Defect and Diffusion Forum Vols. 237-240

Abstract: In the modeling of the oxidation and degradation processes of the superior metallic materials in terms of the high temperature oxidation resistance, being the so-called alumina formers, the matter transport only via grain boundaries of α-Al2O3 is usually taken into account. This paper indicates that such an approach needs to be re-visited. It is pointed out that the following factors should be taken into account (i) diffusion via point defects in the unstable alumina polymorphs (γ, δ, θ) which grow during the early oxidation stages; (ii) dislocation-related processes; (iii) formation of the oxide in three-dimensional defects, being the cracks in the oxide layer; (iv) outward oxide growth resulting from the formation of the new oxide within the existing layer. The mentioned above effects as well as the other ones, related to the interfacial and stress-induced processes are illustrated and discussed. It is pointed out that alumina formers constitute a wide group of materials and several categories should be distinguished among them with respect to their composition and production route on one hand, and the oxidation and degradation mechanisms, on the other hand. It is shown that the comprehensive approach enables better and self-consistent modeling of the oxidation and degradation mechanisms of alumina formers.

Abstract: The effect of hydrogen during annealing of Fe-15.at%Cr alloy on the diffusion profile was investigated concerning the enrichment of chromium and the selective oxidation. Samples were exposed to an annealing gas mixture with different hydrogen contents in an infrared heating furnace and heated to 800oC, kept for 60 seconds and then cooled down to room temperature. After the experiments, field emission scanning electron microscopy (FE-SEM) equipped with electron back-scattering diffraction (EBSD) and x-ray photoelectron spectroscopy (XPS) were employed to characterize the morphology, elemental depth profiles and the chemical states of the elements. The annealing increased the Cr content at the surface in all atmospheres. The increase of hydrogen content in the atmosphere further increased the Cr to Fe ratio in near-surface, and the thickness of the layer affected by the heat treatment. The selective oxidation of chromium occurred as internal Cr2O3 formation, as a function of the Cr content, rather than the oxygen partial pressure. Hydrogen facilitated the diffusion of chromium probably by cleaning of fast diffusion paths.

Abstract: Chromia protective layers are used to prevent corrosion by oxidation of many alloys, such as the stainless steels, for instance. To check if chromia is a barrier to the outward diffusion of iron in these alloys, iron diffusion in chromia was studied in both polycrystals and oxide films formed by oxidation of Ni-30Cr alloy in the temperature range 700-1100°C at an oxygen pressure equal to 10-4 atm. An iron film of about 70 nm thick was deposited on the chromia surface, and after the diffusing treatment, the iron depth profiles were established by secondary ion mass spectrometry (SIMS). Using a solution of the Fick’s second law for diffusion from a thick film, effective or bulk diffusion coefficients were determined in a first penetration domain. Then, Le Claire’s and Hart’s models allowed both the bulk diffusion coefficient and the grain boundary diffusion parameter (aDgbd) to be obtained in a second penetration domain. Iron bulk and grain boundary diffusion does not vary significantly according to the nature-microstructure of chromia. The activation energy of grain boundary diffusion is at least equal or even greater than the activation energy of bulk diffusion, probably on account of segregation phenomena. Iron diffusion was compared to cationic self-diffusion and related to the protective character of chromia.

Abstract: Even though the oxidation behavior of steels is generally considered as to be widely understood, a closer look reveals some open questions, e.g. regarding the influence of the substrate grain size on the overall oxidation kinetics. At temperatures below 570°C the main constituent of the oxide scale formed on top of low alloy steels is magnetite. As shown by gold marker experiments it grows outward and inward at the same time, the latter exhibiting a gradual transition to the more stable spinel compound FeCr2O4. As indicated by intergranular-oxidation attack below the superficial scale, inward oxide growth seems to be driven by oxygen transport along the grain boundaries serving as fast diffusion paths. This is supported by thermogravimetric oxidation tests in air on low-Cr steels with varying grain size: The smaller the grains the higher the oxidation rate. Recently, a numerical model for the diffusive transport processes based on the finite-difference approach has been developed, which distinguishes between fast grain-boundary diffusion and bulk diffusion. Qualitatively, it is capable to predict the relationship between substrate grain size and inward oxide growth kinetics. Together with the thermodynamic tool ChemApp and in combination with a data set for the Fe-Cr-O system the mechanism-based simulation of the overall oxidation process of low-Cr steels is possible.

