Surface analytical investigation of the electrochemical and corrosion behaviour of nanocrystalline FeAl8

From the higher fraction of grain boundaries in nanocrystalline substances a different corrosion behaviour in comparison to the conventional polycrystalline material can be expected, which may be utilised for the development of new corrosion resistant alloys. Therefore, the oxidation behaviour of these two different crystallisation states of FeAl8 was compared by means of electrochemical and surface analytical experiments. The oxide films formed after electrochemical passivation were investigated by Auger Electron Spectroscopy. The application of inelastic peak shape analysis by the method of Tougaard showed, that for both materials the oxide layer may be described by a model of a (below the contamination) buried layer with a thickness of only a few nanometers depending on the preparation conditions. Factor Analysis was applied for the evaluation of the differentiated low energy Auger electron spectra (20–100 eV) as a function of depth profiling sputtering time. For both, the nanocrystalline and the polycrystalline material, the inner part of the oxide layer was enriched in Al, whereas the very outer part (surface region) was enriched in Fe. No differences concerning the sputtering time for removal of the oxide layers were found for the two alloys.     

Changes in the phase composition of oxide layers produced by microarc oxidation on Al–Si and Mg alloys induced by additions of SiO<Subscript>2</Subscript> nanoparticles to the electrolyte

In oxide layers produced by microarc oxidation on Al–Si- and Mg-based alloys, addition of nano-dispersed silicon dioxide to the electrolyte was found to cause a shift in the phase composition toward high-temperature phases and ultra-high-pressure phases. For the first time, ultra-high-pressure phases were detected in oxide layers formed by microarc oxidation on AK6M2 alloy (stishovite) and commercially pure magnesium MG96 (wadsleyite).     

醇氧化为醛的银合金催化剂的量子化学研究

本文用EHMO方法计算了氧原子在晶体银及其合金Ag-X(X=Cu,Mg,Cd,Ga,Ge,In,Sn,Se,Te,Al,Bi,Sb)表面上的吸附态;试图通过计算找出这些金属的电子性质与其对甲醇氧化制甲醛反应中催化活性的关系.计算结果表明:银及其合金的的催化活性与体系的电子最高占有轨道HOMO和最低空轨道LUMO的能量差有关,如果HOMO和LUMO的能量差比纯银体系的能量差小,该合金对反应有较好的催化活性,同时吸附在该合金表面上的氧原子有较大的电荷密度分布.     

Electrochemical behavior of active surface layers in AA8xxx aluminum alloys

AA8xxx alloys employed in the HVAC&R sector (heating, ventilating, air conditioning, and refrigerating) were investigated to highlight the effect of active surface layers in heat-exchanger fins. The local behavior of the surface and the bulk of the alloy sheets was studied by means of an electrochemical microcell in combination with glow-discharge optical-emission spectrometry. Surface layers strongly enhance the electrochemical activity of the fin material. This is related to the segregation of Mg and other elements (Sn) strongly impairing the protective behavior of the oxide film generated during thermomechanical processing.     

Electrochemical and structural characterization of sputter-deposited Mg–Nb and Mg–Nb–Al–Zn alloy films

Nanocrystalline Mg–Nb and Mg–Nb–Al–Zn alloy films were deposited by dc magnetron sputtering on glass and quartz substrates in a wide range of niobium concentrations from 6 to 80 at.%. Structural, electrochemical and corrosion properties of the films were studied by X-ray diffraction, dc voltammetry, electrochemical impedance spectroscopy and electrochemical quartz crystal microbalance. Development of body-centred cubic Nb structure in the Mg–Nb alloy matrix yielded the effects of lattice contraction, grain refining and electrochemical passivity. The measurements showed high corrosion resistance of the films in alkaline solutions when niobium content was one third or more. An increased corrosion resistance was achieved by introducing minor amounts of Al (ca. 2 at.%). In particular, such Al effect was pronounced at lower Nb concentrations (20 to 30 at.%). Semiconductor properties of spontaneously formed oxide on Mg–Nb alloy were studied by Mott–Schottky plots, which indicated highly doped n-type oxide structures on Mg–Nb surface. The paper fills some gap in understanding of niobium–magnesium systems, which show potential for applications in hydrogen storage, switchable mirrors and corrosion protection.     

