The effect of cerium and lanthanum surface treatments on early stages of oxidation of A361 aluminium alloy at high temperature

X-ray photoelectron spectroscopy analysis has been used to study the surface of A361 aluminium alloy after electrodeposition of cerium and lanthanum compounds followed by oxidation tests in air at 100-500 °C for 2 h. Cerium and lanthanum oxide deposits are found on the β-AlFeSi second phase particles and to a lesser extent on the eutectic Al-Si areas, while the α-Al phase is covered with a thin aluminium oxide film. This uneven deposition may be related either to a preferential nucleation and growth process on active interfaces or to the differing electrical conductivity of the phases and intermetallic compounds of the alloy. Initial stages of oxidation of A361 alloy disclosed thickening of the aluminium oxide layer and Mg enrichment at the surface, especially above 400 °C. Rare earth deposits revealed two different effects: reduced Mg diffusion and enhanced thickening of the aluminium oxide film. A distinctive behaviour of Ce oxide appears at 300-500 °C related with Ce(III) to Ce(IV) transition.     

MOCVD外延生长AlGaAs组份的在线监测

利用反射各向异性谱( RAS)和反射谱在线监测了AlxGa1-xAs样品的金属有机化合物汽相淀积(MOCVD)外延生长过程.通过在线监测得到的RAS和反射谱可以敏感地反映出AlxGa1-xAs外延层组份发生的变化,从而优化外延生长工艺.实验表明,反射谱中的振荡周期可以在线计算组份和生长速率,利用反射谱中的振荡的第一个最...     

Interferometric Sensor for Electrochemical Studies of Metallic Alloys in Aqueuos Solution

In a previous study a mathematical model relating surface and bulk behaviors of metals in aqueous solution was developed. The model was established based on principles of holographic interferometry for measuring microsurface dissolution, i.e., mass loss, and on those of electrochemistry for measuring the bulk electronic current, i.e., corrosion current. In the present work, an interferometric sensor was built based on the above model, and the corrosion current density of coated copper and brass in seawater were obtained using this sensor. The interferometric sensor was also utilized for the first time to measure the initial stage of the anodization process (oxidation) of aluminium samples in aqueous solution. This was carried out chemically in different acid concentrations (3.125–25% H₂SO₄) at room temperature. The sensor was further used for observation of catalytic activities, i.e., pitting corrosion, which occurred subsequent to the anodization of the aluminium samples in aqueous solutions, after an oxide film had been formed.     

Understanding the mechanism of aluminium nanoparticle oxidation

Aluminium nanoparticles have gained importance in the last decade because of their increased reactivity as compared with traditional micron-sized particle. The physics of burning of aluminium nanoparticle is expected to be different than that of micron-sized particles, and the current article is motivated by these differences. We have previously measured the size resolved reactivity of nanoaluminium by single-particle mass spectrometry, to which we now add transmission electron microscope (TEM) and an on-line density measurement. The latter two studies revealed the presence of hollow particles following oxidation of nanoaluminium and indicating the significance of diffusion of aluminium in the overall process. Based on experimental evidence, we believe that aluminium nanoparticle oxidation occurs in two regimes. Prior to melting of aluminium slow oxidation occurs through the diffusion of oxygen through the aluminium oxide shell. Above the melting point, we transition to a fast oxidation regime whereby both aluminium and oxygen diffuse through the oxide shell to enhance the oxidation rate.

We also develop a phenomenological model for nanoaluminium oxidation that accounts for the experimentally observed rates, the fact that both fuel and oxidizer are diffusing, and a new effect related to internal pressure gradients. The latter phenomen is based on molecular dynamic simulations suggesting that there are large pressure gradients present inside these particles, with the aluminium core under a positive pressure and the aluminium oxide shell under a negative pressure. We have considered the effect of these pressure gradients on the oxidation process. A power law relation was obtained (t ∝ r ^{1.6± 0.1}) between the time required for oxidation and particle radius.     

