Electrochemical investigation of the influence of laser surface melting on the microstructure and corrosion behaviour of ZE41 magnesium alloy – An EIS based study

Surface melting of a magnesium alloy, ZE41 (4%-Zn, 1%-RE) was performed to achieve electrochemical homogeneity at the surface by microstructure refinement. Large secondary precipitates are particularly known to cause severe pitting in magnesium alloys. The corrosion resistance of the laser treated and untreated alloy was investigated by potentiodynamic polarisation and electrochemical impedance spectroscopy. Contrary to the reported behaviour of other magnesium alloys (such as AZ series alloys), laser surface melting did not significantly improve the corrosion resistance of ZE41. This observation is attributed to the absence of beneficial alloying elements such as Al in ZE41 alloy.     

Influence of pH and chloride ion concentration on the corrosion of Mg alloy ZE41

The influence of pH and chloride ion concentration on the corrosion behaviour of ZE41 was studied using immersion tests and electrochemical measurements. A shorter incubation period to the onset of corrosion; a more negative corrosion potential; and a higher corrosion rate correlated with a higher chloride ion concentration at each pH value and correlated with a lower pH value for each chloride ion concentration. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by a partially protective surface film, with the corrosion reactions occurring predominantly at the breaks or imperfections of the partially protective film. The implication is that the fraction of film free surface increases with decreasing bulk pH and with increasing chloride ion concentration. This is consistent with the known tendency of chloride ions to cause film breakdown and the known instability of Mg(OH)₂ in solutions with pH less than 10.5. The electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.     

Vanadia-based coatings of self-repairing functionality for advanced magnesium Elektron ZE41 Mg–Zn–rare earth alloy

A smart vanadia protective coating of self-repairing functionality that has proven to provide superior corrosion resistance for several magnesium and aluminum alloys has successfully been designed by our group. A newly developed series of magnesium alloys, namely ZE41 alloy, has recently been proposed for automotive, electronics and aerospace applications. The advanced ZE41 alloy possesses very low density, high specific strength, and good castability and weldability characteristics compared to aluminum and steel based alloys. However, the corrosion resistance of ZE41 alloy in the presence of corrosive chloride environment is relatively low. The possibility of utilizing such coatings to add self-repairing functionalities to ZE41 alloy was discussed in this paper. The electrochemical corrosion behavior of the vanadia coatings over ZE41 alloy was investigated in 3.5% NaCl solution using EIS, linear polarization and cyclic voltammetry techniques. The optimum conditions for obtaining protective vanadia coatings of self-repairing abilities and improved localized corrosion resistance were determined. Surface examination of the coatings was investigated using SEM-EDS and macroscopic imaging.     

用热加工图研究铸态和挤压态ZE41A镁合金的热加工性能

采用热加工图研究铸态和挤压态ZE41A镁合金的热变形行为和组织演变。在温度250-450℃,应变速率0.001-1.0s-1的条件下,对铸态和挤压态合金进行抗压测试,建立热加工图。通过显微组织观察确定动态再结晶和不稳定区域。挤压态ZE41A镁合金比铸态合金具有较高的流变应力,较高的能量损耗率和较小的不稳定区域。由于晶粒的细化、材料孔隙度的降低、硬化和强化,挤压态镁合金具有良好的热加工性能。     

Corrosion behaviour of thermally sprayed Al and Al/SiCp composite coatings on ZE41 magnesium alloy in chloride medium

Thermally sprayed Al and Al/SiCp composite coatings have been deposited on ZE41 magnesium alloy and mechanical compaction at room temperature was applied to the Al and Al/SiCp coatings to reduce their porosity. Corrosion behaviour of coated samples was evaluated and compared to that of uncoated substrate in 3.5 wt.% NaCl solution using electrochemical measurements. Al and Al/SiCp composite coatings reduced the corrosion current density of Mg-Zn alloys by three and two orders of magnitude, respectively, and reductions up to four orders of magnitude were obtained after mechanical compaction.     

Sol-gel silica coatings on ZE41 magnesium alloy for corrosion protection

Silica coatings have been applied on the surface of ZE41 magnesium alloy following the organic sol-gel route and the dip-coating technique. Three different concentrations of sol solution and two densification temperatures of the coating (400 °C and 500 °C) were used to optimize the compaction of the coatings and as a result reach the corrosion protection of the metallic substrate tests in 3.5 wt.% NaCl aqueous solution. Crack-free coatings with thickness in the 2-3 μm were obtained on the ZE41 magnesium alloy. The combination of high alkoxide concentration in the sol-gel formulation, and the high sintering temperature (500 °C) leads to coating (D500) with the optimal physical barrier against the corrosion process. This coating was capable of resisting more than 7 days in contact with the aggressive electrolyte suffering minor corrosion degradation. A corrosion mechanism for each of the tested specimens has been proposed.     

