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Coatings for corrosion protection of porous substrates in gasifier components
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Surface & Coatings Technology 202 (2008) 2794 – 2800 www.elsevier.com/locate/surfcoat
Coatings for corrosion protection of porous substrates in gasifier components J. Perez-Mariano ⁎, K.H. Lau, E. Alvarez, R. Malhotra, M. Hornbostel, G. Krishnan, A. Sanjurjo Materials Research Laboratory, SRI International, Menlo Park, CA94025, USA Received 20 June 2007; accepted in revised form 11 October 2007 Available online 17 October 2007
Abstract Deposition of several coatings by Chemical Vapor Deposition in a Fluidized Bed was studied inside porous coupons made of 409 steel. Four different coating systems were studied: TiAlSiN/TiAl/SS409, TiAlSiN/TiAlSi/SS409, TiAlN/Nb/TiAl/SS409 and TiSiN/Nb/TiSi/SS409. Though coating thickness decreased with depth inside the porous filter, the formation of Ti-based ceramic films with thicknesses around 1 micrometer was observed 0.5 mm inside the bulk of the samples. Coated substrates were exposed to a simulated coal gas at 643 K for 300 hours, in order to study their corrosion resistance under conditions that mimic a porous metal particulate filter of a coal gasification system. Some of the coatings did not provide enough protection against corrosion at the bulk of the porous coupons, and iron sulfide crystals were formed that plugged the pores. On the other hand, the TiSiN/Nb/TiSi/SS409 system showed no sign of corrosion. © 2007 Elsevier B.V. All rights reserved. Keywords: Fluidized bed; CVD; Coating; Corrosion; H2S; Coal
1. Introduction Advanced coal gasification systems, such as integrated coal gasification combined cycle (IGCC) processes, offer many advantages over conventional pulverized coal combustors: high energy-conversion efficiencies, reduced pollutant emissions, modular construction, and potentially low capital and operating costs. However, under the highly sulfiding conditions of the high temperature coal gas, the performance of components degrades significantly with time unless expensive high alloy materials are used. The gas composition generally has a low p (O2) and a high p(S2), in contrast to high p(O2) prevalent in many high temperature furnaces and gas turbines. These conditions require development of suitable coatings to impart adequate corrosion resistance and decrease therefore capital and operating costs. One of the components where protective coatings would be of interest is a porous metal particulate filter, which is downstream the heat exchanger in a gasifier unit. This
⁎ Corresponding author. E-mail address: [email protected] (J. Perez-Mariano). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.10.011
component operates at 643 K and is exposed to gas streams containing as much as 2 vol% H2 S. Alloy steels are subject to corrosion in coal gasification environments. Even under conditions where Cr2O3 is stable, sulfidation attack is feasible. The alloying element (i.e. Fe or Ni) that forms a stable sulfide can diffuse out through the oxide scale and eventually form sulfide on its surface, leading to breakaway corrosion. Coatings that can protect Fe-Cr steels in H2S-H2 atmospheres at 643 K are based either in surface modification of steels or in deposition of layers that act as a barrier for the migration of sulfur inward and to Fe transport outward. Several coatings or surface modification processes that have been reported in the literature for the protection of parts in coal gasification applications are: Al, Cr, Si, Ni and/or Ti coatings [1–5], FeAl-based sprayed coatings [6,7], thin layers of Al2O3 [8] or SiO2 [9]. To our knowledge, there are not published studies regarding the deposition of coatings against sulfidation deposited on porous metal substrates, such as the ones described in this paper. This challenging goal requires deposition of resistant coatings inside the structure of the filter in such a uniform way that all the internal surfaces are covered. Coatings here reported were deposited by Chemical Vapor Deposition in a Fluidized Bed Reactor (FBR-CVD). This
J. Perez-Mariano et al. / Surface & Coatings Technology 202 (2008) 2794–2800 Table 1 Coating materials Coating
Diffusion layer
Barrie layer
Ceramic film
A B C D
TiAl TiAlSi TiAl TiSi
Nb Nb
TiAlSiN TiAlSiN TiAlN TiSiN
