氟硅比对AM60B镁合金微弧氧化膜微观结构及耐蚀性的影响

通过改变硅酸盐体系电解液中KF和Na_2Si O_3的浓度,定量分析氟化钾与硅酸钠浓度配比(简称氟硅比)对AM60B镁合金微弧氧化膜层微观结构及耐蚀性的影响。结果表明:主盐Na_2Si O_3在微弧氧化成膜中必不可少。当电解液中含有KF时(氟硅比大于0),随着氟硅比的增大,成膜反应加剧,膜层表面的孔隙率和表面大孔(3μm)数目增加,膜厚增加,且由于KF在电解液中的存在,会生成新物相Mg F_2,促进物相Mg_2Si O_4的形成,这些物相有利于提高膜层的耐蚀性。此外,KF与Na_2Si O_3二者在成膜反应中的竞争与协同作用因氟硅比的变化而变化,致使膜层的微观结构与物相含量随之改变,从而影响膜层的耐蚀性。本研究中,当氟硅比为0.5(KF:Na_2Si O_3=7.5:15)时,由于KF与Na_2Si O_3间良好的协同作用,形成的膜层厚度较大,致密性较好,缺陷较少,此时膜层兼备厚度大与耐蚀性优的特性,是最佳的氟硅比。     

AZ91D镁合金表面微弧氧化陶瓷膜微观结构与组成的研究

采用自制的恒流非对称方波电源用微弧氧化法在AZ91D镁合金表面制备了耐腐蚀陶瓷膜,通过微观分析手段对微弧氧化膜的截面特征、元素成分分布及表面膜的相组成进行了分析,研究了微弧氧化工艺参数对膜层表面形貌、微观结构与组成等的影响.结果表明,提高电流密度会造成组织疏松微孔孔径增大;硅酸盐溶液中微弧氧化制得的陶瓷膜优于铝酸盐溶液.而且电解液中的离子可参与成膜反应,硅酸盐溶液体系镁合金微弧氧化陶瓷层主要由MgO和Mg_2SiO_4相组成,铝酸盐溶液体系微弧氧化膜层主要由MgAl_2O_4相组成.     

二次电压对AZ91D镁合金微弧氧化封孔的影响

目的提高镁合金微弧氧化膜层的耐蚀性。方法在锆盐体系电解液中对AZ91D镁合金进行微弧氧化处理,通过调节二次电压对AZ91D镁合金微弧氧化膜层的孔隙进行封闭,采用XRD、SEM和电化学测试分别对微弧氧化膜层的物相、表面形貌和耐蚀性进行了研究。结果二次电压对膜层的相成分没有影响,主要相组成为MgO、MgF_2、ZrO_2、Mg_2Zr_5O_(12)。随着二次电压的升高,膜层表面放电微孔孔径先减小后增大,孔隙率先降低后升高。与没有二次电压相比,施加二次电压的腐蚀电流降低2~3个数量级,极化电阻升高1~2个数量级,耐蚀性明显提高,且当二次电压为160 V时,膜层的极化电阻最高,耐蚀性最好。结论二次电压能够对AZ91D镁合金微弧氧化膜层的孔隙进行封闭,进而阻止腐蚀液通过微孔进入基体,提高膜层的耐蚀性。     

AM60压铸镁合金表面ZrO_2微弧氧化陶瓷层的制备方法研究

为了改善镁合金的微弧氧化膜层的性能,利用两步法在AM60压铸镁合金表面制备了ZrO_2微弧氧化膜层.研究了膜层在恒电压控制方式下的生长规律,测量了膜层在w(NaCl)=3.5%溶液中的极化曲线,分析了膜层的物相组成.结果表明:在400 V恒压微弧氧化处理时,微弧氧化膜层的平均生长速度最高可达3.5 μm/min:试样在w(NaCl)=3.5%溶液中的极化曲线显示,镁合金经过两步法在锆盐溶液中微弧氧化后,其腐蚀电位正移,腐蚀电流降低,腐蚀速度大幅降低,其耐蚀性得到了大幅度提高.两步法制备微弧氧化膜层主要由ZrO_2、ZrP_2、Mg_3(PO_4)_2、MgF_2相组成,在溶液中添加的Zr元素可以通过微弧氧化处理时的复杂反应进入膜层中,膜层表现出的优异耐蚀性主要源于膜层中存在ZrO_2陶瓷.     

