Surface synthesization of magnesium alloys for improving corrosion resistance and implant applications

In the field of biodegradable material, a new research area has emerged for magnesium (Mg) and its alloys because of its high biocompatibility and biomechanical compatibility. This review summarizes many important types of research that have been done on degradable coatings on magnesium and its alloys for various implant applications. When magnesium alloys come into contact with other metals, they have a low open circuit potential and are consequently prone to galvanic corrosion. When exposed to air or a humid environment, magnesium may rapidly oxidize and generate a thin layer of loose MgO. Its applications were limited due to these drawbacks. Different types of corrosion have been studied in relation to magnesium and its alloys. Several coating methods are described, split into conversion and deposition coatings based on the individual processing procedures employed. This paper covers the most recent advancements in the development of biodegradable Mg alloy coatings over the last decade, revealing that the corrosion resistance of Mg and its alloys increases in most of circumstances due to coatings. Corrosion rate, coating morphology, adhesion, and surface chemistry were identified and explored as significant elements affecting coating performance. Calcium phosphate coatings made by deposition or conversion processes established for orthopedic purposes are the focus of many investigations according to a review of the literature. More research is needed on organic-based biodegradable coatings to improve corrosion resistance. Improved mechanical qualities are also crucial for coating materials. Developing adequate methodologies for studying the corrosion process in depth and over time is still a hot topic of research.     

Micro‐arc oxidation and electrophoretic deposition of nano‐grain merwinite (Ca3MgSi2O8) surface coating on magnesium alloy as biodegradable metallic implant

Magnesium alloys are promising biomaterials as biodegradable implant for orthopedic applications. However, their low corrosion resistance and poor bioactivity have prohibited their implant applications. In order to enhance these two properties, a nano‐grain merwinite coating was prepared on magnesium alloy. Its corrosion and the bioactivity behavior were characterized with electrochemical and immersion tests. The results showed that the nano‐grain merwinite coating can improve both the corrosion resistance and the bioactivity of the magnesium alloy making it an appropriate material for biodegradable bone implants. Copyright © 2014 John Wiley & Sons, Ltd.     

Corrosion behaviour of an Mg–Y–RE alloy used in biomedical applications studied by electrochemical techniques

Magnesium, due to its biocompatibility, a necessity in metabolic processes, and better mechanical properties than polymer, is an ideal candidate for biodegradable implants. The main actual limitation for the use of magnesium alloys is its too fast degradation rate in the physiological environment. The corrosion behaviour of an Mg–Y–RE magnesium alloy in two different physiological solutions (artificial plasma (AP) and simulated body fluid (SBF)) was investigated, using electrochemical impedance spectroscopy (EIS).The investigation showed that SBF is significantly more aggressive than AP with regard to the polished surface. A large difference in the corrosion rate and mechanisms (uniform or localized corrosion) is observed as a function of the buffer capacity of the media, but also of the carbonate and chloride content. For temporary surface protection, the formation of an approximately 350–400 nm dense hydroxide layer is obtained by electrochemical anodising. An increase of the corrosion resistance of the treated alloy for both physiological solutions is obtained, and this is especially noticeable for a long immersion time in AP.     

Characterization and corrosion resistance of pure Mg modified by micro‐arc oxidation using phosphate electrolyte with/without NaOH

Magnesium and its alloys have been suggested as potential absorbable implant materials due to their excellent biodegradability and biocompatibility. Current researchers focus on reducing the rapid corrosion rate of Mg and its alloys by alloying and surface modification. To improve the corrosion resistance, pure Mg is modified by micro‐arc oxidation (MAO) in phosphate electrolyte containing sodium hydroxide and its properties are compared with those formed using only phosphate or sodium hydroxide as electrolytes. A uniform and stable coating layer is formed on Mg after MAO treatment in phosphate electrolyte containing sodium hydroxide. The corrosion resistance of MAO‐coated Mg is evaluated by potentiodynamic polarization study and immersion test. The results reveal that MAO coating enables a good improvement in corrosion resistance, and among them, coatings treated using phosphate electrolyte containing sodium hydroxide offer the best performance. Copyright © 2013 John Wiley & Sons, Ltd.     

