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.     

Effects of Ca addition on microstructure and properties of AZ63 magnesium alloy

1　INTRODUCTIONMg Alalloyshaveaverywideapplicationbecauseoftheirexcellentproperties ,lowmanufacturecost ,easymeltingtechniqueandnoexpensiveele mentscontent[1] .Oneofthiskindalloys ,AZ6 3mag nesiumalloy ,isawidelyappliedmagnesiumsacrifi cialanodewhichisusedextensivelyinundergroundandfreshwateratpresent .However ,compared withthataboard ,homesacrificialanodehassomedemer its :lowcurrentefficiencyandweakprotectionfunc tion ,soinvestigatingthehighdriving potentialandhighefficiencysacrificiala…     

Corrosion resistance of Mg−Al LDH/Mg(OH)2/silane−Ce hybrid coating on magnesium alloy AZ31

An environmentally-friendly hybrid coating on AZ31 magnesium alloy substrates was reported. The synergic effect was studied on Mg−Al-layered double hydroxide Mg−Al LDH/Mg(OH)₂-coated AZ31 magnesium alloy via an in-situ steam coating process and a subsequent combined surface modification of bis-[triethoxysilylpropyl]tetrasulfide (BTESPT) silane and Ce(NO₃)₃. The microstructure and composition characteristics of the hybrid coatings were investigated by means of X-ray diffraction (XRD), scanning electronic microscopy (SEM), Fourier transform infrared spectrophotometry (FT-IR) and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the coated samples was evaluated by potentiodynamic polarization (PDP), electrochemical impedance spectrum (EIS) and hydrogen evolution rate during immersion in 3.5 wt.% NaCl solution. The results show an improved corrosion resistance of the alloy in the presence of BTESPT silane and Ce(NO₃)₃. This is most likely due to the synergistic effect of steam coating and silane coating to enhance the barrier properties of hybrid coating. In addition, the formation mechanism and anti-corrosion mechanism of coatings were discussed.     

Green corrosion inhibitors intercalated Mg∶Al layered double hydroxide coatings to protect Mg alloy

Mg alloy was protected with Mg∶Al layered double hydroxide (LDH) coating intercalated with three green corrosion inhibitors (sodium benzoate,3-amino-propyltriet...     

Zn–Al layered double hydroxides as chloride nanotraps in active protective coatings

Zn–Al layered double hydroxides (LDHs) intercalated with nitrate anions are suggested as chloride nanotraps for organic polymeric coatings. The addition of such nanotraps to a polymer layer drastically reduces the permeability of corrosive chloride anions through the protective coatings. In solution, Zn(2)–Al–NO₃ LDHs are responsive to the concentration of chlorides and the release of nitrates is accompanied by entrapment of chlorides, with the process governed by ion-exchange equilibrium. In particular, a coating modified with LDH–NO₃ was found to exhibit significantly lower permeability to chlorides when compared to both unmodified and LDH–Cl-containing coatings, which proves the applicability of LDHs in delaying coating degradation and corrosion initiation.     

Effect of calcium addition on the corrosion behavior of Mg–5Al alloy

The corrosion behavior of four Mg–5Al–xCa alloys (x = 0.0 to 2.0 wt.%) was evaluated in an alkaline NaCl solution. Surface analyses indicated that the benefits of Ca addition are the refinement of the precipitates and a decrease in grain size. Furthermore, the refinement of the precipitates (Mg₂Ca, Al₂Ca) became more complete with increasing of Ca content. The electrochemical tests revealed that the pitting resistance was improved in Ca-containing specimens. In addition, the polarization resistance of the Mg–5Al specimens increased with increasing Ca content. This is due to the fact that precipitation which is expected to act as a barrier is more continuous over the Mg matrix with a smaller grain size and higher precipitation density.     

