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High Temperature Oxidation Behavior of Al-diffused Fe-Cr-Al Foils
Advanced Materials ' 9 3 , I / A: Ceramics, Powders, Corrosion and Advanced Processing, edited by N. Mizutani et al. Trans. Mat. Res. Soc. Jpn., Volume 14A © 1994 Elsevier Science B.V. All rights reserved.
145
High Temperature Oxidation Behavior of ΑΙ-diffused Fe-Cr-AI Foils Yuji I k e g a m i , Yoshito M a k i t a a n d N o b u y o s h i O k a t o Technical R e s e a r c h Center, N i p p o n Yakin K o g y o Co., Ltd. 4-2 Kojima-cho, K a w a s a k i - k u , K a w a s a k i , J a p a n
Abstract:High t e m p e r a t u r e o x i d a t i o n of Fe-Cr-AI alloy foils containing 7 t o 13 p e r c e n t Al w a s s t u d ied. T h e s e foils w e r e m a d e b y a process w i t h Al coating a n d diffusion a n n e a l i n g . T h e lifetime of foil in air at 1473K w a s greatly e n h a n c e d by increasing Al concentration. T h e g r o w t h r a t e of AI2O3 s c a l e w a s r e d u c e d also b y raising Al levels. Moreover, surface condition of t h e foil significantly influenced t h e o x i d e g r o w t h rate;that is, t h e as-cold-rolled foil s h o w e d p o o r e r o x i d a t i o n resistance t h a n t h e a s diffusion-annealed foil. C1-AI2O3 scales w h i c h s h o w e d lower g r o w t h rates consisted of coarser colum n a r oxide grains, indicating that o x y g e n i n w a r d diffusion along g r a i n b o u n d a r i e s w e r e r e t a r d e d . 1,
INTRODUCTION Fe-Cr-AI alloys h a v e excellent h i g h t e m p e r a t u r e o x i d a t i o n resistance b e c a u s e t h e s e alloys forms p r o t e c t i v e AI2O3 scale u n d e r h i g h tem p e r a t u r e oxidizing e n v i r o n m e n t s . Recently, thin foil of Fe-20Cr-5Al alloy w i t h a small a d d i tion of rare e a r t h m e t a l s h a s b e e n u s e d for a n a u t o m o b i l e catalyst s u b s t r a t e ^ . M a n y s t u d i e s h a v e b e e n focused o n t h e effects of REMs a n d o t h e r reactive e l e m e n t s o n high t e m p e r a t u r e o x i d a t i o n b e h a v i o r of Fe-20Cr-5Al alloy f o i l . H o w e v e r , t h e modification of this alloy foil b y t h e a d d i t i o n of reactive ele m e n t s h a s a limited effect i n t e r m s of e x t e n d i n g lifetime of t h e foil b e c a u s e it greatly d e p e n d s o n Al c o n t e n t s i n t h e foil. In o u r p r e v i o u s w o r k , w e p r o p o s e d a n e w pro cess for p r o d u c i n g h i g h Al containing ferritic stainless steels. T h i s p r o c e s s consists of Al coat ing o n ferritic stainless steel a n d diffusion an nealing . In this study, Fe-Cr-AI foils containing u p t o 13% Al w e r e p r e p a r e d b y t h e s a m e process a n d their o x i d a t i o n b e h a v i o r w a s i n v e s t i g a t e d . ( 2 ) ( 3 ) ( 4 )
(5)
2.
