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Development of coatings for protection in specific high temperature environments
Surface & Coatings Technology 201 (2006) 3872 – 3879 www.elsevier.com/locate/surfcoat
Development of coatings for protection in specific high temperature environments M. Schütze ⁎, M. Malessa, V. Rohr, T. Weber Karl-Winnacker-Institut der DECHEMA e.V., Theodor-Heuss-Allee 25, 60486 Frankfurt am Main, Germany Available online 15 September 2006
Abstract Metallic and intermetallic coatings are widely used in jet engines and land-based gas turbines for oxidation and corrosion protection in the hotter parts of the engines. However there is a significant number of industrial processes where the use of protective coatings at high temperatures could contribute to a significant extension of life-time or an increase in operation temperature and thus efficiency. Examples of such industries are incineration and gasification of waste, biomass and coal, chemical process industries and petrochemical plants where highly aggressive environments are encountered containing species of e.g. carbon, chlorine, sulphur, vanadium or alcalines. Since most of these process environments contain only very low oxygen partial pressures or exhibit high concentrations of extremely aggressive compounds, the conventional, uncoated materials come to their limits. In recent years in laboratory work a number of new types of coatings have been developed for high-temperature applications which include diffusion coatings, overlay coatings and nanotechnological approaches for sealing porosity. In the paper the background of this development and the thermodynamic fundamentals are discussed together with some more recent solutions based on synergistic effects of multi-element coatings. Some results of performance tests of these coatings in sulfidizing, carburizing, chloridizing and vanadate environments will be presented. At the end conclusions can be drawn on the suitability of the different types of coatings for their specific applications. © 2006 Elsevier B.V. All rights reserved. Keywords: Coating design; Multi-element co-diffusion coating; Nano sealant
1. Introduction Metallic and intermetallic coatings have been used for several decades for protection at high temperatures in jet engines and land-based gas turbines with great success. The main purpose of these coatings is to keep oxidation rates in combustion environments on a level which allows reliable operation of the engines for several years without any significant damage to the components. Today's concepts even foresee reconditioning of the surfaces by stripping and renewed application of virgin coatings after certain operation intervals so that long-term resistance of the component to oxidation can be assured by meanwhile established maintenance procedures. Such coatings not only offer protection against oxygen and nitrogen from the intake air environment but also to a certain extent against attack by fuel or air impurities, such as sulfur (SO2 or SO3), chlorine (from maritime environments) or liquid deposits (sulfates). However, these impurities should be present only in low ⁎ Corresponding author. E-mail address: [email protected] (M. Schütze). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.262
amounts as otherwise the coatings would soon come to their limits, and damage cases are occasionally observed due to the action of such impurities. Most of these coatings are based on the formula Me1Me2CrAlX with Me1, Me2 = Ni, Co, Fe and X = Ce, Hf, La, Y, etc. if they are applied as overlay coatings (plasma spray techniques) or on Al and Pt + Al plus substrate alloy if used as diffusion coatings. Recent research has also tried to incorporate reactive elements X into diffusion coatings, however the technical reliability of the manufacturing process has not yet been achieved for such coatings. A common feature of these coatings is that they act as a reservoir phase for the formation of protective and slow growing Al-based oxide scales by reaction with the operation environment, thus providing an environmental barrier against the ingress of more aggressive species down to the metal to be protected and slowing down metal consumption rates by the oxidation process itself. While all these coatings have been developed and used under relatively “mild” environmental conditions mainly with the aim to increase operation temperatures to the mechanical limits of the substrate alloys, more recently a second route of coating deveopment has been taken up again after some initial frustrating
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experiences in the 80s and 90s of the 20th century. This route aims at protection of conventional commercial materials (steel and nickel-base alloys) for application in highly aggressive high temperature environments in the petrochemical and chemical process industries as well as in energy conversion from fossil fuels and also from household and industrial waste as well as from agricultural residues. Such environments are often characterized by low contents of oxygen (so-called reducing environments which hamper the formation of protective oxide scales) and high concentrations of detrimental species like sulfur, chlorine, carbon or vanadium together with alcalines. Some of these species even dissolve protective oxide scales such as alumina or chromia scales so that up to now the upper limit temperatures for the technical components in the plant are not set by their mechanical strength potential but by the metal wastage rates due to high temperature corrosion. In some cases this situation can be very unsatisfactory as, for example, in petrochemical industries where sulfidation may limit operation temperatures to about 400 … 500 °C depending on the materials used and for a similar temperature range in large ship diesel engines due to vanadium pentoxide attack. There may be expensive high alloy materials which could be capable of increasing these limits, however the component size and complexity as well as the rather limited ratio of improved corrosion resistance versus increased costs in most cases exclude solutions based on bulk materials. Under such conditions more and more solutions are becoming attractive which are based on low-cost substrate materials (steels which have sufficient high temperature strength at the temperatures envisaged) and tailormade protective coatings which were developed for use in specific highly aggressive high temperature environments. The present paper aims at a review of the present development stage of such coatings and will elucidate some of the aspects which have to be taken into account when developing and using these coatings for specific applications. In the present paper and historically the term “diffusion coatings” is used for Pack-CVD coatings. There are, however, alternative ways to introduce the
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metallic reservoir element(s) into the metal subsurface zone, which are described in detail e.g. in Ref. [1]. 2. Theoretical considerations in coating design There exist four major criteria for the development of an optimized coating: 1) The coating should form thermodynamically stable protective phases on its surface by the reaction with the process environment. The “classical” protective phases will be Al2O3, Cr2O3 and SiO2 as well as some of the spinels. 2) These protective phases should be slow growing in order to keep coating reservoir depletion rates by the surface reaction on a low level. Again the above-mentioned phases play a key role in high temperature corrosion protection. 3) Interdiffusion between coating and substrate should occur as slowly as possible which suggests the introduction of an interdiffusion barrier or the use of a substrate into which diffusion of the coating species occurs only at a very low rate. 4) The values of the coefficients of thermal expansion of coating, protective surface phase (scale) and substrate should lie as close to each other as possible so that cooling and reheating stresses are minimized in the system during temperature changes or by down times. Generally the system should be strain-tolerant which at least in the case of the coatings could be achieved by the use of ductile alloy phases. The latter is, however, not easy to achieve as almost all reservoir phases in coatings are brittle intermetallics with ductile to brittle transition temperatures clearly above 500 °C. In the following some of these criteria will be detailed further, starting off with considerations on thermodynamic stability of the protective phases formed on top of the coating. These considerations will focus on environments containing S, C or Cl which seem to be the critical elements most often encountered in high temperature plant conditions which can however be present
Fig. 1. Stability diagram for oxidizing–sulfidizing environments at 700 °C [2]. The quadrangle depicts a gasification atmosphere in petrochemistry.
