Electrochemical behavior of AZ91D magnesium alloy in phosphate medium—part I. Effect of pH

The influence of pH on the corrosion behavior of Mg-based AZ91D alloy was investigated in a constant composition phosphate medium using various electrochemical techniques, complemented with surface analysis data. The studied solutions were 0.1 M H₃PO₄, NaH₂PO₄, Na₂HPO₄ and Na₃PO₄ having pH values of 1.8, 4.5, 9.1 and 11.8, respectively. Spontaneous passivation was substantiated from monitoring the continuous positive shift of the open circuit corrosion potential with both immersion time and solution pH. The impedance data indicated more improvement in the insulating properties of the corrosion products formed on the alloy surface with increase in pH. The electrolyte pH plays a determinant influence on surface film properties, as films formed in phosphate solutions with higher pH values are thicker, thus affording better protection for the alloy than those formed in acidic solutions. Good agreement was observed between the results obtained from electrochemical techniques and those from EDX and XRD examinations. The alloy is more susceptible to corrosion in acidic phosphate solutions than in the alkaline ones. Crystalline magnesium (Mg), magnesium hydride (MgH₂) and magnesium oxide (MgO) were found to be the main constituents of the surface film after holding for 2 h in the acidic phosphate medium.     

A silanol-based nanocomposite coating for protection of AA-2024 aluminium alloy

A new hybrid sol–gel type film, composed of tetraethylorthosilicate (TEOS) and tetraocthylorthosilicate (TEOCS), and modified with different nanoparticle systems, has been investigated as a coating for protection of AA-2024-T3 aluminium alloy. The nanoparticle systems considered were either ZrO₂ or CeO₂ or their combination_. The zirconia nanoparticles were prepared from a Zr (IV) propoxide sol (TPOZ), using an organic stabilizer, and the CeO₂ nanoparticles were developed spontaneously after adding cerium nitrate solution to the hybrid sol. The chemical composition and the structure of the hybrid sol–gel films were examined by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The corrosion resistance of the coated AA-2024 alloy was examined by potentiodynamic polarization. The results revealed that, for short exposure times in the electrolyte, incorporation of ZrO₂ or CeO₂ nanoparticles in the hybrid film does not provide an increase in the corrosion resistance of the coated AA-2024 alloy. Further, the resistance was significantly reduced by increasing the nanoparticle content. Conversely, by incorporating both nanoparticles (ZrO₂ and CeO₂), the corrosion resistance of the resulting hybrid films increased slightly. The behavior changed significantly when the coated alloy was exposed to the electrolyte for 5 days. The corrosion resistance of the coatings, unmodified and modified with CeO₂ or ZrO₂ nanoparticles, decreased by two or three orders of magnitude, while the film modified with both nanoparticles (CrO₂ and ZrO₂) showed a relatively high corrosion resistance and responsiveness to activation processes during anodic polarization.     

The corrosion behaviour of rare-earth containing magnesium alloys in borate buffer solution

In this work, the corrosion behaviour of magnesium alloys ZK31, EZ33 and WE54 was studied in sodium borate buffer solution at pH 9.2.The electrochemical processes were studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The composition and morphology of the alloys and corrosion products formed were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).The experimental findings highlighted the differences in the corrosion mechanisms of the different alloys tested. The results showed that the presence of rare-earth elements (RE) only increases the corrosion resistance when present in solid solution, as is the case of the WE54 alloy. At pH 9.2, an amorphous yttrium oxide/hydroxide thick film was formed, which possesses greater stability when compared to magnesium oxide/hydroxide. The role of RE in the corrosion mechanism was discussed.     

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.     

Formation of Ti-Si composite oxide films on Mg-Al-Zn alloy by electrophoretic deposition and anodization

TiO₂ or SiO₂ nanoparticles dispersed in an acetone solvent containing iodine were deposited on Mg-Al-Zn alloy by electrophoretic deposition (EPD). Subsequently, the composite oxide films were formed on the substrate by anodization in KOH-Na₂SiO₃ aqueous solutions containing TiO₂ or SiO₂ nanoparticles. The films formed by EPD were improved binding with the substrate by anodization under high voltages with sparking, and then the anodic films consisted of Si-Mg or Ti-Si-Mg composite oxides. The film thicknesses of TiO₂ and SiO₂ on the alloy increased with anodization time. In polarization tests, the films anodized under high voltages with sparking in the alkaline solutions had high corrosion resistance. Thus, the composite oxide films formed in the present method were successful in providing corrosion resistance to Mg alloy.     

