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Corrosion and failure mechanisms of coatings for gas turbine applications
Thin Solid Fihns, 54 (1978) 359-367 (~ Elsevier Sequoia S.A., Lausanne
Printed in the Netherlands
359
CORROSION AND FAILURE MECHANISMS OF COATINGS FOR GAS TURBINE APPLICATIONS* K. SCHNEIDER, R. BAUER AND H. W. GRUNLING Brown, Boveri & Cie, Mannheim (F.R.G.) (Received April 4, 1978: accepted June 14, 1978)
For stationary gas turbines, coating of the blade material is today a standard procedure to prolong lifetime. It is possible by this method to divide the requirements between the base material (mostly nickel- or cobalt-based alloys, which give mechanical strength) and the coating (which provides resistance to high temperature corrosion and oxidation). However, the separate properties of the coating and base material are not the only advantages. The behavior of the composite may be completely different, especially under actual service conditions where the turbine blade undergoes complex mechanical and thermal stresses simultaneously with the corrosion attack. We show how three coatings (two of chromium and of silicon applied by pack cementation and one of nickel-chromium-silicon applied by plasma spraying) behave under service conditions in industrial gas turbines. It is found that the nickel--chromium-silicon coating is the best coating of these three when it is applied to a material such as Inconel 738 LC with moderately high temperature corrosion resistance. The in-service results and comparison with laboratory tests (corrosion as well as ductility) show that the reason for the excellent behavior of the nickelchromium-silicon coating is not in its corrosion resistance. Here the other two coatings showed less attack in crucible tests after the same exposure times. However, the better ductility and higher toughness of the nickel--chromium-silicon coating prevent the spalling and cracking in service which lead to an early degradation of chromium and silicon coatings. From these r.esults it is concluded that a good coating in industrial gas turbines must show good chemical, physical and mechanical compatibility with the base material to give the required prolongation of blade life to of the order of at least 15 000 h.
1. INTRODUCTION
In stationary gas turbines at the present time the first stages of vanes and blades experience a maximum average temperature of 800-900°C. One of their most * Paper presented at the International Conference on Metallurgical Coatings, San Francisco, California, U.S.A., April 3-7, 1978.
360
K. S('HNI{II)I!R, I~,. B A I t ! R . II. Vv'. (~R('TNI,IN(;
important design properties is the creep and creep rupture behavior of the component material. For these temperatures nickel- or cobalt-based superalloys are used. Although the situation is related to aircraft turbines one major difference is the necessity to design stationary turbine blades or vanes for much longer lifetimes, mostly of the order of 3 × 1 0 4 l 0 s service hours. To reach this goal the crucial factor does not concern the mechanical properties of the superalloys but the resistance to hot corrosion attack of the components. There are two ways to meet the corrosion attack: (1) to develop alloys that are more corrosion resistant (an approach which has led nickel-based superalloys in the direction of increasing chromium content): (2) to divide the requirements, making use of a tess corrosion-resistant base material to give strength and protecting it by a corrosion-resistant coating. The first choice has its problems because the phase stability of chromium-rich alloys, e.~. lncone1939, is more critical ~ and a material that forms chromium oxide is not expected to be oxidation resistant at higher temperatures. Furthermore, chromium-rich alloys, e.~. Inconel 597, are more difficult to forge compared with alloys having a smaller chromium content. Another factor to bear in mind is that in view of the limited chromium supply chromium-rich alloys might become expensive compared with alloys containing less chromium but protected by coatings. The coatings that have proved the most successful in stationary gas turbines today are of the type that form chromium oxide or silicon oxide 2. Simple aluminide coatings give only little protection under the working conditions of industrial gas turbines 3. Pt A14 and M Cr A1 Y coatings are more promising s. In this paper only chromium- and silicon-containing coatings are considered. We shall show how these coatings work in service on Inconel 713 LC alloy and lnconel 738 LC alloy and how their wear can be explained by the results of laboratory tests. 2.
