Whereas alloy development activities have been very successful in improving ..... Figure 16: Cross section of FeCrAl deposited by HVOF exposed to steam for ...
“COATINGS FOR STEAM POWER PLANTS UNDER ADVANCED
CONDITIONS” Alina Agüero and Raul Muelas Instituto Nacional de Técnica Aeroespacial (INTA) Ctra. Ajalvir Km. 4 28850 Torrejón de Ardoz (Madrid) SPAIN Brendon Scarlin / Reinhard Knoedler ALSTOM Baden, SWITZERLAND / Mannheim, GERMANY Abstract To achieve higher power generation efficiency in steam turbines, operating temperatures are expected to rise from 550ºC to 650ºC. Two approaches are being followed in COST 522 in order to accomplish this goal: 1) the development of new materials and 2) for the first time in Europe, the use of oxidation resistant coatings on currently available materials with higher creep strength but inferior steam oxidation resistance, which is the object of this work. This presentation covers the efforts carried out while exploring the suitability of coatings for steam turbine components. In the initial stages of this work, commercially available materials, known to have good high temperature oxidation resistance, were applied by different deposition techniques. These techniques were chosen on the basis of being potentially appropriate for coating very large components: the application of aluminium slurries, the deposition of Al, Cr, Co, Fe and Ni alloyed materials by thermal spray and the application of electroless nickel. The aluminium slurry coating was deposited by brushing, and a series of postheat treatments were evaluated in order to generate a uniform and stable diffusion coating. Thermal spray coatings were deposited by High Velocity Oxygen Flame (HVOF) spraying and optimisation to minimise porosity and maximise adhesion was carried out. Electroless nickel coatings were applied using commercial procedures. The coatings were characterised by metallography, SEM-EDS and XRD and steam oxidation laboratory testing was carried out at 650º C. The preliminary findings showed that some of the studied coatings are very promising. Results of the initial preliminary testing carried out for 10,000 h. will be presented. Finally, mechanisms of protection as well as degradation for the different coatings will also be discussed. Keywords: Steam power plant, high temperature, efficiency, ferritic steels, oxidation resistance, coatings.
1. Introduction and Background Future coal-fired steam power plant will be required to operate with high efficiency and thereby at high temperatures. As a consequence key high temperature components will require not only high creep strength but also a high resistance to oxidation in a steam environment. In particular, components such as steam pipes as well as turbine rotors, casings and blades must be resistant to the growth and also to the exfoliation of oxides. Whereas alloy development activities have been very successful in improving the creep strength (for example the replacement of X20CrMoV 12 1 by P92 or E911), this has generally been achieved
through lowering the chromium content. The consequence has been a worsening of the resistance to steam oxidation [1]. At 650ºC the oxidation of ferritic Cr-steels in steam is much more pronounced than in air [2, 3] and the critical amount of Cr required to form protective Cr2O3 or (Cr, Fe)2O3 scales increases [4, 5]. It has been suggested that H2O reduces the stability of Cr2O3 or causes it to decompose. Other workers suggest that steam causes the dissolution of H+ in Cr2O3, enhancing its ionic conductivity and resulting in a deterioration of its protective properties [6]. A third group suggest that the formation of volatile species such as CrO2OH or CrO2(OH)2 are responsible for the breakdown of the scale [7- 9]. On ferritic steels with a Cr content lower that 10 w%, very thick oxide scales form consisting of a top layer of Fe2O3 and Fe3O4 and an inner zone mainly (Fe,Cr)3O4 spinels. These scales spall causing metal cross-section loss, component blockage and erosion of components located down-stream and also produce a thermal insulating effect resulting in overheating [10-12]. A possible solution may be provided by enhancing the surface oxidation resistance through the application of coatings. The conditions to be satisfied by such coatings are not only high resistance to oxidation growth and spalling during service, but also: - ease of application also for large or complex