Characterization and corrosion properties of Ti‐O/HA composite coatings on Mg‐Zn alloy

Biodegradable magnesium alloys have been widely investigated in the field of biomaterials because they can be gradually dissolved and absorbed by the human body without long‐term existence. However, it was found that bare magnesium implants suffered from rapid corrosion. Surface modification is applied to improve the corrosion resistance and biocompatibility of magnesium implants. In this study, Ti‐O/HA composite coatings including typical flakes and nanofibers were fabricated on the Mg‐Zn alloy. The Ti‐O films were deposited on the magnesium alloy by direct current magnetron sputtering, and subsequently coated with HA flakes and nanofibers by electrochemical deposition, respectively. The obtained coatings were investigated by X‐ray diffraction, Fourier Transform Infrared spectroscopy and scanning electron microscopy. The corrosion resistance was evaluated by potentiodynamic polarization and hydrogen evolution tests in simulated body fluid at 37 °C. The results show that the compact Ti‐O films are composed of particles within the size of 100 nm, the outermost HA coatings are predominantly composed of HA and doped with Na⁺, Mg²⁺ ions and functional groups. The stronger diffraction and broader peak in nanofibers than typical flakes around 25.8° are ascribed to the preferential growth in orientation (002). The morphology of HA coatings changed from typical flakes into nanofibers with the addition of NaF, the mechanism to explain the difference is also discussed. The corrosion resistance was improved significantly by the coatings, the corrosion rates in the 10 days were 4.13, 1.77, 0.96 and 0.85 mm/y, respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

Ca-P Conversion Coating on AZ60 Magnesium Alloy for Biomedical Application

To improve the anti-corrosion behaviors of magnesium alloy in the inner environment of human body,a bioactive Ca-P coating was deposited on the AZ60 magnesium alloy by a novel simple method.The morphologies of the Ca-P coatings formed under different treatment time were studied by scanning electron microscopy（SEM）.The corrosion behaviors of Ca-P coating were investigated by electrochemical polarization test and electrochemical impedance spectroscopy in both 3％（mass fraction） NaCl solution and simulated body fluid（SBF）.Immersion test in SBF was performed to evaluate the corrosion rate of Ca-P coated magnesium alloy.X-Ray diffraction（XRD） analysis result shows that the coating mentioned above mainly consists of dicalcium phosphate dehydrate（CaHPO4·2H2O,DCPD） and β-tricalcium phosphate dehydrate[β-TCP,Ca3（PO4）2],which exhibits good corrosion resistance.After magnesium alloy was immersed in 1 mol/L NaOH solution at 80 ℃ for 2 h,hydroxyapatite [Ca10（PO4）6（OH）2,HA]appeared on the magnesium alloy substrate,which can further decrease the corrosion rate of AZ60 magnesium alloy in SBF.     

Corrosion-resistant composite coatings on biodegradable magnesium alloys: In vitro studies

A composite coating was formed on MA8, MA14, and MA12 magnesium alloys by plasma electrolytic oxidation with subsequent immersion of samples into a superdispersed polytetrafluoroethylene suspension. In vitro volumetry determined that using this coating significantly reduces the magnesium alloy dissolution rate. It was shown that superdispersed polytetrafluoroethylene seals pores of the coating, thus reducing the corrosion rate in an artificial medium that mimics human blood by ionic composition. However, the surface of the calcium phosphate coating (Ca: P = 1.61) containing hydroxyapatite remains open for contact with the environment. The obtained data suggested that the proposed method for surface treatment of MA8, MA14, and MA12 alloys is promising for producing biodegradable protective coatings on magnesium medical implants.     

Corrosion resistance and cytotoxicity of a MgF2 coating on biomedical Mg–1Ca alloy via vacuum evaporation deposition method

To reduce the biocorrosion rate and enhance the biocompatibility by surface modification, MgF₂ coatings were prepared on Mg–1Ca alloy using vacuum evaporation deposition method. The average thickness of the coating was about 0.95 µm. The results of immersion test and electrochemical test indicated that the corrosion rate of Mg–1Ca alloy was effectively decreased after coating with MgF₂. The MgF₂ coating induced calcium phosphate deposition on Mg–1Ca alloy. After 72 h culture, MG63 cells and MC3T3‐E1 cells were well spread on the surface of the MgF₂‐coated Mg–1Ca alloy, while few cells were observed on uncoated Mg–1Ca alloy samples. In summary, MgF₂ coating showed beneficial effects on the corrosion resistance and thus improved cell response of the Mg–1Ca alloy effectively and should be a good surface modification method for other biomedical magnesium alloys. Copyright © 2013 John Wiley & Sons, Ltd.     

