Influence of dielectric barrier plasma treatment of ZnMgAl alloy-coated steel on the adsorption of organophosphonic acid monolayers

The adsorption of octadecyl phosphonic acid (ODPA) on oxide-covered surfaces of ZnMgAl alloy coatings is described as a function of a dielectric barrier discharge (DBD) pretreatment step. The ODPA monolayer formation enables the investigation of the influence of the DBD treatment on the resulting interfacial bond formation and surface coverage. Surface characterisation by means of surface spectroscopy (PM-IRRAS, XPS) and surface electrochemistry (cyclic voltammetry) showed that the DBD pretreatment with Ar, Ar/O₂ and Ar/H₂O gas mixtures leads to improved barrier properties of the adsorbed ODPA monolayer. Moreover, during ODPA monolayer formation from ethanolic solution, a partial etching of the surface oxide layer occurs.     

Application of electrochemical impedance spectroscopy in characterization of structural changes of printing plates

The porous structure of the aluminium oxide surface of lithographic printing plate (PP) has a most significant influence on the quality of the imprints. This study presents the results of application of electrochemical impedance spectroscopy (EIS) in characterization of PPs' porous structures and their changes during chemical processing. Two common PP types—thermal and conventional—were investigated. The influence of the processing solution’s working age on topographical changes of PP surface and associated change in the impedance spectra are investigated and discussed. The equivalent electrical circuit models reproducing the observed EIS spectra are proposed. Based on these models two mechanisms of surface’s topography changes responsible for degradation of PP performance due to the processing are identified and discussed.     

Interfacial microstructure of cold gas spraying coated Al and Mg alloy substrates

The interface between cold gas sprayed zinc-based coatings and selected light weight alloy substrates (aluminium AA7022 and magnesium AZ91) has been investigated by scanning and transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy. In both cases a non-uniform interfacial zone with a thickness of up to 10 microns is evident. It consists of coating and substrate phases as well as an approximately 1 micron thick, partially whirled layer. For the AA7022 substrate the interfacial layer contains nano and submicron sized crystallites of the binary MgZn₂ phase (η) which are embedded in an aluminium solid solution matrix. In the case of AZ91 substrate two ternary phases, Mg₅Zn₂Al₂ (φ) and Mg₁₁Zn₁₁Al₆ (τ) with particle size smaller than 50 nm are evident. The formation of these phases during spraying cannot be explained by diffusion process only, but a local melting is needed. Consequently, the assumption that the high kinetic energy of cold gas sprayed particles causes an intensive mechanical mixing of coating and substrate material accompanied by a high local temperature rise is proved.     

A study of the chemisorption of a series of aminophenols on plasma-grown aluminium and magnesium oxides by IETS

Inelastic electron tunnelling spectroscopy (IETS) has been applied to study the adsorption of a series of aminophenols on plasma-grown aluminium and magnesium oxides. Vapour-phase doping of these surfaces was used and spectra recorded for 1,2-aminophenol, 1,3-aminophenol and 1,4-aminophenol. The tunnel spectra of the 1,2- and 1,3-aminophenols show that their chemisorption at both oxide surfaces is via reaction between phenolic and surface hydroxyl groups. For the 1,4-aminophenol example, adsorbate chemisorption involves transfer of surface-bound protons from the oxide in amino-group protonation. Some of these protons are replaced by deprotonation of the phenol at surface oxide ions. Observed band intensities for all three systems on aluminium oxide are held to be indicative of an adsorbate orientation on the surface which is close to the vertical. In contrast, those for magnesium oxide are more consistent with a non-vertical configuration.     

Monochromatized x‐ray photoelectron spectroscopy of the AM60 magnesium alloy surface after treatments in fluoride‐based Ti and Zr solutions

The behaviour of the 6% aluminium–magnesium alloy (AM60) surface in zirconium or titanium fluoride aqueous acid solutions was studied. X‐ray photoelectron spectroscopy was used to investigate modifications in the surface chemistry with respect to the composition of the surface treatment solution. The surface film is composed of magnesium hydroxide and hydroxyfluoride, zirconium oxide, oxyhydroxide or oxyfluoride, titanium oxide and structural and adsorbed water. Optimal parameters leading to the formation of a zirconium‐ or titanium‐rich film were determined. A mechanism is proposed for the formation of zirconium‐ or titanium‐based films. Copyright © 2005 John Wiley & Sons, Ltd.     

