Microarc oxidation coating formed on SiCw/AZ91 magnesium matrix composite and its corrosion resistance

A ceramic coating was synthesized on the surface of SiCw/AZ91 magnesium matrix composite by means of microarc oxidation (MAO) technique. Scanning electron microscope (SEM) was employed to characterize the surface morphology and cross-section microstructure of the coating. The phase structure and the chemical composition of the ceramic coating were analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the coating was evaluated by immersion test and electrochemical method. The results showed that the corrosion rate of the coated SiCw/AZ91 composite was decreased greatly compared with that of the bare composite because of the protective microarc oxidation coating.     

Effect of zirconia sol in electrolyte on the characteristics of microarc oxidation coating on AZ91D magnesium

The microarc oxidation coatings were prepared on AZ91D magnesium alloy in a Na₂SiO₃-KOH electrolyte with and without zirconia sol, respectively. The effect of zirconia sol as an additive in the electrolyte on the surface morphologies, compositions, structures, and corrosion resistances of the coatings were investigated. It was found that the coating formed in the Na₂SiO₃-KOH electrolyte with zirconia sol has more uniform morphology, less micropores and cracks than that formed in an electrolyte without zirconia sol. The phase compositions of the coatings also varied after addition of zirconia sol in the electrolyte, owing to the participation of zirconia sol in the reaction and its incorporation into the oxide coating, and Zr existed in the form of Mg₂Zr₅O₁₂. The results of potentiodynamic polarization analysis show that the coating formed in the electrolyte with zirconia sol increases significantly corrosion resistance for magnesium alloy.     

2219铝合金搅拌摩擦焊接头微观组织对微弧氧化膜生长的影响

采用微弧氧化方法在硅酸盐电解液里在2219铝合金搅拌摩擦焊接头表面均匀生长一层50 μm陶瓷膜, 分析了铝合金基体和焊缝区陶瓷膜的形貌、相组成和显微硬度分布, 探讨了合金显微组织和微弧氧化膜生长过程的相互影响. 结果表明, 铝合金显微组织对微弧氧化膜的生长影响较小, 铝合金基体和焊缝区的微弧氧化膜特性几乎相同, 陶瓷膜都是由α-Al₂O₃、γ-Al₂O₃和莫来石(3Al₂O₃·2SiO₂)相组成; 不同区域膜层的显微硬度相等, 其平均硬度约为HV 1500. 另外, 微弧放电高温过程对膜/基界面附近的铝合金显微组织没有影响.     

Corrosion behavior of friction stir welded AZ31B magnesium alloy with plasma electrolytic oxidation coating formed in silicate electrolyte

A protective ceramic coating of about 50 μm thick on a friction stir welded (FSW) joint of AZ31B magnesium alloy was prepared by plasma electrolytic oxidation (PEO) in silicate electrolyte. Electrochemical corrosion behavior of uncoated and coated FSW joints was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The equivalent circuits of EIS plots for uncoated and coated FSW magnesium alloy were suggested. The corrosion resistance of FSW magnesium alloy depended on microstructure of the FSW joint. The heat-affected zone with severe grain growth was more susceptible to corrosion than the stir zone and base metal. The PEO coating consisted of a porous outer layer and a dense inner layer. The inner layer of PEO coating played a key role on corrosion protection of the FSW joint of magnesium alloy. Meanwhile, corrosion potential, corrosion current density and impedance at different zones of coated FSW joint were almost the same. The PEO surface treatment significantly improved the corrosion resistance of FSW joints of AZ31B magnesium alloy.     

SiCP/AZ31镁基复合材料微弧氧化膜结构与性能分析

采用微弧氧化表面处理技术在SiC颗粒增强AZ31镁基复合材料表面制备保护性陶瓷膜.分析了陶瓷膜的表面形貌、截面组织和相组成,并测量了膜层的硬度、热震和电化学腐蚀特性.结果表明,陶瓷膜由MgO、Mg₂SiO₄和少量同电解液组成元素相关的相所组成,膜内还残留少量SiC_P增强体.膜层的最高硬度可达到HV800,比复合材料基体提高五倍以上.经过100次热循环(500℃→水淬)后膜层与复合材料结合良好,显示该膜层有较好的抗热震性能.微弧氧化处理后,SiC_P/AZ31镁基复合材料的抗腐蚀能力得到较大提高.     

