Corrosion of magnesium and magnesium –calcium alloy in biologically-simulated environment

A study of biocompatibility and corrosion of both metallic magnesium(Mg) and a magnesium alloy containing 1% calcium(Mg–Ca) were investigated in in vitro culture conditions with and without the presence of bone marrow derived human mesenchymal stem cells(h MSCs).Chemical analysis of the degraded samples was performed using XRD and FEGSEM. The results from the XRD analysis strongly suggested that crystalline phase of magnesium carbonate was present on the surface of both the Mg and Mg–Ca samples. Flame absorption spectrometry was used to analyse the release of magnesium and calcium ions into the cell culture medium. Magnesium concentration was kept consistently at a level ranging from 40 to 80 m M for both Mg and Mg–Ca samples. No cell growth was observed when in direct contact with the metals apart from a few cells observed at the bottom of culture plate containing Mg–Ca alloy. In general, in vitro study of corrosion of Mg–Ca in a biologicallysimulated environment using cell culture medium with the presence of h MSCs demonstrated close resemblances to in vivo corrosion. Although in vitro corrosion of Mg–Ca revealed slow corrosion rate and no immediate cytotoxicity effects to h MSCs, its corrosion rate was still too high to achieve normal stem cell growth when cells and alloys were cultured in vitro in direct contact.     

In vitro corrosion of Mg–1.21Li –1.12Ca –1Y alloy

The influence of the microstructure on mechanical properties and corrosion behavior of the Mg–1.21Li–1.12Ca–1Y alloy was investigated using OM, SEM, XRD, EPMA, EDS, tensile tests and corrosion measurements. The results demonstrated that the microstructure of the Mg–1.21Li–1.12Ca–1Y alloy was characterized by α-Mg substrate and intermetallic compounds Mg2 Ca and Mg24Y5. Most of the fine Mg2 Ca particles for the as-cast alloy were distributed along the grain boundaries, while for the as-extruded along the extrusion direction. The Mg24Y5 particles with a larger size than the Mg2 Ca particles were positioned inside the grains. The mechanical properties of Mg–1.21Li–1.12Ca–1Y alloy were improved by the grain refinement and dispersion strengthening. Corrosion pits initiated at the α-Mg matrix neighboring the Mg2 Ca particles and subsequently the alloy exhibited general corrosion and filiform corrosion as the corrosion product layer of Mg(OH)2and Mg CO3 became compact and thick.     

Development of high-strength,low-cost wrought Mg–2.0 mass% Zn alloy with high Mn content

Mg–Zn–Mn-based alloys have received considerable attention because of their high creep resistance, strength,and good corrosion resistance. The alloying element Mn in Mg–Zn-based alloys is commonly less than 1 wt%. In the present study, the effect of high Mn content(1 wt% and 2 wt%) on the microstructures and mechanical properties of Mg–2Zn–0.3Sr extruded alloy was investigated. The results revealed that the high Mn content significantly increased the ultimate tensile strength, tensile yield strength, compress yield strength, and yield asymmetry of the alloy without affecting its ductility. The dynamically recrystallized(DRXed) grains of Mg–2Zn–0.3Sr were remarkably refined because of the large amount of fine Mn precipitates in the homogenized alloy. The improved strengths were mainly attributed to the fine DRXed grains according to the Hall–Petch effect and to the large amount of spherical and 0001 Mn precipitates through the precipitation and dispersion strengthening. The fine DRXed grains and numerous Mn precipitates effectively suppressed the extension twining, substantially enhanced the compress yield strength, and resulted in improved anisotropy.     

