix diagram of pure Mg (Shaw 2003).

Figure 4: Potential cyclic voltammetry (PCV) test for Mg–0.8Ca alloy at different pH values of HBSS. (a) pH 1.8. (b) pH 5.3. (c) pH 7. (d) pH 8.1.
Figure 4:

Potential cyclic voltammetry (PCV) test for Mg–0.8Ca alloy at different pH values of HBSS.

(a) pH 1.8. (b) pH 5.3. (c) pH 7. (d) pH 8.1.

Under slightly acidic conditions (pH 5.3, letter (b) in Figure 4), the sample also showed dissolution of Mg, during the forward scan, but at a slower rate than at pH 1.8. During the reverse scan, the current density was slightly decreased owing to the formation of a passive layer on the material surface.

In contrast, the behavior of the alloy in both neutral and alkaline pH media (pH 7 and 8.1, denoted by letters (c) and (d) in Figure 4, respectively) was consistent with the presence of a slow, uniform redox reaction and the formation of a passive (Mg(OH)2) layer. Fast corrosion only happened when the pitting potential (Epit) was reached. Eventually, repassivation occurred during the reverse scan. Pitting was likely due to the presence of aggressive Cl− ions in HBSS, which can be adsorbed onto the Mg(OH)2 layer, and lead to the formation of better soluble MgCl2 (Cui et al. 2017). The measurements indicated that the pitting resistance was greater in alkaline medium (pH 8.1, Epit = −1.64 V) than under neutral conditions (pH 7, Epit = −1.72 V, Figure 4). This can be attributed to the increased stability of the passive layer at alkaline pH, as predicted by the pourbaix diagram of pure Mg (Shaw 2003).

3.2.3 Determination of the corrosion rate by Tafel’s extrapolation method

The Tafel curves corresponding to corrosion of the Mg–0.8Ca showed that corrosion current density (icorr) increased anodically as the pH of the HBSS medium decreased (Figure 5). This indicated an increase in the corrosion rate (CR), in accordance with our previous results from immersion and PCV tests.

Figure 5: Tafel curves for Mg–0.8Ca alloy at different pH values of HBSS. (a) pH 1.8. (b) pH 5.3. (c) pH 7. (d) pH 8.1.
Figure 5:

Tafel curves for Mg–0.8Ca alloy at different pH values of HBSS.

(a) pH 1.8. (b) pH 5.3. (c) pH 7. (d) pH 8.1.

The corrosion parameters after Tafel’s Extrapolation are given in Table 2. Corrosion current and rate decreased with increasing pH, in agreement with a similar study in the literature studies (Thirumalaikumarasamy et al. 2014).

Table 2:

Corrosion parameters of Mg–0.8Ca alloy in HBSS at different pH values.

pH of HBSS Ecorr (V) icorr (mA/cm2) CR (mm/year) β a (mV) β c (mV)
1.8 −1.97 ± 0.03 0.32 ± 0.13 7.29 ± 2.2 157.7 ± 20.5 −115.3 ± 31.9
5.3 −1.92 ± 0.01 0.11 ± 0.04 2.59 ± 0.93 140.6 ± 41.2 −107.4 ± 17.0
7 −1.73 ± 0.01 0.05 ± 0.01 1.08 ± 0.38 30.8 ± 2.6 −166.9 ± 9.0
8.1 −1.69 ± 0.02 0.014 ± 0.002 0.31 ± 0.06 31.3 ± 12.2 −144.9 ± 2.0

3.3 Field emission scanning electron microscopy (FESEM) analysis of corrosion products

Morphology of corrosion products that deposited on the alloy surface during immersion were studied using FESEM. At the most acidic pH of 1.8, the Mg–0.8Ca sample surface showed massive accumulation of corrosion products in the form of crystals (Figure 6a). The cracks in the material surface were due to dehydration after the removal of the sample from solution. The same morphology of corrosion products was observed under slightly acidic pH value of 5.3 (Figure 6b), but there were fewer crystals formed than at pH 1.8, in good agreement with results from the immersion test (Figure 1).

Figure 6: Field emission scanning electron microscope (FESEM) images showing the morphology of the corrosion products on Mg–0.8Ca alloy at different pH values of HBSS. Left panels: after 1 day of immersion. Right panels: after 60 days of immersion. (a) pH 1.8. (b) pH 5.3. (c) pH 7 (Mohamed et al. 2019). (d) pH 8.1.
Figure 6:

Field emission scanning electron microscope (FESEM) images showing the morphology of the corrosion products on Mg–0.8Ca alloy at different pH values of HBSS. Left panels: after 1 day of immersion. Right panels: after 60 days of immersion.

(a) pH 1.8. (b) pH 5.3. (c) pH 7 (Mohamed et al. 2019). (d) pH 8.1.

