GW93镁合金点蚀过程的原位监测及点蚀机制

镁合金作为最轻的金属结构材料有很多优异性能,但镁自身化学性质活泼,耐蚀性差,尤其易发生点蚀,破坏性和隐患性非常大。若想降低点蚀对镁合金部件安全服役性能的影响,就需要对镁合金点蚀机制有清楚的认识。然而,适用于其他金属材料的经典的点蚀机制是以形成氧浓差电池为基础,阴极发生的是氧还原反应,而镁合金阴极发生的是析氢反应,因此镁合金的点蚀形成过程尚需深入研究。采用扫描振动电极技术(SVET)原位监测了铸态GW93镁合金在3. 5%NaCl(质量分数)溶液中的点蚀过程,采用SEM观察了腐蚀过程镁合金微观形貌变化,采用电流-时间曲线对比了阴阳极电位对点蚀发展的影响。研究结果表明,点蚀坑外是微阴极,发生析氢反应,点蚀坑内是微阳极,发生镁的溶解反应,随着时间增加,点蚀发展过程是动态变化的。镁合金中第二相所导致的微电偶腐蚀加速效应及氯离子在腐蚀坑内的聚集,两者的协同作用驱动了点蚀不断向基体内部生长。     

Effect of minor content of Gd on the mechanical and degradable properties of as-cast Mg-2Zn-xGd-0.5Zr alloys

RE-containing Mg alloys used as biodegradable medical implants exhibit good promising application due to their good mechanical properties and degradation resistance. In this work, effect of Gd on the microstructure, mechanical properties and biodegradation of as-cast Mg-2Zn-xGd-0.5Zr alloys was investigated. The results showed that there were mainly α-Mg, I-phase, W-phase and MgZn2 phase in Mg-Zn-Gd-Zr alloys. With increase of the Gd content, the strength of the alloys was enhanced due to the second phase strengthening and grain refinement. The degradation resistance of Mg-2Zn-0.5Zr alloy was increased by adding 0.5%–1% Gd due to the uniformly distributed second phases which acted as a barrier to prevent the pitting corrosion. However, increasing Gd content to 2% reduced the degradation resistance of the alloy due to the galvanic corrosion between the matrix and the second phases.The good degradation resistance and mechanical properties of as-cast Mg-2Zn-1Gd-0.5Zr alloy makes it outstanding for biomaterial application.     

T6处理对AZ63镁合金在模拟海水中耐蚀性的影响

为研究热处理工艺对合金耐腐蚀性的影响,采用X射线衍射仪(XRD)、扫描电镜(SEM)、析氢失重以及多种电化学测试等方法研究了经T6处理(固溶处理+人工时效)的AZ63镁合金在模拟海水中的腐蚀行为。结果表明:T6处理(固溶处理+人工时效)镁合金第二相沿晶界呈网状分布,与铸态镁合金相比氢气析出多、腐蚀速度快,并有点蚀产生;T6处理(固溶处理+人工时效)改变了镁合金的组织结构以及分布,增大了稳定相第二相与基体相的接触,降低了合金的耐蚀性。     

Dealloying corrosion of anodic and nanometric Mg41Nd5 in solid solution-treated Mg-3Nd-1Li-0.2Zn alloy

The microstructure and chemical compositions of the solid solution-treated Mg-3Nd-1Li-0.2Zn alloy were characterized using optical microscope,scanning electron microscope(SEM),transmission electron microscope(TEM),electron probe micro-analyzer(EPMA)and X-ray photoelectron spectroscopy(XPS).The corrosion behaviour of the alloy was investigated via electrochemical polarization,electrochemical impedance spectroscopy(EIS),hydrogen evolution test and scanning Kelvin probe(SKP).The results showed that the microstructure of the as-extruded Mg-3Nd-1Li-0.2Zn alloy contained α-Mg matrix and nanometric second phase Mg41 Nd5.The grain size of the alloy increased significantly with the increase in the heat-treatment duration,whereas the volume fraction of the second phase decreased after the solid solution treatment.The surface film was composed of oxides(Nd2O3,MgO,Li2O and ZnO)and carbonates(MgCO3 and Li2CO3),in addition to Nd.The as-extruded alloy exhibited the best corrosion resistance after an initial soaking of 10 min,whereas the alloy with 4h-solution-treatment possessed the lowest corrosion rate after a longer immersion(1 h).This can be attributed to the formation of Nd-containing oxide film on the alloys and a dense corrosion product layer.The dealloying corrosion of the second phase was related to the anodic Mg41Nd5 with a more negative Volta potential relative to α-Mg phase.The preferential corrosion of Mg41Nd5 is proven by in-situ observation and SEM.The solid solution treatment of Mg-3Nd-1Li-0.2Zn alloy led to a shift in corrosion type from pitting corrosion to uniform corrosion under long-term exposure.     