Microstructure,mechanical, and corrosion properties of extruded low-alloyed Mg–xZn–0.2Ca alloys

The microstructure, mechanical, and corrosion properties of extruded low-alloyed Mg-xZn-0.2Ca (x=0, 1.0, 2.0, 3.0) alloys were investigated in this study. Findings from scanning electron microscope, X-ray diffraction and transmission electron microscopy results indicate that the amount of ternary Ca₂Mg₆Zn₃ phase, as the only secondary phase in 1.0Zn, 2.0Zn, and 3.0Zn alloys, gradually increases with the addition of Zn, while the Mg₂Ca phase was observed in the Mg-0.2Ca alloy only. Zn has a strong effect on the orientation and intensity of textures, which also influence mechanical behaviors, as revealed by electron back-scatter diffraction. Among all the alloys, the Mg-2.0Zn-0.2Ca alloy obtains the maximum tensile strength (278 MPa) and yield strength (230 MPa). Moreover, Zn addition has an evident influence on the corrosion properties of Mg-xZn-0.2Ca alloy, and Mg-1.0Zn-0.2Ca alloy exhibits the minimum corrosion rate. This paper provides a novel low-alloyed magnesium alloy as a potential biodegradable material.     

Corrosion behavior of Mg–Y alloy in NaCl aqueous solution

The corrosion behavior of Mg–(0.25, 2.5, 5, 8 and 15)Y alloys in 3.5wt.% NaCl aqueous solution was investigated. It was found that the degree of corrosion deterioration increased with increasing immersion time up to 2 h. Corrosion modes for the alloys with low and high content of Y element were general corrosion and pitting corrosion, respective ly, and the threshold content for the corrosion mode change was 2.5% for the tested alloys. The experimental results showed that the addition of Y refined the grain of the alloy, and the distribution, i.e., continuous or not, of the Mg24Y5 phases had great effect on the corrosion rate and corrosion mode.     

不同冷却方式制备的2507超双相不锈钢在模拟海洋环境中的微观结构和钝化膜对耐蚀性的影响

The effect of microstructure and passive film on the corrosion resistance of 2507 super duplex stainless steel (SDSS) in simulated marine environment was investigated by electrochemical measurements, periodic wet–dry cyclic corrosion test, scanning Kelvin probe force microscopy, atomic force microscopy, and X-ray photoelectron spectrometry. The results show that the occupation ratio of γ phase increases with the decrease in cooling rate, whereas the content of α phase reduces gradually. In addition, the σ precipitated phase only emerges in the annealed steel. The pitting sensitivity and corrosion rate of 2507 SDSS reduce first and then increase as the cooling rate decreases. The σ precipitated phase drastically reduces the protective ability of the passive film and facilitates micro-galvanic corrosion of the annealed steel. For various microstructures, the pits are preferentially distributed within the σ and γ phases. The corrosion resistance of 2507 SDSS prepared by different cooling methods is closely related to the microstructure and structure (stability and homogeneity) of the passive film. Normalized steel shows an optimal corrosion resistance, followed by the quenched and annealed steels.     

Effect of microstructure and passive film on corrosion resistance of 2507 super duplex stainless steel prepared by different cooling methods in simulated marine environment

The effect of microstructure and passive film on the corrosion resistance of 2507 super duplex stainless steel(SDSS) in simulated marine environment was investigated by electrochemical measurements, periodic wet–dry cyclic corrosion test, scanning Kelvin probe force microscopy, atomic force microscopy, and X-ray photoelectron spectrometry. The results show that the occupation ratio of γ phase increases with the decrease in cooling rate, whereas the content of α phase reduces gradually. In addition, the σ precipitated phase only emerges in the annealed steel. The pitting sensitivity and corrosion rate of 2507 SDSS reduce first and then increase as the cooling rate decreases. The σ precipitated phase drastically reduces the protective ability of the passive film and facilitates micro-galvanic corrosion of the annealed steel. For various microstructures, the pits are preferentially distributed within the σ and γ phases. The corrosion resistance of 2507 SDSS prepared by different cooling methods is closely related to the microstructure and structure(stability and homogeneity) of the passive film. Normalized steel shows an optimal corrosion resistance, followed by the quenched and annealed steels.     

Microstructural analyses of aluminum–magnesium–silicon alloys welded by pulsed Nd: YAG laser welding

Revealing grains and very fine dendrites in a solidified weld metal of aluminum–magnesium–silicon alloys is difficult and thus,there is no evidence to validate the micro-and meso-scale physical models for hot cracks. In this research, the effect of preheating on the microstructure and hot crack creation in the pulsed laser welding of AA 6061 was investigated by an optical microscope and field emission electron microscopy. Etching was carried out in the gas phase using fresh Keller’s reagent for 600 s. The results showed that the grain size of the weld metal was proportional to the grain size of the base metal and was independent of the preheating temperature. Hot cracks passed the grain boundaries of the weld and the base metal. Lower solidification rates in the preheated samples led to coarser arm spacing;therefore, a lower cooling rate. Despite the results predicted by the micro and meso-scale models, lower cooling rates resulted in increased hot cracks. The cracks could grow in the weld metal after solidification;therefore, hot cracks were larger than predicted by the hot crack prediction models.     