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as different compounds. The aggressivity of such compounds has to be characterized by the respective activities of these elements present in the process environments, i.e. in the compounds of these environments. The first element to be addressed is sulfur which plays a significant role in corrosion in many high temperature processes such as those in petrochemistry (e.g. H2S corrosion or corrosion in the gasification of refinery residues), in fossil fuel and waste incineration or gasification, and in the combustion of heavy oil in ship diesel engines. The oxygen and sulfur partial pressures (activities) pO2 and pS2 of some of these environments formed the basis for thermodynamic considerations based on stability diagrams of the type shown in Fig. 1 [2]. The latter depicts conditions where pS2 and pO2 lie in a range where protective Al2O3, Cr2O3 and SiO2, can be formed and where the non-protective sulfides of the same alloying elements must be expected. Such diagrams help in identifying alloy or coating compositions of high corrosion resistance and others that will not survive. A particularly beneficial coating element under such conditions is Al with the largest area where the protective oxide is stable over the sulfide while Fe shows the contrary behavior. Diagrams of the type of Fig. 1 can be found in a number of papers in the literature for different temperatures and further elements. These also reveal that another interesting alloying element is Ti [3] which in particular in combination with Al as the intermetallic TiAl phase shows an interesting protection potential especially under sulfidizing environments, as will be further discussed later in this paper. High carbon/low oxygen environments are another critical type of process conditions as encountered, for example, in coal gasification, steam refining and hydro cracking. The critical conditions can be derived from diagrams as a function of carbon activity ac and oxygen partial pressure pO2 [3,4]. Again thermodynamic stability diagrams based on ac and pO2, can be developed, revealing that in particular Al and Si are the coating elements of choice for protection [5]. Finally Cl-containing environments will be considered. A diagram which summarizes different operation conditions based on the chlorine partial pressure pCl2 together with pO2 can be found in Ref. [3]. The key problem of these environments is that instead of solid corrosion products at high temperatures volatile metal chlorides can be formed leading to extreme corrosion rates due to porous non-protective oxide scales and a spongy metal Table 1 Coating elements for protection in different environments from a thermodynamic point of view
Gas phase corrosion S C Cl high pO2 Cl low pO2 H2O below 600 °C H2O above 600 °C Liquid phase corrosion Sulfates Chlorides Vanadates
Al
Cr
Mo
Ni
Si
Fe
++ ++ ++ −− ++ ++
+ + − −− ++ −
+ − −− + + −−
−− − − + ++ −
++ ++ ++ −− ++ +/−
−− −− − −− − −
(+) ? −−
+ + +
? ? −−
− + (−)
+ ? +
−− −− (−)
Fig. 2. SPS-TiAl coating on Alloy 800 H after heating and cooling [12]; SPS: shrouded plasma spraying.
subsurface zone by selective “leaching” of alloying elements. Since traditional stability diagrams can only be calculated for solid and liquid phases (corrosion products) but not for gaseous phases a new type of diagram, called the “quasi stability diagram” [6], had to be developed. This type of diagram again depicts pCl2/ pO2 ranges where protection can be expected since the vapor pressures of the metal chlorides formed stay below a critical level keeping metal wastage rates by evaporation low and ranges where high corrosion rates may occur. Such diagrams have been calculated for all major alloying elements and different temperatures and have recently been further refined taking the influence of component geometry and gas flow rate into account [7]. From these diagrams it can be concluded that in Cl-containing Table 2 Orientational (“maximum") values of the coefficients of thermal expansion (CTE) for several technically relevant substrates and coating phases based on Ref. [13] with further data CTE α [10− 6 K] SiO2 (amorph) HPSN Si SiC AlN TiN MoSi2 Cr2O3, SiO2 (Cryst) Al2O3, Ti Ti Pt Y2O3 Cr IMI 834, NiAl TiO2 Ti3Al (α2) RuAl Pd Ni3Al (adv.) TiSi2 TiAl Co CoAl FeCrAlY
0.94 3.00 4.60 4.94 6.23 7.76 7.94 8.06 8.40 8.60 8.76 8.88 9.47 9.93 10.53 10.94 11.00 11.58 11.88 12.11 12.93 13.88 14.00 14.50
CoO
14.93
CTE α [10− 6 K] Incoloy 956 738 Alloy S, 713 NiPt39Al 617, C-4 CMSX 6, 100 C-276, 230 C-22 NiCrAlY X 600 x-750 NiO, 556, 718 625 Ni 188 800H NiCo17Cr14AlY Fe3Al FeAl2 FeAl Fe–28Al–2Cr Al Fe–28Al–5Cr– 0.1Zr Fe2Al5
15.00 15.29 15.40 15.60 15.64 15.76 15.88 16.11 16.40 16.46 16.58 16.70 17.05 17.23 17.52 17.76 18.29 18.50 19.50 19.50 21.80 22.50 23.80 24.00 24.20
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Fig. 3. Activities of different metal halide phases as a function of temperature for a given powder pack composition [14].
environments with significant amounts of oxygen Al and Si are the coating elements of choice, while Cr and Mo are rather detrimental. At extremely low pO2, however, Mo and Ni seem to be the alloying elements of choice. A somewhat simplified summary of the outcome of the thermodynamic calculations which can actually be performed quite easily by using the respective existing software [8] is given in Table 1. Nevertheless this table can give tendencies which can be used in the theoretical design of coatings together with the other criteria. For a more detailed assessment in particular with regard to use in the respective process environment the abovementioned computer-based calculations (see also Refs. [6,8]) are recommended. So far, only corrosion by the gas phase has been considered but in some applications, in particular in waste incineration, a very severe type of high temperature corrosion is observed by solid or liquid deposits of high chlorine content (mostly KCl, NaCl, CaCl2, ZnCl2 and PbCl2). Furthermore sulfate deposits can lead to significant high temperature attack and there may even be an interaction between sulfates and chlorides. In the case of solid deposits, if they do not form liquid eutectics with the oxide scales and/or the metal, it is mainly the gas phase compound that determines corrosion and the theoretical considerations performed before can be applied again. Liquid phase corrosion is a different mechanism leading to high corrosion rates due to high transport rates in the corrosive compound so that the formation of liquid phases should generally be avoided (be it as eutectics or be it as a single compound). Again thermodynamic calculations can be used to compute ternary phase diagrams. These calculations indicate that Ni should be superior to Fe as the liquid phase area is much larger in the latter case. Cr is also one of the “beneficial” elements under these conditions [9]. Similar considerations can be
Fig. 4. HVOF-TiAl coating (a) and Al–Ti multi-element co-diffusion coating (b) on Alloy 800 [2].
