通过微弧氧化 (MAO) 的方法在Na2SiO3-KOH-NaF电解质溶液中处理AZ31B镁合金,利用SEM、XRD和电化学等表征手段,研究了硅烷偶联剂KH550对MAO膜结构及性能的影响。结果表明,KH-550浓度在0~20 mL/L范围内增加时,MAO膜表面微孔尺寸和粗糙度先减小后增大,膜层厚度和耐蚀性能先增加后降低;引入KH-550后并未改变MAO膜的物相结构。分析认为KH-550通过硅烷醇的吸附和化学作用,增加了阳极表面薄弱区域离子移动的阻力,抑制镁合金在MAO过程的弧光放电,从而提高了膜层的生长效率,细化并均匀化微孔,改善了MAO膜的耐蚀能力。
Influence of the content of coupling reagent KH-550 on the morphology, phase constituent and corrosion resistance in 3.5%NaCl solution of micro-arc oxidation (MAO) coatings, prepared on Mg-alloy AZ31B by a constant voltage mode in an electrolyte of Na2SiO3-KOH-NaF, was investigated by scanning electron microscopy, X-ray diffractometer and electrochemical methods. Results showed that the size of micro pores and the roughness of the MAO coatings are increased firstly and then decreased with the increasing amount of KH-550 in a concentration range of 0~20 mL/L, but its thickness and corrosion resistance show a converse result. However, the phase constituents of the MAO coatings are not changed. The preliminary analysis suggested that KH-550 hinders the ionic migration on certain weak areas, where silanol was adsorbed and/or reacted with, and thereby the arc discharge was modulated during MAO process. Therefore, KH-550 improves the growing efficiency of MAO coating, homogenizes the size and distribution of micro pores, and enhances the corrosion protection ability of the MAO coating on Mg-alloy.
微弧氧化 (MAO) 技术,因其所制备涂层的结构可设计性及性能优势,已成为镁合金腐蚀和磨损防护处理的重要手段之一[1-4]。它是将普通阳极氧化的Faraday区域引入到高压放电区域,在镁合金表面产生微弧放电,通过局部高温、高压等环境因素作用制备硬质陶瓷膜,从而达到工件表面强化的目的[1,5-7]。由于高的电压和电流密度,这种阳极氧化过程常伴随强烈的火花放电现象[5,6]。Khaselev等对镁合金进行恒流阳极氧化处理时,观察到阳极电位达到一定峰值时,便在金属表面开始形成很小的火花,一闪即逝 (平均寿命不超过1 ms);然后,在火花出现的位置出现氧化膜的迅速生长,随后获得了粗糙、多孔的氧化膜;同时,膜的形成过程也伴随着电压波动和剧烈的气体析出。研究人员证实[8,9],火花放电过程影响MAO的生长速度及膜层表面的粗糙度。此外,强烈的火花放电会释放大量的热,导致处理样品表面局部温度过高,容易产生工件烧蚀,在生产中需要大型的冷却设备,提高了生产成本,也给工件的表面处理带来安全隐患[10,11]。因此,文献报道在电解质溶液中引入丙三醇、有机胺和苯二甲酸等物质抑弧,减少工件局部或尖端放电,以利于MAO过程平稳进行。
有机硅烷偶联剂含有两种不同的化学官能团,与水发生水解产生活性的硅烷醇基,容易吸附在材料表面,并与其羟基反应生成共价键,常用于金属表面硅烷化预处理[15,16]。然而,有关硅烷偶联剂引入电解质溶液并起到MAO抑弧作用的研究鲜有报道。近年来,本课题组在镁合金表面MAO处理方面做了一些研究工作[17,18],获得了较低工作电压下 (230~260 V) 耐蚀MAO膜的最佳工艺参数,并提出了一种硅烷偶联剂抑弧的MAO电解质溶液及MAO膜制备方法。由于常用的MAO电解质溶液呈碱性,因此,优选水解后呈碱性及在碱性环境下稳定存在的硅烷偶联剂作为MAO的抑弧组分。本文的研究工作是在基础电解质溶液中引入
基体为AZ31B镁合金,试样尺寸为30 mm×30 mm×2 mm,其化学成分 (质量分数,%) 为：Al 2.94, Zn 0.9,Mn 0.23,Si 0.01,Cu 0.01,Ni 0.00053, Fe 0.003,Mg余量。依次对试样进行碱洗除油→超声清洗→吹干→打磨 (400~1200#SiC砂纸)→丙酮超声→水洗→吹干处理,待用。
利用Starter 2C型pH计和DDS-309+电导率仪 (成都世纪方舟科技有限公司) 分别检测电解质溶液的pH值和电导率,测量结果见
采用非磁性测厚仪 (MiniTest4100) 和表面粗糙度仪 (FTR200) 分别对MAO膜的厚度和表面粗糙度进行测试,每个样品测量12次,去掉最大值和最小值后取平均值作为膜层的最终测量值。
采用扫描电子显微镜 (SEM,VEGA 3 SBU) 表征膜层的表面形貌,加速电压为10~15 kV。利用X射线衍射仪 (XRD,D2 PHASER,Cu-
利用电化学工作站 (CHI660E) 于室温条件下测试样品在3.5%(质量分数) NaCl溶液中的极化曲线。采用标准三电极体系,参比电极为饱和甘汞电极 (SCE),辅助电极为3 cm2的Pt片,工作电极为待测试样,其有效暴露面积为1 cm2。测量时,先对试样的开路电位进行测试,待体系稳定后,进行动电位极化曲线测量,扫描范围为相对开路电位±0.5 V,扫描速率为1 mV/s。为确保测量结果的准确性和可重复性,每个样品测量3次,重复结果相近时以电流密度最大值作为样品的测试结果。测试结束后,利用计算机软件 (CHI) 拟合数据,得出腐蚀电位 (
(2) KH-550含量在1~20 mL/L范围内增加时,MAO膜表面微孔尺寸和粗糙度先减小后增大,膜层厚度和耐蚀性能先增加后降低;添加4~7 mL/L的 KH-550,获得的MAO膜微孔尺寸细小,分布均匀,耐蚀性最好。
The authors have declared that no competing interests exist.
