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Influence of microstructure on the in-vitro degradation behaviour of magnesium alloy
Abstract
The study shows that the microstructural difference between the fine-grained die-cast and coarse-grained sand-cast magnesium-based alloys has no significant effect on the in-vitro degradation behaviour. However, the post-degradation analysis of the alloys suggest that the high volume fraction of secondary phase particles in the die-cast alloy may not be suitable for biodegradable implant applications, primarily due to the high stability of the secondary phase particles in physiological conditions.
... It is well known that alloying elements in alloys may form second phase particles through precipitation, which are usually with noble potentials respect to the magnesium matrix, thus either facilitating the corrosion process, or enriching the corrosion product thereby possibly inhibiting the corrosion rate [15]. Some studies on the in vitro degradation of AZ91 found that with enrichment of aluminum in the grains, a continuous network of stable grain boundary second precipitates could lead to an improvement on the corrosion resistance of the die-cast AZ91 [16]. However, for the sand-cast AZ91, the un-continuous second precipitates acted as the galvanic cathode and accelerated the corrosion of the matrix [17]. ...
... Compared with the magnesium matrix (α phase), Mg 17 Al 12 phase has a higher corrosion potential. Therefore, Mg 17 Al 12 phase may affect the degradation behavior of the matrix of magnesium alloy and leach into the physiological environment accompanied with the degradation of the matrix [16]. However, there are few studies on the degradation behavior of the Mg 17 Al 12 phase. ...
... The content of Mg 17 Al 12 phase is increased with the increase of Al content in Mg-Al-Zn alloys. Due to the different chemical characteristics, Mg 17 Al 12 phase degrades at a different rate from the matrix in body fluid [6,16,24]. Some studies suggested that a galvanic couple between Mg 17 Al 12 particle and the Mg matrix can be formed when the alloy is immersed in body fluid and the Mg 17 Al 12 phase as the cathode can accelerate the corrosion of the matrix. ...
... 23 Additionally, a higher corrosion potential of Mg 17 Al 12 phase than magnesium matrix (a phase) indicates that Mg 17 Al 12 phase may be expected to degrade at a different corrosion rate from that of the a phase and leach into the physiological environment along with the degradation of the matrix. 24 However, there are few direct and systematic studies on the degradation behavior of Mg 17 Al 12 second phase in Mg-Al-Zn alloys. Kannan et al. preliminarily investigated the degradability of Mg 17 Al 12 phase in a simulated body fluid (SBF) using electrochemical measurement, and found that the degradation rate of Mg 17 Al 12 compound was lower than that of the bare magnesium. ...
... The average size of the prepared Mg 17 Al 12 particles was about 10 lm, which was similar to the size of the Mg 17 Al 12 second phase existed in the Mg-Al-Zn alloys. 24,[44][45][46] After stimulation by Mg 17 Al 12 particles for 24 h, the challenged macrophages exhibited morphological changes, including enlarged cell sizes, fuzzy cell contours, and the presence of associated structures [ Fig.5(a)]. SEM observation demonstrated that the internalization of Mg 17 Al 12 particles was triggered by the macrophages through extended pseudopodia [ Fig. 6(a,b,d)] and the encirclement of the particles was emerged through the cell deformation of the macrophage [ Fig.6(c)]. ...
Mg alloys are gaining interest for applications as biodegradable medical implant, including Mg-Al-Zn series alloys with good combination of mechanical properties and reasonable corrosion resistance. However, whether the existence of second phase particles in the alloys exerts influence on the biocompatibility is still not clear. A deeper understanding of how the particles regulate specific biological responses is becoming a crucial requirement for their subsequent biomedical application. In this work, the in vitro biocompatibility of Mg17 Al12 as a common second phase in biodegradable Mg-Al-Zn alloys was investigated via hemolysis, cytotoxicity, cell proliferation and cell adhesion tests. Moreover, osteogenic differentiation was evaluated by the extracellular matrix mineralization assay. The Mg17 Al12 particles were also prepared to simulate the real situation of second phase in the in vivo environment in order to estimate the cellular response in macrophages to the Mg17 Al12 particles. The experimental results indicated that no hemolysis was found and an excellent cytocompatibility was also proved for the Mg17 Al12 second phase when co-cultured with L929 cells, MC3T3-E1 cells and BMSCs. Macrophage phagocytosis co-culture test revealed that Mg17 Al12 particles exerted no harmful effect on RAW264.7 macrophages and could be phagocytized by the RAW264.7 cells. Furthermore, the possible inflammatory reaction and metabolic way for Mg17 Al12 phase were also discussed in detail. This article is protected by copyright. All rights reserved.
Copyright © 2014 Wiley Periodicals, Inc., A Wiley Company.
... In addition, this alloy showed corrosion behavior similar to AZ91, AM50, AZ31 and WE43 alloys [7]. The effect of grain size in vitro degradation behavior of AZ91 Mg alloys was studied and learnt that the grain size had no significant effect on degradation behavior [8]. In a study investigating WE54 alloy, this alloy showed higher corrosion resistance than pure Mg [9]. ...
... Technology Symposium, Pittsburgh, PA, USA, 12-16 October 2014 Mg-Pb-Bi, Au-Cu-Mg, Mg-Cd, Mg-Y and Mg-Zn) production, corrosion and mechanical properties were investigated previously [8,11,12]. It was proved that Li increased ductility and shapability of Mg. ...
In this study, Mg-based biomedical implant plates with controlled porosity and micro-textured surface were designed and manufactured. AZ91 Mg powders of different size distribution were compacted under compaction pressures of 150 and 200 MPa and compaction temperatures of 100 and 150°C. Compacted green plates were then sintered at 380 and 420°C for 40, 60, 120 and 150 minutes. Different aging schemes were also tested, such as aging temperatures of 100 and 200°C and aging time of 60 and 120 minutes. The main interest was to determine an optimum pressure, temperature and time to produce Mg implant plates with necessary levels of porosity and strength. The feasibility study results showed that compaction load has a significant effect on the achievable porosity. Porosity of plates decreases with increasing the compaction pressure. In compaction stage, suggested temperature must be higher than 100°C to ensure strong bonding among Mg powders. In addition, the inert environment is necessary to prevent oxidation during heat treatment. Finally, there is no significant difference between micro-hardness values after sintering in temperatures below eutectic temperature.
... In recent years, a number of Mg alloys have been tested under in vitro and in vivo conditions to understand their corrosion behaviour and mechanisms (Witte et al. 2006;Kannan andRaman, 2008, 2010;Kannan, 2010;Walter and Kannan, 2011). AZ series alloys show a lower corrosion current than that of pure Mg. ...
... In recent years, a number of Mg alloys have been tested under in vitro and in vivo conditions to understand their corrosion behaviour and mechanisms (Witte et al. 2006;Kannan andRaman, 2008, 2010;Kannan, 2010;Walter and Kannan, 2011). AZ series alloys show a lower corrosion current than that of pure Mg. ...
http://www.sciencedirect.com/science/article/pii/B9781845699499500150
... Further, the mechanical properties of magnesium are closer to natural bone [1]. However, pure magnesium degradation is unacceptably high in physiological conditions (i.e., pH level (7.4-7.6) and high chloride concentration) [1,12]. Consequently, the magnesium implant will lose its mechanical integrity before the tissues have sufficiently healed. ...
... However, a major issue with their approach is that the authors have used pure magnesium as the base material. Pure magnesium is highly susceptible to corrosion in SBF [12] and ...
Polylactic acid (PLA) was coated on a biodegradable magnesium alloy, AZ91, using spin coating technique for temporary implant applications. The degradation behaviour of the coated alloy samples was evaluated using electrochemical impedance spectroscopy (EIS) method in simulated body fluid (SBF). EIS results suggested that the PLA coating enhanced the degradation resistance of the alloy significantly. Increase in the PLA coating thickness was found to increase the degradation resistance, but resulted in poor adhesion. Long-term EIS experiments of the PLA coated samples suggested that their degradation resistance gradually decreased with increase in SBF exposure time. However, the degradation resistance of the PLA coated samples was significantly higher than that of the bare metal even after a 48 h exposure to SBF.
... 3) Electrochemical measurement is widely used to measure the in vitro degradation behaviour of Mg alloys. 106,148,[183][184][185] The greatest advantage is that it can be used to obtain the realtime corrosion rate. Changes in corrosion behaviour can be instantaneously observed. ...