INVESTIGATIONS
Uncooled vanes of Inconel 713 LC and lnconel 738 LC* were taken out of 55 MW turbines after 1400 12500 h service and were cut for metallurgical investigations. Under the same service conditions (natural gas fuel, intake air containing less than 10 big cm 3 CI. 0.1 0.4 mg m ~ SO 2) the following coatings were tested on Inconel 713 LC vanes: a silicon type applied by pack cementation: a chromium type applied by pack cementation; Ni Cr Si 2 applied by plasma spraying. The test findings for the last two coatings on an Inconel 738 LC vane are also mentioned. The results gained with the in-service experiments arc compared with laboratory crucible tests under corrosive slag (Table I) at 850 ~C. The three coatings had been applied to small cylinders of lnconel 738 LC 18 mm in diameter, 8 mn'r high). * T h e c h e m i c a l c o m p o s i t i o n s are as follows (w,t.",,): 713 L C 738 L C
C 0.06 0.1
Ni Balance Balance
Co 8.5
('r 12 16.('1
Mo 4.5 1.7
Ti 0.7 3.5
W 2.6
AI 6.0 3.5
361
CORROSION AND FAILURE OF COATINGS FOR GAS TURBINES
TABLE 1 LABORATORYCRUCIBLETEST
Temperature Duration Atmosphere Specimen
850 cC 500, 1000, 2000, 3000, 5000 h Air containing 0.015 vol.% SO 2 and 0.015 vol.% SO 3 ~flow rate 601 h Cylindrical, diameter 8 mm, length 8 ram, central boring
1
Composition oJ the slag (wt.%): NazSO 4 CaSO4.2H20 Fe20 3 ZnSO,- H 2 0
4.3 22.7 22.3 20.6
K2SO , MgO A120 3 SiO z
10.4 2.8 6.5 10.4
To obtain information on the mechanical behavior of the coatings and of composites with the Inconel 738 LC base material, bending tests at room temperature and tensile tests from room temperature up to 850 ' C were conducted. 3. RESULTS
In-service vanes made of the less corrosion-resistant material Inconel 713 LC showed heavy corrosion attack after short service times whatever coatings were applied to them (Fig. 1). However, after service times twice as long Inconel 738 LC showed no attack macroscopically (Fig. 2).
(a)
(b)
(c)
Fig. 1. Coated Incone1713 LC vanes after 5000 h service at 850 °C: (a) nickel-chromium-silicon coating; (b) chromium coating; (c) silicon coating.
Of the various coatings the silicon and chromium coatings tended to spall off whereas the Ni-Cr-Si-coated vanes showed no signs of large areas that had spalled off(Fig. 2). The wear of the silicon coating is shown in Fig. 3. Whereas at the low temperatures ( < 600 °C) that obtain at the root section of
(b)
B a s e Material IN 713 LC
Siliconcoating
Fig. 2. Inconel 738 LC vanes after 12500 h service at 850'C, showing the spalling behavior of the coatings: (a) nickel chromium silicon coating; (b) chromium :oaring. ~'ig. 3. The c o r r o s i o n of a silicon coating o n a n l n c o n e l 713 LC v a n e after 1400 h service at 850 C. T h e n u m b e r s are the V H N h a r d n e s s values of the c o a t i n g .
