Corrosion resistance of plasma-anodized AZ91D magnesium alloy by electrochemical methods

Anodic coatings formed on magnesium alloys by plasma anodization process are mainly used as protective coatings against corrosion. The effects of KOH concentration, anodization time and current density on properties of anodic layers formed on AZ91D magnesium alloy were investigated to obtain coatings with improved corrosion behaviour. The coatings were characterized by scanning electron microscopy (SEM), electron dispersion X-ray spectroscopy (EDX), X-ray diffraction (XRD) and micro-Raman spectroscopy. The film is porous and cracked, mainly composed of magnesium oxide (MgO), but contains all the elements present in the electrolyte and alloy. The corrosion behaviour of anodized Mg alloy was examined by using stationary and dynamic electrochemical techniques in corrosive water. The best corrosion resistance measured by electrochemical methods is obtained in the more concentrated electrolyte 3 M KOH + 0.5 M KF + 0.25 M Na₃PO₄·12 H₂O, with a long anodization time and a low current density. A double electrochemical effects of the anodized layer on the magnesium corrosion is observed: a large inhibition of the cathodic process and a stabilization of a large passivation plateau.     

Electrochemical corrosion behaviour of plasma electrolytic oxidation coatings on AM50 magnesium alloy formed in silicate and phosphate based electrolytes

PEO coatings were produced on AM50 magnesium alloy by plasma electrolytic oxidation process in silicate and phosphate based electrolytes using a pulsed DC power source. The microstructure and composition of the PEO coatings were analyzed by scanning electron microscopy (SEM) and X-ray Diffraction (XRD). The corrosion resistance of the PEO coatings was evaluated using open circuit potential (OCP) measurements, potentiodynamic polarisation tests and electrochemical impedance spectroscopy (EIS) in 0.1 M NaCl solution. It was found that the electrolyte composition has a significant effect on the coating evolution and on the resulting coating characteristics, such as microstructure, composition, coating thickness, roughness and thus on the corrosion behaviour. The corrosion resistance of the PEO coating formed in silicate electrolyte was found to be superior to that formed in phosphate electrolyte in both the short-term and long-term electrochemical corrosion tests.     

Structure and Properties of ZrO2 Ceramic Coatings on AZ91D Mg Alloy by Plasma Electrolytic Oxidation

ZrO₂ ceramic coatings were prepared in situ on an AZ91D Mg alloy by plasma electrolytic oxidation in a K₂ZrF₆ solution. The phase composition and the surface morphology of the coatings were examined with X-ray diffraction and scanning electron microscopy. The thermal shock resistance of the coatings was evaluated by a thermal shock test. The corrosion resistance of the coated samples was examined by the polarizing curve method in a 3.5% NaCl solution. The prepared coating was composed of t -ZrO₂ and a small amount of c -ZrO₂. There were many residual discharging channels on the coating surface. The coated samples showed excellent thermal shock resistance under 500°C, which improved with increasing frequency or decreasing current density or PEO time. Besides, the coating improved the corrosion resistance of AZ91D Mg alloy considerably. In the experiments, the corrosion current density of the coated samples prepared under 1000 Hz was the least, which also decreased with the current density during the PEO process.     

AZ31B镁合金微弧氧化膜复合封孔工艺的研究

对AZ31B镁合金微弧氧化膜复合封孔工艺进行了研究。采用扫描电子显微镜观察了微弧氧化膜的表面形貌,采用X射线衍射仪分析了微弧氧化膜的结构,并采用电化学工作站测试了微弧氧化膜的极化曲线。通过实验得到最佳的微弧氧化电参数为:电流密度3 A/dm²,脉宽100μs,频率800～1 100 Hz。另外,采用质量分数为10%的硅烷溶液-质量浓度为20 g/L的PTFE悬浮液进行复合封孔后,得到的微弧氧化膜的耐蚀性好。     

Growth and Corrosion Characteristics of Plasma Electrolytic Oxidation Ceramic Films Formed on AZ31 Magnesium Alloy

The growth characteristics of oxide ceramic films formed on AZ31 magnesium alloy with plasma electrolytic oxidation (PEO) technique in alkaline silicate solution were investigated. The composition, structure and morphology of the coatings were detected by energy dispersive X-ray spectroscope and scanning electron microscope. The amount of dissolved magnesium in the electrolytes during PEO process was measured by atomic absorption spectrometry. The results indicated that the growth process of PEO films had three stages when applied with constant voltage mode. In the first stage, the growth rate of PEO films was low, and concentrations of elements O, Mg and Si varied slightly. After sparking occurred (the second stage), the PEO films showed higher growth rate due to the high transfer rate of ions and electrons, and the existence of plasma reactions. When the growth rate tended to maintain stable with time, the third stage happened. PEO films exhibited different uniform and pitting-corrosion characteristics in different reaction stages. The films formed at 300 V for 30 min performed best corrosion resistance and the phase of ceramic films was mainly composed of MgSiO3 and forsterite Mg2SiO4.     

