High resolution Auger electron imaging of supported metal particles

High spatial resolution Auger electron spectra and images of supported metal particles have been obtained in a UHV scanning transmission electron microscope. An edge resolution < 3 nm has been achieved. The number of atoms in a small particle can be estimated from the integrated intensity of the Auger electrons. This method is very useful for detecting and measuring particles with sizes smaller than the incident probe size. Ag clusters containing less than 20 atoms have been detected when supported on a thin carbon film.     

Electronic structure of carbynes studied by Auger and electron energy loss spectroscopy

The experimental carbon Auger lineshape for two samples of carbyne has been obtained. The electronic structure of carbynes has been calculated by the self-deconvolution of experimental Auger spectra. Results are discussed in terms of the electronic properties of linear molecules with 2–6 carbon atoms. It is shown that both carbyne samples have a linear structure with cumulene-type bonds but with different numbers of atoms in the chain. The influence of annealing at T < 600°C on the carbyne structure has been studied. The electron energy loss spectrum of carbynes is analyzed.     

Characterisation of the surface films formed on Nb during electrodissolution in aqueous alkali

Anodic polarisation of Nb in warm and concentrated aqueous alkali results in sustained electrodissolution. The process is investigated in NaOH by voltammetric, steady-state and impedance techniques. The j-E steady-state curve is characterised by a current peak and a subsequent plateau extending up to the explored positive limit (8 V_SCE). Upon addition of tartrate to the electrolyte the dissolution current increases markedly. In some NaOH+Na tartrate electrolytes a dissolution-precipitation mechanism sets in, with formation of a surface layer which causes a gradual current decrease; this layer is identified by XRD and elemental analysis as a mixed Na and Nb oxide hydrate. Larger currents may be sustained in KOH+K tartrate solutions without any precipitation process. Impedance diagrams recorded in the plateau region are analysed on the basis of the equivalent circuit resulting from the surface charge model; the oxide formation ratio and the main model parameters are estimated and their temperature dependence is discussed.     

A Comparison of Chromate-Phosphate and Chromate-Free Conversion Coatings for Adhesive Bonding

Three different conversion coatings have been evaluated. One of these is an established chromate-phosphate treatment (BONDERITE 705) whilst the others are chromate-free (BONDERITE 777 and EP2472) and not widely used for adhesive bonding. In the present study, the degree of surface modification introduced by these treatments has been determined using Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). Both initial single lap shear and stressed durability results have been obtained using a single part epoxide adhesive. Degreased-only and grit-blasted adherends were used as controls. Overall, the conversion coatings provided better durability performance than the mechanical treatments. The developmental treatment EP2472, a chromate-free conversion coating, out-performed the established chromate-phosphate process at low applied loads (≤0.5 kN). All three conversion coatings performed similarly at the higher loads (≥ 1 kN).     

A Comparison of Chromate-Phosphate and Chromate-Free Conversion Coatings for Adhesive Bonding

Three different conversion coatings have been evaluated. One of these is an established chromate-phosphate treatment (BONDERITE 705) whilst the others are chromate-free (BONDERITE 777 and EP2472) and not widely used for adhesive bonding. In the present study, the degree of surface modification introduced by these treatments has been determined using Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). Both initial single lap shear and stressed durability results have been obtained using a single part epoxide adhesive. Degreased-only and grit-blasted adherends were used as controls. Overall, the conversion coatings provided better durability performance than the mechanical treatments. The developmental treatment EP2472, a chromate-free conversion coating, out-performed the established chromate-phosphate process at low applied loads (?0.5 kN). All three conversion coatings performed similarly at the higher loads (? 1 kN).     