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performed for sulfates and for vanadates where, however, in the latter case only very limited data exist and some of the conclusions have to be drawn based on the metal vanadate melting points. Again the conclusions have been summarized in Table 1. It should be pointed out that the thermodynamic data do not include the solubilities of the metals or oxides in the liquid phases which in most cases, however are rather high. As a final point on the stability of phases it should be emphasized that, if existing, the theoretical data should be verified by experimentally determined data. For example, for sulfidation and chlorination such data exist [2,3,10,11] which are in quite good agreement with the theoretical considerations. Another extremely important parameter in theoretical coating design is the coefficient of thermal expansion (CTE) which should be deliberately adjusted in order to avoid fracture or spalling of the coating. Fig. 2 shows an example of a coating where the CTE was much lower than that of the austenitic substrate so that during cooling tensile stresses perpendicular and parallel to the coating/ substrate interface were formed, resulting in detachment and fracture of the coating [12]. This situation is confirmed by the (simplified!) summary of the CTEs in Table 2 [13]. As this table reveals, Cr-, NiAl-, Ni3Al-, TiAl- and Ti3Al-based coatings should have CTEs relatively close to ferritic or ferritic–martensitic steels whereas austenitic steels have to be regarded as critical. The behavior of Ni-base alloys is in between the two. Something that should be noted is the high values of Al and of the iron aluminides. Al seems to be less of a problem as it is very
ductile, however the iron aluminides can cause significant problems as a coating phase with respect to mechanical integrity. The last point in this section on theoretical coating design is, in the case of diffusion coatings, the definition of the composition of the powder pack or the reservoir phases in the diffusion process based on thermodynamic calculations. In order to adjust the appropriate metal activities on the metal surface of the component to be coated suitable powder mixtures have to be chosen, in particular if the simultaneous diffusion of several protective metallic elements into the substrate surface is to be performed in a co-diffusion treatment. As will be shown later, such multi-element diffusion coatings can exhibit synergistic positive effects in protection. In this case it is extremely important to design the powder mixture composition in such a way that the desired multielement diffusion coating composition is achieved. The results of the respective thermodynamic calculations are shown for an example of a TiAl diffusion coating in Fig. 3 where the metal halide activities in the powder are depicted as a function of coating temperature. 3. Novel coating manufacturing routes and their advantages For the overlay coating route the “traditional” techniques of atmospheric plasma spraying (APS), shrouded plasma spraying (SPS) and high velocity oxy-fuel spraying (HVOF) have also been investigated for applications of the type addressed by this
Fig. 5. a) Uncoated Alloy 800; b and c) Alloy 800 with Al–Ti co-diffusion coating after 1000 h exposure in low pO2/high pS2/high ac environment at 700 °C [2].
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Fig. 6. Mass change curves for a conventional Al diffusion coating and an Al–Ti co-diffusion coating both on Alloy 800 during exposure in low pO2/high pS2/ high ac environment at 700 °C [14].
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Fig. 8. Low temperature aluminizing coating on P91 [15].
paper. However, it should not be forgotten that overlay coatings are usually not a really “intrinsic” part of the component which means that separation along the coating/substrate interface may occur more easily than for diffusion coatings which are rather a “part of the material”. This is illustrated by Fig. 4 which compares an actually quite dense SPS TiAl coating with an advanced multi-element co-diffusion coating based on Al–Ti [2]. The codiffusion coating shows quite a complex microstructure which, however, avoids any “sharp” phase change from substrate to coating and is free from any physical defects like interlamellar spacings or pores. Such a co-diffusion coating based on Al–Ti is shown in Fig. 5 after exposure in an extremely aggressive environment of refinery residue gasification which is highly sulfidizing and carburizing. The uncoated Alloy 800 already suffers from massive corrosion after 30 h (Fig. 5a) while even after 1000 h the coated surface only shows minor attack (Fig. 5b, c). Furthermore a comparison of the co-diffusion coating with a standard commercial aluminizing coating reveals the obvious synergistic effect of the two-element co-diffusion coating, Fig. 6 [14], since the presence of Ti stimulates the formation of a protective oxide scale at this relatively low temperature of 700 °C where Al alone has difficulties with scale formation. A
further example of the potential of multi-element diffusion coatings is given in Fig. 7 for metal dusting environments where the uncoated material shows a massive metal loss due to metal dusting attack [3,4]. All coatings mentioned in the figure provide a significant improvement. However, with increasing complexity of the coating (i.e. increasing number of different elements) the protective effect increases, which would be even more obvious in this plot if a more appropriate scale had been used for the ordinate. Nevertheless, in recent years there have been efforts to further improve plain aluminide coatings in particular for lowcost applications. As in such cases the microstructure of the steels is thermally less stable new routes for applying diffusion coatings at the relatively low temperature of 650 °C have been developed [15,16]. The results were a very homogeneous Alrich coating on 9–12% Cr high temperature steels, as shown as an example in Fig. 8. These coatings have been tested quite extensively under various conditions (fireside corrosion [17], erosion–corrosion [18], creep [19], waste incineration conditions [17]) where in virtually all cases a significant improvement in corrosion resistance was confirmed without any significant effect on the mechanical properties of the substrate. It was rather that coating life-time was determined by inward diffusion of Al into the substrate so that some thought had to be devoted to the
Fig. 7. Mass change curves without coating and for several diffusion coatings on P91 during exposure in low pO2/high ac environment (metal dusting atmosphere) at 620 °C [3,4]; for details about the coatings see Ref. [4].
Fig. 9. Advanced two-step Cr + Al coating with Cr interdiffusion barrier on P22 [4].
