The effect of phosphate on MAO of AZ91D magnesium using AC power source

A rapid and convenient anodization technology with AC power source to obtain the MAO films formed on magnesium alloy AZ91D in phosphate bath (base electrolyte + Na₃PO₄) with or without aluminate and silicate was studied. The corrosion resistance of the anodic films was studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques and the microstructure and composition of films were examined by SEM and XRD. The results show that Na₃PO₄can promote the occurrence of sparking during the MAO process, while abundant heat generated by sparking might enhance the formation of the glassy phase of the compound when the electrolyte contains the additives of NaAlO₂and Na₂SiO₃simultaneously. The optimized MAO film is ivory‐white smooth by naked eye, while presents porous and microcracks in microscopic scale. The anodic film formed in the alkaline solution with optimized parameters possesses superior corrosion resistance by electrochemical test. The XRD pattern shows that the components of the anodized film consist of MgO, MgAlO₂, and MgSiO₃. No oxide crystal with P element can be found.     

Preparation of hydrophobic anodic film on AZ91D magnesium alloy in silicate solution containing silica sol

Anodic films were prepared on the AZ91D magnesium alloy in the electrolyte of 1.0 M Na₂SiO₃ with and without the addition of silica sol under the constant current density of 20 mA/cm² at 60 °C. The anodic films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that the two main constituents of the anodic films were silicon and oxygen. However, no crystal compound including either silicon or oxygen could be detected by the XRD patterns. The addition of 10 vol.% silica sol increased the thickness of the anodic film and improved the roughness of the film surface. Furthermore, such anodic film revealed some hydrophobic property, which was not observed on the anodic film formed in the base electrolyte without addition of silica sol. And the electrochemical impedance spectroscopy (EIS) results showed that the addition of 10 vol.% silica sol improved the corrosion resistance of the anodic film for the AZ91D Mg alloy obviously.     

Composite cerium oxide/titanium oxide thin films for corrosion protection of AZ91D magnesium alloy via sol–gel process

Anti‐corrosive composite cerium oxide/titanium oxide (CeO₂/TiO₂) thin films were successfully prepared on an AZ91D magnesium alloy substrate by applying cerium oxide (CeO₂) thin films as the inner layer with a sol–gel process. Composition and surface morphology of the thin films were analyzed using X‐ray diffraction (XRD) and scanning electron microscope (SEM). XRD showed that the composite films consisted of cerianite and anatase phases. The wettability of the thin films was evaluated by water contact angles measurements. Potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) tests were used to evaluate the corrosion behavior of the bare substrate and coated samples in 3.5 wt% sodium chloride solution (3.5 wt% NaCl). The results demonstrated that titanium oxide (TiO₂) thin film mainly dominated the corrosion resistance of samples and the composite films with excellent hydrophilicity could significantly improve the corrosion resistance of AZ91D magnesium alloy.     

镁合金表面等离子喷涂Al2O3-TiO2陶瓷涂层的耐腐蚀性研究

采用等离子喷涂技术在AZ31镁合金表面制备Al2O3-13％TiO2陶瓷复合涂层,对涂层的微观组织进行了观察分析,测试了涂层的表面硬度.通过极化曲线和浸泡腐蚀试验,对比研究了镁合金基材及喷涂陶瓷涂层的试样在5％ NaCl溶液中的耐腐蚀性能.结果表明:涂层镁合金试样的硬度和耐腐蚀性优于基体镁合金,但当腐蚀液透过涂层孔隙时...     

Corrosion protection of AZ31 magnesium alloy by silane film of partly hydrolysed BTESPT

Abstract

The corrosion protective behaviour of bis-[triethoxysilylpropyl]tetrasulphide (BTESPT) silane film formed by partly hydrolysed BTESPT on AZ31 Mg alloy was investigated. Fourier transform infrared spectroscopy (FTIR) was used for structural characterisation of the silane film. Scanning electron microscope (SEM) and energy dispersive X-ray (EDS) analysis were used for observation of surface morphology and elements analysis of the film. The corrosion behaviours of bare and the silane treated AZ31 Mg alloy in 3·5 wt-%NaCl solution were studied using electrochemical polarisation test, electrochemical impedance spectroscopy (EIS) and immersion test. The results demonstrate that bare AZ31 Mg alloy endures severe corrosion even in NaCl water solution at pH 12, although the corrosion is lighter than that in neutral and acidic NaCl water solution, and that the BTESPT silane film can improve the corrosion protection performances of AZ31 Mg alloy and a lower corrosion rate correlated with higher pH.     

