Corrosion resistance of plasma-anodized AZ91D magnesium alloy by electrochemical methods

Simulation of Binder Jetting and Analysis of Magnesium Alloy Bonding Mechanism

Binder jetting (3DP) is a kind of additive manufacturing at room temperature and atmospheric environment, which can reduce the risk of magnesium alloy forming. Magnesium alloy powder is bonded to a certain structure by a binder, so the appropriate binder is very important in 3DP. In this study, according to the characteristics of magnesium alloy, a simple and easy-to-obtain water-based low-molecular alcohol binder was used to reduce the difficulty of magnesium alloy 3DP. Additionally, we use COMSOL Multiphysics simulation software to establish a simulation model of the movement and deposition process of the binder. The results show that the increase in jet velocity will increase the quality and saturation of droplets. More importantly, the larger the jet velocity is, the larger the spreading width of the binder droplet after impacting the powder bed, which seriously affects the dimensional accuracy of the green part. In addition, lower binder saturation will weaken the formation of interparticle bonding neck and cannot form a stable structure. Furthermore, we analyzed the bond reactants of the binder and magnesium alloy powder, which eventually decompose into MgO, and the experimental results show that the final sintered sample has considerable performance.

Performance of PEO/Polymer Coatings on the Biodegradability, Antibacterial Effect and Biocompatibility of Mg-Based Materials

Magnesium (Mg) alloys have recently attracted attention in biomedicine as biodegradable materials with non-toxic degradable products. Such compounds have become a frontier in the study of biodegradable materials because of their remarkable biomechanical compatibility and superior biocompatibility. The use of Mg-based implants reduces the negative consequences of permanent biological implants by eliminating the necessity for biomaterial surgery following the healing process. However, the quick deterioration, formation of considerable gas of hydrogen volumes and a rise in the body environment pH are obstacles in the application of Mg as an implant material. Hence, compelling advances for erosion resistance and biocompatibility of magnesium and its alloys are noteworthy. Surface modification may be a practical approach because it improves the erosion resistance compared with extensive preparation of a treated surface for progressed bone recovery and cell attachment. Coating produced by plasma electrolytic oxidation (PEO) seems a compelling method in order to enhance magnesium and the properties of its alloys. PEO-formed coatings cannot provide long-term protection in the physiological environment due to their porous nature. Thus, a polymer coating is applied on the porous PEO-formed coating, which is steadily applied on the surface. Polymer coatings improve the biocompatibility properties of Mg and its alloys and increase corrosion resistance. In this article, the most recent advancements in PEO/polymer composite coatings are reviewed, and the biocompatibility of such coatings is examined.

Chromate-Free Corrosion Protection Strategies for Magnesium Alloys-A Review: Part II-PEO and Anodizing

Although hexavalent chromium-based protection systems are effective and their long-term performance is well understood, they can no longer be used due to their proven Cr(VI) toxicity and carcinogenic effect. The search for alternative protection technologies for Mg alloys has been going on for at least a couple of decades. However, surface treatment systems with equivalent efficacies to that of Cr(VI)-based ones have only begun to emerge much more recently. It is still proving challenging to find sufficiently protective replacements for Cr(VI) that do not give rise to safety concerns related to corrosion, especially in terms of fulfilling the requirements of the transportation industry. Additionally, in overcoming these obstacles, the advantages of newly introduced technologies have to include not only health safety but also need to be balanced against their added cost, as well as being environmentally friendly and simple to implement and maintain. Anodizing, especially when carried out above the breakdown potential (technology known as Plasma Electrolytic Oxidation (PEO)) is an electrochemical oxidation process which has been recognized as one of the most effective methods to significantly improve the corrosion resistance of Mg and its alloys by forming a protective ceramic-like layer on their surface that isolates the base material from aggressive environmental agents. Part II of this review summarizes developments in and future outlooks for Mg anodizing, including traditional chromium-based processes and newly developed chromium-free alternatives, such as PEO technology and the use of organic electrolytes. This work provides an overview of processing parameters such as electrolyte composition and additives, voltage/current regimes, and post-treatment sealing strategies that influence the corrosion performance of the coatings. This large variability of the fabrication conditions makes it possible to obtain Cr-free products that meet the industrial requirements for performance, as expected from traditional Cr-based technologies.