There is increasing interest in the development of bone repair materials for biomedical applications. Magnesium (Mg)-based alloys have a natural ability to biodegrade because they corrode in aqueous media; they are thus promising materials for orthopaedic device applications in that the need for a secondary surgical operation to remove the implant can be eliminated. Notably, Mg has superior biocompatibility because Mg is found in the human body in abundance. Moreover, Mg alloys have a low elastic modulus, close to that of natural bone, which limits stress shielding. However, there are still some challenges for Mg-based fracture fixation. The degradation of Mg alloys in biological fluids can be too rapid, resulting in a loss of mechanical integrity before complete healing of the bone fracture. In order to achieve an appropriate combination of bio-corrosion and mechanical performance, the microstructure needs to be tailored properly by appropriate alloy design, as well as the use of strengthening processes and manufacturing techniques. This review covers the evolution, current strategies and future perspectives of Mg-based orthopaedic implants.
... Over the past few years, alloying has been done to magnesium, and their biodegradation tendency has been evaluated. Those magnesium alloys primarily include AZ (aluminum, zinc) series, calcium-containing and rare-earth containing alloys [22][23][24][25][26][27] . The general degradation rate has been reduced by alloying, but the alloys are susceptible to localized degradation [ 28 , 29 ]. ...
Magnesium is a candidate metal for biodegradable implant applications for its biodegradation tendency and excellent biocompatibility. Unfortunately, the high degradation rate of magnesium and also its localized degradation in physiological conditions are the main issues for its successful implant applications. The degradation rate of magnesium has been reduced to some degree via alloying, but the localized degradation susceptibility is a great concern. For many years, hydroxyapatite (HAp), a biocompatible ceramic material, has been extensively used for bio-implant applications. Recently, a substantial amount of research has been carried out on coating HAp on magnesium-based materials for improved degradation resistance in particular and also to enhance the biocompatibility. This review article focuses on the different methods of HAp coating on magnesium-based materials and also the recent cutting-edge advancements made in the coating process for improved degradation resistance and biocompatibility. The mechanical stability of the HAp coated magnesium-based materials is also discussed.
... In fact, Mg dissolves very rapidly in the pH level (7.4-7.6) and high chloride environment seen in the physiological system. Hence, research has been directed toward reducing the corrosion rate of Mg by various methods (Song & Atrens, 2003;Kannan, 2010). ...
High specific-strength magnesium-based materials are the promising substitute for traditional metallic materials like steel, but their easy corrosivity has caused serious resource waste and safely problems. Chemical conversion coating, as a primer coating for corrosion-resistant surface coating, has a great chance to provide primary and durable corrosion-resistant performance through rationally designed self-healing treatment for preventing further corrosion of damaged sites. Herein, this chapter provides a comprehensive and updated review of the advantages and limitations associated with various chemical conversion coating-based self-healing corrosion protection systems and their mechanisms of action. Self-healing chemical conversion coating tends to be autonomous healing mechanisms with minimal or no intervention, thus possessing the “smart” property. Therefore, self-healing chemical conversion coatings are focused on improving corrosion resistance, self-healing ability, repair durability, cost-efficiency, and environmental-friendliness, instead of the design of stimulation response mechanisms. The hope that obtains high-performance self-healing conversion coatings through tuning simple preparation technologies and multifunctional coating under the premise of simple preparation methods, which will be beneficial for sustainable development in the world.
... 3) Electrochemical measurement is widely used to measure the in vitro degradation behaviour of Mg alloys. 106,148,[183][184][185] The greatest advantage is that it can be used to obtain the realtime corrosion rate. Changes in corrosion behaviour can be instantaneously observed. ...
Introduction A biomaterial is defined as a material intended to interface with biological systems to evaluate, treat, augment or replace any tissue, organ or function of the body. 1, 2 Commonly, pacemakers, stents, sutures, bone plates and screws, needles, knee joints and catheters all constitute biomaterials. Biomaterials are used across a wide range of applications and have become a major industry in the 21 st century. The traditional metallic biomaterial requires that metals are as inert as possible in order to minimise the immune response and reduce the corrosion of the material itself in the physiological environment of the body. Typically, these biomaterials are stainless steels, titanium (Ti) alloys and cobalt-chrome-based alloys. After decades of developing improved corrosion-resistant metallic biomaterials, the design and application of biodegradable metals are currently under the spotlight. A biodegradable material is expected to degrade gradually in vivo, with an appropriate host response elicited by released corrosion products, and then to dissolve completely upon fulfilling the mission of assisting with tissue healing while leaving no implant residues. 3 The materials should be non-toxic or made up of metallic elements which can be metabolised by the human body. Therefore, magnesium (Mg)-based biodegradable alloys are a promising material for clinical applications. Extensive research so far suggests a bright future for biodegradable Mg-based orthopaedic implants. However, new fabrication approaches, updated design strategies and enhanced clinical requirements are emerging. Thus, up-to-date progress and development of biodegradable Mg alloys over recent decades are presented in this review paper by searching google scholar. Firstly, we summarise orthopaedic applications and principles, traditional implant components (especially Ti), potential use of Mg in real clinical applications, degradation mechanisms and role of Mg in the human body. Monolithic Mg alloy components are comprehensively discussed focusing on the effects of alloying elements, microstructural evolution (grain size, second phases, twins, texture, dislocations etc.),
... Strontium alloys are known to be the slowest in degrading. In data published, Zhao et al (39) concluded that Mg2Sr has the highest corrosion resistance with high tensile strength and good compatibility (40). ...
The highly comminuted calcaneal fractures represent a challenge for surgeons and require bone grafts for a good clinical outcome. Postoperative results are generally associated with increased morbidity and long periods of inactivity. The biomedical community promotes the use of artificial materials for grafts in order to achieve improved results. In an era when cosmetic concerns as well as the satisfaction of patients are mandatory and the use of autologous bone grafts is not without complications, an artificial replacement appears to be a favorable option. Synthetic bone grafts are known to fail under stress shield or are associated with systemic side effects. The purpose of the present study was to investigate and determine an already commercially available magnesium (Mg) alloy whose design is most suitable for long-term use. The mechanical properties of Mg1Ca and MgYREZr compared with normal cortical and cancellous bone were assessed. Another discussed aspect was the influence of the alloy in the graft fixation. The results revealed that Mg1Ca and MgYREZr alloys had a low tensile strength of 75 and 250 MPa, respectively. For this reason, it was surmised that MgYREZr alloy could be an optimal choice with favorable corrosion resistance. Since calcaneal fractures are prone to skin necrosis and septic complications, the need for antibacterial procedures and antibiotic prophylaxis is highlighted. Thus, an in vivo attempt was also made to identify the relationship between Mg alloy products and bacterial load. However, the most important feature of the present study was the creation of a 3D model grafting, with an anti-sliding design, which can be potentially used with the preferred Mg alloy in this type of fractures. In conclusion, artificial materials are the future in medicine, replacing the body-limiting capabilities of grafts. They are safe and incur less comorbidities. This method could pave the way for reducing patient discomfort and increasing patient satisfaction. Although further testing is required, this research represents a great starting point for calcaneal fractures.
... [11] For example, in the alloy AZ91, Mg 17 Al 12 phase exhibits a dual purpose during corrosion in simulated body fluid, namely acting as micro-cathodes to accelerate corrosion and/or simultaneously supplying barriers to inhibit corrosion due to its different distributions. [4,12,13] The discontinuous Mg 17 Al 12 phase can work as cathodes and deteriorate the corrosion resistance. Contrary to this, the continuous network of Mg 17 Al 12 phase formed after T6 treatment can act as barriers to the corrosion attack. ...
The influence of intermetallic microstructure on the degradation of Mg-5Nd alloy with different heat treatments was investigated via immersion testing in DMEM + 10 pct FBS under cell culture conditions and subsequent microstructural characterizations. It was found that T4 heat-treated sample had the poorest corrosion resistance due to the lack of finely dispersed precipitates inside grains, continuous lamellar particles along grain boundaries and outer Ca-P layer, and to the formation of a loose corrosion product layer. In contrast, the aged samples exhibited a better corrosion resistance due to their presence and to the formation of a compact corrosion layer. Their degradation behavior largely depended on the intermetallic microstructure. Corrosion was initiated in the matrix around stable globular particles Mg41Nd5 at grain boundaries. In the sample aged at high temperature 245 °C, the coexistence of lamellar Mg41Nd5 particles and their nearby Nd-poor regions enhanced the corrosion. The corrosion first started in such regions. It was shown that those finely dispersed precipitates formed during aging had no influence on the corrosion initiation. However, they indeed affected the subsequent corrosion propagation with the immersion proceeding. They supplied barriers for corrosion propagation and hence were beneficial for improving the corrosion resistance. The continuously distributed lamellar Mg41Nd5 precipitates formed at grain boundaries during aging at 245 °C supplied an additional effective obstacle to corrosion propagation. This was especially beneficial for hindering the corrosion propagation at the later stage of corrosion.
