Corrosion of heat treated magnesium alloy ZE41

This paper investigates the effect of heat treatment upon the corrosion morphology and mechanism of ZE41 alloy. The results of optical and scanning electron microscopy (SEM) together with potentiodynamic polarisation reveal the importance of the microstructure in the initiation and propagation of corrosion in an aqueous environment. The corrosion of the heat-treated alloy is significantly altered due to changes in the microstructure, specifically the Zr-rich regions and the grain boundary T-phase.     

Compositional effects of Zr-rich multi-component brazing alloys on the corrosion of Zr alloy joints

High-temperature corrosion of Zircaloy-4 joints brazed by various Zr(Ti)–Cu–Ni-based multi-component alloys was studied to draw up the compositional guideline of the brazing alloy. From the compositional and microstructural effects of the joints on the corrosion, there was strong evidence for galvanic corrosion susceptibility of primary α-Zr grains (usually Sn-containing) owing to alloying of nobler Ti and its concentration gradient in a joint, inducing a microgalvanic corrosion. The Ti concentration for corrosion inhibition was proposed to be less than about 1.0 at.%. The results clearly demonstrate that the exclusion of Ti is needed for the use of Zr-rich multi-component brazing alloys.     

Pitting corrosion of spray formed Al-Li-Mg alloys

The pitting corrosion behaviour of two new spray formed alloys, designated OX24 and OX27, has been compared with spray formed AA7034 and more corrosion resistant AA5083 by polarisation tests. The new alloys have been designed for use in the aerospace industry where good corrosion resistance and specific mechanical properties are required. OX24 and AA5083 had good corrosion resistance; OX27 pitted immediately showing poor corrosion resistance. However, both OX24 and OX27 show better corrosion resistance than AA7034. Scanning electron microscopy showed that corrosion was associated with intermetallic particles, except in OX24 where initiation appeared not to be associated with the Zr-rich intermetallics.     

Cr含量对CrxMoNbTiZr系高熵合金组织与性能的影响

采用真空电弧熔炼的方法制备了Cr_xMoNbTiZr系高熵合金(x=0, 0.5, 1, 1.5)。利用X射线衍射仪(XRD)、扫描电镜(SEM)、能谱仪(EDS)、显微硬度计以及电化学工作站研究了Cr含量对该高熵合金结构、组织、硬度和耐蚀性能的影响。结果表明,Cr的添加使合金由单相BCC结构转变为富Zr相与富Mo-Nb相的双相BCC结构,随着Cr含量增加,在富Zr相中还有富Cr的Laves相析出;Cr_1.5MoNbTiZr合金具有最高硬度765.53 HV,这是由于第二相析出强化、固溶强化与高熵合金晶格畸变的共同作用;Cr的加入增加了Cr_xMoNbTiZr系高熵合金在质量分数为3.5%NaCl溶液中发生腐蚀倾向,但降低了该系高熵合金的腐蚀速率,同时发现Cr的添加存在一个临界值来保证合金的抗点蚀能力,超过这个临界值合金就会更容易发生点蚀现象。     

In situ monitoring of the microstructural corrosion mechanisms of zinc–magnesium–aluminium alloys using time lapse microscopy

A novel technique has been developed that enables in situ monitoring of the microstructural wet corrosion mechanisms of zinc–(1–2 wt.% magnesium)–(1–2 wt.% aluminium) galvanising alloys using time lapse optical microscopy. The technique enabled the imaging of the progression of anodic attack, the development of corrosion product rings radially to the anode and pH gradients between anodes and cathodes using an indicator. It was found that corrosion initiated in the binary and ternary eutectic regions within the microstructure of the alloy with preferential de-alloying of MgZn₂ lamellae. After eutectic dissolution, anodic attack proceeded on the primary zinc rich dendrites.     

Tracking the corrosion of magnesium sand cast AM50 alloy in chloride environments

An experimental procedure for tracking corrosion on lightweight alloys has been developed using a combination of microscopy and corrosion studies using commercial sand cast magnesium AM50 alloys. Corrosion penetration depths were measured and characterized with CLSM and SEM/XEDS, respectively. Corrosion depths on α-grains in the alloys were expressed as a function of their Al content. Al-rich β-phases and eutectic α-phase microstructures were observed to be most corrosion resistant due to an enrichment of Al, identified with TEM, near the oxide/alloy interface. Sand cast alloys were found to be susceptible to major corrosion events in regions with depleted Al content.     

Microbiologically influenced corrosion of ferritic steel–zirconium-based metal waste form alloy under simulated geological repository environment

The present work focused on investigating the microbiologically influenced corrosion (MIC) susceptibility of ferritic steel–Zr-based metal waste form (MWF) alloy in simulated ground water media, cultured with a common biofilm former Bacillus sp. Total viable count studies showed a good bacterial attachment on the surface of MWF alloy. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) examination of the biofilm MWF surface revealed preferential adhesion of microbes on Fe–Zr-rich intermetallic phases. Anodic polarisation and electrochemical impedance spectroscopic (EIS) studies showed active corrosion potential (E_corr), higher passive current density (I_pass) and decreased polarisation resistance (R_p) values, confirming the corrosion susceptibility of MWF alloy under Bacillus sp. biofilm.     