AZ91D镁合金上钼改性锌系磷化膜的制备、 结构及性能

采用在磷化液中添加钼酸钠及腐蚀抑制剂的方法, 在AZ91D 镁合金表面上制备了均匀细致的锌系复合磷化膜. 用XRD对膜层的化学组成及结构进行了表征,用SEM和EDS对膜层的形貌和组分含量进行分析. 结果表明, 磷化膜主要由Zn3(PO4)2·4H2O和单质Zn组成. 在磷化液中加入钼酸钠使磷化膜组织更加细致而且无裂纹. 磷化液中的钼酸钠含量为1.5 g/L时, 磷化膜的结晶最致密, 单质锌的含量最高, 耐蚀性最好. 还提出了一种快速测量镁合金表面膜层耐蚀性的试验方法, 同时对镁合金上的磷化反应的机理进行了探讨.     

Dépôt de revêtements peu rugueux de diamant sur un alliage de titane

Deposition of smooth diamond coatings on titanium alloy. A new process has been perfected to deposit smooth diamond coatings, at 600 °C, on titanium alloys. Scanning electron microscopy, X-ray diffraction, visible and UV Raman spectroscopy show that these coatings are smooth and mainly composed of crystalline diamond with a fine-grained morphology. The results are compared here to those obtained with classical rough polycrystalline coatings. Optical emission spectroscopy reveals important differences between the plasma species produced for the deposition of these smooth coatings and the plasma species produced for the deposition of both polycrystalline diamond and nanocrystalline films. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASdiamond / coating / titanium alloy / plasma / roughness / Raman spectroscopy / emission spectroscopy     

Characterization and cytocompatibility of polydopamine on MAO‐HA coating supported on Mg‐Zn‐Ca alloy

Magnesium has been suggested as a potential biodegradable metal for the usage as orthopaedic implants. However, high degradation rate in physiological environment remains the biggest challenge, impeding wide clinical application of magnesium‐based biomaterials. In order to reduce its degradation rate and improve the biocompatibility, micro‐arc oxidation coating doped with HA particles (MAO‐HA) was applied as the inner coating, and polydopamine (PDA) film was synthesized by dopamine self‐polymerization as the outer coating. The microstructure evolution of the coating was characterized using scanning electron microscopy (SEM), atomic force microscope (AFM), X‐ray diffraction analyses (XRD), Fourier transform infrared spectroscopy (FT‐IR), and X‐ray photoelectron spectroscopy (XPS). The results showed that PDA film had covered the entire surface of MAO‐HA coating and the pore size of MAO‐HA coating decreased. The root mean square (RMS) roughness of PDA/MAO‐HA coatings was approximately 106.46 nm, which was closer to the optimum surface roughness for cellular attachment as compared with MAO‐HA coatings. Contact angle measurement indicated that the surface wettability had been transformed from hydrophobic to hydrophilic due to the introduction of PDA. The PDA/MAO‐HA coatings exhibited better corrosion resistance in vitro, with the self‐corrosion potential increasing by 150 mV and the corrosion current density decreasing from 2.09 × 10^?5 A/cm ² to 1.46 × 10^?6 A/cm ² . In hydrogen evolution tests, the corrosion rates of the samples coated with PDA/MAO‐HA and MAO‐HA were 4.40 and 5.95 mm/y, respectively. MTS assay test and cell‐surface interactions experiment demonstrated that PDA/MAO‐HA coatings exhibited good cellular compatibility and could promote the adhesion and proliferation of MC3T3‐E1 cells.     

A high performance nano-structure conductive coating on a Crofer22APU alloy fabricated by a novel spinel powder reduction coating technique

A nano-structure conductive coating was fabricated on a Crofer22APU alloy interconnect by an original coating strategy using Mn_0.9Y_0.1Co₂O₄ (MYC) novel spinel nanocrystalline powder. A unique treatment method by which the spinel powder was reduced was used to prepare the green coating. The resulting coating was about 12 μm in thickness, and was composed of MYC nanocrystalline with an average particle size of about 100 nm. The coating was well adhered with the substrate alloy. Less than 4 mΩ cm² of the area specific resistance (ASR) was obtained, and no obvious degradation was observed for a coated alloy (whose coating thickness was about 30 μm) after operated at 800 °C for 538 h under seven thermal cyclings. The coated alloy exhibited excellently electrical performance and long-term stability compared with the uncoated one. The exploration of the novel spinel powder reduction coating technique for alloy interconnect to obtain cheap coatings with excellent microstructure and performance showed a promising prospect for the practical application of solid oxide fuel cells (SOFCs).     