components - good adhesion to the substrate and no need for thermal treatments above the normal tempering temperature - resistance to erosion by solid particles - repairability on site - low cost. Earlier work has been performed in this area in an EPRI programme [13] particularly for boiler tubes of 2.25%Cr steels and austenitic stainless steels. The range of coatings and surface treatments investigated included chromising from an aqueous chromate solution, application of a Cr2O3 ceramic coating, and application of a layer of aluminium, electroless nickel or silicon. Oxidation testing was typically carried out for 2500 hours in a closed-loop steam testing rig at 650°C. For coatings applied using these specific techniques and for these specific testing conditions it was concluded that the only satisfactory behaviour was achieved using the chromising technique. However there is increasing resistance to use of chromate coating processes due to the health and environmental hazards involved, so that alternatives are being sought. In the EPRI investigation it was noted that application conditions were not ideal, so that a continuous defect-free layer was not achieved for all coatings. For example, where the electroless nickel coating was defect free, local behaviour was very good. It was pointed out that “some type of nickel coating may be an effective diffusion barrier”. Other work also covered the deposition of Cr by pack cementation [14] but this process is carried out at temperatures too high (> 800ºC) for the ferritic steels currently employed in the manufacture of steam turbine components In a recent initial feasibility study carried out by our group within the framework of the COST 522 action, a number of commercially available coatings were explored for steam oxidation protection [15, 16]. These included materials known to have good oxidation resistance and deposited by techniques that can be employed to coat large steam turbine components either at the plant or at their location of manufacture, and also taking into consideration economical aspects. The promising results motivated further work and field testing of selected coatings. This work includes the overall results of the joint efforts carried out at INTA in Spain and at ALSTOM Power in Germany and Switzerland. It covers the most promising and/or interesting results obtained when commercially available coatings obtained at INTA or from commercial sources were tested both in the laboratory or in a bypass of a steam turbine. The results have provided some information regarding the mechanism of protection and/or failure of the studied systems.
2. Experimental 2.1 Materials Specimens of P92 (C: 0.1, Mn: 0.5, Si: 0.03, Cr: 8.8, Ni: 0.06, Mo: 0.4, W: 1.8, V: 0.20 w%) were obtained from Nippon Steel Corporation (15x15x1 mm). Specimens of E911 (Cr: 9, Mn: 0.5, Ni: 0.2, W: 1, Mo:1, V: 0.2, Nb: 0.08, N: 0.05, C: 0.1, Si: 0.2 w%) were obtained from the COST 501 programme. Both were sand blasted, ground (Struers 1200) and vapour degreased prior to coating. The Al slurry and the powders for thermal spraying were obtained from commercial sources.
2.2 Slurry Coatings At INTA Process Technology Laboratories, the Al slurry was applied by brush and the coated samples were subjected to a “curing heat treatment” at 350ºC for 30 minutes under air. Diffusion heat treatment was performed under argon flow at 700ºC for 10 h. The Si aluminide coatings were prepared in the same manner by adding Si powder to the commercial slurry before application and ensuring thorough homogenisation of the modified slurry. The commercially applied Al slurries (Sermetel) were obtained from Sermatech, USA.
2.3 Thermal Spray Coatings At INTA Process Technology Laboratories, coatings were deposited by a Sulzer Metco Diamond Jet Hybrid HVOF unit (A-3120) mounted on a 6 axes robot (ABB) and fed by a twin rotation powder feeder. Special (proprietary) stellite and carbide coatings for field tests were applied by commercial HVOF sprayers.
2.4 Electroless Coatings Electroless nickel with a P-content of 12% were applied to different base materials with a thickness of about 50 µm by Degussa, Schwäbisch Gmünd, Germany.
2.5 Characterisation Sample characterisation was carried out by optical and electron microscopy (JEOL JSM-840 equipped with a KEVEX EDS microanalyser) of metallographically polished cross sections before and after exposure and by XRD (PHILLIPS PW 3710).
