Effects of silicate concentration on anodic films formed on AZ91D magnesium alloy in solution containing silica sol

Anodic films were prepared on the AZ91D magnesium alloy in 1.0 M and 1.5 M Na₂SiO₃ with varied silica sol addition under the constant current density of 20 mA/cm² at 18 °C. The surface and cross-section morphologies of the anodic films were characterized by scanning electron microscopy (SEM) and energy dispersion spectrometry (EDS). The results showed that both the surface morphologies and the thickness of the anodic film were affected by the concentration of Na₂SiO₃ and the additions of silica sol. The effects of Na₂SiO₃ concentration and silica sol addition on the solution properties were also investigated. The results showed that the addition of silica sol into Na₂SiO₃ solution could decrease the surface energy and the conductivity of the solution. Moreover, the anodic film formed in 1.5 M Na₂SiO₃ with addition of silica sol was more uniform and compact than that formed in 1.0 M Na₂SiO₃ with addition of silica sol. And the electrochemical impedance spectroscopy (EIS) results also indicated that the anodic film formed in 1.5 M Na₂SiO₃ solution with 5 vol.% silica sol addition could provide higher corrosion resistance than that formed in 1.0 M Na₂SiO₃ with the same silica sol addition for the AZ91D Mg alloy substrate.     

Electroless Ni-Sn-P coating on AZ91D magnesium alloy and its corrosion resistance

An electroless Ni-Sn-P coating was deposited on AZ91D magnesium alloy in an alkaline-citrate-based bath where nickel sulphate and sodium stannate were used as metal ion sources and sodium hypophosphite was used as a reducing agent. The phase structure of the coating was amorphous. SEM and attached EDS observation revealed the presence of dense and uniform nodules in the ternary coating and the content of tin was 2.48wt.%. Both the electrochemical analysis and the immersion test in 10% HCl solution proved that the ternary Ni-Sn-P coating exhibited better corrosion resistance than the Ni-P coating in protecting the magnesium alloy substrate.     

Electroless Ni-P layer with a chromium-free pretreatment on AZ91D magnesium alloy

A phosphate-manganese conversion film was proposed as the pretreatment layer between Ni-P coating and AZ91D magnesium alloy substrate, to replace the traditional chromium oxide plus HF pretreatment. The subsequent Ni-P deposited on the layer was also characterized by its structure, morphology, microhardness and corrosion-resistance. The pretreatment layer on the substrate not only reduces the corrosion of magnesium during Ni-P plating process, but also reduces the potential difference between the matrix and the second phase. Thus, a Ni-P coating with fine and dense structure was obtained on the AZ91D magnesium alloy, which shows better corrosion resistance than the Ni-P with chromium oxide plus HF as pretreatment.     

Environmental friendly anodizing of AZ91D magnesium alloy in alkaline borate-benzoate electrolyte

A kind of environmental friendly anodizing routine for AZ91D magnesium alloy, based on an alkaline borate-sodium benzoate electrolyte (NaBz) was studied. The effect of NaBz on the properties of the anodized film was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrometry (EDS), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), respectively. The results showed that the anodizing process, surface morphology, thickness, phase structure and corrosion resistance of the anodized film were strongly dependent on the concentration of NaBz. In the presence of adequate NaBz, a thick, compact and smoothing anodized film with excellent corrosion resistance was produced. Moreover, the forming mechanism of the anodized film in the presence of NaBz additive was also approached, which was a suppression of arc discharge process by the adsorption of Bz⁻ on the surface of magnesium alloy substrate.     

Formation of an aluminum-alloyed coating on AZ91D magnesium alloy in molten salts at lower temperature

An aluminum-alloyed coating was formed on an AZ91D magnesium alloy in molten salts containing AlCl₃ at a lower temperature of 380 °C. The microstructure and phase constitution of the alloyed layer were investigated by optical microscopy, scanning electron microscopy, energy dispersive spectrum and X-ray diffraction. The nano-hardness of the coating was studied by nanoindentation associated with scanning probe microscopy. The corrosion resistance of the coated specimen was evaluated in a 3.5 wt.% NaCl solution by electrochemical impedance spectroscopy and cyclic potentiodynamic polarization. The results show that the aluminum-alloyed coating consists of Mg₂Al₃ and Mg₁₇Al₁₂ intermetallic layers. The formation of the coating is dictated by the negative standard free energy of the reaction: 2AlCl₃ + 3 Mg = 3MgCl₂ + 2Al. This process is associated with a displacement reaction mechanism and diffusion process that takes place during the molten salt treatment. High activity of Al elements in molten salts contributes to the lower temperature formation of the Al-alloyed coating. The alloyed coating markedly improves the hardness as well as the corrosion resistance of the alloy in comparison with the untreated AZ91D magnesium alloy, which is attributed to the formation of the intermetallic compounds.     

