Effects of applied voltage on the microstructure and properties of hydroxyapatite bioceramic coatings formed on Ti7Cu5Sn titanium alloy by micro-arc oxidation

The effects of the applied voltage on the morphology, composition and corrosion behaviour of Ti7Cu5Sn coated were investigated. At applied voltages lower than 250?V, the composite coatings consist of anatase-TiO₂, rutile-TiO₂, DCPD(CaHPO₄·2H₂O) and a small amount of amorphous calcium phosphate phase. When the applied voltage is increased, the ceramic coatings transform from DCPD (CaHPO₄·2H₂O) to HA (Ca₁₀(PO₄)₆(OH)₂, 300?V), and new phases of Ca₂P₂O₇, CaTiO₃ and TCP(Ca₃(PO₄)₂) form at 350?V. The passive current densities at body potential are one order of magnitude lower than that of the uncoated sample, indicating better corrosion resistance. The MAO film is a tri-layer system: a compact inner layer, a mesosphere porous oxide layer, and an outer layer.     

Formation of bioactive anticorrosion coatings on resorbable implants by plasma electrolytic oxidation

Calcium phosphate coatings (Ca/P = 1.61) containing magnesium oxide MgO and hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ accelerating the growth of bone tissue have been prepared by the method of plasma electrolytic oxidation (PEO) on MA8 magnesium alloy. The phase and element compositions, morphology, and anticorrosion properties of coatings were investigated. Such PEO layers were found to essentially reduce the corrosion rate of magnesium alloy (polarization resistance being increased by two orders). This makes it possible to consider the formed PEO coatings as likely anticorrosion layers for medical bioresorbable implants.     

Fabrication of calcium phosphate/chitosan coatings on AZ91D magnesium alloy with a novel method

Four calcium phosphate/chitosan composite films were fabricated on the surface of micro-arc oxidized (MAO)-AZ91D alloy through electrophoretic deposition (EDP) followed by a conversion process of the coatings in a phosphate buffer solution (PBS). In the EPD process, nano hydroxyapatite (n-HA, Ca₁₀(PO₄) ₆(OH)₂) and Ca(OH)₂ in the layers were obtained from a n-HA/ethanol suspension and a n-HA/chitosan-acetic acid aqueous solution, respectively. After immersion into PBS, brushite (DCPD, CaHPO₄·2H₂O) and new HA were introduced into the deposited layers. The percentage of Ca(OH)₂ in the deposited layers played an important role in developing the new phase in the conversion layers. When the percentages of Ca(OH)₂ in the deposited layers were 32 wt. % and 54 wt. %, the main phase of the conversion layers was DCPD with a little HA. However, when the percentages of Ca(OH)₂ were 64 wt. % and 100 wt. %, the main phase of the conversion layers became HA with a little DCPD. The calcium phosphate/chitosan coatings with more homogeneous bioactive layers and better adhesion strength on MAO-AZ91D alloy substrate were obtained from the electrolyte whose volume percentages of the n-HA/chitosan-acetic acid aqueous solution being 60% and 80%.So, EPD combined with a conversion process into PBS could be a promising method for the preparation of new calcium phosphate/chitosan coatings.     

Duplex-layered manganese phosphate conversion coating on AZ31 Mg alloy and its initial formation mechanism

A duplex-layered phosphate conversion coating was obtained on AZ31 Mg alloy by substituting NaF bath with a citric bath. The morphology, composition and corrosion resistance of the coating were investigated using SEM, EDS, SPM and electrochemical methods. A three-stage mechanism for initial formation of the coating was proposed: Dissolution of the loose oxide film and deposition of Mg₃(PO₄)₂ and AlPO₄, formation of a composite intermediate layer of Mg₃(PO₄)₂, AlPO₄ and Mg(OH)₂, and deposition of manganese phosphate nuclei followed by growth and lamination of the nuclei. The nuclei preferentially deposit at the Al–Mn phase surface and near the grain boundary.     