Effect of Al addition on microstructure and mechanical properties of Mg−Zn−Sn−Mn alloy

The microstructure and properties of the as-cast, as-homogenized and as-extruded Mg−6Zn−4Sn−1Mn (ZTM641) alloy with various Al contents (0, 0.5, 1, 2, 3 and 4 wt.%) were investigated by OM, XRD, DSC, SEM, TEM and uniaxial tensile tests. The results show that when the Al content is not higher than 0.5%, the alloys are mainly composed of α-Mg, Mg₂Sn, Al₈Mn₅ and Mg₇Zn₃ phases_. When the Al content is higher than 0.5%, the alloys mainly consist of α-Mg, Mg₂Sn, MgZn, Mg₃₂(Al,Zn)₄₉, Al₂Mg₅Zn₂, Al₁₁Mn₄ and Al₈Mn₅ phases. A small amount of Al (≤1%) can increase the proportion of fine dynamic recrystallized (DRXed) grains during hot-extrusion process. The room- temperature tensile test results show that the ZTM641−1Al alloy has the best comprehensive mechanical properties, in which the ultimate tensile strength is 332 MPa, yield strength is 221 MPa and the elongation is 15%. Elevated- temperature tensile test results at 150 and 200 °C show that ZTM641−2Al alloy has the best comprehensive mechanical properties.     

Refinement and strengthening mechanism of Mg−Zn−Cu−Zr−Ca alloy solidified under extremely high pressure

Mg?Zn?Cu?Zr?Ca samples were solidified under high pressures of 2–6 GPa. Scanning electron microscopy and electron backscatter diffraction were used to study the distribution of Ca in the microstructure and its effect on the solidification structure. The mechanical properties of the samples were investigated through compression tests. The results show that Ca is mostly dissolved in the matrix and the Mg₂Ca phase is formed under high pressure, but it is mainly segregated among dendrites under atmospheric pressure. The Mg₂Ca particles are effective heterogeneous nuclei of α-Mg crystals, which significantly increases the number of crystal nuclei and refines the solidification structure of the alloy, with the grain size reduced to 22 μm at 6 GPa. As no Ca segregating among the dendrites exists, more Zn is dissolved in the matrix. Consequently, the intergranular second phase changes from MgZn with a higher Zn/Mg ratio to Mg₇Zn₃ with a lower Zn/Mg ratio. The volume fraction of the intergranular second phase also increases to 22%. Owing to the combined strengthening of grain refinement, solid solution, and dispersion, the compression strength of the Mg–Zn–Cu–Zr–Ca alloy solidified under 6 GPa is up to 520 MPa.     

Purifying effect of new flux on magnesium alloy

1　INTRODUCTIONAsoneofthelighteststructurematerials ,mag nesiumalloysofferlightmass(ρ<2 g·cm- 3) ,specif icstrength (higherthanthatofthealuminumalloysandsteels) ,specificstiffness ,excellentmachinabili ty ,superiordampingandmagneticshieldingcapaci ties,whichleadstoagrowinginterestinmagnesiumanditsalloys .Inrecentyears ,magnesiumalloyshavebeenwidelyusedinaviation ,spaceflight,automobileandelectronicsindustries[1] .Since 1990 stheirappli cationhassteadilyextendedanditispredictedthatitwill…     

Role of chlorides on pitting and hydrogen embrittlement of Mg–Mn wrought alloy

The role of chlorides on stress corrosion cracking behavior of Mg–Mn hot rolled alloy was studied in Mg(OH)₂ saturated, 0.01 M and 0.1 M NaCl solutions. The alloy was found to fail by hydrogen embrittlement mechanism both in presence and absence of chlorides. However, the role of chloride has been found to be to damage the passive film, cause pitting and increasing hydrogen embrittlement tendency of the alloy. Crack initiation occurred through pitting and grew in a transgranular manner involving hydrogen.     

Pretreatment-free Ni−P plating on magnesium alloy at low temperatures

Electrochemically promoted electroless plating (EPEP) was used for the application of pretreatment-free Ni?P coating on AM60B magnesium alloy at low temperatures and the obtained coating was characterized by SEM, AFM, EDS and XRD techniques. Compact, uniform, and medium-phosphorus Ni?P coating with mixed crystalline?amorphous microstructure was obtained by applying a cathodic current density of 4 mA/cm² at 50 °C. Also, island-like nickel clusters were deposited on the alloy surface under the same plating condition but without applying the cathodic current. In addition, the durability of the magnesium alloy against corrosion was strongly improved after plating via EPEP technique which was revealed by electrochemical examinations in 3.5% NaCl (mass fraction) corrosive electrolyte. The results of the electrochemical examinations were confirmed by microscopic observations. Thickness, microhardness, porosity and adhesive strength of the deposits were also qualified.     