EXPERIMENTAL
PROCEDURES
2-1 M a t e r i a l s C o m m e r c i a l l y p r o d u c e d Fe-18Cr-3Al-0.1REM alloy strips w i t h a thickness of 0.3mm w e r e u s e d a s a s u b s t r a t e for a l u m i n u m coating. Thickness of Al layers w a s varied t o obtain 7% to 1 3 % Al c o n t a i n i n g alloys. After Al coating,
the strips w e r e cold-rolled t o 0.03mm t o 0 . 1 m m thick a n d t h e n h e a t - t r e a t e d , or diffusion a n nealed, in v a c u u m at 1173K for lOhr. T h i s dif fusion annealing condition w a s chosen t o diffuse Al h o m o g e n e o u s l y t h r o u g h foil thickness. T h e chemical c o m p o s i t i o n s of t h e foils o b t a i n e d a r e given in Table 1. Mechanical a n d physical p r o p e r t i e s of t h e s e foils are described in ref. 6. To s t u d y t h e effects of surface c o n d i t i o n s o n oxi d a t i o n b e h a v i o r of Fe-Cr-AI alloy foils, 0 . 1 m m thick Fe-17Cr-7Al a n d Fe-17Cr-9Al s t r i p s w e r e further cold rolled t o 0.05mm thick. Oxidation resistance of t h e as-rolled foils w e r e c o m p a r e d w i t h t h e a s - d i f f u s i o n - a n n e a l e d foils. 2-2 O x i d a t i o n tests Specimens for oxidation tests w e r e cut into c o u p o n s of 25 X 5 0 m m a n d cleaned ultrasonically in xylene a n d t h e n in acetone. Foils w e r e u s e d in t h e as-diffusion-annealed c o n d i t i o n except o x i d a t i o n test for i n v e s t i g a t i n g t h e effect of sur face conditions. O x i d a t i o n tests w e r e p e r f o r m e d in air at a t e m p e r a t u r e from 1273K t o 1473K. S p e c i m e n s at tached t o a n a l u m i n a h o l d e r w e r e i n s e r t e d into an electric b o x furnace. After oxidized for 24hr, the s p e c i m e n s w e r e r e m o v e d a n d w e i g h e d . This p r o c e d u r e w a s r e p e a t e d e n o u g h t i m e s t o obtain weight-gain-versus-time curves. N o oxide exfoliation w a s o b s e r v e d d u r i n g t h e o x i d a t i o n test; therefore, t h e d a t a o b t a i n e d w e r e u s e d t o d e t e r m i n e t h e o x i d e g r o w t h rate.
146 Table 1 Chemical compositions of Al-diffused
Fe-Cr-Al
/
1
Alloy designation in text
Ο
Compositi on, w t . % AI Ti REM
Fe-17Cr-7AI
Ι 7.4
7.5
Fe-17Cr-9AI Fe-17Cr-11AI Fe-17Cr-13AI
17.5 17.1
9.3 10.9 13.0
3.
16.9
0.04 0.04 0.04 0.04
Ε
0.1
ι
-
11 AI
Ε
0.1 0.1 0.1
ι
0.05mm thick foil oxidized at 1473K
CM
Y
9AI i
Γ
7AI /
ce CD
-
Ρ
2
ο
RESULTS
3-1 Effect of Al and thickness on the lifetime of Fe-Cr-Al foils Figure 1 s h o w s the oxidation behavior of 0.05mm thick Fe-17Cr-7Al, Fe-17Cr-9Al and Fe17Cr-llAl foils. Durations to breakaway oxida tion were prolonged with increasing Al concen tration. Thickening the foil also extended its lifetime as s h o w n in Fig. 2.
5
/L 0
1
1
200
300
O x i d a t i o n t i m e (hr)
Figure 1 Oxidation behavior of 0.05mm thick Al-diffused Fe-Cr-Al foils at 1473K. 8 Ε
3-2 Effect of Al o n parabolic rate constants Figure 3 s h o w s the effect of Al on parabolic rate constants, Kp, for the growth of AI2O3 scale. These data were calculated from oxida tion data at 1423K for 24hr to 96hr. Kp values decreased w i t h increasing Al.
