Influence of microstructure and composition on the corrosion behaviour of Mg/Al alloys in chloride media

The influence of the microstructure and aluminium content of commercial AZ31, AZ80 and AZ91D magnesium alloys was evaluated in terms of their corrosion behaviour in an aerated 3.5 wt.% NaCl solution at 25 °C. The corrosion process was monitored by electrochemical impedance spectroscopy (EIS). The surface was characterized by scanning electron microscopy (SEM), scanning Kelvin probe force microscopy (SKPFM) and low-angle X-ray diffraction (XRD). The extent of corrosion damage was strongly dependent on the aluminium content and alloy microstructure. Two key factors were observed for the lowest corrosion rates, which occurred for the AZ80 and AZ91D two-phase alloys: the aluminium enrichment on the corroded surface for the AZ80 alloy, and the β-phase (Mg₁₇Al₁₂), which acted as a barrier for the corrosion progress for the AZ80 and AZ91D alloys. Surface potential maps suggested that, between the β-phase and the α-matrix, the galvanic coupling was not significant.     

Corrosion behaviour of a Ti-base nanostructure-dendrite composite

It has been reported that the presence of a ductile second phase can significantly improve the mechanical properties of nanocrystalline alloys. The usual low ductility of homogeneous nanocrystalline alloys is greatly enhanced in a promising, newly developed Ti-base bulk alloy with a dendritic phase dispersed in a nanostructured matrix. The corrosion behaviour of this alloy with outstanding mechanical properties has been studied in the present work to determine its suitability for long-term applications.Polarization tests of Ti₆₀Cu₁₄Ni₁₂Sn₄Nb₁₀ copper mould cast specimens indicate good corrosion resistance in acidic media (0.5N H₂SO₄), medium resistance in alkaline (0.5N NaOH) but the alloy undergoes pitting in chloride media (1, 0.1 and 0.01N NaCl). Detailed examination of the surface of specimens after corrosion test by high resolution scanning electron microscopy (HRSEM) disclosed preferential dissolution of the nanocrystalline matrix is cathodic. Contrarily, observations of the uncorroded surface by scanning Kelvin probe force microscopy (SKPFM) suggest that the nature of the matrix is cathodic with respect to the dendrites. This apparent contradiction in results throws some light on the open question of whether the Volta potential difference measured in air between the phases present on the surface of a specimen can be used to determine their relative susceptibility to corrosion in solution under anodic polarization conditions.     

Role of intermetallic phases in localized corrosion of AA5083

The presence of intermetallic inclusions very often plays a crucial role for the susceptibility of different aluminium alloys to localized corrosion attack. The intimate details of localized corrosion of 5083 aluminium alloy have been studied in the present work. Local techniques such as scanning Kelvin probe force microscopy, in situ atomic force microscopy, scanning electron microscopy coupled with energy dispersive spectroscopy were used to investigate the mechanisms and the kinetics of localized corrosion attack. The importance of iron-rich and Mg₂Si intermetallic phases in the initiation of corrosion processes is demonstrated in the paper.The Mg₂Si phase has a potential lower relatively to the matrix. Moreover, the high reactivity of magnesium leads to the dissolution and consequently to the fast dealloying of these intermetallics. However, hydroxide (Mg(OH)₂ and SiO₂·nH₂O) deposits formed during corrosion act as an additional diffusion barrier hindering the deep propagation of pits.The iron containing intermetallics have the potential higher with respect to the aluminum matrix playing the role of effective cathodic centers for oxygen reduction causing anodic polarization and pitting in the surrounding alloy matrix. Dealloying of such intermetallics with subsequent iron enrichment was also revealed. Pitting initiation seems to be statistical and independent of the composition of Fe-rich intermetallics. However, the active growth of the pits prevents initiation of localized corrosion attack in nearby sites. A new pit can start to grow only when a neighbor one becomes passivated.     

