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Al–Mn CVD-FBR protective coatings for hot corrosion application
Surface & Coatings Technology 201 (2007) 4489 – 4495 www.elsevier.com/locate/surfcoat
Al–Mn CVD-FBR protective coatings for hot corrosion application☆ S. Tsipas, J.M. Brossard, M.P. Hierro, J.A. Trilleros, L. Sánchez, F.J. Bolívar, F.J. Pérez ⁎ Universidad Complutense de Madrid. Dpto CC. Materiales e Ingeniería Metalúrgica, Avenida Complutense s/n, Facultad de Ciencias Químicas, 28040 Madrid, Spain Received 1 May 2006; accepted in revised form 13 June 2006 Available online 9 October 2006
Abstract Ferritic steels are usually used in boiler or supercritical steam turbines which operate at temperatures between 600 and 650 °C under pressure. Protective coatings are often applied in order to increase their oxidation resistance and protect them against degradation. In this study, new Al–Mn protective coatings were deposited by CVD-FBR on two ferritic steels (P-92 and HCM12). The initial process parameters were optimized by thermodynamic calculations using Thermo-Calc software. Then, those parameters were used in the experimental procedure to obtain Al–Mn coatings at low temperature and atmospheric pressure. Co-deposition was achieved at moderate temperatures in order to maintain the substrates' mechanical properties. The coatings' microstructure and phase constitution was characterized. Fe–Al intermetallic coatings containing Cr and Mn were obtained. The phase constitution is discussed with reference to the Fe–Al–Mn ternary phase diagram. The effect of diffusion heat treatment on the phase transformations was investigated. © 2006 Published by Elsevier B.V. Keywords: Chemical vapor deposition; Thermodynamic simulation; Al–Mn coating
1. Introduction Increased efficiency of steam turbines for power generation plants means lower fuel consumption and reduced emissions of gaseous pollutants to the atmosphere. The important factors that affect the efficiency of the conventional steam power plants are the temperature and, to a lesser extent, the pressure of the input steam to the turbine. An increase in the steam temperature from 535 °C to 635 °C and the steam pressure from 18.5 to 30 MPa would allow a reduction in fuel consumption and CO2 emission by more than 25% [1]. Ferritic and martensitic heat-resistant steels with Cr content between 9% and 12%, and higher creep strength than the conventional ferritic steels have been developed for uses such as main steam pipes or superheater tubes in power generation boilers [2]. However, the lifetime of boiler materials is determined not only by creep strength but also resistance to steam oxidation. These classes of steels are fairly resistant to oxidation in air or oxygen atmospheres due to the formation of ☆
This paper was presented at the ICMCTF 2006 Conference (In Session B3-12). ⁎ Corresponding author. Tel.: +34 91 39 44215; fax: +34 91 39 33457. E-mail address: [email protected] (F.J. Pérez).
0257-8972/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.06.023
oxide scale of Cr-oxide and/or spinel. However, in steam environments, at higher temperatures, the Cr-oxide scale becomes less protective. Aluminizing onto the surface of iron-, nickel- and cobalt-base superalloys has been proven to be an effective method for improvement in oxidation and corrosion resistance at high temperature [3]. Protective alumina scales formed are well known to have good oxidation resistance at high temperatures and are more protective in steam-containing environment than Cr-oxides scales [4–6]. Japanese patents [7,8] describe the use of Al–Mn alloys for corrosion and oxidation protection but not much is known about the properties of these materials [9,10]. Fe–Al–Mn alloys have also been reported to posses good oxidation resistance at high temperature [11]. Many coating techniques such as chemical vapor deposition (CVD) or pack cementation used for applying protective coatings on steel require high temperatures and/or longer times, which limits their use to substrates that can be exposed to high temperature without degradation of their mechanical properties. CVD in a fluidized bed reactor (FBR) has the advantage of high and uniform mass and heat transfer rates and can be used to deposit Al coatings at low temperature (T b 700 °C) and short times [12–15]. Therefore, CVD-FBR is an important alternative for the deposition of Al coatings on
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ferritic steels, since this technique allows the conservation of microstructure and mechanical properties of these types of materials. This process has been successfully used to carry out aluminizing, chromizing, boriding, depositing silicon and titanium-based coatings [13,16–18]. The aim of the work was to study the feasibility of depositing, Al diffusion coatings containing Mn on ferritic steels (P92 and HCM12) by chemical vapor deposition in fluidized bed reactor (CVD-FBR). The initial parameters of the processes were optimized by thermodynamic calculations using Thermo-Calc software [19]. Then, those parameters were used during the experimental procedure to produce Mn-containing Al coatings at low temperature under atmospheric pressure. The effect of diffusion heat treatment was also studied. The main envisaged application is high temperature oxidation and corrosion protection of ferritic steels. 2. Experimental 2.1. Target materials The composition of the two different ferritic steels used as substrate material for Al–Mn diffusion coatings is presented in Table 1. The samples sheets (10 mm × 10 mm × ξ with 1.5 mm b ξ b 2.5 mm) were polished from 240-grit SiC paper up to 600-grit SiC paper and then ultrasonically cleaned in alcohol, dried and weighed prior to coating. 2.2. Coatings procedure 2.2.1. Thermodynamic calculation Thermodynamic study of phase equilibrium (composition, partial pressure) during CVD was performed using ThermoCalc computer program [19] to determine the feasibility of metal deposition and provide useful guidelines to optimize the deposition conditions. Calculations are based on the free Gibbs energy minimization code and mass conservation rule, in combination with SSUB3 and SSOL2 database (Scientific Group Thermodata Europe) for chloride precursors and substrate definition, respectively. 2.2.2. Experimental conditions The characteristics of the fluidized bed reactor used in this study have been presented in previous works [13]. The bed comprised a powder mixture that contained alumina inert powder (80 wt.%), 10 wt.% of Al (99.5% purity, 200 μm grain size) metal donor and 10 wt.% of Mn (99.5% purity, 100 μm grain size) metal donor. Bed particles were fluidized using Ar flow of 1.5 lt min− 1. The deposition temperature was 560 °C. Table 1 Composition of ferritic steel substrates Material
P-92 HCM12
Element (wt.%) Fe
Si
Cr
Ni
Mn
Mo
W
Cu
Other
87.7 83.5
0.02 0.25
9.07 12.5
0.06 0.34
0.47 0.54
0.46 0.36
1.78 1.9
– 0.85
b0.4