Optimization of AZ31B Magnesium Alloy Anodizing Process in NaOH-Na2SiO3-Na2B4O7 Environmental-Friendly Electrolyte

The optimization of NaOH-Na2SiO3-Na2B4O7 electrolyte for the plasma electrolytic oxidation of AZ31B magnesium alloy was investigated through orthogonal tests. The properties of the anodized films were evaluated by film thickness, roughness measurements, salt spray tests, scanning electron microscopy (SEM), X-ray diffraction (XRD) and potentiodynamic polarization tests, respectively. The orthogonal tests revealed that the optimal formulation of the electrolyte comprised NaOH 45 g/L, Na2SiO3 50 g/L, and Na2B4O7 90 g/L. NaOH exhibited the most significant effect on film thickness, while Na2SiO3 had the greatest effect on corrosion resistance. Moreover, the optimal electrical parameters were also obtained with the values of current density 1 A /dm2, oxidation time 15 min, pulse frequency 200 Hz and duty cycle of 10%. The surface morphology of the anodized coating formed under optimal conditions was uniform and compact. Furthermore, the phase compositions of all samples were mainly composed of MgO and Mg2SiO4. The corrosion potential, corrosion current density and polarization resistance of the prepared coating by plasma electrolytic oxidation improved remarkably compared with that of the substrate.

Plasma Electrolytic Oxidation (PEO) Process-Processing, Properties, and Applications

Plasma electrolytic oxidation (PEO) is a novel surface treatment process to produce thick, dense metal oxide coatings, especially on light metals, primarily to improve their wear and corrosion resistance. The coating manufactured from the PEO process is relatively superior to normal anodic oxidation. It is widely employed in the fields of mechanical, petrochemical, and biomedical industries, to name a few. Several investigations have been carried out to study the coating performance developed through the PEO process in the past. This review attempts to summarize and explain some of the fundamental aspects of the PEO process, mechanism of coating formation, the processing conditions that impact the process, the main characteristics of the process, the microstructures evolved in the coating, the mechanical and tribological properties of the coating, and the influence of environmental conditions on the coating process. Recently, the PEO process has also been employed to produce nanocomposite coatings by incorporating nanoparticles in the electrolyte. This review also narrates some of the recent developments in the field of nanocomposite coatings with examples and their applications. Additionally, some of the applications of the PEO coatings have been demonstrated. Moreover, the significance of the PEO process, its current trends, and its scope of future work are highlighted.

Physical and biological characterizations of TiNbSn/(Mg) system produced by powder metallurgy for use as prostheses material

Titanium scaffolds with non-toxic β stabilizing elements (Nb and Sn), Ti-34Nb-6Sn (TNS), and with magnesium as spacer (TNS/M), were processed by powder metallurgy, and sintered at 800 °C. The X-ray diffraction (XRD) pattern showed that materials are biphasic alloys, presenting 45 to 42% (wt %) in hcp (α-phase) and the rest is bcc (β-phase), and the presence of a slight peak relating to TiO₂ in both materials. Pores of approximately 50 μm for TNS and 300 μm to TNS/M were observed in the micrographic analysis by scanning electron microscopy (SEM). The wettability was higher for TNS/M compared to TNS. The elastic modulus was higher for TNS compared to TNS/M. Stem cells derived from equine bone marrow (BMMSCs) were used for in vitro assays. The morphologic and adhesion evaluation after 72 h, carried out by direct contact assay with the materials showed that the BMMSCs were anchored and adhered to the porous scaffolds, in the way the cytoplasmic extension was observed. The cellular migration, using the "wound healing" method, was significant for the groups treated with conditioned medium with materials in 24 h. Osteogenic differentiation of BMMSCs, assessed by calcium deposition and staining with Alizarin Red, was greater in the conditioned medium with TNS/M in 10 days of culture. Since the biological effects was good and the elastic modulus decreased in the system with magnesium is a promising new content titanium alloy for biomedical application.

Antimicrobial Properties of MgO Nanostructures on Magnesium Substrates

Magnesium (Mg) and its alloys have attracted increasing attention in recent years as medical implants for repairing musculoskeletal injuries because of their promising mechanical and biological properties. However, rapid degradation of Mg and its alloys in physiological fluids limited their clinical translation because the accumulation of hydrogen (H2) gas and fast release of OH- ions could adversely affect the healing process. Moreover, infection is a major concern for internally implanted devices because it could lead to biofilm formation, prevent host cell attachment on the implants, and interfere osseointegration, resulting in implant failure or other complications. Fabricating nanostructured magnesium oxide (MgO) on magnesium (Mg) substrates is promising in addressing both problems because it could slow down the degradation process and improve the antimicrobial activity. In this study, nanostructured MgO layers were created on Mg substrates using two different surface treatment techniques, i.e., anodization and electrophoretic deposition (EPD), and cultured with Staphylococcus aureus in vitro to determine their antimicrobial properties. At the end of the 24-h bacterial culture, the nanostructured MgO layers on Mg prepared by anodization or EPD both showed significant bactericidal effect against S. aureus. Thus, nanostructured MgO layers on Mg are promising for reducing implant-related infections and complications and should be further explored for clinical translation toward antimicrobial biodegradable implants.

