Improve corrosion resistance of magnesium in simulated body fluid by dicalcium phosphate dihydrate coating

A dicalcium phosphate dihydrate (DCPD) coating composed of bar-shaped crystals was deposited on the surface of magnesium in order to slow down the corrosion rate of the substrate. The corrosion resistance of the DCPD-coated specimens was evaluated in a simulated body fluid (SBF) with uncoated specimens as a control. Time-dependent characteristics of specimens and the corresponding SBF were analyzed at 3, 5, 7, 14 and 21 days of immersion. Less weight loss and pH increase were observed for the coated group than the uncoated group. The coating was intact after 3 days of immersion although its dissolution was manifested by XRD examination. Noticeable DCPD dissolution occurred at the 5th day accompanied by a temporary increase in Ca and P concentrations in SBF which otherwise kept decreasing. Despite the dissolution of the coating, some DCPD particles were still observed on the surface of the substrate after 21 days of immersion. In contrast to the coated specimens, a porous layer of Mg(OH)₂ was formed on the surface of uncoated specimens at the 5th day of immersion. It was found that the corrosion rate of the coated group was substantially lower than that of the control.     

二水磷酸氢钙(DCPD)涂层对纯镁在模拟体液中腐蚀行为的影响

使用液相化学沉积技术在纯镁表面制备了二水磷酸氢钙(DCPD)涂层,并通过在SBF中的浸入腐蚀实验研究了涂层对镁耐蚀性的影响.结果表明,涂层提高了镁在仿生环境中的耐腐蚀能力,这种作用随时间的延长而增强;涂层抑制了Mg(OH)2腐蚀产物层的形成;DCPD在SBF中逐步溶解,但是在浸泡21d以后,试样表面仍然残留着DCPD颗粒.     

Preparation and characterization of dicalcium phosphate dihydrate coating on enamel

A coating of 60-80 μm in thickness had been prepared on the surface of enamel by rapid growth of highly crystalline dicalcium phosphate dihydrate crystal (DCPD). The DCPD coating was formed by conversion of calcium-deficient hydroxyapatite (CDHA) in strong acidic conditions. The characterization of the coating was performed using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). No obvious gap between the coating and the enamel was detected by SEM observation, which suggested that the coating adhered well to the enamel. Analysis of FT-IR and XRD revealed that the coating was a good match to DCPD. Moreover, the mean Vicker hardness number (VHN) of the coating was approximately 125, as measured by a microhardness tester.     

Influence of dicalcium phosphate dihydrate coating on the in vitro degradation of Mg-Zn alloy

To reduce the degradation rate and further to improve the biocompatibility of magnesium alloy, dicalcium phosphate dihydrate (CaHPO₄·2H₂O, DCPD) has been fabricated on a kind of magnesium-zinc alloy by way of electrodeposition method. The experimental results of XRD, SEM and EDS showed that the electrodeposited coating on the Mg-Zn alloy mainly contains the flake-like DCPD, along with some octacalcium phosphate (Ca₈(HPO₄)₂(PO₄)₄·4H₂O, OCP). After the in vitro degradation of the coated alloy in modified-simulated body fluid (m-SBF), it was proved that the coating could reduce the degradation rate effectively, and the samples were covered by calcium phosphate salts with a higher Ca/P ratio. Therefore, it indicates that compared with the bare alloy, the DCPD coating rendered a more biocompatible surface, and is a promising coating candidate for biomedical magnesium materials.     

Effect of D-fructose on the in-vitro corrosion behavior of AZ31 magnesium alloy in simulated body fluid

The effect of addingd-fructose to simulated body fluid(SBF) on the corrosion behavior of AZ31 magnesium(Mg) alloy at 37.C and at a pH of 7.4 was studied by potentiodynamic polarization(PDP), electrochemical impedance spectroscopy(EIS), potentiostatic polarization and hydrogen(H2) collecting techniques,Raman spectroscopy technique, scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), X-ray diffraction(XRD), X-ray photoelectron spectroscopy analysis(XPS) and Fourier transformed infrared(FTIR). The results demonstrated that the addition of fructose enhanced the deposition of phosphates forming thick and compact corrosion products, which inhibited the transmission of aggressive ions into the Mg substrate. As a result, both the anodic dissolution of Mg and negative difference effect(NDE) were suppressed. Thus, the corrosion resistance of AZ31 Mgalloy in SBF was significantly improved.     