An irreversible dipping sealing technique for anodized ZE41 Mg alloy

Anodized coatings are porous and sealing has been considered to be an essential step for anodized magnesium alloys. In this study, a new sealing technique was proposed and validated. Anodized ZE41 was dipped in an E-coating bath solution for a few seconds. After baking, the specimen was found to have considerably improved corrosion resistance. The corrosion rate and polarization current densities were found to be more than 3 orders of magnitude lower and the film breakdown potential over 50 mV higher for the anodized ZE41 after sealing in the E-coating bath solution than for the as-anodized ZE41. An irreversible electroless E-coating deposition model and corrosion mitigation mechanism for a sealed specimen are proposed.     

The corrosion performance of anodised magnesium alloys

The corrosion performance of anodised magnesium and its alloys, such as commercial purity magnesium (CP-Mg) and high-purity magnesium (HP-Mg) ingots, magnesium alloy ingots of MEZ, ZE41, AM60 and AZ91D and diecast AM60 (AM60-DC) and AZ91D (AZ91D-DC) plates, was evaluated by salt spray and salt immersion testing. The corrosion resistance was in the sequential order: AZ91D ≈ AM60 ≈ MEZ ? AZ91D-DC ? AM60-DC > HP-Mg > ZE41 > CP-Mg. It was concluded the corrosion resistance of an anodised magnesium alloy was determined by the corrosion performance of the substrate alloy due to the porous coating formed on the substrate alloy acting as a simple corrosion barrier.     

Enhancement of resistance to galvanic corrosion of ZE41 Mg alloy by equal channel angular pressing

The galvanic corrosion behavior of as-received and ECAPed ZE41 Mg alloy coupled with Al7075 alloy is investigated using zero resistance ammeter in three different corrosive environments, 0, 0.1, and 1 M NaCl, to mimic the conditions experienced in engineering applications. The mechanism of galvanic corrosion for the ZE41 Mg–Al7075 aluminum alloy is explained. It is observed that a robust surface film containing a composite layer of oxide/hydroxide of magnesium and aluminum is established in deionized water (0 M). However, only a single layer of magnesium oxide/hydroxide is detected in chloride-containing environments. Equal channel angular pressing (ECAP) improved the resistance to galvanic corrosion by 58% and 54% when compared with the as-cast counterparts in 0 and 1 M NaCl solution, respectively. In contrast, galvanic corrosion resistance decreased by 26% in 0.1 M NaCl after ECAP while the as-received samples evinced pits unfavorable to be used in engineering applications. ECAP is a promising method to combat galvanic corrosion encountered by ZE41 magnesium alloy used in automobiles and components of military vehicles.     

Electrochemical Performance of AZ31 Magnesium Alloy under Different Processing Conditions

为选择一种高性价比的镁电池阳极材料,借助电化学工作站、光学显微镜(OM)、扫描电子显微镜(SEM)和X射线能谱分析(EDS)对不同加工状态的AZ31B镁合金电化学性能进行研究。分别将挤压、轧制、铸轧和铸态AZ31B镁合金作为阳极材料,测试其电化学性能。结果表明,在4种加工状态下,挤压态镁合金是性价比最高的一种阳极材料;其组织由均匀细小的晶粒和第二相组成,拥有最负的平衡电位,最低腐蚀电流密度和最小自腐蚀速率;挤压态样品腐蚀后,表面产生疏松细小,且均匀分布的腐蚀产物,降低了阳极极化,增加了阳极利用率。轧制和铸轧态的AZ31B镁合金的电化学活性和耐蚀性能相对挤压态的较低。铸态AZ31B镁合金由于较粗大的晶粒、第二相和铸造缺陷,表现出不稳定的放电曲线和较正的放电电位。不同状态AZ31镁合金的腐蚀均以点蚀为主。     

Newly developed magnesium alloys for powertrain applications

Several new magnesium alloys have been developed recently for high-temperature applications to obtain an optimal combination of die cast-ability, creep resistance, mechanical properties, corrosion performance, and affordability. Unfortunately, it is difficult to achieve an adequate combination of properties and, in fact, most of the new alloys can only partially meet the required performance and cost. The ZE41 alloy, which is used for most gravity-casting applications, has moderate strength and creep resistance combined with good cast-ability. Although this alloy exhibits poor corrosion resistance, it is still preferred for certain applications. For more information, contact E. Aghion, Dead Sea Magnesium Ltd., P.O. Box 1195, Beer Sheva 84111, Israel; +972-8-628-2422; fax +972-8-628-2431; e-mail eli@dsmag.co.il.     