An additively manufactured Al-14.1Mg-0.47Si-0.31Sc-0.17Zr alloy with high specific strength,good thermal stability and excellent corrosion resistance

A novel Al-14.1 Mg-0.47 Si-0.31 Sc-0.17 Zr alloy was applied in the printing process of selective laser melting(SLM),and the corresponding microstructural feature,phase identification,tensile properties and corrosion behavior of the Al Mg Si Sc Zr alloy were studied in detail.As fabricated at 160 W and 200 mm/s,the Mg content of bulk sample decreased to 11.7 wt%due to the element vaporization at high energy density,and the density of this additively manufactured Al Mg Si Sc Zr alloy was 2.538 g/cm³,which is4.2%8.5%lighter than that of other SLM-processed Al alloys.After heat-treated(HT)at 325℃and 6 h,the microstructure was almost unchanged with an alternate distribution of fine equiaxed crystals and coarse columnar crystals.Nano-sized Al3(Sc,Zr)and Mg₂Si phases precipitated dispersedly in the Al matrix,and the tensile strength increased from 487.6 MPa to 578.4 MPa for precipitation strengthening and fine grain strengthening.With a fine grain size of 2.53μm,an excellent corrosion resistance was obtained for the as-printed(AP)Al Mg Si Sc Zr alloy.While the corrosion resistance of HT sample decreased slightly for the formation of non-dense oxide layer and pitting corrosion induced by diffuse precipitation distribution.This SLM-printed Al Mg Si Sc Zr alloy with high specific strength,good thermal stability and excellent corrosion resistance has broad prospects for the aerospace and automotive applications.     

Initial stages of corrosion within Mg-Zn-Y-Zr alloy in 1 g/l NaCl solution

Corrosion behavior of as-cast and heat treated Mg-Zn-Zr-Y alloy was studied in 1 g/l NaCl solution. The as-cast alloy was the least resistant to pitting corrosion. Its Mg-Zn solid solution matrix was the structural constituent responsible for corrosion susceptibility due to the presence of Zn-lean regions. Homogenization of the Zn distribution within the matrix significantly increased the corrosion resistance of the alloy. Grain boundaries, which contain intermetallic phases, were resistant to corrosion attack, thus impeding corrosion propagation.     

New insights into the effect of Tris-HCl and Tris on corrosion of magnesium alloy in presence of bicarbonate,sulfate,hydrogen phosphate and dihydrogen phosphate ions

In vitro degradation is an important approach to screening appropriate biomedical magnesium(Mg) alloys at low cost. However, corrosion products deposited on Mg alloys exert a critical impact on corrosion resistance. There are no acceptable criteria on the evaluation on degradation rate of Mg alloys. Understanding the degradation behavior of Mg alloys in presence of Tris buffer is necessary. An investigation was made to compare the influence of Tris-HCl and Tris on the corrosion behavior of Mg alloy AZ31 in the presence of various anions of simulated body fluids via hydrogen evolution, p H value and electrochemical tests.The results demonstrated that the Tris-HCl buffer resulted in general corrosion due to the inhibition of the formation of corrosion products and thus increased the corrosion rate of the AZ31 alloy. Whereas Tris gave rise to pitting corrosion or general corrosion due to the fact that the hydrolysis of the amino-group of Tris led to an increase in solution p H value, and promoted the formation of corrosion products and thus a significant reduction in corrosion rate. In addition, the corrosion mechanisms in the presence of Tris-HCl and Tris were proposed. Tris-HCl as a buffer prevented the formation of precipitates of HCO_3~-, SO_4~(2-),HPO_4~(2-) and H_2PO_4~- ions during the corrosion of the AZ31 alloy due to its lower buffering p H value(x.x).Thus, both the hydrogen evolution rate and corrosion current density of the alloy were approximately one order of magnitude higher in presence of Tris-HCl than Tris and Tris-free saline solutions.     