Auger Electron Spectroscopy Analysis of Cross-Section Surface of Oxidized Titanium Carbide Single Crystal

A TiC monocrystal was heated by three thermal cycles with isotherm at 1108 K while exposed to Ar/O₂ mixtures respectively with O₂ contents of 239, 1.0, and 324 ppm. The reactivity was detected with a homemade device based on two identical solid electrolyte oxygen sensors connected to a quadrupole mass spectrometer (QMS). The oxidized sample was cleaved by impact bending under high vacuum and the cross section investigated by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) techniques. Both area and profile AES spectra were acquired by using a 70 nm diameter beam. Spectral changes have been analyzed to identify chemical species present at the TiC/TiO₂ interface. A model for high-temperature oxidation of TiC has been proposed. It implies oxygen diffusion through a protective TiO₂ layer and the existence of two inner interfaces: TiC/TiC_{1− x} O _x and TiC_{1− x} O _x /TiO₂.     

Growth of nanotubes on zirconium in glycerol/fluoride electrolytes

The study examines anodic films formed on bulk and sputtered zirconium in ammonium fluoride/glycerol electrolyte with up to 5 vol.% added water, using film cross-sections prepared by focused ion beam and ultramicrotomy. The findings indicate porous films consisting of zirconium fluoride surrounding zirconia-based nanotubes. The fluoride arises due to faster migration of F⁻ ions relative to O²⁻ ions; its dissolution exposes the nanotubes. Ion beam analyses revealed the highest amount of fluorine for films formed in electrolyte with no water addition. ¹⁸O tracer indicated that water was the primary source of oxygen in the films, which grew at an efficiency of ∼80%.     

Limitations of surface analytical techniques for determining the surface composition of bimetallic particles

Calculations of expected X-ray photoelectron spectroscopy intensity ratios for both homogeneous and segregated bimetallic particles demonstrate that a minimum particle size exists, below which reliable differentiation between homogeneous alloy particles and particles in which one component has segregated to the surface cannot be made. This minimum particle size varies between 3 and 8 nm depending on the specific metal combination in question. In cases of partial segregation of one component, the minimum particle size is even larger. The implications of these results for the study of surface segregation in supported bimetallic catalyst particles are discussed.This work was performed at Sandia National Laboratories for the US Department of Energy under contract DE-AC04-94AL85000.     

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.     

Composition and behaviour of cerium films on galvanised steel

The composition and corrosion performance of galvanised steel treated by immersion in cerium nitrate solution was investigated by electrochemical techniques and surface analysis. The surface film consists of a mixture of Ce(III) and Ce(IV) compounds, being very rich in Ce(III) in the first instants of the deposition process and becoming gradually enriched in the more oxidised form, Ce(IV). The presence of this film on the surface hinders the corrosion reaction by reducing the rate of both the cathodic and the anodic reactions. The film becomes thicker but more uneven when the time of film growth increases, with the development of defects in the film, which in contact with electrolyte behaves anodic with respect to the covered areas of the surface. These thicker films have revealed lower resistance to corrosion initiation.     

Photo-electrochemical investigation of anodic oxide films on cast Ti-Mo alloys. I. Anodic behaviour and effect of alloy composition

The anodic behaviour of cast Ti-Mo alloys, having different Mo contents (6-20 wt.%), was investigated in acidic and neutral aerated aqueous solutions. All sample showed a valve-metal behaviour, owing to formation and thickening of barrier-type anodic oxides displaying interference colours. Growth kinetics of passive films is influenced by both anodizing electrolyte and composition of the starting alloy. This last parameter was found to change also the solid-state properties of the films, explored by photoelectrochemical and impedance spectroscopy experiments. Thicker films (U_f = 8 V/MSE) grown on alloys richer in Mo showed more resistive character and a photocurrent sign inversion under negative bias, that revealed an insulating character, whereas corresponding films grown on alloys with lower Mo content, as well as thinner films, behaved as n-type semiconductors. Results are discussed in terms of formation of a mixed Ti-Mo oxide phase.     