Enrichment,incorporation and oxidation of copper during anodising of aluminium–copper alloys

Porous anodic oxides generated on copper‐containing aluminium alloys are less regular than anodic oxides generated on pure aluminium. Specifically, a porous oxide morphology comprising layers of embryo pores, generated by a cyclic process of oxide film growth and oxygen evolution, is generally observed. In this work, the relation between the oxidation behaviour of copper during anodising and the specific porous oxide film morphology was investigated by electrochemical techniques, transmission electron microscopy and Rutherford backscattering spectroscopy (RBS). It was found that the anodising potential determines the oxidation behaviour of copper, and the latter determines the porous oxide morphology. At low voltage, relatively straight pores with continuous cell walls were obtained on Al? Cu alloys, but selective oxidation of aluminium atoms resulted in the occlusion of copper‐containing metallic nanoparticles in the anodic film. At higher potentials, copper oxidation promoted oxygen evolution within the barrier layer, and generation of a less regular film morphology. RBS, performed on Al? Cu alloy specimens, revealed a high volume fraction of copper atoms in the anodic films generated at low potentials and a reduced amount of copper atoms in the anodic oxide films generated at high potentials. Copyright © 2009 John Wiley & Sons, Ltd.     

Corrosion resistant performances of alkanoic and phosphonic acids derived self-assembled monolayers on magnesium alloy AZ31 by vapor-phase method

Alkanoic and phosphonic acid derived self-assembled monolayers (SAMs) were formed on magnesium alloy by the vapor phase method. AFM and XPS studies showed that SAMs were formed on Mg alloy. The chemical and anticorrosive properties of the SAMs prepared on magnesium alloys were characterized using contact angle measurements, X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. Water contact angle measurements revealed that, although SA and ISA have the same headgroup to anchor to the magnesium alloy surface, the packing density on the magnesium alloy surface could be considerably different. The contact angle hysteresis of SAMs with a carboxylate headgroup is much larger than that of SAMs with a phosphonic acid group. The XPS O 1s peaks indicated more likely a mix of mono-, bi-, or tridentate binding of phosphonic acid SAM to the oxide or hydroxide surface of the Mg alloy. The electrochemical measurements showed that the phosphonic acid derived SAM had better corrosion resistance compared to alkanoic acid derived SAM. The chemical stability of SAMs modified magnesium alloy was investigated using water contact angle and XPS measurements. The water contact angle and XPS measurements revealed that the molecular density of OP and PFEP on magnesium alloy would be higher than those of SA and ISA on magnesium alloy.     

Photocatalytic,phosphorus-doped,anatase oxide layers applicable to titanium implant alloys

Titanium alloys provide excellent corrosion resistance and favorable mechanical properties well suited for a variety of biomaterial applications. The native oxide surface on titanium alloys has been shown to be less than ideal and surface modification is often needed. Previously, an optimized anodization process was shown to form a porous phosphorus-enhanced anatase oxide layer on commercially pure Ti grade 4. The anodized layer was shown to improve osseointegration and to reduce bacteria attachment when photocatalytically activated with UVA preillumination. The primary objective of the present study was to create a similar phosphorus-enhanced anatase oxide layer on series of titanium alloys including commercially pure Ti grade 4, Ti-6Al-7Nb, Ti-6Al-4V ELI, alpha + beta Ti-15Mo, beta Ti-15Mo, and Ti-35Nb-7Zr-5Ta. Phosphorus-enhanced anatase oxide layers were formed on each titanium substrate. Anatase formation was shown to generally increase with oxide thickness, except on substrate alloys containing niobium. Phosphorus uptake was shown to be dependent on the titanium alloy chemistry or microstructure. Anodized layers formed on beta-structured titanium alloys revealed the lowest phosphorus uptake and the most nanosized surface porosity. A methylene blue degradation assay showed anodized layers on commercially pure Ti and both Ti-15Mo alloys to exhibit the highest levels of photocatalytic activity. Given the range of mechanical properties available with the commercially pure Ti and Ti-15Mo alloys, the results of this study indicate the benefits of phosphorus-enhanced anatase oxide coatings may be applicable to a wide variety of biomaterial applications.     

Selected electrochemical properties of Pd–Au alloys: hydrogen absorption and surface oxidation

Hydrogen absorption into and surface oxidation of Pd–Au alloys in acidic solutions were studied by cyclic voltammetry (CV) and chronoamperometry (CA) coupled with the electrochemical quartz crystal microbalance (EQCM). The influence of alloy bulk and surface composition on the process of oxidation of absorbed hydrogen was examined. The stresses induced by hydrogen insertion in Pd–Au alloys were compared with the case of pure Pd. The potential corresponding to the formation of a monolayer of surface oxide was determined for Pd–Au alloys of different surface states. Electrochemical dissolution of Pd–Au alloys was investigated.     