Abstract: Since the introduction of Electroless (EL) coating in 1946 by Brenner and Riddle, the process has been the subject of steady growth. It is one of the most elegant methods available for the production of protective coatings on surface. The technique involves the autocatalytic reduction, at the substrate/solution interface, of cations released from suitable chemical reducing agents with in the EL bath. EL coating technique is simple one, as can be manifested just by controlling pH and temperature of the coating bath. Such coatings are reported to provide excellent physical and mechanical properties. The nickel, silver and copper based alloy and composite EL coatings are being studied at Indian Institute of Technology Roorkee since 1985 and this paper deals with the gist of the same. The structural behavior of Ni-P coatings for different phosphorus contents has been extensively studied. Transmission electron microscopy (TEM) and magnetic movement studies have been used as tools for structural and kinetic studies, respectively. Submicron size coating islands and their transformations have been deduced. The metallography studies using hot stage within TEM to follow the phase transformations occurring at various temperatures have been studied for Ni-B, EL coatings. As a forward step towards composite coatings, Ni-P-C, Ni-P-Al2O3, Ni-P-ZrO2 etc. have been developed by co-deposition techniques. Ag-graphite composite coatings produced by EL technique exhibits ~5 times higher wear resistance and ~ 2 times better corrosion resistance apart from being a good electrical conductor. The tribological behavior of EL Ni-P-X and Ni-P coatings on steel and aluminium substrates in different conditions i.e., as-coated, heat treated at various temperatures at different extents of time with different normal loads, have been studied in terms of dry sliding friction and wear against counter face of case hardened steel. In Ni-P-X nanocoatings (X= ZrO2-Al2O3-Al3Zr), X has been produced in-situ and are composed of nano-sized particles. Such coating could be produced uniformly on carbon fibre of 7µ diameter. Ni-P and Cu are also coated successfully on graphite/ oxide powders of ~ 120µ sizes. Micro-thickness coatings are paving ways to nanocoatings. These are the coatings in which either the thickness of the coating is in nano level or the second phase, that dispersed in the coating matrix is of nano-size. Ni-P-Ferrite nano coatings with thickness less than ~0.1mm thick, is found to exhibit about 20db of absorption of microwave in the rage of 12-18 GHz which can be exploited for radar applications. Here the nano-sized ferrite particles are co-deposited along with the Ni-P EL coatings.

Abstract: Mathematical model of selective and competitive oxidation of multi-component non ideal alloys is used for modelling oxidation of Fe-Cr-Ni alloys. The model is based on: a) the Danielewski-Holly model of interdiffusion, b) the Wagner model of the Ni-Pt alloy oxidation, c) the postulate that the values of fluxes in reacting alloy are limited (the kinetic constraint) and d) the thermodynamics of the Fe-Ni-Cr system. In this paper for the first time modelling of oxidation of a ternary non-ideal alloy based on Danielewski-Holly model is presented. The model is used to predict the evolution of component distributions in the reacting ternary Fe-Cr-Ni alloy. The results of the modelling of oxidation of the 316L stainless steel at 1173 K are presented. We compute the chromium depletion during the long term oxidation. The results allows to conclude that the oxidation reaction is limited by interdiffusion in reacting alloy. The computations demonstrate that the chromium depletion is the key factor affecting the scale stability during the long time exposition.

Abstract: Experimental measurements do not allow for a unique determination of the concentration profiles, e.g., in case of multi-layer systems. The measured concentration of the elements at the alloy/scale interface is an average concentration in an alloy and in a scale near the spot of the beam [1]. The knowledge of the concentration of the elements at the boundary is necessary for the understanding corrosion of alloys. This essential obstacle of experimental techniques can be overcome by computer modelling. Namely, by combining the different methods (non-unique measurement with unique modelling). The Danielewski-Holly model of interdiffusion has a unique solution. This model enables to predict the evolution of component distributions in the reacting alloy. The model is valid for time dependent boundary conditions and consequently can be used for modelling the more complex reactions, eg., the formation of complex oxides. To avoid the nonphysical values of fluxes in reacting alloy the kinetic constraint on all fluxes was introduced, i.e., the flux limitation method. The results of the selective oxidation of the P91 steel (0,1 wt.% C, 8,6 wt.% Cr, 0,25 wt.% Ni) are presented. Calculated concentration profiles are compared with the experimental data. We show the evolution of chromium distribution in oxidizing steel up to 3 000 hours. The computations demonstrate that chromium depletion is the key factor determining the scale composition.

Abstract: The results of the study of the scale growth mechanism on the 50 µm thick foil of Fe18Cr5Al are reported. The experiments were carried out at 1093 K using a two-stage-oxidation approach. In the first stage, the samples were exposed to the SO2 + 1 vol. % O2 mixture for 1.5 h and 24 h in order to produce the primary scale. Subsequently, the samples were cooled down to the room temperature and moved to the furnace where they were exposed to the atmosphere rich in 18O2. The duration of the second stage was 0.5 h and 4.5 h for samples previously oxidized for 1.5 h, 8 h and 72 h, for the samples oxidized previously for 24 h. The elemental distributions were determined using high resolution SIMS, while the scale morphology was observed by means of SEM. In addition the radioisotopic studies using 35SO2 were carried out in order to assess whether sulfide phases developed at the initial stage of oxidation. All the results were interpreted in terms of the transport processes in the oxide scale.