... Hence, alloying elements need to be selected carefully to maintain biocompatibility, because they will also be dissolved in the body fluid during degradation . Furthermore, most Mg alloys are susceptible to localized degradation due to their inhomogeneous microstructure which may impair the mechanical integrity during service (Kannan 2010). In addition, the high electronegative potential of Mg (-2.4 V) makes it difficult to achieve a significantly lower degradation rate just by alloying. ...
The study of innovative biodegradable implant materials is one of the most interesting research topics at the forefront in the area of biomaterials. Biodegradable implant materials in the human body can be gradually dissolved, absorbed, consumed or excreted, so there is no need for the secondary surgery to remove implants after the surgery regions have healed. However, most of the biodegradable materials, usually polymers, do not have good mechanical properties to be reliable for bearing the load of the body. Magnesium and its alloys due to the excellent biodegradability and biocompatibility as well as the suitable mechanical compatibility with human bone are very promising candidates for the development of temporary, degradable implants in load-bearing applications. However, Mg alloys are corrosion susceptible in a biological environment. Besides, the high corrosion rate and the low bioactivity of magnesium implants are the challenging problems, which need to be resolved before employing them in clinical applications. This paper provides a review of state-of-the-art of magnesium alloy implants for orthopedic and tissue engineering applications and describes recent progress in the design of novel structure design Mg alloys and potential approaches to improve their biodegradation performance.
... Al is an effective alloying element in Mg alloys for improving the mechanical properties and corrosion resistance (Alvarez-Lopez et al., 2010;Song et al., 1998;Abidin et al., 2011), which is studied a lot in structure materials filed. In recent years, many researchers have studied on AZ series magnesium alloys, e.g., AZ31 and AZ91, for potential biodegradable implant applications (Alvarez-Lopez et al., 2010;Kannan, 2010;Sunil et al., 2016;Song et al., 2008;Tian et al., 2016;Wen et al., 2009;Witte et al., 2005). When the content of Al is over 2%, the main second phases are Mg 17 Al 12 or Mg 4 Al 3 . ...
Magnesium alloys as a class of biodegradable metals have great potential to be used as implant materials, which attract much attention. In this review, the mechanical properties of magnesium alloys for medical applications are summarized. The methods to improve the mechanical properties of biodegradable magnesium alloys and the mechanical behaviors of Mg alloys in biomedical application are illustrated. Finally the challenges and future development of biodegradable magnesium alloys are presented.
... Therefore, silver is beneficial for functionality as well as for the property profile. Beside the influence of the alloying elements and the material production methods on the alloy microstructure and hence on its degradation [9][10][11][12][13], studies show that magnesium degradation can be influenced by many physiological factors and parameters. This includes the presence of proteins [14,15], the pH value, the buffering effect of bicarbonate salt in combination with CO 2 [16,17] as well as other inorganic salts existing in the body fluid [18,19]. ...
High-resolution analytical methods, including synchrotron infrared microspectroscopy combined with wavelength-dispersive X-ray emission spectroscopy were applied to study the structure and chemical composition of the oxidized layer of pure and Ag-alloyed Mg exposed to cell culture media without and with osteoblasts. Comparative analysis has been done on pure Mg immersed in two different cell culture media: Dulbecco's Modified Eagle's Medium (DMEM) and Roswell Park Memorial Institute medium (RPMI), whereas Mg-xAg binary alloys (x = 2, 4, 6, 8 wt%) were studied after immersion in DMEM. It is shown that the physicochemical formation of degradation products as well as the activity of the biological component is influenced by the addition of silver. It could be demonstrated that the presence of Ag in the Mg alloy enhances the chemical reaction between Mg and C to form amorphous and/or crystalline MgCO3 on account of CaCO3. As a consequence, the further available P and Ca react easily to form Mg-poor amorphous calcium phosphate phases. Osteoblasts actively adjusted these phases towards hydroxyapatite-like phases.
... Kannan investigated the degradability of Mg 17 Al 12 phase in simulated body fluid (SBF) using electrochemical measurements and found that the degradation rate of Mg 17 Al 12 was lower than that of bare Mg. Our previous study demonstrated that pitting corrosion occurs with crackings for Mg 17 Al 12 phase in Hank's solution and degrades much slower than AZ31 alloy and pure Mg [106]. ...
Bone repair materials are rapidly becoming a hot topic in the field of biomedical materials due to being an important means of repairing human bony deficiencies and replacing hard tissue. Magnesium (Mg) alloys are potentially biocompatible, osteoconductive, and biodegradable metallic materials that can be used in bone repair due to their in situ degradation in the body, mechanical properties similar to those of bones, and ability to positively stimulate the formation of new bones. However, rapid degradation of these materials in physiological environments may lead to gas cavities, hemolysis, and osteolysis and thus, hinder their clinical orthopedic applications. This paper reviews recent work on the use of Mg alloy implants in bone repair. Research to date on alloy design, surface modification, and biological performance of Mg alloys is comprehensively summarized. Future challenges for and developments in biomedical Mg alloys for use in bone repair are also discussed.
... In vitro degradation behaviour of pure zinc and Zn-5 Al-4 Mg alloy was evaluated by weight loss analysis and electrochemical methods, i.e., potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS), in simulated body fluid (SBF) maintained at a body temperature of 37.5 ± 0.5 °C and pH of 7.4-7.6. The chemical composition of the SBF is given in Table 2 63 . Prior to the in vitro degradation testing, the samples were ground with SiC paper up to 2500 grit and later polished with 1 μm alumina powder solution, and washed with distilled water and then ultrasonically cleaned in ethanol. ...
In this study, the biocompatibility and in vitro degradation behaviour of a commercial zinc-based alloy (Zn-5 Al-4 Mg) were evaluated and compared with that of pure zinc for temporary orthopaedic implant applications. Biocompatibility tests were conducted using human alveolar lung epithelial cells (A549), which showed that the zinc alloy exhibits similar biocompatibility as compared to pure zinc. In vitro degradation evaluation was performed using weight loss and electrochemical methods in simulated body fluid (SBF) at 37 °C. Weight loss measurements revealed that the degradation of the zinc alloy was slightly lower during the initial immersion period (1–3 days), but marginally increased after 5 and 7 days immersion as compared to pure zinc. Potentiodynamic polarisation experiments showed that the zinc alloy exhibits higher degradation rate than pure zinc. However, electrochemical impedance spectroscopy analysis suggests that pure zinc is susceptible to localized degradation, whereas the zinc alloy exhibited passivation behaviour. Post-degradation analysis revealed localized degradation in both pure zinc and the zinc alloy.
... Corrosion resistance of magnesium alloys is strongly dependent on various parameters such as, [35][36][37][38]: ...
Magnesium alloys are modern biocompatible materials suitable for orthopaedic implants due to their biodegradability in biological environment. Many studies indicate that there is a high demand to design magnesium alloys with controllable in vivo corrosion rates and required mechanical properties. A solution to this challenge can be sought in the development of metal matrix composites based on magnesium alloys with addition of relevant alloying elements and bioceramic particles. In this study, the corrosion mechanisms along with corrosion protection methods in magnesium alloys are discussed. The recently developed magnesium alloys for biomedical applications are reviewed. Special attention is given to the newest research results in metal matrix composites composed of magnesium alloy matrix and calcium phosphates, especially hydroxyapatite or tricalcium phosphate, as the second phase with emphasis on the biodegradation behavior, microstructure and mechanical properties in view of potential application of these materials in bone implants.
... Most of the magnesium alloys that have been studied for biodegradable implant applications contain secondary phase particles : for example, Mg 17 Al 12 (β-phase) in AZ series alloy 11,12) , Mg 12 YNd and Mg 14 YNd 2 in WE43 alloy 13) , and Mg 2 Ca in Mg-Ca alloys 14) . The in vitro and in vivo degradation studies carried out on these secondary phase particles containing magnesium alloys have only focused on the overall corrosion behaviour of the alloy. ...
Magnesium alloys have been extensively studied in recent years for potential biodegradable implant applications. A great deal of work has been done on the evaluation of the corrosion behaviour of magnesium alloys under in vitro and in vivo conditions. However, magnesium alloys, in general, contain secondary phase particles distributed in the matrix and/or along the grain boundaries. Owing to their difference in chemistry in comparison with magnesium matrix, these particles may exhibit different corrosion behaviour. It is essential to understand the corrosion behaviour of secondary phase particles in magnesium alloys in physiological conditions for implant applications. This paper critically reviews the biodegradation behaviour of secondary phase particles in magnesium alloys.
... Alloying of magnesium has been widely studied to decrease its degradation rate [1][2][3][6][7][8][9][10][11]. Elements such as aluminium, zinc, calcium and rare-earths have been added to magnesium and their degradation behaviour has been tested under in vitro and in vivo conditions. ...