L
100
Fe-17Cr-iW oxidized at 1473Κ
ε 0.05mm
χ: Ο)
©
3-3 Effect of surface condition Figure 4 s h o w s the oxidation behavior of 0.05mm thick Fe-17A1-9A1 foils in the as-rolled and in the as-diffusion-annealed condition. Clearly, the as-diffusion-annealed foil s h o w s better oxidation resistance than the as-rolled foil. Parabolic rate constants for the foils were cal culated from the oxidation data during AI2O3 formation and are g i v e n in Table 2. Kp values of the as-diffusion-annealed foil w a s lower than that of the as-rolled foil.
Table 2 Parabolic rate constants at 1474K for Fe-17Cr7A1 and Fe-17Cr-9Al foils. Alloy Fe-17Cr-7AI
Condition as rolled
as diffusion a n n e a l e d Fe-17Cr-9AI as rolled a s diffusion a n n e a l e d
Kp(g%rrrVs) 4.6 χ 10-12 3.9 χ 10 4.7 χ 10-12 1 2
3.2 χ 10 -
1 2
5 100
200
300
O x i d a t i o n t i m e (hr)
Figure 2 Effect of foil thickness on oxidation behavior at 1473K for Fe-17Cr-llAl alloy. 2.0 ι Ε ο
£r
1.5 -
>v
Fe-17Cr-AI foil 0.05mm thick oxidized at 1423K
CN
C
1.0 -
Q.
0.5
10 Al content
15 (%)
Figure 3 Effect of Al o n parabolic rate constants at 1423K for Al-diffused Fe-Cr-Al foils.
147
O x i d a t i o n t i m e (hr)
Figure 4 Oxidation of as-rolled and as-diffu sion-annealed Fe-17Cr-9Al foils at 1473K. 3-4 Oxide morphology FigureS s h o w s SEM images of fracture cross sections of AI2O3 scales formed on the as-rolled and the as-diffusion-annealed Fe-17Cr-7Al foils. In both foils, equiaxed grains were ob served at the outer surface of the scale and columnar grains at the inner. Columnar grains of the as-rolled foil w e r e larger than those of the as-diffusion annealed foil. AI2O3 scales formed on Fe-17Cr-13Al foil had n o equiaxed grains and significantly larger columnar grains as compared with Fe-17Cr-7Al foil as s h o w n in Fig. 6. 4.
Figure 5 Fracture cross sections of AI2O3 scales formed on (a) as-rolled and (b) as-diffusion-an nealed Fe-17Cr-7Al foils oxidized at 1473K for 24hr.
DISCUSSION
The oxidation process for the Fe-Cr-AI foil pro ceed in three s t a g e s . (7)
1st. stage : the g r o w t h of AI2O3 scale governed by the parabolic rate law. 2nd. stage : the growth of C n 0 3 scale beneath AI2O3 layers according to the li near oxidation law after the ex haustion of Al in the foil. 3rd. stage : breakaway oxidation In the second stage, the linear rate constant is said to be proportional to the parabolic rate con stant in the first s t a g e . Therefore, the parabolic rate constant for the growth of AI2O3 scale is one of the most important factors in de termining oxidation resistance of Fe-Cr-AI foil. (7)
Figurée Fracture cross sections of AI2O3 scales formed on (a) Fe-17Cr-7Al foil and (b) Fe-17Cr13A1 foil oxidized at 1423K for 96hr. The foils were as diffusion-annealed before oxidation.