Importance of water content in formation of porous anodic niobium oxide films in hot phosphate-glycerol electrolyte

Niobium has been anodized at a constant current density to 10 V with a current decay in 0.8 mol dm⁻³ K₂HPO₄-glycerol electrolyte containing 0.08-0.65 mass% water at 433 K to develop porous anodic oxide films. The film growth rate is markedly increased when the water content is reduced to 0.08 mass%; a 28 μm-thick porous film is developed in this electrolyte by anodizing for 3.6 ks, while the thickness is 4.6 and 2.6 μm in the electrolytes containing 0.16 and 0.65 mass% water respectively. For all the electrolytes, the film thickness changes approximately linearly with the charge passed during anodizing, indicating that chemical dissolution of the developing oxide is negligible. SIMS depth profiling analysis was carried for anodic films formed in electrolyte containing ∼0.4 mass% water with and without enrichment of H₂¹⁸O. Findings disclose that water in the electrolyte is a predominant source of oxygen in the anodic oxide films. The anodic films formed in the electrolyte containing 0.65 mass% water are practically free from phosphorus species. Reduction in water content increased the incorporation of phosphorus species.     

Inhibition of mild steel corrosion in the presence of fatty acid triazoles

The influence of some fatty acid triazoles namely, 4-Phenyl-5-undecyl-4H- [1,2,4] triazole-3-thiol (PUTT), 5-Heptadec-8-enyl-4-phenyl-4H- [1,2,4] triazole-3-thiol (HPTT), and 5-Dec-9-enyl-4-phenyl-4H- [1,2,4] triazole-3-thiol (DPTT) on the corrosion of mild steel in 1 M hydrochloric acid (HCl) and 0.5 M sulfuric acid (H₂SO₄) was studied by weight loss and potentiodynamic polarization methods. The values of activation energy and free energy of adsorption of all the triazoles were calculated to investigate the mechanism of corrosion inhibition. The potentiodynamic polarization studies were carried out at room temperature, according to which all the compounds were mixed type inhibitors and inhibit the corrosion of mild steel by blocking the active sites of the metal. The adsorption of all the triazoles on mild steel surface in both the acid solutions was found to obey the Langmuir adsorption isotherm. All the compounds showed good inhibition efficiency in both acids. The inhibition efficiency of the compounds was found to vary with their nature and concentration, solution temperature and immersion time. Electrochemical impedance spectroscopy was also used to investigate the mechanism of the corrosion inhibition.     

The growth kinetics and properties of potentiodynamically formed thin oxide films on aluminium in citric acid solutions

The growth kinetics and properties of potentiodynamically formed thin oxide films on Al were investigated in 0.05 M citric acid solutions of different pH (5, 6 and 7) by means of potentiodynamic polarization and a.c. electrochemical impedance spectroscopy (EIS) measurements. Al showed passive behaviour within the pH range that was examined. The potentiodynamic growth of the oxide film on Al takes place due to ionic conductivity under the influence of the high electric field. Characteristic kinetic oxide film growth parameters such as the high-field growth exponential law constants (A and B), ionic conductivity through the oxide film, field strength and half barrier width have been calculated. Impedance measurements were used to determine the parameters related to the characteristic sizes and properties of oxide film. The capacitive response of the impedance spectrum was related to the thickness and dielectric properties of the barrier oxide film. The oxide film resistance values were very high, indicating that the oxide films formed under potentiodynamic conditions are highly uniform in thickness and very resistant. The anodic behaviour of Al in the citric solutions under potentiodynamic conditions were characterized by the rapid growth of the oxide film which diminished the influence of relaxation processes on the growth kinetics and structural characteristics of the aluminium/anodic oxide film/electrolyte system.     

An electrochemical and surface analytical study of the formation of nanoporous oxides on niobium

In the present paper, the anodization of Nb in mixed sulphate + fluoride electrolytes resulting in the formation of a nanoporous oxide film has been studied. Chronoamperometry and electrochemical impedance spectroscopy have been employed to characterise in situ the kinetics of the oxidation process. In addition, the evolution of the layer structure and morphology has been followed by ex situ scanning electron microscopy. Particularly, local electrochemical impedance spectroscopy has been used to discern between the mesoscopic 2D and 3D distributions of time constants at the electrode surface. The similarity between local and global impedance spectra during anodic oxidation of Nb demonstrates the presence of an inherent 3D distribution of the high-frequency time constant, which is interpreted as in-depth variation of the steady state conductivity of the passive film. The experimental and calculational results are discussed in relation to the micro- and nanoscopic structure of the formed oxide.     

