The aim of the present article is to review some of the common corrosion phenomena and describe the predominant corrosion mechanisms in high-temperature and supercritical water. Corrosion in aqueous systems up to supercritical temperatures is determined by several solution-dependent and material-dependent factors. Solution-depending factors are the density, the temperature, the pH value, and the electrochemical potential of the solution, and the aggressiveness of the attacking anions. Material-dependent parameters include alloy composition, surface condition, material purity, and heat treatment. Corrosion phenomena that are observed include intergranular corrosion, pitting, general corrosion, and stress corrosion cracking. The solubility and dissociation of both attacking species and corrosion products play the most important role for corrosion in high-temperature water. Both solubility and dissociation processes are strongly influenced by the density, or the ionic product, respectively, of the solvent. High values of both parameters favor ionic reactions, and thus, accelerate electrochemical forms of corrosion. At low densities, water behaves like a non-polar solvent, and thus, ions associate. In these cases, the concentation of e.g. aggressive H + drops down and thus, solutions containing species such as HCl become neutral and thus less aggressive. Further, corrosion products plug the surface and material loss stops. Materials parameters have influence especially on the initiation of corrosion. In the present article, these factors are linked with the physical and chemical properties of high-temperature and supercritical water. An outlook is also given for future research needs.

Corrosion in high-temperature and supercritical water and aqueous solutions: a review

Multinuclear studies of aluminium compounds

Optically clear monolithic gels (and powders) have been synthesized using various silicon alkoxides and different hydrolysis conditions. Thermal treatments lead to optically clear (transluscent if compacted powders are used) glasses and ceramics. The structure of the gel and its transformation in glass and in crystalline ceramic have been studied by X-ray, DTA, TGA, IR and Raman spectroscopies. Mullite gels show broad spectra similar to alumina gels, whereas germanium substitution leads to narrower bands located at lower frequencies according to the mass effect. The gel structure is of spinel-type. Use of alkoxides with long organic chains favours heterogeneities. Formation of the glass and also of the true spinel structure, does not modify the Raman spectra to any great extent whereas large modifications are observed on IR spectra with the evolution of protonic species. Continuous transformation of spinel into mullite spectra between 1000 and 1600°C, together with band narrowing, indicates a progressive evolution which explains the large nonstoichiometry range. In particular the narrowing of the SiO4 stretching mode (v=970 cm−1) seems to indicate a progressive ordering (history dependent) of the SiO4 environment.

Structure of oxide gels and glasses by infrared and Raman scattering

The mechanical properties of metals change temporarily or permanently under application of electric currents under deformation. This phenomenon is often referred to as electroplasticity. The electroplasticity of metals and their alloys has been investigated under various loading conditions including tensile, compressive, bending, and hardness tests. Electrically-assisted manufacturing (EAM) utilizes this electroplasticity in manufacturing processes. Due to its technical advantages such as enhanced formability and reduced springback, EAM is energy efficient with reduced process time. Recent developments in EAM include various bulk deformation and sheet metal forming processes. This paper summarizes previously reported electroplastic behaviors of various metals or metal alloys and recent EAM processes. In addition, contemporary EAM patents are briefly reviewed.

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A review of electrically-assisted manufacturing

Recent Trends in Metal Alkoxide Chemistry

Double alkoxides of the general formula Mg[Al(OR) 4 ] 2 where R is iso -Pr or sec -Bu were prepared by reacting aluminium and magnesium metals together with the alcohol ROH and purified by vacuum distillation. They were characterised by IR, proton and 27 Al NMR spectroscopy and MW determination. A magnesium aluminium double alkoxide was also prepared by treating the commercially available aluminium alkoxide ‘Aliso B’ [a mixed aluminium ( iso -propoxide)( sec -butoxide)] with magnesium and iso -propanol. Treatment of magnesium aluminium double alkoxides with water and an alkanolamine (preferably triethanolamine) gives a rigid coherent gel. Viscosity measurements and 27 Al NMR spectroscopy suggest that the double alkoxide does not break down to its constituents during hydrolysis. The air-dried gel was shown by XRD to convert quantitatively to spinel on firing to 1500°C. The resistance of the double alkoxide moiety to hydrolysis explains the ease of conversion to spinel on firing. The gel has been used to bind alumina and magnesia grain. Gels suitable for binding refractory grain were obtained only when the alkanolamine content corresponded to one alkanolamine group per metal atom. Electron micrographic and XRD studies showed that in fired refractory pieces, the bonding phase was spinel.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400553007