2-Hydroxy-4-methoxy-acetophenone as an environment-friendly corrosion inhibitor for AZ91D magnesium alloy

The inhibition behavior of 2-hydroxy-4-methoxy-acetophenone (paeonol) as an environment-friendly corrosion inhibitor for AZ91D magnesium alloy was investigated in 0.05 wt.% NaCl solution by means of polarization curve, AC impedance, weight loss measurement, scanning electron microscopy, Fourier transformation infrared spectroscopy, ultraviolet analysis, and computer molecular simulation. The results show that paeonol can inhibit the corrosion of AZ91D. The maximum inhibition efficiency is achieved when paeonol concentration is 50 ppm by weight in this study. It is proposed that paeonol chelates with Mg to form a paeonol-Mg complex mixing with the original Mg(OH)₂ film on the surface to inhibit the anodic dissolution of AZ91D.     

Atmospheric corrosion of field-exposed magnesium alloy AZ91D

The magnesium alloy AZ91D was exposed in three different types of atmospheric environment, viz. urban, rural and marine exposure sites. Corrosion rates, corrosion products formed, and the influence of the microstructure on the corrosion behaviour of the alloy were investigated. The corrosion rate of AZ91D exposed in the marine environment was 4.2 μm/year, and in the rural and urban environments 2.2 and 1.8 μm/year, respectively. The main corrosion product found was magnesium carbonate hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O), which was formed at all three exposure sites. The corrosion attack started in the -phase in larger grains at the boundary between the -phase and the eutectic -/β-phase. Microgalvanic elements were formed with the eutectic -/β-Mg phase as cathodic site and the -Mg grains as anodes. The Al–Mn particles played a minor roll in the initiation process, even though these particles are the most noble in the microstructure and thus the driving force for a corrosion attack around these particles could be expected to be high. A close resemblance was observed between the corrosion mechanisms operating under the field-exposure conditions described here and the mechanisms operating under the previously reported laboratory conditions.     

Roles of β phase in the corrosion process of AZ91D magnesium alloy

For better understanding of the roles of β phase in the corrosion processes of AZ91D alloy, corrosion behavior of a cast (α + β phase) and a homogenized (α phase) AZ91D alloy was investigated in NaCl aqueous solution by gas collection and electrochemical measurements. According to the hydrogen evolution properties during corrosion, two different types of corrosion tend might be differentiated. For type I, hydrogen diffused into alloy, which evoked the decreasing of hydrogen evolution rate (HER) and the weakening of negative difference effect (NDE). For type II, in the corrosion process, hydrogen in the alloys diffused into the product film (P-type semiconductor), which suggested that hydrogen entering the film would be ionized. The generated electrons during ionization decreased the concentration of vacancies in the valence band of the product films, indicating an improvement of corrosion resistance of AZ91D alloy.     

Baking treatment effect on materials characteristics and electrochemical behavior of anodic film formed on AZ91D magnesium alloy

The composition and microstructure of the anodic films formed on AZ91D Mg alloy, with or without baking, were investigated. The associated corrosion behavior of the anodized alloy in 3.5 wt% NaCl solution was also examined using electrochemical impedance spectroscopy (EIS). The results show that MgO was the main component in the anodic film which also contained some Mg(OH)₂, Al₂O₃, Al(OH)₃, and MgAl₂O₄. Both the amorphous and crystalline forms of anodic film were identified. The degree of crystallinity depended on baking temperature, which increased with increasing temperature in the range of 50-250 °C. The amounts of MgO and Al₂O₃ increased as a result of a dehydration reaction. The polarization resistance of anodized Mg alloy was improved significantly by increasing the oxide content in the anodic film. An optimum value of polarization resistance of anodic film was obtained for the alloy baked at 150 °C for 2 h followed by air cooling.     

