AZ91D镁合金微弧氧化膜在不同浓度NaCl溶液中的腐蚀行为(英文)

采用微弧氧化技术(MAO)在镁合金 AZ91D 表面制备微弧氧化陶瓷膜。利用电化学技术和浸泡实验研究该镁合金试样在不同浓度(0.1%,0.5%,1.0%,3.5%和 5.0%,质量分数) NaCl 溶液中的腐蚀行为。结果表明,试样的腐蚀速率随着氯离子浓度的升高而增大。在较高浓度(1.0%,3.5%和 5.0%)的 NaCl 溶液中的主要腐蚀形式是点蚀,而在较低浓度(0.1%和 0.5%)中是全面腐蚀。腐蚀过程可以分为两个阶段:亚稳态蚀点的出现和蚀点的生长。根据腐蚀过程中阻抗谱的特点,对镁合金微弧氧化膜试样在不同浓度 NaCl 溶液中浸泡 120 h 提出了不同的等效电路来模拟其腐蚀行为。     

Tribological and corrosion behavior of PEO coatings with graphite nanoparticles on AZ91 and AZ80 magnesium alloys

The effect of the addition of graphite nanoparticles into the electrolyte used to produce plasma electrolytic oxidation (PEO) coatings on AZ91 and AZ80 magnesium alloys was studied. The corrosion and wear resistances of the obtained coatings were investigated. A solution that contained both phosphates and silicates was used as electrolyte. Moreover, two different PEO treatment times were studied. The corrosion resistance was analyzed with potentiodynamic polarization and EIS tests; the wear resistance was investigated with a flat on ring tribometer. The results were related to the morphology, microstructure, elemental composition and thickness evaluated with SEM analysis. The presence of the graphite nanoparticles increased the thickness, produced a densification of the coating and sealed the pores on the surface, thus improving both the corrosion and wear resistance. The increase in the corrosion and wear resistances was more evident for AZ91 than for AZ80 due to the higher aluminum content.     

镁合金微弧氧化膜耐蚀性表征方法的对比研究

制备结构、性能相近的AZ91D镁合金微弧氧化膜,通过浸泡、点滴及电化学实验表征膜层的耐蚀性,并结合SEM分析膜层腐蚀前后的表面形貌。本研究中6种耐蚀性检测方法的结果均表明：AZ91D镁合金经微弧氧化处理后耐蚀性显著提高; 失重与增重现象的共存使浸泡实验不能准确评定微弧氧化膜层耐蚀性的优劣; 点滴实验可以较快较准确地反映膜层的耐蚀性,但采纳点滴液开始变色的时间点为评价依据更合适,且测试耐蚀性较好的膜层时,点滴液中硝酸的含量提高到标准中的至少两倍时,才能达到快速检测的目的; 循环伏安、Tafel 极化、开路电位和电化学阻抗谱4种电化学实验能反映诸如腐蚀电位、腐蚀电流密度、阻抗值等更多的信息,可以进一步研究膜层的耐蚀原因。膜层的耐蚀性除了与膜厚、化学成分有关外,还与微观结构膜层内部和表面的密切相关。     

Wear and corrosion resistance of AZ91 magnesium alloy coated by pulsed current electrodeposited Ni–Al2O3 nanocomposite

In this work, Ni and Ni–Al₂O₃ nanocomposite coatings were applied on AZ91 magnesium alloy using a pulse plating process and the corrosion resistance of coated samples was evaluated by means of the potentiodynamic polarisation method in 3.5?wt-% NaCl solution. Field emission scanning electron microscopy was employed to identify microstructure and morphology of the coatings. Vickers microhardness and pin-on-disc wear tests were also used to investigate mechanical properties of the coatings. The polarisation test revealed that the pure Ni coating on AZ91 along with the presence of nanoparticles were key factors leading to a reduction in the corrosion current density and the improvement of corrosion resistance so that the corrosion current density of 210.45?µA?cm^?2 for the substrate (AZ91) decreases to 31.92 and 1.54?µA?cm^?2 by applying pure Ni and Ni–Al₂O₃ nanocomposite coatings, respectively. Furthermore, Ni–Al₂O₃ nanocomposite coating increased the microhardness and wear resistance compared to the substrate up to 435 and 340%, respectively.     