When the deposition temperature was reached, a flow of a reactive gas mixture of H2 + HCl was inserted into the bed. The molar ratio of H2/HCl in the reactive gas mixture was 20/1. The reactive gas mixture reacts with the metal donors and generates gas precursors which decompose and cause the deposition of the coating on the surface of the substrate. The reactive H2 + HCl gas mixture was passed through the fluidised bed for the length of the deposition process and was subsequently removed during sample cooling. The deposition time was 2 h. Subsequently, the coated samples were heat treated in a furnace at 700 °C during 2 h under inert gas Ar at atmospheric pressure. 2.3. Characterisation techniques As deposited and heat treated coated substrates were characterised by optical microscopy (OM), scanning electron microscopy (SEM JEOL JM-6400) and energy dispersive X-ray spectroscopy (EDS). Standard metallographic preparation was performed on samples prior to cross-section observations. X-ray diffraction analysis using monochromatic Cu Kα radiation (Philips X'PERT MPD, Kα(Cu) radiation) was performed in order to identify phase composition of coated samples asdeposited and after diffusion treatment. 3. Results and discussion 3.1. Thermodynamics approximation of deposition process In the CVD-FBR process, the gas species generated (precursors), through their absorption at the heated surface of the substrate and their decomposition on the surface will cause the deposition of a stable solid film at the substrate surface. Subsequently, the coating is formed by diffusion into the substrate surface. Gaseous by-products are recombined and desorbed into the gas phase. Consequently, the decomposition of the gas species by dissociation may lead to the formation of atomic Al or Mn and may promote deposition [14,15,20,21]. Thermodynamical calculations were performed in the system Al(s)/M(s)/Ar/HCl/H2. For successful co-deposition of Al and Mn, it is necessary that Al-containing and Mn-containing gas precursors are generated in sufficient quantities. The deposition of Al-diffusion coatings by CVD-FB has been successfully achieved experimentally and it has been verified thermodynamically that gas precursors which aid the deposition processes are generated [15,22]. In addition to the thermodynamics studies, it is considered that Al activity is very high in a CVD-FB and its deposition is very likely [23]. Therefore, in the current study, it was important to establish favourable conditions for Mn co-deposition. Towards this end, the most important Mn-containing gas precursors must be identified and the conditions where they are likely to generate with higher partial pressure established. Initially, the effect of the amount of the metallic donor Al(s) and Mn(s) on the partial pressure of the gas precursors present was evaluated. The calculation was performed for a temperature of 560 °C. This temperature has been found in previous studies to be favourable for Al deposition and in addition it is below the
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Table 2 EDS surface analysis of Al–Mn coatings deposited in CVD-FBR
Fig. 1. Thermodynamic calculation of gas precursors partial pressure at 560 °C in the fluidized bed with respect to the wt.% of Mn in the Al–Mn powder mixture. The molar ratio in the reactive gas mixture was H2/HCl = 20/1.
Al melting point (660 °C) which is the upper temperature limit for Al deposition in the CVD-FB process. The partial pressure of the gas precursors generated (with a partial pressure of N 10− 8 atm) with respect to the wt.% of Mn in the Al–Mn powder mixture is presented in Fig. 1. The calculation was performed for a temperature of 560 °C and a molar ratio of H2/ HCl in the fluidized bed of 20:1. The main Mn-containing gas precursors are MnCl2, MnH and MnCl, and the main Alcontaining gas precursors are AlCl3, AlCl2H and AlCl. The amount of Mn in the Al–Mn powder mixture appears to have no effect on the partial pressure of the gas. Therefore, it could be assumed that even a small amount of Mn will result in the generation of Mn-containing gas-precursors. However, this calculation assumes thermodynamic equilibrium as it does not take into account non-equilibrium conditions as well as other reaction kinetics. Hence, as the optimum amount of Mn in the Al–Mn powder mixture could not be determined thermodynamically, an Al–Mn powder mixture containing 50 wt.% of
Fig. 2. Calculated equilibrium partial pressure of gas precursors for a bed containing an Al–Mn powder mixture of 50 wt.% each and a molar ratio of H2/ HCl of 20:1 as a function of temperature.
Substrate material
Atomic % Al
Cr
Mn
Fe
P-92 HCM12
74.13 69.71
2.65 4.37
1.01 1.42
22.21 24.49
each metal was used for this feasibility study. This Al–Mn powder mixture constituted 20 wt.% of the total bed mass and the remaining 80 wt.% comprised inert alumina, as described in Section 2. Fig. 2 shows the calculated equilibrium gas precursors partial pressure for a bed-containing and Al–Mn powder mixture of 50 wt.% each and a molar ratio of H2/HCl in the fluidized bed of 20:1 as a function of temperature. Between 400 °C and 600 °C, the main Al-containing gas precursors (partial pressure N 10− 8 atm) obtained from this calculation are AlCl3 followed by AlCl2H and AlCl, respectively. The main Mn-containing gas precursors are MnCl2 followed by MnH and MnCl. AlCl3 precursor is the main and the most stable chloride precursor present in all the temperature range studied whereas Al2Cl6 partial pressure decreases with temperature. The partial pressure of all gas precursors increases with temperature expect that of Al2Cl6. This suggests that higher temperature will favour Al–Mn co-deposition. However, an upper limit for temperature
Fig. 3. SEM micrograph of cross-sections of coatings deposited for 2 h at 560 °C in CVD-FBR on (a) P-92 steel substrate and (b) HCM12 steel substrate.
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Fig. 5. XRD spectra for coatings deposited on P-92 steel substrate by CVD-FBR before and after diffusion heat treatment.
Fig. 4. EDS line scan analysis showing atomic % of principal elements present on coatings deposited for 2 h at 560 °C in CVD-FBR on (a) P-92 steel substrate and (b) HCM12 steel substrate.
is set by the onset of melting of aluminum at around 600 °C. Therefore, the temperature that was considered appropriate for this process was 560 °C. The above calculations provide interesting information about gaseous chloride precursors formed in the reactor starting from solid donor and about the evolution of the partial pressure of them versus temperature.
difference in thickness is due to the difference in the two ferritic substrates. HCM12 contains a higher amount of Cr. Growth of the aluminide layer occurs by interdiffusion between Al and Fe. It is possible that the diffusion of Fe and Al is more difficult when the lattice contains a higher amount of Cr. Therefore, interdiffusion occurs with greater difficulty in the HCM12 and the aluminide coating on the HCM12 substrate has smaller thickness than on P-92. Similar effects on the thickness have been observed in other studies [23]. EDS line analysis of the cross-section of both coatings is shown in Fig. 4a and b. Analysis of the line profiles shows that the coatings mainly consist of Fe, Cr and Al. The interface between the coating and the substrate can be clearly recognised by the decrease in the amount of Al and the corresponding increase in Fe content. The atomic percentage of Al present in coating on the P-92 steel is about 72 at.% throughout the thickness of the coating. In the case of the coating on the HCM12 steel, the atomic % is about 70%, slightly lower than in the case of the coating on the P-92 substrate. According to the equilibrium phase diagram [24], atomic percentages in the range 70–75% are commonly associated with the FeAl3 or/and Fe2Al5
3.2. Coatings characterization 3.2.1. As-deposited coatings EDS microanalysis of the surface of the coatings deposited on the two substrates is shown in Table 2. The deposits are free of any chloride impurity remaining from chemical reaction, and the coating is composed of grains rich in aluminum. A small amount of Mn (about 2 at.%) appears to be present in both cases, with slightly higher amount present in the coating deposited on the HCM12 steel. The cross-section of the coatings deposited on P-92 and HCM12 steels are shown in Fig. 3a and b. The coatings are homogenous in both cases and their thickness is about 7 μm in the case of the P-92 substrate and about 5 μm in the case of the HCM12 substrate. Since both coatings where deposited using the same experimental conditions, it can be assumed that the
Fig. 6. XRD spectra for coatings deposited on HCM12 steel substrate by CVDFBR before and after diffusion heat treatment.