a)
~oot
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~hroud
7
,.9
ao
C O R R O S I O N A N D F A I L U R E OF C O A T I N G S F O R G A S T U R B I N E S
363
the vane the coating appears to be one phase, at higher temperatures (750 °C) a coarser structure becomes visible under the same etching conditions. In the center part of the airfoil where the maximum temperature occurs a two-phase structure develops with a few small silicide particles which seem to be oxidized preferentially. A closer look shows that the coating of the root sections is multiphase also. On top of the coatings is a thin oxide layer which is mainly of spinel type. It is doubtful whether silicate 2 forms. Whereas the silicon coating is an interconnected two-phase structure the diffusion chromium coating develops a more layer-like structure 6: an oxide scale, a working 7 layer, an cxor o reservoir and the diffusion zone down to the base material (Fig. 4). The structural appearance is the same whether the in-service temperature is 750~C or 850 °C but the depth of the reservoir layer is smaller for the higher temperature after the same exposure time. With the first two coatings the intermediate stages of consumption are described here: the Ni-Cr-Si plasma-sprayed coating is shown in the later part of its lifetime however (Fig. 5). At the center of the airfoil (850 °C) all the chromium in the coating is consumed leaving an Ni-Si solid solution and particles of aluminum oxides, aluminum nitrides and chromium oxides. At lower temperatures (750 °C) the coating has developed an oxide layer (mainly Cr203) and the part -near the base material is still unattacked. The main difference between the base materials is that with Inconel 713 LC aluminum diffusion into the coating occurred during the annealing treatment and led to ~" precipitation: this is not observed when Incone1738 LC is the vane material. The results of the laboratory slag test are given in Fig. 6 which shows the penetration of corrosion attack as a function of exposure time at 850 °C for Ni-Cr Si, chromium and silicon coatings on Inconel 738 LC. In the initial state all three coatings have a multiphase structure. The N i - C ~ S i coating comprises an Ni-Cr-Si matrix with dispersed particles of high chromium content. In the chromium coating coarse particles of ~-Cr (80~ Cr) and the ~,-Ni-Cr matrix (60'VoCr) are found together with A120 3 (from the chromizing powder). The silicon coating is of a mixed type: a nickel silicide (Ni3Si2) on top and then a dispersion of nickel silicides in an Ni-Si-Cr matrix. It is clear from the diagram that from a corrosion standpoint the sequence of quality of the coatings is (1) silicon, (2) chromium and (3) Ni-Cr-Si. However, on comparing two of the coatings in service and in laboratory tests (Fig. 7) t h e N i - C r - S i coatings were found to be much better than the chromium coatings in service, thereby changing the sequence. In mechanical tests the following differences between the coatings were found. In tensile tests the ductility of coated Inconel 738 LC alloy is not very sensitive to heat treatment (Fig. 8). The ductility values of Inconel 738 LC (uncoated) in the standard heat-treated condition and with annealing treatment by coating cycles fall into the same scatter-band. The ductility of this material when coated with chromium or silicon, however, is lower than the average ductility of uncoated Incohe1738 LC at temperatures between room temperature and 850 °C. The N i - C r Si coating seems to improve the ductility of Inconel 738 LC at higher temperatures. For all the tensile tests shown in Fig. 8 the coatings never failed before the sample.
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.50 p.m
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IN 738 LC, 1 2 500 h service
:'ig. 5. The influence of the base material lespecially the chromium and aluminum content) on the corrosion behavior of a nickel chromium silicon coating.
qg. 4. The corrosion of a chromium diffusion coating on lnconel 738 LC after 1400 h service at 850 :C.
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365
C O R R O S I O N A N D F A I L U R E OF C O A T I N G S F O R G A S T U R B I N E S
40 pm
Ni-Cr-Si Plasma Coating
M a x i m u m attack (pro)
• 300
J
• 200 2o ~
Chromium Diffusion Coating
pm =
100
20 IJm D I
Silicon-Coating
20 pm 0
Initial state
f 500
I 1000
I 2000
Time (h)
Fig. 6. A laboratory crucible test of coatings on lnconel 738 LC at 850°C. % I Elongation at fracture
Corrosion attack
15
~-~ laboratory Ni-Cr-Si - - - - /
/
/;' // /
Scatterband IN 738 LC ~ Ni-Cr-Si
log exposure time m,
/
//
"') 10
~
/
/
//7/./ 0
Temperature 0
100
250
500
750
°C
Fig. 7. Attack of the coatings: a comparison of laboratory tests and in-service behavior. Fig. 8. T h e ductility o f c o a t e d a n d u n c o a t e d I n c o n e l 738 L C as a f u n c t i o n o f test t e m p e r a t u r e : - - n i c k e l - c h r o m i u m - s i l i c o n c o a t i n g : - - . - - , silicon c o a t i n g : , chromium coating.
366
K. SCHNEIDER, R. BAUER, H. W. GRLJNLING
Bending tests at room temperature (Fig. 9) show clearly how the three coatings fail under mechanical stresses. The very hard silicide coating IVHN 1300) develops straight cracks. The chromium coating, consisting of relatively large areas of ~-Cr (VHN 1300}, y solid solution and AI20 3, fails along phase boundaries. The relatively ductile Ni Cr Si coating consisting of small isolated hard brittle chromium-rich particles embedded in a ductile Ni Cr Si matrix, does not fail under the same bending stress but the Inconcl 738 LC base material starts to crack (at smaller elongations). Cracks
b-"-q 50 pm
.=
Cracks
=.