2.6 Steam Oxidation Laboratory Testing Two similar steam oxidation rigs were employed both at INTA and at ALSTOM Power. The design of the rig is shown in figure 1. The oxygen content in the water must be kept to values below the 10 ppb level as the presence of oxygen favours the growth of hematite instead of magnetite. This is achieved by permanently bubbling N2 into de-ionized, distilled water. The water is re-circulated by means of a condenser connected to the reservoir. Prior to testing, air is displaced from the testing chamber by means of N2 which is kept flowing while heating up to the test temperature takes place (approximately 600ºC/h). Once the test temperature is achieved, the N2 flow is cut and the water recirculation pump is turned on. To carry out weight measurements or to remove samples, the samples are cooled to about 300ºC under N2 and removed from the furnace. The reheat cycle (from 300 to the test temperature) is also carried out under N2.
Figure 1: Design of the laboratory set-up of the pure steam oxidation rig.
2.7 Field Testing Field tests were carried out in a steam bypass in the Westfalen power plant within the Komet 650 project [17]. Several alloys as well as coatings are currently under test at nominally 620 and 650 °C. Selected results after 6500 h will be presented.
3. Results Several coatings applied on P92 specimens by three different techniques have been studied and were laboratory tested at 650ºC under pure flowing steam. Commercially available materials containing protective oxide formers (Al, Cr, and Si) were selected also on the basis of good high temperature oxidation resistance (in air). Three families of coatings were studied: slurry aluminides, thermal spray coatings and electroless Ni coatings. Moreover, in the Komet 650 programme, which is coordinated by the RWE Power AG and VGB PowerTech, investigations are focussed on steam oxidation testing in the bypass of the Westfalen B power plant. Samples of 9 to 12%Cr steels (such as E911, P91, P92 and P122), austenitic steels and nickel-based alloys are being exposed for times ultimately up to 24,000 hours nominally at both 620 and 650°C. At intermediate times samples are removed for metallographic examination to determine the rate of oxidation and the tendency to oxide spalling. The effectiveness of some coatings as protection against steam oxidation is also under investigation. Currently testing times have exceeded 12,000 hours.
3.1 Slurry aluminides As deposited coatings characterisation A commercially available aluminium slurry applied on P92 following the manufacturer's recommendations was heat treated at 700ºC under flowing Ar. The resulting structure is shown in figure 2. Electron diffraction carried out in a Transmission Electron Microscope [18] confirmed the presence of an outer Fe2Al5 matrix and inner and thinner FeAl layer as illustrated in the figure. 3.1.1
Samples were also obtained directly coated from a coating vendor using also a slurry aluminium. Their structure was similar to that obtained in the laboratory.
Fe2Al5
FeAl
Figure 2: Slurry aluminide coating on P92 heat-treated at 700ºC
The same Al slurry was modified by adding Si powder in different weight percentages. Si addition may stabilise the coating from inward diffusion, one of the main causes of aluminide coatings failure due to the loss of Al near the surface. Small amounts of Si on steel are also known to reduce the steam oxidation rate. The “as coated samples” were also heat treated at 700ºC under flowing Ar. The microstructure of the 20 w% Si aluminide coating is shown in figure 3 where at least 5 phases can be observed. On the basis of EDS semiquantitative analysis the following phases are proposed: phase I near the surface could be Fe2(Al, Si)5 with some Cr in solution (36 w%Al, 3 w% Cr, 20 w%Si, 40 w%Fe), next to it, in the inwards direction a lighter coloured phase (II) can be attributed to Fe(Al, Si) also with some dissolved Cr (20 w%Al, 3 w% Cr, 22 w%Si, 55 w%Fe) while the next one of darker colour could be Fe2Al5 (46 w%Al, 2 w% Cr, 1 w%Si, 46 w%Fe) since it is poorer in Si. In phase III some precipitates (IV) of Cr(SiAl)2 (12 w%Al, 48 w% Cr, 16 w%Si, 15 w%Fe) can be observed and finally a last lighter colour phase can be attributed to FeAl (20 w%Al, 4 w% Cr, 5 w%Si, 71 w%Fe).