Electrochemical and XPS studies of a Nb-containing Ti-based glass-forming alloy system in H2SO4 solution

The corrosion behaviors of Ti₄₀Zr₂₅Ni_12 -xNb_xCu₃Be₂₀ (x = 0, 4, 8, and 12 at.%) alloys in 0.5 mol/L H₂SO₄ solution were studied, aiming to establish the relationship between Nb content and corrosion resistance. The addition of Nb element gives rise to a clear microstructural evolution, from a completely amorphous structure for the alloys without Nb and with 4% Nb alloys to an amorphous/crystalline composite structure for the alloys with 8% and 12% Nb. The alloy with higher Nb content exhibits better corrosion resistance, which can be attributed to the formation of Ti^4 +-, Zr^4 +-, and Nb^5 +-enriched highly protective surface film in corrosive solutions.     

Surface characteristics and corrosion resistance of sol–gel derived CaO–P2O5–SrO–Na2O bioglass–ceramic coated Mg alloy by different heat-treatment temperatures

To improve the initial corrosion resistance and then make the degradation rate of magnesium alloys to meet the biomedical application, crack-free CaO–P₂O₅–SrO–Na₂O bioglass-ceramic coatings were synthesized on AZ31 magnesium alloy substrates using a sol–gel dip-coating technique followed by a heat-treatment in the temperature range of 400–500 °C. The effects of heat-treatment on the phase constituents, surface characteristics and corrosion resistances of the coatings were investigated. It was shown that the crystallization of Ca₂P₂O₇ occurred after the glass was treated at 400 °C. As the temperature increased from 400 °C to 450 °C, besides main phase Ca₂P₂O₇, β-Ca(PO₃)₂ and Ca₄P₆O₁₉ were identified as minor crystal phases in the glass–ceramic. No new phase was detected with the temperature increasing to 500 °C except for the further crystallization. Meanwhile, the water contact angles of the coatings decreased with the increase of heat-treatment temperature due to the great crystallization. The corrosion resistances of the coated magnesium alloys were studied by electrochemical corrosion techniques in the simulated body fluid. The results revealed that the coating heat-treated at 400 °C exhibited superior corrosion resistance because of less crystallization, suggesting that the calcium phosphate bioglass–ceramic coating can provide effective protection for magnesium alloy substrate to control its initial degradation in vivo and maintain the desired mechanical properties.     

Electrochemical Sensing of Dissolved Hydrogen in Aqueous Solutions as a Tool to Monitor Magnesium Alloy Corrosion

Magnesium and its alloys have been the focus of the development of biodegradable metallic implant materials for years. Since water is reduced to form hydrogen gas during their corrosion, the amount and rate of hydrogen evolution, and therefore the dissolved hydrogen, could be used as an indicator to monitor and compare the corrosion. Here we report on a commercially available Clark‐Type amperometric microsensor and a simple potentiometric sensor for hydrogen to monitor the corrosion of a magnesium alloy in aqueous solutions. The sensors were compared using rare‐earth containing Mg alloy discs (Mg with 4 % Y, 2 % Nd, 0.5 % Ga, 0.5 % Dy) immersed in phosphate buffered saline (pH 7.4) and 3.5 % NaCl.     

Active protective Al–Ce alloy coating electrodeposited from ionic liquid

The electrochemical co-deposition of Al–Ce metallic protective coating with active inhibiting effect was performed for the first time using an ionic liquid as an electrolyte. Cerium was successfully co-deposited with aluminium on surface of Pt and AA2024 aluminium alloy forming uniform films with globular micro-structure and thickness up to 75 μm.Cerium was introduced into the aluminium coating as a potential corrosion inhibitor which can be liberated during sacrificial dissolution of the galvanic layer deposited on the alloy surface. The released inhibitor provides an additional active corrosion protection slowing down the corrosion processes in the defects.     