Corrosion of continuous alumina-fibre reinforced Al-2 wt.% Cu-T6 metal-matrix composite in 3.15 wt.% NaCl solution

The corrosion performance of a continuous alumina-fibre reinforced metal-matrix composite (MMC) and its monolithic matrix alloy (Al-2%Cu-T6) was studied in 3.15 wt.% sodium chloride solution. Corrosion initiation sites, mapping of corrosion current density and pH at corrosion sites, mass loss resulting from immersion, and polarization behaviour were studied. Results show that the MMC exhibited inferior corrosion resistance as compared to its monolithic matrix alloy. Corrosion of the MMC initiated preferentially along the fibre/matrix interface or in regions of plastic deformation. The build-up of acidity at localized corrosion sites on the MMC was enhanced by the formation of micro-crevices caused by fibres left in relief as a result of corrosion.     

Microwave-assisted synthesis of lanthanum conversion coating on Mg–Li alloy and its corrosion resistance

Lanthanum-based conversion coating on Mg–Li alloy has been prepared by a microwave-assisted method. X-ray diffractions (XRD) indicate that the intermetallic compounds of lanthanum are formed on Mg–Li alloy surface. Scanning electron microscopy (SEM) images show that the coating has different morphologies and special structures. The corrosion resistance was assessed by means of potentiodynamic polarization curves and electrochemical impedance spectra (EIS). The results indicate that this coating significantly reduces the corrosion rate of Mg–Li alloy in NaCl solution. A comparing experiment indicates that the coating prepared by microwave-assisted process has superior corrosion resistance to the coating obtained at room temperature.     

Electrodeposition of polypyrrole onto NiTi and the corrosion behaviour of the coated alloy

Polypyrrole (PPy) films were electrodeposited onto nickel--titanium alloy (NiTi) employing sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT or AOT) solutions. Polarizing anodically NiTi samples recovered by PPy in a monomer-free solution increases adhesion of the coating. Electrochemical techniques, scanning electron microscopy (SEM) and element analysis were used in determining the corrosion performance of the coated samples in chloride solution. The polymer improves the corrosion performance at the open circuit potential and at potentials where the bare substrate suffers pitting attack. The improvement in both, adhesion and corrosion performance, is discussed considering substrate/polymer interaction, overoxidation of PPy and the role played by AOT.     

Experimental study of electrochemical corrosion behaviour of bilayer on AZ31B Mg alloy

A bilayer on AZ31B magnesium alloy was prepared by first applying anodization to modify the substrate, and then depositing a self-assembled nanophase particle (SANP) film. Structure and corrosion behaviour are characterized by scanning electron microscopy, energy dispersion spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscope and electrochemical impedance spectroscopy. Experimental results indicate that the bilayer consists of a SNAP/loose layer and a dense layer, and provides the corrosion protection of Mg substrate for more than 354 h in 0.005 M NaCl solution. Therefore, the bilayer is effective to protect Mg alloy against corrosion for a long time.     

Study of the corrosion product films formed on the surface of Mg–xZn alloys in NaCl solution

The corrosion product films formed on the surface of Mg–2Zn and Mg–5Zn alloys in NaCl solution were investigated by electrochemical measurement, scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) analysis. It is found that a compact corrosion product film is formed in the initial stage of immersion, and then the film gradually degrades due to dissolution reaction. The product film formed on Mg–2Zn alloy presents better protection property than that on Mg–5Zn alloy, which can be attributed to the different chemical composition and microstructure of the both alloys.     

Effect of second phases on the corrosion behaviour of wrought Mg-Zn-Y-Zr alloy

The corrosion behaviour of wrought Mg-Zn-Y-Zr alloy was investigated by corrosion morphology observation and electrochemical measurement. The results indicate that the corrosion process can be divided into three stages, corresponding to three types of corrosion features. At the initial stage, corrosion occurred surrounding the second phases, which was driven by galvanic couple effect; at the middle stage, filiform corrosion was found in the central regions of α Mg matrix; at the final stage, pitting corrosion presented around the second phases. The second phases have a great effect on the corrosion process of Mg-Zn-Y-Zr alloy.     

Enhanced corrosion protection of MgO coatings on magnesium alloy deposited by an anodic electrodeposition process

MgO coatings were prepared on magnesium alloy surface by an anodic electrodeposition process in concentrated KOH solution followed by heat treatment in air. The phase composition and microstructure of the as-formed MgO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behavior of the MgO-coated samples was evaluated by electrochemical measurements and immersion tests in Hanks’ solution. The results showed that the MgO-coated Mg alloy exhibited a much superior stability and lower corrosion rate, and thus enabled to improve the corrosion resistance, whereas the bare Mg alloy suffered from severely localized corrosion attack.     