Surface degradation study of magnesium tested in simulated body fluid

Magnesium has attracted a lot of attention as a new class of biodegradable material. The surface properties of magnesium in simulated body fluid (SBF) were investigated in the current research work. Cast magnesium samples with different surface characteristics were prepared which were then placed in SBF for 2, 4 and 8 days. This led to the formation of hydroxylapatite coating on their surfaces. The solution was changed at regular intervals to maintain a pH of 7.6. Other Mg samples were anodized at 40 V and 60 V to create a uniform layer of oxide on them and then their activity in SBF was compared with the casted samples of Mg. It was found that corrosion rate varies as the immersion time increases. Magnesium undergoes localized corrosion with pits and cracks forming on the surface of the samples. This was due to the aggressive corrosive nature of SBF.

The current trends of Mg alloys in biomedical applications-A review

Magnesium (Mg) has emerged as an ideal alternative to the permanent implant materials owing to its enhanced properties such as biodegradation, better mechanical strengths than polymeric biodegradable materials and biocompatibility. It has been under investigation as an implant material both in cardiovascular and orthopedic applications. The use of Mg as an implant material reduces the risk of long-term incompatible interaction of implant with tissues and eliminates the second surgical procedure to remove the implant, thus minimizes the complications. The hurdle in the extensive use of Mg implants is its fast degradation rate, which consequently reduces the mechanical strength to support the implant site. Alloy development, surface treatment, and design modification of implants are the routes that can lead to the improved corrosion resistance of Mg implants and extensive research is going on in all three directions. In this review, the recent trends in the alloying and surface treatment of Mg have been discussed in detail. Additionally, the recent progress in the use of computational models to analyze Mg bioimplants has been given special consideration. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1970-1996, 2019.

The Surface Modification Methods for Constructing Polymer-Coated Stents

Implanting a metal stent plays a key role in treating cardiovascular diseases. However, the high corrosion rate of metal-based devices severely limits their practical applications. Therefore, how to control the corrosion rate is vital to take full advantages of metal-based materials in the treatment of cardiovascular diseases. This review details various methods to design and construct polymer-coated stents. The techniques are described and discussed including plasma deposition, electrospinning, dip coating, layer-by-layer self-assembly, and direct-write inkjet. Key point is provided to highlight current methods and recent advances in hindering corrosion rate and improving biocompatibility of stents, which greatly drives the rising of some promising techniques involved in the ongoing challenges and potential new trends of polymer-coated stents.

Innovative micro-textured hydroxyapatite and poly(l-lactic)-acid polymer composite film as a flexible, corrosion resistant, biocompatible, and bioactive coating for Mg implants

The utility of a novel ceramic/polymer-composite coating with a micro-textured microstructure that would significantly enhance the functions of biodegradable Mg implants is demonstrated here. To accomplish this, bioactive hydroxyapatite (HA) micro-dots can be created by immersing a Mg implant with a micro-patterned photoresist surface in an aqueous solution containing calcium and phosphate ions. The HA micro-dots can then be surrounded by a flexible poly(l-lactic)-acid (PLLA) polymer using spin coating to form a HA/PLLA micro-textured coating layer. The HA/PLLA micro-textured coating layer showed an excellent corrosion resistance when it was immersed in a simulated body fluid (SBF) solution and good biocompatibility, which was assessed by in vitro cell tests. In addition, the HA/PLLA micro-textured coating layer had high deformation ability, where no apparent changes in the coating layer were observed even after a 5% elongation, which would be unobtainable using HA and PLLA coating layers; furthermore, this allowed the mechanically-strained Mg implant with the HA/PLLA micro-textured coating layer to preserve its excellent corrosion resistance and biocompatibility in vitro.

Enhanced corrosion resistance and cytocompatibility of biodegradable Mg alloys by introduction of Mg(OH)2 particles into poly (L-lactic acid) coating

A strategy of suppressing the fast degradation behaviour of Mg-based biomaterials by the introduction of one of Mg degradation products Mg(OH)₂ was proposed according to the following degradation mechanism, Mg + 2H₂O ⇋ Mg(OH)₂ + H₂↑. Specifically, Mg(OH)₂ submicron particles were mixed into poly (L-lactic acid) (PLLA) to synthesize a composite coating onto hydrofluoric acid-pretreated Mg-Nd-Zn-Zr alloy. The in vitro degradation investigations showed that the addition of Mg(OH)₂ particles not only slowed down the corrosion of Mg matrix, but also retarded the formation of gas pockets underneath the polymer coating. Correspondingly, cytocompatibility results exhibited significant improvement of proliferation of endothelial cells, and further insights was gained into the mechanisms how the introduction of Mg(OH)₂ particles into PLLA coating affected the magnesium alloy degradation and cytocompatibility. The present study provided a promising surface modification strategy to tailor the degradation behaviour of Mg-based biomaterials.

