Novel copper immersion coating on magnesium alloy AZ91D in an alkaline bath

Copper immersion coating of magnesium alloys has, to date, been conducted only in acidic baths. This article describes a novel alkaline bath for copper immersion coating on AZ91D magnesium alloy. Prior to the coating process, a chemical etching process of the magnesium substrate was optimized using orthogonal experimental methodology. The copper immersion coating was then investigated with regard to the effect of pH and fluoride content in the deposition bath. It was revealed during the coating process that an increase of pH and fluoride content led to a surface film formation on the magnesium substrate. The surface film formation occurred simultaneously with copper reduction, rendering a controlled magnesium dissolution, thereby a controlled copper deposition. With optimized conditions of chemical etching and immersion coating processes, uniform copper deposits were achieved.     

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.     

AC impedance spectroscopy study of the corrosion behavior of an AZ91 magnesium alloy in 0.1 M sodium sulfate solution

The corrosion behavior of an AZ91 magnesium alloy in 0.1 M sodium sulfate solution at the corrosion potential (E_corr) was investigated using electrochemical impedance spectroscopy (EIS), environmental scanning electron microscopy (ESEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The results showed that when the immersion time was less than 18th, general corrosion occurred on the surface and the main corrosion products were hydroxides and sulfates. The film coverage effect was the main mechanism for the corrosion process of AZ91 alloy. At this stage, the matrix had a better corrosion resistance. With the increasing immersion time, pitting occurred on the surface. At this stage, the corrosion process was controlled by three surface state variables: the area fraction θ₁ of the region controlled by the formation of Mg(OH)₂, the area fraction θ₂ of the region controlled by the precipitation of MgAl₂(SO₄)₄·2H₂O, and the metastable Mg⁺ concentration C_m.     

High corrosion resistance of electroless composite plating coatings on AZ91D magnesium alloys

The process of electroless plating Ni-P on AZ91D magnesium alloys was improved. The Ni-P-ZrO₂ composite coatings and multilayer coatings were investigated based on the new electroless plating process. The coatings surface and cross-section morphologies were observed with scanning electron microscopy (SEM). The chemical compositions were analyzed by EDXS. The corrosion behaviors were evaluated by immersion, salt spray and electrochemical tests. The experimental results indicated that the Ni-P-ZrO₂ composite coatings suffered attack in NaCl solution but displayed passivation characteristics in NaOH and Na₂SO₄ solutions. The corrosion resistance of Ni-P-ZrO₂ coatings was superior to Ni-P coatings due to the effect of ZrO₂ nano-particle. The multilayer coatings consisting of Ni-P-ZrO₂/electroplating nickel/Ni-P (from substrate to surface) can protect magnesium alloys from corroding more than 1000 h for the salt spray test.     

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.     

Controlling the degradation rate of bioactive magnesium implants by electrophoretic deposition of akermanite coating

In order to improve the corrosion resistance and the surface bioactivity of biodegradable magnesium alloys, a nanostructured akermanite (Ca₂MgSi₂O₇) coating was grown on AZ91 magnesium alloy through electrophoretic deposition (EPD) assisted with micro arc oxidation (MAO) method. The crystalline structures, morphologies and compositions of samples were characterized by X–ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The in vitro bio–corrosion (biodegradability) and bioactivity behaviors of samples were investigated by electrochemical and immersion tests. The experimental results indicated that the nanostructured akermanite coating could slow down the corrosion rate and improve the in vitro bioactivity of biodegradable magnesium alloy. Thus, magnesium alloy coated with nanostructured akermanite may be a promising candidate to be used as biodegradable bone implants.     

Electrochemical behavior of magnesium alloys as biodegradable materials in Hank's solution

The electrochemical behavior of extruded AZ31E and AZ91E alloys was investigated in Hank's solution at 37 °C. The behavior of the two alloys was studied with immersion time using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and weight loss tests. It was found that the corrosion resistance of AZ31E alloy is higher than that of AZ91E. Also, the effect of adding different concentrations of a commercial drug called glucosamine sulphate (as inhibitor) to Hank's solution was studied for AZ31E alloy. The corrosion was effectively inhibited by the addition of 0.01 mM glucosamine sulphate that reacts with AZ31E alloy and forms a protective film on its surface. The results were confirmed by surface examination via scanning electron microscope.     

A new phosphate pretreatment process for adhesive bonding of magnesium AZ31 sheets

An economic, high efficiency sodium dihydric phosphate surface pretreatment process (SDPT) was developed to improve the adhesive bond performance of 2 mm thick hot-rolled wrought magnesium AZ31 sheets. A phosphating solution with sodium dihydric phosphate (NaH₂PO₄), additive sodium fluoride (NaF), and accelerator sodium molybdate (Na₂MoO₄), sodium tungstate (Na₂WO₄) and potassium nitrate (KNO₃) were developed to pretreat the magnesium alloys. The content of sodium dihydric phosphate in the phosphating solution was strictly controlled to insure it supplied sufficient acid radical HPO₄ ^2? to phosphate magnesium AZ31 alloy. Furthermore, a suitable H⁺ content to keep the pH values in the range of 5–6 for a phosphating solution was necessary. With this SDPT pretreatment process, a coating consisting of the magnesium phosphate, magnesia (MgO), magnesium hydroxide (Mg(OH)₂), magnesium fluoride (MgF₂) and a minor amount of molybdenum oxide(MoO₃) was formed on the surface of the magnesium AZ31. While the SDPT pretreated adhesive-bonded joints had better initial bond strength than phosphate–permanganate process pretreated joints, the corrosion resistance of SDPT pretreated joints was slightly inferior.     