I II IV
III V
Figure 3: Slurry 20 w% Si aluminide coating on P92 heat treated at 700ºC. I: Fe2(Al, Si)5; II: Fe(Al, Si); III: Fe2Al5; IV: Cr(Si, Al)2 and V: FeAl
Steam Oxidation Laboratory Testing and characterisation of exposed samples Figure 4 shows the mass variation as a function of time during steam oxidation testing. P92, has also been included for comparison purposes. Some of the coated aluminide samples experience an initial weight loss due to losses of undiffused slurry. Thoroughly ground samples did not lose weight. All of the tested coatings showed weight gains considerably lower than that of P92, the base material.
3.1.2
55 50 45 40 P-92
35
Pure Aluminide
30
Mass Variation (mg/cm2)
25
20% Silice-Aluminide
20
throughly Ground Aluminide
15 10 5 0 -5
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
-10 -15
TIME (h)
Figure 4: Mass variation of slurry coatings exposed to steam at 650ºC
P92 experienced a large oxide scale growth as shown in figure 5, with an outer growing layer of Fe2O3-Fe3O4 and inner growing Fe, Cr spinels. There was little evidence of scale spallation up to 10000 h of exposure.
The cross-section of the pure aluminide coating after 10000 h of steam exposure is shown in figure 6. The coating microstructure has evolved to pure FeAl, probably by inward diffusion of Al with a surface content of Al 15 w%. Examination of intermediate time samples showed that the higher aluminide phase disappeared only after 2000-2500 hours. A zone with dark
Figure 5: Cross section of a P92 specimen exposed to 5000 h of steam oxidation at 650ºC
“needle like” precipitates of an unidentified phase rich in Al are also present beneath the coating and within the substrate. This zone was present from the beginning in the unexposed heat treated (for 10 hours) coating and grew in thickness from about 4 µm to 65 µm after 10000h of exposure.
50 µm Figure 6: Cross section of a slurry aluminide coating on P92, exposed to 10000 h of steam oxidation at 650ºC
Cracks perpendicular to the coating surface were present but never reached the substrate up to the tested period. Thermal cycling associated with the quite brittle Fe aluminides phases is probably causing these cracks. XRD of these coatings confirmed the presence of αAl2O3 scale, the most protective of its phases, as shown in figure 7. The steam oxidation behaviour of the samples containing less than 5 % Si was not too different from the pure aluminide coatings, whereas for contents of 10 w% or higher the diffusion rate was modified significantly. For instance, the microstructure of the 20 w% Si aluminide coating tested for 6500 h also showed signs of diffusion but to a much lesser extent (figure 8). Phase I, tentatively identified as Fe2(Al, Si)5 and the closest to the surface has disappeared while II, the next one, Fe(Al, Si) has grown in thickness. Moreover, the thickness of phase III (Fe2Al5) has decreased considerably, while phase V (FeAl) has grown in thickness. The coating is much richer in Al than a pure aluminide exposed to steam for the same period of time and the Al rich precipitate zone only reached 10 µm (at the same exposure, the precipitation zone of the pure aluminide coating was already 60 µm). As with the pure aluminide coating, perpendicular
αAl2O3
Figure 7: XRD of the surface of a pure aluminide coating Exposed to pure steam at 650ºC
cracks develop, but again never reaching the substrate. However, a very thin unidentified phase surprisingly rich in Mn (10-20 w%) and W (6-8 w%) appeared at the coating / substrate interface. Although P92 has 0.5 w% Mn, such as high concentration is difficult to explain. A similar observation of a high Mn concentration in an oxide layer has been recently reported [19]. This layer seems to behave as a diffusion barrier. No signs of coating spallation could be observed for the tested period.