Integrated AFM and SECM for in situ studies of localized corrosion of Al alloys

Rolled 3xxx series Al alloys, e.g., EN AW-3003, are generally used as fin or tube material in heat exchangers for automobiles. With reducing fin thickness, maintaining fin material integrity is of increasing importance. This study aimed at exploring the differences in intrinsic corrosion properties between EN AW-3003 and a newly developed Al-Mn-Si-Zr fin alloy using state-of-the-art local probing techniques. Volta potential mapping of both alloys by scanning Kelvin probe force microscopy (SKPFM) indicates a cathodic behaviour of constituent intermetallic particles (>0.5 μm) relative to the alloy matrix. Compared to EN AW-3003, the Al-Mn-Si-Zr alloy has a smaller number of particles with large Volta potential difference relative to the matrix. In situ atomic force microscopy (AFM) measurements in slightly corrosive solutions showed extensive localized dissolution and deposition of corrosion products on EN AW-3003, and only a small number of corroding sites and “tunnel-like” pits on Al-Mn-Si-Zr. Probing the ongoing localized corrosion process by integrated AFM and scanning electrochemical microscopy (SECM) revealed more extensive local electrochemical activity on EN AW-3003 than on Al-Mn-Si-Zr. In all, the lower corrosion activity and smaller tunnel-like pits resulted in lower material loss of the Al-Mn-Si-Zr alloy, a beneficial property when striving towards thinner fin material.     

Electrochemical microscopy: An approach for understanding localized corrosion in microstructurally complex metallic alloys

A method has been developed to map electrochemical data from constituent particles comprising intermetallic compounds found in aluminum alloys onto real or virtual alloy microstructures with micrometer resolution. This method, referred to as “electrochemical microscopy” facilitates presentation of phase-specific microelectrochemical data by presenting it visually within the context of alloy microstructure. In this paper, several examples of the approach are given using electrochemical reaction rates of the various phases in Al alloy 7075 at the prevailing alloy corrosion potential (−0.8V_sce). In near-neutral dilute chloride solutions, the resulting maps show expected high net anodic reaction rates at Mg₂Si particles and high net cathodic reaction rates at Al₇Cu₂Fe and Al₃Fe particles. When the pH is varied from 2.5 to 12 and chloride concentration is varied from 0.01 to 0.6 M, the maps capture the changes in relative reaction rates. A comparison to corrosion morphologies developed during free corrosion experiments with the alloy show that electrochemical microscopy captures forms and intensity of localized corrosion as well as trends associated with systematic changes in environment chemistry.     

Scanning Kelvin probe force microscopy study of chromium nitrides in 2507 super duplex stainless steel—Implications and limitations

The effect of chromium nitrides on localized corrosion resistance of 2507 super duplex stainless steel was investigated in this study. The Volta potential difference measured with scanning Kelvin probe force microscopy (SKPFM) indicates that chromium nitrides with the size range of 80–230 nm precipitated isothermally at the ferrite/austenite phase boundaries may detrimentally affect the corrosion resistance due to the observed local Volta potential drop at the phase boundaries. Small quenched-in nitrides with the size range of 50–100 nm formed in the centre of the ferrite phase, on the other hand, may have small or no adverse effect on the corrosion resistance since they showed no difference in Volta potential relative to the matrix.     

Effect of laser surface melting on corrosion behaviour of AZ91D Mg alloy in simulated-modified body fluid

High corrosion rate in physiological environment of the body is the major drawback of magnesium alloys for their successful applications as biodegradable orthopaedic implants. In the present study, corrosion behaviour of AZ91D magnesium alloy after laser surface melting (LSM) was studied in modified-simulated body fluid at 37 °C. The improved corrosion resistance of AZ91D alloy using LSM was found to depend on the solidification microstructure in the laser-melted zone. The general and pitting corrosion resistance of laser-treated surface was significantly enhanced due to the refined continuous network of β-Mg₁₇Al₁₂ phases and the increased Al concentration in the laser-melted zone.     