AZ31镁合金表面聚多巴胺/羟基磷灰石复合膜层的耐蚀性研究

通过两步湿化学沉淀法,在AZ31镁合金表面制备了聚多巴胺/羟基磷灰石复合涂层。使用扫描电子显微镜 (SEM),原子力显微镜 (AFM),X射线光电子能谱 (XPS),X射线衍射仪 (XRD) 及傅里叶红外仪 (FT-IR) 等表征了样品的形貌及结构。结果表明,聚多巴胺膜层平滑致密,经聚多巴胺诱导沉积的羟基磷灰石膜为交错排列的片状羟基磷灰石垂直阵列。利用电化学极化曲线和阻抗谱表征了聚多巴胺/羟基磷灰石复合膜的耐蚀性能,结果显示复合膜使AZ31镁合金在SBF溶液中的耐腐蚀性能明显提高。     

AZ31镁合金沉积类金刚石薄膜的表面形貌和腐蚀行为

为了改善镁合金的耐蚀性,扩展其应用范围,采用等离子全方位离子镀膜技术在AZ31镁合金表面沉积了含有Si-N和Si-O的2种类金刚石(Diamond-like carbon,DLC)薄膜,研究了其表面形貌及其在3.5%NaCl溶液中的腐蚀行为,探究了DLC薄膜对AZ31镁合金腐蚀行为的影响。利用SEM和AFM观察了AZ31镁合金表面沉积DLC薄膜的表面形貌,采用电化学法测试表面沉积DLC薄膜的AZ31镁合金在3.5%NaCl溶液中的极化曲线和开路电位,通过拉伸试验测试其在空气和3.5%NaCl溶液中的应力应变。结果表明:镁合金试样表面的DLC薄膜光滑致密,在3.5%NaCl溶液中表面沉积DLC薄膜AZ31镁合金的极化行为与表面未沉积DLC薄膜AZ31镁合金相似,表面沉积DLC薄膜AZ31镁合金电位正向移动,耐蚀性提高;与表面未沉积DLC薄膜AZ31镁合金相比,在空气中,表面沉积DLC薄膜AZ31镁合金极限抗拉强度与其接近,延伸率略低;在3.5%NaCl溶液中,表面沉积DLC薄膜AZ31镁合金极限抗拉强度略有降低,延伸率略高。     

Environmentally friendly anodization on AZ31 magnesium alloy

A novel anodization which is environmentally friendly,low voltage and low energy consumption was developed to improve corrosion resistance of AZ31 magnesium alloy.The corrosion resistance of the anodic films was studied by electrochemical impcdance spectroscopy(EIS) and potentiodynamic polarization techniques.The microstructure and compositions of films were examined by SEM,XPS and XRD.A new kind of organic additive used in the electrolyte is friendly to the environment.The compact, intact and uniform co...     

AZ31B镁合金表面火焰喷涂Al-Mg_2Si涂层的耐腐蚀性能

为提高AZ31B镁合金表面的耐腐蚀性能,用火焰喷涂方法在镁合金表面制备Al-Mg_2Si复合涂层。采用XRD、SEM和EDS分析涂层的物相组成、微观组织及元素分布;通过电化学试验测试样品在3.5%NaCl溶液中的腐蚀电位、腐蚀电流密度;通过3.5%NaCl溶液浸泡试验测试样品的腐蚀速率;并测试涂层的显微硬度。结果表明:涂层中的主要物相有Mg_2Si、Al,组织比较致密,元素分布均匀。Tafel极化曲线测试表明,Al-Mg_2Si涂层样品与AZ31B镁合金样品相比腐蚀电位从-1.489 V正移到-1.366 V,腐蚀电流密度从2.817×10~(-3) A/cm~2降低到1.198×10~(-3) A/cm~2。浸泡试验结果表明,喷涂Al-Mg_2Si的镁合金的腐蚀速率明显低于没有喷涂的镁合金。显微硬度测试表明,涂层的显微硬度集中分布在259~308 HV0.05之间,镁合金为50~60 HV0.05。因此在AZ31B镁合金表面火焰喷涂Al-Mg_2Si涂层可以提高其耐腐蚀性能,表面硬度显著提高。     

阳极氧化法制备TiO2膜在模拟深海热液区的耐腐蚀性能

以恒压阳极氧化方法在钛基体上制备TiO₂氧化膜,使用水热釜模拟深海热液区的条件研究其耐腐蚀性能。采用XRD、SEM、接触角测定仪对氧化膜以及腐蚀试样产物进行晶型、表面结构、化学成分和亲疏水性能测定,使用动电位扫描方法对其进行极化曲线测试。结果表明,钛试样和阳极氧化钛试样在模拟深海环境条件下,经过腐蚀反应在表面都生成了一层非致密的TiO₂ 膜,对基体并不能起到保护作用,而阳极氧化生成的致密TiO₂ 膜对基体能够起到很好的保护作用。经腐蚀后钛试样表面有TiH₂相的形成,腐蚀电位负移0.45 V。而阳极氧化钛试样表面没有TiH₂相的形成,且腐蚀电位负移较小,表现出良好的耐腐蚀性能。     