II V
III
Figure 8: Cross section of a 20 w% Si slurry aluminide coating on P92, exposed to 6400 h of steam oxidation testing at 650ºC 3.1.3
Diffusion studies
In order to determine if the changes in the pure aluminide coatings are due to spallation, interdiffusion with the substrate or both, aluminide coated P92 specimens were held at 650ºC under flowing N2. The samples showed similar behaviour to those exposed to steam, developing cracks and diffusing inwards. After 5500 hours, both N2 and steam exposed samples had a remaining coating thickness of 40 µm and an interdiffusion zone of 55 µm. These results clearly indicate that the coating degradation takes places due to interdiffusion with the substrate and not to spallation. Employing diffusion barriers or adding elements (such as Si) that reduce interdiffusion should therefore ensure longer lives for this type of aluminide coating operating under steam at 650ºC. 3.1.4
Field Tests
In the field test carried out at Westfalen, spallation was often observed in the uncoated samples as shown in figure 9 for P92.
Figure 9: Cross sections of P92 steel exposed at 642°C for 6500 h in a steam bypass in the Westfalen power plant (about 280 µm scale)
This difference from the spallation behaviour observed in the laboratory tests might be attributed to different operating regimes (thermal cycles), to the higher pressure or due to the higher steam velocity in the field. In addition there are some indications, that a higher oxygen concentration is present in the field, which can lead to formation or hematite instead of magnetite. However, these observations must be confirmed by further testing. Al slurry coatings showed only a very thin scale of aluminium oxide on top of the coating after 6500 h at 642 °C in the steam bypass (figure 10). Below the coating, a diffusion zone of about 60 µm forms, where Al has diffused into the base alloy as with the samples tested in the laboratory. The diffusion zone has not increased significantly with respect to the observations after 3300 h. Also the oxidised cracks did not increase neither in number nor in depth. Unprotected E911 develops an oxide layer of about 200 µm after this time.
Figure 10: Al slurry coatings on E911 after 6500 h at 642 °C in a steam bypass in the power plant at Westfalen. The aluminium oxide scale is < 10 µm thick.
3.2 Thermal Spray Coatings Although thermal spray coatings are rather expensive as compared to slurry aluminides, their life may be longer and therefore, their use justified in some specific components. Several commercial powders containing Co, Ni, Fe, Al and Cr were chosen and were deposited by HVOF. 3.2.1
As deposited HVOF coatings characterisation
NiCr (Sulzer-Metco), FeCrAl (TAFA) and AlCoFeCr (SNMI), were deposited on P92 by HVOF. NiCr, was chosen for its high content of Cr (20 w%) and the microstructure of the as-deposited coating is illustrated in figure 11. SEM-EDS analysis showed a 19.8 w% Cr content.
Figure 11: Cross section of as coated NiCr deposited by HVOF on P92
FeCrAl with an even higher Cr content (30 w%) and 5 w% Al showed less initial porosity as shown in figure 12. SEM-EDS analysis indicated 27.5 w% Cr and 5.9 % Al .
Figure 12: Cross section of as coated FeCrAl deposited by HVOF on P92
AlCoFeCr is a brittle quasicrystalline alloy with a very low thermal conductivity (2.3 W/mºK), close to that of traditional thermal barrier coatings based on yttria stabilised zirconia. Tests carried out at INTA have previously shown high resistance to oxidation (air) at 1000ºC and to hot corrosion
Figure 13: Cross section of as coated AlCoFeCr deposited by HVOF on P92
at 900ºC [20]. However, at these temperatures, coating-substrate interdiffusion is important resulting in the transformation of the quasicrystalline phases and therefore forming materials with thermal conductivities typical of metallic alloys. AlCoFeCr deposited by HVOF resulted in a 75 µm layer showing some micro-cracks and porosity as can be seen in figure 13. The SEM-EDS analysis indicated a composition of 58.4 w% Al, 20.13 w% Co, 11.1 w% Fe and 10.4 w% Cr. 3.2.2
Steam Oxidation Laboratory Testing and characterisation of exposed samples
Figure 14 shows the mass variation of the thermal spray coatings as a function of time during steam oxidation testing. The three coatings experienced very little weight gain during 10,000 h of exposure.
