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Sulphide corrosion of pure and chromium-modified β-NiAl intermetallic compound at high temperatures
Materials Science and Engineering, A 120 (1989) 105-109
105
Sulphide Corrosion of Pure and Chromium-modified -NiAI Intermetallic Compound at High Temperatures K. G O D L E W S K I , E. G O D L E W S K A , S. M R O W E C and M. D A N I E L E W S K I
Institute of Materials Science, Academy of Mining and Metallurgy, al. Mickiewicza 30, 30-059 Cracow (Poland) (Received April 6, 1989)
Abstract
The kinetics and mechanism of high-temperature sulphidation of the intermetallic compound fl-NiAl and of pseudobinary NiAI-Cr alloys have been studied as a function of temperature (1073-1273 K) and sulphur pressure (1.6-2 × 103 Pa) in flowing He-Se gas mixtures. The sulphidation process for all the materials studied follows a parabolic rate law after a certain initial period. The rate-determining step of the overall scale growth is the outward diffusion of cations. The scale on pure [3-NiAI was essentially homogeneous and composed of NiAIeS4 sulphospinel only, while that on the NiAI-Cr alloys also contained another spinel phase, (Cr,AI)3S4, the amount of which increased with the chromium content of the alloy. The sulphidation rate of the pseudobinary alloys was higher than that of pure /3-NiAI and increased with chromium content because of the less protective properties of (Cr,AI)~S4 compared with NiAI2S4. The rate of sulphidation for all the materials studied has been found to exceed the oxidation rate of [3-NiAl and F e - C r - A I or C o - C r - A I aluminaforming alloys by at least six orders of magnitude. 1. Introduction
Oxidation of the intermetallic compound fiNiAl, both pure and modified with different additions, has been a subject of numerous investigations because of the great practical importance
TABLE 1
of this material used as scaling-resistant protective coating for nickel-base superaUoys. It is well documented now that rare-earth additions can markedly improve the oxide scale adherence on fl-NiAl and decrease its oxidation rate. Such elements as chromium, platinum or silicon, in turn, are known to ameliorate the hot-corrosion resistance of aluminide coatings [1], this effect being especially strong if they are present in the external part of the coating. The sulphidation behaviour of pure and modified fl-NiAl has not been studied at all, although such data could be of importance for corrosion prevention in sulphur-containing atmospheres. The results of our preliminary investigation concerning sulphidation of pure and chromium-modified /~-NiA1 have been published previously [2]. The aim of the present study was to obtain further information on the sulphidation of pure and chromium-modified fl-NiAl, especially as regards sulphide scale composition and microstructure as well as reaction kinetics and mechanism.
2. Experimental details
The pure intermetallic compound fl-NiAl and pseudobinary NiA1-Cr alloys have been obtained from high-purity metals (99.99% A1; 99.98% Ni; 99.97% Cr) by vacuum induction melting followed by remelting in an argon atmosphere and subsequent annealing in evacuated quartz
Chemical composition of starting umterlals ( a t . % )
NiA1 NiAI-4Cr NiAI-10Cr NiAI-20Cr