Characterization of cast AlSi(Cu) alloys by scanning Kelvin probe force microscopy

The aluminum cast alloys undergo surface treatments involving micro-electrochemistry for improved hardness, wear and/or corrosion resistance. The susceptibility to local galvanic coupling for three different permanent mold cast aluminum alloys, i.e. 99.8 wt.%Al, Al-10 wt.%Si and Al-10 wt.%Si-3 wt.%Cu was investigated using the scanning Kelvin probe force microscopy (SKPFM) technique. All the particles detected in the alloys (i.e. Al-Fe, Al-Fe-Si, Si, Al₂Cu) revealed a positive Volta potential difference relative to the matrix. However, the nobility depended on particles composition. The highest Volta potential differences were determined in the Al-10 wt.%Si-3 wt.%Cu alloy between the copper containing intermetallics and the aluminum matrix whereas the lowest differences were detected for the Al-Fe particles in the 99.80 wt.%Al specimen. Further, the matrix of the 99.8 wt.%Al specimen showed Volta potential differences possibly due to compositional gradients following casting.     

Influences of the quantity of Mg2Sn phase on the corrosion behavior of Mg–7Sn magnesium alloy

The influence of the quantity of the Mg₂Sn phase on the corrosion behavior of different solution temperature treated Mg–7Sn magnesium alloy has been investigated by electrochemical measurements, scanning electron microscope (SEM) observation, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. With the increase of solution temperature, the quantity of Mg₂Sn phase decreased and the tin concentration of matrix increased. The dissolved tin in Mg matrix took part in the film formation and the constituent of film was magnesium oxide and stannic oxide. The corrosion mode and corrosion rate were associated with the quantity of Mg₂Sn phases and tin concentration of the matrix. If most of tin was present as Mg₂Sn, the corrosion mode was pitting corrosion and it accelerated the corrosion rate. If most of tin was dissolved in matrix, the corrosion mode was filiform corrosion and it decreased the corrosion rate. The experiment evidences demonstrated that the corrosion resistance can be improved by increasing the tin concentration of matrix and the lowest corrosion rate was observed for sample solution treated at 540 °C.     

Biodegradable magnesium alloy coated with TiO2/MgO two-layer composite via magnetic sputtering for orthopedic applications: A study on the surface characterization,corrosion, and biocompatibility

In this study, a coating of thin TiO₂ layer and a TiO₂/MgO double layer were created on the surface of AZ91D alloy by magnetic sputtering method in order to improve the corrosion and biocompatibility properties of this alloy. The microstructural studies by field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) showed that coatings were formed continuously and homogeneously on the alloy surface. In the double-layer coating, MgTiO₃ and Mg₂TiO₄ compounds were formed at the coating/substrate interface in addition to TiO₂ and MgO phases as the main phases in the coating structure. The results of corrosion test showed that in general, coating improves the corrosion of AZ91D alloy in simulated-body fluid (SBF). The double-layer coating showed the best corrosion resistance at a corrosion current of 5.743 × 10^?7 μA/cm² and a corrosion potential of ?1.513 V due to its cathodic protection of the substrate and blockage of the path of the corrosive solution towards the substrate. In vitro tests showed that considering the good match between the used materials as the coating and body, no toxic material exits which results in improvement in biocompatibility, adhesion, and bone-cell multiplication.     

Corrosion resistance of plasma-anodized AZ91D magnesium alloy by electrochemical methods

Anodic coatings formed on magnesium alloys by plasma anodization process are mainly used as protective coatings against corrosion. The effects of KOH concentration, anodization time and current density on properties of anodic layers formed on AZ91D magnesium alloy were investigated to obtain coatings with improved corrosion behaviour. The coatings were characterized by scanning electron microscopy (SEM), electron dispersion X-ray spectroscopy (EDX), X-ray diffraction (XRD) and micro-Raman spectroscopy. The film is porous and cracked, mainly composed of magnesium oxide (MgO), but contains all the elements present in the electrolyte and alloy. The corrosion behaviour of anodized Mg alloy was examined by using stationary and dynamic electrochemical techniques in corrosive water. The best corrosion resistance measured by electrochemical methods is obtained in the more concentrated electrolyte 3 M KOH + 0.5 M KF + 0.25 M Na₃PO₄·12 H₂O, with a long anodization time and a low current density. A double electrochemical effects of the anodized layer on the magnesium corrosion is observed: a large inhibition of the cathodic process and a stabilization of a large passivation plateau.     