Corrosion behaviors of Zn/Al-Mn alloy composite coatings deposited on magnesium alloy AZ31B (Mg-Al-Zn)

After being pre-plated a zinc layer, an amorphous Al-Mn alloy coating was applied onto the surface of AZ31B magnesium alloy with a bath of molten salts. Then the corrosion performance of the coated magnesium alloy was examined in 3.5% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results showed that the single Zn layer was active in the test solution with a high corrosion rate while the Al-Mn alloy coating could effectively protect AZ31B magnesium alloy from corrosion in the solution. The high corrosion resistance of Al-Mn alloy coating was ascribed to an intact and stable passive film formed on the coating. The performances of the passive film on Al-Mn alloy were further investigated by Mott-Schottky curve and X-ray photoelectron spectroscopy (XPS) analysis. It was confirmed that the passive film exhibited n-type semiconducting behavior in 3.5% NaCl solution with a carrier density two orders of magnitude less than that formed on pure aluminum electrode. The XPS analysis indicated that the passive film was mainly composed of AlO(OH) after immersion for long time and the content of Mn was negligible in the outer part of the passive film. Based on the EIS measurement, electronic structure and composition analysis of the passive film, a double-layer structure, with a compact inner oxide and a porous outer layer, of the film was proposed for understanding the corrosion process of passive film, with which the experimental observations might be satisfactorily interpreted.     

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.     

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.     

Effect of adding magnesium to lithium on the performance of discharge and hydrogen evolution of the lithium anode

The present study evaluates the effect of magnesium as an inhibitor on the performance of discharge and hydrogen evolution of lithium anode in alkaline electrolyte with additives. The electrochemical behaviors of lithium and lithium–magnesium alloy are assessed by hydrogen evolution rate, discharge current density, anodic potential, and potentiodynamic polarization. For these conditions, the results show that addition of magnesium to lithium enhances the current efficiency. Addition of 0.07 wt% Mg to lithium has minor effect on discharge current and anodic potential of lithium anode. The chemical composition and the morphology of the anode surfaces were evaluated by X-ray diffraction and scanning electron microscopy. The results show that the slow dissolution of lithium–magnesium alloy generates the formation of LiOH, LiOH·H₂O, and Mg(OH)₂. After discharge in saturated alkaline electrolyte with additives, the lithium–magnesium surface is less porous than lithium surface. Hydrogen evolution decrease, prompted by adding magnesium to lithium, is related to surface integrity enhanced by Mg(OH)₂.     

Preparation and corrosion resistance studies of nanometric sol-gel-based CeO2 film with a chromium-free pretreatment on AZ91D magnesium alloy

Magnesium alloy, although valuable, is reactive and requires protection before it can be applied in many fields. In this study, a novel protective environmental-friendly gradient coating was performed on AZ91D magnesium alloy by non-chromate surface treatments, which consisted of phytic acid chemical conversion coating and the sol-gel-based CeO₂ thin film. The surface morphologies, microstructure and composition of the coatings were investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The effects of the concentration, layers, temperature of heat treatment of CeO₂ sol on the anti-corrosion properties of the gradient coating for magnesium were also investigated. The results showed that the gradient coating was mainly composed of crystalline CeO₂. According to the results of electrochemical tests, the corrosion resistance of AZ91D magnesium alloy was found to be greatly improved by means of this new environmental-friendly surface treatment.     

7N01铝合金微弧氧化膜的腐蚀行为

分别采用硅酸盐与磷酸盐?硅酸盐复合电解液在7N01铝合金表面制备了微弧氧化膜(MAO)。采用扫描电子显微镜观察它们的微观形貌,用X射线衍射仪分析了它们的物相,并用极化曲线测量、电化学阻抗谱和盐水浸泡试验考察了它们的防腐蚀性能。结果表明,两种体系所得微弧氧化膜均主要由α-Al2O3和γ-Al2O3相组成。与硅酸盐体系所得膜层相比,磷酸盐?硅酸盐复合体系所得膜层更加致密,防腐蚀作用更好。     

Corrosion behavior of 2024 aluminum alloy anodized in presence of permanganate and phosphate ions

