Search

Oxide thin films have been investigated as protection against corrosion of Fe-based materials in highly carburizing atmospheres at high temperature. The present study describes the structural evolution of an Al oxide/Cr thin film coating deposited on 304 L steel after 50 h of exposure to a CH 4 + H 2 + residual O 2 atmosphere at 1073 K. Comparisons are then made to the structural evolution observed after 20 h of exposure from a previous study [1]. The coatings were produced by reactive magnetron sputtering at low temperature. The resulting thin films had very fine columnar structure, good adhesion to the substrate, amorphous Al oxide and tensile residual stresses. The response of the films to the carburizing atmosphere was evaluated via thermogravimetric analysis (TGA). The exposed samples were analyzed by X-ray diffraction, scanning electron microscopy equipped with energy dispersive analysis, and optical microscopy. Both, the TGA and structural analyses indicate that the presence of the films prevents the massive formation of internal and external corrosion products observed in uncoated 304 L samples exposed in the same conditions. However, the films underwent some structural modifications, which resulted in abundant interdiffusion through the film changing its as-deposited characteristics. [ABSTRACT FROM AUTHOR]

Abstract: The response to metal dusting of Cr/Cr2O3 thin films deposited on 304L stainless steel substrates by reactive magnetron sputtering (RMS) under various deposition conditions was investigated. Selected coated samples and bare substrates were exposed to an atmosphere of CH4 +H2 +residual oxygen at 800°C for up to 20h via thermogravimetric analysis (TGA). The results indicate that the presence of the Cr/Cr oxide film resulted in a clear improvement in resistance against metal dusting of the 304L steel. The protective behavior of the Cr/Cr oxide coatings arises from several effects: decrease of catalytic deposition of carbon on the Cr oxide surface (and expected prevention of inward diffusion of carbon through the Cr oxide), and hindrance of outward Cr and Fe diffusion which were identified as important events in the metal dusting of the bare 304L steel. The structural differences among the various Cr/Cr oxide films prepared in the present conditions played a lesser role in their protective behavior than the presence of the outer Cr oxide layer. [Copyright &y& Elsevier]

Abstract: The role of Cr/Cr oxide thin film coatings on HK40 steel substrates during exposure to C-rich atmospheres was investigated. The coatings were produced via reactive magnetron sputtering as an alternative method to protect these materials against metal dusting. Coated and uncoated substrates were exposed at 1073K to an Ar+CH4 atmosphere with residual oxygen for 10, 30, 60min and 50h. Analysis of the products formed on both samples indicated that attack in the uncoated samples involves both C and O from the atmosphere together with Cr, Fe, and Ni outward diffusion in which the dendritic structure plays an important role. The presence of even a residual amount of oxygen in the carburizing atmosphere had a remarkable effect on the corrosion mechanism in these samples as well. In the coated samples, the Cr/Cr oxide film minimized internal alloy attack not only by limiting carbon ingress, but also minimizing the role of oxygen and outward Cr (Fe and Ni) diffusion from the alloy. [Copyright &y& Elsevier]

Abstract: The evolution of the structure and properties of Cr/Cr oxide thin films deposited on HK40 steel substrates by reactive magnetron sputtering (RMS) was investigated and linked to their potential protective behavior against metal dusting. Deposition time, mode of oxygen feeding, and application of bias voltage were varied to assess their effect on the density, adhesion, and integrity of the films. All the films showed a very fine columnar microstructure and the presence of amorphous Cr oxide. Both, an increasing time and a constant oxygen flow during deposition led to the development of relatively low density films and mud-like cracking patterns. A graded oxygen flow resulted in films with fewer cracks, but a careful control of the oxygen flow is required to obtain films with a truly graded structure. The effect of the bias voltage was much more significant and beneficial. An increasing negative bias voltage resulted in the development of denser films with a transition to an almost crack-free structure and better adhesion. The amorphous oxide resulted in low values of hardness and Young''s modulus. [Copyright &y& Elsevier]