Silicon coating
I I 50 pm
; Chromium coating
Cracks
•
'. 1 50 pm
.= : Ni-Cr-SiCoating
Fig. 9. The cracking behavior of coated Inconel 738 LC in a bending test at room temperature (18~,, elongation).
4. I)ISCUSSION
The results shown here give a good picture of how a coating fails in service in a stationary gas turbine. A corrosion-resistant coating alone (Figs. 6 and 7) cannot protect a material. The coating has to have a certain ductility (Fig. 9) and little tendency to spall off(Fig. 2). With regard to corrosion, silicon coatings seem to be very promising: however, owing to differences in thermal expansion problems could arise in matching the silicon oxides and a coating consisting of a nickel silicide. All mechanical incompatibilities can be avoided by a structure comprising a ductile matrix and hard particles that contain the protective element, as in the NiCr Si coating. Here, however, as in all diffusion coatings, the base material plays an important role. When the coating is applied on Inconel 713 LC, which is a high aluminum alloy compared with Inconel 738 LC, aluminum diffuses into the coating to form Y' precipitates (Fig. 5). It has already been shown in the literature z that the corrosion resistance of an alloy depends on the Cr:A1 ratio, which is lower for lncone1713 LC than for Inconel
CORROSION AND FAILURE OF COATINGS FOR GAS TURBINES
367
738 LC. Because of the high aluminum content the formation of internal aluminum oxides is expected to lead to faster destruction of the coating under the surface. In addition to the effect that a high aluminum alloy can degrade the coating it must be borne in mind that in service erosion and damage by domestic or foreign objects can occur. Here a more ductile coating with a higher toughness behaves better than a brittle coating with better corrosion resistance. In the case of mechanical coating failures the base material plays an important role: if the base material is very sensitive to corrosive attack, e.g. Inconel 713 LC under the conditions in stationary gas turbines, attack begins at spots where the coating is defective and grows laterally and thereby destroys the composite at the interface of the coating and base material. In the examples shown here the combination of an Ni-Cr-Si coating with Inconel 738 LC is rated as the most suitable for stationary gas turbines for two reasons: (1) the coating has a certain high temperature corrosion resistance and, more important, a much better ductility and toughness than the other two coatings: (2) the Incone1738 LC base material does not degrade the corrosion properties of the Ni-Cr Si coating (as Inconel 713 LC does by adding aluminum to the coating elements). REFERENCES 1 T. Geiger, E. Erd6s. M. Lorenz, F. Staub and P. Wellner, Geffigestabilit/it yon hochwarmfesten GuBund Schmiedelegierungen auf Nickelbasis, European Concerted Action, Materials for Gas Turbines, COST50. Project CH 2/1, Final Rep., July 1977. 2 M. Villat and P. Felix, Hochtemperatur-Korrosionsschutzschicht f/Jr Gasturbinen, Teeh. Rundseh. Sulzer, 3 (1976) 97. 3 E. Erd6s, M. Semlitsch and P. Felix, Untersuchung yon Oberflfichenschutzschichten yon Gast urbinenlegierungen, Z. Werkstofftech., 3 (1972) 193. 4 G. Lehnert and H. Kiesheyer. Untersuchungen der HeiBgaskorrosion yon Nickellegierungen mit und ohne Oberfliichenschutz in striSmenden Verbrennungsgasen, European Concerted Action, Materials jbr Gas Turbines, COST50, Project D 1/2, Final Rep., 1977. 5 W. G. Stevens and A. R. Stetson, Controlled composition reaction sintering process for the production of M - C r - A I - Y coatings, Solar Division, Air Force and Marine Laboratories Tech. Rep. AFML-TR-76-91, 1976. 6 R. Bauer, H. W. Gr(inling and K. Schneider, Verhalten yon Diffusionschromschichten unter HeiBgaskorrosion, Dtsch. Ges. Metallkd., Faehberichte zur Tagung Verbundwerkstc~,, Konstanz, 1976, p. 51.