Effect of potassium fluoride on structure and corrosion resistance of plasma electrolytic oxidation films formed on AZ31 magnesium alloy

Plasma electrolytic oxidation films on AZ31 magnesium alloy were prepared in silicate–KOH–glycol (base electrolyte) electrolyte with the addition of different KF concentration. The effect of KF on the characteristic of discharge in electrolytes was studied. The compositions, structures and morphologies of the oxide films formed in different KF concentration were determined by energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Corrosion behavior of the oxide films was evaluated in 3.5 wt.% NaCl solution using potentiodynamic polarization tests, electrochemical impedance spectroscopy and potentiodynamic scanning tests. The films were mainly composed of Mg, Al, O, C, Si as well as a trace of Na and K. Major phases were MgSiO₃ and forsterite Mg₂SiO₄. The appearance of KF in the base electrolyte resulted in the decrease of the equilibrium current density and the appearance of MgF₂ in the films. Corrosion resistance depends on the amount of MgF₂ composition in the films. The films formed in the electrolyte containing 8.0 g/L KF exhibit the highest uniform corrosion resistance and the lowest pitting-corrosion tendency in NaCl solution.     

镁合金等离子喷涂Al/Al_2O_3涂层的耐腐蚀性能

采用等离子喷涂技术在AZ31镁合金表面制备Al/Al_2O_3复合涂层,测试了镁合金及表面喷涂有Al/Al_2O_3复合涂层的镁合金试样的极化曲线,研究了没有涂层、经封孔处理和未经封孔处理的喷涂有复合涂层的镁合金三种试样在浸泡腐蚀和5%NaCl盐雾腐蚀情况下的耐腐蚀性能及其腐蚀行为.结果表明,经封孔处理的Al/Al_2O_3复合涂层镁合金试样在上述腐蚀条件下的耐腐蚀性均优于镁合金和涂层未封孔处理的试样,在浸泡试验中未封孔处理的涂层试样比镁合金腐蚀更加严重,在盐雾试验中却优于镁合金.     

Improvement of corrosion resistance of AZ31 Mg alloy by anodizing with co-precipitation of cerium oxide

Anodizing of AZ31 Mg alloy in NaOH solution by co-precipitation of cerium oxide was investigated. The chemical composition and phase structure of the coating film were determined via optical microscopy, SEM and XRD. The corrosion properties of the anodic film were characterized by using potentiodynamic polarization curves in 17 mmol/L NaCl and 0.1 mol/L Na₂SO₄ solution at 298 K. The corrosion resistance of AZ31 magnesium alloy is significantly improved by adding cerium oxide to alkaline solution. In addition, the surface properties are enhanced and the film contains no crack.     

Corrosion Performance of Anodized AZ91D Magnesium Alloy: Effect of the Anodizing Potential on the Film Structure and Corrosion Behavior

Die-cast AZ91D magnesium alloy samples have been submitted for anodizing at different potentials. Anodizing was conducted in an environmentally friendly solution which comprised 3 M KOH + 1 M Na₂SiO₃ at room temperature. The surface treatment was performed electrolytically at four different potentials: 3, 5, 8, and 10 V. The corrosion resistance was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization curves obtained after 7 days of immersion in a 3.5 wt.% NaCl solution at room temperature. The porosity of the anodic films was estimated by means of the linear polarization method. SEM images revealed that the surface oxide was thicker for the anodic films obtained at 3 and 5 V. The films obtained at these potentials were more porous than those formed at 8 and 10 V. The results showed that the thickness of the anodic film had a significant effect on the corrosion behavior of the AZ91D, whereas the influence of the initial porosity was not significant.     

In situ growth of Mg–Al hydrotalcite conversion film on AZ31 magnesium alloy

In situ growth of Mg–Al hydrotalcite conversion film on AZ31 alloy has been developed by a two-step method. The characteristics of the films were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electronic microscope (SEM) observation, electrochemical and immersion tests. The film formation process was proposed based on the open circuit potential (OCP) measurements and surface analysis. A precursor film with network cracks is first formed and then this film is transformed into a compact and uniform hydrotalcite (Mg₆Al₂(OH)₁₆CO₃·4H₂O) film after the post treatment. This dense Mg–Al hydrotalcite film can provide effective protection to the AZ31 alloy.     

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.     