102 samples were treated by the micro arc oxidation at different bipolar pulsing periods in the alkaline silicate electrolyte. The obtained results demonstrated that the duty cycle has no correlation with the layer growth or the surface roughness. The results were analyzed by the multiple linear regression and then the proper diagrams for thickness and roughness were plotted. The growth mechanism of the ceramic coating was influenced by many complicated and interrelated factors. We suggested a new mechanism to describe the resultant coating phenomenon taking into account the different reactions during the four periods of bipolar pulsing mode. The plasma discharge generator, anodic period, affected significantly in the layer growth and the surface roughness. The cathodic period affected the growth by the etching-protection effect, and affected the surface morphology by the production of hydrogen gaseous sheath. The contribution of the anodic neutral period was caused by the internal etching. The cathodic neutral period significantly affected the surface roughness by releasing the cathodic-hydrogen gaseous sheath, and contributed in the layer growth. The phase formation was also described by the suggested mechanism.
镁合金微弧氧化膜的厚度与电解液的浓度成正比关系,但电解液太 浓,氧化膜层将会出现许多问题,最终影响使用.在硅酸盐电解液中添加丙三醇溶液,研究了其对镁合金AZ91D微弧氧化过程及所获膜层的厚度、表面形貌与相 组成的影响.结果表明:电解液中加入丙三醇能够有效地抑制尖端剧烈放电现象,稳定微弧氧化过程,改善膜层性能;随着丙三醇含量的增加,氧化膜层厚度逐渐降 低;添加丙三醇能使氧化膜层表面孔洞变小,微裂纹数量减少,膜层外观质量得到极大的改善;丙三醇对陶瓷层相组成无明显影响,但MgO含量增 加,Mg2SiO4的含量则有所降低.
以NH4H2PO4、NaF和NaOH组成基础电解液, 采用有机胺作为抑弧剂, 对AZ91D镁合金高压阳极氧化过程进行了研究.结果表明: 有机胺对镁合金的阳极氧化有着显著的抑弧效应, 可使镁合金的阳极火花放电电压提高50～80V.在抑制阳极发生弧光放电的状态下, 镁合金表面可以沉积一层致密、具有较高硬度和优良耐蚀性能的氧化膜层.分析了有机胺对氧化膜层性能和表面形貌的影响以及不同有机胺在镁合金阳极氧化过程中的抑弧能力, 并初步探讨了有机胺在镁合金阳极氧化过程中的抑弧机理.