Characterization and corrosion behavior of ceramic coating on magnesium by micro-arc oxidation

In this study, the commercial pure magnesium was coated in different aqueous solutions of Na₂SiO₃ and Na₃PO₄ by the micro-arc oxidation method (MAO). Coating thickness, phase composition, surface and cross sectional morphology and corrosion resistance of coatings were analyzed by eddy current method, X-ray diffraction (XRD), scanning electron microscope (SEM) and tafel extrapolation method, respectively. The average thickness of the coatings ranged from 52 to 74 μm for sodium silicate solution and from 64 to 88 μm for sodium phosphate solution. The dominant phases on the coatings were detected as spinal Mg₂SiO₄ (Forsterite) and MgO (Periclase) for sodium silicate solution and Mg₃(PO₄)₂ (Farringtonite) and MgO (Periclase) for sodium phosphate solution. SEM images reveal that the coating is composed of two layers as of a porous outer layer and a dense inner layer. The corrosion results show the coating consisting Mg₂SiO₄ is more resistant to corrosion than that containing Mg₃(PO₄)₂.     

Effect of heat treatment on the structure and in vitro bioactivity of microarc-oxidized (MAO) titania coatings containing Ca and P ions

Microarc oxidized (MAO) coating containing TiO₂ and amorphous calcium phosphate was formed on Ti6Al4V in an electrolyte containing EDTA-Ca and phosphate. Subsequent heat treatment has significant effects on the structure and in vitro bioactivity of the MAO coating. After heat treatment (400-800 °C), the crystallinity of TiO₂ increases, and micropore numbers of the MAO coating decline. Moreover, Ca₃(PO₄)₂ is formed on the surfaces of the MAO coatings after heat treatment at 700 and 800 °C. The SEM and ICP-OES results indicate that the abilities of apatite-forming and Ca and P releasing of the MAO coating decrease after heat treatment. The apatite-forming ability of the MAO coating is associated with the crystallinities of titanium oxide and calcium phosphate. The MAO coating containing TiO₂ with a low crystallinity and amorphous calcium phosphate facilitates the apatite formation in vitro. In addition, the induced biomimetic apatite by the MAO coating without heat treatment exhibits carbonated structure, controllable crystallinity and pore networks on the nanometer scale.     

电压对AZ31镁合金微弧氧化涂层微观结构及腐蚀性能的影响

为研究过程参数对镁合金微弧氧化涂层的微观结构及耐腐蚀性能的影响,在AZ31镁合金基体上,采用不同电压,在电解液磷酸三钠(Na3PO4)中制备微弧氧化涂层.采用扫描电子显微镜(SEM)及光学显微镜,分析膜层腐蚀前后的微观组织结构;通过X射线衍射仪(XRD)分析涂层样品腐蚀前后的相组成.采用动电位极化曲线和电化学阻抗谱(EIS)测试对涂层的耐腐蚀性能进行评价.结果表明:AZ31镁合金微弧氧化涂层主要由Mg3(PO4)2,MgO,Mg和少量MgAl2O4组成,腐蚀产物由Mg(OH)2,quintinite和Ca10(PO4)6 (OH)2组成.在电压为325 V,频率3 000 Hz,氧化时间为5 min下制备的微弧氧化涂层具有最致密均匀的微观形貌和最小的腐蚀电流密度,因此表现出最强的耐腐蚀性能.     

Enhanced corrosion resistance of high strength Mg–3Al–1Zn alloy sheets with ultrafine grains in a phosphate-buffered saline solution

The corrosion behaviour of ultrafine grained AZ31Mg alloy sheets with very high strength, which were prepared by high-ratio differential speed rolling (HRDSR) technique, was studied in a phosphate-buffered saline solution. The corrosion resistance was greatly improved after HRDSR. This result was attributed to the enhanced stability of the Mg(OH)₂ layer due to the grain refinement and precipitation of various types of P-containing compounds on the stabilised Mg(OH)₂ layer. The HRDSR technique has a good potential to be used for the development of magnesium sheets with good combination of mechanical and biocorrosion properties.     

