Electrochemical behavior of magnesium alloys in simulated body fluids

Potentiodynamic electrochemical technique was utilized to study the corrosion behavior of magnesium alloys in simulated body fluids （SBFs）. The influence of materials, solutions and their temperature on corrosion rate was mainly discussed. The results demonstrate that the free corrosion potential （Ecorr） of AZ31 and AZ91 alloys rises rapidly at initial stage, and then stabilizes at some value. Ecorr of WE43 alloy increases continuously. While Ecorr of AZ91 alloy with macro-arc oxidation （MAO） coating decreases drastically in 3 min, and then fluctuates between -1 607 mV and -1 503 mV. The WE43 alloy has better corrosion resistance in Hank＇s solution, compared with AZ31 and AZ91 alloys. Corrosion rates of the alloys are sensitive to the chemical composition and temperature of SBFs. A thin MgF： film slightly improves corrosion resistance. An MAO coating on AZ91 alloy significantly reduces corrosion rate and enhances Ecorr. Pitting corrosion occurs on both AZ31 and WE43 alloys in Hank＇s solution.     

Wear resistance of ceramic coating on AZ91 magnesium alloy by micro-arc oxidation

The ceramic coating formed on AZ91 magnesium alloy by micro-arc oxidation （MAO） was characterized. The results show that the ceramic coating （3.4-23 μm in thickness）on the surface ofAZ91 alloy was attained under different micro-arc oxidation treatment conditions, which consist mainly of MgO, Mg2SiO4 and MgSiO3 phases. Nano-hardness in a cross-sectional specimen was determined by nano-indentation experiment. The MAO coatings exhibit higher hardness than the substrate. Dry sliding wear tests for the MAO coatings and AZ91 alloy were also carded out using an oscillating friction and wear tester in a ball-on-disc contact configuration. The wear resistance of the MAO coatings is improved respectively under different treatment time as a result of different structures of ceramic coatings formed on AZ91 alloy.     

Corrosion resistance of composite coating on magnesium alloy using combined microarc oxidation and inorganic sealing

The combined microarc oxidation (MAO) and inorganic sealing process was used to deposit a composite coating to improve the corrosion resistance of AZ31 magnesium alloy.The surface morphologies of the resulting duplex coatings were studied by SEM.Furthermore,the corrosion resistance of the coated Mg alloy substrates was investigated using electrochemical workstation and dropping corrosion test.The results show that the composite coating surface consists of Mg,Si,O and Na.It is difficult to deposit inorganic coating on a thick MAO coating surface.As the composite coating was solidified by CO2 under 175 °C,it exhibits a better corrosion resistance than the MAO monolayer,owing to the thick and compact inorganic coating.     

Improving tribological and corrosion resistance of Ti6Al4V alloy by hybrid microarc oxidation/enameling treatments

A promising duplex coating was prepared by microarc oxidation(MAO) and enameling processes onto polished Ti6A14V alloy. The TiO_2 ceramic coating deposited by MAO was characterized and then combined with an enameling treatment in order to improve the tribological and corrosion resistance of Ti6A14V alloy. The morphology, phase composition, and hardness of MAO and MAO/enameling-coated Ti6A14V alloy were evaluated by scanning electron microscopy(SEM), X-ray diffraction(XRD), and Vickers microhardness tester, respectively.The tribological performance was investigated using a ballon-disk tribometer. The corrosion resistance was studied using immersion tests and potentiodynamic polarization.Wear tests show that the enamel coating on the MAOcoated surface causes a reduction in the friction coefficient.Immersion tests demonstrate that the duplex coating is more effective in improving the corrosion resistance of Ti6A14V than the MAO coating especially at high temperature(80 ℃). Potentiodynamic polarization curves reveal that the corrosion potential of the duplex coating increases by about 250 mV and the corrosion current density is slightly lower than that of the MAO coating. The duplex coating is superior to the stand-alone MAO coating in improving the tribological and corrosion behavior of Ti6A14V.     

Effects of increase extent of voltage on wear and corrosion resistance of micro-arc oxidation coatings on AZ91D alloy

The effects of increase extent of voltage on the wear resistance and corrosion resistance of micro-arc oxidation(MAO) coatings on AZ91D magnesium alloy were investigated in silicate electrolyte.The results show that with increasing extent of voltage, both of the thickness and bonding force of MAO coatings first increase,and then decrease.These parameters are all up to their maximum values when the increase extent of voltage is 20 V.The roughness of the coatings always increases.The coating has the best c...     