In this article, the effects of two inorganic corrosion inhibitors, permanganate and phosphate anions, on morphology, composition, thickness, and corrosion resistance of the anodic film formed on 2024 aluminum alloy in sulfuric acid were investigated. Surface morphology of the oxide films were studied by field emission scanning electron microscopy. Electrochemical impedance spectroscopy and potentiodynamic polarization tests were conducted to assess the corrosion resistance of the coatings. These analyses showed that the presence of the mentioned inhibitors changed the coating morphology, especially in the case of phosphate ions. Corrosion results indicated that using inhibitors during the anodizing process increased the corrosion resistance of the anodized samples. The increases were approximately 81% and 97% for 0.05 M of permanganate and phosphate ions, respectively, compared with the anodized coatings in the absence of additives. An increase in the inhibitor concentration results in better corrosion resistance of the aluminum anodic layer.     

十二烷基苯磺酸钠增强AZ91D镁合金阳极氧化膜耐蚀性研究

在碱性硅硼电解液中对AZ91D镁合金进行电化学阳极氧化处理,考察十二烷基苯磺酸钠(SDBS)添加剂对镁合金阳极氧化膜微观结构和耐腐蚀性能的影响。结果表明,电解液中加入SDBS可以增大氧化膜电阻,提高阳极氧化电压。SDBS使镁合金阳极氧化过程中熔融物的流动性得到提升,流平性更好,从而得到微孔少、裂纹小及平整性好的氧化膜。SDBS质量浓度为0.4 g/L时所得的氧化膜具有最正的开路电位和最大的膜层电子传递电阻,镁合金基底具有最好的耐腐蚀性能。     

Formation of a compact oxide layer on AZ91D magnesium alloy by microarc oxidation via addition of cerium chloride into the MAO electrolyte

Magnesium and its alloys have been used in many industries, but they are reactive and require protection against aggressive environments. In this study, a compact and relatively pore-free oxide coating was formed on AZ91D magnesium alloy to improve its corrosion resistance by means of the microarc oxidation (MAO) process via the addition of CeCl₃ as an additive into the MAO electrolyte. Morphologies and compositions of the coatings were studied with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. It was found that with the addition of CeCl₃ into the MAO electrolyte, the obtained coating showed more uniform morphology compared with the coating produced in the CeCl₃ free electrolyte. EDS analysis confirmed the presence of cerium in the coating formed in the electrolyte with CeCl₃. Polarization tests results showed that the corrosion current density of the coating formed in the CeCl₃ containing electrolyte was less than that of the coating formed in the electrolyte without CeCl₃. Furthermore, electrochemical impedance spectroscopy (EIS) tests indicated that the coating formed in the electrolyte with the addition of CeCl₃ improved the corrosion resistance of the substrate significantly.     

Effect of Na2SiO3 concentration on corrosion resistance and wear resistance of MAO ceramic film on the Al-Mg-Sc alloy

Although Al-Mg-Sc alloy was widely applied to aviation aerospace field, they were vulnerable to local corrosion and wear in the process of long-term service in severe environmental conditions. In this paper, micro-arc oxidation (MAO) ceramic films on Al-Mg-Sc alloy substrate were prepared in electrolyte solutions with different Na₂SiO₃ concentrations, and the corrosion resistance and wear resistance of the MAO samples were studied. The experimental results of potentiodynamic polarization (PDS) and long-term immersion tests indicated that the MAO ceramic film prepared in 10 g/L Na₂SiO₃ electrolyte solution had the best corrosion resistance, as manifested by no obvious cracks, serious collapse and corrosion products on the sample surface and no deep cracks and corrosion paths in the cross-sectional area. The increase of Na₂SiO₃ concentration in electrolyte solution also improved the wear resistance of MAO ceramic film, as manifested by low wear depth (10 μm)and width (1 mm) of the MAO ceramic film prepared in 10 g/L Na₂SiO₃ electrolyte solution against GCr15 steel ball. Studies in mechanisms suggested that as the Na₂SiO₃ concentration in the electrolyte increased, the MAO ceramic film became denser, which could prevent the penetration of corrosive medium, promote the generation of the anti-wear layer with SiO₂ as the main component to enhance the wear resistance. MAO ceramic film formed in Na₂SiO₃ electrolyte solution provided good protective performance for Al-Mg-Sc alloy in the corrosion and wear conditions, which had a broader application prospect.     