Plasma-Sprayed Hydroxyapatite Coating for Improved Corrosion Resistance and Bioactivity of Magnesium Alloy

In the present study, the corrosion resistance and bioactivity of AZ91HP magnesium alloy were improved by plasma spraying hydroxyapatite (HA) coating. X-ray diffraction measurements indicated that the coating formed amorphous and little β-Ca₃ (PO₄)₂ besides of HA. The corrosion resistance and bioactivity of the coating and magnesium alloy in simulated body fluid were investigated using immersion test. The coating showed lower corrosion rate and better bioactivity than magnesium alloy. The coating significantly improved the hydrophilicity of Mg alloy. The prothrombin time of the coating was 18 s, and the prothrombin time of Mg alloy was 11 s, so the coating had better anticoagulant activity.     

Effect of microplasma modes and electrolyte composition on micro-arc oxidation coatings on titanium for medical applications

A method of micro-arc oxidation (MAO) has been used for the obtaining of the bioactive calcium-phosphate coatings on the surface of nanostructured titanium. A homogeneous alkaline electrolyte containing phosphate ions and calcium (II) complexes with EDTA was used. An effect of changes of current modes on the coating characteristics has been studied. Obtained coatings have the molar ratio Ca/P up to 1.5 and include the phase of calcium phosphate β-Ca₃(PO₄)₂. The adhesion strength of coatings to the titanium substrate is in the range 10-35 MPa, the thickness is up to 100 μm. The experiments in vivo have been carried out. They have shown 75% probability of new bone tissue growth on coatings with roughness of 2.5-5.5 μm.     

Effects of Al3+ concentration in hydrothermal solution on the microstructural and corrosion resistance properties of fabricated MgO ceramic layer on AZ31 magnesium alloy

Porous magnesium oxide (MgO) ceramic layers were prepared on AZ31 magnesium alloy via microarc oxidation (MAO) and sustained hydrothermal treatment at 90°C for 18 h in alkaline zinc nitrate solution with varying Al³⁺ concentrations. Increasing the Al³⁺ concentration advanced the formation of layered double hydroxide (LDH) films on the MgO ceramic layers, enabling the sealing of micropoles and cracks via in situ growing. Electrochemical and immersion test results indicated that the MAO/hydrothermal treatment-prepared LDH/MgO composite coatings achieved increased corrosion resistance than single MAO ceramic layers, due to the sealing effect and ion-exchange capacity of LDHs. Furthermore, as Al³⁺ concentration in hydrothermal solution increased, the anticorrosion properties of the composite coatings were enhanced. Concludingly, LDH/MgO composite coatings could provide augmented long-term anticorrosion protection for magnesium alloys compared with single MAO ceramic layers.     

Influence of hydrothermal treatment temperature on oxidation modification of C/C composites with aluminum phosphates solution by a microwave hydrothermal process

Carbon/carbon (C/C) composites were modified with an aluminum phosphates solution by a novel microwave hydrothermal (MH) process in order to improve their low temperature oxidation resistance. Results show that a H₃PO₄ or HPO₃ continuous molten layer with some regular, white cubic Al(PO₃)₃ crystallites are obtained on the surface of the modified composites. The anti-oxidation property of the composites after modification improves with the increase of the MH temperature from 393 to 473 K. The oxidation rate is almost constant after oxidation at 873 K for 6 h. The formation of annular structure of Al(PO₃)₃ is helpful to improve the oxidation resistance of the composites.     

Apatite formation in Hanks' solution on β-Ca2SiO4 films prepared by MOCVD

β-Ca₂SiO₄ film in a single phase was prepared by metalorganic chemical vapor deposition (MOCVD) and immersed in Hanks' solution to evaluate the apatite formation ability. Apatite formed on the surface of the β-Ca₂SiO₄ film after immersion for 1 d. A dense apatite layer covered the β-Ca₂SiO₄ film surface after immersion for 7 d. CO₃²⁻ and PO₄³⁻ groups were identified by Fourier transform-infrared spectroscopy (FT-IR) after immersion for 1 d. The shape of apatite changed from granular to needle-like to densely packed granular with increasing immersion time from 1 d to 14 d.     