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Lauwerens et al [9] deposited Al and Mn phases (with no intermetallic bonds) by means of PVD on the surface of sheet steel which converted upon annealing to Al8Mn5 and Fe2Al5 with some Mn dissolved in it. Therefore, it is possible that the Mn present is in solid solution in the FeAl3 and Fe2Al5 phases. 3.2.2. After diffusion heat treatment In order to promote the transformation of the Fe2Al5 coating to iron-rich phases (Fe3Al and/or FeAl) and improve the mechanical properties of the coating a thermal treatment was conducted on deposited samples. The heat treatment was performed under flowing argon at 700 °C for 2 h, followed by cooling to room temperature under Ar gas atmosphere. According to the aluminum–iron phase diagram [24], several intermetallic can be formed from Al–Fe diffusion couple. The cross-sections of P-92 and HCM12 samples after diffusion heat treatment are shown in Fig. 7a and b. The vertical lines show the regions where the EDS line scan analysis was performed. For the coated P-92 substrate, two regions can be clearly distinguished: an outer porous region and an inner interdiffusion zone. The outer porous region also contains some round precipitates (lighter in color). The HCM12 substrate
Fig. 7. SEM micrograph of cross-sections of coatings deposited onto (a) P-92 steel substrate and (b) HCM12 steel substrate, after diffusion heat treatment for 2 h at 700 °C in Ar atmosphere. Lines show regions where the EDS line scan analysis was performed.
phases. The structures of those phases can contain up to 6.4 and 6.2 at.% of Cr respectively without altering their lattice parameter significantly [25]. Therefore, it can be assumed that the Cr present in the coating is in solid solution. For coatings deposited by CVD techniques, during aluminizing of iron, the Fe2Al5 phase forms preferentially even at atomic concentrations where it appears that the FeAl3 or FeAl2 phase should be more stable [26]. The Mn content in the coating appears to be low in both coatings. X-ray analysis reveals that the Aluminide coating on P-92 consists of a mixture of Fe2Al5 and FeAl3 phases, whereas the aluminide coating on HCM12 mainly consists of Fe2Al5 (see Figs. 5 and 6). This is consistent with the EDS analysis that reveals a slightly higher atomic % of Al present in the coating on P-92 (see Fig. 4 and Table 2). However, the coating constitution as it appears on the EDS line scan (see Fig. 4a) is quite homogenous, and it is not possible to identify a distinct regions where the two phases are present. X-ray diffraction reveals no Mn-rich or Mn-containing phases in the as-deposited coating. However, a small amount of Mn appears to be present in the coating, as shown by the EDS scans. In the Al-rich corner of the ternary phase diagram Al–Fe Mn no ternary intermetallic phases are found to be in equilibrium [27]. However, it has been reported [9] that the solid solubility of Mn in FeAl3 phase is about 6 wt.%, and
Fig. 8. EDS line scan analysis showing atomic % of principal elements present on coatings deposited onto (a) P-92 steel substrate and (b) HCM12 steel substrate, after diffusion heat treatment for 2 h at 700 °C in Ar atmosphere.
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Fig. 9. SEM micrograph of the cross-section of heat treated coating deposited on HCM12 substrate. The arrowheads point at the regions where EDS point analysis was performed.
similarly has an outer porous region that contains round precipitates and an inner interdiffusion zone. EDS line analysis (see Fig. 8a and b) reveals that Fe has diffused outwards and Al inwards. From the atomic percentages, it is possible to postulate that FeAl and Fe3Al intermetallics compounds are the main constituents of the coatings after diffusion heat treatment. Fig. 9 is a SEM micrograph of the cross-section of heat treated coating deposited on HCM12 substrate. The arrowheads point to the regions where EDS point analysis was performed. Point EDS analysis of the round precipices (see Table 3) reveals that they contain about 57 at.% Al, 36 at.% Fe, 5 at.% Cr and 1 at.% Mn. The EDS line profile for P-92 that goes through one of the precipitates clearly shows an increase in the Fe and Cr content and a decrease in the Al content in the precipitate with respect to the surrounding area (see Fig. 8a). The exact formation mechanism and phase constitution of these precipitates is not clear at this stage. The darker outer region that contains the precipitates consists of atomic percentage of about 66 at.% Al, 32 at.% Fe and 2 at.% Cr (see Table 3) and therefore it is possible to postulate that this region possibly consists of the Fe2Al5 or the FeAl3 phase. Moving towards the substrate, the atomic concentration of Al decreases and the intermetallic compounds present are principally FeAl and Fe3Al. Figs. 5 and 6 show the X-ray diffraction spectra for the coatings after diffusion heat treatment. In the coating deposited on HCM12 substrate, the identified phases were Fe2Al5 and FeAl. No or very little Fe3Al appears to be present. B-2 type structures FeAl and Fe3Al also exhibit a solid solubility limit of about 6 wt.% for Mn [28] and therefore the Mn present is
probably in solid solution in this phases. In P-92, the phases that were identified were Fe2Al5 and FeAl, and it is possible that a small amount of Fe3Al was also present. Some small diffraction peaks could not be identified with certainty (they are indicated with a star in the spectra). It is possible that these correspond to the stable compound Al6Mn or to the compound with the empirical formula Al58Cr10Fe32; however, it was not possible to identify any compound with certainty and further research is necessary. The formation and growth of intermetallic compounds is governed by diffusion and by the reactions occurring at the interface [29]. In this case, the formation of a coating of FeAl, Fe3Al and solid solution of Al in Fe was determined by the interdiffusion of Fe, Cr, Al and Mn. The diffusion mechanism may be by both inward Al and outward Fe and Cr diffusion. The formation of Cr- and Fe-rich precipitates may be due to thermodynanmic stability of the precipitate phase and/or diffusion mechanisms. These Fe-rich intermetallic compounds show better corrosion behaviour than Fe2Al5 and so are more desirable in the final coating. 3.3. Thermodynamics approximation of coating behaviour in steam environment Since the envisaged application of the coating studied is high temperature oxidation protection of ferritic steels, specifically in steam-containing environments, a thermodynamic calculation using Thermo-Calc was performed in order to investigate the stable solid compounds formed in such environment. The input data was guided by the experimentally obtained coatings. Specifically, the weight percentages derived form EDS analysis (shown in Table 2) for the elements present in the coating, together with the weight gain after the deposition process for each of the two coated substrates where used as input to calculate the thermodynamically stable compound in a 20% steam environment in the temperature range of 500–800 °C. Results are shown in Fig. 10. The stable oxides formed are Al2FeO4, Cr2FeO4, FeO and Al2MnO4. These results indicate
Table 3 EDS point analysis of precipitates and darker region in Al–Mn coatings deposited on HCM12 after diffusion heat treatment Region
Precipitate Darker region
Atomic % Al
Cr
Mn
Fe
W
57.68 66.22
4.96 2.26
0.7 –
36.16 31.53
0.50 –
Arrowheads in Fig. 9 indicate the exact points where EDS point analysis was performed.