Oberfl?chensegregation an Au-Pd-Legierungen

Summary Measurements of the surface composition of Au-Pd binary alloys performed by Ion Scattering Spectroscopy (ISS) and X-ray Photoelectron-Spectroscopy (XPS) are presented. A set of 11 alloys with different bulk composition was used for these studies. Argon bombardment induced alternation of the surface composition due to preferential sputtering effects and surface segregation due to temperature excursions up to 500 °C have been studied. For quantitative evaluation of the ISS spectra pure metal standards and also calculated scattering cross sections were used. In general, little gold enrichment due to preferential sputtering and strong surface segregation of gold at elevated temperature was found by ISS measurements. The thickness of the altered layer is about 1 or 2 monolayers.     

Nano-scale dissolution phenomena in Al–Cu–Mg alloys

Localized corrosion of aluminum alloys is a major issue worldwide and in spite of decades of work, several questions still remain unresolved. In this study we focus on key issues in the context of localized corrosion of Al–Cu–Mg/Al–Mg–Cu alloys that have not been adequately addressed. By careful electrochemical exposure along with high-resolution electron microscopy, we reveal that microstructural features down to a few nanometers in size can behave as unique electrochemical entities. In addition, not only is this critical to emerging damage accumulation models, but we also reveal that significant dissolution can occur at potentials below the breakdown potential of the bulk alloy. This work has potentially wide consequences in the interpretation of Al alloy corrosion and alloys design for corrosion resistance.     

贮氢合金表面处理改善Ni/MH电池1C充放电性能

研究了贮氨合金两种表面化学处理方法对MH电极活化性能及Ni／MH电池IC充放电性能的影响：第一种处理方法是贮氢合金在6th。l·L－’KOH溶液中80T处理sh，第二种处理方法是在含有0．04mol·L－‘KBH4的6mol·L’‘KOH溶液中80t处理sh．通过MH电极的放电容量、充放电过程中电极极化和电化学阻抗谱测试评价了上述化学处理对电极活化性能的影响．电子探针表面分析表明化学处理后贮氢合金表面由于铝元素的优先溶解形成一层具有较高电催化活性的富镍表面层，它是改善电极活化性能的主要原因·以处理的贮氨合金为负极材料的Ni／MH电池具有较高IC充放电循环寿命和1．ZV以上放电容量．     

Evolution of surface characteristics of two industrial 7xxx aluminium alloys exposed to humidity at moderate temperature

This study analyses the evolution of surface characteristics of two industrial high-strength 7xxx aluminium alloys with a focus on alloy composition and environmental parameters. Based on storage and transport conditions of as-machined products, the effect of humidity—as liquid and vapour phase—on the natural oxide layer has been studied. The evolution of the natural oxide layer has been analysed by scanning electron microscopy and X-ray photoelectron spectroscopy. The growth behaviour of the surface layer is dominated by environmental conditions, while microgalvanic activity depends mainly on the alloys' chemical composition and differs significantly for tested alloys. Scanning transmission electron microscopy images demonstrated that the long-term exposure at moderate temperatures affects the microstructure near the surface, which differs for the analysed alloy compositions. An anomalous precipitation of zinc-rich particles at the surface and along the precipitate-free zone is observed for the alloy with higher Zn/Mg ratio and lower Cu content.     

Effects of Ca and Ag addition and heat treatment on the corrosion behavior of Mg-7Sn alloys in 3.5 wt.% NaCl solution

The effects of 1 wt.% Ca or 1 wt.% Ca + 1 wt.% Ag addition and heat treatment on the corrosion behavior of Mg-7Sn (wt.%) alloy in 3.5 wt.% NaCl solution were investigated by electrochemical measurements and immersion tests. The alloys were characterized by optical microscope (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). It was found that all alloys were corroded by pitting corrosion and grain boundary corrosion and further corroded with time going. Loose layers of compounds, including Mg (OH)₂, MgO, SnO₂, and other compounds containing Ca and Ag elements, were calibrated on the surface of corroded Mg-7Sn-1Ca-1Ag alloy. The Ca addition improved the corrosion resistance of Mg-7Sn alloy due to the formation of relatively stable compounds containing calcium element and grains refinement. Furthermore, the solid solutioned alloys obtained a superior corrosion resistance due to the dissolve of eutectic Mg₂Sn phase and homogenization treatment. However, the aging treatment is slightly detrimental to the corrosion resistance of Mg-7Sn alloys with the formation of Mg₂Sn precipitates. In conclusion, the aged Mg-7Sn-1Ca-1Ag alloy exhibited a better corrosion resistance and a noticeable micro-hardness property compared with those of as-cast Mg-7Sn alloy. And this study provides an important idea for the research on the comprehensive properties of Mg-Sn alloys.     