At neutral pH of 7 (Figure 6c), corrosion products were of needle-shaped morphology, in agreement with a detailed study from our lab (Mohamed et al. 2019). At pH 8.1, morphology of the corrosion products was quite different, exhibiting rod-like structures (Figure 6d). Thus, the pH of the surrounding medium affects the morphology of corrosion products formed from Mg–Ca alloys. Needle- and rod-shaped morphologies of the corrosion products were manifested with the strong alkalization of the medium, while globular habit of corrosion products predominated in acidic media.

FESEM surface analysis clearly shows an increase in the amount of corrosion products on the samples’ surfaces with increasing immersion time, consistent with the weight gain of samples during immersion (Figure 1). Furthermore, the extensive growth of the corrosion products was witnessed at the prolonged immersion period of 60 days (Figure 6, right panels).

3.4 Chemical compositional analysis of corrosion products by energy dispersive spectroscopy (EDX) and X-ray diffraction (XRD) techniques

EDX analysis of corrosion products on the surface of Mg–0.8Ca samples after the two-month immersion period showed significant signals of Ca, Mg and O peaks in all the samples; indicating Ca–P products. This was further supported by XRD analysis (Figure 7).

Figure 7: XRD analysis of corrosion products on Mg–0.8Ca alloy after 60-day immersion in HBSS of different pH values. (a) pH 1.8. (b) pH 5.3. (c) pH 7. (d) pH 8.1.
Figure 7:

XRD analysis of corrosion products on Mg–0.8Ca alloy after 60-day immersion in HBSS of different pH values.

(a) pH 1.8. (b) pH 5.3. (c) pH 7. (d) pH 8.1.

XRD analysis confirmed that the formed corrosion products on the Mg–0.8Ca alloy surface in a simulated body fluid (HBSS) at different pH values were mainly magnesium phosphates [Mg3(PO4)2] matching with the reference ICDD: 00-011-0235, calcium phosphates [Ca3(PO4)2] matching with ICDD: 00-009-0169, magnesium hydroxide [Mg(OH)2] matching with ICDD: 00-044-1482 and carbonated hydroxyapatite [Ca10(PO4)3(CO3)3(OH)2] matching with ICDD: 00-019-0272.

It was noticed that the signal for magnesium hydroxide (Mg(OH)2) was the strongest at alkaline pH of 8.1, and the weakest at the most acidic pH of 1.8. This is consistent with the stability of Mg(OH)2 at high pH values as indicated by the pourbaix diagram of Mg (Shaw 2003).

4 Conclusions

The present work studied the effect of the pH of Hank’s balanced salt solution (HBSS, a simulated body fluid) on the degradation kinetics of Mg–0.8Ca orthopedic implants. The results indicate:

  1. The pH of the immersion fluid strongly affects the degradation kinetics of Mg–0.8Ca alloy, with an increase in degradation rate as the pH of the medium decreases. This was consistently seen in both, immersion and electrochemical tests.

  2. Electrochemical tests predicted a very high degradation rate of about 7.29 ± 2.2 mm/year in case of the highly acidic pH 1.8, and a relatively low rate of about 0.31 ± 0.06 mm/year for the alkaline pH 8.1. The reduced degradation rate at higher pH was mainly attributed to the increasing stability of the Mg(OH)2 passive layer formed on the alloy surface when the pH is shifted to the alkaline region. Under acidic conditions, rapid dissolution of Mg took place and no signs of passivation were observed.

  3. The pH of the HBSS medium reaches a constant alkaline value (∼pH 10) at the end of the 60-day immersion period for all the tested media regardless of their initial pH value. This was mainly due to the saturation of Mg2+ ions in the solution and formation of the Mg(OH)2 passive layer.

  4. The corrosion products formed at different pH values were magnesium and calcium phosphates, together with magnesium hydroxide and carbonated hydroxyapatite. The morphology of the formed Ca–P products differs according to the pH of the medium.

Corresponding author: Aya Mohamed, Materials Engineering Department, German University in Cairo (GUC), New Cairo, 11835, Egypt, E-mail:

Acknowledgments

The authors thank the Helmholtz–Zentrum Geesthacht for the alloys synthesis and casting.

  1. Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

  3. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Received: 2020-02-01
Accepted: 2020-08-12
Published Online: 2020-10-20
Published in Print: 2020-11-18

© 2020 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.1515/corrrev-2020-0008
Keywords for this article
biodegradable; electrochemical polarization means; immersion test; orthopedics; pH; simulated body fluid

Articles in the same Issue

  1. Frontmatter
  2. Reviews
  3. The science of alternating current-induced corrosion: a review of literature on pipeline corrosion induced due to high-voltage alternating current transmission pipelines
  4. Stress corrosion cracking and precipitation strengthening mechanism in TWIP steels: progress and prospects
  5. Original articles
  6. Effect of pH on the degradation kinetics of a Mg–0.8Ca alloy for orthopedic implants
  7. Enhancement of barrier and anti-corrosive performance of zinc-rich epoxy coatings using nano-silica/graphene oxide hybrid
  8. Nonlinear trending of corrosion of high nickel alloys in extended marine and atmospheric exposures
  9. Effect of small amounts of chalcogen alloying elements on the oxidation resistance of copper
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