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Fig. 10. Non-optimized SiO2-based “nano”-sealant on HVOF coating/13Cr steel substrate manufactured through the sol–gel route [20], top view.
development of an interdiffusion barrier. The idea of such barriers is not really new but was revived in Ref. [4], leading to the two-step diffusion coating in Fig. 9. The effect of this “chromium barrier”, known as being effective in the case of austenitic substrates, has not yet been finally confirmed for ferritics, but the concept of a slowly diffusing element like Cr as a barrier for the faster diffusing Al would seem sensible. The Cr barrier is applied in a first diffusion step at high temperature (1000–1100 °C) while the Al follows in a second diffusion step at lower temperature (650–750 °C). Both temperature ranges correspond to the conventional heat treatment steps of these steels so that the coating process can be combined with the standard heat treatment procedure. The overlay-type coatings should not be completely forgotten in the development of protective surfaces for components in the industrial areas addressed by this paper although they may have certain disadvantages, as discussed above. Usually diffusion coatings have to be applied off-site, i.e. the components may have to be welded on-site to build up the final structure of the plant. In this case coatings would disturb the welding process so that these parts of the components have to be left unprotected until the welding procedure has taken place. After this the welded areas should be protected by overlay coatings. Moreover in the case of
Fig. 12. HVOF-NiCrSi coating on heat-resistant steel after exposure under simulated waste incineration conditions (N2–10H2O–5O2–0,25HCl in vol.% + deposit 14NaCl–13KCl–27K2SO4–16ZnSO4·7H2O–30Na2SO4 in wt.%) for 288 h with protective SiO2/B2O3 sealant on the right side and discontinuous non-protective sealant on the left side [20].
repair or post-operational surface protection overlay coatings have clear advantages over diffusion coatings although development is going on where by a slurry or an electrolytically applied diffusion reservoir on the component surface diffusion coatings could even be applied on-site. One of the drawbacks of thermal spray coatings can be open porosity which deteriorates the protective effect and may lead to undercorrosion, i.e. corrosion along the coating/substrate interface. Most recent development work aims at a nanoparticle-based approach of a sealant to close the porosity of thermal spray coatings and in particular of the HVOF-TiAl coating surface [20]. These SiO2 particles are produced by the sol–gel route where the sol is directly applied to the HVOF coatings. After gelation the coating is heat-treated at 700 °C so that the nanoparticles sinter together to form a continuous layer. As Fig. 10 shows, only optimized sol precursors lead to satisfactory results as significant shrinkage of the sealant can take place during the heat treatment. Although the optimized sealant may still contain a few cracks, Fig. 11, it can significantly help to maintain the protective effect of the HVOF coating under conditions of waste incineration, Fig. 12. Finally it should be mentioned that high quality HVOF composite coatings consisting of a ductile and a brittle phase have the potential for massive improvement of corrosion resistance even in the almost “hopeless” case of high temperature vanadate corrosion [20]. 4. Summary
Fig. 11. Optimized SiO2-based sealant on HVOF coating/13Cr steel substrate manufactured through the sol–gel route [20], cross-section.
The present paper has undertaken the attempt to outline some of the more recent developments in coating research for application in very aggressive high temperature environments. It has transpired that there are quite a few promising results which address the specific situations in the petrochemical and chemical process industries, energy conversion of fossil fuels, refinery residues and waste, and of combustion of heavy oil in ship diesel engines. A valuable tool in developing such coatings is a predevelopmental theoretical design of the coatings and the coating procedure in particular for diffusion coatings. The latter offer some significant advantages over the overlay coatings, in
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particular when the synergistic effects of advanced multi-element co-diffusion coatings are taken into account. However, even the overlay coatings also have potential under the conditions mentioned here. This potential is due to more recent developments in nanoparticle-based sealants or to composite coatings of phases with different mechanical and chemical properties. At the present stage none of these new advanced coatings have been applied yet on a large industrial scale. However, industry should no longer hesitate to make use of their potential. References [1] M.G. Hocking, V. Vasantasree, P.S. Sidky, Metallic and Ceramic CoatingsProduction, High Temperature Properties and Applications, Longman, London, 1989. [2] T. Weber, C. Rosado, M. Schütze, NACE CORROSION/2002, Paper No. 02376, NACE International, Houston, 2002. [3] M. Schütze, Corrosion Resistance at Elevated Temperatures in Highly Aggressive Environments, in: D. Shoesmith, G. Cragnolino (Eds.), Proc. Res. Topical Symp. 2005 “Corrosion Resistant Materials for Extreme Conditions”, NACE Int., Houston, 2005, p. 1. [4] C. Rosado, M. Schütze, Mater. Corros. 54 (2003) 831. [5] W. Schendler, in: D.B. Meadowcroft, M.I. Manning (Eds.), Corrosion Resistant Materials for Coal Conversion Systems, Appl. Sci., London, 1983, p. 201.
[6] [7] [8] [9] [10] [11] [12]
[13] [14] [15] [16] [17] [18] [19] [20]
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R. Bender, M. Schütze, Mater. Corros. 54 (2003) 567. S. Doublet, M. Schütze, in preparation. Factsage Software, GTT, Herzogenrath (Germany). M. Schütze, M. Spiegel, Proc. CD-ROM GfKORR Annual Meeting 2005, Frankfurt/M. B. Aumüller, T. Weber, M. Schütze, Proc. Int. Thermal Spray Conf., 2002, p. 23. S. Doublet, Doctoral Dissertation, RWTH Aachen (Germany), 2006. T. Weber, M. Schütze, Tagungsband 6. Industriefachtagung “Oberflächen– und Wärmebehandlungstechnik” und zum 8. Werkstofftechnischen Kolloquium, Chemnitz 2005. M. Schütze, High Temperature Corrosion of Advanced Materials and Protective Coatings, North-Holland Publ., Amsterdam, 1992, p. 32. T. Weber, M. Schütze, EUROCORR 2001, Proc. CD-ROM, AIM, Milano 2001. V. Rohr, A. Donchev, M. Schütze, A. Milewska, F.J. Perez, Corros. Eng. Sci. Technol. 40 (2005) 226. V. Rohr, M. Schütze, E. Fortuna, D.N. Tsipas, A. Milewska, F.J. Perez, Mater. Corros. 56 (2005) 874. J. Kalivodova, D. Baxter, M. Schütze, V. Rohr, Mater. Corros. 56 (2005) 882. E. Huttunen-Saarivirta, S. Kalidakis, F.H. Stott, V. Rohr, M. Schütze, Mater. Corros. 56 (2005) 897. L. Nieto Hierro, V. Rohr, P.J. Ennis, M. Schütze, W.J. Quadakkers, Mater. Corros. 56 (2005) 890. M. Malessa, T. Weber, M. Schütze, unpublished results.