Study of the near‐surface of hot‐ and cold‐rolled AlMg0.5 aluminium alloy

Rolling is known to alter the surface properties of aluminium alloys and to introduce disturbed near‐surface microcrystalline layers. The near‐surfaces of mostly higher alloyed materials were investigated by various techniques, often combined with a study of their electrochemical behaviour. Cross‐sectional transmission electron microscopy (TEM), after ion milling or ultramicrotomy, indicated the presence of disturbed layers characterized by a refined grain structure, rolled‐in oxide particles and a fine distribution of intermetallics. Those rolled‐in oxide particles reduce the total reflectance of rolled Al alloys. Furthermore, various depth profiling techniques, such as AES, XPS, SIMS and qualitative glow discharge optical emission spectroscopy (GD‐OES) have been used to study the in‐depth behaviour of specific elements of rolled Al alloys. Here, the surface and near‐surface of AlMg0.5 (a commercially pure rolled Al alloy with addition of 0.5 wt.% Mg) after hot and cold rolling, and with and without additional annealing is studied with complementary analytical techniques. Focused ion beam thinning is introduced as a new method for preparing cross‐sectional TEM specimens of Al surfaces. Analytical cross‐sectional TEM is used to investigate the microstructure and composition. Measuring the total reflectance of progressively etched samples is used as an optical depth profiling method to derive the thickness of disturbed near‐surface layers. Quantitative r.f. GD‐OES depth profiling is introduced to study the in‐depth behaviour of alloying elements, as well as the incorporation of impurity elements within the disturbed layer. The GD‐OES depth profiles, total reflectance and cross‐sectional TEM analyses are correlated with SEM/energy‐dispersive x‐ray observations in GD‐OES craters. Copyright © 2005 John Wiley & Sons, Ltd.     

ALSV investigation of the phase composition of electrolytic Cu + Sn alloys

Anodic linear sweep voltammograms (ALSVs) have been recorded for thin layers of Cu + Sn alloys electrochemically deposited on graphite from a pyrophosphate bath. Three characteristic peaks were found. The first peak, appearing in the ALSVs of all samples, could be ascribed both to pure Cu (in the deposit obtained at low cathodic polarization) and to the η-Cu₆Sn₅ phase, while one of the other two peaks appearing only in the samples obtained at low cathodic polarization should reflect the presence of the -Cu₃Sn phase. The other peak, appearing only in the samples obtained at high cathodic polarization, is likely to reflect the presence of the tin-rich β-solid solution. The phase composition, in terms of the content of different phases, was determined as a function of the thickness of the alloy as well as of the deposition potential.     

Metals and alloys bonded on solid polymer electrolyte for electrochemical reduction of pure benzaldehyde without liquid supporting electrolyte

Metals and alloys bonded on the solid polymer electrolyte (SPE) Nafion® 117 were studied as both the electrodes and electrolyte for the electrochemical reduction of pure benzaldehyde without liquid supporting electrolyte. The results indicated that SPE electrodes modified with metals such as Pt, Ni, Pb, Cu and Ag by the ion exchange chemical deposition method had a more stable structure and could provide a larger electrochemical active surface area than those prepared by other methods. The composition of reducing agents and the pH value have a significant effect on the characteristics of the prepared SPE electrodes. In this study a novel method was developed to prepare a Pt+Pb/Nafion® electrode which formed a protective Pt layer on the surface of Pb/Nafion®. The results of scanning electron microscopy further confirmed that Pt+Pb/Nafion® electrodes had obvious advantages for the electrochemical reduction of benzaldehyde. The results also revealed that the current efficiencies of benzylalcohol production at various SPE electrodes decreased in the order Pt+Pb/Nafion® > Pb/ Nafion® > Ni/Nafion® > Cu/Nafion® > Ag+Cu/Nafion® > Ag/Nafion® > Pt/Nafion®.     