0921-5093/89/$3.50
Cr
Ni
AI
S
-3.87 9.42 19.36
48.20 46.42 41.44 37.93
51.79 49.70 49.13 42.70
5 5 5 5
Fe x x x x
10 -5 10 -5 10 -5 10 -5
8 8 8 8
x x x x
Co 10 -5 10 -5 10 -5 10 -5
3 3 3 3
x x x x
10 -4 10 - 4 10 - 4 10 -4
© Elsevier Sequoia/Printed in The Netherlands
106
ampoules at 1273 K for 100 h. The chemical compositions of all the materials studied are collected in Table 1. Electron microprobe and X-ray analyses have revealed that at room temperature all chromiumcontaining alloys were two-phase alloys, as illustrated in Fig. 1. However, at sulphidation temperatures the A1Ni-4er alloy was homogeneous since, in agreement with the fl-NiAl-ot-Cr phase diagram [3], both phases exhibit limited mutual solubilities that increase with increasing temperature. The AI:Ni atomic ratio in the fl-NiA1 phase was in the range 1.04-1.17. The sulphidation kinetics was measured as a function of temperature (1073-1273 K) and sulphur pr0sosure( 1.6-2 x 103 Pa) in a special microthermogravimetric apparatus, the details of which together with the experimental procedure are described elsewhere [4]. It should be added that for these measurements the carrier gas (helium) was deoxidized in heated columns filled ~With iron wool. The oxygen level in the carder gas, measured by means of an electrochemical probe, ranged from 10 -~5 to 10-~°Pa. At low sulphur pressures, not exceeding about 50 Pa, it was impossible, however, to follow sulphidation kinetics due tO the passivation of the samples (formation bf an ~-A1203 film). Therefore in these conditions the sulphidation reaction was initiated at a higher sulphur pressure (e.g. 500 P a ) a n d it was subsequently continued at lower pressures as illustrated in Fig. 2(c). The phase and chemical composition o f the scales were studied b3/Various X-ray techniques. For this purpose the scale was carefully detached from the metallic core and the diffraction patterns were taken from its both sides• In addition, the scale was ground to obtain powder patterns. The surface Of the metallic core was also analyzed. The X~ray diffraction studies were supplemented with
'
kC
?_
3.0
~<
1.0 (a)
0.5
NiAt • 1273K, o 1273K, • 1173K ' • 1173K,
b
Fig. 1. M i c r o s t r u c t u r e s o f N i A I - C r
(b) NiAI-10Cr; (c) NiAI-4Cr.
c alloys: (a) N i A 1 - 2 0 C r ;
psi= 2x103 Po ps'= 5.102 Pe p~2=2x103Pe ~2 2 ps2=5.10 Pa
o
lC
(b) 0
05
1.0 t . 10-4 , s
J
2
15
15
~c
O
o
1.5
15
(C) 40 ~jrn
1.0 t . I l T 4, s
2C
x
~.-Cr
/
2.0
% p-NiA[
o NiAI - 2OCt *NiA[ - 10Or
5.0
"k
J
I3
~
tl/2.10 -2. s
Fig. 2. Parabolic plots for the sulphidation of (a) N i A l - C r pseudobinary alloys at T = 1273 K and Ps2 = 2 x 103 Pa,' (b) pure ~-NiA1 at T = 1173 K and 1273 K and Ps2 = 5 x 102 Pa and 2 x 103 Pa, (c) pure ~8-NiAI at T = 1273 K and different sulphur pressures.
107
quantitative electron probe microanalysis taken from metallographic cross-sections prepared in water-free lubricants and with energy dispersive X-ray analysis from the fractured scales. The sample preparation procedure for all these analyses was designed in such a way as to avoid the presence of humidity which would decompose some hygroscopic compounds, such as A12S3, if present. The scale morphology was studied by scanning electron microscopy.
-2
-3
NiAI z, Fe-234Cr-186At • Fe48At • Fe-gAI o
-4
~-~.
