中国腐蚀与防护学报, 2017, 37(3): 227-232
doi: 10.11902/1005.4537.2016.016
KH-550对AZ31B镁合金表面微弧氧化膜结构及性能的影响

Effect of KH-550 Content on Structure and Properties of a Micro-arc Oxidation Coating on Mg-alloy AZ31B
崔学军1,2,, 代鑫1, 郑冰玉1, 张颖君1,2

摘要:

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。

关键词: 镁合金 ; 涂层 ; 阳极氧化 ; 硅烷偶联剂 ; 抑弧效应

Abstract:

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.

Key words: magnesium alloy ; coating ; plasma electrolytic oxidation ; silane coupling agent ; restraining sparking

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[7]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[12]、有机胺[13]和苯二甲酸[14]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[19]。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γ-氨丙基三乙氧基硅烷 (KH-550),研究其用量对MAO膜结构及性能的影响规律,为提高MAO膜的耐蚀耐磨性能提供技术参考。

1 实验方法

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。

采用QX-30型MAO成套设备对镁合金样品进行MAO处理,镁合金样品用铝线连接,作为阳极;不锈钢筒 (Φ150 mm×300 mm) 为阴极。采用恒压控制模式,设定电压为230 V,频率为300 Hz,占空比为30%;同时开启搅拌和水冷却系统,电解液温度控制在40 ℃以内,氧化时间为10 min。基础电解液由15 g/L Na2SiO3+20 g/L KOH+3 g/L NaF组成。向基础电解液中添加不同量的硅烷偶联剂KH-550,在相同电参数条件下对镁合金样品进行MAO处理,添加浓度及电解液其它参数见表1。

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,测量结果见表1。

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-Kα) 分析MAO样品的物相组成。

表1 电解质溶液的pH值和电导率与KH-550浓度的关系
Table 1 Effects of additive amount of KH-550 on pH value and conductivity of electrolyte solution
Number Content of KH-550mLL-1 pHvalue Conductivityms
a 0 13.77 23.94
b 1 13.50 23.31
c 4 13.61 23.24
d 7 13.38 23.19
e 10 13.38 22.81
f 20 13.16 23.08

表1 电解质溶液的pH值和电导率与KH-550浓度的关系

Table 1 Effects of additive amount of KH-550 on pH value and conductivity of electrolyte solution

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 (Ecorr)、腐蚀电流密度 (Icorr) 和Tafel斜率 (bc) 等腐蚀过程动力学参数。

2 结果与讨论
2.1 表面形貌

镁合金经MAO处理后,可在试样表面形成一层白色的陶瓷膜。当引入不同浓度的KH-550时,将所得MAO样品的颜色进行比较,可见随着KH-550浓度的增大,MAO膜的颜色随之加深,并呈现出淡黄色。

图1是不同KH-550浓度下所得MAO膜的表面形貌。所有样品的表面都显示出MAO膜的特征结构:具有密集的微孔及“火山口状”熔融物[9,20]。未添加KH-550时 (图1a和b),MAO膜表面的微孔分布不均匀,结构凹凸起伏显著;随着KH-550添加量的增加 (图1c~l),MAO膜表面的微孔数量逐渐减少,微孔分布更加均匀,这应与KH-550的抑弧效应有关。

图1 电解液中添加不同浓度KH-550时所得MAO膜的表面形貌

Fig.1 Surface morphologies of AZ31B alloy after MAO treatments in electrolyte solutions with various concentrations of KH-550: (a, b) 0%; (c, d) 1%; (e, f) 4%; (g, h) 7%; (i, j) 10%; (k, l) 20%

2.2 厚度

图2是KH-550含量与MAO膜厚度的关系。未引入KH-550时,膜层的厚度约为9.9 μm;当添加1~4 mL/L的KH-550时,膜层厚度迅速增大至12.9 μm。此后,随着KH-550的浓度增加到20 mL/L,膜层厚度值相对变化较小,甚至略有降低。