Biodegradable behaviors of AZ31 magnesium alloy in simulated body fluid

Magnesium alloys have unique advantages to act as biodegradable implants for clinical application. The biodegradable behaviors of AZ31 in simulated body fluid (SBF) for various immersion time intervals were investigated by electrochemical impedance spectroscopy (EIS) tests and scanning electron microscope (SEM) observation, and then the biodegradable mechanisms were discussed. It was found that a protective film layer was formed on the surface of AZ31 in SBF. With increasing of immersion time, the film layer became more compact. If the immersion time was more than 24 h, the film layer began to degenerate and emerge corrosion pits. In the meantime, there was hydroxyapatite particles deposited on the film layer. The hydroxyapatite is the essential component of human bone, which indicates the perfect biocompatibility of AZ31 magnesium alloy.     

Effect of corrosion on mechanical behaviors of Mg-Zn-Zr alloy in simulated body fluid

The main purpose of this paper is to investigate the effect of corrosion on mechanical behaviors of the Mg-Zn-Zr alloy immersed in simulated body fluid （SBF） with different immersion times. The corrosion behavior of the materials in SBF was determined by immersion tests. The surfaces of the corroded alloys were examined by SEM. The tensile samples of the extruded Mg-2Zn-0.8Zr magnesium alloy were immersed in the SBF for 0, 4, 7, 10, 14, 21 and 28 d. The tensile mechanical behaviors of test samples were performed on an electronic tensile testing machine. SEM was used to observe the fracture morphology. It was found that with extension of the immersion time, the ultimate tensile strength （UTS）, yield strength （YS） and elongation （EL） of the Mg-2Zn-0.8Zr samples decreased rapidly at first and then decreased slowly. The main fracture mechanism of the alloy transformed from ductile fracture to cleavage fracture with the increasing immersion times, which can be attributed to stress concentration and embrittlement caused by pit corrosion.     

镁合金微弧电泳复合膜层的微观结构和抗腐蚀性能

采用恒压模式在硅酸盐系电解液中制备镁合金微弧氧化陶瓷层,对比研究了微弧电泳和直接电泳镁合金的截面形貌、结合力大小以及抗腐性能差异.结果表明:在镁合金微弧氧化陶瓷层的表面可制备电泳有机层,简化了电泳工艺;在微弧电泳复合膜层间形成机械咬合力和化学键力,附着力等级可达1级;经800 h中性盐雾腐蚀试验后,复合膜层腐蚀增重量和样品表面的形貌均没有明显的变化;与微弧氧化陶瓷层和直接电泳有机层相比,微弧电泳复合膜层的电化学稳定性显著增强,腐蚀电流相分别减少了约5个和2个数量级.     

Electrochemical behavior of biocompatible AZ31 magnesium alloy in simulated body fluid

Dense oxidation coatings have been successfully developed on biocompatible AZ31 magnesium alloy, using microarc oxidation technique, to improve the corrosion resistance. Three different deposition voltages of 250, 300, and 350 V have been employed. The effect of voltage on the coating corrosion resistance has been evaluated through electrochemical experiments in a simulated body fluid (SBF) up to 7 days. Potentiodynamic polarization and electrochemical impedance spectroscopy scans were performed in the SBF solution, followed by optical microscopy surface inspection. The results indicate that the corrosion rates of the coatings are in the order of 250 < 300 < 350 V after immersion for 7 days, and the charge transfer resistance (R _ct) of the three samples is in the order of 250 > 300 > 350 V. Both the electrochemical tests and the surface inspection suggest that the 250 V coating has the highest corrosion resistance, with lowest corrosion current density, highest R _ct, and the best surface quality.     

In-vitro degradation behaviour of WE54 magnesium alloy in simulated body fluid

Rare earths containing magnesium alloy, WE54, exhibited a marginally higher in-vitro degradation resistance than that of pure magnesium. Heat-treatment procedure had an influence on the degradation behaviour. However, comparing with AZ91 magnesium alloy the in-vitro degradation resistance of WE54 magnesium alloy was significantly lower, which suggests that the passivating capacity of rare earths is inferior to that of aluminium under in-vitro condition.     