Development of coatings for protection in specific high temperature environments M. Schütze ⁎, M. Malessa, V. Rohr, T. Weber Karl-Winnacker-Institut der DECHEMA e.V., Theodor-Heuss-Allee 25, 60486 Frankfurt am Main, Germany Available online 15 September 2006
Abstract Metallic and intermetallic coatings are widely used in jet engines and land-based gas turbines for oxidation and corrosion protection in the hotter parts of the engines. However there is a significant number of industrial processes where the use of protective coatings at high temperatures could contribute to a significant extension of life-time or an increase in operation temperature and thus efficiency. Examples of such industries are incineration and gasification of waste, biomass and coal, chemical process industries and petrochemical plants where highly aggressive environments are encountered containing species of e.g. carbon, chlorine, sulphur, vanadium or alcalines. Since most of these process environments contain only very low oxygen partial pressures or exhibit high concentrations of extremely aggressive compounds, the conventional, uncoated materials come to their limits. In recent years in laboratory work a number of new types of coatings have been developed for high-temperature applications which include diffusion coatings, overlay coatings and nanotechnological approaches for sealing porosity. In the paper the background of this development and the thermodynamic fundamentals are discussed together with some more recent solutions based on synergistic effects of multi-element coatings. Some results of performance tests of these coatings in sulfidizing, carburizing, chloridizing and vanadate environments will be presented. At the end conclusions can be drawn on the suitability of the different types of coatings for their specific applications. © 2006 Elsevier B.V. All rights reserved. Keywords: Coating design; Multi-element co-diffusion coating; Nano sealant
1. Introduction Metallic and intermetallic coatings have been used for several decades for protection at high temperatures in jet engines and land-based gas turbines with great success. The main purpose of these coatings is to keep oxidation rates in combustion environments on a level which allows reliable operation of the engines for several years without any significant damage to the components. Today's concepts even foresee reconditioning of the surfaces by stripping and renewed application of virgin coatings after certain operation intervals so that long-term resistance of the component to oxidation can be assured by meanwhile established maintenance procedures. Such coatings not only offer protection against oxygen and nitrogen from the intake air environment but also to a certain extent against attack by fuel or air impurities, such as sulfur (SO2 or SO3), chlorine (from maritime environments) or liquid deposits (sulfates). However, these impurities should be present only in low ⁎ Corresponding author. E-mail address: [email protected] (M. Schütze). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.262
amounts as otherwise the coatings would soon come to their limits, and damage cases are occasionally observed due to the action of such impurities. Most of these coatings are based on the formula Me1Me2CrAlX with Me1, Me2 = Ni, Co, Fe and X = Ce, Hf, La, Y, etc. if they are applied as overlay coatings (plasma spray techniques) or on Al and Pt + Al plus substrate alloy if used as diffusion coatings. Recent research has also tried to incorporate reactive elements X into diffusion coatings, however the technical reliability of the manufacturing process has not yet been achieved for such coatings. A common feature of these coatings is that they act as a reservoir phase for the formation of protective and slow growing Al-based oxide scales by reaction with the operation environment, thus providing an environmental barrier against the ingress of more aggressive species down to the metal to be protected and slowing down metal consumption rates by the oxidation process itself. While all these coatings have been developed and used under relatively “mild” environmental conditions mainly with the aim to increase operation temperatures to the mechanical limits of the substrate alloys, more recently a second route of coating deveopment has been taken up again after some initial frustrating
M. Schütze et al. / Surface & Coatings Technology 201 (2006) 3872–3879
experiences in the 80s and 90s of the 20th century. This route aims at protection of conventional commercial materials (steel and nickel-base alloys) for application in highly aggressive high temperature environments in the petrochemical and chemical process industries as well as in energy conversion from fossil fuels and also from household and industrial waste as well as from agricultural residues. Such environments are often characterized by low contents of oxygen (so-called reducing environments which hamper the formation of protective oxide scales) and high concentrations of detrimental species like sulfur, chlorine, carbon or vanadium together with alcalines. Some of these species even dissolve protective oxide scales such as alumina or chromia scales so that up to now the upper limit temperatures for the technical components in the plant are not set by their mechanical strength potential but by the metal wastage rates due to high temperature corrosion. In some cases this situation can be very unsatisfactory as, for example, in petrochemical industries where sulfidation may limit operation temperatures to about 400 … 500 °C depending on the materials used and for a similar temperature range in large ship diesel engines due to vanadium pentoxide attack. There may be expensive high alloy materials which could be capable of increasing these limits, however the component size and complexity as well as the rather limited ratio of improved corrosion resistance versus increased costs in most cases exclude solutions based on bulk materials. Under such conditions more and more solutions are becoming attractive which are based on low-cost substrate materials (steels which have sufficient high temperature strength at the temperatures envisaged) and tailormade protective coatings which were developed for use in specific highly aggressive high temperature environments. The present paper aims at a review of the present development stage of such coatings and will elucidate some of the aspects which have to be taken into account when developing and using these coatings for specific applications. In the present paper and historically the term “diffusion coatings” is used for Pack-CVD coatings. There are, however, alternative ways to introduce the
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metallic reservoir element(s) into the metal subsurface zone, which are described in detail e.g. in Ref. [1]. 2. Theoretical considerations in coating design There exist four major criteria for the development of an optimized coating: 1) The coating should form thermodynamically stable protective phases on its surface by the reaction with the process environment. The “classical” protective phases will be Al2O3, Cr2O3 and SiO2 as well as some of the spinels. 2) These protective phases should be slow growing in order to keep coating reservoir depletion rates by the surface reaction on a low level. Again the above-mentioned phases play a key role in high temperature corrosion protection. 3) Interdiffusion between coating and substrate should occur as slowly as possible which suggests the introduction of an interdiffusion barrier or the use of a substrate into which diffusion of the coating species occurs only at a very low rate. 4) The values of the coefficients of thermal expansion of coating, protective surface phase (scale) and substrate should lie as close to each other as possible so that cooling and reheating stresses are minimized in the system during temperature changes or by down times. Generally the system should be strain-tolerant which at least in the case of the coatings could be achieved by the use of ductile alloy phases. The latter is, however, not easy to achieve as almost all reservoir phases in coatings are brittle intermetallics with ductile to brittle transition temperatures clearly above 500 °C. In the following some of these criteria will be detailed further, starting off with considerations on thermodynamic stability of the protective phases formed on top of the coating. These considerations will focus on environments containing S, C or Cl which seem to be the critical elements most often encountered in high temperature plant conditions which can however be present
Fig. 1. Stability diagram for oxidizing–sulfidizing environments at 700 °C [2]. The quadrangle depicts a gasification atmosphere in petrochemistry.