图2 KH-550含量与MAO膜厚度的关系

Fig.2 Effects of the content of KH-550 on the thickness ofMAO coating

2.3 粗糙度

图3是KH-550含量与MAO膜表面粗糙度的关系。未引入KH-550时,膜层的粗糙度平均值约为0.59 μm,但其测量值在0.52~0.66 μm范围内波动较大。引入1~20 mL/L的KH-550后,膜层的粗糙度值明显降低,其平均值在0.48~0.55 μm范围内变化,低于未引入KH-550时MAO膜的粗糙度。

图3 KH-550含量与MAO膜粗糙度的关系

Fig.3 Effects of the content of KH-550 on the roughness of MAO coating

2.4 XRD谱

图4是添加7 mL/L KH-550前后MAO样品的XRD谱。MAO样品的主要物相为Mg和MgO。Mg的衍射峰主要来自基体,而MgO来自MAO膜。由图4可见,引入KH-550后并未改变MAO样品的物相组成,但相应衍射峰强度显著降低,尤其是Mg的衍射峰。预示Mg和MgO两种物相的含量都降低,一方面是膜层增厚,探测到的基体相减少;另一方面,引入KH-550后,增加了膜层中非晶相的含量,从而减少了MgO晶粒的数量。

图4 添加7 mL/L KH-550前后MAO样品的XRD谱

Fig.4 XRD patterns of AZ31B alloy after MAO treatments in electrolyte solutions with and without 7 mL/L KH-550

2.5 极化曲线

图5是MAO样品在3.5%NaCl溶液中的极化曲线。可见,所有MAO样品的阳极极化曲线在极化初期近似为直线,表明MAO样品随电位正移,其Icorr增加较快,腐蚀速率显著增加;阴极极化曲线变化较小。与未引入KH-550的样品相比,引入KH-550的MAO样品的Icorr值变化较小,但Ecorr正移,说明样品的腐蚀倾向降低[21];同时,也表明样品的表面状态发生了变化,如微孔数量降低、膜层致密性增加等。

图5 MAO处理样品在3.5%NaCl溶液中的极化曲线

Fig.5 Polarization curves of MAO treated AZ31B alloy in 3.5%NaCl solution

表2图5极化曲线相对应的拟合电化学参数值
Table 2 Fitting results of the polarization curves
Concentration of KH-550 / % -EcorrmV Icorr μAcm-2 -bc mVdec-1
0 1522 0.415 231
1 1446 0.159 263
4 1416 0.110 261
7 1380 0.120 197
10 1306 0.178 253
20 1324 0.570 287

表2图5极化曲线相对应的拟合电化学参数值

Table 2 Fitting results of the polarization curves

对阴极极化曲线进行拟合,计算出Icorrbc,结果见表2。可见,未引入KH-550时,样品的EcorrIcorr分别为-1522 mV和0.415 μAcm-2;引入1 mL/L KH-550时,其Ecorr正移76 mV,而Icorr也呈现出减小的趋势。这说明引入KH-550提高了MAO样品的耐蚀性能。随着KH-550从1 mL/L增加到20 mL/L,Ecorr先正移后负移,而Icorr先减小后增大,表明KH-550的添加量有一个最佳范围,不宜过量。

2.6 KH-550的作用机理

KH-550是一种氨丙基三乙氧基硅烷,与水发生水解反应,生成氨丙基三乙氧基硅烷醇,反应式见式 (1)。硅烷醇不稳定,醇基端能与无机材料表面的羟基反应生成共价键,从而达到材料改性的目的[16]

H 2 N ( C H 2 ) 3 Si ( O C 2 H 5 ) 3 + H 2 O = H 2 N ( C H 2 ) 3 Si ( OH ) 3 + C H 3 C H 2 OH (1)

MAO膜的形成是一个复合化学、电化学、热化学与等离子体化学的复杂过程[22,23],其相组成多为金属氧化物且表面含有大量的羟基。当MAO处理样品被置于硅烷溶液中时,硅烷醇便与涂层表面的羟基发生化学反应,长碳链分子附着在涂层表面,增加附着区域膜层周围离子移动的阻力,从而增大了MAO的击穿电压。若MAO为恒压控制,便降低了电流,从而抑制火花放电。