Enhanced corrosioon resistance by sol‐gel‐based ZrO2‐CeO2 coatings on magnesium alloys

The physical, chemical and mechanical properties of magnesium alloys make them attractive materials for automotive and aerospace applications. However, these materials are susceptible to corrosion and wear. This work discusses the potential of using sol‐gel based coatings consisting of ZrO₂ and 15 wt.% of CeO₂. The CeO₂ component provides enhanced corrosion protection, while ZrO₂ impart corrosion as well as wear resistance. Coating deposition was performed by the dip coating technique on two magnesium alloy substrates with different surface finishes: AZ91D (as‐casted, sand‐blasted, and machined) and AZ31 (rolled and machined). All as‐deposited coatings (xerogel coatings) were then subjected to 10 h annealilng: a temperature of 180°C was applied to the AZ91D alloy and 140°C to the AZ31 alloy. Morphological and structural properties of the annealed coatings were investigated by scanning electron microscopy, atomic force microscopy and transmission electron microscopy. Coating composition was examined using energy dispersive X‐ray analysis. Adhesion of the annealed ZrO₂‐CeO₂ coatings on the substrates, assessed by scratch tests, showed critical loads indicative of coating perforation of up to 32 N. Hardness and elasticity, measured using depth‐sensing nanoindentation tests, gave a hardness and elastic modulus of 4.5 GPa and 98 GPa, respectively. Salt spray corrosion tests performed on these coatings showed superior corrosion resistance for AZ91D (as‐casted and machined) and AZ31 (machined), while severe corrosion was observed for the AZ31 (rolled) and AZ91D (sand‐blasted) magnesium alloy substrates.     

Thickness effects on corrosion and wear resistance properties of micro-arc discharge oxide coatings on AZ91D magnesium alloys

The microarc oxidation coatings with difference thickness were synthesized on AZ91D magnesium alloy. The microstructure and phase structure of the coatings were analyzed using SEM and XRD, the tribological properties and corrosion resistance behaviour of the coatings were also investigated. The results show that the coating contains two layers, a porous outer layer and relatively dense inner layer. The microhardness of the MAO coatings is four to six times higher than that of the magnesium alloy substrate. The MAO coatings have much better wear-resistance and corrosion resistance abilities than those of magnesium alloy substrate, but possess higher friction coefficient. The results further indicate that there is an optimization thickness for corrosion and wear resistance.     

AZ91D镁合金微弧氧化工艺参数的优化

利用自制微弧氧化装置在硅酸盐体系中对AZ91D镁合金进行微弧氧化处理.采用4因素3水平正交试验,从考察膜层厚度、表面粗糙度和耐蚀性出发,确定了AZ91D镁合金在硅酸盐体系中的最佳工艺参数.结果表明：在最佳工艺条件下,微弧氧化膜呈多孔结构、孔径较小,裂纹较少,分布均匀,膜层较为致密;微弧氧化膜由MgO、Mg2SiO4、MgAl2O4和少量的SiO2组成;室温下,在质量分数为3.5%的NaCl中性溶液中浸泡168 h后,膜层表面未出现明显的点蚀现象,耐蚀性较镁合金基体有了很大提高.     

Effect of Gd on microstructure and stress corrosion cracking of the AZ91-extruded magnesium alloy

AZ91 and AZ91–xGd (x = 0.5, 1.0, 1.5 wt%) magnesium alloys are extruded into plates. The addition of Gd promotes the formation of Al₂Gd, effectively reducing the volume fraction of the β-Mg₁₇Al₁₂ phase and making the banded structures of the extruded magnesium alloys thinner. The corrosion weight loss tests and electrochemistry analyses demonstrate that Gd significantly improves the pitting resistance of the AZ91 in 3.5-wt% NaCl solution saturated with Mg(OH)₂. Slow strain rate tensile tests show that in a corrosive environment, compared with AZ91, the elongation to failure of the AZ91–1.0Gd alloy is increased by 47%, and the alloy exhibits excellent stress corrosion resistance in this study. The fracture mode of AZ91 is changed from typical intergranular fracture to a mixture of transgranular and intergranular fracture in the corrosion solution by adding Gd. The mechanism of Gd to improve the stress corrosion resistance of the AZ91 magnesium alloy is that Gd increases the corrosion resistance, especially the pitting of AZ91.     

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.     

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.     