.... /
~_e o. /
-E
/
o/,°
/ /o/O O/O
/ //
/ log ps2,Pa
3. R e s u l t s and d i s c u s s i o n
Some of the results of sulphidation rate measurements are presented in Fig. 2 in parabolic plots. As can be seen, after the initial period of about 2 h, the sulphidation process for all the materials follows an approximately parabolic rate law. It is also evident that the rate of sulphide corrosion of NiA1-Cr pseudobinary alloys is higher than that of pure fl-NiA1 and it increases with chromium content in these materials. The influence of sulphur pressure on the sulphidation rate is presented in Fig. 3, whereas Fig. 4 shows the pressure dependence of the sulphidation rate obtained in this study for fl-NiA1 at 1273 K in He-S2 atmospheres and reported for Fe-9AI, Fe-8A1 and Fe-23.4Cr-18.6A1 alloys sulphidized at 1173K in HES-H2 atmospheres [5,6]. It is clearly seen that the corrosion resistance of the materials studied increases with decreasing sulphur pressure but even at rather low pressure (5 Pa) the rate of sulphidation is still about four orders of magnitude higher than the oxidation rate of alumina-forming alloys. On the other hand, sulphidation rates for pure fl-NiA1 at 1273 K (Fig. 4) are significantly lower (by 2-3
/
-~
Fig. 4. Pressuredependenceof the parabolic rate constant for pure fl-NiAl sulphidized at 1273K in He-S2 mixtures (this study) and for Fe-9A1, Fe-18Al and Fe-23.4Cr-18.6Al alloys sulphidizedat 1173K in H2-H2S mixtures[5, 6]. orders of magnitude) than those for Fe-A1 and F e - C r - A I alloys at 1173 K at the same sulphur pressure. At a sulphur pressure of 1.6Pa at 1273 K the sulphidation rate of pure fl-NiA1 is even comparable with the oxidation rate of Ni-30Cr alloy [7] which is one of the scalingresistant chromia formers. It has been found that the morphology and phase composition of scales are very complex. For illustration, Figs. 5-7 show the cross-sections of scales formed on pure fl-NiAl and the NiA1-Cr alloys. It can be seen that all the materials undergo internal sulphidation. Preferential sulphidation of aluminium results initially in the formation of nickel-rich fl-NiAl (fl-NiAINi) and then of an Nia A1 layer containing AI2$3 precipitates (internal sulphidation zone). The presence of a continuous AI2 $3 layer situated next to the metallic core could be detected only after quite a long sulphidation time (Fig. 8).
1273K
-•-5
spinet
" -6
o~
/
f
• NiAt20Cr/
-7 I- ,/,,"
scale
2S4
• .,! !10cq ".,At /
l.z"
tog ps2,Pa Fig. 3. Effectof sulphur pressure on the sulphidationrate of NiAI-Cr alloysat 1173 and 1273K.
~3-NiAI
lot lotion zone
Fig. 5. Cross-sectionof sulphidescale formedon pure/~-NiAI at 1173K and Ps2 = 2 x l03 Pa.
108
i3S 2
•
At2S3
3spinets SC
internc sutfido zone [3-NiAt I #-NiA[,
,12S4 ~,[)3S4 +
A[2S3
90 80 70 60 50 40 30 20 1() 0 (a) 28"(Co~)
Fig. 6. Cross-section of sulphide scale formed on NiAI-4Cr alloy at 1273 K and Ps~ = 5 x 102 Pa. o NiAtzSz. • A|2S3
utfospinet
r At 13S4
90 80 70 60 50 40 30 2'0 10
0
2e. Fig. 8. X-ray diffraction patterns obtained after sulphidation of pure fl-NiA1 under the conditions given in Fig. 2(c). (a) Surface of the metallic core; (b) powder pattern of the sulphide scale.
Fig. 7. Cross-section of sulphide scale formed on NiA1-20Cr alloy at 1173 K and Psz = 2 x 103 Pa.