Effects of different inorganic anions on equipment material corrosion behaviour in subcritical water oxidation

In this work, possible corrosion mechanisms of Fe- and Ni-based alloys are discussed which are protected by Cr₂O₃, NiO and MoO₂ surface layers. But chloride ions can dissolve these oxide films and there is a strong synergistic effect between hydronium ions and oxygen, leading to severe local alloy corrosion. On the other hand, titanium and Zr-3 alloys show good corrosion resistance in acidic oxidising subcritical water containing different inorganic salts. A double-layer oxide film (TiO and TiO₂) is formed on the surface of TA2 and TA9 alloys, while a triple oxide layer (TiO, Ti₂O₃ and TiO₂) is formed on TA10 surfaces in such aqueous solutions containing chloride and sulphate ions. In addition to the two oxide layers, Ti₃(PO₄)₄ deposits are also formed on the surface of TA2 and TA9 alloys when the subcritical water contains phosphates. Moreover, (TiO)₂P₂O₇ deposits form besides Ti₃(PO₄)₄ layers on the surface when the TA10 alloy is oxidised under the latter conditions.     

Corrosion processes of Mg–Y–Nd–Zr alloys in Na2SO4 electrolyte

The corrosion behavior of Mg–Y–Nd–Zr (WE43 commercial alloy) was investigated in Na₂SO₄ electrolyte using potentiodynamic polarization curves, X-Ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) depth profiles, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS) analyzes. SEM and EDS data show that Nd-rich precipitates are mainly located at the grains boundaries. Zr/Y-rich zones are distributed inside the most of the grains. XPS study indicates a depletion of Mg on surface that could be attributed to Mg dissolution and an enrichment of the addition element oxides. XPS and ToF-SIMS analyzes demonstrate that the corrosion films are made up of a magnesium hydroxide (Mg(OH)₂) outer layer and an inner layer containing magnesium oxide (MgO), yttrium oxide (Y₂O₃) and hydroxide (Y(OH)₃), mixed with a small amount of MgH₂, zirconium oxide (ZrO₂) and neodymium oxide (Nd₂O₃). The Y₂O₃ and Y(OH)₃ signals increase slightly in the inner layer towards the corrosion film/alloy interface. Unlike these compounds, ZrO₂ and Nd₂O₃ compound signals are constant inside the inner layer. It is concluded that: (i) neodymium, zirconium and yttrium play a key role in the slightly improved corrosion resistance of the alloy and (ii) the cathodic reaction is slower on WE43 than on pure Mg and AZ91.     

Anti-corrosion microarc oxidation coatings on SiCP/AZ31 magnesium matrix composite

The corrosion-resistant ceramic coatings up to 80 μm thick were fabricated on SiC_P/AZ31 magnesium matrix composite by microarc oxidation (MAO) technique in Na₃PO₄ + KOH + NaF solution. The microstructure, composition and phase constituent of ceramic coatings were analyzed by SEM and XRD, and the electrochemical corrosion behaviour of coatings was evaluated by the electrochemical polarization method. The thicker coating is compact and displays a good adhesion to the composite substrate. The ceramic coatings consist of MgO, Mg₂SiO₄, MgF₂, Mg₃(PO₄)₂, furthermore, a few residual SiC phases were also found in the coatings by means of SEM observation and EDX analysis. Most of SiC reinforced particles in the oxidized composite substrate have transformed into the oxides under microarc discharge sintering, but a few residual SiC reinforcements in the MAO coatings have not disrupted the continuity of coatings. So the corrosion resistance of the SiC_P/AZ31 composite is greatly improved by MAO surface treatment, however, the corrosion resistance of coated composite also depends on the coating thickness.     