Comparison of Electrochemical Methods for the Evaluation of Cast AZ91 Magnesium Alloy

Linear polarization is a potentiodynamic method used for electrochemical characterization of materials. Obtained values of corrosion potential and corrosion current density offer information about material behavior in corrosion environments from the thermodynamic and kinetic points of view, respectively. The present study offers a comparison of applications of the linear polarization method (from -100 mV to +200 mV vs. E_OCP), the cathodic polarization of the specimen (-100 mV vs. E_OCP), and the anodic polarization of the specimen (+100 mV vs. E_OCP), and a discussion of the differences in the obtained values of the electrochemical characteristics of cast AZ91 magnesium alloy. The corrosion current density obtained by cathodic polarization was similar to the corrosion current density obtained by linear polarization, while a lower value was obtained by anodic polarization. Signs of corrosion attack were observed only in the case of linear polarization including cathodic and anodic polarization of the specimen.

Effect of current density on the microstructure and corrosion resistance of microarc oxidized ZK60 magnesium alloy

The application of magnesium alloys as biomaterials is limited by their poor corrosion behavior. Microarc oxidation (MAO) treatment was used to prepare ceramic coatings on ZK60 magnesium alloys in order to overcome the poor corrosion resistance. The process was conducted at different current densities (3.5 and 9.0 A/dm(2)), and the effect of current density on the process was studied. The microstructure, elemental distribution, and phase composition of the MAO coatings were characterized by scanning electron microscopy, energy-dispersive x-ray spectrometry, and x-ray diffraction, respectively. The increment of current density contributes to the increase of thickness. A new phase Mg2SiO4 was detected as the current density increased to 9.0 A/dm(2). A homogeneous distribution of micropores could be observed in the coating produced at 3.5 A/dm(2), while the surface morphology of the coating formed at 9.0 A/dm(2) was more rough and apparent microcracks could be observed. The coating obtained at 3.5 A/dm(2) possessed a better anticorrosion behavior.

Biomedical coatings on magnesium alloys - a review

This review comprehensively covers research carried out in the field of degradable coatings on Mg and Mg alloys for biomedical applications. Several coating methods are discussed, which can be divided, based on the specific processing techniques used, into conversion and deposition coatings. The literature review revealed that in most cases coatings increase the corrosion resistance of Mg and Mg alloys. The critical factors determining coating performance, such as corrosion rate, surface chemistry, adhesion and coating morphology, are identified and discussed. The analysis of the literature showed that many studies have focused on calcium phosphate coatings produced either using conversion or deposition methods which were developed for orthopaedic applications. However, the control of phases and the formation of cracks still appear unsatisfactory. More research and development is needed in the case of biodegradable organic based coatings to generate reproducible and relevant data. In addition to biocompatibility, the mechanical properties of the coatings are also relevant, and the development of appropriate methods to study the corrosion process in detail and in the long term remains an important area of research.

Enhancing biocompatibility and corrosion resistance of Mg implants via surface treatments

Oxide coating layers were formed on a pure magnesium (Mg) substrate through anodization and micro-arc oxidation (MAO) in order to enhance the biocompatibility and reduce the degradation rate. A thin, smooth MgO coating layer was formed after the anodization. On the other hand, when the Mg was treated using the MAO process, a relatively thick, rough MgO layer was formed. The corrosion properties were investigated using electrochemical and ion release tests in a simulated body fluid. Both the anodization and the MAO treatment enhanced the corrosion resistance of the Mg specimens. However, the MgO layers that formed on the surface were not stable enough to render favorable environments for cell growth. The anodized and MAO-treated specimens were post-treated in a cell-culturing medium in order to improve the stability of the coating layer. The biocompatibility was evaluated using in vitro cell tests, including cell attachment, DNA measurement, and alkaline phosphatase (ALP) activity tests. The DNA levels of the surface-treated Mg were about 6–10 times higher than the bare Mg. The ALP activity levels were also more than double after either the anodization or the MAO followed by the post-treatments. These results demonstrated that the biocompatibility and the corrosion resistance of Mg were significantly improved by the series of surface treatments.