HIGH TEMPERATURE SULFIDATION BEHAVIOUR OF AN Fe-Mn-Al ALLOY A Thesis submitted in fulfillment of the requirements for admission to the Degree of Master of Science by Noreen Sa Na Quan December, 1984 School of Chemical Engineering and Industrial Chemistry The University of New South Wales STATEMENT This is to certify that the work presented in this thesis was carried out in the School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, and has not been submitted previously to any other university or technical institution for a degree or award. Noreen Sa Na Quan (i) ABSTRACT The aim of this research was to investigate the sulfidation behaviour of a ferrous alloy containing 4.5 w/o (weight percent) manganese, 8.8 w/o aluminium and 0.36 w/o carbon. The alloy was sulfidised at 973 K, 1073 K and 1173 K in H2/H2S atmospheres corresponding to sulfur partial pressures between 10 -8 atm and 10 -4 atm. The weight gain of each sample over a period of time was measured gravimetrically. X-ray diffraction analyses, microscopic techniques and electron probe microanalysis were used to analyse the scales formed. Two main types of reaction kinetics were observed. On the one hand, slow parabolic weight uptake kinetics were observed when the combination of temperature and sulfur partial pressure was _ i not too severe at T = 973 K when P <10 atm and at T = b2 1073 K when P = 10 -7 atm. Under these conditions, a thin b2 uniform reaction product layer of oc - MnS and in one instance Al2S3 as well was formed. When only <x - MnS was formed, the sulfidation rate of the alloy was approximately of the same magnitude as the sulfidation rate of pure Mn (5) On the other hand, at higher temperatures and sulfur partial -5 -4 pressures i.e. at T = 973 K when P = 10 atm, 10 atm and _2 at T = 1073 K when 10 < P_ < 10 4 atm and at T = 1173 K b2 when 10 < P <10 atm, breakdown of the protective S2 oc - MnS scale was observed with the formation of large sulfide nodules. This was associated with initially accelerating reaction followed by fast linear kinetics. The (ii) large nodules were found to contain Fe(Mn)S and a voluminous, porous two phase reaction product layer at their base which was found to be Fe(Mn)Al2S4 plus Mn(Fe)S. This pattern of behaviour was ascribed to mechanical failure of the initially formed protective scales allowing gas access to the under­ lying metal which leads to nucleation and growth of nodules. Behaviour of a somewhat intermediate type was observed at T = 973 K and P = 10 ^ atm. z (iii) LIST OF TABLES Page Table 1 Mean Rate Constants obtained during 35 the sulfidation of Fe-Al alloys and Pure Fe. Table 2 Rate Constants obtained during sulfidation 51 of Fe-Mn alloys, Pure Iron and Pure Manganese. Table 3 Values of the Parabolic Rate Constant, 53 k , for growth of oc - MnS on Mn. Table 4 Sulfidation Rate Constants of 63 Fe-25Mn-5Al alloy. Table 5 Parabolic Rate Constants of 63 Fe-25Mn-20Al. Table 6 Comparison of Sulfidation Rate 73 Constants at 973 K. Table 7 Comparison of Sulfidation Rate 74 Constants at 1073 K. Table 8 Standard Free Energies of sulfide 77 formation. Table 9 Equilibrium data for sulfide formation. 79 Table 10 Experimental conditions. 80 Table 11 Sulfidation Rate Constants of 94 Fe-4.5Mn-8.8A1-0.36C. Table 12 Parabolic Rate Constants of Iron and 138 Fe-4.5Mn-8.8A1-0.36C alloy. (iv) Page Table 13 Parabolic Rate Constants for 142 sulfidation at T = 973 K. Table 14 Parabolic Rate Constants at 142 T = 1073 K and P = 10_7 atm. z Table 15: Linear Rate Constants obtained by 149 sulfidation of Fe-4.5Mn-8.8Al and Fe-25Mn-5Al. (V) LIST OF FIGURES Page Figure 1 Activity gradients of both metal 6 and oxygen established across the scale. Figure 2 Diffusion of metal through scale. 7 Figure 3 Ellingham Diagram 14 Figure 4 FeS phase diagram in the Fe^_^S 19 region. Figure 5 Autoradiogram of cross-section of 30 sulfide scale on iron at 973 K. Figure 6 Reaction Mechanism (schematic). 