Controlled synthesis of microarc oxidation coating on Ti6Al4V alloy and its antifriction properties

Abstract

Ceramic coatings were fabricated on a Ti6Al4V alloy surface by microarc oxidation (MAO) in Na₂ SiO₃ – (NaPO₃)₆ aqueous solutions with and without NaAlO₂ additive using an AC power supply. The effect of NaAlO₂ on microstructure, composition, and homogeneity of ceramic coatings were characterised using SEM, XRD, and EPMA. The antifriction property of the coatings with optimised microstructure sliding against SAE 52 100 steel ball was investigated on a pin-on-disc friction and wear tester. The results show that the addition of NaAlO₂ into Na₂ SiO₃ – (NaPO₃)₆ solution assists the formation of more dense, uniform, and thicker coatings and increases rutile TiO₂ content in the coatings. The optimised coating sliding against the steel has a friction coefficient as low as 0.2 – 0.3 at an applied load of 0.5 N and sliding cycle below 2500, which is much smaller than that of uncoated Ti6Al4V against the same counterpart. The transferring of material from the softer steel ball onto the coating surface is the main wear event, while the microarc oxidation coating is characterised by slight abrasive wear and adhesive wear.     

TC4钛合金表面氧化亚铜掺杂微弧氧化层的结构和性能

在电解液中添加不同浓度的氧化亚铜微粒,然后在TC4钛合金表面制备掺杂铜氧化物的微弧氧化层。在模拟海水中进行微弧氧化层的摩擦磨损和抗菌实验,并使用扫描电镜(SEM)、X射线衍射仪(XRD)、X光电子能谱仪(XPS)和显微硬度仪等手段对比研究了掺杂氧化亚铜的微弧氧化层的微观结构和性能。结果表明：掺杂氧化亚铜的微弧氧化层表面呈多孔形貌特征,但是微孔的数量较少和孔径尺寸较小,氧化亚铜微粒在膜层中以氧化铜和氧化亚铜两种形式存在;与未掺杂氧化亚铜的微弧氧化层相比,添加不同浓度氧化亚铜的微弧氧化层在模拟海水中的抗磨损性能和抗菌性能显著提高,但是微弧氧化层中的铜元素使其耐蚀性有所降低。     

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.     

AM60镁合金Cr涂层制备及其耐腐蚀性研究

为了提高AM60镁合金的耐腐蚀性能,采用机械涂覆的方法在合金表面制备Cr涂层。通过XRD、视频显微镜、SEM、显微硬度分析等方法对表面涂层的物相、截面形貌、涂层的显微硬度等进行表征,利用电化学工作站对涂覆Cr前后的AM60镁合金的耐蚀性能进行分析。结果表明:AM60镁合金表面成功涂覆了Cr涂层,所制备涂层与基体结合致密,涂覆效果较好;同时,涂层的显微硬度高达到1 132 HV,较基体提高了1.96倍;球料比为10∶1和20∶1时,球磨时间为20 h和15 h时所制备的膜层耐腐蚀性能较好,和基体相比,所制备样品的自腐蚀电流密度均降低了3个数量级,自腐蚀电位均大幅提高,阻抗谱半径也均增加,在模拟海水中的耐腐蚀性能都得到明显改善。因此,在该实验条件下,Cr涂层的最佳制备工艺为:球料比为10∶1,球磨时间为20 h。      

镁合金无铬微弧氧化新工艺

利用正交试验对MB2镁合金无铬、磷、镁微弧氧化成膜工艺进行了研究,同时利用表面分析技术,分析了氧化膜层的显微硬度、截面形貌和相结构,采用动电位扫描法考察了氧化膜的耐腐蚀性能.研究的最佳工艺条件为:30 g/L KOH,45g/L Al(OH)3,2 g/L K2SiO3,2g/L添加剂M,电流密度65 mA/cm2,温度45℃.该微弧氧化新工艺能在镁合金上形成银灰色的氧化膜层,其显微硬度值及耐腐蚀性远优于传统含铬工艺DOW17所形成的膜层;微弧氧化膜主要由MgO,MgAl2O4,Al2O3组成,具有多孔结构,孔径较为均匀,分为内外两层,外层为疏松层,内层为与基体结合牢固的致密层;在成膜过程中,电解液的铝盐浓度和微弧氧化电流密度是影响性能的主要因素.     