Effect of anodic oxidation on the corrosion behavior of Ti-based materials in simulated physiological solution

Anodic oxidation in acetic acid was investigated as a means of improving the corrosion resistance, in simulated physiologic solution, of titanium and two titanium-based alloys, Ti–6Al–7Nb and Nitinol. The composition and the thickness of the surface layers formed by anodization were analyzed using X-ray photoelectron spectroscopy. The electrochemical characteristics were investigated by linear polarization, cyclic polarization, and electrochemical impedance spectroscopy at the open circuit potential. Anodization of all these three metals resulted in the formation of TiO₂ as the main oxide. These layers improved the corrosion behavior in simulated physiological solution, as evidenced by significant increase in polarization resistance and decrease in corrosion current density. Electrochemical impedance results were interpreted based on the two-layer structure of the passive film. Anodization has thus been shown to provide a simple and effective means of improving the corrosion behavior of titanium-based alloys in simulated physiological solution.     

Electrochemical corrosion behavior of magnesium and titanium alloys in simulated body fluid

The electrochemical behavior of AZ91D and Ti–6Al–4V alloys was investigated in simulated body fluid (SBF) at 37 °C. The aim of the present study was to evaluate their corrosion performance through the analysis of corrosion resistance variation with time, using electrochemical impedance spectroscopy (EIS) tests and corrosion current density using potentiodynamic polarization measurements. Very low current density was obtained for Ti–6Al–4V alloy compared to that of AZ91D alloy, indicating a typical passive behavior for Ti alloy. EIS results exhibited high corrosion resistance indicating a highly stable film on titanium alloy compared to magnesium alloy in SBF.     

镁合金表面处理技术进展

综述了近年来镁合金表面处理技术的进展,包括镁合金电镀、化学镀、化学氧化、等离子电解氧化、硅烷化处理和表面沉积羟基磷酸钙涂层等。     

Enhanced corrosion resistance properties of radiofrequency cold plasma nitrided carbon steel: Gravimetric and electrochemical results

Cold plasma nitriding treatment was performed to improve the corrosion resistance of C38 carbon steel. Nitriding process was conducted using a radiofrequency nitrogen plasma discharge for different times of treatment on non-heated substrates. The modification of the corrosion resistance characteristic of the C38 steel due to the treatment in acid medium (1 M HCl) were investigated by gravimetric and electrochemical tests such as potentiodynamic polarisation curves and electrochemical impedance spectroscopy (EIS). It was shown that the plasma nitriding treatment improves the corrosion resistance. Indeed, in the gravimetric tests, nitrided samples showed lower weight loss and lower corrosion rate in comparison to untreated one. In the Tafel polarisation tests, the nitrided samples showed greatly reduced corrosion current densities, anodic dissolution and also retarded the hydrogen evolution reaction. Using EIS method, an adequate structural model of the interface was used and the values of the corresponding parameters were calculated and discussed. The results obtained from weight loss and electrochemical studies were in reasonable agreement. X-ray photoelectron spectroscopy (XPS) was carried out to establish the mechanism of corrosion inhibition of nitrided C38 steel in 1 M HCl medium. The enhancement of the corrosion resistance is believed to be related to the iron nitride compound layer formed on the C38 steel surface during plasma nitriding, which protected the underlying metal from corrosive attack in the aggressive solutions.     

汽车用AZ91D镁合金表面Ni-SiO_2纳米复合镀层的制备与研究

为改善和提高汽车支架类零件常用的AZ91D镁合金表面耐蚀性能,采用两步法在其表面电沉积Ni-SiO_2复合镀层。设计了正交试验,以SiO_2质量分数和表面粗糙度作为评价指标,运用正交试验法确定了施镀工艺参数的影响主次顺序,并得到最优施镀工艺参数为:镀液中SiO_2颗粒质量浓度20 g/L、电流密度8 A/dm~2、超声波功率300 W、镀液温度55℃。结果表明:采用最优施镀工艺制备出Ni-SiO_2纳米复合镀层,其表面平整、致密,腐蚀均匀,腐蚀速率为65 g/(m~2·d),明显低于镁合金基体的96 g/(m~2·d)。Ni-SiO_2纳米复合镀层能够提供有效的表面防护,改善和提高镁合金耐蚀性能。     

