The degradation of SiO2 coatings deposited on alloys by metal organic chemical vapour deposition (MOCVD) in sulphidizing high‐temperature environments is determined by delamination and crack formation. With increasing water concentration during deposition, the crack density in silica decreases and the critical thickness for delamination of SiO2 coatings increases. This improvement is supposed to be caused by compositional changes in the SiO2 coating. In this study presence of water and silanol groups as measured by Fourier transform infrared spectroscopy(FTIR) and the Si:O ratio as measured by XPS are discussed in relation to the protective properties. The FTIRmeasurements show that the coatings deposited in more humid environments contain more silanol groups and have lower stress levels. The coatings obtained under all deposition conditions consisted of stoichiometric SiO2.0 as determined by XPS. The presence of silanol groups reduces the viscosity of the coating, and stress relaxation by viscous flow becomes enhanced, thereby improving the coating performance.

A b s t r a c t . Vanadium oxide catalysts of the monolayer type have been prepared by means of chemisorption of vanadate(V)-anions from aqueous solutions and by chemisorption of gaseous V,O,(OH),. Using A1,0,, Cr,03, TiO,, CeO, and ZrO,, catalysts with an approximately complete monomolecular layer of vanadium(V) oxide on the carrier oxides can be prepared, if temperature is not too high. Divalent metal oxides like CdO and Zno may already form threedimensional surface vanadates at moderate temperature.The thermal stability of a monolayer catalyst is related t o the parameter z/a, i. e. the ratio of the carrier cation charge to the sum of ionic radii of carrier cation and oxide anion. Thus, monolayer catalysts will be thermally stable only under the condition that z/a is not too high (aggregated catalyst) nor too small (ternary compound formation). Vanadiumoxid Monoschichtkatalysatoren. I. Darstellung, Charakterisierung und thermische StabilitiitI n h a l t s i i b e r s i c h t . Durch Chemisorption von Vana,dat(V)-Anionen aus waBrigen Losungen, bzw. Cheniisorption von gasformigem V,O,(OH), warden Va.nadinoxidkatalysatoren des Monoschichttyps dargestellt. Mit A1,0,, Cr,O,, TiO,, CeO, nnd IntroductionVanadium oxides have been used extensively as catalysts in gas phase oxidation of hydrocarbons. I n spite of extensive studies [l-121 the interpretation of the role of vanadium in the mechanisms of catalytic oxidations is still contradictory.One of the problems is the difficulty in discriminating between surface atoms and the atoms in the interior of the oxide phase.HANKE et al.[13] suggested that more favourable conditions for studying the role of transition metal ions as active centres may be present, if the metal 26 F. ROOZEBOON, T. FRAXSEN, P. MARS, and P. J. GELLKNCS oxide can be obtained as a monomolecular layer, in which all atoms are dispersed on a carrier material. I n this case all atoms may take part in catalytic reactions. However, the active component is chemically influenced by the carrier, i. e., it has an intimate contact with the latter species without loosing its surface character. This is reflected by the ~ta~bility towards certain reactions of the monolayer vanadium(V) oxide compared to pure V,O,, such as reduction by hydrocarbons [ I l , 141 and dissolution in aqueous ammonia [14].The chemical influence of the carrier is not necessarily a disadvantage: by carrier selection the selectivity and activity may be influenced [lSJ. Thus, the importance of monolayer catalysts is based on the large surface area exposed per amount of active component and on the influence of the carrier on catalytic activity and selectivity.However, monolayer formation and monolayer stability are limited and depend strongly on the nature of the carrier and on temperature. In preparing monolayer catalysts it is essential that no ternary compounds of the type M,V,O, (which we shall call a three dimensional "salt") nor solid solutions are formed between the two oxides by interdiffusion of the cations. Otherwise the active c...