60 55 50
Weight Variation (mg/cm2)
45 40 35
AlF eC o C r F eC rAl
30
NiC r 25
P -92
20 15 10 5 0 -5
0
2000
4000
6000
8000
10000
TIM E (h)
Figure 14: Mass variation of HVOF coatings exposed to steam at 650ºC
The cross section micrograph of NiCr exposed to 10000 h is shown in figure 15 along with the EDS mapping of the principal elements. A very thin oxide scale has developed on its surface.
Figure 15: Cross section of NiCr deposited by HVOF exposed to steam for 10,000 h at 650ºC
EDS analysis shows this layer is probably a chromium oxide. As can be seen, there is no significant thickness loss and the Cr content remains unchanged except in the zone near the substrate interface where some coating-substrate interdiffusion has taken place. The FeCrAl coating remained totally unaffected as shown in figure 16. No changes in the coating composition or evidence of coating-substrate interaction could be observed. An even thinner oxide layer rich in Al could be seen at the coating surface, but in this case the EDS point analysis does not allow differentiation between Al oxide or perhaps an Al, Cr spinel.
Cr
Al
10 µm
O
Figure 16: Cross section of FeCrAl deposited by HVOF exposed to steam for 10,000h at 650ºC
AlCoFeCr, the thermal barrier candidate, did show evidence of coating-substrate interdiffusion as seen in figure 17. No initial quasicrystalline coating is left. Instead, an outer layer with a much lower Al concentration (30 w%) and Kirkendall porosity is present, on top of a second layer, richer in Fe. These two layers are probably the result of Al inward diffusion, leaving a depleted layer on top and forming an Fe aluminide at the coating-substrate interface. Moreover, Al rich needle-like precipitates, identical to those observed in the slurry aluminide coatings are present in the substrate immediately adjacent to the coating. However, no signs of substrate oxidation could be observed.
Figure 17: Cross section of AlCoFeCr deposited by HVOF exposed to steam for 10,000h at 650ºC
Field Tests A few proprietary coatings were applied by HVOF spraying onto E911 and exposed to steam in the bypass at the power plant in Westfalen. These coatings proved to be very oxidation resistant similar to the results obtained in the laboratory for the HVOF coatings, and only very thin oxide scales could be observed (figures 18a and b).
3.2.3
Figure 18a: Chromium carbide containing nickel coating on E911 after 6500 h at 642°C at the Westfalen power plant.
3.3
Figure18b: Stellite coating on E911 after 6500 h at 642 °C at the Westfalen power plant.
Electroless nickel coatings
Electroless nickel with a P-content of 12% was applied onto E911 with a thickness of 40 – 60 µm. After exposure times of up to 5000 h in the laboratory at 600 and 650 °C the oxidation stability of these coatings is seen to be very high although the presence of cracks that reached the substrate was observed. Figure19 shows that only thin oxide layers (predominantly Fe-oxides) grow on top of the coating.
Figure 19: Electroless nickel on E911 after 4350 h at 600 °C exposed to steam. Near the crack thin oxide scales can be seen.