Pressure effects on aluminium oxidation kinetics using X-ray photoelectron spectroscopy and parallel factor analysis

The effects of water vapour pressure on oxidation kinetics of aluminium have been studied using X-ray photoelectron spectroscopy (XPS) and three-way parallel factor analysis (PARAFAC). While the first technique is a powerful experimental tool for surface oxidation studies, the PARAFAC technique is a sophisticated analytical tool for analysing XPS data. The XPS Al(2p) and O(1s) core level have been used to follow the oxide film growth on clean surfaces at room temperature as a function of oxidation time (ranging from 1 to 60 min) and pressure of water vapour (ranging from 2.0×10⁻⁶ to 6.5×10⁻⁴ Pa). The growth of thin oxide films on aluminium surfaces has been found to follow the Cabrera–Mott inverse logarithmic law in all pressure ranges studied. The pressure effects have shown that the defect formation reaction at the oxide film/gas interface is the rate determining process in the aluminium oxidation. The pressure dependence of oxidation kinetics can be explained on the basis of metal vacancies in the defect structure of thin aluminium oxide films.     

Effect of polymorphic phase transformations in alumina layer on ignition of aluminium particles

The mechanism of aluminium oxidation is quantified and a simplified ignition model is developed. The model describes ignition of an aluminium particle inserted in a hot oxygenated gas environment: a scenario similar to the particle ignition in a reflected shock in a shock tube experiment. The model treats heterogeneous oxidation as an exothermic process leading to ignition. The ignition is assumed to occur when the particle's temperature exceeds the alumina melting point. The model analyses processes of simultaneous growth and phase transformations in the oxide scale. Kinetic parameters for both direct oxidative growth and phase transformations are determined from thermal analysis. Additional assumptions about oxidation rates are made to account for discontinuities produced in the oxide scale as a result of increase in its density caused by the polymorphic phase changes. The model predicts that particles of different sizes ignite at different environment temperatures. Generally, finer particles ignite at lower temperatures. The model consistently interprets a wide range of the previously published experimental data describing aluminium ignition.     

Electrogenerated chemiluminescence of chosen terbium complexes deposited into Al2O3 layer at the aluminium cathode

Cathodic electrogenerated luminescence of terbium(III)-(acetylacetonate)₃-1,10-phenanthroline, terbium(III)-(salicylate)₃ and terbium(III)-(salicylate)₃-1,10-phenanthroline complexes were studied. In this experiment terbium complexes were coated upon aluminium porous oxide layer by physical adsorption from methanolic or acetonate solution. The hexagonally ordered porous aluminium oxide has been prepared in a two-step anodization process in sulphuric acid solution. The primary step of cathodic ECL in this method is tunnel emission of hot electrons through barrier oxide layer. Electrochemiluminescence was achieved by recombination of the radical species, such as reduced ligand's aromatic moiety and strong oxidizing agent, i.e. sulphate radicals. This study was conducted to investigate ECL spectra, kinetic and integral ECL intensity relative to the porous aluminium oxide layer thickness.     

Inhibition of corrosion of aluminium in presence of fluorescein in basic medium

The influence of Fluorescein in conjunction with calcium oxide on the corrosion of Aluminium in 1.0 N NaOH was studied by galvanostatic studies and weight loss studies. It has been found that the inhibition of corrosion of aluminium increased with the increasing concentration of the inhibitor. The maximum inhibition efficiency of fluorescein alone was found to be 30.80%. However, the addition of calcium oxide increased the maximum inhibition efficiency to 53.71%. The corrosion process was found to be under the anodic control, in the presence or in the absence of inhibitor. The inhibition was found to be mixed type. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, Inida, Nov. 28–30, 2003.     

Characterisation of the anodic layers formed on 2024 aluminium alloy, in tetraborate electrolyte containing molybdate ions

Anodic layer growth on 2024 aluminium alloy at 70 °C, under 40 V, during 60 min, in 50 g L⁻¹ di-sodium tetraborate solution containing di-sodium molybdate from 0.1 to 0.5 M (pH 10) is examined. Anodising behaviours strongly depend on additive concentration. Development of anodic films is favoured with weak molybdate additions (<0.3-0.4 M). The film thicknesses increase and the porosity of anodic layers decreases. Molybdenum (+VI), detected by X-ray photoelectron spectroscopy (XPS) analysis, is present in the anodic films and the Mo incorporation, studied by energy dispersive spectroscopy (EDS) analysis, increases with molybdate concentration. However, for high molybdate concentrations (>0.4 M), anodising behaviour becomes complex with the formation of a blue molybdenum oxide at the cathode. The growth of aluminium oxide is hindered. As the anodic layers are thinner, the Mo(+VI) incorporation significantly decreases. These two configurations implicate different corrosion performances in 5% sodium chloride solution at 35 °C. As the alkaline anodic layer formed with 0.3 M molybdate species is the thickest and the Mo incorporation is the more pronounced, its corrosion resistance is the highest. The effect of morphology and composition of anodic films on pitting corrosion is also discussed.     