Fig. 10. Thermodynamically stable compounds formed by exposing Mncontaining Al deposited coating in a 20% steam environment in the temperature range of 500–800 °C.
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that even with a small amount of Mn present in the coating the formation of Al2MnO4 oxide is possible and this may have a beneficial effect on the steam oxidation resistance of ferritic steels [30]. 4. Conclusions Thermodynamic simulations were performed in order to investigate the optimum conditions for deposition of aluminide coatings containing Mn in an CVD-FBR. The main gas precursors and phases formed at equilibrium in the temperature range 400–600 °C were investigated. In addition, the effect of different metal donor amounts on gas partial pressures was also studied. In light of these results, possible gas precursors which may play a significant role in the deposition of Al and/or Mn were discussed briefly and optimum conditions for the deposition process were identified. The CVD-FBR has been shown to be a powerful and effective technique to obtain Mn-containing aluminide coatings on ferritic steels. Coatings with thickness of 7 μm and 5 μm were achieved on P-92 and HCM12 ferritic steels respectively at low temperatures (560 °C) and short time (2 h). The coatings consisted mainly of Fe2Al5 and FeAl3 phases with some Mn in solid solution. A further diffusion heat treatment at 700 °C converted Al-rich coatings into other Fe–Al intermetallic compounds (FeAl and Fe3Al) and resulted in the formation of some Cr-rich precipitates. These coatings could be potential candidates for steam oxidation protection of ferritic steels. Acknowledgments This work was supported by SUNASPO, European Commission funded RTN project no. HPRN-CT-2001-002001, which is gratefully acknowledged. References [1] J. Zurek, M. Michalik, F. Schmitz, T.U. Kern, L. Singheiser, W.J. Quadakkers, Oxid. Met. 63 (2005) 401.
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[2] T. Ishitsuka, Y. Inoue, H. Ogawa, Oxid. Met. 61 (2004) 125. [3] K.L. Wang, F.S. Chen, G.S. Leu, Mater. Sci. Eng., A 357 (2003) 27. [4] Y. Zhang, B.A. Pint, J.A. Haynes, P.F. Tortorelli, Oxid. Met. 62 (2004) 103. [5] I.G. Wright, B.A. Pint, P.F. Tortorelli, Oxid. Met. 55 (2001) 333. [6] G.S. Han, W.D. Cho, Oxid. Met. 58 (2002) 391. [7] K. Shiotani, Patent Appl. Num. JP06220613, (1993). [8] T. Ikeda, K. Irie, and T. Ayabe, Patent Appl. Num. JP04246166, (1991). [9] W. Lauwerens, A. De Boeck, M. Thijs, S. Claessens, M. Van Stappen, P. Steenackers, Surf. Coat. Technol. 146 (2001) 27. [10] J.C. Li, S.H. Nan, Q. Jiang, Surf. Coat. Technol. 106 (1998) 135. [11] J.W. Lee, C.C. Wu, T.F. Liu, Scr. Mater. 50 (2004) 1389. [12] K.H. Lau, A. Sanjurjo, B.J. Wood, Surf. Coat. Technol. 54 (1992) 234. [13] F.J. Perez, M.P. Hierro, F. Pedraza, C. Gomez, M.C. Carpintero, Surf. Coat. Technol. 121 (1999) 151. [14] D.N. Tsipas, K.G. Anthymidis, Y. Flitris, J. Mater. Process. Technol. 134 (2003) 145. [15] N. Voudouris, C. Christoglou, G.N. Angelopoulos, Surf. Coat. Technol. 141 (2001) 275. [16] K.G. Anthymidis, E. Stergioudis, and D.N. Tsipas, 51 (2001) 156. [17] A. Sanjurjo, B.J. Wood, K.H. Lau, G.T. Tong, D.K. Choi, M.C.H. McKubre, H.K. Song, N. Church, Surf. Coat. Technol. 49 (1991) 110. [18] A. Sanjurjo, B.J. Wood, K.H. Lau, G.T. Tong, D.K. Choi, M.C.H. McKubre, H.K. Song, D. Peters, N. Church, Surf. Coat. Technol. 49 (1991) 103. [19] J.O. Andersson, T. Helander, L.H. Hoglund, P.F. Shi, B. Sundman, CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 26 (2002) 273. [20] K.G. Anthymidis, N. Maragoudakis, G. Stergioudis, O. Haidar, D.N. Tsipas, Mater. Lett. 57 (2003) 2399. [21] Z.D. Xiang, J.S. Burnell-Gray, P.K. Datta, J. Mater. Sci. 36 (2001) 5673. [22] F. Pedraza, C. Gomez, M.C. Carpintero, M.P. Hierro, F.J. Perez, Surf. Coat. Technol. 190 (2005) 223. [23] F.J. Perez, M.P. Hierro, F. Pedraza, C. Gomez, M.C. Carpintero, J.A. Trilleros, Surf. Coat. Technol. 122 (1999) 281. [24] R. Bouchet, R. Mevrel, Acta Mater. 50 (2002) 4887. [25] M. Palm, J. Alloys Compd. 252 (1997) 192. [26] L. Levin, A. Ginzburg, L. Klinger, T. Werber, A. Katsman, P. Schaaf, Surf. Coat. Technol. 106 (1998) 209. [27] Alloy Phase Diagrams, 10th ed., ASM Handbook, vol. 3, ASM International, Materials Park, Ohio, USA, 1992, p. 512. [28] A.O. Mekhrabov, and M.V. Akdeniz, 47 (1999) 2067. [29] H. Mehrer, Mater. Trans. 37 (1996) 1259. [30] A. Dias, V.D.C. Lins, Corros. Sci. 40 (1998) 271.