Improvement of corrosion resistance of AZ91D magnesium alloy by holmium addition

The corrosion behaviour of two Mg–9Al–Ho alloys (Mg–9Al–0.24Ho and Mg–9Al–0.44Ho) was evaluated by general corrosion measurements and electrochemical methods in 3.5% NaCl solution saturated with Mg(OH)₂. The experimental results were compared with that of Mg–9Al alloy without Ho addition. Various corrosion rate tests showed that the addition of Ho obviously enhanced corrosion resistance of Mg–9Al alloy. The microstructure of the three magnesium alloys and the morphology of their corrosion product film were examined by Electron Probe Microanalysis (EPMA) and Energy Dispersion Spectroscopy (EDS). The alloys with Ho addition showed a microstructure characterized by α phase solid solution, which was surrounded by some β phase and grain-like Ho-containing phase. The improvement of corrosion resistance of the Mg–9Al–Ho alloys could be explained by the fact that the deposited Ho-containing phases were less cathodic. Moreover, the corrosion product films on the Ho-containing alloy surface demonstrated their ability to restrain further corrosion.     

Corrosion behavior of laser melted AZ91HP magnesium alloy

AZ91HP magnesium alloy was melted by CO₂ laser. Compared with as‐received Mg alloy, the grain of the melted layer was refined significantly and the content of Al was increased. The corrosion resistance of the melted layer was improved because of the grain refinement, the redistribution of β‐Mg₁₇Al₁₂ and the increasing of the Al content. As compared to the non‐overlapping zone, the overlapping zone of the melted layer was liable to be corroded.     

Effects of electrolytes on properties of anodic coatings formed on AZ91HP magnesium alloy

In a basic solution containing 10 g/L NaOH, the thickness, morphology and corrosion resistance of anodic coatings formed in solutions without and with additions of Na2SiO3, Na2CO3, Na2B4O7 and Na2SnO3 were separately determined and evaluated. The results show that the electrolytes used above take part in the coating formation and the obtained anodic coatings contain elements from the electrolytes. These electrolytes, especially Na2SiO3 and Na2B4OT, contribute to the film thickness. The coating uniformity obtained by addition of Na2SiO3 is the best, while Na2CO3, Na2B4O7 and Na2SnO3 decrease the number of pores per area on the coatings surface though they worsen the uniformity of the anodic coatings. In addition, the anodic coatings formed in solutions with addition of Na2CO3 or Na2SnO3 exhibit loose and net-like structure. Na2SiO3 and Na2B4O7 can significantly improve the corrosion resistance of the anodic coatings while Na2CO3 and Na2SnO3 have minor effects on improving the coating corrosion resistance.     

氨基酸类有机添加剂对镁合金阳极氧化的影响及其作用机制研究

为了进一步改善镁合金表面阳极氧化膜的质量和性能,在Na OH+Na_2SiO_3基础电解液中添加氨基酸类有机添加剂对AZ31镁合金进行阳极氧化,研究了氨基酸类有机添加剂对镁合金阳极氧化过程及氧化膜厚度、表面形貌、结构及耐蚀性的影响。探讨了有机添加剂在镁合金阳极氧化中的作用机制。结果表明:不同的氨基酸对镁合金阳极氧化过程及氧化膜性能影响不尽相同,其中乙二胺四乙酸和L-鸟氨酸醋酸盐可显著提高阳极氧化的击穿电压,起到明显的抑弧作用,大幅度提高了氧化膜的厚度、致密性和耐蚀性能。有机添加剂主要通过缓蚀、抑弧和表面活性剂3方面的综合作用来影响镁合金阳极氧化的过程及氧化膜的性能。     

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.     

Chemical conversion coating on AZ31B magnesium alloy and its corrosion tendency

The morphology change of the magnesium matrix after pre-treatment and the mor-phology as well as the phase composition of chemical conversion coating formed by phosphate were studied using scanning electron microscope and X-ray diffraction. The corrosion resistance of the coating was studied by salt spray and damp test, and the corrosion tendency during salt immersion test was analyzed. The results show that the phase composition before and after pre-treatment is almost change- less, and the deep microflaw appears between α and β phases during acidic pickling. The phosphate conversion coating is mainly composed of Mg, MgO, and some amor-phous phase, and it can provide a good protection for the AZ31B alloy. Results from corrosive morphology indicate that the growth and the corrosion resistance of the phosphate conversion coating are related to the forming process of the AZ31B matrix.     