I n the case of pure fl-NiAl sulphidation (Fig. 5), the scale was essentially homogeneous and composed of NiA12S4 phase only. However, at sulphur pressures exceeding 5 Pa, large spherical nodules of nickel sulphide appeared on the scale surface. Analogous nodules were also observed on the surface of scales formed on pseudobinary alloys at pressures higher than 5 Pa and temperatures exceeding 1073 K. The shape of these nodules suggests the presence of a liquid phase on the scale surface under the reaction conditions, which is in agreement with the melting point of Ni3S2, i.e. 1079 K. The absence of this nickel sulphide on the scale surface at a sulphur pressure of about 5 Pa and temperature of 1273 K can be accounted for by its thermodynamic instability. The scales formed on the pseudobinary alloys containing small and medium amounts of chromium (4% and 10%) were heterogeneous and
composed of N i A I 2 S 4 and (Cr,A1)3S 4 spinels with N i 3 S 2 nodules on the surface (Fig. 6). The scale formed on NiAI-20Cr alloy was also heterogeneous and consisted of (Cr,A1)3S4 and Ni3S2 in a stratified structure. For illustration, Fig. 7 shows very distinct Ni3 $2 bands separating much thicker (Cr,AI)3S 4 layers. The stratified morphology of the scale may indicate a non-stationary process of scale growth, as reported by other authors for oxide systems [8]. Marker experiments have shown that the scales on all the studied materials except NiA1-20Cr, grow essentially by outward diffusion of cations. It has been stated here already that the sulphidation rates of//-NiA1 and NiA1-Cr alloys are several orders of magnitude higher than the oxidation rates of alumina-forming alloys due to the fact that, in sulphidizing atmospheres, heterogeneous scales with poor protective properties are formed on these materials. As a matter of fact, the sulphide scale on pure //-NiAI is essentially
109
homogeneous but instead of AI2 S3 its main component is NiAI2S4 sulphospinel. Higher sulphidation rates of pseudobinary NiA1-Cr alloys in comparison with pure fl-NiAl result from the formation of the second (Cr,AI)3 $4 spinel phase in the scale, the contribution of which increases with the chromium content in the alloy. In contrast to NiAI2S4, this phase is highly defective [9] and it can dissolve a great amount of nickel [10]. The (Cr,A1)3S4 sulphospinel, due to its high diffusivity for cations, is considered to be the most undesirable scale component, being responsible for the rapid degradation of the NiAI-Cr pseudobinary alloys, thus making them actually unsuitable for the protection of materials in highly sulphidizing environments.
4. Summarizing remarks Sulphide corrosion of the intermetallic compound fl-NiA1 and the NiAI-Cr pseudobinary alloys at temperatures of 1073-1273 K and sulphur pressures of 1.6-2 x 103 Pa follows an approximately parabolic rate law, thus being diffusion-controlled. The sulphidation rates for fl-NiAl and NiAI-Cr alloys are several orders of magnitude higher than the oxidation rates for fl-NiAl and alumina-forming alloys. The NiA1-Cr alloys sulphidize faster than pure fl-NiAl, the reaction rate increasing with the chromium content of the alloy. Rapid degradation of these materials results from the poor protective properties of sulphospinels (NiAlzS 4 and (Cr,AI)3S4) which form the main part of sulphide scales. The contribution of (Cr,A1)3S4 in the sulphide scale
increases with chromium content in the NiAI-Cr alloys. This cation-deficient sulphospinel is the most undesirable scale component, being responsible for the relatively high sulphidation rates of the NiAI-Cr alloys, thus making them unsuitable for highly sulphidizing environments. In addition to the sulphospinel phases, the sulphide scales on all the materials studied contain A12S3 (in the internal sulphidation zone) and Ni3S2 nodules on the surface. A continuous A12S3 layer adjacent to the metallic core develops on pure//-NiA1 at low sulphur pressures after long sulphidation times, and it coexists with the outer layer built of NiAI2S4.
Acknowledgments This work has been carried out under contract CPB-R No. 6.6 supported by the Ministry of Progress in Science and Technology.