Development of HA-CNTs composite coating on AZ31 Magnesium alloy by cathodic electrodeposition. Part 2: Electrochemical and in-vitro behavior

The carbon nano-tubes (CNTs) reinforced hydroxyapatite (HA), with various functionalized CNTs concentration ranging from 0 to 1.5?wt%, were deposited on AZ31 magnesium alloy by direct and pulse cathodic electrodeposition methods. The corrosion resistance of the coatings was tested in simulated body fluid (SBF) using different electrochemical methods such as open circuit potential, polarization and electrochemical impedance spectroscopy. The in-vitro behavior, changes in solution pH as well as the amount of evolved hydrogen of these coatings were also evaluated during five days immersion in SBF. The results indicated that the pulse deposited HA having 1% CNTs coating was the optimum condition which decreased the corrosion current density of AZ31 magnesium alloy from 44.25?µA/cm² to 0.72?µA/cm². Moreover, it stabilized the alkalization behavior of AZ31 alloy and caused a tenfold decrease in the amount of hydrogen generation in SBF. Additionally, the formation of new hydroxyapatite layer on the surface of the pre-exist coatings after five days immersion in SBF was confirmed by SEM characterization.     

Low-temperature fabrication of TiO<Subscript>2</Subscript> film on flexible substrate by atmospheric roll-to-roll CVD

Titanium dioxide (TiO₂) thin film was fabricated using titanium isopropoxide as a precursor through an atmospheric low-temperature roll-to-roll chemical vapor deposition method. TiO₂ was deposited on the PET substrate in the temperature range of room temperature to 100°C, and the working pressure was 740 Torr. The surface morphology of TiO₂ thin film was analyzed by field emission scanning electron microscopy and a 2D surface profiler. The results revealed that the growth rate of TiO₂ film was 31 nm/min at 100°C, and it also showed that the surface is uniform and smooth. Moreover, the lowest root mean square roughness (R _q) value of 1.87 nm was obtained for TiO₂ film prepared at 100°C. The composition of TiO₂ film was confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The film showed very good chemical and optical properties while increasing the substrate deposition temperature. The UV–Vis spectroscopy analysis revealed that TiO₂ films exhibited excellent optical transmittance, more than 91% observed in the visible region.     

Single frequency electrochemical impedance investigation of zero charge potential for different surface states of Cu–Ni alloys

Adsorption of sulphate anions onto polycrystalline pure copper, pure nickel and two copper–nickel alloys was explored by performing zero charge potential E _PZC measurements. E _PZC measurements were performed by differential capacitance analysis using single?frequency electrochemical impedance spectroscopy. E _PZC and surface charge of the studied materials were analysed for different surface states: bare—immediately after cathodic polarisation and after 5 h of immersion in a 0.1 M Na₂SO₄ solution. According to the surface state and nature of the studied materials, the choice of the appropriate measuring frequency was investigated. Specific methodologies dedicated to the study of electrode/electrolyte interfaces are then presented for bare metal, metals covered by corrosion products and metals covered by passive film. In the presence of passive film, the space charge capacitance interferes with the differential capacitance measurements, and its influence is discussed in the framework of semiconductor electrochemistry.     

Special Features of Oxide Layer Formation on Magnesium Alloys during Plasma Electrolytic Oxidation

The process of the oxidation of magnesium alloys in a silicate electrolyte during plasma electrolytic oxidation is investigated. An anomalous form of the chronogram of the formation voltage of the oxide layer in the electrolytes with the highest silicate concentration (approximately 0.15 M Na₂SiO₃ · 5H₂O) is detected. X-ray diffraction analysis, scanning electron microscopy with energy dispersive X-ray spectroscopy analysis, and thickness gauges are used to characterize the surface microstructure, phase composition, and thickness, respectively. Mechanisms for the initial period of PEO and the “insular” growth were described. During the “insular” growth, islands consisting of vitrified components of the electrolyte are growing on the original smooth surface.     

Long-term corrosion resistance and fast mineralization behavior of micro-nano hydroxyapatite coated magnesium alloy in vitro

To improve the long-term corrosion resistance of biodegradable AZ31 magnesium alloy, the micro-nano structural hydroxyapatite (HA) coating was fabricated on AZ31 substrate by hydrothermal treatment. The compact and high crystallinity HA coating prepared at 120 °C had excellent electrochemical properties. Moreover, the cell viability experiment revealed that the micro-nano structure coating was conducive to the viability and proliferation of MC3T3-E1 osteoblasts. The immersion experiment in simulated body fluid (SBF) solution showed that the micro-nano structural HA coatings could quickly induce the production of HA mineralization, and then the mineralization evolved into a compact mineralized layer on the surface of coated sample, which provided a long-term protection for the specimen. Even after 147 days of immersion, the coated samples remained the relatively complete macroscopic shape, the corrosion rates were lower than 0.500 mm/y and the pH values of the SBF solution maintained in the range of 7.10–7.80, suggesting when these coated AZ31 magnesium alloys were used as degradable biomaterial implants, they could provide a long-term mechanical support during the healing of damaged bones.     