Magnesium and its alloys have been widely studied in recent years for load-bearing biodegradable implant applications due to their biodegradability and biocompatibility. Unfortunately, there are two major concerns with these materials, i.e., high degradation rate and localized degradation susceptibility which can affect the in-service mechanical integrity. This review paper focuses on the potential use of calcium phosphates as biocompatible coatings on magnesium-based materials to control their degradation rates and also delay their localized degradation tendency.
... Together with mass decreasing the driving parameters improve, this is connected mainly to the dynamic behavior of the vehicles. A need for reduction of the transportation vehicle mass is very important, because more and more transportation vehicles is equipped with additive accessories (like airbags, safety belts, raising and lowering system for car windows, etc.) which increases mass and have influence not only on safety, but also on usable attractiveness of these vehicles [4][5][6][7][13][14][15][16][17][18]. Volkswagen was the first to apply magnesium in the automotive industry on its Beetle model, which used 22 kg magnesium in each car of this model [2]. ...
Purpose: In this paper there is presented the corrosion behavior of cast magnesium alloys in as cast state and after heat treatment.Design/methodology/approach: The following results concern scanning electron microscopy investigations in the SE as well BSE observation mode, for better phase contrast results, also qualitative microanalysis was applied for chemical composition investigations of the surface. Pitting corrosion resistance was carried out using the potentiodynamic electrochemical method (direct current), based on anodic polarisation curve. Based on the achieved anodic polarisation curves, using the Tefel extrapolation method near to the corrosion potential, the quantitative data were determined, which describe the electrochemical corrosion process of the investigated alloys: value of the corrosion potential Ecor (mV), polarisation resistance Rp (kΩ/cm2), corrosion current density icor (μA/cm2), corrosion rate Vp (mm/year) as well the mass loss Vc (g/m2).Findings: Surface morphology of the samples after corrosion test performed after and before heat treatment show irregular shaped pinholes and numerous cracks on the material surface layer.Research limitations/implications: The applied cooling rate and alloy additions seems to be a good compromise for properties and microstructures, nevertheless further tests should be carried out in order to examine different cooling rates and parameters of solution treatment process and aging process.Practical implications: Investigation results concerning the surface layer presents some interesting findings connected to the layer morphology, which can be of high interest for practical application for the reason of better layer quality as well as surface layer properties. Limitation of surface damage including irregular shaped pinholes and numerous cracks is of very high importance for decreasing the influence of pitting corrosion onto the surface layer corrosion resistance in very width range o applications.Originality/value: The value of this paper is to define the influence of heat treatment parameters and aluminium addition on corrosion resistance properties of magnesium-aluminium cast alloys.
... The degradation mechanisms of the pure Mg, the PEO coated and the PEO-PLLA coated samples are schematically shown in Fig. 7. It is well known that high-chloride concentration, as in the SBF, causes high localized degradation in pure magnesium [25]. The native film (MgO) and the degradation product film, Mg(OH) 2 , formed in aqueous solution are not protective in chloride-containing solution [26], and as a result a high level of localized degradation was observed within 48 h of exposure to SBF. ...
Magnesium, a light-weight engineering metal, is a potential biomaterial for orthopaedic biodegradable mini-implant applications due to its compatible mechanical properties, biodegradability and biocompatibility. However, magnesium-based implants will become a reality only if the degradation rate of magnesium is controlled. Alloying has shown to reduce the degradation rate of magnesium to some degree, but magnesium alloys are highly susceptible to localised degradation, which can affect the mechanical integrity during service. In orthopaedic applications, mechanical integrity of the implant is a key factor to be considered, especially during the initial service period. Hence, surface modification techniques to delay the general and localised degradation behaviour of magnesium-based materials have gained increased interest in recent years. This chapter reviews the use of biodegradable polymers as coating materials on magnesium-based materials for enhancing the general and localised degradation resistance.
... Because of the partly protective surface film, it would be expected that microstructure effects would be less pronounced than in chloride solutions, and this was observed by Zainal Abidan et al. [129] (see Table 8). Nevertheless, micro-galvanic acceleration of corrosion by second phases has been reported [131,135,138,126,[142][143][144]. The good performance, of biodegradable magnesium implants from 1878 to 1981, was reviewed by Witte [14]. ...
This chapter reviews our understanding of Mg corrosion and the measurement of Mg corrosion. There is some emphasis on the use of Mg alloys for biodegradable medical applications. Mg melt purification using Zr has been shown to provide the opportunity to produce ultra-high-purity Mg alloys, which could lead to stainless Mg. Nor's solution may be a good start model for the study of Mg for biodegradable medical implant applications. The uni-positive Mg+ corrosion mechanism is consistent with the know corrosion data, although it is clear that self-corrosion is also important.
... In addition, the degradation process of Mg is accompanied by hydrogen evolution and solution alkalization which can delay the healing and cause implant loosening [6]. Therefore many in vitro studies are focused on adjusting the type and concentration of alloying elements to achieve the suitable mechanical properties of the material along with the optimized degradation profile [7][8][9][10][11][12][13][14][15][16]. Another significant problem is to find the right comparison guidelines between in vitro and in vivo studies [17][18][19][20][21]. ...
Magnesium and its alloys have considerable potential for orthopedic applications. During the degradation process the interface between material and tissue is continuously changing. Moreover, too fast or uncontrolled degradation is detrimental for the outcome in vivo. Therefore in vitro setups utilizing physiological conditions are promising for the material / degradation analysis prior to animal experiments. The aim of this study is to elucidate the influence of inorganic salts contributing to the blood buffering capacity on degradation. Extruded pure magnesium samples were immersed under cell culture conditions for 3 and 10 days. Hank’s balanced salt solution without calcium and magnesium (HBSS) plus 10% of fetal bovine serum (FBS) was used as the basic immersion medium. Additionally, different inorganic salts were added with respect to concentration in Dulbecco’s modified Eagle’s medium (DMEM, in vitro model) and human plasma (in vivo model) to form 12 different immersion media. Influences on the surrounding environment were observed by measuring pH and osmolality. The degradation interface was analyzed by electron-induced X-ray emission (EIXE) spectroscopy, including chemical-element mappings and electron microprobe analysis, as well as Fourier transform infrared reflection micro-spectroscopy (FTIR).
... For instance, since AZ91 magnesium alloy after die-casting possessed finer grain size and more homogeneous distribution of intermetallic phases around the boundary in comparison with conventional casting counterpart, excellent corrosion resistance was observed [Zucchi et al., 2006]. On the contrary, the investigation of AZ91 Mg alloys fabricated by different processes [Bobby, 2010] demonstrated different consequences. The Nyquist graphs (a plot between real and imaginary components of impedance) of die-cast, as-cast AZ91 Mg alloy samples and pure magnesium as a reference, shown in Fig. 2.1, indicated that the two alloys had much better corrosion properties than pure magnesium due to reduced corrosion by the addition of aluminium. ...
... However, the high degradation rate of magnesium in physiological conditions hinders its use as a biodegradable implant due to premature failure concerns [2]. Alloying [3][4][5] and various coatings, including bioceramics [6][7][8] and biodegradable polymers [9,10], have been extensively studied in recent years for controlling the degradation rate of magnesium. Particularly, coating of calcium phosphates (CaPs) on magnesium-based materials has gained increased interest [8,11,12] since they are proven coating materials for titanium-based and stainless steel biomaterials [13,14]. ...
An attempt was made to develop a self-dissolution assisted coating on a pure magnesium metal for potential bone fixation implants. Magnesium phosphate cement (MPC) was coated successfully on the magnesium metal in ammonium dihydrogen phosphate solution. The in vitro degradation behaviour of the MPC coated metal was evaluated using electrochemical techniques. The MPC coating increased the polarisation resistance (RP) of the metal by ~150% after 2 h immersion in simulated body fluid (SBF) and reduced the corrosion current density (icorr) by ~80%. The RP of the MPC coated metal remained relatively high even after 8 h immersion period. However, post-degradation analysis of the MPC coated metal revealed localized attack. Hence, the study suggests that MPC coating alone may not be beneficial, but this novel coating could provide additional protection if used as a precursor for other potential coatings such as biodegradable polymers or calcium phosphates.
... Alloying of magnesium has been widely studied to decrease its degradation rate [1][2][3][6][7][8][9][10][11]. Elements such as aluminium, zinc, calcium and rare-earths have been added to magnesium and their degradation behaviour has been tested under in vitro and in vivo conditions. ...
Magnesium alloys are attractive for use as biodegradable materials for temporary implant applications. However, the high localized degradation of magnesium alloys in physiological conditions is a major concern, which can affect the mechanical integrity of the implant during service. Calcium phosphate (CaP) coating is a suitable method to delay the initiation of localized attack in magnesium alloys. This paper will discuss the challenges and opportunities in electrochemically coating CaP on magnesium and its magnesium alloys for biodegradable implant applications.