technique takes advantage of the excellent mass and heat transfer of fluidized beds, thus increasing deposition rates and homogeneity [10]. Since it is not a line-of-sight deposition method, substrates with complex shapes can be coated. 2. Experimental The FBR-CVD system has been explained in detail before [11] and will be only briefly described here. It consists of a tubular quartz reactor (ID = 4.2 cm) with a porous plate that supports the particles (alumina with sizes in the range 150 – 212 microns) and distributes the gas. The fluidized bed's height was approximately 16 cm. The fluidizing stream used was a mixture of Ar and H2. An Ar current was bubbled in liquid TiCl4 (kept at 318 K) and the saturated gas was added to the main stream before entering the reactor. Ar was also flown through a fixed bed of AlCl3 (kept at 368 K) and then also added to the main stream. A third Ar flow was bubbled in liquid SiHCl3 (kept in an ice bath) and then mixed with the main stream. The Nb precursor used was NbCl5; it was added as a solid to the bed using a quartz tube. NH3 was also supplied directly into the fluidized bed. The total gas flow was 8.3 L/min (10 cm/s). Samples (50 × 20 × 3 mm) were custom produced by sintering 409 stainless steel (SS409) powder in a furnace under hydrogen at 1473 K. SS409 powder was supplied by Ametek, had a particle size b150 microns and the following composi-
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tion: 11.43 wt% Cr, 0.023 wt% C, 0.19 wt% Ni, 0.69 wt% Si, 0.13 wt% Mn, 0.009 wt% S, 0.70 wt% Nb, 0.33 wt% O, 0.019 wt% P, 0.022 wt% N and Fe (bal.). As we found in previous studies [12], due to its low C content this steel is an excellent base material for diffusion coatings. During coating deposition, the porous specimens were hanging vertically and allowed to move freely in the fluidized bed and directly heated by RF induction. The deposition process consisted of 3 steps. First, Ti, Al or Si were diffused into the samples during 8 hours through in-situ reduction of the corresponding chlorides by H2 at a temperature of 1223 K. Second, in some cases a Nb interlayer was deposited at the same temperature by reaction of NbCl5 (1.5 g total, regularly supplied for 30 minutes) with H2 at 1023 K. Finally, a TiN-based ceramic was deposited by reaction during 6 hours of TiCl4, AlCl3 and/or SiHCl3 with NH3. Table 1 summarizes the 4 combination of coating materials discussed in this paper. A horizontal electrically-heated quartz tube furnace was used to expose coupons to simulated gasifier environments for 300 hours (see Fig. 1). A gas mixture that simulated a typical gas stream in a coal gasifier plant was supplied to the system at a rate of 120 sccm. Its composition (25.7% H2, 38.9% CO, 17.3% CO2, 1.4% H2S and balance steam) was achieved by mixing streams from two cylinders containing H2:CO:CO2 and H2S:H2. Both flows were controlled by mass flow controllers. A heated evaporator was used to produce steam with controlled rate of injection of liquid water by a computer-controlled syringe pump. The tube carrying steam and other gases was heated to prevent further condensation. The furnace had three independently-controlled heating zones which permitted a uniform temperature profile (± 5 K). EDX surface analyses were performed at an acceleration potential of 20 kV. XRD measurements were carried out in a Philips XRG 3100 apparatus with a Cu cathode, at a step size of
Fig. 1. Schematic diagram of the bench-scale exposure test system.
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Fig. 2. Typical surface appearance of a coated 409 porous sample (run D). The inset shows the appearance of an uncoated sample (scale bar in the inset corresponds to 2 microns).
0.05° and at 4 s per step. Thermochemical calculations were performed by means of HSC software v5.11 [13]. 3. Results and discussion A typical surface overview of a porous steel coupon after deposition is presented in Fig. 2, showing globular growth morphology typical of CVD coatings. According to SEM observations, the macroscopic porosity of coated filters was the same as in uncoated samples (see inset in Fig. 2). As described in the experimental section, the porous samples were prepared by sintering SS409 powders at 1473 K, whereas the maximum temperature during deposition was 1223 K, too low to appreciate any sintering effect. In our experiments, as a first deposition step, we co-diffused combinations of Ti, Al and Si because these elements are reported to increase steel sulfidation resistance [2,4]. Fig. 3
Fig. 4. Thermodynamic estimations of Nb deposition yield after reduction of NbCl5 by H2 in a system with elemental composition Ar:Nb:Cl:H in ratios 500:1:5:x (x = 500, 250, 125).
shows a cross-section detail of a TiAlSiN/TiAlSi/SS409 coupon that was fractured under liquid nitrogen to expose internal coated surfaces and cross sections of the coating. The corresponding EDX maps for Si, Ti and Fe are also presented. As can be seen, the material is composed of 3 zones: the SS409 substrate at the most internal part, an intermediate Si-rich diffusion layer and an external TiAlSiN coating. In previous studies it was determined that the Si-rich layer contains iron silicides (mainly Fe3Si) [14,15]. Diffusion of Si can then be considered as a thermodynamic sink for Fe, due to the strong interaction between them (ΔHf[Fe3Si] = -22.4 kcal/mol). This bond reduces Fe activity and thus the outward diffusion of this element to react with S species. Ti is a low diffusing element in steels at 1223 K. During the diffusion step, the surface concentration is enriched in Ti but this element does not diffuse
Fig. 3. SEM view of a fractured zone (specimen from run B, depth in the sample: 250 μm) and EDX map analyses for Si, Ti and Fe.