The behavior of uncoated and coated pure Fe exposed to an atmosphere of CH 4 + H 2 + residual O 2 at 1073 K for various times was studied by thermogravimetric analysis. For the uncoated Fe, extensive coking and carburizing was preceded by the formation and reduction of Fe oxides that occurred in the first 5 h of exposure and produced a highly un-protective porous Fe-rich structure on the surface. The metastable Fe-O-C diagram for the present temperature indicates that Fe oxides may be stable until carbon accumulation due to CH 4 decomposition builds up to a level high enough to start reduction of the oxides. Regardless of the Fe oxides that form, eventually wustite will be present during reduction, and its conversion to Fe involves a change in volume that results in the porous Fe observed, which is evidently non-protective. Thus, Fe-based materials exposed to carburizing-oxidizing atmospheres will be always potentially susceptible to this adverse effect. The presence of a Cr/Al oxide thin film coating applied by d.c. magnetron sputtering helped to decrease coking and carburization serving as a diffusion barrier for outward Fe and to avoid formation of the oxides, and for inward C in the coking and carburizing stage. [ABSTRACT FROM AUTHOR]

Abstract: Cr oxide/Cr thin films have been deposited on HK40 steel substrates in order to evaluate their behavior under metal dusting conditions. The films were produced by reactive magnetron sputtering under various conditions of oxygen flow, work pressure, power and bias voltage to find the optimum deposition conditions to form an adequate dense structure with good adhesion to the substrate. The coatings were extensively characterized by scanning electron microscopy, x-ray diffraction and atomic force microscopy. The most promising structures were subjected to metal dusting conditions in a thermobalance (TGA) to evaluate their response to this type of corrosion. The results indicate that lower applied power and slowly graduated feeding of oxygen resulted in coatings with less residual stresses, higher density and better adherence. The TGA experiments showed an improvement in resistance to metal dusting of the coated samples compared to an uncoated one. [Copyright &y& Elsevier]

Abstract: The presence of some surface oxides in materials exposed to metal dusting has been proven to be an effective method to prevent this type of corrosion due to the very low diffusivity of carbon in oxides. However, the surface oxide films have to be dense and adhere well to the component to be protected. Reactive magnetron sputtering (RMS) is a promising method to produce these types of oxide layers due to its flexibility in terms of process parameters and resulting structures. In the present work, Al oxide/Al layers have been deposited by RMS on HK40 substrates under various conditions in order to develop the most adequate structure for protection against metal dusting. Some coated substrates were subjected to metal dusting conditions in a thermobalance. The microstructure of the coatings before and after metal dusting was characterized by scanning electron microscopy, atomic force microscopy and x-ray diffraction. Bias voltage was found to be the most important deposition variable for the development of dense adherent films. The coatings showed a protective behavior during the carburization experiments, however more work is needed to optimize the structure of the coatings for more effective protection. [Copyright &y& Elsevier]

Abstract: Metal dusting is a corrosion process that occurs in highly carburizing atmospheres. It represents significant economical losses in the petrochemical, iron reduction, and heat treatment industries. In the present work, PVD coatings of Cr adhesion layers+protective chromium oxides have been applied to HK40 substrates and their response to metal dusting measured by thermogravimetric analysis. Coatings were produced by reactive magnetron sputtering under different processing conditions. For the adhesion layer, the effect of power and surface roughness on the quality of the adhesion layer was analyzed. In the case of the oxide deposit, the composition of the reactive (Ar+O2) mixture was varied. Thermogravimetric analysis of uncoated and coated samples was carried out in carburizing conditions. The microstructure of the samples was analyzed by scanning electron microscopy+energy dispersive analysis and X-ray diffraction. The results indicate that the quality of the coatings, particularly their density and adhesion, are more important than the thickness of the layer deposited for effective protection against metal dusting. [Copyright &y& Elsevier]