Printed in the Netherlands
359
CORROSION AND FAILURE MECHANISMS OF COATINGS FOR GAS TURBINE APPLICATIONS* K. SCHNEIDER, R. BAUER AND H. W. GRUNLING Brown, Boveri & Cie, Mannheim (F.R.G.) (Received April 4, 1978: accepted June 14, 1978)
For stationary gas turbines, coating of the blade material is today a standard procedure to prolong lifetime. It is possible by this method to divide the requirements between the base material (mostly nickel- or cobalt-based alloys, which give mechanical strength) and the coating (which provides resistance to high temperature corrosion and oxidation). However, the separate properties of the coating and base material are not the only advantages. The behavior of the composite may be completely different, especially under actual service conditions where the turbine blade undergoes complex mechanical and thermal stresses simultaneously with the corrosion attack. We show how three coatings (two of chromium and of silicon applied by pack cementation and one of nickel-chromium-silicon applied by plasma spraying) behave under service conditions in industrial gas turbines. It is found that the nickel--chromium-silicon coating is the best coating of these three when it is applied to a material such as Inconel 738 LC with moderately high temperature corrosion resistance. The in-service results and comparison with laboratory tests (corrosion as well as ductility) show that the reason for the excellent behavior of the nickelchromium-silicon coating is not in its corrosion resistance. Here the other two coatings showed less attack in crucible tests after the same exposure times. However, the better ductility and higher toughness of the nickel--chromium-silicon coating prevent the spalling and cracking in service which lead to an early degradation of chromium and silicon coatings. From these r.esults it is concluded that a good coating in industrial gas turbines must show good chemical, physical and mechanical compatibility with the base material to give the required prolongation of blade life to of the order of at least 15 000 h.
1. INTRODUCTION
In stationary gas turbines at the present time the first stages of vanes and blades experience a maximum average temperature of 800-900°C. One of their most * Paper presented at the International Conference on Metallurgical Coatings, San Francisco, California, U.S.A., April 3-7, 1978.
360
K. S('HNI{II)I!R, I~,. B A I t ! R . II. Vv'. (~R('TNI,IN(;
important design properties is the creep and creep rupture behavior of the component material. For these temperatures nickel- or cobalt-based superalloys are used. Although the situation is related to aircraft turbines one major difference is the necessity to design stationary turbine blades or vanes for much longer lifetimes, mostly of the order of 3 × 1 0 4 l 0 s service hours. To reach this goal the crucial factor does not concern the mechanical properties of the superalloys but the resistance to hot corrosion attack of the components. There are two ways to meet the corrosion attack: (1) to develop alloys that are more corrosion resistant (an approach which has led nickel-based superalloys in the direction of increasing chromium content): (2) to divide the requirements, making use of a tess corrosion-resistant base material to give strength and protecting it by a corrosion-resistant coating. The first choice has its problems because the phase stability of chromium-rich alloys, e.~. lncone1939, is more critical ~ and a material that forms chromium oxide is not expected to be oxidation resistant at higher temperatures. Furthermore, chromium-rich alloys, e.~. Inconel 597, are more difficult to forge compared with alloys having a smaller chromium content. Another factor to bear in mind is that in view of the limited chromium supply chromium-rich alloys might become expensive compared with alloys containing less chromium but protected by coatings. The coatings that have proved the most successful in stationary gas turbines today are of the type that form chromium oxide or silicon oxide 2. Simple aluminide coatings give only little protection under the working conditions of industrial gas turbines 3. Pt A14 and M Cr A1 Y coatings are more promising s. In this paper only chromium- and silicon-containing coatings are considered. We shall show how these coatings work in service on Inconel 713 LC alloy and lnconel 738 LC alloy and how their wear can be explained by the results of laboratory tests. 2.