148 Parabolic r a t e c o n s t a n t s for Al-diffused foils in this s t u d y are g i v e n in Table 3 c o m p a r e d w i t h d a t a in t h e literature for oxidation of c o m m e r cially p r o d u c e d Fe-20Cr-5Al alloy foils and A l - d e p o s i t e d stainless steel f o i l . Kp v a l u e s for t h e Al-diffused foils are l o w e r t h a n t h o s e of Fe-20Cr-5Al foils a n d a g r e e well w i t h t h a t for A l - d e p o s i t e d stainless steel foil. A s s h o w n in Fig. 5 a n d 6, AI2O3 scales w i t h l o w oxide g r o w t h rates h a d coarse columnar grains. By c o m p a r i n g t h e activation energies ob t a i n e d in this s t u d y ( s e e Figure 7) w i t h t h e d a t a of ref.10,11 a n d 12, b o t h Al o u t w a r d diffusion t h r o u g h t h e b u l k oxide a n d o x y g e n i n w a r d dif fusion along grain b o u n d a r i e s could control a AI2O3 scale g r o w t h . C o a r s e 01-AI2O3 g r a i n s h a v e fewer p a t h s for o x y g e n diffusion, leading to a l o w o x i d e g r o w t h rate. It is n o t clear w h a t d e t e r m i n e s the grain size of C1-AI2O3 scales. T h i n film of a l u m i n u m oxide w a s detected b y AES in the as-diffusion-an n e a l e d foil prior t o oxidation test. It is consid ered t h a t this film could p r e v e n t transitional o x i d e f o r m a t i o n at t h e early s t a g e of oxidation a n d p r o m o t e coarse c o l u m n a r a - A b C b s c a l e s . Increasing Al concentration in t h e foil m a y m o d ify the thin film a n d reduce the s u b s e q u e n t oxide g r o w t h rate in air while forming coarse oxide grains. (9)
(10)
4. C O N C L U S I O N S H i g h t e m p e r a t u r e o x i d a t i o n b e h a v i o r of Aldiffused Fe-Cr-Al alloy foils containing u p to 13% Al w a s i n v e s t i g a t e d . ( l ) D u r a t i o n s t o b r e a k a w a y o x i d a t i o n of t h e foil w e r e p r o l o n g e d b y increasing t h e Al concentra tion a n d also b y thickening the foil. (2)The p a r a b o l i c r a t e c o n s t a n t for t h e AI2O3 g r o w t h d e c r e a s e d w i t h increasing Al content. (3)The foil in t h e as-rolled c o n d i t i o n exhibited p o o r e r o x i d a t i o n resistance t h a n t h a t in t h e asdiffusion-annealed condition. ( 4 ) A b 0 3 scales f o r m e d o n t h e foil w i t h a l o w e r g r o w t h r a t e h a d coarser oxide grains, indicat ing t h a t o x y g e n i n w a r d diffusion along grain boundaries were retarded.
Table 3 Parabolic r a t e c o n s t a n t s in air at 1423K. Alloy
Kp(10 - i 2 g 2 / 4 / s e c ) c m
Fe-17Cr-7to13AI:
1.0 to 1.8
Fe-20Cr-5AI(ref.8):
5.2 to 6.2
AI
1.1
10 •11,
ο Ε υ
10 -12
OU 10"
ο
•
17Cr-7AI 1 7 C r - 1 1 Al
activation energy 17Cr-7AI :351kJ/mol 17Cr-11AI:323kJ/mol
10* 6.5
7.0 1 /Temperature
8.0
7.5 (10 /K) 4