Study on the anodic film formation process of AZ91D magnesium alloy

The formation of anodic film of AZ91D magnesium alloy has been investigated by means of electrochemical impedance spectroscopy (EIS), cyclic voltammetry, anodic polarization curve, current-time transients and SEM technique. The results show that, under our experimental conditions, the formation of AZ91D anodic film follows the mechanism of 3D nucleation with diffusion controlled growth. With the increase of applied anodizing potential, the nucleation type of anodic film changes from progressive to instantaneous. The results also show that the initially formed anodized film is threadlike and porous, and high potential is essential for the formation of good anodic film with excellent properties.     

Influence of molybdenum species on growth of anodic titania

TiMo, bcc, solid-solution alloys, containing 11.5-37.0 at.% molybdenum, have been anodised galvanostatically in 0.1 mol dm⁻³ ammonium pentaborate and 1.0 mol dm⁻³ phosphoric acid electrolytes, with resultant anodic films characterised by scanning electron microscopy, transmission electron microscopy, Rutherford backscattering spectroscopy and glow discharge optical emission spectroscopy. Uniform amorphous films are formed at high current efficiency to >100 V, with formation ratios of 2.3 and 2.2 nm V⁻¹ in the respective electrolytes, contrasting with the amorphous-to-crystalline transition of anodic titania on titanium that occurs at ∼20-50 V. Apart from minor incorporation of electrolyte species, the films comprise an outer layer of TiO₂ and an inner oxide layer containing Ti⁴⁺ and Mo⁶⁺ ions. The films grow by migration of both cations and anions, with Ti⁴⁺ ions migrating faster than Mo⁶⁺ ions that is related to the energies of Ti⁴⁺O and Mo⁶⁺O bonds.     

Electrodeposition of casein coatings on zinc alloy

The electrochemical behavior of zinc in caseinate solution at pH 10 was investigated. Current–potential curves and weight loss measurements by atomic absorption showed that the anodic dissolution rate of the metal is higher than that obtained in NaOH solution (pH 10). Anodic treatment of zinc alloy at constant potential in such solutions induces the growth of a coating which presents, after drying, strong adherence and a good protection effect in aggressive solution (0.2 M NaCl+0.2 M Na₂SO₄). IR and ESCA spectrophotometric analysis showed that the zinc ions mainly participated in the growth of a casein coating, probably in a complexed form.     

Influence of the interface width on the adhesion strength of copper films deposited on negatively biased carbon steel substrates

In order to improve the adhesion of copper films deposited by DC magnetron sputtering on carbon steel substrates, a negative bias voltage was applied to the substrate during the film deposition process. The scratch test was used to evaluate the adhesion strength of the films on the substrates. Chemical element identification and interface width measurement were carried out by Auger electron spectroscopy. The experimental results show that a variation of the bias voltage causes a change in the behaviour of the interface width similar to that of the critical load. The size of the interface width is obtained from Auger elemental depth profiles by measuring the depth of the interface between the coating and the substrate. It had a value of 45 min for an unbiased substrate and increased to 310 min at a bias of 450 V. In the latter case, the interface is relatively wide and the effects of diffusion and physical mixing of materials at the interface become preponderant. Then, the interface width decreased to 130 min at 600 V in which case it gets narrower and the phenomenon of film densification becomes prominent. In all cases, the substrate temperature generated by the bias voltage also has an effect. Moreover, it was observed in this study that the critical load increases with the size of the interface width. As a result, the application of a bias voltage contributes positively to the enlargement of the interface and consequently enhances the adhesion strength.