Anodizing a AZ91D magnesium alloy in environmentally friendly borate-terephthalic acid (TPA) electrolyte was studied. The effect of TPA on the anodizing process and the properties of the resultant anodized film were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrometry (EDS), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The results showed that the anodizing process, the surface morphology, thickness, phase structure, and corrosion resistance of the anodized film were strongly dependent on the concentration of TPA. In the presence of adequate TPA, a moderate anodizing process was obtained. The current density of the anodizing process was reduced and excessive sparking in the anodizing process was obviously inhibited. In the presence of TPA, the quality of the anodized film improved. The film became more compact and smooth in structure. The thickness of the film decreased slightly. The interface between the anodized film and the magnesium substrate became indistinct indicating a better adhesion between them. The corrosion resistance of the anodized film was obviously enhanced. From these highly positive results, TPA can be used as an effective additive for the anodizing treatment of magnesium alloy. The proposed anodizing process is of importance to make the existing anodizing process 'greener' and to improve the quality of the anodized film. 2011-03-16 2011-07-06 2011-07-27 ZHANG Zhao Email: email@example.com The project was supported by the National Natural Science Foundation of China (50771092, 21073162), Science and Technology Commission of Shanghai Municipality, China (08JC1421600) and Science and Technology Bureau of Jiaxing Municipality, China (2008AZ2018). LIU Yan, WEI Zhong-Ling, YANG Fu-Wei, ZHANG Zhao. Anodizing of AZ91D Magnesium Alloy in Borate-Terephthalic Acid Electrolyte[J]. , (10): 2385-2392. doi: 10.3866/PKU.WHXB20110931
A hydrophobic surface was fabricated through the micro-arc oxidation (MAO) and subsequent stearic acid surface modification of AZ31 Mg alloy, achieving a maximum water contact angle of 151.5° after 10h of modification. Through surface analyses and investigation into the corrosion behavior, this hydrophobicity is ascribed to a combination of the MAO coating’s rough micro-pore structure and the formation of a stearic acid monolayer through bidentate bonding. The hydrophobic MAO coating effectively inhibits corrosion of the Mg alloy, especially pitting corrosion, and demonstrates that surface modification could further increase or accelerate the application of MAO treatment to Mg alloys.
In this work, the factors affecting the discharge of AZ91 magnesium alloy were investigated during microarc oxidation, such as electrolyte configuration, electrical pulse parameters, and physical properties of the coating in the early discharge. Based on metal and gas electronics, a mechanism of thermal electron emission causing the discharge along the dielectric surface was proposed at the solid-liquid-gas interface. It was in good agreement with the experimental results. Due to the tunnel effect and impurity ionization, the electrons could form an electron current along the surface of the dielectric, i.e. the sediment at the initial electrochemical stage and the metal oxide ceramic coating formed at the post stage of plasma reaction during microarc oxidation. The local high-density filamentary current caused thermal electron emission. Emitted electrons impacted the molecule in gas bubbles which was formed on the surface of the metal oxide coating. Thus the impact ionization happened leading to the formation of plasma. The formation of plasma was influenced by the current density, the defects of local uneven electrical conductivity, the impurity level, the resistance of the coating, and the ionization coefficient of the metal oxide coating.
This paper overviews the relatively new surface engineering discipline of plasma electrolysis, the main derivative of this being plasma electrolytic deposition (PED), which includes techniques such as plasma electrolytic oxidation (PEO) and plasma electrolytic saturation (PES) processes such as plasma electrolytic nitriding/carburizing (PEN/PEC). In PED technology, spark or arc plasma micro-discharges in an aqueous solution are utilised to ionise gaseous media from the solution such that complex compounds are synthesised on the metal surface through the plasma chemical interactions. The physical and chemical fundamentals of plasma electrolysis are discussed here. The equipment and deposition procedures for coating production are described, and the effects of electrolyte composition and temperature on ignition voltage, discharge intensity and deposited layer thickness and composition are outlined. AC-pulse PEO treatment of aluminium in a suitable passivating electrolyte allows the formation of relatively thick (up to 50002μm) and hard (up to 2302GPa) surface layers with excellent adhesion to the substrate. A 10–2002μm thick surface compound layer (1200HV) and 200–30002μm inner diffusion layer with very good mechanical and corrosion-resistant properties can also be formed on steel substrates in only 3–502min by use of the PEN/PEC saturation techniques. Details are given of the basic operational characteristics of the various techniques, and the physical, mechanical and tribological characteristics of coatings produced by plasma electrolytic treatments are presented.
Magnesium alloys are advanced light structural and functional materials being increasingly used in the automotive, aerospace, electronic, and energy industries. However, their corrosion performance at the current stage of development is still not good enough for increasingly diverse practical applications. The Cooperative Research Centre for Cast Metals Manufacturing (CAST) in Australia is one of the most active research organizations in the world established to cope with the problems associated with the development and application of advanced light metals. Corrosion and prevention of magnesium and its alloys is an important part of CAST's research program. This paper presents a brief summary of recent research achievements by CAST and relevant research work in this area in the world. This overview covers anodic hydrogen evolution, estimation of corrosion rate, corrosion of aluminum- and non-aluminum-containing magnesium alloys, influences of composition and microstructure on corrosion, corrosion of a die-cast magnesium alloy, galvanic corrosion, coolant corrosion, and an aluminum-alloyed coating. The aim of this overview is to deepen the current understanding of corrosion and protection of magnesium and its alloys and to provide a base for future research work in this field.
This review summarizes the experience of using plasma electrolyte methods to improve the properties of magnesium alloys intended for designing components of various devices, medical equipment, etc. The emphasis is placed on the formation of coatings by both the “standard” microarc oxidation (MAO) method and its version using electrolytes containing suspended powders of different natures and degrees of dispersion. This modification can significantly expand the range of application of magnesium alloys in medicine, biology, the instrument-making industry, mechanical engineering, and living systems technology.