Plasma anodized ZE41 magnesium alloy sealed with hybrid epoxy-silane coating

Anodic coatings on magnesium ZE41 alloy were formed by DC plasma electrolytic oxidation (PEO) in spark regime in solution composed of NaOH, Na₂SiO₃ and KF. The positive effect of poly(ethylene oxide) addition into the anodizing electrolyte on PEO process, anodic film porosity and its protective performance was described. Anodic films were sealed with hybrid epoxy-silane formulation. The corrosion behavior of the coated ZE41 was studied through electrochemical impedance spectroscopy (EIS) in 0.6 M NaCl solution. Resulting duplex PEO/epoxy-silane coating provides good protective performance without significant signs of corrosion during 1 month of immersion test.     

The effect of fluoride on the electrochemical behavior of Ti and some of its alloys for dental applications

Titanium is the best metal for making dental implants and restorations. In the last decade, new titanium alloys have been developed in different areas of dentistry. Concurrently, treatments using fluoride supplementation, such as odontology fluoride containing gels, have also been widely used in odontology. The aim of this study is to investigate the electrochemical behaviour of a new titanium alloy containing Cu and Ag, in fluoride‐containing media, and compare it with the behavior of Ti and Ti6Al4V, which are used frequently as biomaterials. Open circuit potential, polarization resistance and electrochemical impedance spectroscopy measurements revealed that the corrosion resistance of titanium and its alloys is controlled by the fluoride ion concentration and the pH of the solution. The presence of F^? ions in neutral solution does not hinder the formation of a protective layer of Ti and its alloys. Thus, the corrosion resistance of Ti is maintained in this medium. However, the corrosion of Ti and its alloys are enhanced in an acidic environment, because F^? ions in the solution combines with H⁺ ion to form HF, even in low fluoride concentration.     

Electrochemical behaviors of magnesium alloy with phosphate conversion coating in NaCl solutions

A composite conversion coating was prepared on magnesmm alloy by the only one-step immersion treatment.The characteristics of the conversion coating were investigated by scanning electron microscopy(SEM)and X-ray photoelectron spectroscopy(XPS).The results indicate that the composite conversion coating consists of magnesmm hydroxide,magnesmm phosphate and manganese phosphate.The electrochemical behavior of the conversion coating was investigated systematically by electrochemical impedance spectroscopy(EIS) and potentiodynamic polarization measurement in different NaCl solutions.Polarization measurements and EIS results reveal that the magnesium alloy with the conversion coating have better corrosion resistance compared to the bare magnesium alloy in these conditions.And the corrosion rate of the magnesium alloy with conversion coating increases consistently with the chloride ion concentration.In alkaline conditions,the magnesium alloy with conversion coating has superior corrosion resistance by the synergistic effects between Mg(OH)_2 film and conversion coating.Moreover,the electrochemical corrosion mechanism of the magnesium alloy was analyzed with respect to the conversion coating in a Cl~- containing environment.     

Corrosion and corrosion testing of magnesium alloys

The corrosion behaviour of magnesium alloys is not substantially comparable to other metals, such as iron, nickel and copper. It is always accompanied by hydrogen evolution. More hydrogen is evolved at a more positive potential or a higher anodic current density. The ‘strange’ hydrogen evolution behaviour is a common phenomenon for magnesium alloys and it is called negative difference effect (NDE). The NDE continues to receive considerable discussion. Furthermore, the corrosion behaviour of magnesium alloys depends mainly on the pH value of the surrounding electrolyte. Voluminous reaction products, formed in neutral electrolytes, lead to a diffusion‐controlled dissolution on the surface of the underlying magnesium alloy. Therefore, influences from structure and alloying are suppressed very strongly. In alkaline environments, passivation occurs as a result of the formation of a hydroxide layer on the magnesium surface. Therefore, differences in the corrosion behaviour between the alloys are hardly detectable. Measurable effects can only be detected using very ‘aggressive’ corrosion conditions. Present methods do not adequately take into account the specific character of the corrosion of magnesium alloys. It can be better characterized using a rotating disc electrode for electrochemical measurements, which enables model defined flow conditions on the surface. Furthermore, the application of electrochemical noise offers the possibility of a simple and sensitive assessment of the corrosion susceptibility of magnesium alloys. Due to the high sensitivity of this measurement procedure, it is also possible to carry out examinations under more practical conditions.     