时效态Mg-Gd-Y-Nd合金的组织结构及耐腐蚀性能

镁合金中加入Gd、Y、Nd等元素能提高其耐蚀性能,但目前对同时加入3种元素的情况研究较少。采用熔炼法制备了Mg-10Gd-3Y-Nd合金,并对其进行时效处理。利用金相显微镜、扫描电镜、X射线衍射仪等研究了时效态Mg-10Gd-3Y-Nd合金的微观组织及物相。通过静态失重法研究了时效态合金在不同浓度NaCl溶液中的耐腐蚀性能。结果表明:时效态合金主要由基体α-Mg和晶间共晶相(Mg_5Gd、Mg_(24)Y_5)组成,Gd能够良好地固溶于镁合金基体中;随着NaCl溶液浓度的增加,时效态合金的腐蚀速率增加,腐蚀深坑严重,颗粒状腐蚀物逐渐转为粉状;第二相颗粒的存在能阻止腐蚀过程向更深的晶粒区域发展,降低了腐蚀程度,提高了合金的耐腐蚀性。     

Evaluating the improvement of corrosion residual strength by adding 1.0 wt.% yttrium into an AZ91D magnesium alloy

The influence of yttrium on the corrosion residual strength of an AZ91D magnesium alloy was investigated detailedly. Scanning electron microscope was employed to analyze the microstructure and the fractography of the studied alloys. The microstructure of AZ91D magnesium alloy is remarkably refined due to the addition of yttrium. The electrochemical potentiodynamic polarization curve of the studied alloy was performed with a CHI 660b electrochemical station in the three-electrode system. The result reveals that yttrium significantly promotes the overall corrosion resistance of AZ91D magnesium alloy by suppressing the cathodic reaction in corrosion process. However, the nucleation and propagation of corrosion pits on the surface of the 1.0 wt.% Y modified AZ91D magnesium alloy indicate that pitting corrosion still emerges after the addition of yttrium. Furthermore, stress concentration caused by corrosion pits should be responsible for the drop of corrosion residual strength although the addition of yttrium remarkably weakens the effect of stress concentration at the tip of corrosion pits in loading process.     

Corrosion mechanism applicable to biodegradable magnesium implants

Much of our understanding of the Mg corrosion mechanism is based on research using aggressive chloride based solutions like 3% NaCl, which are appropriate for understand the corrosion for applications such as auto construction. The chloride ions tend to cause break down of the partly protective surface film on the Mg alloy surface. The corrosion rate increases with exposure time until steady state is reached, which may take several weeks. An overview is provided of the aspects which determine the corrosion of Mg alloys: (i) measurement details; (ii) impurity elements Fe, Ni, Cu and Co; (iii) second phases; (iv) surface films and surface condition and (v) stress corrosion cracking (SCC). This understanding is used to help understand Mg corrosion for Mg as a biodegradable implant for medical applications. Solutions that elucidate these applications tend to form surface films and the corrosion rate tends to decrease with immersion time.     

In vitro corrosion of magnesium alloy AZ31 --- a synergetic influence of glucose and Tris

Biodegradable Mg alloys have generated great interest for biomedical applications. Accurate predictions of in vivo degradation of Mg alloys through cost-effective in vitro evaluations require the latter to be conducted in an environment close to that of physiological scenarios. However, the roles of glucose and buffering agents in regulating the in vitro degradation performance of Mg alloys has not been elucidated. Herein, degradation behavior of AZ31 alloy is investigated by hydrogen evolution measurements, pH monitoring and electrochemical tests. Results indicate that glucose plays a content-dependent role in degradation of AZ31 alloy in buffer-free saline solution. The presence of a low concentration of glucose, i.e. 1.0 g/L, decreases the corrosion rate of Mg alloy AZ31, whereas the presence of 2.0 and 3.0 g/L glucose accelerates the corrosion rate during long term immersion in saline solution. In terms of Tris-buffered saline solution, the addition of glucose increases pH value and promotes pitting corrosion or general corrosion of AZ31 alloy. This study provides a novel perspective to understand the bio-corrosion of Mg alloys in buffering agents and glucose containing solutions.     