Heterogeneous crystallization of dicalcium phosphate dihydrate on titanium surfaces

The kinetics of nucleation and crystal growth of dicalcium phosphate dihydrate (DCPD, CaHPO_4. 2H₂O) on titanium powders with different grain sizes have been investigated using the constant composition crystal growth method at 37°C and pH = 5.50. Nucleation is independent of titanium powder size while crystal growth rate is strongly size-dependent. A minimum relative supersaturation ratio, _min, required in the system for a detectable crystal growth rate to occur, taking into account the presence of the foreign substrate in the system, was determined. Based on the kinetic data and on the various characterizations by X-ray photoelectron spectroscopy and by scanning electron microscopy, we propose a crystal growth kinetic equation as a function of two parameters: the size of the substrate and the excess of supersaturation (–_min) which is squared, indicating a spiral growth mechanism. © 1999 Kluwer Academic Publishers     

AZ31镁合金表面激光熔覆Al-TiC复合涂层微观组织与腐蚀性能

为有效改善AZ31镁合金表面的腐蚀性能,本文采用激光熔覆技术在AZ31镁合金表面成功制备了无缺陷的Al-TiC复合涂层。研究了不同成分含量的Al-TiC复合涂层的相组成、微观组织和耐腐蚀性能的影响。结果表明：在Al-TiC复合涂层内形成了大量的Al₁₂Mg₁₇、Mg₂Al₃和TiC相。复合涂层内微观组织呈现出连续网络状分布特征。随着Al-TiC混合粉末中Al含量的减小,复合涂层中Al₁₂Mg₁₇、Mg₂Al₃和TiC相的含量呈递增趋势,网络状分布的微观组织结构变得更加均匀连续。复合涂层与AZ31基体之间形成了良好的冶金结合界面。激光熔覆制备的Al-TiC复合涂层耐腐蚀性能较AZ31基体显著提升。自腐蚀电位由基体的-1.563 V提升至-1.144 V,自腐蚀电流由基体的1.55×10^-4 A减小至2.63×10^-6 A。     

Homogeneous corrosion of high pressure torsion treated Mg-Zn-Ca alloy in simulated body fluid

Magnesium alloys have got extensive attention as biodegradable implant materials due to their biodegradability in the physiological environment and similar elastic modulus to natural bone. But their poor corrosion resistance is a dominant problem that limits their clinical application due to the inhomogeneous distribution of the second phase. Nevertheless, after high pressure torsion (HPT) treatment, the second phase became nano-sized particles and distributed uniformly in grain interiors instead of along grain boundaries. The immersion tests indicated that the HPT-treated sample exhibited homogeneous corrosion resulting from the uniform distribution of the second phase. The results of the potentiodynamic polarization experiments showed that, compared with the as-cast alloy, the corrosion current density of the HPT-treated alloy decreased from 5.3 × 10^− 4 A/cm² to 3.3 × 10^− 6 A/cm².     

陈化对AZ91D镁合金上磷化膜的影响及磷化膜退化机制的研究

以磷酸盐化学转化膜为研究体系,采用交流阻抗(EIS)及红外(FT-IR)分析方法及检测手段,研究陈化时间对AZ91D镁合金磷化膜性能的影响及磷化膜的腐蚀退化机制.研究发现(1) EIS图谱和FT-IR图谱对比结果表明陈化时间对AZ91D磷化膜层性能有影响,磷化膜经陈化72h后更稳定.(2)磷化试样在硼酸缓冲液中浸泡实验的EIS测试表明,磷化膜的腐蚀退化机制分3个阶段:浸泡初期,中期和后期.浸泡初期(5～10h),电解质缓慢向磷化膜渗入,且有少量电解质浸入内层界面;浸泡中期(30～70h),随着内层界面腐蚀反应的发生,腐蚀产物开始对外层晶体层产生破坏作用,使缺陷扩大,导致电解质开始大量进入到内层界面;浸泡后期(80～120h),内层腐蚀反应达到平衡,膜层电阻等各项参数不再改变.     

Electrodeposition of dicalcium phosphate dihydrate coatings on stainless steel substrates

Cathodic reduction of an aqueous solution containing dissolved calcium and phosphate ions results in the deposition of micrometer thick CaHPO₄·2H₂O (dicalcium phosphate dihydrate) coatings on stainless steel substrates. The coating obtained at a low deposition current (8 mA cm^???2) comprises lath-like crystallites oriented along 020. The 020 crystal planes are non-polar and have a low surface energy. At a high deposition current (12 mA cm^???2), platelets oriented along 121? are deposited. CaHPO₄·2H₂O is an important precursor to the nucleation of hydroxyapatite, the inorganic component of bones. Differently oriented CaHPO₄·2H₂O coatings transform to hydroxyapatite with different kinetics, the transformation being more facile when the coating is oriented along 121?. These observations have implications for the development of electrodeposited biocompatible coatings for metal endoprostheses for medical applications.     