38 Figure 7 Manganese concentration vs lattice 44 parameter of (X -manganous sulfide in 1 atm of sulfur vapour at 1073 K, 1173 K, 1273 K and 1373 K. Figure 8 Change in the lattice parameters 44 with sulfur pressures of 1 atm, 10 atm, 10 ^ atm, 3.5 x 10 ^ atm, all at 1273 K. Figure 9 Lattice parameters (a0,c0) of ferrous 45 sulfide at various pressures at 1273 K. Figure 10 Equilibrium phase diagram of an 45 FeS-MnS system. Figure 11 Changes in (Ns/Npe + Nm^) in sulfides 46 with manganese content at various sulfur pressures at 1273 K. (vi) Page Figure 12 Arrhenius plots of the self diffusion 61 coefficient of Mn in the <*: - MnS scales. Figure 13 Schematic assembly of sulfidation 82 rig used. Figure 14 Linear plots, weight gain vs time, 89 for the sulfidation of the alloy at 973 K in 10_8^ Pq ^ 10~4 atm. b2 -4 Figure 15 Parabolic plots, mg2 - cm vs time 90 (min) for the sulfidation of the alloy — 8 at 973 K in P = 10 atm and 10 -5 atm. 2 -4 Figure 16 Parabolic plots, mg2 cm vs time (min) 91 for the sulfidation of the alloy at 973 K and P = 10 ^ atm. b2 -4 Figure 17 Parabolic plots, mg2 cm vs time (min) 92 for the sulfidation of the alloy at 973 K and P = 10 6 atm. b2 Figure 18 Linear plots, weight gain vs time, for 95 the sulfidation of the alloy at 1073 K in 10 ^ P 10 4 atm. Figure 19 Linear plot , weight gain vs time, for 96 the sulfidation of the alloy at 1073 K in P = 10 ^ atm. b2 -4 Figure 20 Parabolic plots, mg2 cm vs time (min) 97 for the sulfidation of the alloy at 1073 K in P = 10 atm. b2 (vii) Page Figure 21 Linear plots, weight gain vs time, 98 for the sulfidation of the alloy at 1173 K in 10"7^T P ^ 10-4 atm. b2 -1 Figure 22 Plots of - lnk^ vs 1/T (k ) for the 101 -4 alloy sulfidised at P = 10 atm. b2 -1 Figure 23 Plots of -lnk^ vs 1/T (k ) for the 102 -5 alloy sulfidised at P = 10 atm. b2 Figure 24 Plots of -log k, vs log (P ) for the 104 b2 alloy sulfidised at 1173 K and 1073 K. Figure 25 Micrograph of scale formed on alloy 106 sulfidised at T = 973 K and P = 10 ^ b2 atm for 7.4 hours. Figure 26 Micrograph of scale formed on edge of 107 specimen sulfidised at T = 973 K and -7 P = 10 atm for 4.3 hours. b2 Figure 27 Micrograph inner layer of scale formed 107 on edge of alloy sulfidised at T = 973 K and P = 10 7 atm for 4.3 hours. b2 Figure 28 Micrograph of outermost scale formed 107 on edge of specimen sulfidised at T = 973 K and P = 10 7 atm for 4.3 b2 hours. Figure 29 Electron probe microanalysis of 108 specimen sulfidised at T = 973 K and -7 10 atm. (Viii) Page Figure 30 Electron probe microanalysis of 109 specimen sulfidised at T = 973 K -7 and P = 10 atm. b2 Figure 31 Micrograph of scale formed on alloy 112 sulfidised at T = 973 K and P = b2 10~6 atm for 4.6 hours. Figure 32 Micrograph of scale formed on alloy 112 sulfidised at T = 973 K and P = -6 2 10 atm for 4.6 hours. Figure 33 Tiny nodule formed on alloy sulfidised 112 at T = 973 K and P^ = 10 ^ atm for 4.6 hours. Figure 34 Electron microprobe analysis of scale 113 formed on nodule of specimen sulfidised at 973 K and P = 10 ^ atm. b2 Figure 35 Electron microprobe analysis of scale 114 formed on flat surface of specimen sulfidised at 973 K and P = 10 ^ atm. b2 Figure 36 Micrograph of scale formed on edge of 116 specimen sulfidised at T = 973 K and -5 P =10 atm for 4.8 hours. b2 Figure 37 Micrograph of scale formed on edge of 116 specimen sulfidised at T = 973 K and -5 P =10 atm for 4.8 hours. b 2 Figure 38 Micrograph of scale formed on flat 116 surface of specimen sulfidised at T = 973 K and P = 10 ^ atm for 4.8 hours. b2 (ix) Page Figure 39 Micrograph of scale formed on flat 116 surface of specimen sulfidised at T = 973 K and P = 10 atm for 4.8 b2 hours. Figure 40 Electron microprobe analysis of scale 117 formed on flat surface of specimen sulfidised at 973 K and P = 10 atm. b2 Figure 41 Micrograph of scale formed on edge of 119 specimen sulfidised at T = 973 K and -4 P = 10 atm for 4.9 hours, z Figure 42 Micrograph of scale formed on specimen 119 sulfidised at T = 973 K and P = 10 ^ b2 atm for 4.9 hours. Figure 43 Micrograph of scale formed on alloy 119 sulfidised at T = 973 K and P = 10 z atm for 4.9 hours. Figure 44 Micrograph of scale formed on alloy 119 sulfidised at T = 973 K and P = 10 z atm for 4.9 hours.