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as different compounds. The aggressivity of such compounds has to be characterized by the respective activities of these elements present in the process environments, i.e. in the compounds of these environments. The first element to be addressed is sulfur which plays a significant role in corrosion in many high temperature processes such as those in petrochemistry (e.g. H2S corrosion or corrosion in the gasification of refinery residues), in fossil fuel and waste incineration or gasification, and in the combustion of heavy oil in ship diesel engines. The oxygen and sulfur partial pressures (activities) pO2 and pS2 of some of these environments formed the basis for thermodynamic considerations based on stability diagrams of the type shown in Fig. 1 [2]. The latter depicts conditions where pS2 and pO2 lie in a range where protective Al2O3, Cr2O3 and SiO2, can be formed and where the non-protective sulfides of the same alloying elements must be expected. Such diagrams help in identifying alloy or coating compositions of high corrosion resistance and others that will not survive. A particularly beneficial coating element under such conditions is Al with the largest area where the protective oxide is stable over the sulfide while Fe shows the contrary behavior. Diagrams of the type of Fig. 1 can be found in a number of papers in the literature for different temperatures and further elements. These also reveal that another interesting alloying element is Ti [3] which in particular in combination with Al as the intermetallic TiAl phase shows an interesting protection potential especially under sulfidizing environments, as will be further discussed later in this paper. High carbon/low oxygen environments are another critical type of process conditions as encountered, for example, in coal gasification, steam refining and hydro cracking. The critical conditions can be derived from diagrams as a function of carbon activity ac and oxygen partial pressure pO2 [3,4]. Again thermodynamic stability diagrams based on ac and pO2, can be developed, revealing that in particular Al and Si are the coating elements of choice for protection [5]. Finally Cl-containing environments will be considered. A diagram which summarizes different operation conditions based on the chlorine partial pressure pCl2 together with pO2 can be found in Ref. [3]. The key problem of these environments is that instead of solid corrosion products at high temperatures volatile metal chlorides can be formed leading to extreme corrosion rates due to porous non-protective oxide scales and a spongy metal Table 1 Coating elements for protection in different environments from a thermodynamic point of view
Gas phase corrosion S C Cl high pO2 Cl low pO2 H2O below 600 °C H2O above 600 °C Liquid phase corrosion Sulfates Chlorides Vanadates
Al
Cr
Mo
Ni
Si
Fe
++ ++ ++ −− ++ ++
+ + − −− ++ −
+ − −− + + −−
−− − − + ++ −
++ ++ ++ −− ++ +/−
−− −− − −− − −
(+) ? −−
+ + +
? ? −−
− + (−)
+ ? +
−− −− (−)
Fig. 2. SPS-TiAl coating on Alloy 800 H after heating and cooling [12]; SPS: shrouded plasma spraying.
subsurface zone by selective “leaching” of alloying elements. Since traditional stability diagrams can only be calculated for solid and liquid phases (corrosion products) but not for gaseous phases a new type of diagram, called the “quasi stability diagram” [6], had to be developed. This type of diagram again depicts pCl2/ pO2 ranges where protection can be expected since the vapor pressures of the metal chlorides formed stay below a critical level keeping metal wastage rates by evaporation low and ranges where high corrosion rates may occur. Such diagrams have been calculated for all major alloying elements and different temperatures and have recently been further refined taking the influence of component geometry and gas flow rate into account [7]. From these diagrams it can be concluded that in Cl-containing Table 2 Orientational (“maximum") values of the coefficients of thermal expansion (CTE) for several technically relevant substrates and coating phases based on Ref. [13] with further data CTE α [10− 6 K] SiO2 (amorph) HPSN Si SiC AlN TiN MoSi2 Cr2O3, SiO2 (Cryst) Al2O3, Ti Ti Pt Y2O3 Cr IMI 834, NiAl TiO2 Ti3Al (α2) RuAl Pd Ni3Al (adv.) TiSi2 TiAl Co CoAl FeCrAlY
0.94 3.00 4.60 4.94 6.23 7.76 7.94 8.06 8.40 8.60 8.76 8.88 9.47 9.93 10.53 10.94 11.00 11.58 11.88 12.11 12.93 13.88 14.00 14.50
CoO
14.93
CTE α [10− 6 K] Incoloy 956 738 Alloy S, 713 NiPt39Al 617, C-4 CMSX 6, 100 C-276, 230 C-22 NiCrAlY X 600 x-750 NiO, 556, 718 625 Ni 188 800H NiCo17Cr14AlY Fe3Al FeAl2 FeAl Fe–28Al–2Cr Al Fe–28Al–5Cr– 0.1Zr Fe2Al5
15.00 15.29 15.40 15.60 15.64 15.76 15.88 16.11 16.40 16.46 16.58 16.70 17.05 17.23 17.52 17.76 18.29 18.50 19.50 19.50 21.80 22.50 23.80 24.00 24.20
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Fig. 3. Activities of different metal halide phases as a function of temperature for a given powder pack composition [14].
environments with significant amounts of oxygen Al and Si are the coating elements of choice, while Cr and Mo are rather detrimental. At extremely low pO2, however, Mo and Ni seem to be the alloying elements of choice. A somewhat simplified summary of the outcome of the thermodynamic calculations which can actually be performed quite easily by using the respective existing software [8] is given in Table 1. Nevertheless this table can give tendencies which can be used in the theoretical design of coatings together with the other criteria. For a more detailed assessment in particular with regard to use in the respective process environment the abovementioned computer-based calculations (see also Refs. [6,8]) are recommended. So far, only corrosion by the gas phase has been considered but in some applications, in particular in waste incineration, a very severe type of high temperature corrosion is observed by solid or liquid deposits of high chlorine content (mostly KCl, NaCl, CaCl2, ZnCl2 and PbCl2). Furthermore sulfate deposits can lead to significant high temperature attack and there may even be an interaction between sulfates and chlorides. In the case of solid deposits, if they do not form liquid eutectics with the oxide scales and/or the metal, it is mainly the gas phase compound that determines corrosion and the theoretical considerations performed before can be applied again. Liquid phase corrosion is a different mechanism leading to high corrosion rates due to high transport rates in the corrosive compound so that the formation of liquid phases should generally be avoided (be it as eutectics or be it as a single compound). Again thermodynamic calculations can be used to compute ternary phase diagrams. These calculations indicate that Ni should be superior to Fe as the liquid phase area is much larger in the latter case. Cr is also one of the “beneficial” elements under these conditions [9]. Similar considerations can be
Fig. 4. HVOF-TiAl coating (a) and Al–Ti multi-element co-diffusion coating (b) on Alloy 800 [2].