Corrosion mechanism of micro-arc oxidation treated biocompatible AZ31 magnesium alloy in simulated body fluid

The rapid degradation of magnesium(Mg) based alloys has prevented their further use in orthopedic trauma fixation and vascular intervention,and therefore it is essential to investigate the corrosion mechanism for improving the corrosion resistance of these alloys. In this work, the effect of applied voltage on the surface morphology and the corrosion behavior of micro-arc oxidation(MAO) with different voltages were carried out to obtain biocompatible ceramic coatings on AZ31 Mg alloy. The effects of applied voltage on the surface morphology and the corrosion behavior of MAO samples in the simulated body fluid(SBF) were studied systematically. Scanning electron microscope(SEM) and X-ray diffractometer(XRD)were employed to characterize the morphologies and phase compositions of coating before and after corrosion. The results showed that corrosion resistance of the MAO coating obtained at 250 V was better than the others in SBF. The dense layer of MAO coating and the corrosion precipitation were the key factors for corrosion behavior. The corrosion of precipitation Mg(OH)2and the calcium phosphate(Ca–P) minerals on the surface of MAO coatings could enhance their corrosion resistance effectively. In addition, the mechanism of MAO coated Mg alloys was proposed.     

Effect of solidification cooling rate on microstructure and tribology characteristics of Zn–4Sialloy

The main objective of this work was to modify the microstructure and enhance the tribological properties of a new Zn–4Si alloy through a high solidification cooling rate (SCR). According to the results, by increasing the SCR from 2.0 to 59.5°C/s the average size of primary Si particles and that of the grains reduced from 76.1 and 3780μm to less than about 14.6 and 460μm, respectively. Augmenting the SCR also enhanced the microstructural homogeneity, decreased the porosity content (by 50%), and i...     

Rapid cooling effect during solidification on macro-and micro-segregation of as-cast Mg–Gd alloy

The high performance of as-cast Mg-RE alloys is always related to their high RE additions.However,RE elements can be readily segregated in Mg alloys and the segregation becomes more significant with the increasing RE content.In this research,the effect of cooling rate on the macro-and micro-segregation in the as-cast Mg-8 Gd alloy was studied.The Gd content at the bottom of the fabricated ingot with the cooling rate of 4.6-6.9℃/s was～1.7 times of that at the top and coarse eutectics as well as some non-equilibrium phases of α-Gd,MgGd,Mg_2 Gd were distributed along the grain boundaries.The formation mechanisms of the specific gravity segregation and grain boundary segregation were also proposed.Upon the application of the water-cooled copper mold with the cooling rate of 27-200℃/s,only fine Mg5 Gd and some nanoscale metastable β_1 and β' phases were found to disperse uniformly in the grain interior,thus the homogeneity of the composition,microstructure,and performance within the whole ingot was considerably improved.It is expected that these results will facilitate the processing design for the fabrication of the highly-homogenized Mg-RE alloy castings.     

摩擦表面合金化法开发一种 Mg–Zn 表面合金:模拟体液中的体外降解研究

A new variant of friction-assisted process named friction surface alloying (FSA) for developing surface alloys was demonstrated in the present work. In FSA, the dispersed phase is melted and allowed to react with the matrix material to form an alloy at the surface of a metallic substrate. In the present work, magnesium (Mg) sheets and zinc (Zn) powder were selected, and fine grained (~3.5 μm) Mg–Zn surface alloy with improved hardness was produced by FSA. X-ray diffraction studies confirmed the formation of intermetallic phases of Mg and Zn at the surface. From the in vitro degradation studies carried out by immersing in simulated body fluids, a lower corrosion rate was observed for the Mg–Zn surface alloy compared with pure Mg. The surface morphologies after immersion studies indicated large degraded areas on the base Mg compared with Mg–Zn. The results demonstrate the potential of FSA in developing Mg-based surface alloys without melting the substrate to impart better surface properties.     

