Defect and Diffusion Forum Vols. 289-292

Abstract: High temperature reactivity of materials under oxidizing atmospheres is based on the formation of protective oxide scales. The protectiveness is obtained when the thermally grown oxide scales are dense, continuous and adherent to the metallic substrates (even during thermal shocks); as a matter of fact, the growth of the scale has to be governed by diffusion of species across the growing scale. The diffusing species are coming from the substrate (metallic ions) and/or from the oxidizing atmosphere (oxygen ions). The understanding of growth mechanisms can be reached by making two stage oxidation experiments, using oxygen isotopes. The experiment consists in oxidizing first the metallic substrate in 16O2, evacuating the oxygen after the desired time, and replacing it by 18O2. The distribution of oxygen isotopes given by secondary ion mass spectrometry (SIMS) across the oxide scale informs about the growth mechanisms: anionic transport, cations diffusion or mixed diffusion processes. The use of marker experiment is able to determine the oxygen diffusion coefficients within the growing scales. In this case, a thick scale is grown under 16O2, followed by a shorter diffusion experiment in 18O2. The distribution of 18O isotope across the scale by plotting ln (18O intensity) versus x (depth in oxide) allows determining the oxygen effective diffusion coefficient in the scale, according to the classical Fick’s law solution. A more accurate analysis of these profiles can provide information about bulk and grain boundary diffusion of oxygen. The results can be related to kinetics according to Wagner’s theory. The proposed work consists firstly in making a state of the art review about oxygen diffusion in thermally grown oxide scales, and secondly in connecting the so-obtained outcome (effective, bulk and grain boundary diffusion) to kinetics results. The proposed oxides are chromia, alumina and zirconia.

Abstract: The development of new power generation plants firing fossil fuel is aiming at achieving higher thermal efficiencies of the energy conversion process. The major factors affecting the efficiency of the conventional steam power plants are the temperature and, to a lesser extent, the pressure of the steam entering the turbine. The increased operating temperature and pressure require new materials that have major oxidation resistance. Due to this problem, in the last years numerous studies have been conducted in order to develop new coatings to enhance the resistance of steels with chromium contents between 9 and 12% wt against steam oxidation in order to allow operation of steam turbines at 650 0C. In this study, Si protective coatings were deposited by CVD-FBR on ferritic steel P-91. These type of coatings have shown to be protective at 650 0C under steam for at least 3000 hours of laboratory steam exposure under atmospheric pressure. Morphology and composition of coatings were characterized by different techniques, such as scanning electron microscopy (SEM), electron probe microanalysis, and X-ray diffraction (XRD). The results show a substantial increase of steam oxidation protection afforded by Si coating by CVD-FBR process.

Abstract: Nitriding by low energy high flux processing has been carried out at about 400°C in fcc metal substrates (pure Ni, Ni-20Cr model alloy and a conventional AISI 304L stainless steel). The gradual ingress of this element into the structures will be shown to depend markedly on the chemical composition of the substrate. The associated expansion of the fcc lattices and surface roughness will be discussed in this work with the support of X-ray diffraction, atomic force, scanning and transmission electron microscopy techniques. In light of the resulting composition, microstructures and thickness of nitrided layers, some preliminary results of their behaviour under isothermal oxidation conditions at high temperatures will be discussed. The high temperatures will provoke decomposition of the expanded austenite into a conventional gamma phase and some chromium nitrides. Trapping of chromium therefore shall explain a reduction of the high temperature oxidation resistance against the untreated substrates.

Abstract: Oxidation mechanism of nonstoichiometric cuprous oxide (Cu2 yO) has been studied as a function of temperature (973-1273 K) and oxygen pressure (1-105 Pa), using microthermogravimetric, marker and two-stage oxidation techniques. The last method consisted in using heavy oxygen isotope 18O in order to confirm the results, obtained with marker technique. It has been found that in early stages of reaction not exceeding 30 hours, CuO formation on the surface Cu2 yO follows approximately cubic rate law. In later stages, during long-term oxidation, lasting hundreds of hours, oxidation process follows strictly parabolic kinetics with the rate, being independent of Cu2 yO pre-treatment. Marker and two-stage oxidation experiments have shown that the growth process of CuO layer proceeds by the outward diffusion of cations, clearly indicating that cation sublattice of CuO is predominantly defected.