镁合金阳极氧化

镁合金以优良的物理、机械性能得到了广泛的应用,而耐蚀性差制约其进一步应用.对镁合金进行适当的表面处理能够提高耐蚀性.阳极氧化膜具有与基体金属结合力强、良好的耐蚀性及耐磨损等优点,同时具有多孔结构,能够按照要求进行着色/封孔处理,并能为进一步涂覆有机涂层提供优良底层,阳极氧化工艺是目前普遍应用的表面处理方法.本文介绍了镁合金阳极氧化工艺的发展概况,阳极氧化工艺的成膜机理,膜的耐蚀性影响因素等.     

Natural Silk Film for Magnesium Protection: Hydrophobic/Hydrophilic Interaction and Self‐Healing Effect

Biodegradable implants are required in order to provide successful treatment of injuries. Temporary magnesium‐based implants with particular properties are needed in cases when it is desirable not only to maintain vital activity, but also to initiate the self‐healing process of damaged bones or tissues as well. Unfortunately, the use of magnesium alloys is limited due to the fast biodegradability of the applied material. The aim of this research is to improve the corrosion resistance of magnesium alloys by sonochemical treatment in silk solution followed by additional layer‐by‐layer deposition of natural silk on the magnesium surface. The sonication process is carried out at a frequency of 20 kHz during 5–10 min, while the duration of the silk layer deposition is 15 min. The corrosion behavior of magnesium substrates modified by natural silk layer‐by‐layer assembly is studied. Magnesium substrates sonochemically treated in silk solution demonstrate three times better corrosion resistance compared to control samples sonochemically treated in water. Additional deposition of a silk layer enhances obtained corrosion resistance by 18 times, resulting in a 54‐fold increase overall.     

Self-healing hybrid coating of phytic acid/silane for improving the corrosion resistance of magnesium alloy

In order to improve the corrosion resistance of a biodegradable magnesium alloy, a series of phytic acid/3-aminopropyltrimethoxysilane (γ-APS) hybrid coatings was prepared on AZ31 magnesium alloys by dipping the magnesium alloy into the mixing solution of phytic acid and γ-APS. During the preparation of hybrid coatings, the pH values of the mixing solutions greatly affected the uniformity of the coatings and subsequently influenced their corrosion resistance. Electrochemical tests indicated that the hybrid coating prepared in the solution of pH = 8.0 could highly improve corrosion resistance of AZ31 magnesium alloys. Meanwhile, corrosion current density of the hybrid coating coated sample was significantly decreased from the uncoated sample of 138.1 ± 11.9 to 8.5 ± 0.8 μA cm^?2. Immersion test in simulated body fluid revealed that the cracks on the surface of the hybrid coating gradually healed up during the lengthy immersion.     

Preparation and characterization of a double-layer coating on magnesium alloy AZ91D

A double-layer coating was prepared on AZ91D alloy by plasma electrolytic oxidation (PEO) plus electroless plating (EP). The plasma eletrolytic oxidation film was prepared in a silicate bath as an inner layer of the coating. Electroless plated Ni-P layer grew from the pores of the PEO film in a nickelous acetate bath and formed as the outer layer of the coating. The microstructure and crystallographic structure was observed with FESEM and XRD. The corrosion resistance of the double-layer coating was evaluated by means of chronopotentiometric (E-t), potentiodynamic polarization (E-i), neutral salt spray test and electrochemical impedance spectroscopy (EIS) test. Compared with the data of as-cast AZ91D magnesium, the open circuit potential of the double-layer coated AZ91D alloy increased by 1.1815 V, while the self-corrosion current density decreased by two orders of magnitude. E-i, EIS result showed that the corrosion resistance of magnesium alloy AZ91D was improved by the double-layer coating. The salt spray test and polarization test results show that the pitting corrosion resistance of AZ91D alloy was improved greatly. An equivalent circuit was proposed to fit the impedance diagrams of AZ91D alloy with the coating.     