镁合金样品在MAO过程中作为阳极,带正电,因此,电解质溶液中的负电离子,如硅烷醇,便在高电场作用下,迅速向镁合金表面移动。在MAO初期,硅烷醇在镁合金表面预先形成一层保护膜,而表面薄弱区域更容易聚集较多的硅烷醇,从而避免了微弧放电过程过早发生在薄弱区域。随着微弧电压的增加,当大部分薄弱区域达到耐击穿极限条件时,MAO过程开始。由于硅烷醇的吸附和硅烷膜的作用,这个MAO过程将是一个均匀的火花放电过程,因此,得到的MAO膜表面微孔细小,分布更均匀 (图1d)。与抑弧前的膜层相比,抑弧后膜层的粗糙度降低 (图3)。由于硅烷醇抑制弧光放电,且减少了尖端放电,从而提高了MAO过程的成膜效率,进而增加了膜层的厚度 (图2)。

引入过量的硅烷偶联剂,将降低溶液的电导率 (表1),增加电解质溶液的电能损耗,从而降低了恒压控制条件下所得膜层的厚度 (图2)。同时,过量的硅烷醇吸附在MAO膜表面的某些薄弱区域,致使膜层非薄弱区域优先在大电流条件下击穿,导致MAO膜微孔尺寸增大 (图1f),MAO膜对镁合金基体的腐蚀防护能力降低 (图5和表2)。

3 结论

(1) AZ31B镁合金进行MAO处理时,在电解质溶液中添加KH-550对MAO膜的物相组成几乎无影响。

(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。

(3) KH-550能抑制镁合金在MAO过程中的弧光放电,避免尖端放电,提高膜层生长效率,使得微孔细化且分布更均匀,强化了MAO膜对镁合金的腐蚀防护能力。

The authors have declared that no competing interests exist.

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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.
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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: zhangzhao@zju.edu.cn 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
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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.
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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.
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[1] Yerokhin A L, Nie X, Leyland A, et al.Plasma electrolysis for surface engineering[J]. Surf. Coat. Technol., 1999, 122: 73
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.
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[2] Song G L.Recent progress in corrosion and protection of magnesium alloys[J]. Adv. Eng. Mater., 2005, 7: 563
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.
DOI:10.1002/adem.200500013      URL     [本文引用:]
[3] Vladimirov B V, Krit B L, Lyudin V B, et al.Microarc oxidation of magnesium alloys: A review[J]. Surf. Eng. Appl. Electrochem., 2014, 50: 195
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.
DOI:10.3103/S1068375514030090      URL     [本文引用:]
[4] Zhang L, Zhang J Q, Chen C F, et al.Advances in microarc oxidation coated AZ31 Mg alloys for biomedical applications[J]. Corros. Sci., 2015, 91: 7
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.
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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.
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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.
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[8] Duan H P, Yan C W, Wang F H.Growth process of plasma electrolytic oxidation films formed on magnesium alloy AZ91D in silicate solution[J]. Electrochim. Acta, 2007, 52: 5002
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.
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[21] Yagi S, Sengoku A, Kubota K, et al.Surface modification of ACM522 magnesium alloy by plasma electrolytic oxidation in phosphate electrolyte[J]. Corros. Sci., 2012, 57: 74
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.
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[22] Lu X P, Blawert C, Kainer K U, et al.Investigation of the formation mechanisms of plasma electrolytic oxidation coatings on Mg alloy AM50 using particles[J]. Electrochim. Acta, 2016, 196: 680
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.
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[23] Veys-Renaux D, Rocca E, Martin J, et al.Initial stages of AZ91 Mg alloy micro-arc anodizing: Growth mechanisms and effect on the corrosion resistance[J]. Electrochim. Acta, 2014, 124: 36
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.
DOI:10.1016/j.electacta.2013.08.023      URL     [本文引用:1]
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关键词(key words)
镁合金
涂层
阳极氧化
硅烷偶联剂
抑弧效应

magnesium alloy
coating
plasma electrolytic oxida...
silane coupling agent
restraining sparking

作者
崔学军
代鑫
郑冰玉
张颖君

CUI Xuejun
DAI Xin
ZHENG Bingyu
ZHANG Yingjun