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Fig. 5. SEM view of a polished zone (coating C, depth in the sample: 50 μm) and EDX linescan analyses for Ti, Fe, Nb and Cr.
inward as much as Si. As a consequence, in the corresponding EDX map Ti was not detected as deep inside the sample as Si. An Al-rich zone was not observed (EDX map not shown) because the concentration of Al precursors used in our experiments was low and Al is a fast diffusing element in steels. Nb is known to decrease the corrosion rate of Fe-based alloys in H2/H2O/H2S atmospheres [16]. In some experiments, we carried out a second step in which we deposited a Nb interlayer with the expectation of slowing outward Fe diffusion and inward S diffusion during exposure to an H2S-containing gas
mixture. Since Nb deposition by FBR-CVD is not reported in the literature, before any experiment we carried out thermodynamic simulations to determine its feasibility. Roughly, we found the equilibrium composition for different mixture ratios of reactants, i.e. the ratios of reactants that have the minimum Gibbs free energy. Fig. 4 depicts the percentage of NbCl5(g) in the simulation inputs that appeared as Nb(s) in the outputs. The results showed that NbCl5(g) can be reduced to Nb(s) by H2 at H/Cl atomic ratios as low as 25 with a yield above 90%. Due to the good mass and heat transfer of a fluidized bed, it can be
Fig. 6. SEM view of a fractured zone (coating C, depth in the sample: 500 μm) and EDX map analyses for Ti, Fe and Nb.
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Fig. 7. Evolution of Ti, Si, Nb, Fe and Cr concentrations with depth in a coated sample from run D. EDX top measurements were carried out at different locations of the cross-section exposed after fracturing the sample.
assumed that it operates under conditions close to equilibrium. Thermodynamic simulations are therefore a powerful predictive tool in FBR-CVD processes. With the NbCl5 feeding rate and the H2 flow used in our experiments, average H/Cl ratio was above 600. Under these conditions, Nb deposition is expected to be very efficient. Fig. 5 shows a cross-section micrograph of a fractured and polished TiAlN/Nb/TiAl/SS409 sample. The zone presented in the figure is relatively close to the external surface of the sample. The substrate, the ceramic film and the Nb interlayer (thin white
line between the coating and the substrate) can be observed in the micrograph. According to unpublished XRD results regarding the same coatings deposited under the same conditions on solid 409 steel coupons, the Nb in the interlayer was not nitrided and remained as a metal. The multilayer structure is confirmed by EDX linescan analyses across the coating depicted in the same figure. Fig. 6 shows a cross-section micrograph of the same TiAlN/Nb/TiAl/SS409 fractured sample, but unpolished. The corresponding EDX elemental maps for Ti, Fe and Nb are also presented in the figure. Fe is mainly detected in the substrate
Fig. 8. XRD diffraction pattern of the corroded bulk in a sample from run C, compared to XRD measurements on crystals found in coated 409 stainless steel solid coupons exposed to the same corrosion test at 1173 K (unpublished results).
J. Perez-Mariano et al. / Surface & Coatings Technology 202 (2008) 2794–2800
Fig. 9. Enlarged view of a zone in the bulk of a sample from run B that suffered sulfidation during exposure to the corrosion test.
surface exposed as a consequence of the fracture. On the other hand, Ti and Nb EDX maps match the areas where coating remained after fracture. Ti signal is more intense than Nb signal, because the overlying Ti-based ceramic film (clearly appreciated in the micrograph) is thicker than the Nb interlayer, which cannot be seen in the unpolished sample. Note that the area shown in Fig. 6 is deeper inside the coupon and the coating is therefore
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thinner than in Fig. 5. A coating thickness around 1 micron was found at a depth of 500 microns, whereas the thickness close to the surface was around 6 microns (corresponding to deposition rates of 0.15 and 1 micron/hour). Despite the expected decrease in coating thickness with depth inside the porous substrate, the above-discussed results indicate that infiltration of the porous substrate was achieved and that multiple layers can be deposited by FBR-CVD on the internal surfaces of a filter. To be sure that the infiltration was achieved along the cross-section of the porous samples, EDX analyses were carried out at several depths of fractured specimens. Fig. 7 depicts the evolution of Ti, Si, Nb, Fe and Cr concentration with depth from a sample from run TiSiN/ Nb/TiSi/SS409 (on the x-axis, 0 corresponds to the porous coupon external face and 1300 microns to its center in the bulk). Nitrogen was not considered for quantification, so the sum of these 5 elements in each measurement was 100. In thin films, EDX surface analysis integrates the film composition and part of the substrate. Thus, higher Ti concentration is indicative of thicker films. As expected, Ti concentration decreases with depth, because deposition inside the coupon is less favored (gas reactants are depleted) than at the external surface. However, Ti concentration at the bulk of the sample is still high (roughly 50 at%) and Si and Nb are detected at all depths, with concentrations above 2 at% and 1 at%. The evolution of Ti and Si concentration with depth suggest that the Si precursor is preferably depleted than the Ti
Fig. 10. SEM view of a cross-section of samples from runs A (top left), B (top right), C (bottom left) and D (bottom right) after exposure to the corrosion test.