References 1 R. Streiff, Rapport DRET, AEPA 80 043, Universit6 de Provence, MarseiUe, 1984. 2 E. Godlewska, K. Godlewski and S. Mrowec, Mater. Sci. Eng., 87(1987) 183. 3 I. I. Kornilov and R. S. Minc, Dokl. Akad. Nauk S.S.S.R., 94 (1954) 1085. 4 S. Rusiecki, A. Wojtowicz, S. Mrowec and K. Przybylski, Solid State Ionics, 21 (1986) 273. 5 P. C. Patnaik and W. W. Smeltzer, Oxid. Met., 23 (1985) 53. 6 T. Narita, K. Przybylski and W. W. Smeltzer, Oxid. Met., 22 (1984) 181. 7 S. Mrowec and K. Przybylski, High Temp. Mater. Processes, 6 (1984) 1. 8 J. P. Larpin, personal communication, 1988. 9 I. Nakatani, J. Solid State Chem., 35 (1980) 50. 10 E. Erdos and A. Rahmel, Oxid. Met., 26(1986) 101.
105
Sulphide Corrosion of Pure and Chromium-modified -NiAI Intermetallic Compound at High Temperatures K. G O D L E W S K I , E. G O D L E W S K A , S. M R O W E C and M. D A N I E L E W S K I
Institute of Materials Science, Academy of Mining and Metallurgy, al. Mickiewicza 30, 30-059 Cracow (Poland) (Received April 6, 1989)
Abstract
The kinetics and mechanism of high-temperature sulphidation of the intermetallic compound fl-NiAl and of pseudobinary NiAI-Cr alloys have been studied as a function of temperature (1073-1273 K) and sulphur pressure (1.6-2 × 103 Pa) in flowing He-Se gas mixtures. The sulphidation process for all the materials studied follows a parabolic rate law after a certain initial period. The rate-determining step of the overall scale growth is the outward diffusion of cations. The scale on pure [3-NiAI was essentially homogeneous and composed of NiAIeS4 sulphospinel only, while that on the NiAI-Cr alloys also contained another spinel phase, (Cr,AI)3S4, the amount of which increased with the chromium content of the alloy. The sulphidation rate of the pseudobinary alloys was higher than that of pure /3-NiAI and increased with chromium content because of the less protective properties of (Cr,AI)~S4 compared with NiAI2S4. The rate of sulphidation for all the materials studied has been found to exceed the oxidation rate of [3-NiAl and F e - C r - A I or C o - C r - A I aluminaforming alloys by at least six orders of magnitude. 1. Introduction
Oxidation of the intermetallic compound fiNiAl, both pure and modified with different additions, has been a subject of numerous investigations because of the great practical importance
TABLE 1
of this material used as scaling-resistant protective coating for nickel-base superaUoys. It is well documented now that rare-earth additions can markedly improve the oxide scale adherence on fl-NiAl and decrease its oxidation rate. Such elements as chromium, platinum or silicon, in turn, are known to ameliorate the hot-corrosion resistance of aluminide coatings [1], this effect being especially strong if they are present in the external part of the coating. The sulphidation behaviour of pure and modified fl-NiAl has not been studied at all, although such data could be of importance for corrosion prevention in sulphur-containing atmospheres. The results of our preliminary investigation concerning sulphidation of pure and chromium-modified /~-NiA1 have been published previously [2]. The aim of the present study was to obtain further information on the sulphidation of pure and chromium-modified fl-NiAl, especially as regards sulphide scale composition and microstructure as well as reaction kinetics and mechanism.