Effect of surface composition on electronic structure, stability, and electrocatalytic properties of Pt-transition metal alloys: Pt-skin versus Pt-skeleton surfaces

The surface properties of PtM (M = Co, Ni, Fe) polycrystalline alloys are studied by utilizing Auger electron spectroscopy, low energy ion scattering spectroscopy, and ultraviolet photoemission spectroscopy. For each alloy initial surface characterization was done in an ultrahigh vacuum (UHV) system, and depending on preparation procedure it was possible to form surfaces with two different compositions. Due to surface segregation thermodynamics, annealed alloy surfaces form the outermost Pt-skin surface layer, which consists only platinum atoms, while the sputtered surfaces have the bulk ratio of alloying components. The measured valence band density of state spectra clearly shows the differences in electronic structures between Pt-skin and sputtered surfaces. Well-defined surfaces were hereafter transferred out from UHV and exposed to the acidic (electro)chemical environment. The electrochemical and post-electrochemical UHV surface characterizations revealed that Pt-skin surfaces are stable during and after immersion to an electrolyte. In contrast all sputtered surfaces formed Pt-skeleton outermost layers due to dissolution of transition metal atoms. Therefore, these three different near-surface compositions (Pt-skin, Pt-skeleton, and pure polycrystalline Pt) all having pure-Pt outermost layers are found to have different electronic structures, which originates from different arrangements of subsurface atoms of the alloying component. Modification in Pt electronic properties alters adsorption/catalytic properties of the corresponding bimetallic alloy. The most active systems for the electrochemical oxygen reduction reaction are established to be the Pt-skin near-surface composition, which also have the most shifted metallic d-band center position versus Fermi level.     

Characterization of electrodeposited Co + Ni alloys by application of the ALSV technique Professor Aleksandar Despic to commemorate his 70th birthday

Anomalous codeposition of Co and Ni onto a gold RDE was investigated in a solution containing simple sulfate salts with the addition of sodium citrate. It was shown that the dependence of the percentage of Co in the deposit on the percentage of Co in the bath follows the shape found in the literature, with the percentage of Co in the deposit being slightly higher than in electrolytes containing pure simple salts. Alloy layers of different composition, electrodeposited at constant charge Q_dep = 1 C cm⁻² (thickness 0.34 μm) were submitted to anodic dissolution at a sweep rate of 1 mV s⁻¹ (ALSV technique) in a solution of 1 M NaCl, pH 2. All samples were found to dissolve through a single anodic peak, indicating that both constituents of the alloy dissolve simultaneously. Alloys with higher Ni content (above 40at.%) were found to dissolve at potentials more positive than the potential of pure Ni dissolution as a consequence of the Gibbs energy change of formation of electrodeposited solid solution type Co + Ni alloys. The composition of electrodeposited alloys was determined by the atomic absorption technique. An attempt was made to obtain a correlation between the peak potentials of anodic dissolution of alloy samples and the composition of alloys, to determine the composition of the alloy from the peak potential of its dissolution. It is found that such a correlation can be used only for strictly defined conditions of alloy deposition and dissolution, caused by the contribution of the Gibbs energy change of formation of electrodeposited alloys. Also, the presence of a CoNi₃ ordered structure in the system is not detected as a separate ALSV peak, but its existence could be the cause of the shape of the Gibbs energy change with composition of the alloy for alloys electrodeposited at low current density.     

Electrochemical behavior of Pd–Rh alloys

Pd–Rh alloys were prepared by electrochemical codeposition. Bulk compositions of the alloys were determined by the energy dispersive X-ray analysis method, while surface compositions were determined from the potential of the surface oxide reduction peak. Cyclic voltammograms, recorded in 0.5 M H₂SO₄ for Pd–Rh alloys of different bulk and surface compositions, are intermediate between curves characteristic of Pd and Rh. The influence of potential cycling on electrochemical properties and surface morphologies of the alloys was studied. Due to electrochemical dissolution of metals, both alloy surface and bulk become enriched with Pd. Carbon oxides were adsorbed at a constant potential from the range of hydrogen adsorption. The presence of adsorbed CO₂ causes remarkable diminution of hydrogen adsorption but it does not significantly influence hydrogen insertion into the alloy bulk. On the other hand, in the presence of adsorbed CO, both hydrogen absorption and adsorption are strongly suppressed. Oxidative removal of the adsorbates results in a characteristic voltammetric peak, whose potential increases with the decrease in Rh surface content. Electron per site (eps) values calculated for the oxidation of the adsorbates change with alloy surface composition, more for CO₂ than CO adsorption, indicating the variation of the structure and composition of CO₂ and CO adsorption products. The course of the dependence of eps values on surface composition suggests that the products of CO₂ and CO adsorption on Pd–Rh alloys are similar but not totally identical.