... From microstructure point of view, it was obtained a dendritic structure characteristic for casting alloys. The addition of Ca in Mg-Ca alloys refined the grain size, but also modified Mg 2 Ca phase from course lamellar shape to short bar shape, which is in line with the other results [4,5,6]. ...
The magnesium alloys has been intensively studied for their suitable mechanical properties, excellent biocompatibility and their ability to biodegrade in biological environments. Although magnesium biodegradable implants possess many desirable properties, it is important that the alloy is able to be tolerated by the body- the constitutional elements of magnesium-based alloys should be toxic free. In this study two binary magnesium alloys Mg-Ca0,8 and Mg-Ca1,8 were experimentally obtained by casting and was characterized in order to investigate the microstructure, mechanical properties and how alloying elements influenced the characteristics of this new alloys potentially used for orthopedic implants.
... The degradation mechanisms of the pure Mg, the PEO coated and the PEO-PLLA coated samples are schematically shown in Fig. 7. It is well known that high-chloride concentration, as in the SBF, causes high localized degradation in pure magnesium [25]. The native film (MgO) and the degradation product film, Mg(OH) 2 , formed in aqueous solution are not protective in chloride-containing solution [26], and as a result a high level of localized degradation was observed within 48 h of exposure to SBF. ...
An attempt was made to seal the porous silicate-based plasma electrolytic oxidation (PEO) layer on pure magnesium (Mg) with a biodegradable polymer, poly(L-lactide) (PLLA), to delay the localized degradation of magnesium-based implants in body fluid for better in-service mechanical integrity. Firstly, a silicate-based PEO coating on pure magnesium was performed using a pulsed constant current method. In order to seal the pores in the PEO layer, PLLA was coated using a two-step spin coating method. The performance of the PEO─PLLA Mg was evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The EIS results showed that the polarization resistance (Rp) of the PEO─PLLA Mg was close to two orders of magnitude higher than that of the PEO Mg. While the corrosion current density (icorr) of the pure Mg was reduced by 65% with the PEO coating, the PEO─PLLA coating reduced the icorr by almost 100%. As expected, the Rp of the PEO─PLLA Mg decreased with increase in exposure time. However, it was noted that the Rp of the PEO─PLLA Mg even after 100 h was six times higher than that of the PEO Mg after 48 h exposure, and did not show any visible localized attack.
... Consequently there is an (uphill) diffusion flux from the smaller phases to the bigger ones. This process is often called coarsening or Ostwald-ripening, and minimizes the total interfacial energy of the system [22][23][24][25]. ...
Purpose: In this work there are presented microstructure investigation results of the long aged Ag-Cu
alloy used for monetary production. The purpose of this work was to determine the microstructural
phase changes after 30 year ageing time, with appliance of transmission electron microscopy. Mainly
the possibility of spinodal decomposition process occurrence was investigated.
Design/methodology/approach: The investigations were performed using optical microscopy
for the microstructure determination. By mind of the transmission and scanning electron microscopy
the phase determination was possible to achieve. Morphology investigation of the Ag-Cu matrix and
phase identification using electron diffraction, EBSD technique and SEM phase contrast methods was
applied.
Findings: After the long time ageing time and plastic deformation of the material there are
morphological different areas of the Agα and Cuβ phase detected.
Research limitations/implications: The investigated material samples were examined
metallographically using light microscope, SEM, TEM with different image techniques. The hardness
was measured using the Rockwell hardness tester, also EDS microanalysis and electron diffraction was
performed.
Practical implications: As an implication for practice use there is the possibility of application of long
term ageing for mechanical properties improvement by natural ageing method. Also the comparison of
microstructure change and deformation after long term ageing can deliver a new scientific view on the
processes occurred in the microstructure over a long time period - spinodal decomposition can act as
an example of this. Some other investigations should be performed in the future, but the knowledge
found in this research shows an interesting investigation direction, where a low cost but long term
treatment operations can be applied.
Originality/value: The combination of TEM investigation for silver containing alloys makes the
investigation very attractive for electronic, chemical and monetary industry branches.
... 46,60,64,69,71,75,82,88,92,93,125,126,129,131,136,137,142,151,154,[168][169][170][171][172][173] However, both techniques are often used within a single study to provide complementary results regarding corrosion behavior. 63,80,127,132,143,144,155,157,[174][175][176][177][178][179][180][181][182][183][184] The primary advantage in utilizing these electrochemical techniques rather than the immersion methods described above is the ability to gain instantaneous data on the corrosion rate of the materials, while also elucidating some of the mechanisms resulting in this corrosion behavior. 145,161 However, it remains unclear as to whether these mechanisms are relevant to the corrosion that occurs in an in vivo environment. ...
Magnesium (Mg) has a long history of investigation as a degradable biomaterial. Physicians first began using Mg for biomedical applications in the late 19th century. Experimentation continued with varying levels of success until the mid-20th century when interest in the metal waned. In recent years the field of Mg-based biomaterials has once again become popular, likely due to advancements in technology allowing improved control of corrosion. Although this has led to success in vascular applications, continued difficulties in predicting and controlling the corrosion rate of Mg in an intraosseous environment has impeded the development of Mg-based biomaterials for orthopedic applications. In this review, an initial summary of the basic properties and the physiological role of Mg are followed by a discussion of the physical characteristics of the metal which lend it to use as a degradable biomaterial. A description of the historical and modern applications for Mg in the medical field is followed by a discussion of the methods used to control and assess Mg corrosion, with an emphasis on alloying. The second part of this review concentrates on the methods used to assess the corrosion and biocompatibility of Mg-based orthopedic biomaterials. This review provides a summary of Mg as a biomaterial from a biological perspective. © 2014 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2014.
... Furthermore, the microstructure of the alloy caused by the production process is important. It is reported that grain refinement decreased the corrosion rate [55,76,84,85]. Equal-channel angular pressing (ECAP) is previously used to produce homogenous ultrafine grains [76,84]. ...
The clinical application of degradable orthopedic magnesium implants is a tangible vision in medical science. This interdisciplinary review discusses many different aspects of magnesium alloys comprising the manufacturing process and the latest research. We present the challenges of the manufacturing process of magnesium implants with the risk of contamination with impurities and its effect on corrosion. Furthermore, this paper provides a summary of the current examination methods used in in vitro and in vivo research of magnesium alloys. The influence of various parameters (most importantly the effect of the corrosive media) in in vitro studies and an overview about the current in vivo research is given.
... alloys and polymers. However, the major issue of using magnesium as a biodegradable implant is that the degradation rate of pure magnesium in physiological conditions is extremely high [9], and produces hydrogen gas, which can create gas pockets around the implant [6]. There has been a large amount of work done to improve the degradation performance of biodegradable magnesium through alloying [10][11][12][13] and coatings [14][15][16][17][18]. ...
In this study, the in vitro degradation behaviour of AZ91 magnesium alloy with two different surface finishes was investigated using electrochemical impedance spectroscopy (EIS) in simulated body fluid (SBF). The polarisation resistance (Rp) of the rough surface alloy immersed in SBF for 3 h was ~30% lower as compared to that of the smooth surface alloy. After 12 h immersion in SBF, the Rp values for both the surface finishes decreased and were also similar. However, localised degradation occurred sooner, and to a noticeably higher severity in the rough surface alloy as compared to the smooth surface alloy.
... Many previous works reported that fine-grained magnesium alloys, such as AZ31 [14,15], AZ91 [16,17] and Mg-Ca alloys [6], could exhibit a significantly reduced corrosion rate compared to the coarse-grained one. On the basis of this assumption, the further decreased corrosion rate is expected for AZ31 alloy after grain refinement by enough pass number of ECAP and BP-ECAP processing. ...
The equal channel angular pressing (ECAP) technique with and without back pressure (BP) was introduced in this paper to prepare biomedical AZ31 magnesium alloy, with the effect of pass number (from 1 to 4) on the corrosion properties as well as in vitro biocompatibility being investigated. The results indicated that ECAPed or BP-ECAPed AZ31 alloys exhibited similar corrosion rate to that of the as-extruded one, but the corrosion rate slightly increased after 1–2 passes ECAP or BP-ECAP and further decreased after 4-pass procedure. Additionally, severe local corrosion was observed for the 1–3 passes ECAPed or BP-ECAPed AZ31 alloy samples. Compared to the as-extruded AZ31 alloy, the samples after ECAP or BP-ECAP procedure showed much smaller sized corrosion pits on the surface after removing the corrosion product. The surface analysis after 20 days immersion in Hank's solution revealed that the composition of the corrosion product consisted of C, O, Mg, P, Ca whereas only weak signal of Mg(OH)2 could be detected beside the dominant α(Mg) peak by X-ray diffraction. The cytotoxicity results suggested that the multi-pass ECAPed or BP-ECAPed AZ31 alloy exhibited Grade I–II cytotoxicity according to ISO 10993-5: 1999.