纯Ti表面微弧氧化-碱热处理仿生陶瓷膜的制备及耐蚀性

采用微弧氧化-碱热处理在纯Ti表面制备了含有羟基磷灰石(HA)的仿生陶瓷膜。利用SEM,XRD和电化学工作站等手段研究了膜层的形貌、物相及其耐蚀性。结果表明:在乙酸钙-磷酸二氢钙电解液体系中微弧氧化(MAO),纯Ti表面形成一层含Ca和P的TiO2多孔陶瓷膜。经水热处理后,膜层表面的孔洞变小、致密性增加,膜层中还出现了鳞状、层片状以及针棒状的HA。在Hank's模拟体液中,MAO膜和微弧氧化-碱热处理(MAOAH)膜均表现出较好的耐蚀性。MAO膜经模拟体液腐蚀后,形成了缺钙型HA(Ca8.86(PO4)6(H2O2)2)和CaTiO3;而模拟体液中的阴离子与MAOAH膜层的氧化物作用使膜层孔洞直径和深度增加。     

Self-healing mechanism of a protective film prepared on a Ce(NO3)3-pretreated zinc electrode by modification with Zn(NO3)2 and Na3PO4

Self-healing mechanism of a protective film against corrosion of zinc at scratches in an aerated 0.5 M NaCl solution was investigated by polarization measurements, X-ray photoelectron spectroscopy (XPS) and electron-probe microanalysis (EPMA). The film was prepared on a zinc electrode by treatment in a Ce(NO₃)₃ solution and addition of aqueous solutions containing 9.98 or 19.9 μg/cm² of Zn(NO₃)₂ · 6H₂O and 55.2 μg/cm² of Na₃PO₄ · 12H₂O. After the coated electrode was scratched with a knife-edge crosswise and immersed in the NaCl solution for many hours, polarization measurements, observation of pit formation at the scratches, XPS and EPMA were carried out. This film was remarkably protective and self-healing against zinc corrosion on the scratched electrode. The cathodic and anodic processes of zinc corrosion were markedly suppressed by coverage of the surface except for scratches with a thin Ce₂O₃ layer containing a small amount of Ce⁴⁺ and the surface of scratches with a layer composed of Zn₃(PO₄)₂ · 4H₂O, Zn(OH)₂ and ZnO mostly.     

PREPARATION OF MICROARC OXIDATION COATING ON (Al2O3-SiO2)sf/AZ91D MAGNESIUM MATRIX COMPOSITE AND ITS ELECTROCHEMICAL IMPEDANCE SPECTROSCOPIC ANALYSIS\par

在硅酸盐电解液体系中, 采用交流微弧氧化方法在增强体体积分数为33%的 (Al₂O₃-SiO₂)_sf/AZ91D镁基复合材料表面制备出完整的保护性氧化膜. 利用SEM, EDS和XRD分析了氧化膜的形貌、成分和相组成, 测量了膜层的显微硬度分布. 采用电化学阻抗谱(EIS)评价了微弧氧化表面处理前后复合材料的电化学腐蚀性能, 确立了不同浸泡时间对应的等效电路. 结果表明, 微弧氧化膜主要由MgO和Mg₂SiO₄相组成, 最大硬度达到1017 HV. 氧化膜电化学阻抗模值|Z|与镁合金基体相比大幅度提高, 耐腐蚀性能明显高于基体. 在3.5%NaCl溶液里浸泡96 h后, EIS出现感抗弧, 显示膜内部开始出现点蚀破坏. 氧化膜耐蚀性由膜内致密层特性所决定.     

TEM Analysis on Micro-Arc Oxide Coating on the Surface of Magnesium Alloy

By micro-arc oxidation (MAO), the oxide coatings were prepared on the surface of magnesium alloys in a composite electrolytic solution. The microstructures of the coating layer and the interface between coating and substrate were analyzed by using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The oxide coating consists of two layers (the outer and the inner layer). Although both layers are composed of microcrystalline MgO and amorphous phase, the inner layer is more compact and rich in fluorine with a thickness of about 1-2 μm. Fluorine plays an important role in the inner dense layer formation. The inner layer, like a barrier wall, blocks the thickness of the oxide coating to increase and improves corrosion resistance. The formation mechanism of the inner layer is also discussed.