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4. Conclusions
Fig. 11. Enlarged view of a zone in the bulk of a sample from run D after exposure to the corrosion test.
precursor. The enthalpy of formation of TiCl4(g) is -182 kcal/mol and the one of SiHCl3(g) -119 kcal/mol. TiCl4 is a far more stable molecule than SiHCl3 and it is therefore expected that this last one is preferably depleted. After the corrosion test, we found that in most cases the bulk of the sample was attacked and crystals were formed that blocked the pores. EDX showed that the only 2 elements in the crystals were Fe and S, and XRD measurements revealed two iron sulfide phases: pyrrhotite (Fe1-xS) and FeS (see Fig. 8). These results are in agreement with findings reported in the literature for a similar corrosion test [7] and with our unpublished results regarding a similar corrosion test in coated 409 stainless steel solid coupons (also presented in Fig. 8). An enlarged view of a zone that suffered sulfidation is presented in Fig. 9, showing iron sulfide crystals that grew in the pores of the coupon. Fig. 10 shows bulk-to-surface SEM cross-sections of all samples. The formation of iron sulfide crystals was observed specially at the bulk of some samples, where coatings are thinner than at zones closer to the surface and therefore it is more likely that any defect or areas less-exposed to the reactants during deposition act as initiation spots for sulfidation. Once initiated, corrosion will rapidly proceed and as a result the pore in that area will be blocked by iron sulfide crystals. If this process takes place over an extended area, the filter is unacceptable because it will fail in service. As seen in Fig. 10, whereas in the TiAlSiN/TiAl/SS409 sample most of the coupon was corroded, samples TiAlSiN/TiAlSi/SS409 and TiAlN/Nb/TiAl/SS409 suffered less severe attacks: crystals were only observed in approximately one third of the filter cross-section. Thus, both the Si diffusion layer and the Nb barrier provided a protective effect. The combination of both effects (sample TiSiN/Nb/TiSi/SS409) resulted in enough protection both at the surface and at the bulk of the coupon. In this sample we did not observe any iron sulfide crystal (see Fig. 11 for a high magnification view of an area at the center of the specimen) and EDX analyses showed no indication of S content in any zone.
Chemical Vapor Deposition in a Fluidized Bed Reactor is a feasible process to deposit conformal coatings inside the structure of porous steel filters. This is a versatile technique that allows three consecutive steps in the same experiment: surface modification of steel by diffusion of Ti-Al-Si, deposition of a Nb interlayer and deposition of a titanium nitride film with some Al and/or Si content. Due to depletion of reactants, deposition rates decrease with depth inside the bulk of the samples from 1 micron/hour (close to surface) to 0.15 micron/hour (depth of 0.5 mm). As a consequence, the middle of a coated coupon is more prone to sulfidation than the zone close to the surface. The mechanism of corrosion involves the formation of iron sulfide crystals that block the pores of the filter. The TiAlSiN/TiAl/SS409 system showed extended corrosion after exposure to a simulated coal gas at 643 K for 300 hours. The TiAlSiN/TiAlSi/SS409, and the TiAlN/Nb/ TiAl/SS409 systems were only attacked at the bulk. The TiSiN/ Nb/TiSi/SS409 system showed no sign of corrosion and is a promising candidate for filters downstream of the heat recovery unit in coal gasification systems. Acknowledgements This research was supported by the US Department of Energy through contract number DE-FC26-03NT41616. References [1] K. Natesan, Surf. Coat. Technol. 56 (1993) 185. [2] J. Jimenez-Soler, J.F. Norton, M. Nöth, M. Schütze, Mater. Corros. 50 (1999) 394. [3] K. Natesan, Corrosion Resistance of Iron Aluminides.” Report by Argonne National Laboratory under Contract No. W31-109-Eng-38, 2001. [4] B.A. Pint, Y. Zhang, P.F. Tortorelli, J.A. Haynes, I.G. Wright, Mater. High Temp. 18 (2001) 185. [5] L. Xuefeng, Surf. Coat. Technol. 183 (2004) 212. [6] K.R. Luer, J.N. DuPont, A.R. Marder, Corrosion 56 (2000) 189. [7] C. Xiao, W. Chen, Surf. Coat. Technol. 201 (2006) 3625. [8] H.D. van Corbach, V.A.C. Haanappel, T. Fransen, P.J. Gellings, Thin Solid Films 239 (1994) 31. [9] R. Hofman, J.G.F. Westheim, T. Fransen, P.J. Gellings, J. Phys., IV C9 (1993) 865. [10] A. Sanjurjo, M.C.H. Mckubre, G.D. Craig, Surf. Coat. Technol. 39 (1989) 691. [11] J. Perez-Mariano, K.H. Lau, A. Sanjurjo, J. Caro, J.M. Prado, C. Colominas, Surf. Coat. Technol. 201 (2006) 2174. [12] N. Priyantha, P. Jayaweera, A. Sanjurjo, K. Lau, F. Lu, K. Krist, Surf. Coat. Technol. 163-164 (2003) 31. [13] A. Roine, Outokumpu HSC Chemistry for Windows, Outokumpu Research Oy, Pori, Finland, 2002. [14] A. Sanjurjo, S. Hettiarachchi, K.H. Lau, P. Cox, B. Wood, Surf. Coat. Technol. 54–55 (1992) 224. [15] F.J. Bolívar, L. Sánchez, S.A. Tsipas, M.P. Hierro, J.A. Trilleros, F.J. Pérez, Surf. Coat. Technol. 201 (2006) 3953. [16] W. Kai, D.L. Douglas, Oxid. Met. 39 (1993) 317.