Electroless deposition of Ni–P–B4C composite coating on AZ91D magnesium alloy and investigation on its wear and corrosion resistance

In the present work, the effect of applying ternary Ni–P–B₄C composite coating from an electroless plating bath containing sulfate nickel, sodium hypophosphate and suspended B₄C particles, on the corrosion and wear resistance of an AZ91D, high aluminum cast magnesium alloy, was investigated. Regarding low corrosion resistance of magnesium alloys, chromium oxide plus HF (Hydro Fluoric Acid) pretreatment was applied to prepare the substrate for coating treatment in electroless bath. The pH value and temperature of the electroless bath were 9 and 82 °C, respectively. The coating was characterized for its micro structure, morphology, microhardness, wear and corrosion resistance. SEM (Scanning Electron Microscope) observation showed dense and coarse nodules in the ternary composite coating and the cross section of Ni–P–B₄C coating offered presence of well dispersed B₄C particles in the coating. The hardness of the Ni–P–B₄C composite coatings was around 1200 MPa, more than what can be obtained for Ni–P coatings (about 700 MPa). The wear test which was carried out by using pin on disc method, showed that ternary Ni–P–B₄C composite coating had a good wear resistance and more superior than Ni-P coating. The polarization test results for ternary Ni–P–B₄C composite coating exhibited good corrosion resistance properties in protecting the AZ91D magnesium alloy, but not better than Ni–P coating.     

Surface characteristics and electrochemical corrosion behavior of a pre-anodized microarc oxidation coating on titanium alloy

A porous bioactive titania coating on biomedical β titanium alloy was prepared by pre-anodization followed by micro arc oxidation technology. The effects of pre-anodization on the phase constituent, morphology and electrochemical corrosion behavior of the microarc oxidation coating were investigated. The results show that pre-anodization has less influence on the phase constituent and the surface morphology of the microarc oxidation coating, but improves the inner layer density of the microarc oxidation coating. The decrease of plasma discharge strength due to the presence of the pre-anodized oxide film contributes to the formation of the compact inner layer. The pre-anodized microarc oxidation coating effectively inhibits the penetration of the electrolyte in 0.9% NaCl solution and thus increases the corrosion resistance of the coated titanium alloy in physiological solution.     

LY12铝合金微弧氧化陶瓷层的结构和性能

分析了LY12铝合金微弧氧化陶瓷膜的形貌、组成和结构,研究了氧化膜的硬度、与基体的结合强度以及在油润滑和干摩擦这两种条件下的摩擦学行为.结果表明,铝合金微弧氧化膜可分为疏松层和致密层,疏松层由α-Al2O3、γ-Al2O3以及Al-Si-O相组成,致密层由α-Al2O3和γ-Al2O3组成,致密层中α-Al2O3的含量远远高于疏松层.从表层到基体,微弧氧化膜的断面显微硬度先增大后减 小.微弧氧化膜与铝合金基体结合紧密.随着膜厚度的增加,氧化膜的临界载荷线性增加.氧化膜具有优良的抗磨性能,油润滑条件下的摩擦系数仅为干摩擦下的1/10.     

Effect of chloride ion level on the corrosion performance of MAO modified AZ31 alloy in NaCl solutions

Microarc oxidation coatings were fabricated on AZ31 magnesium alloy in the electrolyte of sodium phosphate. Potentiodynamic polarization and electrochemical impedance spectroscopy tests were employed to investigate the electrochemical corrosion behavior. The corroded surface was characterized by an optical microscope and X-ray diffraction. The influence of chloride ion concentration on the corrosion resistance of microarc oxidation coated AZ31 alloy is discussed. The corrosion current density enlarged ratio and the charge transfer resistance reduced ratio indicated that the extent of the corrosion damage of microarc oxidation coated AZ31 alloy is much higher when chloride ion concentration is greater than 5%. A corrosion mechanism related to the effect of chloride ion concentrations on the corrosion behavior is proposed.     

Effects of nano-Fe2O3 powder on thermal emission property of microarc oxidation coating on titanium alloy

Abstract

A ceramic coating was formed on the titanium alloy by microarc oxidation in an electrolyte containing nano-Fe₂O₃, emulsifier OP-10 and sodium phosphate. The composition, surface and cross-sectional morphology and the element compositions of the coatings were characterised by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis system. The spectral emissivity of the coatings was measured by a Fourier transform spectrometer apparatus. The bonding strength between the coating and the titanium alloy was studied by tensile strength test. The thermal shock resistance of the coatings was also evaluated. The results showed that nano-Fe₂O₃ was incorporated into the coating, and the coating had high emission at the wavelength range of 3–20 μm. The bonding strength was 33·2 MPa, and after being subjected to severe thermal shocking for 50 cycles, little peeling-off of the coating occurred.     

Fluoride treatment and in vitro corrosion behavior of an AZ31B magnesium alloy

As a new class of biodegradable material, magnesium alloys have attracted much attention in recent years. In order to improve the corrosion resistance, a fluoride coating was prepared on the surface of AZ31B magnesium alloy. The surface characterization analysis showed a dense coating with some irregular pores was formed. The TF-XRD analysis indicated that the coating was mainly composed of MgO and MgF₂. Electrochemical and immersion tests proved that the fluoride conversion coating significantly improved the corrosion resistance of AZ31B. Three-point bending test revealed that the degradation behavior of the fluoride treated AZ31B could meet the requirement as a biodegradable material.     

