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

Aluminum oxide is a relatively cheap, abundant material that is widely used for plasma- spray applications. This material, however, exists in many crystallographic modifications with different properties. In addition, most of these modifications are metastable and cannot be used in applications employed at elevated temperatures. Usually γ, δ, or other phases form after spraying, while α phase (corundum) is often the most desirable phase due to high corrosion resistance and hardness. This paper first reviews the method of α stabilization in the as- sprayed materials offered in literature. Then, as an example, it summarizes the results of an extensive study of chromia additions to alumina. Chromia was chosen because of its complete solid solubility in alumina and its crystal lattice type, which is similar to that of alumina. It was demonstrated that the addition of approximately 20 wt% chromia results in the formation of one solid solution of (Al- Cr)2O3 in the α- modification. Finally, this paper discusses the thermal stability of various alumina phases. Phase change routes of heating for different starting alumina modifications are discussed, and a case study of alumina- chromia is presented. Both types of as-sprayed structures, a mixture of α, δ, and γ phases, and 100% (Al- Cr)2O3 were annealed up to 1300 °C and the phase composition checked. At lower temperatures and shorter holding times, the amount of α phase decreases while another metastable θ phase appears, and the fraction of γ + δ, if present, increases. At temperature above 1100 °C, the amount of α phase increases again.

Alumina-base plasma-sprayed materials part I: Phase stability of alumina and alumina-chromia

The effects of chromia (Cr 2 O 3 ) additions on the microstructural evolution and the mechanical properties of alumina (Al 2 O 3 ) were investigated. When small amounts ( 2 O 3 were added in samples hot pressed at 1500°C, the grain size distribution became bimodal; large platelike grains were dispersed in a relatively small grained matrix. The large grains were composed of a core region that is free of Cr and a surrounding shell region rich in Cr. The interface between the core and the shell was composed of misfit dislocations. The high diffusion rate of Cr ions through the surface of alumina was attributed to this microstructural evolution. The mechanical properties of the specimens were strongly influenced by this microstructural change. The fracture toughness and the flaw tolerance (R-curve behavior) of Al 2 O 3 were improved markedly by the formation of the large platelike grains. The hardness and the elastic modulus also increased, however, the fracture strength decreased by the addition of Cr 2 O 3 .

Effect of Cr2O3 addition on microstructural evolution and mechanical properties of Al2O3

Crystal structure and properties of Al2O3-Cr2O3 solid solutions with different Cr2O3 contents

Hot corrosion behavior of Al2O3–Cr2O3 refractory by molten glass at 1200 °C under static condition

Preparative procedures, grain growth, densification and change in corundum lattice dimensions in sintered compacts made from chromium (III) oxide/aluminium (III) oxide mixtures were evaluated in a study of some alumina-chrome refractories. High-energy milling using “A17 reactive” alumina gave densification at low temperature and rapid grain growth, also a low variability in modulus of rupture. X-ray diffraction studies confirmed solid solution formation, the corundum lattice dimensions decreasing with 7 wt% chromium (III) oxide, other compositions giving lattice expansion. The modulus of rupture at 1150 °C for a series of ethyl silicate-bonded alumina-chrome refractories containing 5 to 12 wt% chromium (III) oxide was also minimal at 7 wt% chromium (III) oxide. The composition of this series is typical of ethyl silicate-bonded alumina-chrome refractories used in the steel industry.

Preparation and properties of some alumina-chrome refractories

Refractory oxides containing barium

Incorporating chromium(III) oxide in alumina refractories improves resistance to thermal-shock damage and slag attack. In manufacturing aluminachrome refractories, the chromium(III) oxide can be added to a coarse alumina as a fine chrome ore [1] or to corundum as a purified chrome ore [2]. When preparing ethyl-silicate-bonded alumina-chrome refractories [3] the source of chromium is usually chromium(III) oxide. In our earlier work [4, 5] on alumina-chrome refractories the effect of powder preparation procedures and composition, compaction pressure, and temperature and time of sintering, on the formation of solid solutions in sintered compacts prepared from two aluminium(III) oxide materials (MA95, average particle size 4 #m angular; A17, reactive average particle size 2/~m round) and a chromium(III) oxide material (M100, average particle size 0.4/~m) was studied. These results are summarized in Table I. The low density of some sintered compacts prepared from MA95 material may be due to loss of chromium, which could occur by the reaction

Solid-state chemistry of alumina-chrome refractories

On a etudie des refractaires alumine-chrome contenant de 0 a 15% en poids de Cr 2 O 3 . On examine le broyage et le melange des poudres, la densification, la microstructure, la croissance de grain et les proprietes mecaniques des compacts frittes et les modifications de parametres cristallins du corindon.

H. G. Emblem

Papers

Preparation and properties of some alumina-chrome refractories

Refractory oxides containing barium

Solid-state chemistry of alumina-chrome refractories

Characterisation and properties of alumina-chrome refractories

Aluminium-barium oxides as binders for refractory grain mixes