Microstructure and Properties of As-Cast Mg–2.0Sn–1.0Zn–1.0Y–0.3Zr Alloys at Different Extrusion Ratios

Herein, the effect of hot extrusion with different extrusion ratios (λ = 6, 8, 10, and 12) on the microstructure evolution and properties of as-cast Mg–2.0Sn–1.0Zn–1.0Y–0.3Zr magnesium alloys, using optical microscopy (OM), scanning electron microscopy (SEM), immersion corrosion and electrochemical corrosion experiment, and tensile testing, is investigated. The results show that the Mg₁₄SnY and Mg₆SnY precipitated phases exist in the alloy before and after extrusion. After hot extrusion, the second phase of the alloy is broken into particles along the extrusion direction, whereas the grain size is significantly reduced, and dynamic recrystallization and deformed grains exist in the microstructure. The mechanical properties of the extruded alloy improve, but the corrosion resistance weakens. When the extrusion ratio is λ = 10, the extruded alloy exhibits relatively good mechanical properties and corrosion resistance. The corrosion behaviors of the extruded alloys are affected by both the grain size and galvanic corrosion. In the initial stage of corrosion, intergranular corrosion plays a major role in reducing the corrosion resistance of the extruded alloys. With prolonged corrosion time, galvanic corrosion has a more significant effect on weakening the corrosion resistance of the extruded alloys.     

Structure,mechanical and corrosion properties of extruded Mg-Nd-Zn,Mg-Y-Zn and Mg-Y-Nd alloys

Rare earth elements are known to improve both mechanical and corrosion properties. However, it highly depends on the final microstructure conditions of prepared material. During extrusion, intermetallic phases may be redistributed, partially dissolved or on the contrary, precipitated. The knowledge of the impact of extrusion on the individual alloys is therefore essential for their application. In this work, three magnesium alloys (Mg-4Y-3RE, Mg-2Y-1Zn, Mg-3Nd-0.5Zn) were prepared by an extrusion process. Microstructure, mechanical and corrosion properties were compared with extruded pure Mg. The advantages and disadvantages of individual alloys are discussed. Based on the obtained results, the Mg-4Y-3RE alloy seems to exert the best mechanical and corrosion properties. Other materials were characterised with anisotropy of mechanical properties and much higher corrosion rate.     

<Emphasis Type="Italic">In vitro</Emphasis> corrosion of magnesium alloy AZ31 — a synergetic influence of glucose and Tris

Biodegradable Mg alloys have generated great interest for biomedical applications. Accurate predictions of in vivo degradation of Mg alloys through cost-effective in vivo evaluations require the latter to be conducted in an environment close to that of physiological scenarios. However, the roles of glucose and buffering agents in regulating the in vivo degradation performance of Mg alloys has not been elucidated. Herein, degradation behavior of AZ31 alloy is investigated by hydrogen evolution measurements, pH monitoring and electrochemical tests. Results indicate that glucose plays a content-dependent role in degradation of AZ31 alloy in buffer-free saline solution. The presence of a low concentration of glucose, i.e. 1.0 g/L, decreases the corrosion rate of Mg alloy AZ31, whereas the presence of 2.0 and 3.0 g/L glucose accelerates the corrosion rate during long term immersion in saline solution. In terms of Tris-buffered saline solution, the addition of glucose increases pH value and promotes pitting corrosion or general corrosion of AZ31 alloy. This study provides a novel perspective to understand the bio-corrosion of Mg alloys in buffering agents and glucose containing solutions.     

均匀化对EK31镁合金组织与腐蚀性能的影响

为了研究均匀化热处理对含有稀土元素Er的EK31镁合金组织及其耐蚀性能的影响,本文利用光学显微镜(OM),扫描电镜(SEM),X射线衍射仪(XRD)和化学工作站研究了铸态EK31镁合金在均匀化温度为300,400和500℃,保温时间由0.5 h至8 h热处理后的组织结构和在3.5 wt.%的Na Cl水溶液中的腐蚀行为.结果表明,EK31镁合金主要由基体α-Mg晶粒和β-Zr核相组成.经过300℃×6 h均匀化处理后,合金基体α-Mg中的β-Zr核相溶解扩散,在3.5%Na Cl水溶液中浸泡96 h后的腐蚀速率达到最低,为4.3×10-3mg/(cm2·h);当均匀化温度为400和500℃时,保温时间超过6 h后,在晶界附近富集的稀土元素Er生成Mg24Er5相,使该合金的腐蚀速率上升到1.1×10-2mg/(cm2·h).然而,与其它镁合金相比较,EK31镁合金具有优异的耐蚀性能,其原因为稀土元素Er在晶界附近形成富Er区以及Mg24Er5相的阻碍作用.     