镁合金微弧氧化膜结构及耐蚀性的初步研究

空气中由于AZ91D镁合金耐腐蚀性差,影响实际应用.为了弄清腐蚀原因,增加应用效果,作者利用扫描电镜和X射线衍射分析了AZ91D镁合金表面微弧氧化膜的形貌、结构和相组成,并对氧化膜的耐蚀性作了初步试验分析.研究表明,AZ91D微弧氧化膜呈3层结构,外层氧化膜存在一些孔洞;中间层氧化膜疏松、具有较大厚度;内层氧化膜与基体金属结合紧密.氧化膜主要由MgO,MgSiO3,MgAl2O4,Mg3(PO4)2组成.经1周3%NaCl溶液浸泡试验,结果表明微弧氧化膜可以较大程度地提高AZ91D镁合金的耐蚀性,但氧化膜表面富含Si,P的颗粒是易发生腐蚀的电化学活性点,导致氧化膜发生局部腐蚀.     

The influence of HF treatment on corrosion resistance and in vitro biocompatibility of Mg-Zn-Zr alloy

The samples made of a Mg-2.5wt.%Zn-0.5wt.%Zr alloy were immersed in the 20% hydrofluoric acid (HF) solution at room temperature for different time, with the aim of improving the properties of magnesium (Mg) alloy in applications as biomaterials. The corrosion resistance and in vitro biocompatibility of untreated and fluoride-coated samples were investigated. The results show that the optimum process is to immerse Mg alloys in the 20% HF solution for 6 h. After the immersion, a dense magnesium fluoride (MgF₂) coating of 0.5 μm was synthesized on the surface of Mg-Zn-Zr alloy. Polarization tests recorded a reduction in the corrosion current density from 2.10 to 0.05 μA/cm² due to the MgF₂ protective coating. Immersion tests in the simulated body fluid (SBF) also reveal a much milder corrosion on the fluoride-coated samples, and its corrosion rate was calculated to be 0.05 mm/yr. Hemolysis test suggests that the conversion coated Mg alloy has no obvious hemolysis reaction. The hemolysis ratio (HR) of the samples decreases from 11.34% to 1.86% with the HF treatment, which meets the requirements of biomaterials (HR < 5%). The coculture of 3T3 fibroblasts with Mg alloy results in the adhesion and proliferation of cells on the surface of fluoride-coated samples. All the results show that the MgF₂ conversion coating would markedly improve the corrosion resistance and in vitro biocompatibility of Mg-Zn-Zr alloy.     

电解液组成对AZ91D镁合金微弧氧化的影响

在含有NaAlO2、KF的电解质溶液中,采用恒电流方式对AZ91D镁合金进行微弧氧化获得陶瓷膜.研究了电解液组分及浓度对陶瓷氧化膜厚度及表面形貌的影响,同时,采用动电位极化曲线及电化学交流阻抗评价了陶瓷氧化膜的耐蚀性.研究发现:NaAlO2单独存在时即可产生火花放电现象,但得到的氧化膜较薄;氟化钾的加入可以显著增加氧化膜厚度,膜厚的增长速度与氟化钾的加入量呈线性关系.SEM表面形貌分析表明:电解质浓度较低时产生的氧化膜宏观上较粗糙、微观上颗粒结合紧密;高浓度时得到的氧化膜宏观上细致光滑,微观上存在明显的孔洞和放电隧道,呈熔融状态结合在一起.动电位极化曲线及电化学交流阻抗的测试一致表明,经微弧氧化处理后的镁合金耐蚀性显著提高.     

Biocorrosion resistance of coated magnesium alloy by microarc oxidation in electrolyte containing zirconium and calcium salts

The key to use magnesium alloys as suitable biodegradable implants is how to adjust their degradation rates. We report a strategy to prepare biocompatible ceramic coating with improved biocorrosion resistance property on AZ91D alloy by microarc oxidation （MAO） in a silicate-K2ZrF6 solution with and without Ca（H2PO4）2 additives. The microstructure and biocorrosion of coatings were characterized by XRD and SEM, as well as electrochemical and immersion tests in simulated body fluid （SBF）. The results show that the coatings are mainly composed of MgO, Mg2SiO4, m-ZrO2 phases, further Ca containing compounds involve the coating by Ca（H2PO4）2 addition in the silicate-K2ZrF6 solution. The corrosion resistance of coated AZ91D alloy is significantly improved compared with the bare one. After immersing in SBF for 28 d, the Si-Zr5-CaO coating indicates a best corrosion resistance performance.