Thin alumina films were deposited at low temperatures (290-420 °C) on stainless steel, type AISI 304. The deposition process was carried out in nitrogen by metal organic chemical vapour deposition using aluminium tri-sec-butoxide. The film properties including the protection of the underlying substrate against high temperature corrosion, the chemical composition of the film, the microstructure, and the refractive index were investigated. The activation energy for the heterogeneous reaction was 83 _+ 5 kJ mol ~. Corrosion experiments, performed at 450 C in a hydrogen sulphide containing gas, showed that the amount of corrosion products of an alumina film (0.20 + 0.05 mg cm 2) AISI 304 combination decreased with increasing deposition temperature. The alumina films, even those deposited at 420'C, exhibited an amorphous structure, in agreement with the index of refraction. Transmission electron microscopy analysis revealed that extremely fine y-alumina was formed. Only OH groups were found as an impurity in the oxide film. No carbon was detected.

combustion and coal gasification, waste incinerators, fossil-fuel-fired boilers, and gas turbines is hightemperature corrosion. Most materials, when exposed at high temperatures to aggressive gaseous compounds, such as hydrogen sulfide, oxygen and hydrogen chloride or chlorine, are rapidly attacked. 1-3 Less information exists considering the performance of materials in coal gasification atmospheres containing small amounts of hydrogen chloride gas or chlorine.Additional corrosion problems have been associated with high concentrations of chlorine. For example, the chlorine content of British coal lies between 0.02 and 0.75%, 4 which corresponds to 150 to 1,000 ppm hydrogen chloride gas, which is unusually high compared with coals from most of the world. The chlorine was present mostly as inorganic chloride compounds, such as sodium, potassium, or calcium. It has been reported that the chlorine is partly bonded as sodium chloride, as well as a part present as chloride ions weakly bound to the coal matter. The chlorine in coal is rapidly released as hydrogen chloride gas during the initial stage of gasification or combustion processes.The mechanisms of attack by oxygen and/or sulfur are reasonably well understood, but the corrosion rate and mechanism may be altered by the high chlorine content. Presently, the effect of chlorine on the corrosion mechanism is still unclear. There is a well-accepted relationship between the corrosion rate ABSTRACT The effect of chlorine on the high-temperature corrosion of high-alloy steels Monit † , Al29-4C, Sanicro † 28 (UNS N08028), Sanicro † 31, Incoloy † 800H (UNS N08810) and AISI 310 (UNS 531000), and of low-alloy steels 10CrMo9.10, Mod.9Cr1Mo, X20CrMoV12.1, and 15Mo3 in atmospheres with low oxygen and high sulfur partial pressures were investigated. The addition of 500 ppm HCl (pCl 2 = 2.5 x 10 -20 atm) to the oxidizing/sulfidizing atmosphere at 450°C increases the corrosion rate and the void fraction in the sulfide scale and decreases the adherence causing spallation of the scale. Depending on the experimental procedure, locally condensed chloride compounds could be formed. For AISI 310, additional experiments on the effect of temperature and pCl 2 on the corrosion rate were performed. With increasing temperature or pCl 2 , the corrosion rate increased. Only the high-alloy steels showed promising results. Some models are discussed to explain the enhanced corrosion rate due to the presence of chlorine.KEY WORDS: corrosive atmospheres, high-temperature corrosion, hydrogen chloride, sulfidation, thermogravimetry FIGURE 9. Electron micrograph (SEI) of the surface of a 10CrMo9.10 alloy for 70 h at 450°C in an oxidizing/sulfidizing environment.

A simple mathematical model of the metal organic chemical vapour deposition (MOCVD) process is presented. This model consists of two coupled reaction schemes. The first is based on the basic equation for a plug flow reactor with homogeneous reactions. It is suggested that the decomposition of the metal organic precursor (in this case, aluminium-tri-sec-butoxide, ATSB) is irreversible and will form an intermediate I, which becomes the reactant of the irreversible reaction producing oxide (alumina). The second reaction scheme for the heterogeneous reaction deals with the equation for mass transfer with and without a homogeneous reaction. This part of the model is also known as the 'film model'.Using the mathematical model, some consequences of changing process parameters and material properties are discussed in relation to the deposition rate of alumina. It is found that the temperature, the gas flow and the position in the reaction tube are important parameters of the model. Also material properties, such as the activation energies and pre-exponential factors of the homogeneous and heterogeneous reactions, will effect the deposition rate. Notwithstanding the simplicity of this model, it explains the behaviour and probably predicts the effect of changing parameters on the deposition rate of thin alumina films.