超声波对AZ91D镁合金电镀铜的影响

利用超声波辅助电镀技术在AZ91D镁合金表面电镀铜。通过极化曲线、SEM、结合力测试和NaCl溶液浸泡试验,研究了超声波对镁合金电镀铜的影响。试验表明,无超声波辅助电镀所得铜层表面晶粒粗糙,结合力较差;而利用超声波辅助电镀技术所得铜层表面晶粒均匀、平整,结合力强,耐蚀性较好。SEM观察表明,超声波功率对镀层性能也有一定影响,80W超声功率下得到的镀层表面比40W超生功率的平整。     

Influence of substrate composition on corrosion protection of sol–gel thin films on magnesium alloys in 0.6 M NaCl aqueous solution

The corrosion protection behaviour of organic–inorganic hybrid thin films on AZ31 and AZ61 magnesium alloy substrates has been studied. These films were prepared by a sol–gel dip-coating method. The organopolysiloxane precursors were γ-methacryloxypropyltrimethoxysilane (MAPTMS) and tetramethoxysilane (TMOS). An attempt was made to determine the possible relationships between the degradation of the sol–gel film and composition of the metal substrate during the exposure of the metal/coating system to 0.6 M NaCl aqueous solutions. For this purpose electrochemical impedance spectroscopy (EIS) and hydrogen evolution measurements were applied. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the sol–gel films formed on the surface of AZ61 alloy were far more perfect and uniform than those formed on the AZ31 alloy. This behaviour was attributed to the effect of the native oxide film initially present on the surface of the AZ61 alloy, which inhibited the attack of magnesium. Results indicated that the sol–gel coated AZ61 substrate tended to develop corrosion products slower than the sol–gel coated AZ31 substrate, trend that could change by prolonging exposure time. After 6 days of immersion, a clear inhibitive effect of the corrosion products formed during the test was observed in the case of the sol–gel coated AZ31, but not with the coated AZ61 alloy substrate, a phenomenon explained by the carbonate enrichment observed by XPS.     

AZ31B镁合金表面磷化/阴极电泳复合涂层耐蚀性能研究

通过AZ31B镁合金表面磷化预处理及电泳涂装获得了复合电泳涂层,并对复合涂层的耐蚀性能进行评价。磷化膜内层连续致密,外层粗大的晶粒使表面粗糙,提高了与电泳涂层的结合力;获得复合涂层的附着力达到0级,硬度达到5H,冲击性能大于90kg·cm,耐盐雾试验600h不起泡。因此,磷化预处理的丁二烯阴极电泳涂层对镁合金起到了良好的装饰和防护作用。     

镁合金电沉积镍形成机理及电镀工艺的研究

分析了浸锌层的作用机理,利用恒电势下电流密度随时间的变化和开路电势随时间变化的曲线,研完了镍在浸锌层上的形成机理.研究发现:镍的电沉积形核过程为连续形核,超电势增加时,晶核数迅速增加;镍层的生长过程遵循VolmerWeber模式;浸锌层能明显改善镀层结合力.在镀镍溶液中加入晶粒细化剂,细化了镀层晶粒,提高了镀层性能.X射线衍射表明:晶粒细化剂加入后,镍层晶粒明显细化,平均柱径约为26 nm,镍层表现为(111)择优取向.纳米晶镍层具有优良的性能,15μm厚的镀层24 h中性盐雾实验的保护等级为9级,热震和划痕试验表明结合力良好.     

Effect of naturally formed oxide films and other variables in the early stages of Mg-alloy corrosion in NaCl solution

The influence of initial surface conditions on the subsequent corrosion behaviour of AZ31 and AZ61 magnesium alloys in 0.6 M NaCl solution has been studied using electrochemical impedance spectroscopy. For obtaining the different surface conditions, some of the specimens were immersion tested with the surface in the as-received condition, while others were tested immediately after mechanical polishing, and part of the polished specimens after six months of exposure to the laboratory atmosphere. Considering the evolution of the high-frequency capacitive arc of the Nyquist diagram, whose diameter is related to the corrosion process, a clear effect of the initial surface conditions is observed only in the early stages of testing. This effect is especially significant for the freshly polished specimens.     

醋酸镍对AZ 63B镁合金微弧氧化膜耐蚀性的影响

在硅酸盐体系电解液中添加适量的醋酸镍及配位剂后,对AZ 63B镁合金进行微弧氧化处理,在AZ 63B镁合金表面制备了微弧氧化膜。研究了醋酸镍对微弧氧化膜耐蚀性的影响。结果表明:在电解液中添加醋酸镍,经微弧氧化后,镁合金表面生成了含Ni化合物的咖啡色氧化膜,耐蚀性得以提高。     

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.