Preparation and characterization of oxide films containing crystalline TiO2 on magnesium alloy by plasma electrolytic oxidation

Oxide films have been produced on AM60B magnesium alloy using plasma electrolytic oxidation process in an alkaline phosphate electrolyte with and without addition of titania sol. The microstructure and composition of the oxide films were analyzed by Scanning Electron Microscope (SEM), X-ray Photoelectron Spectroscope (XPS) and X-ray Diffraction (XRD). The corrosion resistances of the oxide films were evaluated using potentiodynamic polarization measurements in 3.5 wt% NaCl solution. It is found that the oxide film containing crystalline rutile and anatase TiO₂ compounds are produced in an alkaline phosphate electrolyte with addition of titania sol. The oxide film formed in electrolyte with addition of titania sol has more uniform morphology with less structural imperfections than that formed in electrolyte without addition of titania sol. The results of potentiodynamic polarization analysis show that the oxide film formed in the present modified electrolyte is successful in providing superior corrosion resistance for magnesium alloy.     

Effect of electrolyte additives on performance of plasma electrolytic oxidation films formed on magnesium alloy AZ91D

Various plasma electrolytic oxidation (PEO) films were prepared on magnesium alloy AZ91D in a silicate bath with different additives such as phosphate, fluoride and borate. Effects of the additives on chemical composition and corrosion resistance of the PEO films were examined by means of scanning electron microscopy (SEM), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution. The results showed that the PEO films obtained in solutions with both borate and fluoride had better corrosion resistance. In order to understand the corrosion mechanism of PEO films on magnesium alloy AZ91D, electronic property of the magnesium electrode with PEO films was studied by Mott-Schottky approach in a solution containing borate and chloride. The results indicated that magnesium electrodes with and without PEO films all exhibited n-type semiconducting property. However, in comparison with the magnesium electrode treated in solutions containing phosphate or borate, the electrode treated in solutions containing both borate and fluoride (M-film) had lower donor concentration and much negative flat band potential; therefore, the M-film had lower reactivity and higher corrosion resistance.     

铬酸盐对镁合金微弧氧化膜耐蚀性的影响

研究在硅酸盐溶液中添加K2CrO4对AZ91D镁合金微弧氧化膜耐蚀性的影响。结果表明：在含有K2CrO4的硅酸盐系溶液中进行微弧氧化处理的镁合金表面能够获得黄绿色的氧化膜层,主要由MgO、Mg2SiO4、MgAl2O4及MgCr2O4等耐蚀物相组成,其中具有尖晶石结构的MgCr2O4对提高膜层耐蚀性有极大的促进作用,同...     

The influence of Ag and Si additions on the electrochemical behavior in extruded Mg-Zn alloys

Development of new wrought magnesium alloys for the automotive industry has increased in recent years, due to their high potential as structural materials for low density and high strength/weight ratio demands. However, the poor mechanical properties and low corrosion resistance of the magnesium alloys have led to a search for new kinds of magnesium alloys, for better strength, ductility, and high corrosion resistance. The main objective of this research is to investigate the corrosion behavior of new magnesium alloys: Mg-Zn-Ag (ZQ) and Mg-Zn-Si (ZS). The ZQ6X and ZS6X alloys were prepared using the hot extrusion method. AC and DC polarization tests were carried out on the extruded rods, which contain different amounts of silver or silicon. The microstructure was examined using optical and electron microscopy (SEM) and energy dispersive spectrometry (EDS). The results showed that the silver addition decreased corrosion resistance. The addition of silicon also affected corrosion behavior. These results can be explained by the effects of alloying elements on the microstructure of Mg-Zn alloys, such as grain size and precipitates caused by the change in precipitation, and by recrystallization behavior.     

改进的等离子体电解工艺制备氧化锆陶瓷薄膜及其性能

为改善氟锆酸盐电解液的老化现象,以氟锆酸盐为主电解液,在其中加入少量NaH2PO4 H3P04,对AZ31镁合金进行等离子体电解氧化处 理,制得氧化锆陶瓷薄膜.用扫描电镜、X射线衍射及能谱分析检测了添加剂对陶瓷薄膜的形貌特征、元素及物相组成的影响.结果表明:添加剂改 变了电解液体系的放电特性,能有效抑制氟锆酸盐电解液的老化;添加剂的加入能够提高膜中氧化锆的含量,且降低了F在膜中的含量.制得的氧化 锆薄膜能有效提高AZ31镁合金的耐腐蚀性能.     

镁合金表面防护处理技术的研究现状与前景展望

镁合金因其具有优良的性能而日益受到重视,但是镁合金耐腐蚀性较差限制了它的应用。综述了镁合金环保型防护处理技术,包括化学氧化处理、阳极氧化、微弧氧化、金属镀层等技术,分析了镁合金表面防腐处理技术的发展趋势。