Plasma electrolytic oxidation (PEO) or micro-arc oxidation (MAO) coatings on magnesium and its alloys have gained increased attention in the past decade for their versatility. Unfortunately, due to their porous nature, they only provide short-term protection against corrosion of magnesium and its alloys. Hence, these coatings depend on other techniques for enhancing their corrosion protection efficiency. Particularly, effective sealing of the pores in the PEO/MAO coatings seems to be the research focus in recent years. This chapter discusses about the different techniques and advances made in improving the performance of PEO/MAO coatings on magnesium and its alloys.KeywordsPlasma electrolytic oxidationMicro-arc oxidationMg alloyCorrosion resistanceRecent progress
Corrosion and corrosion-related mechanical behaviors of Ti-microalloyed AZ31 Mg alloy (AZ31Ti) in simulated body fluid (SBF) under a dynamic environment were investigated. AZ31 Mg alloy was used as a control alloy. Microstructure analysis of the samples was performed by using a scanning electron microscope and an x-ray diffractometer. Mass loss measurements and corrosion-related tensile tests were carried out by immersing the samples in the SBF solution at 37.5±0.5 °C for 24, 72, and 336 h under dynamic conditions. Potentiodynamic polarization and electrochemical impedance spectroscopy measurements were also employed in the SBF solution at 37.5±0.5 °C. Microstructural studies showed that the β (Mg17Al12) intermetallic phases in the AZ31 alloy are dispersed in the microstructure and formed as relatively angular particles, and that the dimensions of the β phases transformed to a smaller size and globular form with Ti microalloying. While the tensile strength and hardness values of AZ31 and AZ31Ti alloys were similar to each other, Ti microalloying showed a considerable increase in the yield strength and elongation. This study suggests that microalloying of AZ31 alloy with Ti is beneficial in terms of their corrosion resistance and corrosion-related mechanical properties in an SBF environment under dynamic conditions.
The potential of Magnesium alloy as bio-absorbable or biodegradable implants for biomedical applications has been extensively studied. This paper summarizes the history and current status of Magnesium as a bio-absorbable implant material discussed in the development of a Magnesium alloy AZ91D that demonstrates promising degradation behavior relative to the commercially available Magnesium and its alloys. The paper also explains the mechanical and tribological properties which is necessary to classify biomaterial for their suitable use in medical industries.
Despite the high potential of biodegradable magnesium (Mg) alloys as a new generation of biomaterials for orthopaedic and cardiovascular implantation, their high corrosion rates in body fluid limits their suitability for clinical applications. Extensive research has been performed to decrease the corrosion rate of Mg-based biomaterials. Researchers have also been working to develop new testing and assessment techniques to evaluate the corrosion rate and other in vitro and in vivo properties of their modified Mg alloys. The objective of this review is to present the principles and operation procedures of commonly used standard methods for assessment of Mg-based biomaterials from bench to clinic. The pros and cons of each of these methods are discussed, together with factors for consideration to choose the right methodology. This review also presents the current state and challenges in understanding the testing of Mg-based biomaterials.
A systematic electrochemical study was performed to understand the influence of HEPES buffer concentration on the biodegradation behaviour of pure magnesium in a pseudo-physiological solution (Earle’s balanced salt solution (EBSS)). Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation experiments suggest that HEPES accelerates the degradation of magnesium. While 5% CO2 in EBSS reduced the polarisation resistance (Rp) of magnesium by ∼79%, addition of HEPES (25 mM) to EBSS decreased the Rp of magnesium by ∼98% and escalated the corrosion current (icorr) by over an order of magnitude as compared to that in EBSS. Increase in HEPES concentration (50 mM and 100 mM) further increased the degradation of magnesium. Interestingly, the bulk pH difference between EBSS+HEPES and EBSS+ CO2, before and after the electrochemical testing, was only marginal. However, the polarisation curves and Fourier transform infrared (FTIR) spectroscopy analysis suggest that HEPES affected the formation of insoluble salt layer (phosphate and carbonate) on the surface of magnesium. This effect can be attributed to the increase in the ionic strength of the solution due to HEPES addition.
Silver alloys, due to its specific properties are widely used in different branches of industry, with approximately 95% of the world silver production is used in the photographic industry and for jeweller production. Moreover Poland is a significant producer of silver in the world, and takes the 6th place on the list of the world's silver producers with the KGHM Polish Copper Company production on the first place among global companies providing silver on the market, with an annual production at 1281 tones. Because of this impotent role of silver, this work presents microstructure and mechanical properties investigations results of the long aged Ag-Cu alloys used mainly for mint monetary production. The purpose of this work was to determine the microstructural changes after 32 and 40 year of natural ageing time, with appliance of transmission electron microscopy as well as light microscopy. A very important issue - one of the investigations directions is also the possibility of study of spinodal decomposition process, which occurred in this alloy. After the long-time ageing of the material, there are detected morphological different areas of the Ag-α and Cu-β phase. As an implication for appliance in real conditions - as coin metal, or in electronic and chemical industry branches, there is the possibility of application of long term ageing for mechanical properties improvement by natural ageing method. Some other investigations should be performed in the future, but the knowledge found in this research shows an interesting investigation direction, where a low cost but long term treatment operations can be applied.
Calcium phosphate layer was coated onto AZ31 magnesium substrate with an insertion of a titania buffer layer by sol-gel method. The calcium phosphate layer was employed to enhance the bioactivity of the magnesium alloy, and the titania buffer layer was inserted to improve the bonding strength between the calcium phosphate layer and magnesium substrate, as well as to prevent the corrosion of the magnesium substrate. The structure and composition of the sol-gel coatings were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The bonding strength, corrosion behaviors and the hydrogen evolution rate of the samples were also studied. The results showed that the sol-gel coatings improved the corrosion resistance and reduced the hydrogen evolution rate of the magnesium alloy in simulated body fluid (SBF) solution. The titania buffer layer could significantly improve the bonding strength and the corrosion resistance.
The effect of a hybrid coating, calcium phosphate (Cap) + polylactic acid (PLA), on a magnesium alloy on its in vitro degradation (general and localized) behaviour was studied for potential load-bearing biodegradable mini-implant applications. CaP was coated on a magnesium alloy, AZ91, using an electrochemical deposition method. A spin coating method was used to coat PLA on the CaP coated alloy. In vitro degradation performance of the alloy with hybrid coating was evaluated using electrochemical impedance spectroscopy (EIS) in simulated body fluid (SBF). The EIS results showed that the hybrid coating enhanced the degradation resistance of the alloy by more than two-order of magnitude as compared to the bare alloy and one-order of magnitude higher than that of the CaP coated alloy, after 1 h exposure in simulated body fluid (SBF). Long-term (48 h) EIS results also confirmed that the hybrid coating performed better than the bare alloy and the CaP coated alloy. Importantly, the hybrid coating improved the localized degradation resistance of the alloy significantly, which is critical for better in service mechanical integrity.
Nano- or micropowders of Eu2O3 were added to MgB2, resulting in a composition of (MgB2)0.975(EuO1.5)0.025. Pristine and doped samples were prepared using spark plasma sintering and tested for (i) Vickers hardness, (ii) pH evolution in phosphate-buffered saline solution, (iii) corrosion resistance (Tafel polarization curves), (iv) cytotoxicity (in vitro tests), and (v) antibacterial activity. Eu2O3 addition influenced the investigated properties. Solutions of MgB2-based samples show a relatively high saturation pH of 8.5. This value is lower than that of solutions incubated with Mg or other Mg-based biodegradable alloys reported in the literature. MgB2-based samples have lower electro-corrosion rates than Mg. Their Vickers hardness is 6.8–10.2 GPa, and these values are higher than those of biodegradable Mg-based alloys. MgB2 has low in vitro biocompatibility, good antibacterial activity against Escherichia coli, and mild activity against Staphylococcus aureus. Our results suggest that MgB2-based materials deserve attention in biomedical applications, such as implants or sterile medical instruments.
The aim of this work was to understand the effect of microgalvanic degradation on secondary phase particles in magnesium alloys under in vitro condition. Pure magnesium and Mg17 Al12 (β-phase) were galvanically coupled in simulated body fluid and the degradation behavior was studied using electrochemical impedance spectroscopy. The galvanic coupling produced a phosphate/carbonate layer on the β-phase, which initially increased the degradation resistance. However, the deposited phosphate/carbonate layer rapidly degraded once the galvanic coupling was removed, and β-phase exhibited similar degradation resistance to that of pure magnesium. A phenomenological model has been presented, demonstrating the galvanic coupling effect. © 2014 Wiley Periodicals, Inc. J Biomed Mater Res Part A, 2014.