XPS study of the effect of aluminium on the atmospheric corrosion of the AZ31 magnesium alloy

The surface characteristics and corrosion behaviour of the AZ31 magnesium alloy exposed to a high relative humidity (RH) atmosphere were investigated. During the first 15 days of humidity test at 98% RH and 50 °C, a significant increase of magnesium carbonate and a decrease of magnesium oxide were detected on the surface film by XPS; after this stage, increased exposure times did not produce substantial changes on the relative amounts of these compounds. The surface film of commercially pure magnesium, also examined for comparison purposes, revealed more magnesium carbonate and less magnesium oxide compared with the AZ31 alloy. Unlike the AZ31 alloy, the surface of pure Mg disclosed almost complete substitution of MgO by magnesium carbonate after 30 days of exposure time. Mass gain values of tested specimens and scanning electron microscope characterisation of corroded surfaces indicated lower corrosion susceptibility of the AZ31 alloy compared with the commercially pure Mg, suggesting superior chemical stability of the oxide/hydroxide film formed over the magnesium–aluminium alloy surface. XPS and energy dispersive X‐ray (EDX) analyses did not revealed any substantial enrichment of aluminium in the corrosion products film on the AZ31 alloy after 30 days of testing. Copyright © 2008 John Wiley & Sons, Ltd.     

Static SIMS studies of carboxylic acids on gold and aluminium–magnesium alloy surfaces

Static secondary ion mass spectrometry was used to study the surface reactions and lateral distributions of fatty carboxylic acid molecules on sputter‐deposited gold and aluminium surfaces, as well as commercial aluminium–magnesium alloy surfaces, cleaned using UV/ozone. Films were prepared by spin coating dilute solutions of stearic acid and lauric acid onto the above surfaces. These carboxylic acids were shown to react with the oxide formed on the aluminium and aluminum–magnesium alloy substrates to produce a deprotonated acid anion, stabilized by the formation of a magnesium soap on the aluminium–magnesium alloy surface. Secondary ion imaging of stearic acid films revealed the formation of C‐type crystals. Copyright © 2003 John Wiley & Sons, Ltd.     

Anodic film formation on binary Al?Mg and Al?Ti alloys in nitric acid

Nitric acid is commonly used for surface treatments of aluminium alloys. It is used to clean the surfaces after alkaline etching; it has application in chemical polishing and is also used for electrograining. The majority of these treatments undergo the application of anodic polarisation that results in formation of anodic oxide film. However, little is known about the behaviour of aluminium containing magnesium or titanium in solid solution under such conditions. To reveal the effects of magnesium and titanium alloying additions on anodic film formation in nitric acid, Al‐1800 ppm Mg and Al‐800 ppm Ti alloys were investigated. It was found that porous alumina film developed on the surfaces with reduced efficiency of 40%, due to the reactive nature of nitric acid to alumina. The presence of magnesium and titanium in aluminium had little influence on the efficiency of film growth, as confirmed by the relatively similar thicknesses of oxide formed on binary alloys and aluminium. However, incorporation of magnesium ions into the alumina film led to development of a high‐population density of localised voids near the alloy/film interface. An increased titanium content was found in the film regions close to the alloy/film interface, indicating its oxidation. Copyright © 2009 John Wiley & Sons, Ltd.     

Growth mechanism of anodic oxide films on pure aluminium in aqueous acidic and alkaline solutions

The present work was conducted to explore the growth mechanism of anodic oxide films on pure aluminium in aqueous acidic and alkaline solutions by using a.c. impedance spectroscopy and a beam deflection technique. From the analyses of a.c impedance data, it was found that the reciprocal capacitance of anodic oxide film on pure aluminium increased linearly with increasing film formation potential in both acidic and alkaline solutions, indicating a linear increase in the film thickness with film formation potential. However, as the film formation potential increased, the resistance of anodic oxide film decreased in acidic solution, while it increased in alkaline solution. From the measurements of the deflection, the deflection was observed to move towards only a compressive direction with time in acidic solution, but it showed a transition in the direction of movement from compressive to tensile in alkaline solution. Based upon the above experimental results, it is suggested that the movement of oxygen vacancy through the oxide film contributes to the growth of anodic oxide film on pure aluminium in acidic solution, but the movement of both aluminium vacancy and oxygen vacancy accounts for that oxide film growth in alkaline solution. Received: 12 August 1997 / Accepted: 9 October 1997     