梯度多孔Ti的微弧氧化研究

在硫酸电解液中对致密Ti和梯度多孔Ti样品进行微弧氧化研究。分析了孔隙特性、电流密度和电解液组成对微弧氧化过程中起始电压、击穿电压、起弧时间、氧化膜形貌和厚度的影响,并测量了微弧氧化膜的物相组成。结果表明:与致密Ti样品相比,梯度多孔Ti样品的起始电压和击穿电压提高、起弧时间延长,氧化膜层厚度增加。随着电流密度的增加,梯度多孔Ti微弧氧化反应剧烈,表面膜层的微孔直径增大,孔洞变小,膜层表面粗糙度增加,膜层变厚。当在硫酸电解液中加入少量硝酸镧后,微弧氧化起始电压和击穿电压提高、起弧时间延长,表面氧化膜的平均孔径从200nm增加到2μm左右,膜层厚度从27.6μm增加到35.6μm,膜层表面粗糙度增加。XRD分析表明,微弧氧化膜主要由Ti、锐钛矿TiO2和金红石TiO2相组成,其中以锐钛矿TiO2为主。     

复合电解液中ZK60镁合金的微弧氧化机理

在优化的铝酸钠-磷酸钠复合电解液体系中,以ZK60镁合金为研究对象进行微弧氧化实验,并结合电压-时间曲线和微弧氧化各个阶段膜层的微观形貌以及物相分析等方面对复合电解液体系中微弧氧化过程及成膜机理进行了探讨。结果表明,该体系下微弧氧化过程分为了氧化膜生成阶段、微弧氧化膜层快速生长阶段和电压微小下降过程以及微弧氧化膜层的修整阶段。膜层的物相分析表明微弧氧化初期膜层主要成分为MgO,Mg和MgZn2,中后期膜层中的主体相为尖晶石结构的MgAl2O4和方镁石结构的MgO,微弧氧化的最后阶段对膜层的物相组成没有影响。     

SiC颗粒增强的AZ91D镁合金微弧氧化膜层的结构与性能研究

在硅酸盐体系的电解液中加入SiC粉末,对AZ91D镁合金进行微弧氧化处理.用SEM、XRD分析加入SiC粉末对微弧氧化膜层的表面形貌特征和物相成分的影响.结果表明,加入SiC粉末后微弧氧化膜层表面的微孔变少且直径减小;膜层物相成分中增加了SiC相.在w(NaCl)=3.5%溶液中的电化学腐蚀实验结果表明,加入SiC粉末后所制备的膜层的耐腐性能显著提高.在硅酸盐体系的电解液中加入SiC粉末(浓度为5g/L),频率600Hz、占空比30%、平均电流密度15A/dm<'2>时,可获得耐腐蚀性能较好的微弧氧化膜层.     

ZK60镁合金微弧氧化改性研究

在NaOH电解液中,对ZK60镁合金进行微弧氧化处理。研究了微弧氧化过程的电压-时间曲线、微弧氧化电流、氧化时间对微弧氧化膜层的显微形貌和厚度的影响,测试了氧化膜的耐蚀性能。研究结果表明:随着微弧氧化电流和时间的增加,表面膜层厚度增加,但膜层中的微孔直径增加,表面粗糙度增加,氧化膜质量降低。在NaOH电解液中,微弧氧化电流为3A、氧化3min后,ZK60镁合金表面形成的氧化膜质量最好,厚度约为19.8μm。XRD分析表明微弧氧化处理后试样表面膜层由MgO相组成。耐腐蚀测试表明微弧氧化后样品的质量出现先增加而后降低的现象,其失重和析氢量均比未微弧氧化样品少,同时溶液pH值变化较慢,这说明微弧氧化后样品的耐腐蚀性提高。     