Al–Mn CVD-FBR protective coatings for hot corrosion application☆ S. Tsipas, J.M. Brossard, M.P. Hierro, J.A. Trilleros, L. Sánchez, F.J. Bolívar, F.J. Pérez ⁎ Universidad Complutense de Madrid. Dpto CC. Materiales e Ingeniería Metalúrgica, Avenida Complutense s/n, Facultad de Ciencias Químicas, 28040 Madrid, Spain Received 1 May 2006; accepted in revised form 13 June 2006 Available online 9 October 2006
Abstract Ferritic steels are usually used in boiler or supercritical steam turbines which operate at temperatures between 600 and 650 °C under pressure. Protective coatings are often applied in order to increase their oxidation resistance and protect them against degradation. In this study, new Al–Mn protective coatings were deposited by CVD-FBR on two ferritic steels (P-92 and HCM12). The initial process parameters were optimized by thermodynamic calculations using Thermo-Calc software. Then, those parameters were used in the experimental procedure to obtain Al–Mn coatings at low temperature and atmospheric pressure. Co-deposition was achieved at moderate temperatures in order to maintain the substrates' mechanical properties. The coatings' microstructure and phase constitution was characterized. Fe–Al intermetallic coatings containing Cr and Mn were obtained. The phase constitution is discussed with reference to the Fe–Al–Mn ternary phase diagram. The effect of diffusion heat treatment on the phase transformations was investigated. © 2006 Published by Elsevier B.V. Keywords: Chemical vapor deposition; Thermodynamic simulation; Al–Mn coating
1. Introduction Increased efficiency of steam turbines for power generation plants means lower fuel consumption and reduced emissions of gaseous pollutants to the atmosphere. The important factors that affect the efficiency of the conventional steam power plants are the temperature and, to a lesser extent, the pressure of the input steam to the turbine. An increase in the steam temperature from 535 °C to 635 °C and the steam pressure from 18.5 to 30 MPa would allow a reduction in fuel consumption and CO2 emission by more than 25% [1]. Ferritic and martensitic heat-resistant steels with Cr content between 9% and 12%, and higher creep strength than the conventional ferritic steels have been developed for uses such as main steam pipes or superheater tubes in power generation boilers [2]. However, the lifetime of boiler materials is determined not only by creep strength but also resistance to steam oxidation. These classes of steels are fairly resistant to oxidation in air or oxygen atmospheres due to the formation of ☆
This paper was presented at the ICMCTF 2006 Conference (In Session B3-12). ⁎ Corresponding author. Tel.: +34 91 39 44215; fax: +34 91 39 33457. E-mail address: [email protected] (F.J. Pérez).
0257-8972/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.06.023
oxide scale of Cr-oxide and/or spinel. However, in steam environments, at higher temperatures, the Cr-oxide scale becomes less protective. Aluminizing onto the surface of iron-, nickel- and cobalt-base superalloys has been proven to be an effective method for improvement in oxidation and corrosion resistance at high temperature [3]. Protective alumina scales formed are well known to have good oxidation resistance at high temperatures and are more protective in steam-containing environment than Cr-oxides scales [4–6]. Japanese patents [7,8] describe the use of Al–Mn alloys for corrosion and oxidation protection but not much is known about the properties of these materials [9,10]. Fe–Al–Mn alloys have also been reported to posses good oxidation resistance at high temperature [11]. Many coating techniques such as chemical vapor deposition (CVD) or pack cementation used for applying protective coatings on steel require high temperatures and/or longer times, which limits their use to substrates that can be exposed to high temperature without degradation of their mechanical properties. CVD in a fluidized bed reactor (FBR) has the advantage of high and uniform mass and heat transfer rates and can be used to deposit Al coatings at low temperature (T b 700 °C) and short times [12–15]. Therefore, CVD-FBR is an important alternative for the deposition of Al coatings on
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ferritic steels, since this technique allows the conservation of microstructure and mechanical properties of these types of materials. This process has been successfully used to carry out aluminizing, chromizing, boriding, depositing silicon and titanium-based coatings [13,16–18]. The aim of the work was to study the feasibility of depositing, Al diffusion coatings containing Mn on ferritic steels (P92 and HCM12) by chemical vapor deposition in fluidized bed reactor (CVD-FBR). The initial parameters of the processes were optimized by thermodynamic calculations using Thermo-Calc software [19]. Then, those parameters were used during the experimental procedure to produce Mn-containing Al coatings at low temperature under atmospheric pressure. The effect of diffusion heat treatment was also studied. The main envisaged application is high temperature oxidation and corrosion protection of ferritic steels. 2. Experimental 2.1. Target materials The composition of the two different ferritic steels used as substrate material for Al–Mn diffusion coatings is presented in Table 1. The samples sheets (10 mm × 10 mm × ξ with 1.5 mm b ξ b 2.5 mm) were polished from 240-grit SiC paper up to 600-grit SiC paper and then ultrasonically cleaned in alcohol, dried and weighed prior to coating. 2.2. Coatings procedure 2.2.1. Thermodynamic calculation Thermodynamic study of phase equilibrium (composition, partial pressure) during CVD was performed using ThermoCalc computer program [19] to determine the feasibility of metal deposition and provide useful guidelines to optimize the deposition conditions. Calculations are based on the free Gibbs energy minimization code and mass conservation rule, in combination with SSUB3 and SSOL2 database (Scientific Group Thermodata Europe) for chloride precursors and substrate definition, respectively. 2.2.2. Experimental conditions The characteristics of the fluidized bed reactor used in this study have been presented in previous works [13]. The bed comprised a powder mixture that contained alumina inert powder (80 wt.%), 10 wt.% of Al (99.5% purity, 200 μm grain size) metal donor and 10 wt.% of Mn (99.5% purity, 100 μm grain size) metal donor. Bed particles were fluidized using Ar flow of 1.5 lt min− 1. The deposition temperature was 560 °C. Table 1 Composition of ferritic steel substrates Material
P-92 HCM12
Element (wt.%) Fe
Si
Cr
Ni
Mn
Mo
W
Cu
Other
87.7 83.5
0.02 0.25
9.07 12.5
0.06 0.34
0.47 0.54
0.46 0.36
1.78 1.9
– 0.85
b0.4