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performed for sulfates and for vanadates where, however, in the latter case only very limited data exist and some of the conclusions have to be drawn based on the metal vanadate melting points. Again the conclusions have been summarized in Table 1. It should be pointed out that the thermodynamic data do not include the solubilities of the metals or oxides in the liquid phases which in most cases, however are rather high. As a final point on the stability of phases it should be emphasized that, if existing, the theoretical data should be verified by experimentally determined data. For example, for sulfidation and chlorination such data exist [2,3,10,11] which are in quite good agreement with the theoretical considerations. Another extremely important parameter in theoretical coating design is the coefficient of thermal expansion (CTE) which should be deliberately adjusted in order to avoid fracture or spalling of the coating. Fig. 2 shows an example of a coating where the CTE was much lower than that of the austenitic substrate so that during cooling tensile stresses perpendicular and parallel to the coating/ substrate interface were formed, resulting in detachment and fracture of the coating [12]. This situation is confirmed by the (simplified!) summary of the CTEs in Table 2 [13]. As this table reveals, Cr-, NiAl-, Ni3Al-, TiAl- and Ti3Al-based coatings should have CTEs relatively close to ferritic or ferritic–martensitic steels whereas austenitic steels have to be regarded as critical. The behavior of Ni-base alloys is in between the two. Something that should be noted is the high values of Al and of the iron aluminides. Al seems to be less of a problem as it is very
ductile, however the iron aluminides can cause significant problems as a coating phase with respect to mechanical integrity. The last point in this section on theoretical coating design is, in the case of diffusion coatings, the definition of the composition of the powder pack or the reservoir phases in the diffusion process based on thermodynamic calculations. In order to adjust the appropriate metal activities on the metal surface of the component to be coated suitable powder mixtures have to be chosen, in particular if the simultaneous diffusion of several protective metallic elements into the substrate surface is to be performed in a co-diffusion treatment. As will be shown later, such multi-element diffusion coatings can exhibit synergistic positive effects in protection. In this case it is extremely important to design the powder mixture composition in such a way that the desired multielement diffusion coating composition is achieved. The results of the respective thermodynamic calculations are shown for an example of a TiAl diffusion coating in Fig. 3 where the metal halide activities in the powder are depicted as a function of coating temperature. 3. Novel coating manufacturing routes and their advantages For the overlay coating route the “traditional” techniques of atmospheric plasma spraying (APS), shrouded plasma spraying (SPS) and high velocity oxy-fuel spraying (HVOF) have also been investigated for applications of the type addressed by this
Fig. 5. a) Uncoated Alloy 800; b and c) Alloy 800 with Al–Ti co-diffusion coating after 1000 h exposure in low pO2/high pS2/high ac environment at 700 °C [2].
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Fig. 6. Mass change curves for a conventional Al diffusion coating and an Al–Ti co-diffusion coating both on Alloy 800 during exposure in low pO2/high pS2/ high ac environment at 700 °C [14].
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Fig. 8. Low temperature aluminizing coating on P91 [15].
paper. However, it should not be forgotten that overlay coatings are usually not a really “intrinsic” part of the component which means that separation along the coating/substrate interface may occur more easily than for diffusion coatings which are rather a “part of the material”. This is illustrated by Fig. 4 which compares an actually quite dense SPS TiAl coating with an advanced multi-element co-diffusion coating based on Al–Ti [2]. The codiffusion coating shows quite a complex microstructure which, however, avoids any “sharp” phase change from substrate to coating and is free from any physical defects like interlamellar spacings or pores. Such a co-diffusion coating based on Al–Ti is shown in Fig. 5 after exposure in an extremely aggressive environment of refinery residue gasification which is highly sulfidizing and carburizing. The uncoated Alloy 800 already suffers from massive corrosion after 30 h (Fig. 5a) while even after 1000 h the coated surface only shows minor attack (Fig. 5b, c). Furthermore a comparison of the co-diffusion coating with a standard commercial aluminizing coating reveals the obvious synergistic effect of the two-element co-diffusion coating, Fig. 6 [14], since the presence of Ti stimulates the formation of a protective oxide scale at this relatively low temperature of 700 °C where Al alone has difficulties with scale formation. A
further example of the potential of multi-element diffusion coatings is given in Fig. 7 for metal dusting environments where the uncoated material shows a massive metal loss due to metal dusting attack [3,4]. All coatings mentioned in the figure provide a significant improvement. However, with increasing complexity of the coating (i.e. increasing number of different elements) the protective effect increases, which would be even more obvious in this plot if a more appropriate scale had been used for the ordinate. Nevertheless, in recent years there have been efforts to further improve plain aluminide coatings in particular for lowcost applications. As in such cases the microstructure of the steels is thermally less stable new routes for applying diffusion coatings at the relatively low temperature of 650 °C have been developed [15,16]. The results were a very homogeneous Alrich coating on 9–12% Cr high temperature steels, as shown as an example in Fig. 8. These coatings have been tested quite extensively under various conditions (fireside corrosion [17], erosion–corrosion [18], creep [19], waste incineration conditions [17]) where in virtually all cases a significant improvement in corrosion resistance was confirmed without any significant effect on the mechanical properties of the substrate. It was rather that coating life-time was determined by inward diffusion of Al into the substrate so that some thought had to be devoted to the
Fig. 7. Mass change curves without coating and for several diffusion coatings on P91 during exposure in low pO2/high ac environment (metal dusting atmosphere) at 620 °C [3,4]; for details about the coatings see Ref. [4].
Fig. 9. Advanced two-step Cr + Al coating with Cr interdiffusion barrier on P22 [4].