This is most pronounced in the vicinity of cracks in the nickel coating, as the mapping shows in detail in figure 20. It is therefore important for future work to make efforts to avoid such cracks. At the interface between nickel and base alloy also thin oxides has been formed around the crack. These inner oxides are formed by Fe-, Ni- and Cr –oxides
Figure 20: EDS mapping of an electroless Ni coating on E911 exposed to steam for 4350 h at 600ºC
4. Discussion The principal coating degradation mechanism observed for the alumina-former coatings, both slurry aluminide or HVOF deposited AlCoFeCr coatings is the loss of Al content due to coating-substrate interdiffusion since there is no evidence of scale spalling or degradation. αAl2O3 seems therefore to form despite the relatively low temperature and to be inert to steam under the testing conditions. Longer exposure will reveal if the pure aluminide slurry coatings will be protective during the expected 100,000 h of service or if more stable coatings such as the Si aluminides will be required. In these Si aluminide coatings, the presence of Si has caused the formation of a diffusion barrier layer at the coating substrate interface. This layer is rich in Mn and its phase composition will be determined by means of electron diffraction and TEM microanalysis. Moreover longer term effect of cracks needs to be determined. As mentioned, the HVOF deposited AlCoFeCr coating intended to be used as a thermal barrier coating, was not stable at 650ºC since Al inward interdiffusion took place. Thus, the initial coating gains thermal conductivity as it loses its quasicrystalline phases. Regarding the chromia formers, some interesting results have been observed as chromia formed on NiCr (20 w%Cr) seems to be very stable to steam oxidation. No evidence of scale degradation nor Cr depletion near the scale could be observed and the coating did not seem to experience changes during the 10,000 h steam exposure period, other than a very limited degree of coating-substrate interdiffusion (and at a zone remote from the surface). A possible explanation for this observation may be in the role of Fe in the degradation mechanism of chromia. Its absence in NiCr may prevent the formation of less stable (Fe, Cr) spinels. This hypothesis needs to be explored in more detail. Results seem to indicate that the FeCrAl coating formed alumina instead of chromia despite the relatively low temperature, as well as the low Al content (5 w% as opposed to 30 w% of Cr). There is however some precedent in this selective Al oxidation for Fe-13Cr-4Al alloys as I. Kvernes and collaborators observed the formation of Al2O3 on this alloy at 940ºC when exposed to air with up to a 2 v% in moisture. At lower temperature, the oxidation resistance of this alloy was also excellent but only Cr2O3, Fe2O3 and (Fe, Cr)2O3 were observed [21]. Further analysis will be carried out in order to determine the composition of the oxide formed under the present testing conditions, that is if it is pure Al- oxide or an Al, Cr spinel.
Electroless nickel coatings suffer from iron diffusion through the nickel layer. As a consequence, iron oxides are formed at the nickel/steam interface. This is most pronounced in the vicinity of cracks in the nickel coating. However, since the iron oxide scale is very thin, even after longer times, electroless nickel could serve as protective coating, especially if the coating can be applied almost crack –free. The field test results so far obtained are in agreement with the laboratory tests regarding the bahaviour of the slurry aluminide coatings. However, uncoated materials spalled in the field testing whereas under laboratory conditions spalling was seldom observed for samples of equal exposure time even though the oxide scale thickness was similar in both samples. Higher flow rates as well as high pressure may be the cause of this difference. Differences in steam chemistry may also play a role (oxygen content of feedwater).
5. Conclusions and Outlook The use of coatings on steam turbine components can greatly reduce steam oxidation at 600-650ºC for durations in excess of 10,000 h. Longer exposure testing is ongoing, both in the laboratory and under operating conditions. The principal mechanism of the slurry aluminide coating degradation is coating-substrate interdiffusion and efforts have already begun in order to prevent or retard it. Thermal spray coatings are more stable than slurry aluminides. Their higher cost may be justified by longer lifetime for certain components. Chromia formed in alloys not containing Fe is very stable and formation of volatile chromium oxide or hydroxide containing species does not seem to occur in these alloys. Evaporation of chromium volatile species in Cr-containing steels under steam may be rather due to Cr, Fe spinels instead of chromium oxide. Further studies will be carried out in order to validate this hypothesis. Nickel coatings can also be a viable solution. The oxidation rate is greatly diminished by nickel coatings. However, it is not clear whether the necessary freedom from cracking can be achieved. It must also be tested in longterm experiments, whether the thin iron oxide layers, which form at the interface nickel/steam tend to spall off. Quasicrystalline AlCoFeCr can not be employed as a thermal barrier coating at 650ºC since it degrades due to coating substrate interdiffusion. Traditional thermal barrier coatings will be deposited on P92 and tested under steam at 650ºC There are few results with Al-coatings available from field tests. These tests show a good correlation with the laboratory tests, although spallation in field tests is more pronounced than in the laboratory. Thus, coating can avoid both excessive oxide growth and spallation. This helps to achieve a higher power plant lifetime and efficiency by avoiding damage of turbine components by spalled particles and by providing better heat transfer conditions (thinner scales).