医用钛表面TiO_2纳米管/HA复合膜层于Tyrode’s生理溶液中电化学腐蚀行为

钛及其合金具有良好的力学性能和生物相容性,被广泛地用作医用人工植入体.然而,钛植入体在人体内的生理环境中必然发生腐蚀,金属离子的溶出和积累可产生毒副作用.本文应用电化学方法对医用钛金属作表面改性,提高其生物活性,应用Tafel极化曲线和电化学阻抗(EIS)研究其耐蚀性能及腐蚀电化学行为.结果表明,在钛基TiO2纳米管阵列膜层上沉积构筑HA涂层之后,由于表面阻挡层的强化,TiO2涂层在Tyrode’s生理溶液中的耐蚀性有所提高.     

Preparation and Characterization for the Electrodeposited Hybrid Coating of Calcium Phosphate/Chitosan on Ti Alloy Surface

Hydroxyapatite (HA) formulated as Ca₁₀(PO₄)₆(OH)₂ becomes a favorable material for implants because of its chemical similarity to the calcium phosphate minerals present in biological hard tissue. Many efforts have been made in recent years in the development of processing methods for depositing hydroxyapatite on implant alloy substrate in order to have high strength, good processability, suitable specific density, excellent corrosion resistance in the physiological environment and good affinity to the living body. The plasma spray technique is commonly used in the HA coating on implants. The major problem for the plasma spray, however, is the decomposition and phase transformation of hydroxyapatite during the spray coating process. Electrochemical techniques including electrophretic deposition and electro-deposition are being developed as an alternative method for producing hydroyapatite coated composite material. It is very desirable at present to further strengthen the coating and bond it to the metal substrate, and to increase the bioactivity of hydroxyapatite coatings as well, which is very important for forming a strong chemical bond with natural bone as an implant material.     

Bioactivity Performance of Pure Mg after Plasma Electrolytic Oxidation in Silicate-Based Solutions

The biodegradable metals, including magnesium (Mg), are a convenient alternative to permanent metals but fast uncontrolled corrosion limited wide clinical application. Formation of a barrier coating on Mg alloys could be a successful strategy for the production of a stable external layer that prevents fast corrosion. Our research was aimed to develop an Mg stable oxide coating using plasma electrolytic oxidation (PEO) in silicate-based solutions. 99.9% pure Mg alloy was anodized in electrolytes contained mixtures of sodium silicate and sodium fluoride, calcium hydroxide and sodium hydroxide. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), contact angle (CA), Photoluminescence analysis and immersion tests were performed to assess structural and long-term corrosion properties of the new coating. Biocompatibility and antibacterial potential of the new coating were evaluated using U2OS cell culture and the gram-positive Staphylococcus aureus (S. aureus, strain B 918). PEO provided the formation of a porous oxide layer with relatively high roughness. It was shown that Ca(OH)₂ was a crucial compound for oxidation and surface modification of Mg implants, treated with the PEO method. The addition of Ca²⁺ ions resulted in more intense oxidation of the Mg surface and growth of the oxide layer with a higher active surface area. Cell culture experiments demonstrated appropriate cell adhesion to all investigated coatings with a significantly better proliferation rate for the samples treated in Ca(OH)₂-containing electrolyte. In contrast, NaOH-based electrolyte provided more relevant antibacterial effects but did not support cell proliferation. In conclusion, it should be noted that PEO of Mg alloy in silicate baths containing Ca(OH)₂ provided the formation of stable biocompatible oxide coatings that could be used in the development of commercial degradable implants.     

AZ91镁合金表面稀土转化膜的制备及耐蚀性能研究

采用在镁合金表面形成无毒、无污染的稀土铈转化膜的方法解决AZ91镁合金表面的腐蚀问题。确定了最佳成膜工艺参数，讨论了处理液的浓度、成膜温度和成膜时间等因素对转化膜耐蚀性的影响。利用湿热实验、阳极极化曲线的测定等实验方法评价了转化膜对镁合金表面的防护作用。结果表明，在潮湿温热条件下稀土铈转化膜试样仍能保持膜层的完整性并具有较高的覆盖度，腐蚀现象不明显。腐蚀电势升高，出现钝化现象，腐蚀电流密度下降，稀土铈转化膜可以提高AZ91镁合金的耐蚀性能。用扫描电镜观察了膜的微观形貌，稀土铈转化膜是由基膜和附着的细小颗粒组成，最佳工艺形成的铈转化膜无破碎现象，对AZ91镁合金表面的腐蚀过程的发生有明显的抑制作用。