Surface & Coatings Technology 202 (2008) 2794 – 2800 www.elsevier.com/locate/surfcoat
Coatings for corrosion protection of porous substrates in gasifier components J. Perez-Mariano ⁎, K.H. Lau, E. Alvarez, R. Malhotra, M. Hornbostel, G. Krishnan, A. Sanjurjo Materials Research Laboratory, SRI International, Menlo Park, CA94025, USA Received 20 June 2007; accepted in revised form 11 October 2007 Available online 17 October 2007
Abstract Deposition of several coatings by Chemical Vapor Deposition in a Fluidized Bed was studied inside porous coupons made of 409 steel. Four different coating systems were studied: TiAlSiN/TiAl/SS409, TiAlSiN/TiAlSi/SS409, TiAlN/Nb/TiAl/SS409 and TiSiN/Nb/TiSi/SS409. Though coating thickness decreased with depth inside the porous filter, the formation of Ti-based ceramic films with thicknesses around 1 micrometer was observed 0.5 mm inside the bulk of the samples. Coated substrates were exposed to a simulated coal gas at 643 K for 300 hours, in order to study their corrosion resistance under conditions that mimic a porous metal particulate filter of a coal gasification system. Some of the coatings did not provide enough protection against corrosion at the bulk of the porous coupons, and iron sulfide crystals were formed that plugged the pores. On the other hand, the TiSiN/Nb/TiSi/SS409 system showed no sign of corrosion. © 2007 Elsevier B.V. All rights reserved. Keywords: Fluidized bed; CVD; Coating; Corrosion; H2S; Coal
1. Introduction Advanced coal gasification systems, such as integrated coal gasification combined cycle (IGCC) processes, offer many advantages over conventional pulverized coal combustors: high energy-conversion efficiencies, reduced pollutant emissions, modular construction, and potentially low capital and operating costs. However, under the highly sulfiding conditions of the high temperature coal gas, the performance of components degrades significantly with time unless expensive high alloy materials are used. The gas composition generally has a low p (O2) and a high p(S2), in contrast to high p(O2) prevalent in many high temperature furnaces and gas turbines. These conditions require development of suitable coatings to impart adequate corrosion resistance and decrease therefore capital and operating costs. One of the components where protective coatings would be of interest is a porous metal particulate filter, which is downstream the heat exchanger in a gasifier unit. This
⁎ Corresponding author. E-mail address: [email protected] (J. Perez-Mariano). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.10.011
component operates at 643 K and is exposed to gas streams containing as much as 2 vol% H2 S. Alloy steels are subject to corrosion in coal gasification environments. Even under conditions where Cr2O3 is stable, sulfidation attack is feasible. The alloying element (i.e. Fe or Ni) that forms a stable sulfide can diffuse out through the oxide scale and eventually form sulfide on its surface, leading to breakaway corrosion. Coatings that can protect Fe-Cr steels in H2S-H2 atmospheres at 643 K are based either in surface modification of steels or in deposition of layers that act as a barrier for the migration of sulfur inward and to Fe transport outward. Several coatings or surface modification processes that have been reported in the literature for the protection of parts in coal gasification applications are: Al, Cr, Si, Ni and/or Ti coatings [1–5], FeAl-based sprayed coatings [6,7], thin layers of Al2O3 [8] or SiO2 [9]. To our knowledge, there are not published studies regarding the deposition of coatings against sulfidation deposited on porous metal substrates, such as the ones described in this paper. This challenging goal requires deposition of resistant coatings inside the structure of the filter in such a uniform way that all the internal surfaces are covered. Coatings here reported were deposited by Chemical Vapor Deposition in a Fluidized Bed Reactor (FBR-CVD). This
J. Perez-Mariano et al. / Surface & Coatings Technology 202 (2008) 2794–2800 Table 1 Coating materials Coating
Diffusion layer
Barrie layer
Ceramic film
A B C D
TiAl TiAlSi TiAl TiSi
Nb Nb
TiAlSiN TiAlSiN TiAlN TiSiN
technique takes advantage of the excellent mass and heat transfer of fluidized beds, thus increasing deposition rates and homogeneity [10]. Since it is not a line-of-sight deposition method, substrates with complex shapes can be coated. 2. Experimental The FBR-CVD system has been explained in detail before [11] and will be only briefly described here. It consists of a tubular quartz reactor (ID = 4.2 cm) with a porous plate that supports the particles (alumina with sizes in the range 150 – 212 microns) and distributes the gas. The fluidized bed's height was approximately 16 cm. The fluidizing stream used was a mixture of Ar and H2. An Ar current was bubbled in liquid TiCl4 (kept at 318 K) and the saturated gas was added to the main stream before entering the reactor. Ar was also flown through a fixed bed of AlCl3 (kept at 368 K) and then also added to the main stream. A third Ar flow was bubbled in liquid SiHCl3 (kept in an ice bath) and then mixed with the main stream. The Nb precursor used was NbCl5; it was added as a solid to the bed using a quartz tube. NH3 was also supplied directly into the fluidized bed. The total gas flow was 8.3 L/min (10 cm/s). Samples (50 × 20 × 3 mm) were custom produced by sintering 409 stainless steel (SS409) powder in a furnace under hydrogen at 1473 K. SS409 powder was supplied by Ametek, had a particle size b150 microns and the following composi-