Effect of Al2Cu precipitates size and mass transport on the polarisation behaviour of age-hardened Al–Si–Cu–Mg alloys in 0.05 M NaCl

The electrochemical behaviour of age-hardened Al–Si–Cu–Mg alloys was investigated in a 0.05 M NaCl solution under controlled mass transport conditions using a rotating disk electrode. This work aimed at getting better understanding of the effect of the alloy microstructure, in particular the size distribution of Al₂Cu phase, on the corrosion behaviour of the alloy. Three different size distributions of the Al₂Cu phase were obtained through appropriate heat treatments. The cathodic reduction of oxygen was found to occur mainly on the Al₂Cu phases acting as preferential cathodes. Small sized Al₂Cu phases were found to promote at high rotation rates a transition from a 4 electron to a 2 electron dominated oxygen reduction mechanisms.     

Study on the phase structure and electrochemical properties of RE0.93Mg0.07Ni2.96Co0.60Mn0.37Al0.17 hydrogen storage alloy

In our endeavor to improve overall properties of the La-Mg-Ni-Co type alloys, RE_0.93Mg_0.07Ni_2.96Co_0.60Mn_0.37Al_0.17 hydrogen storage electrode alloy with low magnesium content was obtained by inductive melting. The phase structure and electrochemical characteristics of the alloy were investigated by XRD and electrochemical measurement. The results indicate that RE_0.93Mg_0.07Ni_2.96Co_0.60 Mn_0.37Al_0.17 alloy has multi-phase microstructure containing the CaCu₅ structure of LaNi₅ phase as matrix phase and a little of LaNi₃ phase as the secondary phase. The maximum discharge capacity of RE_0.93Mg_0.07Ni_2.96Co_0.60Mn_0.37Al_0.17 alloy reaches 359 mAh/g, which is 7.2% higher than that of commercial AB₅ alloy electrode. The discharge capacity of RE_0.93Mg_0.07Ni_2.96Co_0.60Mn_0.37Al_0.17 alloy electrode at 233 K is up to 147 mAh/g, which is 308.3% higher than that of commercial AB₅ alloy electrode. Meanwhile, the discharge capacity of RE_0.93Mg_0.07Ni_2.96Co_0.60Mn_0.37Al_0.17 alloy can reach 92.7% of commercial AB₅ alloy after 100 charge/discharge cycles.     

AC impedance spectroscopy study of the corrosion behavior of an AZ91 magnesium alloy in 0.1 M sodium sulfate solution

The corrosion behavior of an AZ91 magnesium alloy in 0.1 M sodium sulfate solution at the corrosion potential (E_corr) was investigated using electrochemical impedance spectroscopy (EIS), environmental scanning electron microscopy (ESEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The results showed that when the immersion time was less than 18th, general corrosion occurred on the surface and the main corrosion products were hydroxides and sulfates. The film coverage effect was the main mechanism for the corrosion process of AZ91 alloy. At this stage, the matrix had a better corrosion resistance. With the increasing immersion time, pitting occurred on the surface. At this stage, the corrosion process was controlled by three surface state variables: the area fraction θ₁ of the region controlled by the formation of Mg(OH)₂, the area fraction θ₂ of the region controlled by the precipitation of MgAl₂(SO₄)₄·2H₂O, and the metastable Mg⁺ concentration C_m.     

Improvement in the cycle life of LaB5 metal hydride electrodes by addition of ZnO to alkaline electrolyte

The improvement in the cycle life of a metal-hydride electrode, LaNi_3.35Co_0.75Mn_0.4Al_0.3, brought about by the addition of ZnO to the alkaline electrolyte has been investigated using measurements based upon in situ electrical resistance, corrosion, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction and inductively-coupled plasma-atomic emission spectroscopy. It was found that Zn is underpotentially deposited on and subsequently alloyed with the subject electrode upon repeated charge-and-discharge cycles. The presence of Zn extends the cycle life of the LaNi_3.35Co_0.75Mn_0.4Al_0.3 electrode by inhibiting the disintegration and lowering the corrosion rate of the alloy particles.     

