Please use this identifier to cite or link to this item: http://localhost:8081/xmlui/handle/123456789/1189
Authors: Saxena, D
Issue Date: 1986
Abstract: The high temperature oxidation of a large number of metals and alloys in air and oxygen environment have been investigated extensively. It is now well established that the introduction of small amount of rare earth/re active metal additions ( <1% ) enhances the adherence of the scale and thereby scaling resistance of mainly a-alumina forming Fe, Co and Ni base alloys in oxidising atmosphere. Recently, the increasing demand of the alloys for use at high temperature in sulphur bearing atmospheres has stimulated the interest in sulphidation and simul taneous oxidation-sulphidation. The earlier work reported was mainly confined to effect of high sulphur pressure on sulphidation of common metals (Fe,Co,Ni ) and their alloys. However, little work has been reported on low sulphur pressures, which are generally encountered in practice as e.g. thermal power plants, petroleum industry etc. The rate of sulphidation of common metals and their alloys was found to be several order of magnitude higher than the oxidation rates. This has been attributed to porous morphology of the sulphide scales and absence of the subscale as found in oxidising conditions. Beneficial effect of reactive/rare earth elements in a-alumina scale forming alloys are hardly investigated in sulphidising environment. (v). In view of the above a systematic investigation on the effect of alloying additions like yttrium, yttria and different low concentrations of zirconium (0.2 - 1% ) in Fe-15Cr-4Al alloy under sulphidising condition was undertaken. Limited study was also undertaken on simul taneous oxidising-sulphidising atmosphere. The base alloy Fe-15Cr-4Al alloys with 1%Y, 0.2-1% Zr additions were prepared either by vacuum induction melting or non-consumable arc melting in argon atmosphere. These alloys were homogenised at 1000°C for 20 hours in vacuum then hot rolled to about 1mm thickness strips. The speci mens of size (10x5x1mm) were cut and polished. The polished and weighed specimens were taken for kinetic studies. For this study thermogravimetry setup, consisting of borosilicate helical glass spring balance was used. The weight gain vs time data was continuously recorded by monitoring the extension of the spring of cathetometer at fixed interval of time. The different sulphidising conditions were generated by changing the composition of Hg-H^S gas mixture, whereas the oxidising sulphidising conditions were obtained by H^-H^O-H^ mixture. All the alloys under investigation were sulphidised at different temperatures,700,350 and 1000°C respectively and the partial pressure of sulphur (P_ ) was varied -9 -3 2 from 10 to 10 atm. Thermogravijiietric data for all alloys was fitted by using least (vi) square method. Parabolic rate law came out as best flit curve in all alloys under above conditions. The para bolic rate constants (Kp )were calculated and their strong dependence on Ps was observed in all alloys but the extent of dependence varied. The activation energy plots (log K vs 1/T )are linear for all alloys and there is sligh/variation in activation energies observed with the addition of the dopants • Multilayer sulphide scales were found to form on all alloys and their different layers were characterised by using X-ray diffractometry, EDAX- area analysis, Electron probe microanalysis. X-ray images of Fe,Cr,Al,Y,Zr and S elements were also taken for knowing the distri bution of elements in sulphide scale. Generally triplex and duplex sulphide scales were found at high and low sulphur pressures respectively. The outer layer consisted mainly of iron sulphide. Whereas the layer just above the substrate was found to contain aluminium sulphide with little amount of iron and chromium sulphides. The zirconium and yttrium were only detected in the innermost scale. The middle layer was found to have higher amount of chromium sul phide with iron and aluminium sulphides . The sulphide scale was in general extremely fragile and crystalline. In some specimens, spelling of the scale was observed during cooling of the sample. In case of (vii) base alloy outgrowth along the edges could be observed. The addition of yttrium and zirconium led to improvement in sulphidation resistance as indicated by weight chan ges adherency of the scale and its morphology. In case of zirconium the higher concentrations were .less effective in increasing the sulphidation resistance. ..............
Other Identifiers: Ph.D
Research Supervisor/ Guide: Me, M. L.
Parkash, S. P.
metadata.dc.type: Doctoral Thesis
Appears in Collections:DOCTORAL THESES (chemistry)

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