Abstract: This paper originates from a large EU programme designed to produce high performance coatings with superior mechanical and corrosion resistance properties to enhance high temperature corrosion behaviour of TiAl based alloy (Ti45Al8Nb).The paper here is concerned with studies of the hot corrosion behaviour of Ti – Aluminide alloys (Ti45Al8Nb mainly -TiAl with small amount 2 (Ti3Al)) coated with three coatings: (1) TiAlYN/CrN coating, (2) CrAl with 2%YN addition [1,2] (3) TiAlN with Al2O3 overlayer. Hot corrosion was induced by spraying salts mixture of 80%Na2SO4/20%NaCl with melting point around 700oC [1] [2] on heated sample surfaces, and subsequently heating the sprayed samples in a furnace at 750oC for various times. The extent of degradation, assessed by weight changes and examination by OM (Optical Microscope), SEM (Scanning Electron Microscope), and EDX (Energy Dispersive X-Ray Analysis) indicated that all samples suffered hot corrosion attack to various degrees. TiAlN with Al2O3 overlayer had the best corrosion resistance.We shall discuss these results within a mechanistic framework of hot corrosion degradation and describe their implications.

Abstract: The aim of the present work is to introduce a thermodynamic model to describe the growth of an oxide layer on a metallic substrate. More precisely, this paper offers a study of oxygen dissolution into a solid, and its consequences on the apparition of mechanical stresses. They strongly influence the oxidation processes and may be, in some materials, responsible for cracking. To realize this study, mechanical considerations are introduced into the classical diffusion laws. Simulations were made for the particular case of uranium dioxide, which undergoes the chemical fragmentation. According to our simulations, the hypothesis of a compression stress field into the oxidised UO2 compound near the internal interface is consistent with the interpretation of the mechanisms of oxidation observed experimentally.

Abstract: Thermal spray coatings provide good tribological and corrosion-resistant properties. Coatings with carbides or diborides improve resistance to oxidation. However, depending on the thermal spray conditions some carbide and diboride oxidation takes place. The aim of this study is to analyze the oxidation of TiC or TiC+TiB2 powders embedded into a stainless steel matrix. The starting powders were obtained by Self-propagating High-temperature Synthesis (SHS). Oxidation was studied at two temperatures, 700°C and 800°C. The results aid our understanding of the powders’ behaviour during coating and use. An open electric furnace was used, and the samples were analyzed using a SEM coupled with an EDS. Thermal treatment time ranged from 2 minutes to 9 hours. A continuous oxidized layer was observed for the TiC particles. This was not the case for TiB2. The decomposition/oxidation of TiB2 begins before the oxidation of TiC. An oxidized layer of around 4 m is formed at a temperature of 800°C after 690 s, when using a powder of 45 m mean size.

Abstract: The steam oxidation behaviour at 800°C of aluminized HCM12A ferritic-martensitic steel has been studied. The aluminization process used was CVD in fluidized bed reactor (CVD-FBR), using a reactive bed modified with Ce or La particles. The obtained coatings were mainly composed of (Fe,Cr)2Al5 intermetallic phase. Long term oxidation (1000h) behaviour of the coated HCM12A was studied in 100%H2O atmosphere. By the application of the protective coating, the ferritic-martensitic steel oxidation rate is reduced considerably because of the alternately formation of Al2O3 and Cr2O3 + (Fe,Mn)3O4 protective scales on the substrate surface due to the diffusion processes that take place during the exposure at high temperatures in combination with the aggressive environment.

Abstract: The influence of Al and Al-Si coatings on the corrosion behaviour of HCM12 in molten KCl-ZnCl2 mixture at 650°C in air has been characterized by electrochemical impedance spectroscopy (EIS). Al and Al/Si protective coatings were developed by chemical vapour deposition in fluidized bed reactor (CVD-FBR) at moderate temperature to respect to mechanical properties of substrate. Scanning electron microscopy (SEM) was used to analyse the damage on the HCM12 electrode surface. Al-Si coating was found to be more resistant to the molten chlorides attack than Al coating; and both coatings increased the corrosion resistance of HCM12 in these conditions. The surface composition has been determined by X-ray diffraction (XRD).

Abstract: The kinetics and mechanism of metal dusting corrosion of 9Cr-1Mo steel, commonly used in CCR platforming units, have been studied as a function of temperature (773 – 1173 K) in propane-butane atmosphere, being the mixture of 70 vol. % of propane and 30 vol. % of butane with the total pressure equal 105 Pa. The kinetics of corrosion have been studied thermogravimetrically in the apparatus enabling the mass changes of corroded sample to be followed continuously with the accuracy of the order of 10-6 g. It has been found that metal dusting corrosion in this atmosphere, modeling in some way industrial environments in petrochemical industry, is complex and two-stages of linear kinetics may be distinguished. In the first stage, which may be considered as an incubation period, the reaction proceeds with rather low rate, which increases dramatically in the second stage, the beginning of which depends strongly on temperature. Linear course of reaction indicates that chemical reactions and not diffusion processes determine the rate of corrosion. This conclusion is confirmed by the fact, that the layer of corrosion products is not compact but considerably porous.