2. Experimental details
The pure intermetallic compound fl-NiAl and pseudobinary NiA1-Cr alloys have been obtained from high-purity metals (99.99% A1; 99.98% Ni; 99.97% Cr) by vacuum induction melting followed by remelting in an argon atmosphere and subsequent annealing in evacuated quartz
Chemical composition of starting umterlals ( a t . % )
NiA1 NiAI-4Cr NiAI-10Cr NiAI-20Cr
0921-5093/89/$3.50
Cr
Ni
AI
S
-3.87 9.42 19.36
48.20 46.42 41.44 37.93
51.79 49.70 49.13 42.70
5 5 5 5
Fe x x x x
10 -5 10 -5 10 -5 10 -5
8 8 8 8
x x x x
Co 10 -5 10 -5 10 -5 10 -5
3 3 3 3
x x x x
10 -4 10 - 4 10 - 4 10 -4
© Elsevier Sequoia/Printed in The Netherlands
106
ampoules at 1273 K for 100 h. The chemical compositions of all the materials studied are collected in Table 1. Electron microprobe and X-ray analyses have revealed that at room temperature all chromiumcontaining alloys were two-phase alloys, as illustrated in Fig. 1. However, at sulphidation temperatures the A1Ni-4er alloy was homogeneous since, in agreement with the fl-NiAl-ot-Cr phase diagram [3], both phases exhibit limited mutual solubilities that increase with increasing temperature. The AI:Ni atomic ratio in the fl-NiA1 phase was in the range 1.04-1.17. The sulphidation kinetics was measured as a function of temperature (1073-1273 K) and sulphur pr0sosure( 1.6-2 x 103 Pa) in a special microthermogravimetric apparatus, the details of which together with the experimental procedure are described elsewhere [4]. It should be added that for these measurements the carrier gas (helium) was deoxidized in heated columns filled ~With iron wool. The oxygen level in the carder gas, measured by means of an electrochemical probe, ranged from 10 -~5 to 10-~°Pa. At low sulphur pressures, not exceeding about 50 Pa, it was impossible, however, to follow sulphidation kinetics due tO the passivation of the samples (formation bf an ~-A1203 film). Therefore in these conditions the sulphidation reaction was initiated at a higher sulphur pressure (e.g. 500 P a ) a n d it was subsequently continued at lower pressures as illustrated in Fig. 2(c). The phase and chemical composition o f the scales were studied b3/Various X-ray techniques. For this purpose the scale was carefully detached from the metallic core and the diffraction patterns were taken from its both sides• In addition, the scale was ground to obtain powder patterns. The surface Of the metallic core was also analyzed. The X~ray diffraction studies were supplemented with
'
kC
?_
3.0
~<
1.0 (a)
0.5
NiAt • 1273K, o 1273K, • 1173K ' • 1173K,
b
Fig. 1. M i c r o s t r u c t u r e s o f N i A I - C r
(b) NiAI-10Cr; (c) NiAI-4Cr.
c alloys: (a) N i A 1 - 2 0 C r ;
psi= 2x103 Po ps'= 5.102 Pe p~2=2x103Pe ~2 2 ps2=5.10 Pa
o
lC
(b) 0
05
1.0 t . 10-4 , s
J
2
15
15
~c
O
o
1.5
15
(C) 40 ~jrn
1.0 t . I l T 4, s
2C
x
~.-Cr
/
2.0
% p-NiA[
o NiAI - 2OCt *NiA[ - 10Or
5.0
"k
J
I3
~
tl/2.10 -2. s
Fig. 2. Parabolic plots for the sulphidation of (a) N i A l - C r pseudobinary alloys at T = 1273 K and Ps2 = 2 x 103 Pa,' (b) pure ~-NiA1 at T = 1173 K and 1273 K and Ps2 = 5 x 102 Pa and 2 x 103 Pa, (c) pure ~8-NiAI at T = 1273 K and different sulphur pressures.
107
quantitative electron probe microanalysis taken from metallographic cross-sections prepared in water-free lubricants and with energy dispersive X-ray analysis from the fractured scales. The sample preparation procedure for all these analyses was designed in such a way as to avoid the presence of humidity which would decompose some hygroscopic compounds, such as A12S3, if present. The scale morphology was studied by scanning electron microscopy.
-2
-3
NiAI z, Fe-234Cr-186At • Fe48At • Fe-gAI o
-4
~-~.