When the deposition temperature was reached, a flow of a reactive gas mixture of H2 + HCl was inserted into the bed. The molar ratio of H2/HCl in the reactive gas mixture was 20/1. The reactive gas mixture reacts with the metal donors and generates gas precursors which decompose and cause the deposition of the coating on the surface of the substrate. The reactive H2 + HCl gas mixture was passed through the fluidised bed for the length of the deposition process and was subsequently removed during sample cooling. The deposition time was 2 h. Subsequently, the coated samples were heat treated in a furnace at 700 °C during 2 h under inert gas Ar at atmospheric pressure. 2.3. Characterisation techniques As deposited and heat treated coated substrates were characterised by optical microscopy (OM), scanning electron microscopy (SEM JEOL JM-6400) and energy dispersive X-ray spectroscopy (EDS). Standard metallographic preparation was performed on samples prior to cross-section observations. X-ray diffraction analysis using monochromatic Cu Kα radiation (Philips X'PERT MPD, Kα(Cu) radiation) was performed in order to identify phase composition of coated samples asdeposited and after diffusion treatment. 3. Results and discussion 3.1. Thermodynamics approximation of deposition process In the CVD-FBR process, the gas species generated (precursors), through their absorption at the heated surface of the substrate and their decomposition on the surface will cause the deposition of a stable solid film at the substrate surface. Subsequently, the coating is formed by diffusion into the substrate surface. Gaseous by-products are recombined and desorbed into the gas phase. Consequently, the decomposition of the gas species by dissociation may lead to the formation of atomic Al or Mn and may promote deposition [14,15,20,21]. Thermodynamical calculations were performed in the system Al(s)/M(s)/Ar/HCl/H2. For successful co-deposition of Al and Mn, it is necessary that Al-containing and Mn-containing gas precursors are generated in sufficient quantities. The deposition of Al-diffusion coatings by CVD-FB has been successfully achieved experimentally and it has been verified thermodynamically that gas precursors which aid the deposition processes are generated [15,22]. In addition to the thermodynamics studies, it is considered that Al activity is very high in a CVD-FB and its deposition is very likely [23]. Therefore, in the current study, it was important to establish favourable conditions for Mn co-deposition. Towards this end, the most important Mn-containing gas precursors must be identified and the conditions where they are likely to generate with higher partial pressure established. Initially, the effect of the amount of the metallic donor Al(s) and Mn(s) on the partial pressure of the gas precursors present was evaluated. The calculation was performed for a temperature of 560 °C. This temperature has been found in previous studies to be favourable for Al deposition and in addition it is below the
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Table 2 EDS surface analysis of Al–Mn coatings deposited in CVD-FBR
Fig. 1. Thermodynamic calculation of gas precursors partial pressure at 560 °C in the fluidized bed with respect to the wt.% of Mn in the Al–Mn powder mixture. The molar ratio in the reactive gas mixture was H2/HCl = 20/1.
Al melting point (660 °C) which is the upper temperature limit for Al deposition in the CVD-FB process. The partial pressure of the gas precursors generated (with a partial pressure of N 10− 8 atm) with respect to the wt.% of Mn in the Al–Mn powder mixture is presented in Fig. 1. The calculation was performed for a temperature of 560 °C and a molar ratio of H2/ HCl in the fluidized bed of 20:1. The main Mn-containing gas precursors are MnCl2, MnH and MnCl, and the main Alcontaining gas precursors are AlCl3, AlCl2H and AlCl. The amount of Mn in the Al–Mn powder mixture appears to have no effect on the partial pressure of the gas. Therefore, it could be assumed that even a small amount of Mn will result in the generation of Mn-containing gas-precursors. However, this calculation assumes thermodynamic equilibrium as it does not take into account non-equilibrium conditions as well as other reaction kinetics. Hence, as the optimum amount of Mn in the Al–Mn powder mixture could not be determined thermodynamically, an Al–Mn powder mixture containing 50 wt.% of
Fig. 2. Calculated equilibrium partial pressure of gas precursors for a bed containing an Al–Mn powder mixture of 50 wt.% each and a molar ratio of H2/ HCl of 20:1 as a function of temperature.
Substrate material
Atomic % Al
Cr
Mn
Fe
P-92 HCM12
74.13 69.71
2.65 4.37
1.01 1.42
22.21 24.49
each metal was used for this feasibility study. This Al–Mn powder mixture constituted 20 wt.% of the total bed mass and the remaining 80 wt.% comprised inert alumina, as described in Section 2. Fig. 2 shows the calculated equilibrium gas precursors partial pressure for a bed-containing and Al–Mn powder mixture of 50 wt.% each and a molar ratio of H2/HCl in the fluidized bed of 20:1 as a function of temperature. Between 400 °C and 600 °C, the main Al-containing gas precursors (partial pressure N 10− 8 atm) obtained from this calculation are AlCl3 followed by AlCl2H and AlCl, respectively. The main Mn-containing gas precursors are MnCl2 followed by MnH and MnCl. AlCl3 precursor is the main and the most stable chloride precursor present in all the temperature range studied whereas Al2Cl6 partial pressure decreases with temperature. The partial pressure of all gas precursors increases with temperature expect that of Al2Cl6. This suggests that higher temperature will favour Al–Mn co-deposition. However, an upper limit for temperature
Fig. 3. SEM micrograph of cross-sections of coatings deposited for 2 h at 560 °C in CVD-FBR on (a) P-92 steel substrate and (b) HCM12 steel substrate.
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Fig. 5. XRD spectra for coatings deposited on P-92 steel substrate by CVD-FBR before and after diffusion heat treatment.
Fig. 4. EDS line scan analysis showing atomic % of principal elements present on coatings deposited for 2 h at 560 °C in CVD-FBR on (a) P-92 steel substrate and (b) HCM12 steel substrate.
is set by the onset of melting of aluminum at around 600 °C. Therefore, the temperature that was considered appropriate for this process was 560 °C. The above calculations provide interesting information about gaseous chloride precursors formed in the reactor starting from solid donor and about the evolution of the partial pressure of them versus temperature.
difference in thickness is due to the difference in the two ferritic substrates. HCM12 contains a higher amount of Cr. Growth of the aluminide layer occurs by interdiffusion between Al and Fe. It is possible that the diffusion of Fe and Al is more difficult when the lattice contains a higher amount of Cr. Therefore, interdiffusion occurs with greater difficulty in the HCM12 and the aluminide coating on the HCM12 substrate has smaller thickness than on P-92. Similar effects on the thickness have been observed in other studies [23]. EDS line analysis of the cross-section of both coatings is shown in Fig. 4a and b. Analysis of the line profiles shows that the coatings mainly consist of Fe, Cr and Al. The interface between the coating and the substrate can be clearly recognised by the decrease in the amount of Al and the corresponding increase in Fe content. The atomic percentage of Al present in coating on the P-92 steel is about 72 at.% throughout the thickness of the coating. In the case of the coating on the HCM12 steel, the atomic % is about 70%, slightly lower than in the case of the coating on the P-92 substrate. According to the equilibrium phase diagram [24], atomic percentages in the range 70–75% are commonly associated with the FeAl3 or/and Fe2Al5
3.2. Coatings characterization 3.2.1. As-deposited coatings EDS microanalysis of the surface of the coatings deposited on the two substrates is shown in Table 2. The deposits are free of any chloride impurity remaining from chemical reaction, and the coating is composed of grains rich in aluminum. A small amount of Mn (about 2 at.%) appears to be present in both cases, with slightly higher amount present in the coating deposited on the HCM12 steel. The cross-section of the coatings deposited on P-92 and HCM12 steels are shown in Fig. 3a and b. The coatings are homogenous in both cases and their thickness is about 7 μm in the case of the P-92 substrate and about 5 μm in the case of the HCM12 substrate. Since both coatings where deposited using the same experimental conditions, it can be assumed that the
Fig. 6. XRD spectra for coatings deposited on HCM12 steel substrate by CVDFBR before and after diffusion heat treatment.