Effect of rapid cold stamping on fracture behavior of long strip S′ phase in Al−Cu−Mg alloy

High-angle annular dark-field scanning transmission electron microscopy and selected area electron diffraction techniques were used to study the mechanism that underlies the influence of rapid cold-stamping deformation on the fracture behavior of the elongated nanoprecipitated phase in extruded Al−Cu−Mg alloy. Results show that the interface between the long strip-shaped S′ phase and the aluminum matrix in the extruded Al−Cu−Mg alloy is flat and breaks during rapid cold-stamping deformation. The breaking mechanisms are distortion and brittle failure, redissolution, and necking. The breakage of the long strip S′ phase increases the contact surface between the S′ phase and the aluminum matrix and improves the interfacial distortion energy. This effect accounts for the higher free energy of the S′ phase than that of the matrix and creates conditions for the redissolution of solute atoms back into the aluminum matrix. The brittle S′ phase produces a resolved step during rapid cold-stamping deformation. This step further accelerates the diffusion of solute atoms and promotes the redissolution of the S′ phase. Thus, the S′ phase necks and separates, and the long strip-shaped S′ phase in the extruded Al−Cu−Mg alloy is broken into a short and thin S′ phase.     

Comparison on corrosion resistance and surface film of pure Mg and Mg−14Li alloy

To study different corrosion resistances and surface film types of hexagonal close-packed (HCP) pure Mg and body-centered cubic (BCC) Mg−14wt.%Li alloy in 0.1 mol/L NaCl, a series of experiments were conducted, including hydrogen evolution, mass loss, in-situ electrochemical testing combined with Raman spectroscopy and microstructural observation. The results indicate that the corrosion resistance of pure Mg is superior to that of Mg−14Li, and the protective function of the surface films on both magnesium systems is elevated within 16 h of immersion in 0.1 mol/L NaCl. An articulated, thick, and needle-like surface film containing Li₂CO₃ on Mg−14Li, different from the typically thin, flaky Mg(OH)₂ film on pure Mg, is confirmed via scanning electron microscopy (SEM). However, both surface films can be broken down at a high anodic over-potential. Thus, different corrosion resistances of the two Mg systems are ascribed to various protective films forming on their surfaces.     

Effects of processing parameters on microstructure of semi-solid magnesium alloy

In this paper, the effects of pouring temperature of magnesium melt, preheating temperature of the barrel of the screw mixer, and shear rate on the solidified microstructures of semi-solid slurry were investigated by a mechanical stirring semi-solid process. The appropriate processing parameters of slurry preparation were obtained, and the mold filling ability of semi-solid slurry for thin-walled casting was examined. Results indicate that the solid volume fraction of non-dendritic microstructure increases with a decrease in pouring temperature of magnesium melt and the barrel preheating temperature of the screw mixer. Also the grain size of primary α-phase is reduced. Furthermore, the solid volume fraction of semi-solid nondendritic structure decreases with an increase of shear rate. The fine and round granular microstructure with 30～50 μm in size of semi-solid AZ91D magnesium alloy was presented. Finally, a 1.0 mm thin-walled casting with a clear contour and good soundness was successfully made by semi-solid rheo-diecasting.     