Fabrication and corrosion behavior of HA/Mg-Zn biocomposites

The thermal-treated hydroxyapatite (HA) particles, Mg and Zn powders were used to prepare the HA/Mg-Zn composites with different HA contents by means of powder metallurgy technology. The microstructures, formation phases, and corrosion behaviors in simulated body fluid (SBF) were studied in comparison with pure magnesium and HA/Mg composites fabricated by the same preparation technology. As a result, no evident reaction happened between HA particles and Mg matrix during sintering process, and Zn atoms diffused into Mg matrix to form a single phase Mg-Zn alloy matrix. The addition of HA particles changed the corrosion mechanism of Mg matrix. During the corrosion process, HA particles would adsorb and Ca²⁺ ions efficiently and induce the deposition of Ca-P compounds on the surface of composites. HA could improve the corrosion resistance of magnesium matrix composites in SBF and restrain the increase of pH of SBF. Furthermore, the addition of Zn was favorable to improve the corrosion resistance of HA/Mg composites due to the densification of composites and the formation of Mg-Zn alloy matrix.     

Corrosion behavior of Al–60 vol.% SiCp composites in NaCl solution

In this study, corrosion behavior of pure Al and Al–4 wt.% Mg alloy matrix composites, comprising 60 vol.% SiC particles, has been investigated. Composites were produced by pressure infiltration technique at 750 °C. The corrosion tests were carried out in 3.5 wt.% NaCl environment up to 28 days. The weight loss of the composites increased with increasing duration time up to 3–5 days then remained constant. Scanning electron microscopy (SEM) analysis showed that Al–4 wt.% Mg alloyed matrix composite exhibited higher corrosion resistance than pure Al matrix composite although potentiodynamic polarisation measurements showed higher i_corr values of Al–4 wt.% Mg alloyed matrix composites than pure Al matrix composites. Experimental results revealed that precipitation of Mg₂Si as a result of reaction between Al–Mg alloy and SiC particle has a beneficial effect on corrosion resistance of Al–4Mg alloy matrix composites due to interruption of the continuity of the matrix channels within the pressure infiltrated composites.     

新型Mg-8Li-5Al-5Ca合金的微观组织、力学及耐腐蚀性能

设计了新型高钙铝比Mg-8Li-5Al-5Ca合金,通过常温拉伸、失重法、pH测定和电化学测试等方法研究了合金的常温力学性能和耐腐蚀性能。采用扫描电镜(SEM)和X射线衍射(XRD)分析了基体和腐蚀产物相结构、合金显微组织以及腐蚀形貌。研究结果表明,这种镁锂合金形成Al2Ca相包围双基体(α-Mg+β-Li)的结构,挤压后基体组织和第二相粒子均明显细化。Mg-8Li-5Al-5Ca合金的耐腐蚀性能优于一般镁锂合金,且随着挤压比的增大进一步提升。该合金的力学性能协调了镁锂合金的优良塑性和高钙铝比镁合金的高强度,拥有较高的抗拉强度(222 MPa)和延伸率(8.3%)。     

具有低自腐蚀电位的AlMgZnSnPbCuMnNi高熵合金的制备及其电化学性能

设计了一类AlMgZnSnPbCuMnNi系列等摩尔比合金,采用大气造渣保护熔炼方法制备了合金铸锭,铸锭尺寸为Φ10×60mm。采用电化学工作站测定了所制备的等摩尔比合金的电化学性能,采用XRD和SEM分析了合金微观结构。研究表明,从AlMg合金到AlMgZnSnPbCuMnNi合金,随组元数的增加,高熵合金的自腐蚀电位大致呈现先下降后上升的趋势,而自腐蚀电流密度呈现先上升后下降的趋势。在AlMgZnSnPbCuMnNi高熵合金中,尽管AlMgZn的含量远小于传统的牺牲阳极材料,但自腐蚀电位仍低达-1.33V,自腐蚀电流密度也很低,为3.0×10-5A/cm2,均低于目前使用的铝合金与锌合金牺牲阳极材料,为进一步研究开发阳极材料提供了数据支持。