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.

The metal organic chemical vapour deposition (MOCVD) of amorphous alumina films on steel was performed in nitrogen at atmospheric pressure. This MOCVD process is based on the thermal decomposition of aluminiumtri-sec-butoxide (ATSB). The effect of the deposition temperature (within the range 290-420 °C), the precursor vapour pressure (5.33 x 10-3_2.67 x 10-2 kPa), and the gas flow (6.5-12.5 1 mint) of the MOCVD process have been studied in relation to corrosion properties at high temperatures. The corrosion experiments were performed at 450 °C in a gas atmosphere containing 1% H2S, 1% H20, 19% H2, and balanced Ar. It was found that the amount of corrosion products on an alumina film (0.20__. 0.05 mg cm-2)-AISI 304 combination decreased with increasing deposition temperature of the coating. This was more pronounced for the products formed through the coating owing to a certain porosity. The crack density, where products were also formed, was almost unaffected.

Amorphous alumina films were deposited by metal-organic chemical vapour deposition (MOCVD) on stainless steel, type AISI 304. The MOCVD experiments were performed in nitrogen at low and atmospheric pressures. The effects of deposition temperature, growth rate and film thickness on the mechanical properties have been studied. The experiments were performed with the dynamic ultra-micro-hardness tester, DUH-200, and the scanning scratch tester, SST-II)]. both developed by Shimadzu. The DUH-200 is associated with crack formation during indentation. This technique involves a qualitative method to study the crack behaviour of the thin alumina films as well as a method to estimate the fracture toughness of the film and the film/substrate interface. The experiments performed with the SST-101 are based on the estimation of the film adhesion to the substrate by determining a critical load; the load where the film starts to spall or to delaminate. The best mechanical properties were obtained using low deposition rates and high deposition temperatures. Therefore, low-pressure MOCVD is recommended in addition to the deposition of alumina films at high temperatures.