.... /
~_e o. /
-E
/
o/,°
/ /o/O O/O
/ //
/ log ps2,Pa
3. R e s u l t s and d i s c u s s i o n
Some of the results of sulphidation rate measurements are presented in Fig. 2 in parabolic plots. As can be seen, after the initial period of about 2 h, the sulphidation process for all the materials follows an approximately parabolic rate law. It is also evident that the rate of sulphide corrosion of NiA1-Cr pseudobinary alloys is higher than that of pure fl-NiA1 and it increases with chromium content in these materials. The influence of sulphur pressure on the sulphidation rate is presented in Fig. 3, whereas Fig. 4 shows the pressure dependence of the sulphidation rate obtained in this study for fl-NiA1 at 1273 K in He-S2 atmospheres and reported for Fe-9AI, Fe-8A1 and Fe-23.4Cr-18.6A1 alloys sulphidized at 1173K in HES-H2 atmospheres [5,6]. It is clearly seen that the corrosion resistance of the materials studied increases with decreasing sulphur pressure but even at rather low pressure (5 Pa) the rate of sulphidation is still about four orders of magnitude higher than the oxidation rate of alumina-forming alloys. On the other hand, sulphidation rates for pure fl-NiA1 at 1273 K (Fig. 4) are significantly lower (by 2-3
/
-~
Fig. 4. Pressuredependenceof the parabolic rate constant for pure fl-NiAl sulphidized at 1273K in He-S2 mixtures (this study) and for Fe-9A1, Fe-18Al and Fe-23.4Cr-18.6Al alloys sulphidizedat 1173K in H2-H2S mixtures[5, 6]. orders of magnitude) than those for Fe-A1 and F e - C r - A I alloys at 1173 K at the same sulphur pressure. At a sulphur pressure of 1.6Pa at 1273 K the sulphidation rate of pure fl-NiA1 is even comparable with the oxidation rate of Ni-30Cr alloy [7] which is one of the scalingresistant chromia formers. It has been found that the morphology and phase composition of scales are very complex. For illustration, Figs. 5-7 show the cross-sections of scales formed on pure fl-NiAl and the NiA1-Cr alloys. It can be seen that all the materials undergo internal sulphidation. Preferential sulphidation of aluminium results initially in the formation of nickel-rich fl-NiAl (fl-NiAINi) and then of an Nia A1 layer containing AI2$3 precipitates (internal sulphidation zone). The presence of a continuous AI2 $3 layer situated next to the metallic core could be detected only after quite a long sulphidation time (Fig. 8).
1273K
-•-5
spinet
" -6
o~
/
f
• NiAt20Cr/
-7 I- ,/,,"
scale
2S4
• .,! !10cq ".,At /
l.z"
tog ps2,Pa Fig. 3. Effectof sulphur pressure on the sulphidationrate of NiAI-Cr alloysat 1173 and 1273K.
~3-NiAI
lot lotion zone
Fig. 5. Cross-sectionof sulphidescale formedon pure/~-NiAI at 1173K and Ps2 = 2 x l03 Pa.
108
i3S 2
•
At2S3
3spinets SC
internc sutfido zone [3-NiAt I #-NiA[,
,12S4 ~,[)3S4 +
A[2S3
90 80 70 60 50 40 30 20 1() 0 (a) 28"(Co~)
Fig. 6. Cross-section of sulphide scale formed on NiAI-4Cr alloy at 1273 K and Ps~ = 5 x 102 Pa. o NiAtzSz. • A|2S3
utfospinet
r At 13S4
90 80 70 60 50 40 30 2'0 10
0
2e. Fig. 8. X-ray diffraction patterns obtained after sulphidation of pure fl-NiA1 under the conditions given in Fig. 2(c). (a) Surface of the metallic core; (b) powder pattern of the sulphide scale.
Fig. 7. Cross-section of sulphide scale formed on NiA1-20Cr alloy at 1173 K and Psz = 2 x 103 Pa.