The AZ31 Mg alloy is highly corrodible in vivo. Microarc oxidation (MAO), among many other methods for producing a protective coating on AZ31, is effective and economic in reducing the corrosion rate. In this review, we aim to summarize the latest achievements and strategies for the MAO parameters as well as other factors that influence the corrosion properties of microarc oxidized AZ31. We also review the influence of the MAO parameters on the residual stress in the oxide coating on aluminum-containing Mg alloys. Finally, we outlook the existing issues and challenges in the biomedical applications of the microarc oxidized AZ31.
Abstract Magnesium and its alloys are increasingly being used for applications where weight reduction is important. The article discusses anodized coatings to protect these lightweight materials from corrrosion. Of the three anodic coatings available for magnesium alloys, a new chromium-free inorganic coating is the best. It gives enhanced stand-alone corrosion protection, excellent paint adhesion, and superior wear resistance.
The anodic behavior of pure Mg, binary Mg-Al alloys (2 at.%, 5 at.% and 8 at.% of Al) and intermetallic Mg 17 Al 12 was studied in a solution containing 3 M KOH+0.6 M KF+0.21 M Na 3 PO 4 , with and without addition of 0.4 M and 1.1 M of aluminate. Constant voltage polarization revealed primary and secondary passivity of Mg and Mg-Al alloys. At a constant applied current, the voltage increases linearly with time until it reaches values of breakdown. The breakdown voltage increases with the Al content in the alloy and the aluminate concentration in the solution.
In order to get a clear picture for describing the growth process of the oxide film formed on magnesium alloy AZ91D under plasma electrolytic oxidation (PEO) in alkaline silicate solution, the characteristics of PEO films formed at different reaction stages were systemically investigated. The results of morphologies, compositions and electronic properties indicated that the PEO films had a different growth behavior as the PEO treatment proceeding. At the initial stage (before the occurrence of sparking), the growth rate of PEO films was low, the elements (O, Mg, Al and Si) contents were varied obviously and the donor concentration in the film was kept at a high level. After sparking occurred, the PEO films showed a higher growth rate due to the high transfer rate of ions and electrons and the existence of plasma reactions; simultaneously, the films formed on 伪- and 尾-phase exhibited different growth rate. With treated time increased, the thickness of PEO films and transfer resistance to ions and electrons were also increased; thereby, the growth rate of the PEO films was decreased gently.
Plasma electrolytic oxidation (PEO) on a newly designed ACM522 magnesium die-casting alloy has been conducted in an aqueous phosphate solution and the morphology and corrosion resistance of the PEO films were investigated in detail. By covering the whole surface of the substrate with the PEO film, the corrosion resistance was significantly improved; the modified surface endured a salt spray test for 168h. It was also clarified that traces of local breakdown caused by excessive PEO can be a cause of pitting corrosion, resulting in significant decline in corrosion resistance with PEO time.
The growth of plasma electrolytic oxidation (PEO) coatings can be considered a complex process that includes discharge breakdown, sintering, and deposition process. In this work, inert SiO2and La2O3particles were used as tracers to investigate the formation mechanisms of PEO coatings on Mg alloy AM50. The growth direction and kinetics of the coating formation are primarily controlled by the intensity and the number of discharges. High-intensity discharges enable the inward growth of the PEO coating rapidly. Low-intensity discharges allow the outward growth of the coating at a slow speed. At the initial stage of a treatment, conversion products form locally around the intermetallics and disseminate gradually. Discharges appear after reaching the breakdown potential, leading to rapid growth of the coating. The outward growth of the layer is non-uniform because the protruding conversion products are the last locations converted by the discharges. Inward growth of the layer occurs preferentially around intermetallic phases and the formation of the inner layer is related to the inward growth.
In the framework of the new ecological regulations, micro-arc oxidation (MAO) appears as an alternative to usual processes in the field of corrosion protection of Mg alloys. In this work, the initial stages of anodic layer growth in KOH-based electrolytes are studied up to and beyond the initiation of the micro-arc regime. The properties of the first anodized film preceding the occurrence of the dielectric breakdown (corresponding to the start of the micro-arc regime) are mainly determined by the incorporation of additives (fluorides or silicates) in the film, as shown by in situ electrochemical measurements. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy reveal both the change of morphology and chemical state of silicate and fluoride in the anodized layer before and after the micro-arc regime. In terms of electrochemical behaviour, investigated by stationary methods and electrochemical impedance spectroscopy (EIS) in reference corrosive water, the anodic film grown in the silicate medium provides the best corrosion resistance thanks to a thick layer containing Mg2SiO4, whose degradation products seal the porosities of the coating.