Electrochemical performance of microarc oxidation films formed on AZ91D alloy in two group electrolytes

The phase composition and electrochemical performances of microarc oxidation（MAO） films prepared on AZ91D alloy by using step-down current method in a phosphate electrolyte（P-film） and silicate electrolyte（Si-film） were studied. The results show that P-film is mainly composed of Mg, MgAl2O4 and MgO, and Si-film is composed of Mg2SiO4 and MgO. There clearly exists a fluoride-enriched zone with the thickness of about 1 - 2 μm for P-film and 0.7 - 1μm for Si-film at the MAO coating/substrate interface. The electrochemical tests show that both P-film and Si-film can enhance the corrosion resistance of AZ91D magnesium alloy significantly. The corrosion failure process of the two films in 5% （mass fraction） NaCl solution is quite different.     

Influence of Voltage on the Corrosion and Wear Resistance of MicroArc Oxidation Coating on Mg-8Li-2Ca Alloy

Calcium phosphate(CaP) coatings were prepared on Mg–8Li–2Ca magnesium alloy by micro-arc oxidation(MAO) in an alkaline Na_3PO_4–Ca[C_3H_7O_6P] base solution at the different applied voltages. Scanning electron microscope and X-ray diffraction were employed to characterize the microstructure and phase composition of the coatings, respectively. The corrosion resistance of the coatings was assessed by potential dynamic polarization curves, electrochemical impedance spectroscopy and hydrogen evolution experiment in simulated body fluids solution. The friction and wear properties were evaluated by friction and wear testing machine. The results demonstrate that the coating surface is porous and mainly composed of MgO, Ca_5(PO_4)_3(OH) and CaH_2P_2O_5. With the increase in voltage, the corrosion resistance and wear resistance of the MAO coating are both enhanced. The corrosion current density of the MAO coating decreases about two orders of the magnitude compared to the substrate. Additionally, wear and corrosion mechanisms are discussed.     

Electroless nickel-plating on die cast magnesium alloy AZ91D

Electroless nickel-plating on die cast magnesium alloy AZ91D was investigated. Growth of the electroless nickel-plating coating was characterized using scanning electron microscopy. Corrosion resistance of the coating was evaluated by open circuit potential and potentiodynamic polarization curves in 3.5% （mass fraction） NaC1 solution. The results show that plating deposition is initiated on the crevices and then spread onto primary a phase. The corrosion potentials for die cast magnesium alloy AZ91D and nickel-plating coating are about -1.45 V and -0.36 V （vs. SCE）, respectively. No discoloration, cracks, blisters, or peeling appear by heat-quench test. The results show that the corrosion potential of Ni-P coating is increased by 1 000 mV and corrosion resistance for die cast magnesium alloy AZ91D is improved. The adhesion between the coating and the substrate is excellent. Electroless nickel plating is a promising method to enhance magnesium alloys resistance for attacking.     

Effects of rare earths on the microarc oxidation of a magnesium alloy

The effects of rare earths on the properties of the microarc oxidation（MAO） coating on a magnesium alloy were investigated by means of scanning electron microscopy（SEM）,energy dispersive X-ray spectroscopy（EDS）,and electrochemistry methods.The results show that a nice and compact MAO coating was successfully obtained when the magnesium alloy was treated in nitrate solutions as the pre-treatment of MAO.However,the MAO was not successfully completed for the silicate electrolytes with the addition of rare earths.After the magnesium alloy being treated by rare earth nitrate,the obtained MAO coating has advantages such as uniform distribution of thickness,improved corrosion resistance,and nice-uniform surface,as compared with the untreated magnesium alloy.In addition,the time of non-ESP,the voltage and current density of the MAO process obviously decrease.Cerium oxide doped on the surface of the magnesium alloy can significantly improve the corrosion resistance of the MAO coating and decrease the current density of the MAO process,as compared with lanthanum oxide,whereas the doped rare earths have no significant effect on the components of the MAO coating.     

Influencing factors of surface roughness of MAO ceramic coating on AZ91D

To investigate the treating parameters＂ influence on the surface roughness of the MAO ceramic coating on AZ91D, experiments were implemented in the alkaline electrolyte by using a pulse power source with positive and negative pulse, and the surface roughness was measured and analyzed by using a Times roughness-meter and an optical microscope. The machining parameters＇ influencing rule on the coating surface roughness was investigated. The result indicates that the influence of all parameters is interactive, while the positive voltage and the electrolyte concentration, or increasing the frequency and the positive and negative voltage ratio are appropriate, the coating surface roughness will be improved.     