Variation of hydrogen level in magnesium alloy melt

M agnesium alloy is one of the lightest metals used in industry. It offers numerous merits in physical, mechanical and casting properties: high specific strength and stiffness, good castability suitable for high pressure die casting, low density, high damping capacity, good thermal and electrical conductivity etc. [1] However, microporosity in the castings reduces mechanical properties [2-4] such as ultimate strength, yield strength, ductility and fatigue resistance. There are mainly two viewp…     

Research on the diffusion bonding of superplastic magnesium alloy

The elevated temperature tensile experiments have been carried out on the magnesium alloy and results indicate that the magnesium alloy has excellent superplastic property.Gleebe-1500 testing machine was used in the diffusion bonding experiment on the superplastic magnesium alloy.Then,the shear stength of the joints under different conditions is obtained through shear testing and the optimum processing parameters for the diffusion bonding are achieved.By metallurgical microscope and scanning electron microscope (SEM),it is revealed that the micromechanism of diffusion bonding is the slide of grain boundaries caused by the growth of grains and atom diffusion of the superplastic magnesium alloy.     

Characterization of the chemical conversion films that form on Mg−Al alloy in colloidal silica solution

Chemical conversion treatment of Mg−Al alloy (AZ91) using colloidal silica as an alternative to chromate conversion was investigated as a function of solution pH, temperature, solution conditions, and treatment time. The solution used for the colloidal silica coating consisted of colloidal silica, titanium sulfate, and cobalt ions to maintain good anti-corrosion and adhesion properties. Adding CoSO₄ to the colloidal silica solution enhanced the adhesion force between the silica film and magnesium substrate. The optimum conditions for the chemical conversion treatment solution were pH 2, 90-sec treatment, and 25°C.     

Hydrogen-induced decomposition of Zr-rich cores in an Mg−6Zn−0.6Zr−0.5Cu alloy

This study presents the first experimental evidence of a hydrogen-induced decomposition reaction in an Mg–6Zn–0.6Zr–0.5Cu alloy from combined transmission electron microscopy and atom-probe tomography characterization. The reaction takes place due to the presence of H in the Mg matrix, causes the decomposition of pre-existing, high-temperature Zr–Zn intermetallic rods into Zr-rich hydride and β′ (Zn₃Mg₂), and forms novel composite precipitates in the Zr-rich cores of the alloy during ageing at 180 °C. The stoichiometry of the Zr–Zn rods was found to be Zn₃(Zr_1?x, Mg_x)₂, rather than Zn₂Zr₃, although both have a similar tetragonal crystal structure. The intrinsic link between the high-temperature Zr–Zn rods and the subsequent elongated composite precipitates, as depicted by the reaction, highlights the importance of engineering the Zr–Zn rod microstructures to control the final precipitate microstructure and effectively strengthen the Zr-rich cores, and hence the advanced Mg alloys.     

Effects of sodium tungstate on characteristics of microarc oxidation coatings formed on magnesium alloy in silicate-KOH electrolyte

Oxide coatings on AM60B magnesium alloy were prepared using the microarc oxidation（MAO） technique in silicate-KOH electrolyte with addition of 0-6.0 g/L Na2WO4. The MAO processes in base electrolyte with different concentrations of Na2WO4 were studied. The microstructure, compositions and mechanical tribological characteristics of the oxide coatings were also investigated by SEM, XRD, XPS, microhardness analysis and ball-on-disc friction testing, respectively. It is found that the addition of Na2WO4 into the base electrolyte has direct effect on the characteristics of voltage-time curves and breakdown voltage in MAO process. The number of micropores at top of the coating surface is increased by the addition of Na2WO4. The fraction of forsterite Mg2SiO4 in the oxide coating increases with increasing concentration of Na2WO4 in base electrolytes. Furthermore, the microhardness and wear resistance of oxide coatings are enhanced as well.     

Effect of temperature on mechanical behavior of AZ31 magnesium alloy

Strain rate sensitivity and tension/compression asymmetry of AZ31 magnesium alloy at different temperatures and strainrates were investigated.Both of mechanical behaviors are temperature dependent.Strain rate sensitivity increases with increasingtemperature.Thermally activated slip is the source of strain rate sensitivity.At the temperature below or near 373 K,strain ratesensitivity is very little.Tension/compression asymmetry in yielding decreases with increasing temperature.Twinning is the reasonof tension/compression asymmetry.At the temperature above or near 573 K,the material shows little tension/compressionasymmetry of the flow stress.