微弧氧化膜层对压铸镁合金拉伸性能的影响

利用自主开发的微弧氧化工艺处理了实际镁合金压铸试样,并探讨了微弧氧化膜层对镁合金基体拉伸性能的影响。研究发现,微弧氧化处理的镁合金压铸件试样表面的膜层厚度均匀、色泽均一;膜层含有MgO、MgAl2O4及Mg2SiO4等物相,且MgO含量随处理时间的延长而增加;处理时间合适时,膜层几乎不影响镁合金基体的拉伸性能,但随着处理时间的增加,由于膜层中存在的诸多缺陷而使基体的拉伸性能开始有所下降。     

不同加压方式下镁合金微弧氧化膜结构及耐蚀性的变化规律

在硅酸盐体系电解液中采用恒压、变压两种加压方式对AZ91D镁合金进行不同时间的微弧氧化处理,研究膜层的厚度、表面形貌、物相组成及耐蚀性随时间的变化规律,对膜层中微孔的大小、数量及表面孔隙率进行定量统计,比较两加压方式下膜层的差异。结果表明:两加压方式下,膜层结构随时间的变化规律基本相同,其变化幅度在变压方式下相对较大;随着处理时间的增加,膜层的厚度不断增大,微孔数量不断减少,较大微孔不断增多,表面孔隙率不断增大,而物相组成却不变;恒压方式下膜层的耐蚀性不断增强,变压方式下则先增大后减小;与变压方式相比,恒压方式下膜层生长前期的生长速率较大,更有利于工业化生产。     

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.     

Effect of TiO2 nanoparticles on the microstructure and corrosion behavior of MAO coatings on magnesium alloy

The microstructure and corrosion behavior of the micro‐arc oxidation (MAO) coatings modified by TiO₂ on AZ91 magnesium alloys were investigated by SEM, EDS, XRD, electrochemical analysis and drop test, respectively. The modified MAO coatings were consisted of not only MgAl₂O₄ and MgO, which usually could be found in MAO coatings on the AZ91 alloys, but also a small quantity of Ti. With increasing the contents of TiO₂ nanoparticles from 0 to 4.8 g/L in the electrolyte, the coatings thickness increased from 19 to 24 µm, and the color of the MAO coating changed from light gray to dark. The addition of TiO₂ could effectively decrease the number of the pores and improve the density of the coatings. The corrosion potential of the modified coatings positively shifted about 180 mV and the corrosion current density declined and the drop time increased. The mechanism of the corrosion resistances of the modified coatings lay in the fact that TiO₂ nanoparticles could increase the thickness and the density of the coatings on the AZ91 magnesium alloys.     

Study of the structure and corrosion behavior of PEO coatings on AM50 magnesium alloy by electrochemical impedance spectroscopy

In this work coatings were developed on the surface of AM50 magnesium alloy using four different electrolytes containing 10 wt.% each of K₃PO₄ and Na₃PO₄ in combination with either potassium or sodium hydroxides. Electrolyte conductivity and breakdown voltage were measured in order to correlate the property of the coating to the nature of electrolyte. Further, the coatings were examined using scanning electron microscopy for surface morphology and cross sectional investigation, X-ray diffraction for phase determination, and electrochemical impedance spectroscopy for corrosion resistance evaluation. The effect of employing different ions in the electrolytes results in different surface morphologies, chemical phases and, consequently, the corrosion resistance of the coatings. The EIS results indicate the presence of porous and compact layers in the structure of the PEO coatings, whilst the overall coating resistance mainly results from the compact layer, the role of the porous layer as a barrier against corrosion is negligible. Finally, a correlation between the passive current density of the bare alloy and the corrosion resistance of the PEO coating is proposed.     