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Fig. 10. Non-optimized SiO2-based “nano”-sealant on HVOF coating/13Cr steel substrate manufactured through the sol–gel route [20], top view.
development of an interdiffusion barrier. The idea of such barriers is not really new but was revived in Ref. [4], leading to the two-step diffusion coating in Fig. 9. The effect of this “chromium barrier”, known as being effective in the case of austenitic substrates, has not yet been finally confirmed for ferritics, but the concept of a slowly diffusing element like Cr as a barrier for the faster diffusing Al would seem sensible. The Cr barrier is applied in a first diffusion step at high temperature (1000–1100 °C) while the Al follows in a second diffusion step at lower temperature (650–750 °C). Both temperature ranges correspond to the conventional heat treatment steps of these steels so that the coating process can be combined with the standard heat treatment procedure. The overlay-type coatings should not be completely forgotten in the development of protective surfaces for components in the industrial areas addressed by this paper although they may have certain disadvantages, as discussed above. Usually diffusion coatings have to be applied off-site, i.e. the components may have to be welded on-site to build up the final structure of the plant. In this case coatings would disturb the welding process so that these parts of the components have to be left unprotected until the welding procedure has taken place. After this the welded areas should be protected by overlay coatings. Moreover in the case of
Fig. 12. HVOF-NiCrSi coating on heat-resistant steel after exposure under simulated waste incineration conditions (N2–10H2O–5O2–0,25HCl in vol.% + deposit 14NaCl–13KCl–27K2SO4–16ZnSO4·7H2O–30Na2SO4 in wt.%) for 288 h with protective SiO2/B2O3 sealant on the right side and discontinuous non-protective sealant on the left side [20].
repair or post-operational surface protection overlay coatings have clear advantages over diffusion coatings although development is going on where by a slurry or an electrolytically applied diffusion reservoir on the component surface diffusion coatings could even be applied on-site. One of the drawbacks of thermal spray coatings can be open porosity which deteriorates the protective effect and may lead to undercorrosion, i.e. corrosion along the coating/substrate interface. Most recent development work aims at a nanoparticle-based approach of a sealant to close the porosity of thermal spray coatings and in particular of the HVOF-TiAl coating surface [20]. These SiO2 particles are produced by the sol–gel route where the sol is directly applied to the HVOF coatings. After gelation the coating is heat-treated at 700 °C so that the nanoparticles sinter together to form a continuous layer. As Fig. 10 shows, only optimized sol precursors lead to satisfactory results as significant shrinkage of the sealant can take place during the heat treatment. Although the optimized sealant may still contain a few cracks, Fig. 11, it can significantly help to maintain the protective effect of the HVOF coating under conditions of waste incineration, Fig. 12. Finally it should be mentioned that high quality HVOF composite coatings consisting of a ductile and a brittle phase have the potential for massive improvement of corrosion resistance even in the almost “hopeless” case of high temperature vanadate corrosion [20]. 4. Summary
Fig. 11. Optimized SiO2-based sealant on HVOF coating/13Cr steel substrate manufactured through the sol–gel route [20], cross-section.
The present paper has undertaken the attempt to outline some of the more recent developments in coating research for application in very aggressive high temperature environments. It has transpired that there are quite a few promising results which address the specific situations in the petrochemical and chemical process industries, energy conversion of fossil fuels, refinery residues and waste, and of combustion of heavy oil in ship diesel engines. A valuable tool in developing such coatings is a predevelopmental theoretical design of the coatings and the coating procedure in particular for diffusion coatings. The latter offer some significant advantages over the overlay coatings, in
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particular when the synergistic effects of advanced multi-element co-diffusion coatings are taken into account. However, even the overlay coatings also have potential under the conditions mentioned here. This potential is due to more recent developments in nanoparticle-based sealants or to composite coatings of phases with different mechanical and chemical properties. At the present stage none of these new advanced coatings have been applied yet on a large industrial scale. However, industry should no longer hesitate to make use of their potential. References [1] M.G. Hocking, V. Vasantasree, P.S. Sidky, Metallic and Ceramic CoatingsProduction, High Temperature Properties and Applications, Longman, London, 1989. [2] T. Weber, C. Rosado, M. Schütze, NACE CORROSION/2002, Paper No. 02376, NACE International, Houston, 2002. [3] M. Schütze, Corrosion Resistance at Elevated Temperatures in Highly Aggressive Environments, in: D. Shoesmith, G. Cragnolino (Eds.), Proc. Res. Topical Symp. 2005 “Corrosion Resistant Materials for Extreme Conditions”, NACE Int., Houston, 2005, p. 1. [4] C. Rosado, M. Schütze, Mater. Corros. 54 (2003) 831. [5] W. Schendler, in: D.B. Meadowcroft, M.I. Manning (Eds.), Corrosion Resistant Materials for Coal Conversion Systems, Appl. Sci., London, 1983, p. 201.
[6] [7] [8] [9] [10] [11] [12]
[13] [14] [15] [16] [17] [18] [19] [20]
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R. Bender, M. Schütze, Mater. Corros. 54 (2003) 567. S. Doublet, M. Schütze, in preparation. Factsage Software, GTT, Herzogenrath (Germany). M. Schütze, M. Spiegel, Proc. CD-ROM GfKORR Annual Meeting 2005, Frankfurt/M. B. Aumüller, T. Weber, M. Schütze, Proc. Int. Thermal Spray Conf., 2002, p. 23. S. Doublet, Doctoral Dissertation, RWTH Aachen (Germany), 2006. T. Weber, M. Schütze, Tagungsband 6. Industriefachtagung “Oberflächen– und Wärmebehandlungstechnik” und zum 8. Werkstofftechnischen Kolloquium, Chemnitz 2005. M. Schütze, High Temperature Corrosion of Advanced Materials and Protective Coatings, North-Holland Publ., Amsterdam, 1992, p. 32. T. Weber, M. Schütze, EUROCORR 2001, Proc. CD-ROM, AIM, Milano 2001. V. Rohr, A. Donchev, M. Schütze, A. Milewska, F.J. Perez, Corros. Eng. Sci. Technol. 40 (2005) 226. V. Rohr, M. Schütze, E. Fortuna, D.N. Tsipas, A. Milewska, F.J. Perez, Mater. Corros. 56 (2005) 874. J. Kalivodova, D. Baxter, M. Schütze, V. Rohr, Mater. Corros. 56 (2005) 882. E. Huttunen-Saarivirta, S. Kalidakis, F.H. Stott, V. Rohr, M. Schütze, Mater. Corros. 56 (2005) 897. L. Nieto Hierro, V. Rohr, P.J. Ennis, M. Schütze, W.J. Quadakkers, Mater. Corros. 56 (2005) 890. M. Malessa, T. Weber, M. Schütze, unpublished results.