Measurement of the double layer capacitance of aluminium samples by holographic interferometry

In the present investigation, holographic interferometry was utilized for the first time to measure the double layer capacitance of aluminium samples during the initial stage of anodization processes in an aqueous solution without any physical contact. The anodization process (oxidation) of the aluminium samples was carried out chemically in different sulphuric acid concentrations (0.5–3.125% H₂S0₄) at room temperature. In the meantime, a method of holographic interferometry was used to measure the thickness of anodization (oxide film) of the aluminium samples in aqueous solutions. Along with the holographic measurement, a mathematical model was derived in order to correlate the double layer capacitance of the aluminium samples in solutions to the thickness of the oxide film of the aluminium samples which forms due to the chemical oxidation. The thickness of the oxide film of the aluminium samples was measured by real-time holographic interferometry. Consequently, holographic interferometry is found to be very useful for surface finish industries, especially for monitoring the early stage of anodization processes of metals, in which the thickness of the anodized film as well as the double layer capacitance of the aluminium samples can be determined in situ. In addition, a comparison was made between the obtained data of the double layer capacitance from the holographic measurements and the double layer capacitance data obtained from measurements of electrochemical impedance spectroscopy. The comparison indicates that there is good agreement between the data from both techniques.     

Measurement of the a.c. impedance of aluminium samples by holographic interferometry

In the present investigation, holographic interferometry was utilized for the first time to measure the alternating current (a.c.) impedance of aluminium samples during the initial stage of anodization processes in aqueous solution without any physical contact. The anodization process (oxidation) of the aluminium samples was carried out chemically in different sulphuric acid concentrations (0.5–3.125% H₂SO₄) at room temperature. In the mean time, a method of holographic interferometric was used to measure the thickness of anodization (oxide film) of the aluminium samples in aqueous solutions. Along with the holographic measurement, a mathematical model was derived in order to correlate the a.c. impedance of the aluminium samples in solutions to the thickness of the oxide film of the aluminium samples which forms due to the chemical oxidation. The thickness of the oxide film of the aluminium samples was measured by the real-time holographic interferometry. Consequently, holographic interferometry is found very useful for surface finish industries especially for monitoring the early stage of anodization processes of metals, in which the thickness of the anodized film as well as the a.c. impedance of the aluminium samples can be determined in situ. In addition, a comparison was made between the a.c. impedance values obtained from the holographic interferometry measurements and from measurements of electrochemical impedance spectroscopy. The comparison indicates that there is good agreement between the data from both techniques.     

Optical corrosion meter

In a previous study, a mathematical model relating surface and bulk behaviours of metals in aqueous solution has been developed. The model was established based on principles of holographic interferometry for measuring microsurface dissolution, i.e. mass loss, and on those of electrochemistry for measuring the bulk electronic current, i.e. corrosion current. In the present work, an optical corrosion meter was built based on the above model. The corrosion meter consists of an electrochemical cell in which the sample is immersed in aqueous solution. Furthermore, the corrosion meter has a holographic camera with a thermoplastic film for in situ processing of holograms in order to obtain real-time holographic interferoms of the sample in the electrochemical cell. Results of the present work indicate that optical holography is a very useful technique for measuring the corrosion current density of different alloys in aqueous solutions. As a result, the corrosion current density of aluminium, stainless steel, and low-carbon steel in 1 M KCl, 1 M NaCl, and 1 M NaOH solutions were obtained. A comparison between the corrosion data of samples showed that the corrosion current density of the stainless steel in 1 M NaCl is nearly three-folds higher than that of the aluminium in 1 M KCl and the low-carbon steel in 1 M NaOH.     