Thermodynamical stabilities of four conventional second phases as well as magnesium
matrix in bioabsorbable magnesium alloys were investigated theoretically via
computer calculationmethod. Model of individual phase and systems including phase
and four water molecular (phase-4H2O) were established to simulate the in vitro and
in vivo environment. Local orbital density functional theory approach was applied to
calculate the total energy for the individual phase and phase-4H2Osystem. The results
demonstrated that all the second phases possessed higher phase stability compared
with magnesium matrix, but the phase stability was quite different for different types
of second phases or second phase-4H2O systems. Furthermore, a schematic process
of inflammation reaction caused by magnesium alloy implants was proposed for the
further evaluation on biocompatibility of different second phases.
In this study, a magnesium alloy (AZ91) was coated with calcium phosphate using potentiostatic pulse-potential and constant-potential methods and the in vitro corrosion behaviour of the coated samples was compared with the bare metal. In vitro corrosion studies were carried out using electrochemical impedance spectroscopy and potentiodynamic polarization in simulated body fluid (SBF) at 37 °C. Calcium phosphate coatings enhanced the corrosion resistance of the alloy, however, the pulse-potential coating performed better than the constant-potential coating. The pulse-potential coating exhibited ~ 3 times higher polarization resistance than that of the constant-potential coating. The corrosion current density obtained from the potentiodynamic polarization curves was significantly less (~ 60%) for the pulse-deposition coating as compared to the constant-potential coating. Post-corrosion analysis revealed only slight corrosion on the pulse-potential coating, whereas the constant-potential coating exhibited a large number of corrosion particles attached to the coating. The better in vitro corrosion performance of the pulse-potential coating can be attributed to the closely packed calcium phosphate particles.
A simple method of hydroxyapatite (HA) coating deposition on Mg substrates at 37 °C is proposed. It was established that variation of ionic composition of the initial solution leads to the deposition of coatings with various phase composition, i.e. DCPD, DCPD + HA, HA which decreased corrosion rate of Mg. The paper also discusses the crystallization of dicalcium phosphate dehydrate (DCPD) and HA coatings on Mg substrates obtained by dipping method under the permanent magnetic field (0.3 T) in the neighborhood of the north and the south pole. A difference in particle morphology and crystal texture of precipitates in the north pole and the south pole proximity was observed. Lattice parameters of DCPD coatings obtained near opposite magnet poles were calculated using XRD results. It was found that the proximity to the south pole of magnet increases the crystallinity of calcium–phosphates. Increase of crystallite sizes in (0 2 0) and (0 4 0) plane was observed for DCPD in the presence of magnetic field.
A new type of patented biodegradable biomedical magnesium alloy Mg–Nd–Zn–Zr (hereafter, denoted as JDBM) was prepared in this study. The biocorrosion properties of the as-extruded JDBM alloy were investigated in simulated body fluid (SBF) by hydrogen evolution, mass loss and electrochemical tests. The biocorrosion properties of as-extruded AZ31 and as-extruded WE43 alloys as well as the mechanical properties at room temperature were also studied in order to compare with the novel JDBM biodegradable biomedical magnesium alloy. The results show that the as-extruded JDBM alloy not only owns much better mechanical properties at room temperature but also exhibits much better biocorrosion properties in SBF.
The microstructure and mechanical properties of as-extruded Mg-8Y-1Er-2Zn (wt%) alloy containing long period stacking ordered (LPSO) phase are comparatively investigated before and after corrosion in a simulated body fluid (SBF) at 37°C. The as-extruded alloy consists of a long strip-like 18R-LPSO phase and some fine lamellae grains formed by primary recrystallization during the extrusion process. The hydrogen evolution volume per day fluctuates between 0.21 and 0.32ml/cm(2) in the immersion test for 240h, and the corresponding corrosion rate is calculated as 0.568mm/y. The corrosion product is determined as Mg(OH)2, whilst a Ca(H2PO4)2 compound is also observed on the surface of the samples. The corrosion site preferentially occurs at the interface between LPSO phase and Mg matrix. Before immersing, the tensile yield strength (TYS), ultimate tensile strength (UTS) and elongation of the alloy are 275MPa, 359MPa, and 19%, respectively. More attractively, these mechanical properties can be maintained even after immersing in SBF for 240h (TYS, UTS and elongation are 216MPa, 286MPa and 6.8%, respectively) because of the existence of high anti-corrosion LPSO phase.
The corrosion behavior of an AZ91 magnesium alloy in 0.1M sodium sulfate solution at the corrosion potential (Ecorr) was investigated using electrochemical impedance spectroscopy (EIS), environmental scanning electron microscopy (ESEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The results showed that when the immersion time was less than 18th, general corrosion occurred on the surface and the main corrosion products were hydroxides and sulfates. The film coverage effect was the main mechanism for the corrosion process of AZ91 alloy. At this stage, the matrix had a better corrosion resistance. With the increasing immersion time, pitting occurred on the surface. At this stage, the corrosion process was controlled by three surface state variables: the area fraction θ1 of the region controlled by the formation of Mg(OH)2, the area fraction θ2 of the region controlled by the precipitation of MgAl2(SO4)4·2H2O, and the metastable Mg+ concentration Cm.
As a lightweight metal with mechanical properties similar to natural bone, a natural ionic presence with significant functional roles in biological systems, and in vivo degradation via corrosion in the electrolytic environment of the body, magnesium-based implants have the potential to serve as biocompatible, osteoconductive, degradable implants for load-bearing applications. This review explores the properties, biological performance, challenges and future directions of magnesium-based biomaterials.
Recent studies indicate that there is a high demand to design magnesium alloys with adjustable corrosion rates and suitable mechanical properties. An approach to this challenge might be the application of metal matrix composite (MMC) based on magnesium alloys. In this study, a MMC made of magnesium alloy AZ91D as a matrix and hydroxyapatite (HA) particles as reinforcements have been investigated in vitro for mechanical, corrosive and cytocompatible properties. The mechanical properties of the MMC-HA were adjustable by the choice of HA particle size and distribution. Corrosion tests revealed that HA particles stabilised the corrosion rate and exhibited more uniform corrosion attack in artificial sea water and cell solutions. The phase identification showed that all samples contained hcp-Mg, Mg(17)Al(12), and HA before and after immersion. After immersion in artificial sea water CaCO3 was found on MMC-HA surfaces, while no formation of CaCO3 was found after immersion in cell solutions with and without proteins. Co-cultivation of MMC-HA with human bone derived cells (HBDC), cells of an osteoblasts lineage (MG-63) and cells of a macrophage lineage (RAW264.7) revealed that RAW264.7, MG-63 and HBDC adhere, proliferate and survive on the corroding surfaces of MMC-HA. In summary, biodegradable MMC-HA are cytocompatible biomaterials with adjustable mechanical and corrosive properties.
The electrochemical behavior of magnesium in aerated 0.5 M Na2SO4 solution was investigated by plotting steady-state current—voltage curves and by measurement of electrochemical impedance.The low value of capacity associated with the high frequency loop, and the increase in protection with immersion time provide evidence for the existence of a protective layer over the surface. The layer was thought to consist of Mg(OH)2 and/or MgO. The protective action of this film is dependent on anodic or cathodic polarization, the latter affording better protection.The electrochemical determination of the corrosion rate was based on the Stern and Geary relationship, into which were introduced the value of the charge transfer resistance (diameter of the high frequency loop in the impedance diagram) and the anodic and cathodic Tafel coefficients. The results obtained are in agreement with those obtained by direct assay of dissolved magnesium by atomic absorption spectroscopy.
Applications of magnesium alloys as biodegradable orthopaedic implants are critically dependent on the mechanical integrity of the implant during service. In this study, the stress corrosion cracking susceptibility of sand-cast Mg–Al–Zn alloy in modified-simulated body fluid was evaluated using the slow strain rate test method. The study suggests that the stress corrosion cracking susceptibility of the sand-cast magnesium alloy is not substantial and this aspect should not be a concern for its implant applications.
The corrosion behavior of skins and the bulk of die cast and thiocast AXJ530 Mg alloy with its growing use in automotive industry because of its good mechanical properties is discussed. General Motors has developed an Mg-Al-Ca-Sr AXJ530 alloy for use in the automotive engines to improve creep resistance and to study the corrosion property of the skin and bulk of die cast and thiocast AXJ530 Mg alloy by electrochemical impedance spectroscopy (EIS). Thiocasting process induces strong modifications in in the microstructure of AXJ530 alloy in terms of distribution, composition, and volume fraction of the constituent phases. The results show that the corrosion rate of thiocast alloy is 35% less than any other die cast alloy.