I n the case of pure fl-NiAl sulphidation (Fig. 5), the scale was essentially homogeneous and composed of NiA12S4 phase only. However, at sulphur pressures exceeding 5 Pa, large spherical nodules of nickel sulphide appeared on the scale surface. Analogous nodules were also observed on the surface of scales formed on pseudobinary alloys at pressures higher than 5 Pa and temperatures exceeding 1073 K. The shape of these nodules suggests the presence of a liquid phase on the scale surface under the reaction conditions, which is in agreement with the melting point of Ni3S2, i.e. 1079 K. The absence of this nickel sulphide on the scale surface at a sulphur pressure of about 5 Pa and temperature of 1273 K can be accounted for by its thermodynamic instability. The scales formed on the pseudobinary alloys containing small and medium amounts of chromium (4% and 10%) were heterogeneous and
composed of N i A I 2 S 4 and (Cr,A1)3S 4 spinels with N i 3 S 2 nodules on the surface (Fig. 6). The scale formed on NiAI-20Cr alloy was also heterogeneous and consisted of (Cr,A1)3S4 and Ni3S2 in a stratified structure. For illustration, Fig. 7 shows very distinct Ni3 $2 bands separating much thicker (Cr,AI)3S 4 layers. The stratified morphology of the scale may indicate a non-stationary process of scale growth, as reported by other authors for oxide systems [8]. Marker experiments have shown that the scales on all the studied materials except NiA1-20Cr, grow essentially by outward diffusion of cations. It has been stated here already that the sulphidation rates of//-NiA1 and NiA1-Cr alloys are several orders of magnitude higher than the oxidation rates of alumina-forming alloys due to the fact that, in sulphidizing atmospheres, heterogeneous scales with poor protective properties are formed on these materials. As a matter of fact, the sulphide scale on pure //-NiAI is essentially
109
homogeneous but instead of AI2 S3 its main component is NiAI2S4 sulphospinel. Higher sulphidation rates of pseudobinary NiA1-Cr alloys in comparison with pure fl-NiAl result from the formation of the second (Cr,AI)3 $4 spinel phase in the scale, the contribution of which increases with the chromium content in the alloy. In contrast to NiAI2S4, this phase is highly defective [9] and it can dissolve a great amount of nickel [10]. The (Cr,A1)3S4 sulphospinel, due to its high diffusivity for cations, is considered to be the most undesirable scale component, being responsible for the rapid degradation of the NiAI-Cr pseudobinary alloys, thus making them actually unsuitable for the protection of materials in highly sulphidizing environments.
4. Summarizing remarks Sulphide corrosion of the intermetallic compound fl-NiA1 and the NiAI-Cr pseudobinary alloys at temperatures of 1073-1273 K and sulphur pressures of 1.6-2 x 103 Pa follows an approximately parabolic rate law, thus being diffusion-controlled. The sulphidation rates for fl-NiAl and NiAI-Cr alloys are several orders of magnitude higher than the oxidation rates for fl-NiAl and alumina-forming alloys. The NiA1-Cr alloys sulphidize faster than pure fl-NiAl, the reaction rate increasing with the chromium content of the alloy. Rapid degradation of these materials results from the poor protective properties of sulphospinels (NiAlzS 4 and (Cr,AI)3S4) which form the main part of sulphide scales. The contribution of (Cr,A1)3S4 in the sulphide scale
increases with chromium content in the NiAI-Cr alloys. This cation-deficient sulphospinel is the most undesirable scale component, being responsible for the relatively high sulphidation rates of the NiAI-Cr alloys, thus making them unsuitable for highly sulphidizing environments. In addition to the sulphospinel phases, the sulphide scales on all the materials studied contain A12S3 (in the internal sulphidation zone) and Ni3S2 nodules on the surface. A continuous A12S3 layer adjacent to the metallic core develops on pure//-NiA1 at low sulphur pressures after long sulphidation times, and it coexists with the outer layer built of NiAI2S4.
Acknowledgments This work has been carried out under contract CPB-R No. 6.6 supported by the Ministry of Progress in Science and Technology.
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