INVESTIGATIONS
Uncooled vanes of Inconel 713 LC and lnconel 738 LC* were taken out of 55 MW turbines after 1400 12500 h service and were cut for metallurgical investigations. Under the same service conditions (natural gas fuel, intake air containing less than 10 big cm 3 CI. 0.1 0.4 mg m ~ SO 2) the following coatings were tested on Inconel 713 LC vanes: a silicon type applied by pack cementation: a chromium type applied by pack cementation; Ni Cr Si 2 applied by plasma spraying. The test findings for the last two coatings on an Inconel 738 LC vane are also mentioned. The results gained with the in-service experiments arc compared with laboratory crucible tests under corrosive slag (Table I) at 850 ~C. The three coatings had been applied to small cylinders of lnconel 738 LC 18 mm in diameter, 8 mn'r high). * T h e c h e m i c a l c o m p o s i t i o n s are as follows (w,t.",,): 713 L C 738 L C
C 0.06 0.1
Ni Balance Balance
Co 8.5
('r 12 16.('1
Mo 4.5 1.7
Ti 0.7 3.5
W 2.6
AI 6.0 3.5
361
CORROSION AND FAILURE OF COATINGS FOR GAS TURBINES
TABLE 1 LABORATORYCRUCIBLETEST
Temperature Duration Atmosphere Specimen
850 cC 500, 1000, 2000, 3000, 5000 h Air containing 0.015 vol.% SO 2 and 0.015 vol.% SO 3 ~flow rate 601 h Cylindrical, diameter 8 mm, length 8 ram, central boring
1
Composition oJ the slag (wt.%): NazSO 4 CaSO4.2H20 Fe20 3 ZnSO,- H 2 0
4.3 22.7 22.3 20.6
K2SO , MgO A120 3 SiO z
10.4 2.8 6.5 10.4
To obtain information on the mechanical behavior of the coatings and of composites with the Inconel 738 LC base material, bending tests at room temperature and tensile tests from room temperature up to 850 ' C were conducted. 3. RESULTS
In-service vanes made of the less corrosion-resistant material Inconel 713 LC showed heavy corrosion attack after short service times whatever coatings were applied to them (Fig. 1). However, after service times twice as long Inconel 738 LC showed no attack macroscopically (Fig. 2).
(a)
(b)
(c)
Fig. 1. Coated Incone1713 LC vanes after 5000 h service at 850 °C: (a) nickel-chromium-silicon coating; (b) chromium coating; (c) silicon coating.
Of the various coatings the silicon and chromium coatings tended to spall off whereas the Ni-Cr-Si-coated vanes showed no signs of large areas that had spalled off(Fig. 2). The wear of the silicon coating is shown in Fig. 3. Whereas at the low temperatures ( < 600 °C) that obtain at the root section of
(b)
B a s e Material IN 713 LC
Siliconcoating
Fig. 2. Inconel 738 LC vanes after 12500 h service at 850'C, showing the spalling behavior of the coatings: (a) nickel chromium silicon coating; (b) chromium :oaring. ~'ig. 3. The c o r r o s i o n of a silicon coating o n a n l n c o n e l 713 LC v a n e after 1400 h service at 850 C. T h e n u m b e r s are the V H N h a r d n e s s values of the c o a t i n g .
a)
~oot
~,irfoil
~hroud
7
,.9
ao
C O R R O S I O N A N D F A I L U R E OF C O A T I N G S F O R G A S T U R B I N E S
363
the vane the coating appears to be one phase, at higher temperatures (750 °C) a coarser structure becomes visible under the same etching conditions. In the center part of the airfoil where the maximum temperature occurs a two-phase structure develops with a few small silicide particles which seem to be oxidized preferentially. A closer look shows that the coating of the root sections is multiphase also. On top of the coatings is a thin oxide layer which is mainly of spinel type. It is doubtful whether silicate 2 forms. Whereas the silicon coating is an interconnected two-phase structure the diffusion chromium coating develops a more layer-like structure 6: an oxide scale, a working 7 layer, an cxor o reservoir and the diffusion zone down to the base material (Fig. 4). The structural appearance is the same whether the in-service temperature is 750~C or 850 °C but the depth of the reservoir layer is smaller for the higher temperature after the same exposure time. With the first two coatings the intermediate stages of consumption are described here: the Ni-Cr-Si plasma-sprayed coating is shown in the later part of its lifetime however (Fig. 5). At the center of the airfoil (850 °C) all the chromium in the coating is consumed leaving an Ni-Si solid solution and particles of aluminum oxides, aluminum nitrides and chromium oxides. At lower temperatures (750 °C) the coating has developed an oxide layer (mainly