Developing Mg–Zn surface alloy by friction surface allosying: In vitro degradation studies in simulated body fluids

A new variant of friction-assisted process named friction surface alloying(FSA) for developing surface alloys was demonstrated in the present work. In FSA, the dispersed phase is melted and allowed to react with the matrix material to form an alloy at the surface of a metallic substrate. In the present work, magnesium(Mg) sheets and zinc(Zn) powder were selected, and fine grained(～3.5 μm) Mg–Zn surface alloy with improved hardness was produced by FSA. X-ray diffraction studies confirmed the formation of intermetallic phases of Mg and Zn at the surface. From the in vitro degradation studies carried out by immersing in simulated body fluids, a lower corrosion rate was observed for the Mg–Zn surface alloy compared with pure Mg. The surface morphologies after immersion studies indicated large degraded areas on the base Mg compared with Mg–Zn. The results demonstrate the potential of FSA in developing Mg-based surface alloys without melting the substrate to impart better surface properties.     

Corrosion behavior of as-cast Mg–8Li–3Al+xCe alloy in 3.5wt% NaCl solution

Mg–8Li–3Al+xCe alloys (x = 0.5wt%, 1.0wt%, and 1.5wt%) were prepared through a casting route in an electric resistance furnace under a controlled atmosphere. The cast alloys were characterized by X-ray diffraction, optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The corrosion behavior of the as-cast Mg–8Li–3Al+xCe alloys were studied under salt spray tests in 3.5wt% NaCl solution at 35°C, in accordance with standard ASTM B–117, in conjunction with potentiodynamic polarization (PDP) tests. The results show that the addition of Ce to Mg–8Li–3Al (LA83) alloy results in the formation of Al₂Ce intermetallic phase, refines both the α-Mg phase and the Mg₁₇Al₁₂ intermetallic phase, and then increases the microhardness of the alloys. The results of PDP and salt spray tests reveal that an increase in Ce content to 1.5wt% decreases the corrosion rate. The best corrosion resistance is observed for the LA83 alloy sample with 1.0wt% Ce.     

Preparation, microstructure and degradation performance of biomedical magnesium alloy fine wires

With the development of new biodegradable Mg alloy implant devices, the potential applications of biomedical Mg alloy fine wires are realized and explored gradually. In this study, we prepared three kinds of Mg alloy fine wires containing 4 wt% RE(Gd/Y/Nd) and 0.4 wt% Zn with the diameter less than 0.4 μm through casting, hot extruding and multi-pass cold drawing combined with intermediated annealing process. Their microstructures, mechanical and degradation properties were investigated. In comparison with the corresponding as-extruded alloy, the final fine wire has significantly refined grain with an average size of 3–4 μm, and meanwhile shows higher yield strength but lower ductility at room temperature. The degradation tests results and surface morphologies observations indicate that Mg–4Gd–0.4Zn and Mg–4Nd–0.4Zn fine wires have similar good corrosion resistance and the uniform corrosion behavior in SBF solution. By contrast, Mg–4Y–0.4Zn fine wire shows a poor corrosion resistance and the pitting corrosion behavior.     

Effect of benzaldehyde on the electrodeposition and corrosion properties of Ni–W alloys

The effect of different concentrations of benzaldehyde on the electrodeposition of Ni–W alloy coatings on a mild steel substrate from a citrate electrolyte was investigated in this study. The electrolytic alkaline bath (pH 8.0) contained stoichiometric amounts of nickel sulfate, sodium tungstate, and trisodium citrate as precursors. The corrosion resistance of the Ni–W-alloy-coated specimens in 0.2 mol/L H₂SO₄ was studied using various electrochemical techniques. Tafel polarization studies reveal that the alloy coatings obtained from the bath containing 50 ppm benzaldehyde exhibit a protection efficiency of 95.33%. The corrosion rate also decreases by 21.5 times compared with that of the blank. A higher charge-transfer resistance of 1159.40 Ω·cm2 and a lower double-layer capacitance of 29.4 μF·cm-2 further confirm the better corrosion resistance of the alloy coating. X-ray diffraction studies reveal that the deposits on the mild steel surface are consisted of nanocrystals. A lower surface roughness value (R_max) of the deposits is confirmed by atomic force microscopy.     