Transport properties of p-type semiconducting oxides—influence of aliovalent impurities

The effect of aliovalent impurities on the chemical diffusion coefficient of an oxide AO having a p-type semiconducting behavior has been determined. This treatment shows that for an oxide doped by a monovalent impurity the chemical diffusion coefficient in the extrinsic range is equal to the diffusion coefficient of the prevailing defects. In the case of doping by higher-valent ions, it predicts an increase of the chemical diffusion coefficient. The above predictions are in agreement with the experimental results obtained from electrical conductivity measurements on single crystals ofcobaltous oxide doped either by chromium or aluminium or lithium. In the case of pure or chromium doped nickel oxide, this analysis is in qualitative agreement with the experimental results but lack of data does not allow any calculation.     

Electrochemical impedence spectroscopy versus optical interferometry techniques during anodization of aluminium

In the present investigation, holographic interferometry was utilized for the first time to measure in situ the thickness of the oxide film, alternating current (AC) impedance, and double layer capacitance of aluminium samples during anodization processes in aqueous solution without any physical contact. The anodization process (oxidation) of the aluminium samples was carried out by the electrochemical impedance spectroscopy (EIS), in different concentrations of sulphuric acid (1.0–2.5% H₂SO₄) at room temperature. In the mean time, the real-time holographic interferometric was used to measure the thickness of anodized (oxide) film of the aluminium samples in aqueous solutions. Also, mathematical models were applied to measure the AC impedance, and double layer capacitance of aluminium samples by holographic interferometry, during anodization processes in aqueous solution. Consequently, holographic interferometric is found very useful for surface finish industries especially for monitoring the early stage of anodization processes of metals, in which the thickness of the anodized film, the AC impedance, and the double layer capacitance of the aluminium samples can be determined in situ. In addition, a comparison was made between the electrochemical values obtained from the holographic interferometry measurements and from measurements of EIS. The comparison indicates that there is good agreement between the data from both techniques.     

Continuous X-ray-induced Auger electron spectroscopy

Bremsstrahlung emission from a tungsten anode has been used, for the first time, to obtain X-ray-induced Auger electron spectra (CXAES) in a conventional X-ray photoelectron spectrometer. Auger KLL signal and background intensities obtained from a silicon—aluminium sample have been compared with those obtained un der the same experimental conditions but using a standard aluminium anode. For such an aluminium anode, the respective contributions of the bremsstrahlung and the characteristic radiation to the C_KVV intensity have also been established.     

Sol-gel based alumina powders with catalytic applications

The sol-gel process provides a new approach to the preparation of oxide materials and offers many advantages for making catalysts. Since homogeneous mixing can be achieved at the molecular scale, the chemical reactivity of the oxide surface can be greatly enhanced; thus powders with high surface area and optimized pore size distribution can be obtained at low temperatures. In the present work NiO/Al₂O₃ sol-gel catalysts were obtained by simultaneous gelation of aluminium isopropoxide and nickel nitrate. A comparative study with pure sol-gel alumina was also realized. By physical-structural studies the changes induced by the introduction of the Ni precursor, before and after aluminium alkoxide hydrolysis were highlighted. The introduction of Ni at the beginning of the reaction favors γ-Al₂O₃ crystallization. When Ni is added at the end of reaction, it delays the alumina crystallization and induces the disorder of the lattice. The obtained Ni doped sol-gel derived alumina has been used as catalyst in the finished form for glycerol reforming to generate H₂ for fuel cell applications. Some evaluation results of Ni-doped alumina combined with TiO₂ in photocatalytic glycerol reforming reaction have been included.     

Measurement of the corrosion rate of aluminium in sodium hydroxide using holographic interferometry

The application of holographic interferometry to the measurement of the corrosion rate of aluminium in sodium hydroxide is investigated. Details of the fabrication of the corrosion cell and the experimental procedure are given. Thickness loss of aluminium was found for different dissolution times and compared with the conventional weight-loss method using a microbalance.     