Threshold Al KLL Auger spectra of oxidized aluminium foils

Threshold Al KLL Auger electron spectroscopy and K‐edge x‐ray absorption fine structure spectroscopy have been used to examine technical purity (99.5%) aluminium foil before and after chemical treatment that altered the thickness and degree of hydroxylation of the oxidized layer. Comprehensive surface chemical characterization was effected by means of monochromatized Al Kα‐excited photoelectron spectroscopy. Threshold Al KL_{2, 3}L_{2, 3} spectra were obtained for three of the foils investigated and these spectra were in broad agreement with those observed previously for pure Al foil. The relative intensities of the spectral components for two of the foils were clearly consistent with the previously proposed assignment of the resonantly enhanced Auger component, situated between those arising from the metal and Al(III) oxide, to a thin interfacial layer. The threshold Auger spectra from the aluminium foil bearing the thickest and most hydroxylated oxidized layer were not obviously consistent with the interfacial layer model but O K‐edge spectra revealed that this surface layer was fundamentally different from the others and could have had a greater interfacial surface area. Copyright © 2003 John Wiley & Sons, Ltd.     

Improvement in corrosion resistance of micro arc oxidation coating formed on AZ91D magnesium alloy via applying a nano-crystalline sol–gel layer

Although magnesium is used in many industries, it is reactive and requires protection against aggressive environments. In this study, oxide coating was formed on AZ91D magnesium alloy using micro-arc oxidation (MAO) process in an alkaline electrolyte. Then, in order to seal the pores in the oxide film, a sol–gel layer was applied to the surface of the MAO coating by dipping. For investigation of heat treatment temperature of the sol–gel layer on the properties of the coatings, two different temperatures (150 and 350 °C) were chosen. Surface morphologies and compositions of the coatings were analyzed by Scanning Electron Microscope and X-ray Diffraction (XRD). Surface roughness of the coatings was also measured. The corrosion behavior of the coatings was evaluated with Electrochemical Impedance Spectroscopy and potentiodynamic polarization tests in 3.5 wt%NaCl solution. The porosity percent of the coatings was measured by potentiodynamic polarization tests results. It is found that the sol–gel layers successfully cover the pores of the MAO coatings. The results of the corrosion tests show that the sol–gel layers significantly increase the corrosion resistance of the substrate by reducing the percent of the porosity. The grain size measurements by XRD analysis shows that the grain size of the sol–gel layer heated in 350 °C is about 50 nm.     

X-ray photoelectron and scanning Auger electron spectroscopy study of electrodeposited ZnCr coatings on steel

Zn–Cr alloyed coatings electrochemically deposited are of high interest for leading steel manufacturing companies because of their novel properties and high corrosion resistance compared with conventional Zn coatings on steel. For tuning and optimizing the properties of the electrodeposited Zn–Cr coatings, a broad range of the deposition conditions must be studied. For this reason, two different types of material were investigated in this study, one with a low electrolyte temperature and one with an elevated electrolyte pH, compared with the standard values. Because different corrosion performance and delamination behaviour of the layers were observed for the two types, advanced surface analysis was conducted to understand the origin of this behaviour and to discover differences in the formation of the coatings. The topmost surface, the shallow subsurface region, and the whole bulk down to the coating–steel interface surface were analysed in detail by X-ray photoelectron spectroscopy (XPS) and high-resolution scanning Auger electron spectroscopy to determine the elemental and the chemical composition. For better understanding of the resulting layer structure, multiple reference samples and materials were measured and their Auger and XPS spectra were fitted to the experimental data. The results showed that one coating type is composed of metallic Zn and Cr, with oxide residing only on the surface and interface, whereas the other type contains significant amounts of Zn and Cr oxides throughout the whole coating thickness.     

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.     