AZ 31镁合金阳极氧化工艺参数的优化研究

采用正交实验对AZ 31镁合金在碱性电解液中进行阳极氧化的工艺参数进行优化。考察了氧化时间、电流密度、脉冲频率和占空比对阳极氧化膜性能的影响,获得最佳阳极氧化工艺参数为:氧化时间15min,电流密度1.0A/dm2,脉冲频率200Hz,占空比10%。采用X射线衍射仪(XRD)、扫描电镜(SEM)和动电位极化曲线等检测手段研究了阳极氧化膜的结构、表面形貌和耐蚀性。结果表明:经优化工艺制得的阳极氧化膜,其主要成分为MgO,Al2O3和MgAl2O4,膜层孔隙分布均匀、致密,耐蚀性大幅提高。     

Effects of pH and temperature on the deposition properties of stannate chemical conversion coatings formed by the potentiostatic technique on AZ91 D magnesium alloy

The effects of pH and temperature of a stannate bath on the quality of stannate chemical conversion coatings formed on AZ91 D magnesium alloy by using the potentiostatic polarization technique at E = −1.1 V were investigated in order to improve uniformity and corrosion protection performance of the coating films. It was found that the uniformity and corrosion resistance of coating films deposited by potentiostatic polarization were closely associated with pH and temperature of the coating bath. The pH and temperature to obtain the best coating film were investigated as a function of corrosion protection performance evaluated by curves of potentiodynamic anodic polarization conducted in borate buffer solution. Scanning electron microscope observation and electrochemical corrosion tests of the stannate-coated samples confirmed significant improvement in uniformity and corrosion resistivity of coating films deposited by the potentiostatic technique by modifying the pH and temperature of the coating bath. It was also found that uniformity and corrosion resistivity of the coating films deposited by the potentiostatic technique were considerably improved compared to those of coatings deposited by the simple immersion method at the best conditions of pH and temperature of the coating bath.     

Effect of impurities on the electrochemical behaviour of aluminium electrodes anodized in phosphoric acid

The electrochemical behaviour of anodic films formed on pure aluminium (99.999% and 1100) and 6061-Al alloy substrates in phosphoric acid has been investigated in an aluminium saturated phosphate solution of pH 6.23. The polarization resistance data obtained after one day immersion in the electrolyte showed a strong correlation with the pit number, assessed from an outdoor atmospheric exposure test after 4 months. The cathodic polarization measurements of anodized aluminium in the electrolyte were also carried out. Electrochemical behaviour, eg corrosion, hydrogen evolution reaction, was interpreted in terms of “active sites” within the anodic film. It is suggested that the active sites were mainly formed in the presence of impurities or alloy constituents in aluminium substrates.     

3种溶液体系中镁合金微弧氧化研究第一部分——氧化膜的相组成及其耐蚀性

利用直流脉冲方法在3种溶液体系中于AZ91D镁合金表面制得了微弧氧化陶瓷膜,分析了各膜层的厚度、显微硬度、相组成和耐蚀性能。结果表明,不同体系中的膜层增厚速率不同,形成膜层的相关成分也不同。通过比较微弧氧化前后镁合金的动电位极化曲线和交流阻抗发现,处理后的AZ91D镁合金的耐蚀性得到了明显改善。     

Corrosion behavior study of AZ91 magnesium alloy coated with methyltriethoxysilane doped with cerium ions

The present work aims to investigate the corrosion behavior of AZ91 magnesium alloy treated with a 4% (v/v) methyltriethoxysilane (MTES) alcohol solution, with and without an alkaline pretreatment. The corrosion resistance was assessed by electrochemical impedance spectroscopy (EIS) and current densities were monitored by potentiodynamic polarization curves during immersion in a 0.1 M Na₂SO₄ solution. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to perform a surface analysis. The electrochemical results showed an improvement of anticorrosion properties of AZ91. Furthermore, alkaline pretreatment enhances adhesion between silane film and substrate surface. This can be attributed to a surface enrichment in hydroxyl groups after the alkaline step, which increases formation of Si–O–Mg covalent bonds. The addition of Ce(NO₃)₃ to the MTES bath was evaluated, and it was found that the electrochemical response depends on the cerium ions concentration used. It was shown that the addition of 6.0 × 10⁻⁵ M of Ce(NO₃)₃ to a MTES bath improves corrosion resistance. Higher concentration of cerium ions lead to destabilizing the siloxane network, decreasing the efficiency of the silane coatings.