氟化钾对AZ91D镁合金微弧氧化膜的生长及微观结构的影响

通过在NaOH和Na_2SiO_3组成的基础电解液中,分别不加及加入KF,对AZ91D镁合金进行微弧氧化处理,研究了KF的有无对镁合金微弧氧化膜的生长、微观结构及耐蚀性能的影响。结果表明:与不加KF相比,加入KF后,试样的起弧电压明显降低,击穿变得剧烈,试样表面火花较大,膜层的生长速率明显提高,膜层厚度显著增大,表面孔隙率稍有增大,但表面微孔数量减少。KF的加入有利于MgF_2、MgAl_2O_4的生成,与同样来自电解液的Si、O两元素相比,F~-更易被基体中的Mg所吸附,也容易通过已成膜层迁移到膜层的内部。电解液中含有KF时,膜层厚度显著增大,MgAl_2O_4物相含量增加,并生成新物相MgF_2,这些都有利于膜层耐蚀性的提高。     

K2ZrF6对镁合金微弧氧化膜抗点燃性能的影响

目的 降低镁合金在高温环境下起火燃烧的风险,同时探索微弧氧化膜是否具有抗点燃的热防护功能.方法 采用添加不同含量K2ZrF6的碱性硅酸盐电解液体系,在AZ91D镁合金表面制备微弧氧化膜,研究不同浓度K2ZrF6对微弧氧化膜抗点燃性能的影响.采用扫描电镜观察燃烧前微弧氧化膜的微观结构变化,结合XRD分析涂层的相组成,并借助抗点燃性能测试装置,考察微弧氧化膜的起燃时间.结果 添加K2ZrF6制备的微弧氧化膜主要由MgO、Mg2SiO4、MgF2和ZrO2组成.随着K2ZrF6含量的增加,膜层中大尺寸缺陷减少,致密性和厚度显著提高,表面粗糙度先增加、后降低.添加0～10 g/L K2ZrF6的微弧氧化膜的致密性较差,这类微弧氧化层在火焰的作用下,会形成大量的烧蚀孔洞和纵向扩展裂纹;添加15、20 g/L K2ZrF6的微弧氧化膜,具有更为致密的内层,这类微弧氧化膜直到基体完全熔化变形,才会出现横向扩展裂纹,导致涂层失效.AZ91D镁合金经过含15 g/L K2ZrF6的电解液微弧氧化处理后,起燃时间最长,抗点燃性最好.结论 K2ZrF6浓度对微弧氧化膜抗点燃性的影响主要体现在对其致密性的影响上,内层的致密结构能够减少在高温火焰作用下形成的烧蚀孔洞,以及延缓裂纹的产生,从而阻碍熔融的液态镁和镁蒸气向外扩散,减缓氧化反应的程度,提高涂层的抗点燃性.     

电压增幅对镁合金微弧氧化膜层性能的影响

研究了硅酸盐体系微弧氧化过程中,电源电压增幅对放电火花形态及AZ91D镁合金膜层的厚度、表面形貌和耐蚀性有影响。结果表明,随着脉冲电压增加,电弧的弧斑亮度增强、尺寸变大,但数目减少,大弧倾向增加;微弧氧化膜层的厚度增厚,但成膜速率降低;膜层表面熔融物颗粒增大,表面孔径增加,粗糙度增加;腐蚀率呈现出先减小后增加的趋势。当电压增幅为100~150 V时,其过程稳定性、成膜速率、膜层外观质量和耐蚀性等方面的综合性能相对最优。     

高低电压下不同厚度微弧氧化膜抗蚀电化学响应对比研究

目的 对比研究具有不同厚度的微弧氧化膜的抗蚀电化学响应.方法 通过调整电压,在硅酸盐电解液体系中,于AZ91D镁合金表面制备具有不同厚度的微弧氧化膜层.利用SEM、EPMA和XRD研究膜层的微观形貌、元素及物相组成,进而采用循环伏安(CV)法、动电位极化曲线和电化学阻抗谱(EIS),对比研究该膜层的抗蚀电化学响应.结果 AZ91D镁合金经微弧氧化处理后,其抗蚀性得以显著提高,但不同厚度的膜层的抗蚀电化学响应不同.相比低电压下制备的薄膜,高电压下制备的厚膜的抗蚀物相含量高,CV曲线环面积和腐蚀电流密度均降低了约1个数量级,线性极化电阻增大了约3.7倍,且其阻抗模值更高,膜层抗蚀性更优.两膜层的腐蚀失效过程不同.厚膜的腐蚀过程主要经历了2个阶段:腐蚀介质逐渐渗入膜层和腐蚀介质渗透至膜基界面侵蚀基体.薄膜的腐蚀过程经历了3个阶段:腐蚀介质逐渐渗入膜层、腐蚀介质渗透至膜基界面侵蚀基体和膜层完全失效.结论 厚膜呈现出较弱的腐蚀倾向和优异的抗蚀性能,三种电化学测试的结果都能较好地相互印证,但在揭示膜层的腐蚀过程和腐蚀机制时各自的深入程度不同.     