Cr203) and the part -near the base material is still unattacked. The main difference between the base materials is that with Inconel 713 LC aluminum diffusion into the coating occurred during the annealing treatment and led to ~" precipitation: this is not observed when Incone1738 LC is the vane material. The results of the laboratory slag test are given in Fig. 6 which shows the penetration of corrosion attack as a function of exposure time at 850 °C for Ni-Cr Si, chromium and silicon coatings on Inconel 738 LC. In the initial state all three coatings have a multiphase structure. The N i - C ~ S i coating comprises an Ni-Cr-Si matrix with dispersed particles of high chromium content. In the chromium coating coarse particles of ~-Cr (80~ Cr) and the ~,-Ni-Cr matrix (60'VoCr) are found together with A120 3 (from the chromizing powder). The silicon coating is of a mixed type: a nickel silicide (Ni3Si2) on top and then a dispersion of nickel silicides in an Ni-Si-Cr matrix. It is clear from the diagram that from a corrosion standpoint the sequence of quality of the coatings is (1) silicon, (2) chromium and (3) Ni-Cr-Si. However, on comparing two of the coatings in service and in laboratory tests (Fig. 7) t h e N i - C r - S i coatings were found to be much better than the chromium coatings in service, thereby changing the sequence. In mechanical tests the following differences between the coatings were found. In tensile tests the ductility of coated Inconel 738 LC alloy is not very sensitive to heat treatment (Fig. 8). The ductility values of Inconel 738 LC (uncoated) in the standard heat-treated condition and with annealing treatment by coating cycles fall into the same scatter-band. The ductility of this material when coated with chromium or silicon, however, is lower than the average ductility of uncoated Incohe1738 LC at temperatures between room temperature and 850 °C. The N i - C r Si coating seems to improve the ductility of Inconel 738 LC at higher temperatures. For all the tensile tests shown in Fig. 8 the coatings never failed before the sample.
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Thickness I
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Base material
Fo
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Diffusion zone
r
IN 713 LC, 5 0 0 0 h service
50 pm
Hottest aart of airfoil eading adge T = 850°C
.50 p.m
Mrfoit 3ear ;hroud, eading .~dge r _ 750 °C
IN 738 LC, 1 2 500 h service
:'ig. 5. The influence of the base material lespecially the chromium and aluminum content) on the corrosion behavior of a nickel chromium silicon coating.
qg. 4. The corrosion of a chromium diffusion coating on lnconel 738 LC after 1400 h service at 850 :C.
o co
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c 0
E
#
3rowth zone
z
4,
;=
7,
z
4a
365
C O R R O S I O N A N D F A I L U R E OF C O A T I N G S F O R G A S T U R B I N E S
40 pm
Ni-Cr-Si Plasma Coating
M a x i m u m attack (pro)
• 300
J
• 200 2o ~
Chromium Diffusion Coating
pm =
100
20 IJm D I
Silicon-Coating
20 pm 0
Initial state
f 500
I 1000
I 2000
Time (h)
Fig. 6. A laboratory crucible test of coatings on lnconel 738 LC at 850°C. % I Elongation at fracture
Corrosion attack
15
~-~ laboratory Ni-Cr-Si - - - - /
/
/;' // /
Scatterband IN 738 LC ~ Ni-Cr-Si
log exposure time m,
/
//
"') 10
~
/
/
//7/./ 0
Temperature 0
100
250
500
750
°C
Fig. 7. Attack of the coatings: a comparison of laboratory tests and in-service behavior. Fig. 8. T h e ductility o f c o a t e d a n d u n c o a t e d I n c o n e l 738 L C as a f u n c t i o n o f test t e m p e r a t u r e : - - n i c k e l - c h r o m i u m - s i l i c o n c o a t i n g : - - . - - , silicon c o a t i n g : , chromium coating.
366
K. SCHNEIDER, R. BAUER, H. W. GRLJNLING
Bending tests at room temperature (Fig. 9) show clearly how the three coatings fail under mechanical stresses. The very hard silicide coating IVHN 1300) develops straight cracks. The chromium coating, consisting of relatively large areas of ~-Cr (VHN 1300}, y solid solution and AI20 3, fails along phase boundaries. The relatively ductile Ni Cr Si coating consisting of small isolated hard brittle chromium-rich particles embedded in a ductile Ni Cr Si matrix, does not fail under the same bending stress but the Inconcl 738 LC base material starts to crack (at smaller elongations). Cracks
b-"-q 50 pm
.=
Cracks
=.