碳纳米管对AZ31镁合金抗腐蚀性能影响与机制研究

研究碳纳米管对镁合金抗腐蚀性能的影响,在氩气保护下,采用搅拌铸造法制备碳纳米管增强AZ31镁基复合材料,观察并分析其显微组织,并进行盐水静态浸渍试验,研究碳纳米管的加入量对复合材料在3.5%NaCl(质量分数,下同)溶液中腐蚀行为的影响,测试浸泡过程中3.5%NaCl介质的pH值变化;利用扫描电子显微镜对复合材料腐蚀形貌进行观察和分析;并对复合材料的抗腐蚀机制进行分析。结果表明:碳纳米管的加入能有效地提高复合材料的抗腐蚀性能,当碳纳米管加入量为1.5%(质量分数)时,平均腐蚀速率由AZ31基体3.2068mg/(m2·s)降为1.1069mg/(m2·s)。     

XRD studies of electrochemically hydrided–dehydrided thin films of the alloy AZ31 and AZ31 with Ti

A magnetron sputtered thin films of the AZ31 alloy and AZ31 alloy with Ti capped with Pd were electrochemically hydrogenated and dehydrogenated in a 3 M KOH solution. A phase composition and structure of the films were studied by XRD. It has been determined that the behaviour of magnetron sputtered alloy AZ31 during electrochemical charging with hydrogen was alike that of pure Mg. The shift of the XRD peak Mg (0 0 0 2) to lower angles indicates that a hydrogen solid solution in the AZ31 alloy was formed along with MgH₂. When the AZ31 alloy with 18 at.% of Ti was electrochemically hydrogenated the whole film was transformed into hydride. The minor part of the hydride was in the nanocrystalline state with a structure of rutile and a major part of the hydride was in the amorphous state. After dehydrogenation only a part of the alloy recovered and the rest remained in the state of amorphous hydride. A positive shift of peak Pd (1 1 1) was observed in all of the XRD patterns for hydrogenated films. At least partially the shift should be associated with the compressive stresses in the top-cap layer of Pd, which arose due to the hydrogenation of the AZ31 alloy.     

Enhanced corrosion resistance of magnesium and its alloys through the formation of cerium (and aluminium) oxide surface films

Cerium (and aluminium) oxide layers were formed on magnesium and its alloys (AZ91) by chemical surface treatment with or without subsequent annealing. The corrosion behaviour modifications provided by the formation of these surface films were studied by means of different electrochemical and surface analysis techniques. The electrochemical behaviour, studied in sodium sulphate (Na₂SO₄) solution, showed (i) a marked shift of the corrosion potential towards more positive values, (ii) a slight inhibition of the cathodic reaction and (iii) a significant decrease of the anodic dissolution current. X‐ray photoelectron spectroscopy (XPS) was used for the characterisation of the composition of the deposited films and of the changes in the film composition during the electrochemical corrosion tests. The components of some oxide films are cerium dioxide (CeO₂), aluminium oxide (Al₂O₃) and aluminium hydroxide (Al(OH)₃). Other metallic mixed oxide films were obtained as a function of the solution composition. Very little (or no) change in the oxide film composition during the cathodic and anodic polarization experiments was observed from XPS measurements. Chemical treatment provides thick and moderately adherent protective oxide films. Annealing under oxygen further improves the beneficial effect of the chemical treatment.     

Plasma electrolytic oxidation of AZ91D magnesium alloy with different additives and its corrosion behavior

Plasma electrolytic oxidation (PEO) of Mg‐based AZ91D alloys was investigated using 50 Hz AC anodizing technique in an alkaline borate solution, which contained a new kind of organic additive and without F, P, and Cr. The anodizing technological parameters have been optimized and a kind of ivory‐white smooth anodic film with high corrosion resistance was obtained. It was found that the formation of the anodic films was always coupled with sparking and oxygen evolution, whose intensity changed with the additive and anodizing voltage. All EIS plots have two capacitive loops and one low frequency inductive component. Two capacitive arcs present the barrier and porous layer of the PEO film and the inductive component in the low frequency domain is a complex behavior due to the porous structure connected to the electrolyte. EIS plots and fitting results show that a self‐sealing process of the PEO firm with different additives takes place in the beginning of immersion time, then corrosion attack becomes a preponderant process to promote the degradation of the film. Tafel results show that PEO treatment decreases the corrosion current density by four, even five orders of magnitude, while additives content does not affect strongly the electrochemical corrosion behavior. Salt spray test shows that the PEO film formed with NaAlO₂ and Na₂SiO₃ presents good corrosion resistance, over 600 h without any sealing treatment. The difference of corrosion resistance arose by additives examined by electrochemical techniques and salt spray test does not show strict corresponding relationship.