Studies on adhesion characteristics and corrosion behaviour of vinyltriethoxysilane/epoxy coating protective system on aluminium

The corrosion stability of vinyltriethoxysilane/epoxy coating protective system on aluminium is strongly related to the strength of bonds forming at the metal/organic coating interface. This article is a study of adhesion, composition, electrochemical and transport properties of epoxy coatings electrodeposited on bare aluminium and aluminium pretreated by vinyltriethoxysilane (VTES) during exposure to 3% NaCl. The VTES film was deposited on aluminium surface from 2% vinyltriethoxysilane solution during 30 s. From the values of adhesion strength (pull-off test), time dependence of pore resistance and coating capacitance of epoxy coating (impedance measurements) and diffusion coefficient of water through epoxy coating (gravimetric liquid sorption measurements), the influence of VTES sublayer on the corrosion stability of the electrodeposited epoxy coating was shown.The work discusses the role of the VTES pretreatment in the enhanced adhesion and corrosion stability of epoxy cataphoretic coating. The electrochemical results showed that the aluminium pretreatment by VTES film improved barrier properties of epoxy coating (greater pore resistance and lower coating capacitance). The lower value of diffusion coefficient of water through epoxy coating indicates the lower porosity, while the smaller adhesion reduction points to better adhesion of epoxy coating on aluminium pretreated by VTES film. The composition of the deposited coatings investigated by XPS enabled the clarification of the bonding mechanism.     

Sonochemical synthesis of aluminium and aluminium hybrids for remediation of toxic metals

Spherical shaped nano-size aluminium oxide and its hybrids with indole and indole derivatives have been synthesized using sol–gel and post grafting methods coupled with sonication (Branson Digital SonifierS-250D; 20 kHz; 40%) for the remediation of toxic metals (lead and mercury). Different spectroscopic techniques (FTIR, SEM, BET, XRD, and XPS) have been applied to assess the properties of synthesized aluminium oxide and its hybrids. FTIR spectra showed the absorption bands of aluminium oxide (Al-O-Al) and aluminium hybrids (Al-O-C) at 800–400 cm⁻¹ and 1650–1100 cm⁻¹ region, respectively. SEM showed spherical shaped clusters of aluminium oxide which changed into the net-shape structure after the hybrid synthesis. It is worth noting that sonication energy increases the total surface area of aluminium oxide when it gets hybridized with indole and its derivatives from 82 m²/g to 167 m²/g; it also improved the product yield from 68% to 78%. Simultaneously, FTIR, SEM and BET analysis of non-sonicated aluminium oxide and its hybrids were also recorded for comparison. While XRD and XPS analysis were only conducted for sonicated aluminium oxide and its hybrids to manifest the structural and compositional properties. XRD patterns indexed as the cubic crystal system with an average 41 nm crystallite size of sonicated aluminium oxide which remains unaffected after hybrid synthesis. A survey scan under XPS confirmed the presence of all expected elements (aluminium, oxygen, carbon, nitrogen) and deconvolution of each recorded peak showed binding of element with its neighboring elements. The performance of aluminium oxide and its hybrids synthesize with and without sonication are also evaluated using a time-dependent batch adsorption protocol optimize for one hour. The maximum adsorption of lead (37%) and mercury (40%) are found onto sonicated aluminium oxide. The sonicated aluminium hybrids showed 43–63% of lead and 55–67% of mercury at pH 7. The fitness of experimental data using adsorption kinetics and isotherms revealed that adsorption follows Pseudo-second-order kinetic, Langmuir, and Freundlich isotherms.     

The use of X-ray photoelectron spectroscopy in corrosion science

Most metals in commercial use are protected from reaction with an environment at ambient temperature by an oxide layer of ~5 nm in thickness. This layer may increase in thickness, by solid state diffusion, at high (>300°C) temperature or may dissolve and be replaced by soluble species by electrochemical reactions in aqueous environments. These are the twin processes of corrosion and the problem of preventing such destructive transformations had led to a longstanding interest by corrosion scientists in the thin film: and indeed in sub-monolayers. Prior to the introduction of the techniques of surface derived ion and electron spectroscopy the chemical analyses of films, which had known physical and electronic characteristics, were unavailable. This review attempts to specify the requirements of an analytical technique for thin films and examines the present role of XPS in this context. The areas covered will include elemental analysis, evaluation of valence state, non-destructive information, elemental distribution and finally the use of ion-etch depth profiling. Examples are drawn from corrosion problems in both aqueous and gaseous environments.