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tion: 11.43 wt% Cr, 0.023 wt% C, 0.19 wt% Ni, 0.69 wt% Si, 0.13 wt% Mn, 0.009 wt% S, 0.70 wt% Nb, 0.33 wt% O, 0.019 wt% P, 0.022 wt% N and Fe (bal.). As we found in previous studies [12], due to its low C content this steel is an excellent base material for diffusion coatings. During coating deposition, the porous specimens were hanging vertically and allowed to move freely in the fluidized bed and directly heated by RF induction. The deposition process consisted of 3 steps. First, Ti, Al or Si were diffused into the samples during 8 hours through in-situ reduction of the corresponding chlorides by H2 at a temperature of 1223 K. Second, in some cases a Nb interlayer was deposited at the same temperature by reaction of NbCl5 (1.5 g total, regularly supplied for 30 minutes) with H2 at 1023 K. Finally, a TiN-based ceramic was deposited by reaction during 6 hours of TiCl4, AlCl3 and/or SiHCl3 with NH3. Table 1 summarizes the 4 combination of coating materials discussed in this paper. A horizontal electrically-heated quartz tube furnace was used to expose coupons to simulated gasifier environments for 300 hours (see Fig. 1). A gas mixture that simulated a typical gas stream in a coal gasifier plant was supplied to the system at a rate of 120 sccm. Its composition (25.7% H2, 38.9% CO, 17.3% CO2, 1.4% H2S and balance steam) was achieved by mixing streams from two cylinders containing H2:CO:CO2 and H2S:H2. Both flows were controlled by mass flow controllers. A heated evaporator was used to produce steam with controlled rate of injection of liquid water by a computer-controlled syringe pump. The tube carrying steam and other gases was heated to prevent further condensation. The furnace had three independently-controlled heating zones which permitted a uniform temperature profile (± 5 K). EDX surface analyses were performed at an acceleration potential of 20 kV. XRD measurements were carried out in a Philips XRG 3100 apparatus with a Cu cathode, at a step size of
Fig. 1. Schematic diagram of the bench-scale exposure test system.
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Fig. 2. Typical surface appearance of a coated 409 porous sample (run D). The inset shows the appearance of an uncoated sample (scale bar in the inset corresponds to 2 microns).
0.05° and at 4 s per step. Thermochemical calculations were performed by means of HSC software v5.11 [13]. 3. Results and discussion A typical surface overview of a porous steel coupon after deposition is presented in Fig. 2, showing globular growth morphology typical of CVD coatings. According to SEM observations, the macroscopic porosity of coated filters was the same as in uncoated samples (see inset in Fig. 2). As described in the experimental section, the porous samples were prepared by sintering SS409 powders at 1473 K, whereas the maximum temperature during deposition was 1223 K, too low to appreciate any sintering effect. In our experiments, as a first deposition step, we co-diffused combinations of Ti, Al and Si because these elements are reported to increase steel sulfidation resistance [2,4]. Fig. 3
Fig. 4. Thermodynamic estimations of Nb deposition yield after reduction of NbCl5 by H2 in a system with elemental composition Ar:Nb:Cl:H in ratios 500:1:5:x (x = 500, 250, 125).
shows a cross-section detail of a TiAlSiN/TiAlSi/SS409 coupon that was fractured under liquid nitrogen to expose internal coated surfaces and cross sections of the coating. The corresponding EDX maps for Si, Ti and Fe are also presented. As can be seen, the material is composed of 3 zones: the SS409 substrate at the most internal part, an intermediate Si-rich diffusion layer and an external TiAlSiN coating. In previous studies it was determined that the Si-rich layer contains iron silicides (mainly Fe3Si) [14,15]. Diffusion of Si can then be considered as a thermodynamic sink for Fe, due to the strong interaction between them (ΔHf[Fe3Si] = -22.4 kcal/mol). This bond reduces Fe activity and thus the outward diffusion of this element to react with S species. Ti is a low diffusing element in steels at 1223 K. During the diffusion step, the surface concentration is enriched in Ti but this element does not diffuse
Fig. 3. SEM view of a fractured zone (specimen from run B, depth in the sample: 250 μm) and EDX map analyses for Si, Ti and Fe.