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Lauwerens et al [9] deposited Al and Mn phases (with no intermetallic bonds) by means of PVD on the surface of sheet steel which converted upon annealing to Al8Mn5 and Fe2Al5 with some Mn dissolved in it. Therefore, it is possible that the Mn present is in solid solution in the FeAl3 and Fe2Al5 phases. 3.2.2. After diffusion heat treatment In order to promote the transformation of the Fe2Al5 coating to iron-rich phases (Fe3Al and/or FeAl) and improve the mechanical properties of the coating a thermal treatment was conducted on deposited samples. The heat treatment was performed under flowing argon at 700 °C for 2 h, followed by cooling to room temperature under Ar gas atmosphere. According to the aluminum–iron phase diagram [24], several intermetallic can be formed from Al–Fe diffusion couple. The cross-sections of P-92 and HCM12 samples after diffusion heat treatment are shown in Fig. 7a and b. The vertical lines show the regions where the EDS line scan analysis was performed. For the coated P-92 substrate, two regions can be clearly distinguished: an outer porous region and an inner interdiffusion zone. The outer porous region also contains some round precipitates (lighter in color). The HCM12 substrate
Fig. 7. SEM micrograph of cross-sections of coatings deposited onto (a) P-92 steel substrate and (b) HCM12 steel substrate, after diffusion heat treatment for 2 h at 700 °C in Ar atmosphere. Lines show regions where the EDS line scan analysis was performed.
phases. The structures of those phases can contain up to 6.4 and 6.2 at.% of Cr respectively without altering their lattice parameter significantly [25]. Therefore, it can be assumed that the Cr present in the coating is in solid solution. For coatings deposited by CVD techniques, during aluminizing of iron, the Fe2Al5 phase forms preferentially even at atomic concentrations where it appears that the FeAl3 or FeAl2 phase should be more stable [26]. The Mn content in the coating appears to be low in both coatings. X-ray analysis reveals that the Aluminide coating on P-92 consists of a mixture of Fe2Al5 and FeAl3 phases, whereas the aluminide coating on HCM12 mainly consists of Fe2Al5 (see Figs. 5 and 6). This is consistent with the EDS analysis that reveals a slightly higher atomic % of Al present in the coating on P-92 (see Fig. 4 and Table 2). However, the coating constitution as it appears on the EDS line scan (see Fig. 4a) is quite homogenous, and it is not possible to identify a distinct regions where the two phases are present. X-ray diffraction reveals no Mn-rich or Mn-containing phases in the as-deposited coating. However, a small amount of Mn appears to be present in the coating, as shown by the EDS scans. In the Al-rich corner of the ternary phase diagram Al–Fe Mn no ternary intermetallic phases are found to be in equilibrium [27]. However, it has been reported [9] that the solid solubility of Mn in FeAl3 phase is about 6 wt.%, and
Fig. 8. EDS line scan analysis showing atomic % of principal elements present on coatings deposited onto (a) P-92 steel substrate and (b) HCM12 steel substrate, after diffusion heat treatment for 2 h at 700 °C in Ar atmosphere.
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Fig. 9. SEM micrograph of the cross-section of heat treated coating deposited on HCM12 substrate. The arrowheads point at the regions where EDS point analysis was performed.
similarly has an outer porous region that contains round precipitates and an inner interdiffusion zone. EDS line analysis (see Fig. 8a and b) reveals that Fe has diffused outwards and Al inwards. From the atomic percentages, it is possible to postulate that FeAl and Fe3Al intermetallics compounds are the main constituents of the coatings after diffusion heat treatment. Fig. 9 is a SEM micrograph of the cross-section of heat treated coating deposited on HCM12 substrate. The arrowheads point to the regions where EDS point analysis was performed. Point EDS analysis of the round precipices (see Table 3) reveals that they contain about 57 at.% Al, 36 at.% Fe, 5 at.% Cr and 1 at.% Mn. The EDS line profile for P-92 that goes through one of the precipitates clearly shows an increase in the Fe and Cr content and a decrease in the Al content in the precipitate with respect to the surrounding area (see Fig. 8a). The exact formation mechanism and phase constitution of these precipitates is not clear at this stage. The darker outer region that contains the precipitates consists of atomic percentage of about 66 at.% Al, 32 at.% Fe and 2 at.% Cr (see Table 3) and therefore it is possible to postulate that this region possibly consists of the Fe2Al5 or the FeAl3 phase. Moving towards the substrate, the atomic concentration of Al decreases and the intermetallic compounds present are principally FeAl and Fe3Al. Figs. 5 and 6 show the X-ray diffraction spectra for the coatings after diffusion heat treatment. In the coating deposited on HCM12 substrate, the identified phases were Fe2Al5 and FeAl. No or very little Fe3Al appears to be present. B-2 type structures FeAl and Fe3Al also exhibit a solid solubility limit of about 6 wt.% for Mn [28] and therefore the Mn present is
probably in solid solution in this phases. In P-92, the phases that were identified were Fe2Al5 and FeAl, and it is possible that a small amount of Fe3Al was also present. Some small diffraction peaks could not be identified with certainty (they are indicated with a star in the spectra). It is possible that these correspond to the stable compound Al6Mn or to the compound with the empirical formula Al58Cr10Fe32; however, it was not possible to identify any compound with certainty and further research is necessary. The formation and growth of intermetallic compounds is governed by diffusion and by the reactions occurring at the interface [29]. In this case, the formation of a coating of FeAl, Fe3Al and solid solution of Al in Fe was determined by the interdiffusion of Fe, Cr, Al and Mn. The diffusion mechanism may be by both inward Al and outward Fe and Cr diffusion. The formation of Cr- and Fe-rich precipitates may be due to thermodynanmic stability of the precipitate phase and/or diffusion mechanisms. These Fe-rich intermetallic compounds show better corrosion behaviour than Fe2Al5 and so are more desirable in the final coating. 3.3. Thermodynamics approximation of coating behaviour in steam environment Since the envisaged application of the coating studied is high temperature oxidation protection of ferritic steels, specifically in steam-containing environments, a thermodynamic calculation using Thermo-Calc was performed in order to investigate the stable solid compounds formed in such environment. The input data was guided by the experimentally obtained coatings. Specifically, the weight percentages derived form EDS analysis (shown in Table 2) for the elements present in the coating, together with the weight gain after the deposition process for each of the two coated substrates where used as input to calculate the thermodynamically stable compound in a 20% steam environment in the temperature range of 500–800 °C. Results are shown in Fig. 10. The stable oxides formed are Al2FeO4, Cr2FeO4, FeO and Al2MnO4. These results indicate
Table 3 EDS point analysis of precipitates and darker region in Al–Mn coatings deposited on HCM12 after diffusion heat treatment Region
Precipitate Darker region
Atomic % Al
Cr
Mn
Fe
W
57.68 66.22
4.96 2.26
0.7 –
36.16 31.53
0.50 –
Arrowheads in Fig. 9 indicate the exact points where EDS point analysis was performed.