微弧氧化技术在变形镁合金改性中的应用

简述了关注变形镁合金及其耐腐蚀性的必要性。对变形镁合金进行微弧氧化有报道的电解液体系包括硅酸盐体系、磷酸盐体系和复合盐体系。在硅酸盐电解液体系中微弧氧化研究最多的镁合金是AZ31系列，研究多集中在电解液的配方上；对MB8微弧氧化研究则侧重在工艺过程及工艺条件上。磷酸盐电解液体系微弧氧化的研究不多。比较有研究前景的电解液体系是复合盐体系。指出应该加强对实际的变形镁合金构件的微弧氧化技术的开发。     

几种添加剂作用的微弧氧化膜表面结构及防腐性能

为定性比较添加剂对涂层防腐性能的作用,通过恒压微弧氧化(MAO)方法在AZ31B镁合金表面制备氧化陶瓷膜,采用扫描电子显微技术(SEM)、中性盐雾试验(NSS)等手段,考察了KOH及添加剂Na_2B_4O_7、C_6H_5Na_3O_7和EDTA-2Na浓度(质量浓度)对MAO膜表面形貌、防腐性能、粗糙度和厚度的影响。结果表明:单一组分Na_2SiO_3电解液因较高起弧电压而未能在260 V恒压条件下获得具有"火山口"形貌特征的MAO膜,其防腐性能较差。适量KOH因较低微弧等离子体诱发电压和OH-较快的放电作用,提高了涂层的防腐性能。在优化的Na_2SiO_3-KOH体系中引入10~15 g/L Na_2B_4O_7,因其特殊的形成过程及其"火山喷射状"的微结构,且获得的MAO膜具有自封孔结构,提高了其对镁合金的点腐蚀防护性能。C_6H_5Na_3O_7和EDTA-2Na具有抑弧效应,获得的MAO膜表面微孔分布均匀,但降低了MAO膜的厚度、粗糙度以及防腐性能。     

Conversion coating treatment for AZ91 magnesium alloys by a permanganate‐REMS bath

A permanganate‐rare earth metal salt (REMS) chemical conversion bath was applied to a sample of AZ91 magnesium alloy in this study, a red‐brown conversion coating formed subsequently on the sample surface. The test results of this coating with a scanning electron microscope (SEM) showed that there existed net‐like cracks on the surface of the treated magnesium alloy. With the analyses of X‐ray Diffraction (XRD) and X‐ray Photoelectron Spectroscopy (XPS), a further study of this coating indicated that the coating was structurally amorphous and mainly composed of CeO₂, MnO, MnO₂, MgO, Mg(OH)₂ and MgAl₂O₄. Furthermore, the electrochemical polarization tests showed that compared with the samples treated by the chrome‐based method, the open‐circuit potential of the magnesium alloy coated in permanganate‐REMS bath moved from ? 1.34 V_SCE to ? 1.28 V_SCE and the anodic current density of the alloy, at the same potential, decreased evidently in simulated sweat fluid. The cracks in the chemical conversion coating should be caused by the phase structure of the magnesium alloy. During the chemical conversion process, the localized corrosion micro‐cell led to the formation of the net‐like cracks on the surface. Simultaneously, the dehydration of the surface coating after treatment also accelerated the formation of the cracks at the coating surface.     

Characterization and corrosion studies of fluoride conversion coating on degradable Mg implants