Characterization and wear resistance of macro-arc oxidation coating on magnesium alloy AZ91 in simulated bedy fluids

The mechanical characteristics ofthe macro-arc oxidation(MAO) coating on Mg alloy AZ91 were examined by means of nano scratch tester.The corrosion and erosion corrosion behavior of AZ91 with and without MAO coating were investigated by using potentiodynamic electrochemical technique and micro-abrasion tribometer in simulated body fluids,respectively.The influence of HCO3-ions on the erosion corrosion was discussed.The results show that the coating and its substrate are in a pronounced bond.The MAO coating inereases1-2 orders of magnitude of the corrosion resistance of AZ91 alloy.HCO3-ions enhance the corrosion rates of the AZ91 alloys more significantly than the alloys with MAO coating.However,there exists an obvious passivation process of AZ91 without coating in the HCO3-solutions.Moreover,an MgCO3 film formed in HCO3-containing solutions leads to an enhancement in micro-wear resistance.MAO coating deteriorates the erosion corrosion resistance of AZ91 alloy due to the formation of oxidation debris resulted from the broken MAO coating.     

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。     

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.     

微弧氧化处理对AZ91D镁合金腐蚀行为的影响

采用碱性硅酸盐溶液,在AZ91D镁合金试样表面制得微弧氧化膜,并利用电化学阻抗方法对镁合金及微弧氧化处理试样在3.5%NaCl溶液中的腐蚀行为进行比较研究.结果表明,镁合金经微弧氧化处理后,腐蚀电位和膜层阻抗均有一定程度的提高.但在浸泡过程中,微弧氧化处理试样的电化学参数呈现出不同的变化规律,初期波动较大,后期则逐渐降低,趋向稳定.     

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.     

SiCw/AZ91镁基复合材料的微弧氧化行为及涂层的耐腐蚀性能

采用微弧氧化表面处理技术在SiCw/AZ91镁基复合材料表面制备保护性涂层.通过与AZ91镁合金对比,研究镁基复合材料的微弧氧化行为及其形貌特征,并采用电化学方法评价了微弧氧化涂层的耐腐蚀性能.结果表明,SiC晶须的存在影响了基底材料表面阻挡层的形成,使复合材料的微弧氧化行为不同于基体合金.与合金相比,在恒电流模式下进行微弧氧化的过程中复合材料的电压随时间的演变趋势不够理想,而且在相同工艺条件下复合材料的起弧时间比合金要长.复合材料在微弧氧化过程中偶尔会出现烧蚀现象.虽然SiC晶须会影响复合材料表面涂层的形成,微弧氧化处理仍然能够增强镁基复合材料的耐腐蚀性能,使其自腐蚀电位提高,腐蚀电流降低.当采用恒压模式制备涂层时,涂层耐腐蚀性能随电压的提高而增强.     

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.     

微弧表面处理对AZ31B镁合金耐腐蚀及耐腐蚀疲劳性能的影响

采用微弧表面处理技术(微弧氧化MAO和微弧复合MCC)在AZ31B镁合金基体上制备出不同断面结构的防护涂层。通过电化学腐蚀及腐蚀疲劳测试方法,研究了MAO、MCC涂层的电化学腐蚀及腐蚀疲劳性能。结果表明,生长10 min的MAO涂层具有较好的耐电化学腐蚀性能。MAO涂层表面存在微孔和微裂纹,在应力条件下微孔和微裂纹作为疲劳断裂的裂纹萌生点,可加速裂纹的萌生与扩展,使其腐蚀疲劳寿命相较AZ31B合金基体降低了55%。而具有MCC涂层的AZ31B合金试样腐蚀疲劳极限为(64.0±5.4) MPa,比AZ31B合金基体提高了59%。在低应力载荷下(＜80 MPa),微弧复合涂层试样的腐蚀疲劳强度得到明显提高。     

(Al_2O_3-SiO_2)_(sf)／AZ91D镁基复合材料微弧氧化膜的制备及电化学阻抗谱分析

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(Al_2O_3-SiO_2)_(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Mg_2SiO_4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.     

Enhanced corrosion resistance of micro-arc oxidation coated magnesium alloy by superhydrophobic Mg−Al layered double hydroxide coating

To further enhance the corrosion resistance of the porous micro-arc oxidation (MAO) ceramic layers on AZ31 magnesium alloy, superhydrophobic Mg−Al layered double hydroxide (LDH) coating was fabricated on MAO-coated AZ31 alloy by using in-situ growth method followed by surface modification with stearic acid. The characteristics of different coatings were investigated by XRD, SEM and EDS. The effect of the hydrothermal treatment time on the formation of the LDH coatings was studied. The results demonstrated that the micro-pores and cracks of MAO coating were gradually sealed via in-situ growing LDH with prolonging hydrothermal treating time. Electrochemical measurement displayed that the lowest corrosion current density, the most positive corrosion potential and the highest impedance modulus were observed for superhydrophobic LDH/MAO coating compared with those of MAO coating and LDH/MAO coating. Immersion experiment proved that the superhydrophobic LDH/MAO coating with the active anti-corrosion capability significantly enhanced the long-term corrosion protection for MAO coated alloy.