Acknowledgements AA and RM (INTA) wish to acknowledge A. Del Olmo and T. Atance for their invaluable technical assistance, M.C. García for her contribution to the XRD work and A. Sánchez for supporting this project. RK (ALSTOM) wishes to thank G. Scheffknecht, G. Stamatelopoulos and Q. Chen for their support of the project and for valuable discussions. The authors would like to thank the Management Committee of the COST 522 action and the national funding bodies for financial support.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
R. Knödler, P.J. Ennis, BALTICA V Conference, Porvoo, June 2001, Proceedings p. 355 J.P.T. Vossen, P. Gawenda, K. Rahts, M. Röhrig, M. Schorr, M. Schütze, Materials at High Temperatures 14 ( 1997) 3. K. Zabelt, Technische Überwachung 39 (1998) 11C. I. Kvernes, M. Oliviera and P. Kofstad, Corros. Sci., 17 (1977) 2371 S. Jianian, Z. Longjiang and L. Tiefan, Oxid. Met., 48 (1997) 347 T. Norby, J. Phys. 3 (1993) 99 H. Asteman, J.-E. Svensson, M. Norelli, L.-G. Johansson, Oxidation of Metals 54 (2000), 11 H.C. Graham and H.H. Davies, J. Am Ceram. Soc., 54 (1971) 89 C. A. Stearns, F.J. Kohl and G.C: Fryburg, J. Electrochem. Soc., 121 (1974) 945. C. Berger C., R.B. Scarlin, K.H. Mayer., D.V. Thornton and S.M. Beech in Materials for Advanced Power Engineering, D. Coutsouradis et al Eds., (Kluwer Academic Publ., Dordrecht, Boston, New York, 1994) Part I, pp 47 W.J. Quaddakers, P.J. Ennis, Materials for Advanced Power Engineering 1998, Part I, Liege, Belgium, J. Lecomte-Beckers, F. Schubert and P.J. Ennis, Eds., (Forschungszentrum Jülich Gmbh, 1998), pp.123 Y. Fufuda, K. Tamura and T. Sato, Materials for Advanced Power Engineering 1998, Part I, Liege, Belgium, J. Lecomte-Beckers, F. Schubert and P.J. Ennis, Eds., (Forschungszentrum Jülich Gmbh, 1998), pp.461 EPRI Report CS-1811, Project 644-1, Corrosion problems in coal-fired boiler superheater tubes: Steam-side oxidation and exfoliation, April 1981 J. Stringer , Surface and Coating Technology, 108-109 (1998) 1 A. Agüero, F.J. García de Blas, R. Muelas, A. Sanchez y S. Tsipas, Materials Science Forum, 369-372 (2001) 939-94 A. Agüero R. Muelas and A. Román., Surface Modification Technologies XIV, T. S. Sudarshan and M. Jeandin Eds., (ASM Inernational, Materials Park, Ohio, 2001), pp. 92 Köster, P. Moser, H. Bergmann, J. Jacobs, VGB Power Tech 9 (2001),64 P. Vallés, INTA, Madrid, 2001, Unpublished Results R.J. Ehlers, W.J. Quadakkers, Oxidation of ferritic 9-12%Cr steels in steam-containing atmospheres at 550 to 650°C, Jülich Report 3883, ISSN 0944-2952, 2001 (in German) Sánchez A., Garcia de Blas F.J., Algaba J.M., Alvarez J, Garcia Poggio M.C., Agüero A., Proceedings of Materials Research Society 1998 Fall Meeting, J.M. Dubois, P.A. Thiel, A.P. Tsai and K. Urban Eds., Boston, Massachussets, November 1998, pp 447-458. I.Kvernes, M.Oliveira and P.Kofstad, Corrosion Science, 17 (1977) 237