J. Perez-Mariano et al. / Surface & Coatings Technology 202 (2008) 2794–2800
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Fig. 5. SEM view of a polished zone (coating C, depth in the sample: 50 μm) and EDX linescan analyses for Ti, Fe, Nb and Cr.
inward as much as Si. As a consequence, in the corresponding EDX map Ti was not detected as deep inside the sample as Si. An Al-rich zone was not observed (EDX map not shown) because the concentration of Al precursors used in our experiments was low and Al is a fast diffusing element in steels. Nb is known to decrease the corrosion rate of Fe-based alloys in H2/H2O/H2S atmospheres [16]. In some experiments, we carried out a second step in which we deposited a Nb interlayer with the expectation of slowing outward Fe diffusion and inward S diffusion during exposure to an H2S-containing gas
mixture. Since Nb deposition by FBR-CVD is not reported in the literature, before any experiment we carried out thermodynamic simulations to determine its feasibility. Roughly, we found the equilibrium composition for different mixture ratios of reactants, i.e. the ratios of reactants that have the minimum Gibbs free energy. Fig. 4 depicts the percentage of NbCl5(g) in the simulation inputs that appeared as Nb(s) in the outputs. The results showed that NbCl5(g) can be reduced to Nb(s) by H2 at H/Cl atomic ratios as low as 25 with a yield above 90%. Due to the good mass and heat transfer of a fluidized bed, it can be
Fig. 6. SEM view of a fractured zone (coating C, depth in the sample: 500 μm) and EDX map analyses for Ti, Fe and Nb.
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Fig. 7. Evolution of Ti, Si, Nb, Fe and Cr concentrations with depth in a coated sample from run D. EDX top measurements were carried out at different locations of the cross-section exposed after fracturing the sample.
assumed that it operates under conditions close to equilibrium. Thermodynamic simulations are therefore a powerful predictive tool in FBR-CVD processes. With the NbCl5 feeding rate and the H2 flow used in our experiments, average H/Cl ratio was above 600. Under these conditions, Nb deposition is expected to be very efficient. Fig. 5 shows a cross-section micrograph of a fractured and polished TiAlN/Nb/TiAl/SS409 sample. The zone presented in the figure is relatively close to the external surface of the sample. The substrate, the ceramic film and the Nb interlayer (thin white
line between the coating and the substrate) can be observed in the micrograph. According to unpublished XRD results regarding the same coatings deposited under the same conditions on solid 409 steel coupons, the Nb in the interlayer was not nitrided and remained as a metal. The multilayer structure is confirmed by EDX linescan analyses across the coating depicted in the same figure. Fig. 6 shows a cross-section micrograph of the same TiAlN/Nb/TiAl/SS409 fractured sample, but unpolished. The corresponding EDX elemental maps for Ti, Fe and Nb are also presented in the figure. Fe is mainly detected in the substrate
Fig. 8. XRD diffraction pattern of the corroded bulk in a sample from run C, compared to XRD measurements on crystals found in coated 409 stainless steel solid coupons exposed to the same corrosion test at 1173 K (unpublished results).
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Fig. 9. Enlarged view of a zone in the bulk of a sample from run B that suffered sulfidation during exposure to the corrosion test.
surface exposed as a consequence of the fracture. On the other hand, Ti and Nb EDX maps match the areas where coating remained after fracture. Ti signal is more intense than Nb signal, because the overlying Ti-based ceramic film (clearly appreciated in the micrograph) is thicker than the Nb interlayer, which cannot be seen in the unpolished sample. Note that the area shown in Fig. 6 is deeper inside the coupon and the coating is therefore
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thinner than in Fig. 5. A coating thickness around 1 micron was found at a depth of 500 microns, whereas the thickness close to the surface was around 6 microns (corresponding to deposition rates of 0.15 and 1 micron/hour). Despite the expected decrease in coating thickness with depth inside the porous substrate, the above-discussed results indicate that infiltration of the porous substrate was achieved and that multiple layers can be deposited by FBR-CVD on the internal surfaces of a filter. To be sure that the infiltration was achieved along the cross-section of the porous samples, EDX analyses were carried out at several depths of fractured specimens. Fig. 7 depicts the evolution of Ti, Si, Nb, Fe and Cr concentration with depth from a sample from run TiSiN/ Nb/TiSi/SS409 (on the x-axis, 0 corresponds to the porous coupon external face and 1300 microns to its center in the bulk). Nitrogen was not considered for quantification, so the sum of these 5 elements in each measurement was 100. In thin films, EDX surface analysis integrates the film composition and part of the substrate. Thus, higher Ti concentration is indicative of thicker films. As expected, Ti concentration decreases with depth, because deposition inside the coupon is less favored (gas reactants are depleted) than at the external surface. However, Ti concentration at the bulk of the sample is still high (roughly 50 at%) and Si and Nb are detected at all depths, with concentrations above 2 at% and 1 at%. The evolution of Ti and Si concentration with depth suggest that the Si precursor is preferably depleted than the Ti
Fig. 10. SEM view of a cross-section of samples from runs A (top left), B (top right), C (bottom left) and D (bottom right) after exposure to the corrosion test.