Figure 7 A plot of logKp vs. t h e reciprocal of the absolute t e m p e r a t u r e for 0.05mm thick Fe17Cr-7Al a n d F e - 1 7 C r - U A l foils. REFERENCES 1. N . N o n n e n m a n n : SAE p a p e r 850131(1985) 2. T. K a w a s a k i a n d K. Ishii: P r o c e e d i n g s of International Conference o n Stainless Steel, ISIJ, (1991), 1205 3. K. O h m u r a et al: ibid, (1991), 1212 4. N . H i r a m a t s u et al: ibid,(1991),1227 5. Y. I k e g a m i et al: ibid,(1991),1235 6. Y. I k e g a m i et al: N i p p o n Yakin Technical Report, 2(1993), 71 7. K. Ishii a n d K. K a w a s a k i : J. J a p a n Inst. Metals, 56(1992), 854 8. D. R. Sigler: Oxid. Met., 36(1991), 57 9. A. A n d o et al: N i s s h i n Steel Technical Report, 65(1992), 1 10. Y. Oishi a n d W. D. Kingery: J. C h e m . Phys., 33(1960), 480 11. A. E. P a l a d i n o a n d W. D . Kingery: ibid, 37 (1962), 957 12. H . A. Wang a n d F. A. Kroger: J. A m . C e r a m . S o c , 63(1980)613
145
High Temperature Oxidation Behavior of ΑΙ-diffused Fe-Cr-AI Foils Yuji I k e g a m i , Yoshito M a k i t a a n d N o b u y o s h i O k a t o Technical R e s e a r c h Center, N i p p o n Yakin K o g y o Co., Ltd. 4-2 Kojima-cho, K a w a s a k i - k u , K a w a s a k i , J a p a n
Abstract:High t e m p e r a t u r e o x i d a t i o n of Fe-Cr-AI alloy foils containing 7 t o 13 p e r c e n t Al w a s s t u d ied. T h e s e foils w e r e m a d e b y a process w i t h Al coating a n d diffusion a n n e a l i n g . T h e lifetime of foil in air at 1473K w a s greatly e n h a n c e d by increasing Al concentration. T h e g r o w t h r a t e of AI2O3 s c a l e w a s r e d u c e d also b y raising Al levels. Moreover, surface condition of t h e foil significantly influenced t h e o x i d e g r o w t h rate;that is, t h e as-cold-rolled foil s h o w e d p o o r e r o x i d a t i o n resistance t h a n t h e a s diffusion-annealed foil. C1-AI2O3 scales w h i c h s h o w e d lower g r o w t h rates consisted of coarser colum n a r oxide grains, indicating that o x y g e n i n w a r d diffusion along g r a i n b o u n d a r i e s w e r e r e t a r d e d . 1,
INTRODUCTION Fe-Cr-AI alloys h a v e excellent h i g h t e m p e r a t u r e o x i d a t i o n resistance b e c a u s e t h e s e alloys forms p r o t e c t i v e AI2O3 scale u n d e r h i g h tem p e r a t u r e oxidizing e n v i r o n m e n t s . Recently, thin foil of Fe-20Cr-5Al alloy w i t h a small a d d i tion of rare e a r t h m e t a l s h a s b e e n u s e d for a n a u t o m o b i l e catalyst s u b s t r a t e ^ . M a n y s t u d i e s h a v e b e e n focused o n t h e effects of REMs a n d o t h e r reactive e l e m e n t s o n high t e m p e r a t u r e o x i d a t i o n b e h a v i o r of Fe-20Cr-5Al alloy f o i l . H o w e v e r , t h e modification of this alloy foil b y t h e a d d i t i o n of reactive ele m e n t s h a s a limited effect i n t e r m s of e x t e n d i n g lifetime of t h e foil b e c a u s e it greatly d e p e n d s o n Al c o n t e n t s i n t h e foil. In o u r p r e v i o u s w o r k , w e p r o p o s e d a n e w pro cess for p r o d u c i n g h i g h Al containing ferritic stainless steels. T h i s p r o c e s s consists of Al coat ing o n ferritic stainless steel a n d diffusion an nealing . In this study, Fe-Cr-AI foils containing u p t o 13% Al w e r e p r e p a r e d b y t h e s a m e process a n d their o x i d a t i o n b e h a v i o r w a s i n v e s t i g a t e d . ( 2 ) ( 3 ) ( 4 )
(5)
2.