Fluoride conversion coating was synthesized on magnesium (Mg) by immersion treatment in hydrofluoric acid (HF) at room temperature, with the aim of improving the corrosion resistance of Mg in applications as degradable implant material. After an immersion period of 24 h in 48% HF, the samples carried a bronze color, and the conversion coating was dense and free of cracks. Field-emission scanning-electron microscopy (FE-SEM) of the cross-section revealed a coating thickness of about 1.5 μm. Atomic-force microscopy (AFM) recorded an average surface roughness of ∼ 21 nm for the coated sample, similar to that of the untreated one (∼ 17 nm). The coating was mainly composed of magnesium fluoride (MgF₂) as identified by thin-film X-ray diffractometry (TF-XRD), consistent with compositional analysis using X-ray photoelectron spectroscopy (XPS). The MgF₂ was in the form of crystallites of a few nm. A small amount of oxygen was present inside the coating, suggesting that some F⁻ ions are replaced by hydroxyl (OH⁻) ions in the MgF₂ structure, or that a small amount of Mg(OH)₂ was present. The corrosion resistance of untreated and conversion coated Mg in Hanks' solution was studied using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization tests, and immersion tests. EIS results showed a polarization resistance of 0.18 kΩ cm² for the untreated Mg and 5.2 kΩ cm² for the coated sample, giving an improvement of about 30 times. Polarization tests also recorded a reduction in corrosion current density from 400 μA/cm² to 10 μA/cm², showing an improvement of about 40 times. The galvanic effect between untreated and fluoride-coated Mg samples was small. Immersion tests in Hanks' solution also resulted in a much milder and more uniform corrosion damage on the fluoride-coated samples. The results of the present study showed that fluoride coating by conversion treatment is a simple and promising way of enhancing the corrosion resistance of Mg in Hanks' solution, or that it may be employed as a pretreatment step for subsequent coating.     

Corrosion protection of AZ91 magnesium alloy by anodizing in niobium and zirconium-containing electrolytes

A new Nb + Zr-based anodized coating was designed for the corrosion protection of AZ91 magnesium alloy. Polarization curves and electrochemical impedance diagrams plotted in Na₂SO₄ electrolyte showed its high protective effect. Analysis of the chemical composition by X-ray photoelectron spectroscopy indicated that the coating mainly consisted of (i) magnesium metaborate and metaphosphate, (ii) MgF₂ and ZrF₄, and (iii) Nb₂O₅, ZrO₂ and MgO. A higher concentration of fluorine at both interfaces and an enrichment in Zr compared to Nb were revealed by SEM and EDS analyses. Thus, Zr-based compounds and MgF₂ play a key role in the anti-corrosion ability of the coating.     

Comparison of electrochemical corrosion behaviour of MgO and ZrO2 coatings on AM50 magnesium alloy formed by plasma electrolytic oxidation

Two types of PEO coatings were produced on AM50 magnesium alloy using pulsed DC plasma electrolytic oxidation process in an alkaline phosphate and acidic fluozirconate electrolytes, respectively. The phase composition and microstructure of these PEO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behaviour of the coated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in neutral 0.1 M NaCl solution. The results showed that PEO coating prepared from alkaline phosphate electrolyte consisted of only MgO and on the other hand the one formed in acidic fluozirconate solution was mainly composed of ZrO₂, MgF₂. Electrochemical corrosion tests indicated that the phase composition of PEO coating has a significant effect on the deterioration process of coated magnesium alloy in this corrosive environment. The PEO coating that was composed of only MgO suffered from localized corrosion in the 50 h exposure studies, whereas the PEO coating with ZrO₂ compounds showed a much superior stability during the corrosion tests and provided an efficient corrosion protection. The results showed that the preparation of PEO coating with higher chemical stability compounds offers an opportunity to produce layers that could provide better corrosion protection to magnesium alloys.     

Properties of fluoride film and its effect on electroless nickel deposition on magnesium alloys

The physical characteristics and microstructure of the fluoride film formed during activation were investigated using SEM,XPS and SAM,and its stability in electroless nickel(EN) bath was analyzed.The effects of the fluoride film on EN deposition were studied additionally.The results show that the fluoride film on magnesium alloys is a kind of porous film composed of MgF2 with thickness of 1.6-3.2 μm.The composition of the activation bath and pretreatment of EN processing have influence on the composition of the fluoride film.The fluoride is stable and dissolves little in EN bath;as a result,the fluoride film can protect magnesium substrate from the corrosion of EN bath.The composition of fluoride determines the initial deposition of EN and part of the fluoride film finally exists as inclusion in EN coating.