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4. Conclusions
Fig. 11. Enlarged view of a zone in the bulk of a sample from run D after exposure to the corrosion test.
precursor. The enthalpy of formation of TiCl4(g) is -182 kcal/mol and the one of SiHCl3(g) -119 kcal/mol. TiCl4 is a far more stable molecule than SiHCl3 and it is therefore expected that this last one is preferably depleted. After the corrosion test, we found that in most cases the bulk of the sample was attacked and crystals were formed that blocked the pores. EDX showed that the only 2 elements in the crystals were Fe and S, and XRD measurements revealed two iron sulfide phases: pyrrhotite (Fe1-xS) and FeS (see Fig. 8). These results are in agreement with findings reported in the literature for a similar corrosion test [7] and with our unpublished results regarding a similar corrosion test in coated 409 stainless steel solid coupons (also presented in Fig. 8). An enlarged view of a zone that suffered sulfidation is presented in Fig. 9, showing iron sulfide crystals that grew in the pores of the coupon. Fig. 10 shows bulk-to-surface SEM cross-sections of all samples. The formation of iron sulfide crystals was observed specially at the bulk of some samples, where coatings are thinner than at zones closer to the surface and therefore it is more likely that any defect or areas less-exposed to the reactants during deposition act as initiation spots for sulfidation. Once initiated, corrosion will rapidly proceed and as a result the pore in that area will be blocked by iron sulfide crystals. If this process takes place over an extended area, the filter is unacceptable because it will fail in service. As seen in Fig. 10, whereas in the TiAlSiN/TiAl/SS409 sample most of the coupon was corroded, samples TiAlSiN/TiAlSi/SS409 and TiAlN/Nb/TiAl/SS409 suffered less severe attacks: crystals were only observed in approximately one third of the filter cross-section. Thus, both the Si diffusion layer and the Nb barrier provided a protective effect. The combination of both effects (sample TiSiN/Nb/TiSi/SS409) resulted in enough protection both at the surface and at the bulk of the coupon. In this sample we did not observe any iron sulfide crystal (see Fig. 11 for a high magnification view of an area at the center of the specimen) and EDX analyses showed no indication of S content in any zone.
Chemical Vapor Deposition in a Fluidized Bed Reactor is a feasible process to deposit conformal coatings inside the structure of porous steel filters. This is a versatile technique that allows three consecutive steps in the same experiment: surface modification of steel by diffusion of Ti-Al-Si, deposition of a Nb interlayer and deposition of a titanium nitride film with some Al and/or Si content. Due to depletion of reactants, deposition rates decrease with depth inside the bulk of the samples from 1 micron/hour (close to surface) to 0.15 micron/hour (depth of 0.5 mm). As a consequence, the middle of a coated coupon is more prone to sulfidation than the zone close to the surface. The mechanism of corrosion involves the formation of iron sulfide crystals that block the pores of the filter. The TiAlSiN/TiAl/SS409 system showed extended corrosion after exposure to a simulated coal gas at 643 K for 300 hours. The TiAlSiN/TiAlSi/SS409, and the TiAlN/Nb/ TiAl/SS409 systems were only attacked at the bulk. The TiSiN/ Nb/TiSi/SS409 system showed no sign of corrosion and is a promising candidate for filters downstream of the heat recovery unit in coal gasification systems. Acknowledgements This research was supported by the US Department of Energy through contract number DE-FC26-03NT41616. References [1] K. Natesan, Surf. Coat. Technol. 56 (1993) 185. [2] J. Jimenez-Soler, J.F. Norton, M. Nöth, M. Schütze, Mater. Corros. 50 (1999) 394. [3] K. Natesan, Corrosion Resistance of Iron Aluminides.” Report by Argonne National Laboratory under Contract No. W31-109-Eng-38, 2001. [4] B.A. Pint, Y. Zhang, P.F. Tortorelli, J.A. Haynes, I.G. Wright, Mater. High Temp. 18 (2001) 185. [5] L. Xuefeng, Surf. Coat. Technol. 183 (2004) 212. [6] K.R. Luer, J.N. DuPont, A.R. Marder, Corrosion 56 (2000) 189. [7] C. Xiao, W. Chen, Surf. Coat. Technol. 201 (2006) 3625. [8] H.D. van Corbach, V.A.C. Haanappel, T. Fransen, P.J. Gellings, Thin Solid Films 239 (1994) 31. [9] R. Hofman, J.G.F. Westheim, T. Fransen, P.J. Gellings, J. Phys., IV C9 (1993) 865. [10] A. Sanjurjo, M.C.H. Mckubre, G.D. Craig, Surf. Coat. Technol. 39 (1989) 691. [11] J. Perez-Mariano, K.H. Lau, A. Sanjurjo, J. Caro, J.M. Prado, C. Colominas, Surf. Coat. Technol. 201 (2006) 2174. [12] N. Priyantha, P. Jayaweera, A. Sanjurjo, K. Lau, F. Lu, K. Krist, Surf. Coat. Technol. 163-164 (2003) 31. [13] A. Roine, Outokumpu HSC Chemistry for Windows, Outokumpu Research Oy, Pori, Finland, 2002. [14] A. Sanjurjo, S. Hettiarachchi, K.H. Lau, P. Cox, B. Wood, Surf. Coat. Technol. 54–55 (1992) 224. [15] F.J. Bolívar, L. Sánchez, S.A. Tsipas, M.P. Hierro, J.A. Trilleros, F.J. Pérez, Surf. Coat. Technol. 201 (2006) 3953. [16] W. Kai, D.L. Douglas, Oxid. Met. 39 (1993) 317.