Effects of benzotriazole on anodized film formed on AZ31B magnesium alloy in environmental-friendly electrolyte

An environmental-friendly electrolyte of silicate and borate, which contained an addition agent of 1H-benzotriazole (BTA) with low toxicity (LD50 of 965 mg/kg), was used to prepare an anodized film on AZ31B magnesium alloy under the constant current density of 1.5 A/dm² at room temperature. Effects of BTA on the properties of the anodized film were studied by scanning electron microscopy (SEM), energy dispersion spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), loss weight measurement, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), respectively. The results demonstrated that anodized growth process, surface morphology, thickness, phase structure and corrosion resistance of the anodized film were strongly dependant on the BTA concentration, which might be attributed to the formation of an BTA adsorption layer on magnesium substrate surface. When the BTA concentration was 5 g/L in the electrolyte, a compact and thick anodized film could provide excellent corrosion resistance for AZ31B magnesium alloy.     

AZ91D镁合金电弧喷涂铝锌工艺的研究

选择不同的喷涂工艺参数,在镁合金表面喷涂铝锌丝材。采用中性盐雾试验,对不同工艺参数得到的涂层的耐腐蚀性进行了比较。结果表明电弧喷涂铝锌工艺参数对涂层质量有一定影响。试验确定的最佳喷铝锌工艺参数:电压27V,喷涂电流100A,空气压力0.5MPa,喷涂距离100mm。     

Broad-beam laser cladding of Al-Si alloy coating on AZ91HP magnesium alloy

In the present study, an attempt was made to improve the wear resistance and the corrosion resistance of AZ91HP magnesium alloy by laser cladding Al-Si eutectic alloy. The results showed that the clad layer mainly consisted of Mg₂Si, Mg₁₇Al₁₂ and Mg₂Al₃ phases. The microstructure of the bonding zone changed from columnar grains to equiaxial grains along the direction of heat-flow. The heat-affected zone consisted of α-Mg and α-Mg + β-Mg₁₇Al₁₂ eutectic. The formation of multiple Mg intermetallic compounds allowed the clad layer to exhibit higher hardness, better wear resistance and corrosion resistance.     

电解质对AZ91D镁合金微弧氧化的影响

在Na2SiO3和NaAlO2为主成膜剂的硅铝复合电解液中,利用交流脉冲电源对AZ91D镁合金进行微弧氧化处理,研究主成膜剂含量的变化对微弧氧化过程及膜层特性的影响规律。利用扫描电镜(SEM)和膜层测厚仪分别研究了膜层的微观形貌和膜层厚度,通过电化学阻抗谱(EIS)测试膜层在3.5%NaCl中性溶液中的耐蚀性能。结果表明,随着主成膜剂含量的增加,微弧氧化过程中起弧电压和终止电压均呈下降的变化趋势,而膜层耐蚀性则基本呈先增大后降低的变化趋势,膜厚的变化趋势与其耐蚀性一致;Na2SiO3含量的变化对膜层内部致密层和外部疏松层的耐蚀性均有影响,而NaAlO2含量的变化则主要影响膜层内部致密层的耐蚀性;适量的主成膜剂含量是获得致密耐蚀膜层的关键。     

Gas-tungsten arc welding of AZ91 magnesium alloy

The gas-tungsten arc (GTA) welding behaviors of the commercial AZ91 magnesium alloy were examined in terms of process efficiencies and microstructure characteristics. This study focused on the effects of GTA welding process parameters (like welding current in the range of 100/300 A and welding speed in the range of 3.33/13.33 mm/s) on energy absorption by the substrate material. The dependences of arc and welding efficiency on the used process parameters were presented. The measurements revealed that the arc efficiency values ranged from 0.63 to 0.88. Melting efficiency was found to rise with both increasing welding current and speed. The analyses revealed a strong influence of the GTA welding process on the width and depth of the fusion zone and also on the refinement of the microstructure in the fusion zone. The results of dendrite arm size (DAS) measurements were presented. Additionally, the presence of a partially melted zone (PMZ) was disclosed.