Nonaqueous synthesis of metal oxide nanoparticles:Review and indium oxide as case study for the dependence of particle morphology on precursors and solvents

Nonaqueous solution routes to metal oxide nanoparticles are a valuable alternative to the well-known aqueous sol-gel processes, offering advantages such as high crystallinity at low temperatures, robust synthesis parameters and ability to control the crystal growth without the use of surfactants. In the first part of the review, we give an overview of the various nonaqueous routes to metal oxides, their surface functionalization and their assembly into well-defined nanostructures. However, we will strongly focus on surfactant-free processes developed in our group. Within the various reaction systems such as metal halides—benzyl alcohol, metal alkoxides—benzyl alcohol, metal alkoxides—ketones, metal acetylacetonates—benzyl alcohol and metal acetylacetonates—benzylamine we will discuss representative examples in order to show the versatility of this approach. The careful characterization of the organic species in the final reaction mixtures provides information about possible condensation mechanisms. Depending on the system several reaction pathways have been postulated: (i) elimination of organic ethers as result of condensation between two metal alkoxide precursors; (ii) C–C bond formation between the alkoxy ligand of the metal alkoxide precursor and the solvent benzyl alcohol under formation of a metal hydroxyl species, which can undergo further condensation; (iii) ketimine and aldol-like condensation steps, which in the metal acetylacetonate systems are preceded by a solvolysis of the precursor, involving C–C bond cleavage. In the second part of the paper we will focus on the synthesis of indium oxide nanoparticles using different precursors and solvents. Indium oxide represents an instructive example how the oxide precursors and the solvents influence the particle morphology. These findings make it possible to tailor particle size and shape of a particular metal oxide by the appropriate choice of the reaction system.     

An IETS study of the adsorption of 2,3-dihydroxynaphthalene, 1,2-dihydroxybenzene, 1,3-dihydroxybenzene and 1,4-dihydroxybenzene on thin-film plasma-grown aluminium and magnesium oxides

Inelastic Electron Tunnelling Spectroscopy (IETs) has been applied to study the adsorption of 2,3-dihydroxynaphthalene, 1,2-dihydroxybenzene, 1,3-dihydroxybenzene and 1,4-dihydroxybenzene onto plasma-grown thin-film partially hydroxylated magnesium and aluminium oxides. For both 2,3-dihydroxynaphthalene and 1,2-dihydroxybenzene on aluminium oxide it is found that adsorbate chemisorption involves reaction of the two hydroxyl groups present in the adsorbate to form a di-anion in the case of the former and both the mono- and di-anion for the latter. The tunnel spectra for both compounds on magnesium oxide indicate that the di-anion is formed. Adsorption at the oxide surfaces for these two adsorbates involves adsorbate deprotonation with the formation, at the oxide surface, of molecular water which is subsequently desorbed and pumped away during sample junction preparation. For the 1,3- and 1,4-dihydroxy systems, on both oxides, the presence of a strong ν(OH) band at ≈3650 cm⁻¹ suggests that only one of the hydroxyl groups present in both systems is involved in adsorbate deprotonation interactions at the respective oxide surfaces, with the second hydroxyl group present contributing to the enhanced substrate oxide ν(OH) envelope observed.     

Electron Rutherford back‐scattering case study: oxidation and ion implantation of aluminium foil

Electron Rutherford back scattering (ERBS) is a new spectroscopy for determining the composition of surfaces. In this work the surface sensitivity of ERBS was investigated by changing the entrance and exit angle of the electron beam while keeping the scattering angle constant. It was found that in this way the surface sensitivity of the technique can be varied considerably. We use aluminium as a test case for ERBS, as it is well studied. The technique has been used to investigate the oxide film of aluminium foil as manufactured and the native oxide (Al₂O₃) film formed on a clean aluminium surface exposed to air. We have also used ERBS to investigate the presence of Xe, implanted during the sputter cleaning process, at a variety of depths within an aluminium matrix. Copyright © 2007 John Wiley & Sons, Ltd.     

Layer-like fatigue is induced during mechanical pulping

The question whether fatigue is induced during mechanical pulping was addressed experimentally. The grinding process was interrupted to image partly ground spruce samples. The grinding was performed at five different feed velocities using two different grindstones. This approach allowed creating an in situ snapshot of the developing grinding zone in the wood samples. The depth profiles of the stiffness modulus and nm-scale pores, close to and within, the grinding zone were quantified by ultrasonic pitch-catch measurements and thermoporosimetry. To perform these profiling measurements, wood material was iteratively removed layer-by-layer with a microtome from the sample surface after taking the snapshot. The grinding-induced changes in cell morphology inside the sample were imaged using microcomputed tomography, whereas the changes on the surface of the samples were imaged with optical microscopy and SEM. A layer that penetrated 0.5–1.5 mm into the sample exhibiting up to 80% decreased stiffness modulus—compared to the unaltered sample parts—was detected when the Wave-type grindstone was employed. The corresponding layer thickness was 0.3 mm with the conventional grindstone. The results match previously measured temperature profiles, and confirm the Atack-May hypothesis that grinding induces a fatigue layer. Confirming this old, widely used hypothesis is significant for the field of energy efficiency research related to mechanical pulping and may provide new opportunities for grinding research.     