The high strength to weight ratio of magnesium alloys makes them extremely attractive for applications in transport or aerospace technology. However, their corrosion behavior is a major issue and one reason why they are still not as popular as aluminum alloys. This papers reviews the corrosion mechanisms of magnesium and provides the basis for the design of new alloys with improved corrosion properties.
Magnesium (Mg) alloys as well as experimental alloys are emerging as light structural materials for current, new, and innovative
applications. This paper describes the influence of the alloying elements and the different casting processes on the microstructure
and performance of these alloys and corrosion. It gives a comprehensible approach for the resistance of these alloys to general,
localized and metallurgically influenced corrosion, which are the main challenges for their use. Exposure to humid air with
∼65% relative humidity during 4 days gives 100–150 nm thickness. The film is amorphous and has an oxidation rate less than
0.01 µm/y. The pH values between 8.5 and 11.5 correspond to a relatively protective oxide or hydroxide film; however above
11.5 a passive stable layer is observed. The poor corrosion resistance of many Mg alloys can be due to the internal galvanic
corrosion caused by second phases or impurities. Agitation or any other means of destroying or preventing the formation of
a protective film leads to increasing corrosion kinetics. The pH changes during pitting corrosion can come from two different
reduction reactions: reduction of dissolved oxygen (O) and that of hydrogen (H) ions. Filiform corrosion was observed in the
uncoated AZ31, while general corrosion mainly occurred in some deposition coated alloys. Crevice corrosion can probably be
initiated due to the hydrolysis reaction. Exfoliation can be considered as a type of intergranular attack, and this is observed
in unalloyed Mg above a critical chloride concentration.
The corrosion of a magnesium alloy containing rare earth elements (WE43 type alloy) was studied in 0.05 and 0.5M Na2SO4 or 0.1 and 1M NaCl solutions using electrochemical techniques: linear polarization resistance, potentiodynamic polarization, impedance
measurements. The electrolytes favoured anodic magnesium oxidation but the presence of rare earth elements improved the tendency
of magnesium to passivation. The dissolution rates in chlorides were higher than in sulphates because chlorides, in contrast
to sulphates, interfered with the formation and maintenance of a protective layer of corrosion products which decreased the
severity of the attack. The effects of galvanic corrosion due to cathodic intermetallic precipitates at grain boundaries were
particularly evident in chloride media at long testing times.
The corrosion behaviour of pure magnesium in aerated and deaerated Na2SO4 solutions (0.01 and 0.1 M) was investigated by steady-state current–voltage curves and electrochemical impedance measurements with a rotating disc electrode. It was shown that the anodic current densities were lower and the resistance values higher in deaerated media. It was demonstrated that the absence of HCO3− due to the absence of CO2, which is always present in the natural environment was responsible for the results observed. The magnesium surface was covered with a porous film (MgO, Mg(OH)2) which became thicker with time. The protection afforded by the layer was low and the corrosion rate remained constant as the immersion time increased. In addition, experiments carried out at different NaHCO3 concentrations in deaerated media revealed that the corrosion rate of magnesium was accelerated when the NaHCO3 concentration increased due to dissolution of the film (MgO, Mg(OH)2). It was concluded that O2 did not influence magnesium corrosion.
The effect of microconstituents on the corrosion and electrochemical behaviour of AZ91D alloy prepared by die-casting and ingot casting route has been investigated in 3.5% NaCl solution at pH 7.25. The experimental techniques used include constant immersion technique, in-situ corrosion monitoring, and potentiodynamic polarisation experiments. Surface examination and analytical studies were carried out using optical and scanning electron microscopy, EDX and XRD. The corrosion behaviour of microconstituents namely primary α, eutectic α and β phases was significantly different. Coring of aluminum showed influence on corrosion behaviour more significantly in ingot material. Areas with aluminium concentration less than about 8% were found to be prone to corrosion attack compared with either those with higher amount of aluminium or β phase. Die-cast material with smaller grain size and fine β phase offered marginally lower corrosion rate and better passivation compared with the ingot. In die-cast and ingot, hydrogen evolution took place preferentially on β phase. XRD pattern of non-corroded and corroded surface revealed the removal of β phase from alloy surface during corrosion. The corrosion products for ingot consisted of Mg(OH)2 with small amounts β phase, magnesium-aluminium oxide and MgH2 while for die-cast, the product showed a highly amorphous structure.
The present work is dedicated to a study of the corrosion resistance of AZ91D (91% Mg) alloy in wet environments. Three industrial alloys obtained by die-casting or sand casting were subjected to salt spray corrosion tests (ASTM-B117 standard) and immersion tests. Weight loss kinetic curves were measured. Surface analysis was performed by X-ray photoelectron diffraction (XPS). After corrosion the sand cast alloy presents a surface mainly enriched in hydroxides and carbonates while the die-cast alloy presents a surface enriched also in mixed Mg–Al oxides. The quantitative analysis of the rate Mg/Al shows an enrichment in aluminium for the die-cast alloys in comparison to the sand cast alloy.
Current tissue engineering strategies focus on the replacement of pathologically altered tissues by the transplantation of cells in combination with supportive biocompatible scaffolds. Scaffolds for tissue engineering strategies in musculoskeletal research require an appropriate mechanical stability. In recent studies, considerable attention has thus been given to magnesium alloys as biodegradable implants. The aim of this study was to characterize the biocompatibility of magnesium scaffolds by the inflammatory host response. Open porous scaffolds made of the magnesium alloy AZ91D were implanted into the distal femur condyle of rabbits and were compared to autologous bone, which was transplanted into the contralateral condyle in a 3 and 6 months follow-up group. After 3 months, magnesium scaffolds were already largely degraded and most of the original magnesium alloy has disappeared. Concomitantly, a fibrous capsule enclosed the operation site. Histological analysis revealed that the magnesium scaffolds caused no significant harm to their neighboring tissues. This study shows that even fast degrading magnesium scaffolds show a good biocompatibility and react in vivo with an appropriate inflammatory host response. Magnesium alloy based implants are therefore a very promising approach in the development of mechanically suitable and open porous scaffolds for the replacement of subchondral bone in cartilage tissue engineering.
To understand the in vitro degradation mechanism of magnesium alloy, electrochemical experiments viz., electrochemical impedance spectroscopy and potentiodynamic polarization, were carried out on AZ91 magnesium alloy under different experimental conditions. The study suggests: (i) the body temperature decreases significantly the corrosion resistance of the alloy, (ii) alkali-treatment of the alloy enhances the corrosion resistance, and (iii) although chloride in simulated body fluid minimizes the corrosion resistance, the presence of other constituents viz., phosphate, calcium, and carbonate, enhances the film forming tendency and hence increases the corrosion resistance of the alloy.
The electrochemistry of the corrosion process of a magnesium rare-earth-alloy is studied in detail in simulated body fluid (m-SBF) over the first 5 days. The aim is to investigate the corrosion mechanism under in vitro conditions. For this purpose we also used electrolytes that contain only some of the components of SBF, they were compared to SBF to investigate the influence of the different ions in SBF. The influence of albumin on the corrosion process was studied with a solution containing m-SBF and albumin in physiological concentration. For this study, impedance spectroscopy series measurements were performed. Additional results were gained from polarization curves. We conclude from the study that the corrosion resistance is significantly lower in m-SBF than in simple isotonic NaCl-solution. Albumin may form a blocking layer on the surface in the first hours of exposure. The formed corrosion layers consisting of amorphous apatite have only a low protective ability. Further results show that the corrosion processes in SBFs follow a linear time-law. The results elucidate critical factors and mechanisms of the electrochemical corrosion process of magnesium rare-earth alloys in SBFs, this understanding is crucial for a successful application of Mg alloys in biomedical applications.
The successful applications of magnesium-based alloys as degradable orthopaedic implants are mainly inhibited due to their high degradation rates in physiological environment and consequent loss in the mechanical integrity. This study examines the degradation behaviour and the mechanical integrity of calcium-containing magnesium alloys using electrochemical techniques and slow strain rate test (SSRT) method, respectively, in modified-simulated body fluid (m-SBF). Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) results showed that calcium addition enhances the general and pitting corrosion resistances of magnesium alloys significantly. The corrosion current was significantly lower in AZ91Ca alloy than that in AZ91 alloy. Furthermore, AZ91Ca alloy exhibited a five-fold increase in the surface film resistance than AZ91 alloy. The SSRT results showed that the ultimate tensile strength and elongation to fracture of AZ91Ca alloy in m-SBF decreased only marginally (approximately 15% and 20%, respectively) in comparison with these properties in air. The fracture morphologies of the failed samples are discussed in the paper. The in vitro study suggests that calcium-containing magnesium alloys to be a promising candidate for their applications in degradable orthopaedic implants, and it is worthwhile to further investigate the in vivo corrosion behaviour of these alloys.