Nucleation and crystal growth of dicalcium phosphate dihydrate on titanium powder

The nucleation and crystal growth of dicalcium phosphate dihydrate (CaHPO₄ · 2H₂O DCPD or brushite) on titanium powder surface has been studied in metastable supersaturated solutions at 37°C and pH=5.50. These experiments were carried out using the constant composition method. In order to determine the effect of substrate size on the kinetics of brushite formation, we used four ranges of titanium particle size (<20 m, 20–45 m, 63–90 m and 90–125 m). The induction period , before the initial crystal growth of DCPD, varied markedly with the relative supersaturation . The initial growth rate R _C of brushite is strongly size-dependent, whereas is relatively constant for the different particle sizes. Plots of Ln R _C versus Ln , for each particle size, gave an apparent order of 3.4±0.2 for the crystallization of brushite on titanium surface. The interfacial energy estimate of 22.9±1.2 mJ m^-2, from the kinetic results, was in the same order of magnitude than that for sparingly soluble salts. Observations by scanning electron microscopy showeed many platelets of DCPD covering the titanium powder surface.     

AZ91D镁合金锌系磷化膜成膜机理和生长过程的研究

以磷酸盐化学转化膜为研究体系,采用SEM、XRD、OCP等分析方法及检测手段,研究AZ91D镁合金系磷酸盐化学转化膜的成膜机理、膜层结构及生长过程。研究发现AZ91D镁合金在磷化液中成膜过程分5个阶段:初始成核(1～5s)、基体快速溶解(5～60s)、晶体快速生长(1～2min)、膜层稳态生长(2～10min)和膜层沉积溶解平衡阶段(10min以后)。AZ91D镁合金表面的磷酸盐晶核的形成并非在金属进入溶液的最初时刻一次形成,是分批形成。最先形成的晶核逐渐长大,新的晶核不断生成,磷酸盐晶粒对其表面的覆盖度逐渐增大,直至各个晶粒逐渐长大相互接界,将其表面完全覆盖,结晶过程结束。晶核的形成未优先发生在基体金属的晶界上,随着晶核的生长和外延而形成磷化膜。     

Corrosion process of pure magnesium in simulated body fluid

The chemical and physical processes of magnesium in simulated body fluid (SBF) were investigated. The corrosion rate of magnesium was measured after 3, 5, 7, 14 and 21 days of immersion, respectively. It was found that the corrosion rate decreased with increasing immersion time, while the pH of SBF changed inversely. Network-like cracks and pits were the main damages resulting from corrosion, and the localized buildup of chloride ions was the major cause of pit formation.     

Calcium phosphate coating on magnesium alloy for modification of degradation behavior

Magnesium alloy has similar mechanical properties with natural bone, but its high susceptibility to corrosion has limited its application in orthopedics. In this study, a calcium phosphate coating is formed on magnesium alloy (AZ31) to control its degradation rate and enhance its bioactivity and bone inductivity. Samples of AZ31 plate were placed in the supersaturated calcification solution prepared with Ca(NO₃)₂, NaH₂PO₄ and NaHCO₃, then the calcium phosphate coating formed. Through adjusting the immersion time, the thickness of uniform coatings can be changed from 10 to 20 μm. The composition, phase structure and morphology of the coatings were investigated. Bonding strength of the coatings and substrate was 2–4 MPa in this study. The coatings significantly decrease degradation rate of the original Mg alloy, indicating that the Mg alloy with calcium phosphate coating is a promising degradable bone material.     

Influence of dicalcium phosphate dihydrate coating on the <Emphasis Type="Italic">in vitro</Emphasis> degradation of Mg-Zn alloy

To reduce the degradation rate and further to improve the biocompatibility of magnesium alloy, dicalcium phosphate dihydrate (CaHPO₄·2H₂O, DCPD) has been fabricated on a kind of magnesium-zinc alloy by way of electrodeposition method. The experimental results of XRD, SEM and EDS showed that the electrodeposited coating on the Mg-Zn alloy mainly contains the flake-like DCPD, along with some octacalcium phosphate (Ca₈(HPO₄)₂(PO₄)₄·4H₂O, OCP). After the in vitro degradation of the coated alloy in modified-simulated body fluid (m-SBF), it was proved that the coating could reduce the degradation rate effectively, and the samples were covered by calcium phosphate salts with a higher Ca/P ratio. Therefore, it indicates that compared with the bare alloy, the DCPD coating rendered a more biocompatible surface, and is a promising coating candidate for biomedical magnesium materials.