Spinel Formation from Magnesium Aluminium Double Alkoxides

The results of a selective study of the literature on transition metal binder-hard metals and the parameters that determine the choice of suitable binders are reported. Thermodynamic and kinetic considerations have played a major role in the selection of suitable binders, with the electron theory of metals playing a minor role. It was deduced from the study that the electronic configuration of metals could be a major controlling factor in the thermo-kinetic reactions that take place during sintering in hard metal systems. A set of rules for binder metal selection is proposed based on this study. A modification of these rules would seem partly to explain ductilities currently obtained in certain intermetallics. The application of these rules would point to suitable binder metal(s) selection possibilities that are currently unexplored and predict alloy selection that could improve the ductility of brittle intermetallics.

Proposed selection rules for suitable binders in cemented hard metals with possible applications for improving ductility in intermetallics

Studies of cavitation in Y-TZP during superplastic flow have been made for both tensile and compressive deformation conditions. It was observed that the morphologies of cavities near the fracture faces of tensile specimens varied markedly with testing conditions and in most cases differed from those near the gauge heads. Two quite different forms of cavitation behaviour were observed leading to high and low strains to failure, respectively. For optimum conditions of superplastic flow, of high temperature/low strain rate (low stress), when large elongations were observed, cavities were either spherical or elongated parallel to the tensile axis. Those near the fracture face interlinked in a plastic (necking) mode to give transverse cavities and subsequent failure. At high strain rate/low temperature (high stress), transverse intergranular cracking played a dominant role in failure at low elongations. For intermediate conditions of temperature/strain rate, elongated cavities developed parallel to the tensile axis, but near the fracture face these usually interlinked by transverse cracking. These conditions were associated with intermediate elongations to failure. For the assessment of cavity growth mechanisms, artificial pores were introduced into fine grain Y-TZP specimens and changes in their shape and size during tensile or compressive deformation were investigated. Results show that the change of pore volume, in the superplastic regime, is controlled by plastic deformation of the matrix and can be described by the relationship of dR/dɛ = ;ηR, where ɛ is the true strain, η the cavity growth rate parameter and R is the radius of the pore.

Cavitation behaviour in fine grain 3Y-TZP during tensile and compressive superplastic flow

The formation of mullite via gels prepared from technical ethyl silicate and aluminium chlorides has been studied. Normally, gels prepared specifically with the oxide stoichiometry of mullite (3Al2O3·2SiO2) do not form the mineral mullite on firing to 1200° C in the absence of a mineralizer. However, when the stoichiometric gel is homogeneous (achieved by acidic or neutral catalysts during the gel preparation) firing at 1200° C can lead to an almost quantitative yield of mullite. For a homogeneous gel, the presence of strontium or caesium salts, or an organo-tin compound such as dibutyltin diacetate or dibutyltin oxide during the gel preparation promotes almost quantitative conversion to mullite at about 1000° C. There is a threshold concentration under which conversion to mullite is incomplete, some cristobalite being formed. For the organo-tin compounds, the type of aluminium chloride is unimportant and the way in which water for the hydrolysis step is added is also unimportant. When the gel is non-homogeneous, the product obtained on firing contains cristobalite andα-alumina orγ-alumina, with little mullite, even if strontium or caesium salts, or organ-otin compounds are present. A ceramic bond is formed from alumina and some other refractory grains during firing.

T. J. Davies

Papers

Spinel Formation from Magnesium Aluminium Double Alkoxides

Proposed selection rules for suitable binders in cemented hard metals with possible applications for improving ductility in intermetallics

Cavitation behaviour in fine grain 3Y-TZP during tensile and compressive superplastic flow

Mullite formation from ethyl silicate and aluminium chlorides

Refractory oxides containing barium