Comparison of corrosion behaviors of AZ31, AZ91, AM60 and ZK60 magnesium alloys

The corrosion behaviours of four kinds of rolled magnesium alloys of AZ31, AZ91, AM60 and ZK60 were studied in 1 mol/L sodium chloride solution. The results of EIS and potentiodynamic polarization show that the corrosion resistance of the four materials is ranked as ZK60＞AM60＞AZ31＞AZ91. The corrosion processes of the four magnesium alloys were also analyzed by SEM and energy dispersive spectroscopy(EDS). The results show that the corrosion patterns of the four alloys are localized corrosion and the galvanic couples formed by the second phase particles and the matrix are the main source of the localized corrosion of magnesium alloys. The corrosion resistance of the different magnesium alloys has direct relationship with the concentration of alloying elements and microstructure of magnesium alloys. The ratio of the β phase in AZ91 is higher than that in AZ31 and the β phase can form micro-galvanic cell with the alloy matrix, as a result, the corrosion resistance of AZ31 will be higher than AZ91. The manganese element in AM60 magnesium alloy can form the second phase particle of AlMnFe, which can reduce the Fe content in magnesium alloy matrix, purifying the microstructure of alloy, as a result, the corrosion resistance of AM60 is improved. However, due to the more noble galvanic couples of AlMnFe and matrix, the microscopic corrosion morphology of AM60 is more localized. The zirconium element in ZK60 magnesium alloy can refine grain, form stable compounds with Fe and Si, and purify the composition of alloy, which results in the good corrosion resistance of ZK60 magnesium alloy.     

镁合金材料表面处理技术研究新动态

对镁合金材料近年来在表面微弧氧化、表面超疏水膜层、激光表面改性以及溶胶-凝胶涂层四个方面的研究动态进行了简要综述。镁合金材料采用双极性和混合(单极和双极的组合)电流模式微弧氧化处理的膜层生长速率较快,膜层更致密且硬度更高,膜层的耐磨性和耐腐蚀性能更好。在高浓度苛性碱为主的强碱性溶液中添加适量的添加剂,经短时间(~3 min)微弧氧化处理,即可获得中性盐雾试验达200 h以上的致密耐腐蚀膜层。采用水热法、电化学刻蚀、微弧氧化和电沉积等方法,可在镁合金材料表面形成具有微纳米多级结构的粗糙表面,再用低表面能物质对粗糙表面进行修饰,可在镁合金表面获得超疏水膜层,从而提高镁合金的耐腐蚀性能。镁合金材料激光表面改性处理可改善其表面成分,细化晶粒,使组成相分布更均匀以及提高表层的固溶度极限,从而提高镁合金材料的耐腐性能、摩擦磨损抗力和疲劳强度。溶胶-凝胶有机/无机杂化涂层与镁合金基材良好的附着力,不仅可提高镁合金的耐腐蚀性能,还可以使镁合金具有抗氧化、耐磨损、防水性以及其他性能。     

镁合金在碳酸饮料中的降解与人体补镁初探

提出了通过镁产品在饮用水中的镁离子释放作为人体补镁手段的新思路,分析了镁及其合金在碳酸饮料中的降解机理、溶液镁离子浓度和pH值变化规律,探讨了镁合金产品作为人体补镁来源的可行性。结果表明,Mg的降解速度最快,AZ31次之,AZ91最低,表面均形成了抑制降解的(Ca,Mg)CO3腐蚀层;铝元素可有效降低镁的释放速度和碳酸饮料的pH值增幅;3种镁合金在容量为600 mL的碳酸饮料中浸泡2～4 h所释放的镁离子基本能满足每日人体所需补镁量,此时pH值变化较小,对碳酸饮料的品质无明显影响。     

Formation and properties of stannate conversion coatings on AZ61 magnesium alloys

The effects of solution composition and temperature on the microstructure and corrosion resistance of stannate conversion coatings on AZ61 magnesium alloys were investigated. The conversion coating consisted of a porous layer as under layer intimately contacted with the magnesium plate and a hemispherical particle layer as major overlay formed right on top of the porous layer. During the coalescence of the hemispherical particles to form a complete coating on the magnesium alloy, some sites of discontinuity inevitably left and determined the corrosion resistance of the coating evaluated using a salt spray test. Increasing bath stannate ion concentration and lowering bath pH increased the population density of the hemispherical particles whose size was accordingly reduced. The corrosion resistance of the conversion coating was improved with finer particles, which were preferably formed at less alkaline solution with higher stannate ion contents. Furthermore, the conditions favoring the formation of finer particles also reduced the immersion time necessary for producing the conversion coating with optimal corrosion resistance.