Silicon coating
I I 50 pm
; Chromium coating
Cracks
•
'. 1 50 pm
.= : Ni-Cr-SiCoating
Fig. 9. The cracking behavior of coated Inconel 738 LC in a bending test at room temperature (18~,, elongation).
4. I)ISCUSSION
The results shown here give a good picture of how a coating fails in service in a stationary gas turbine. A corrosion-resistant coating alone (Figs. 6 and 7) cannot protect a material. The coating has to have a certain ductility (Fig. 9) and little tendency to spall off(Fig. 2). With regard to corrosion, silicon coatings seem to be very promising: however, owing to differences in thermal expansion problems could arise in matching the silicon oxides and a coating consisting of a nickel silicide. All mechanical incompatibilities can be avoided by a structure comprising a ductile matrix and hard particles that contain the protective element, as in the NiCr Si coating. Here, however, as in all diffusion coatings, the base material plays an important role. When the coating is applied on Inconel 713 LC, which is a high aluminum alloy compared with Inconel 738 LC, aluminum diffuses into the coating to form Y' precipitates (Fig. 5). It has already been shown in the literature z that the corrosion resistance of an alloy depends on the Cr:A1 ratio, which is lower for lncone1713 LC than for Inconel
CORROSION AND FAILURE OF COATINGS FOR GAS TURBINES
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738 LC. Because of the high aluminum content the formation of internal aluminum oxides is expected to lead to faster destruction of the coating under the surface. In addition to the effect that a high aluminum alloy can degrade the coating it must be borne in mind that in service erosion and damage by domestic or foreign objects can occur. Here a more ductile coating with a higher toughness behaves better than a brittle coating with better corrosion resistance. In the case of mechanical coating failures the base material plays an important role: if the base material is very sensitive to corrosive attack, e.g. Inconel 713 LC under the conditions in stationary gas turbines, attack begins at spots where the coating is defective and grows laterally and thereby destroys the composite at the interface of the coating and base material. In the examples shown here the combination of an Ni-Cr-Si coating with Inconel 738 LC is rated as the most suitable for stationary gas turbines for two reasons: (1) the coating has a certain high temperature corrosion resistance and, more important, a much better ductility and toughness than the other two coatings: (2) the Incone1738 LC base material does not degrade the corrosion properties of the Ni-Cr Si coating (as Inconel 713 LC does by adding aluminum to the coating elements). REFERENCES 1 T. Geiger, E. Erd6s. M. Lorenz, F. Staub and P. Wellner, Geffigestabilit/it yon hochwarmfesten GuBund Schmiedelegierungen auf Nickelbasis, European Concerted Action, Materials for Gas Turbines, COST50. Project CH 2/1, Final Rep., July 1977. 2 M. Villat and P. Felix, Hochtemperatur-Korrosionsschutzschicht f/Jr Gasturbinen, Teeh. Rundseh. Sulzer, 3 (1976) 97. 3 E. Erd6s, M. Semlitsch and P. Felix, Untersuchung yon Oberflfichenschutzschichten yon Gast urbinenlegierungen, Z. Werkstofftech., 3 (1972) 193. 4 G. Lehnert and H. Kiesheyer. Untersuchungen der HeiBgaskorrosion yon Nickellegierungen mit und ohne Oberfliichenschutz in striSmenden Verbrennungsgasen, European Concerted Action, Materials jbr Gas Turbines, COST50, Project D 1/2, Final Rep., 1977. 5 W. G. Stevens and A. R. Stetson, Controlled composition reaction sintering process for the production of M - C r - A I - Y coatings, Solar Division, Air Force and Marine Laboratories Tech. Rep. AFML-TR-76-91, 1976. 6 R. Bauer, H. W. Gr(inling and K. Schneider, Verhalten yon Diffusionschromschichten unter HeiBgaskorrosion, Dtsch. Ges. Metallkd., Faehberichte zur Tagung Verbundwerkstc~,, Konstanz, 1976, p. 51.