Fig. 10. Thermodynamically stable compounds formed by exposing Mncontaining Al deposited coating in a 20% steam environment in the temperature range of 500–800 °C.
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that even with a small amount of Mn present in the coating the formation of Al2MnO4 oxide is possible and this may have a beneficial effect on the steam oxidation resistance of ferritic steels [30]. 4. Conclusions Thermodynamic simulations were performed in order to investigate the optimum conditions for deposition of aluminide coatings containing Mn in an CVD-FBR. The main gas precursors and phases formed at equilibrium in the temperature range 400–600 °C were investigated. In addition, the effect of different metal donor amounts on gas partial pressures was also studied. In light of these results, possible gas precursors which may play a significant role in the deposition of Al and/or Mn were discussed briefly and optimum conditions for the deposition process were identified. The CVD-FBR has been shown to be a powerful and effective technique to obtain Mn-containing aluminide coatings on ferritic steels. Coatings with thickness of 7 μm and 5 μm were achieved on P-92 and HCM12 ferritic steels respectively at low temperatures (560 °C) and short time (2 h). The coatings consisted mainly of Fe2Al5 and FeAl3 phases with some Mn in solid solution. A further diffusion heat treatment at 700 °C converted Al-rich coatings into other Fe–Al intermetallic compounds (FeAl and Fe3Al) and resulted in the formation of some Cr-rich precipitates. These coatings could be potential candidates for steam oxidation protection of ferritic steels. Acknowledgments This work was supported by SUNASPO, European Commission funded RTN project no. HPRN-CT-2001-002001, which is gratefully acknowledged. References [1] J. Zurek, M. Michalik, F. Schmitz, T.U. Kern, L. Singheiser, W.J. Quadakkers, Oxid. Met. 63 (2005) 401.
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[2] T. Ishitsuka, Y. Inoue, H. Ogawa, Oxid. Met. 61 (2004) 125. [3] K.L. Wang, F.S. Chen, G.S. Leu, Mater. Sci. Eng., A 357 (2003) 27. [4] Y. Zhang, B.A. Pint, J.A. Haynes, P.F. Tortorelli, Oxid. Met. 62 (2004) 103. [5] I.G. Wright, B.A. Pint, P.F. Tortorelli, Oxid. Met. 55 (2001) 333. [6] G.S. Han, W.D. Cho, Oxid. Met. 58 (2002) 391. [7] K. Shiotani, Patent Appl. Num. JP06220613, (1993). [8] T. Ikeda, K. Irie, and T. Ayabe, Patent Appl. Num. JP04246166, (1991). [9] W. Lauwerens, A. De Boeck, M. Thijs, S. Claessens, M. Van Stappen, P. Steenackers, Surf. Coat. Technol. 146 (2001) 27. [10] J.C. Li, S.H. Nan, Q. Jiang, Surf. Coat. Technol. 106 (1998) 135. [11] J.W. Lee, C.C. Wu, T.F. Liu, Scr. Mater. 50 (2004) 1389. [12] K.H. Lau, A. Sanjurjo, B.J. Wood, Surf. Coat. Technol. 54 (1992) 234. [13] F.J. Perez, M.P. Hierro, F. Pedraza, C. Gomez, M.C. Carpintero, Surf. Coat. Technol. 121 (1999) 151. [14] D.N. Tsipas, K.G. Anthymidis, Y. Flitris, J. Mater. Process. Technol. 134 (2003) 145. [15] N. Voudouris, C. Christoglou, G.N. Angelopoulos, Surf. Coat. Technol. 141 (2001) 275. [16] K.G. Anthymidis, E. Stergioudis, and D.N. Tsipas, 51 (2001) 156. [17] A. Sanjurjo, B.J. Wood, K.H. Lau, G.T. Tong, D.K. Choi, M.C.H. McKubre, H.K. Song, N. Church, Surf. Coat. Technol. 49 (1991) 110. [18] A. Sanjurjo, B.J. Wood, K.H. Lau, G.T. Tong, D.K. Choi, M.C.H. McKubre, H.K. Song, D. Peters, N. Church, Surf. Coat. Technol. 49 (1991) 103. [19] J.O. Andersson, T. Helander, L.H. Hoglund, P.F. Shi, B. Sundman, CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 26 (2002) 273. [20] K.G. Anthymidis, N. Maragoudakis, G. Stergioudis, O. Haidar, D.N. Tsipas, Mater. Lett. 57 (2003) 2399. [21] Z.D. Xiang, J.S. Burnell-Gray, P.K. Datta, J. Mater. Sci. 36 (2001) 5673. [22] F. Pedraza, C. Gomez, M.C. Carpintero, M.P. Hierro, F.J. Perez, Surf. Coat. Technol. 190 (2005) 223. [23] F.J. Perez, M.P. Hierro, F. Pedraza, C. Gomez, M.C. Carpintero, J.A. Trilleros, Surf. Coat. Technol. 122 (1999) 281. [24] R. Bouchet, R. Mevrel, Acta Mater. 50 (2002) 4887. [25] M. Palm, J. Alloys Compd. 252 (1997) 192. [26] L. Levin, A. Ginzburg, L. Klinger, T. Werber, A. Katsman, P. Schaaf, Surf. Coat. Technol. 106 (1998) 209. [27] Alloy Phase Diagrams, 10th ed., ASM Handbook, vol. 3, ASM International, Materials Park, Ohio, USA, 1992, p. 512. [28] A.O. Mekhrabov, and M.V. Akdeniz, 47 (1999) 2067. [29] H. Mehrer, Mater. Trans. 37 (1996) 1259. [30] A. Dias, V.D.C. Lins, Corros. Sci. 40 (1998) 271.