Microstructure and corrosion behavior of plasma electrolytic oxidation coated magnesium alloy pre-treated by laser surface melting

An AZ91D magnesium alloy was treated using duplex techniques of laser surface melting (LSM) and plasma electrolytic oxidation (PEO). The microstructure, composition and corrosion behavior of the laser melted surface, PEO coatings, LSM–PEO duplex coatings as well as the as-received specimen were characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electrochemical corrosion tests, respectively. Especially, the effect of LSM pre-treatment on the microstructure, composition and corrosion resistance of the PEO coatings was investigated. Results showed that the corrosion resistance of AZ91D alloy was marginally improved by LSM due to the refinement of grains, redistribution of β-phase (Mg₁₇Al₁₂) and increase of Al on the surface. Both the PEO and duplex (LSM–PEO) coatings improved significantly the corrosion resistance of the AZ91D alloys, while the duplex (LSM–PEO) coating exhibited better corrosion resistance compared with the PEO coating.     

Effects of electric parameters on corrosion resistance of anodic coatings formed on magnesium alloys

Anodic coatings were obtained by micro-arc oxidation on AZ91HP magnesium alloys in a solution containing 10 g/L NaOH and 8 g/L phytic acid. The effects of electric parameters including frequency, final voltage, duty cycle and current density on the corrosion resistance of anodic coatings formed on the magnesium alloys were investigated by using an orthogonal experiment of four factors with three levels. The results show that the final voltage plays a main role on the coating properties. The orders of affecting corrosion resistance and coating thickness are separately ranked from high to low as, final voltage>duty cycle>current density>frequency and final voltage>current density>frequency>duty cycle. The final voltage influences the corrosion resistance of the anodized samples mainly by changing the surface morphology and coating thickness.     

AZ91D镁合金微弧氧化膜的腐蚀行为研究

利用双向全波脉冲电源对AZ91D镁合金在硅酸盐体系中进行了微弧氧化处理,通过电化学阻抗谱(EIS)测试、极化曲线分析并结合XRD和SEM等分析方法对微弧氧化处理的镁合金腐蚀行为进行了研究.结果表明,微弧氧化膜表面分布着几微米的微孔,微弧氧化膜中主要含有MgF2,Mg2SiO4和Al2O3.AZ91D镁合金经过微弧氧化处理之后,耐蚀性能明显提高,自腐蚀电流密度降低3个数量级,自腐蚀电位高出约300 mV,阻抗值高出3个数量级,研制的微弧氧化膜对镁合金具有很好的防腐保护性能.     

SiO2 based hybrid inorganic–organic films doped with TiO2–CeO2 nanoparticles for corrosion protection of AA2024 and Mg-AZ31B alloys

Hybrid sol–gel coatings provide an approach as protective layers on metals. In this work, corrosion protection of aluminium and magnesium alloys by SiO₂-methacrylate coatings doped with TiO₂–CeO₂ nanoparticles was studied. The films show an improvement of the barrier properties at initial immersion. The reactivity of both alloys produces a deterioration of the protection with longer immersion, although TiO₂–CeO₂ nanoparticles let to observe signals of self-healing effect. Aluminium oxide/sol–gel interface was found to be stable. In combination with excellent paint adhesion on sol–gel films, these coatings can be a promising alternative pre-treatment for high strength aluminium alloys prior to painting.     

Influences of NH4 + ions and thin electrolyte layers (TEL) thickness on the corrosion behavior of AZ91D magnesium alloy

In this study, the influences of NH₄ ⁺ ions and the thin electrolyte layer (TEL) thickness on the corrosion behavior of the AZ9D magnesium alloy in NH₄⁺‐containing environments were investigated by electrochemical measurements and surface characterization. The experimental results indicate that NH₄⁺ greatly accelerates the corrosion of AZ91D magnesium alloy whether in a bulk solution or in a TEL. As the TEL thickness decreasing, the corrosion resistance of the AZ91D magnesium alloy is strengthened. According to the corrosion morphology, electrochemical analysis, and characterization analysis of corrosion products, the corrosion mechanism of AZ91D under TELs with different thicknesses is divided into three stages: (a) uniformly distributed corrosion pits with deep depth and large size when TEL thickness is higher than or equal to 500 μm; (b) slightly corrosion with randomly distributed corrosion pits and the accumulation of small amount of corrosion product when TEL thickness is between 100 μm and 200 μm; (c) no corrosion pit with only deposition of corrosion product when TEL thickness is lower than or equal to 100 μm. Among which, the roles of NH₄⁺, TEL thickness, and corrosion product are emphasized and discussed in the corrosion process of AZ91D magnesium alloy under TEL with different thicknesses.     

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.