T. Fransen

Oxidation kinetics of chromium and morphological phenomena

The oxidation kinetics of chromium at 900°C are independent of the oxygen partial pressure. Although this observation gives evidence for a defect mechanism where chromium interstitials account for the chromium transport in the oxide scale, the experimental phenomena do not support one single model. The occurrence of oxide whiskers and oxide ridges are explained by the energy of activation for the breakup of the oxidant molecule. Large oxide pegs are formed at metal multiple-grain junctions after scale breakdown.

Properties of alumina films by atmospheric pressure metal-organic chemical vapour deposition

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 aluminum 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-1. 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 γ-alumina was formed. Only OH groups were found as an impurity in the oxide film. No carbon was detected.

Corrosion Kinetics of Low- and High-Alloy Steels in Chlorine-Containing Gas Atmospheres

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 AISI310 (UNS 531000), and o...

The mechanical properties of thin alumina films deposited by metal- organic chemical vapour deposition

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-101, 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.

Properties of alumina films prepared by metal-organic chemical vapour deposition at atmospheric pressure in hte presence of small amounts of water

Thin alumina films were deposited on stainless steel, type AISI 304. The deposition process was carried out in nitrogen with low partial pressures of water (0–2.6 × 10−2 kPa (0−0.20 mmHg)) by metal-organic chemical vapour deposition (MOCVD) with aluminium-tri-sec-butoxide (ATSB) as the precursor. Also results are presented regarding the alumina deposition in the presence of small amounts of 2-butanol. The film properties, including the protection of the underlying substrate against aggressive gas compounds such as sulphur at high temperatures, the chemical composition, the microstructure, and the refractive index were investigated as a function of the water vapour pressure. In contrast with the results of stress reduction in silica films by the addition of small amounts of water to the deposition process, no significant effect on the internal stress in alumina films was found. TEM analysis showed that extremely fine grains of γ- Al2O3/AlO(OH) were formed, in agreement with the refractive index. Only an increase of the OH groups was found if water was added to the process, which also was the only impurity in the oxide film. Carbon was not detected.

Cracking and delamination of metal organic vapour deposited alumina and silica films

Amorphous alumina and silica films were deposited on AISI by thermal decomposition at atmospheric pressure of aluminium-tri-sec-butoxide and di-acetoxy-di-tertiary-butoxided-silane respectively. Above a critical coating thickness of the oxide films, cracking and delamination occurred during the deposition process. These were due to intrinsic rather than to thermal stresses. Cracking and delamination do not occurs simultaneously. The fracture toughness of the film, the substrate and the interface is an important factor. After delamination along the substrate-film interface, the films curled to scrolls, indicating stress. A complete explanation of the stress gradient formed in the film during the deposition process is not yet available.

High-temperature corrosion and mechanical properties of protective scales on Incoloy 800H: The influence of preoxidation and ion implantation

Coatings, obtained by preoxidation of Incoloy 800H at low PO2 show good sulphidation resistance due to the higher chromia content in the oxide scale. Yttrium-ion implantation of Incoloy 800H has also a beneficial effect on sulphidation, if preoxidation is applied. The reason for this is presumably the segregation of yttrium to grain boundaries of the oxide. Furthermore, the oxidation kinetics of Incoloy 800H are independent of the partial pressure of the oxygen. Mechanical testing of the preformed oxide scale/substrate combinations in air at 600°C by means of constant-extension-rate experiments shows that preoxidation at low partial pressures of oxygen leads to earlier scalecracking.

THE PROTECTION OF ALLOYS AGAINST HIGH TEMPERATURE SULPHIDATION BY SiO2-COATINGS DEPOSITED BY MOCVD

Silica coatings have been deposited on various alloys by MOCVD (Metal Organic Chemical Vapor Deposition) to protect them against high temperature corrosion in coal gasification environments. DiAcetoxyDitertiaryButoxySilane (DADBS) has been used as a metal organic precursor at deposition temperatures between 773 - 873 °K and amorphous layers were produced with a growth rate of about 1 μm. h-1. These coatings have been tested at 823°K in a sulphiclizing atmosphere with a low oxygen (9.3 10 -29 bar) and a high sulphur partial pressure (1.2 10 bar). In this environment the sulphidation resistance of various alloys has improved by a factor of at least 100 by the coating. The observed corrosion reaction is local and is explained by a model in which in the first stage cracks are formed due to mechanical stresses in the coating. In the second stage metal sulphides are formed by outward diffusion of metal and inward diffusion of sulphur through the cracks. When stainless steels are used as the alloy the outer layer consists of FeS and the lower one of CrS.

Properties of protective oxide scales containing cerium on Incoloy 800H in oxidizing and sulfidizing environments. I. Constant-extension-rate study of mechanical properties

The mechanical properties of ceramic coatings containing cerium oxide, prepared by the sol-gel method and used to protect Incoloy 800H against aggressive environments, are reported. Deformation and cracking behavior in oxidizing and sulfidizing environments has been investigated by constant-extension-rate tests. Extension rates were between 9.3×10−6 and 3.7×10−7 sec−1 at 823 <T<973 K. Under these conditions, cerium oxide sol-gel-coated specimens do not show any failure at extensions of 1.0% or more, but in hydrogen, sulfide failure is found at lower extensions than in air.

Al2O3 coatings against high temperature corrosion deposited by metal-organic low pressure chemical vapour deposition

Metal-organic chemical vapour deposition of thin amorphous films of Al2O3 on steels was performed at low pressure. Aluminium tri-sec-butoxide (ATSB) was used as a precursor. The effects of the deposition temperature (200–380 °C), the deposition pressure (0.17–1.20 kPa) and the ATSB concentration ((5.5−33.5) × 10−4 kPa) were studied with respect to the growth rate of the coating and the sulphidation properties at high temperatures. The sulphidation experiments were performed for 70 h at 450 °C in a gas atmosphere consisting of 1% H2S, 1.5% H2O, 19% H2, Ar balance. From the results and scanning electron microscopy observations the best process conditions were determined.