Microstructural characteristics and electrical resistivity of rapidly solidified Co-Sn alloys

The rapid solidification behavior of Co-Sn alloys was investigated by melt spinning method.The growth morphology of αCo phase in Co-20% Sn hypoeutectic alloy changes senistively with cooling rate.A layer of columnar αCo dendrite forms near the roller side at low colling rates.This region becomes small and disappears as the cooling rate increases and a kind of very fine homogeneous microstructure characterized by the distribution of equiaxed αCo dendrites in γCo3Sn matrix is subsequently produced.For Co-34.2% Sn eutectic alloy,anomalous eutectic forms within the whole range of cooling rates.The increase of cooling rate has two obvious effects on both alloys:one is the microstructure refinement,and the other is that it produces more crystal defects to intensify the seattering of free electrons,leading to a remarkable increase of electrical resistivity,Under the condition that the grain boundary reflection coefficient γ approaches 1,the resistivity of rapidly solidified Co-Sn alloys can be predicted theoretically.     

Microstructural analyses of aluminum–magnesium–silicon alloys welded by pulsed Nd: YAG laser welding

Revealing grains and very fine dendrites in a solidified weld metal of aluminum–magnesium–silicon alloys is difficult and thus,there is no evidence to validate the micro-and meso-scale physical models for hot cracks. In this research, the effect of preheating on the microstructure and hot crack creation in the pulsed laser welding of AA 6061 was investigated by an optical microscope and field emission electron microscopy. Etching was carried out in the gas phase using fresh Keller's reagent for 600 s. The results showed that the grain size of the weld metal was proportional to the grain size of the base metal and was independent of the preheating temperature. Hot cracks passed the grain boundaries of the weld and the base metal. Lower solidification rates in the preheated samples led to coarser arm spacing; therefore, a lower cooling rate. Despite the results predicted by the micro and meso-scale models, lower cooling rates resulted in increased hot cracks. The cracks could grow in the weld metal after solidification; therefore, hot cracks were larger than predicted by the hot crack prediction models.     

Effect of nonmetallic inclusions on localized corrosion of spring steel

Certain inclusions in high-strength 60 Si2Mn–Cr spring steel result in poor resistance to localized corrosion. In this work, to study the effect of inclusions on the localized corrosion behavior of spring steel, accelerated corrosion tests were performed by immersing spring steel in 3 wt% FeCl_3 solution for different times. The results show that severe corrosion occurred in areas of clustered Ca S inclusions. Sulfide inclusions containing Ca and Mg induced the strongest localized corrosion susceptibility. For the case of(Ca,Mn,Mg)S inclusions, the ability to induce localized corrosion susceptibility is ranked as follows: Mg S Ca S Mn S. Moreover, Ca S,(Ca,Mn)S, and(Ca,Mn,Mg)S inclusions were mainly responsible for inducing environmental embrittlement.     

Hot deformation behavior of 51.1Zr–40.2Ti–4.5Al–4.2V alloy in the single β phase field

The hot deformation behavior of a newly developed 51.1Zr–40.2Ti–4.5Al–4.2 V alloy was investigated by compression tests in the deformation temperature range from 800 to 1050 ℃ and strain rate range from 10-3to 100 s-1. At low temperatures and high strain rates, the flow curves exhibited a pronounced stress drop at the very beginning of deformation, followed by a slow decrease in flow stress with increasing strain. The magnitude of the stress drop increased with decreasing deformation temperature and increasing strain rate. At high temperatures and low strain rates, the flow curves exhibited typical characteristics of dynamic recrystallization. A hyperbolic-sine Arrhenius-type equation was used to characterize the dependences of the flow stress on deformation temperature and strain rate. The activation energy for hot deformation decreased slightly with increasing strain and then tended to be a constant value. A microstructural mechanism map was presented to help visualize the microstructure of this alloy under different deformation conditions.