AZ91D镁合金表面不同树脂体系富镁涂层的保护性能

采用划叉浸泡实验,电化学交流阻抗(electrochemical impedance spectroscopy,EIS),开路电位(open circuit potential,OCP)及动电位扫描研究了不同类型的环氧树脂对于AZ91D镁合金的表面的富镁涂层的保护性能的影响。结果表明环氧618-593构成的富镁涂层防护性能较差;环氧6101-TY650制备的富镁涂层可明显改善涂层对破损处镁合金基体的保护作用,但涂层本身长期防护性能较差;环氧618-T31构成的富镁涂层对AZ91D镁合金的防护作用较强,适宜制备镁合金表面的富镁涂层。3种环氧涂料中加入镁粉颗粒制备的富镁涂层均可对缺陷处裸露的AZ91D镁合金基体提供保护,从而延长漆膜的破坏时间。涂层中的镁粉颗粒被激活后,为镁合金的基体提供了一定程度的阴极保护作用,减缓了镁合金基体的腐蚀。     

Influence of second phase particles on initial electrochemical properties of AA7010-T76

The initial behavior of the open-circuit potential of differently prepared AA7010 samples was investigated using conventional immersion experiments and microcapillary experiments. For macroscopic samples a strong increase of the open-circuit potential can be observed, which is due to selective magnesium dissolution from the Mg₂Si phase leaving a remnant silicon oxide particle. This silicon oxide particle is smaller than the initial Mg₂Si phase and thus a micro-crevice is formed, being the initiation site of localized corrosion of the surrounding matrix. Optical micrographs, SEM and EDX confirm this interpretation of the open-circuit potential behavior.     

Electrochemical reactivity of trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate ionic liquid on glassy carbon and AZ31 magnesium alloy

The electrochemical behaviour of the ionic liquid trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate ([P_6,6,6,14][(ⁱC₈)₂PO₂]) over different potential ranges, on both an inert substrate (glassy carbon), and AZ31 magnesium alloy has been investigated. On the glassy carbon electrode the ionic liquid exhibits an electrochemical window of 4 V in an argon environment. Anodic cycling of AZ31 in the ionic liquid resulted in a reduction in current density of over two orders of magnitude after one cycle and also stifled ionic liquid oxidation at approximately 2.0 V, indicating a passive response in the ionic liquid as a result of the deposition of a surface film. In contrast, cathodic cycling of AZ31 led to an increasing current density response with progressive cycles. The understanding of this electrochemical behaviour will allow for future optimisation of electrochemical parameters to obtain a robust and corrosion protective film on AZ31 alloy.     

Zinc–magnesium-pigment rich coatings for corrosion protection of aluminum alloys

Chromate (Cr(VI))-based pigments have been widely used for corrosion protective coatings because of their outstanding protection efficiency especially for aluminum alloy products. However, due to environmental issues associated with Cr VI, more and more requests are being made for alternative solutions. In the presented work zinc was modified by alloying with magnesium to achieve a combination of properties – cathodic protection and less reactivity during production, storage and application of the pigments. The magnesium content leads to a lowering of the electrochemical potential which allows the cathodic protection of aluminum alloys. zinc–magnesium pigments were prepared in different compositions with special attention to the intermetallic phases MgZn, Mg₂Zn₃, and MgZn₂. Pigments were produced and a zinc–magnesium rich coating was formulated and compounded. Pickled samples of AA 2024 unclad were coated and the corrosion behavior investigated. A durability of more than 10,000 h in salt spray test could be achieved.     

钼酸盐对镁合金在模拟冷却水中腐蚀的抑制作用

采用Tafel极化曲线、电化学阻抗谱方法研究了钼酸钠对AZ40镁合金在模拟冷却水中腐蚀的抑制作用,利用扫描电子显微镜(SEM)和能量色散谱(EDS)表征了试样的形貌及组成。结果表明,钼酸钠对镁合金在模拟冷却水中的腐蚀具有一定的抑制作用,钼酸钠浓度为1000 μg·g^-1时缓蚀效果最佳。钼酸钠的加入减弱了腐蚀性Cl^-在试样/介质界面的吸附,同时参与镁合金试样在模拟冷却水中表面膜的生成,使表面膜变得致密,从而较好地抑制镁合金的腐蚀;其缓蚀作用属于阳极抑制型缓蚀作用。