Electron transport and redox reactions in carbon-based molecular electronic junctions

A unique molecular junction design is described, consisting of a molecular mono- or multilayer oriented between a conducting carbon substrate and a metallic top contact. The sp2 hybridized graphitic carbon substrate (pyrolyzed photoresist film, PPF) is flat on the scale of the molecular dimensions, and the molecular layer is bonded to the substrate via diazonium ion reduction to yield a strong, conjugated C-C bond. Molecular junctions were completed by electron-beam deposition of copper, titanium oxide, or aluminium oxide followed by a final conducting layer of gold. Vibrational spectroscopy and XPS of completed junctions showed minimal damage to the molecular layer by metal deposition, although some electron transfer to the molecular layer resulted in partial reduction in some cases. Device yield was high (>80%), and the standard deviations of junction electronic properties such as low voltage resistance were typically in the range of 10-20%. The resistance of PPF/molecule/Cu/Au junctions exhibited a strong dependence on the structure and thickness of the molecular layer, ranging from 0.13 ohms cm2 for a nitrobiphenyl monolayer, to 4.46 ohms cm2 for a biphenyl monolayer, and 160 ohms cm2 for a 4.3 nm thick nitrobiphenyl multilayer. Junctions containing titanium or aluminium oxide had dramatically lower conductance than their PPF/molecule/Cu counterparts, with aluminium oxide junctions exhibiting essentially insulating behavior. However, in situ Raman spectroscopy of PPF/nitroazobenzene/AlO(x)/Au junctions with partially transparent metal contacts revealed that redox reactions occurred under bias, with nitroazobenzene (NAB) reduction occurring when the PPF was biased negative relative to the Au. Similar redox reactions were observed in PPF/NAB/TiO(x)/Au molecular junctions, but they were accompanied by major effects on electronic behavior, such as rectification and persistent conductance switching. Such switching was evident following polarization of PPF/molecule/TiO2/Au junctions by positive or negative potential pulses, and the resulting conductance changes persisted for several minutes at room temperature. The "memory" effect implied by these observations is attributed to a combination of the molecular layer and the TiO2 properties, namely metastable "trapping" of electrons in the TiO2 when the Au is negatively biased.     

A study by IETS of the chemisorption of benzotriazole and benzimidazole on plasma-grown aluminium and magnesium oxides

The inelastic electron tunnelling spectra of benzotriazole and benzimidazole suggest that these are adsorbed on thin-film plasma-grown magnesium oxide by deprotonation thus forming an anionic surface species. Conversely, the formation of a cationic adsorbate seems likely for both systems on the corresponding less basic aluminium oxide surface.     

Evolution of surface chemistry during sintering of water-atomized iron and low-alloyed steel powder

Water-atomized iron and steel powder is commonly used as the base material for powder metallurgy (PM) of ferrous components. The powder surface chemistry is characterized by a thin surface oxide layer and more thermodynamically stable oxide particulates whose extent, distribution, and composition change during the sintering cycle due to a complex set of oxidation–reduction reactions. In this study, the surface chemistry of iron and steel powder was investigated by combined surface and thermal analysis. The progressive reduction of oxides was studied using model sintering cycles in hydrogen atmospheres in a thermogravimetric (TG) setup, with experiments ended at intermediate steps (500–1300°C) of the heating stage. The surface chemistry of the samples was then investigated by means of X-ray photoelectron spectroscopy (XPS) to reveal changes that occurred during heating. The results show that reduction of the surface oxide layer occurs at relatively lower temperature for the steel powder, attributed to an influence of chromium, which is supported by a strong increase in Cr content immediately after oxide layer reduction. The reduction of the stable oxide particulates was shifted to higher temperatures, reflecting their higher thermodynamic stability. A complementary vacuum annealing treatment at 800°C was performed in a furnace directly connected to the XPS instrument allowing for sample transfer in vacuum. The results showed that Fe oxides were completely reduced, with segregation and growth of Cr and Mn oxides on the particle surfaces. This underlines the sequential reduction of oxides during sintering that reflects the thermodynamic stability and availability of oxide-forming elements.