Corrosion properties of Nd:YAG laser-GMA hybrid welded AA6061 Al alloy and its microstructure

The paper presents the corrosion behaviour of the Nd:YAG laser-gas metal arc (GMA) welds of AA6061-T6 alloy. The laser-GMA hybrid welding enhances the corrosion susceptibility of AA6061 alloy. The surface morphology observation and composition analysis were investigated by the scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy. An increasing of the precipitate phase is observed in the weld fusion zone (WFZ). The WFZ suffers more severe pitting and cracks are associated with pitting. It is proposed that the increased precipitate phase increases the galvanic corrosion couples and results in the aggravation of pitting and cracking in the WFZ.     

Effect of antimony, bismuth and calcium addition on corrosion and electrochemical behaviour of AZ91 magnesium alloy

This study investigated the effect of antimony, bismuth and calcium addition on the corrosion and electrochemical behaviour of AZ91 magnesium alloy in 3.5% NaCl solution. Techniques including constant immersion, electrochemical potentiodynamic polarisation, scanning electron microscopy (SEM), energy dispersed spectroscopy (EDS) and X-ray diffraction (XRD) were used to characterise electrochemical and corrosion properties and surface topography. It was found that corrosion attack occurred preferentially on Mg₃Bi₂ and Mg₃Sb₂ particles while Mg₁₇Al₈Ca_0.5 and Mg₂Ca phases showed no detrimental effect on corrosion. Combined addition of small amounts of bismuth and antimony to the AZ91 alloy resulted in significant increase in corrosion rate.     

Microstructure and corrosion performance of a cold sprayed aluminium coating on AZ91D magnesium alloy

A pure Al coating was deposited on AZ91D magnesium alloy through cold spray (CS) technique. The microstructure of the coating was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the grain interfaces and subgrains formed close to the particle/particle boundaries. Electrochemical tests revealed that the cold sprayed pure Al coating had better pitting corrosion resistance than bulk pure Al with similar purity in neutral 3.5 wt.% NaCl solution. In addition, a mass-transfer step was found to be involved in the corrosion during 10 days immersion.     

Investigation of the corrosion behaviour of AA 2024-T3 in low concentrated chloride media

Aluminium alloy (AA) 2024-T3 is an important engineering material due to its widespread use in the aerospace industry. However, it is very prone to localized corrosion attack in chloride containing media, which has been mainly associated to the presence of coarse intermetallics (IMs) in its microstructure. In this work the corrosion behaviour of AA 2024-T3 in low concentrated chloride media was investigated using microscopy and electrochemical methods. TEM/EDS observations on non-corroded samples evidenced the heterogeneous composition within the IMs. In addition, SEM observations showed that intermetallics with the same nominal composition present different reactivity, and that both types of coarse IMs normally found in the alloy microstructure are prone to corrosion. Moreover, EDS analyses showed important compositional changes in corroded IMs, evidencing a selective dissolution of their more active constituents, and the onset of an intense oxygen peak, irrespective to the IM nature, indicating the formation of corrosion products. On the other hand, the results of the electrochemical investigations, in accordance with the SEM/EDS observations, evidenced that IMs corrosion dominates the electrochemical response of the alloy during the first hours of immersion in the test electrolyte.     

Corrosion relationships as a function of time and surface roughness on a structural AE44 magnesium alloy

Pit initiation, growth, and coalescence corrosion mechanisms of an AE44 magnesium alloy subjected to a salt-water environment were quantified. Stereological quantities were evaluated using optical microscopy, scanning electron microscopy, and laser beam profilometry. Three corrosion mechanisms clearly arose: pitting, intergranular, and general. Pitting began as the result of localized galvanic dissolution between the intermetallics and magnesium matrix. Intergranular corrosion arose as pits coalesced. General corrosion arose by dissolution and regeneration of a Mg(OH)₂ film at a continuous rate. Stereological quantification demonstrated that the corrosion pit number density and pit radius size distribution initially increased before decreasing due to pit coalescence.     

Anodic dissolution of Al sacrificial anodes in NaCl solution containing Ce

The corrosion behaviour of Al–Zn–In sacrificial anodes has been investigated in a sodium chloride solution containing CeCl₃. Scanning electron microscopy, energy-dispersive X-ray analysis, and inductively coupled plasma mass spectrometry have been employed to gain knowledge of the micro-morphology and corrosion process of the Al alloy. Cerium, both as the alloy element and as the additive in the NaCl solution, improves the electrochemical properties of the Al–Zn–In alloy. The activation of Ce in the Al–Zn–In alloy in the NaCl solution has been studied.