EXPERIMENTAL
PROCEDURES
2-1 M a t e r i a l s C o m m e r c i a l l y p r o d u c e d Fe-18Cr-3Al-0.1REM alloy strips w i t h a thickness of 0.3mm w e r e u s e d a s a s u b s t r a t e for a l u m i n u m coating. Thickness of Al layers w a s varied t o obtain 7% to 1 3 % Al c o n t a i n i n g alloys. After Al coating,
the strips w e r e cold-rolled t o 0.03mm t o 0 . 1 m m thick a n d t h e n h e a t - t r e a t e d , or diffusion a n nealed, in v a c u u m at 1173K for lOhr. T h i s dif fusion annealing condition w a s chosen t o diffuse Al h o m o g e n e o u s l y t h r o u g h foil thickness. T h e chemical c o m p o s i t i o n s of t h e foils o b t a i n e d a r e given in Table 1. Mechanical a n d physical p r o p e r t i e s of t h e s e foils are described in ref. 6. To s t u d y t h e effects of surface c o n d i t i o n s o n oxi d a t i o n b e h a v i o r of Fe-Cr-AI alloy foils, 0 . 1 m m thick Fe-17Cr-7Al a n d Fe-17Cr-9Al s t r i p s w e r e further cold rolled t o 0.05mm thick. Oxidation resistance of t h e as-rolled foils w e r e c o m p a r e d w i t h t h e a s - d i f f u s i o n - a n n e a l e d foils. 2-2 O x i d a t i o n tests Specimens for oxidation tests w e r e cut into c o u p o n s of 25 X 5 0 m m a n d cleaned ultrasonically in xylene a n d t h e n in acetone. Foils w e r e u s e d in t h e as-diffusion-annealed c o n d i t i o n except o x i d a t i o n test for i n v e s t i g a t i n g t h e effect of sur face conditions. O x i d a t i o n tests w e r e p e r f o r m e d in air at a t e m p e r a t u r e from 1273K t o 1473K. S p e c i m e n s at tached t o a n a l u m i n a h o l d e r w e r e i n s e r t e d into an electric b o x furnace. After oxidized for 24hr, the s p e c i m e n s w e r e r e m o v e d a n d w e i g h e d . This p r o c e d u r e w a s r e p e a t e d e n o u g h t i m e s t o obtain weight-gain-versus-time curves. N o oxide exfoliation w a s o b s e r v e d d u r i n g t h e o x i d a t i o n test; therefore, t h e d a t a o b t a i n e d w e r e u s e d t o d e t e r m i n e t h e o x i d e g r o w t h rate.
146 Table 1 Chemical compositions of Al-diffused
Fe-Cr-Al
/
1
Alloy designation in text
Ο
Compositi on, w t . % AI Ti REM
Fe-17Cr-7AI
Ι 7.4
7.5
Fe-17Cr-9AI Fe-17Cr-11AI Fe-17Cr-13AI
17.5 17.1
9.3 10.9 13.0
3.
16.9
0.04 0.04 0.04 0.04
Ε
0.1
ι
-
11 AI
Ε
0.1 0.1 0.1
ι
0.05mm thick foil oxidized at 1473K
CM
Y
9AI i
Γ
7AI /
ce CD
-
Ρ
2
ο
RESULTS
3-1 Effect of Al and thickness on the lifetime of Fe-Cr-Al foils Figure 1 s h o w s the oxidation behavior of 0.05mm thick Fe-17Cr-7Al, Fe-17Cr-9Al and Fe17Cr-llAl foils. Durations to breakaway oxida tion were prolonged with increasing Al concen tration. Thickening the foil also extended its lifetime as s h o w n in Fig. 2.
5
/L 0
1
1
200
300
O x i d a t i o n t i m e (hr)
Figure 1 Oxidation behavior of 0.05mm thick Al-diffused Fe-Cr-Al foils at 1473K. 8 Ε
3-2 Effect of Al o n parabolic rate constants Figure 3 s h o w s the effect of Al on parabolic rate constants, Kp, for the growth of AI2O3 scale. These data were calculated from oxida tion data at 1423K for 24hr to 96hr. Kp values decreased w i t h increasing Al.
L
100
Fe-17Cr-iW oxidized at 1473Κ
ε 0.05mm
χ: Ο)
©
3-3 Effect of surface condition Figure 4 s h o w s the oxidation behavior of 0.05mm thick Fe-17A1-9A1 foils in the as-rolled and in the as-diffusion-annealed condition. Clearly, the as-diffusion-annealed foil s h o w s better oxidation resistance than the as-rolled foil. Parabolic rate constants for the foils were cal culated from the oxidation data during AI2O3 formation and are g i v e n in Table 2. Kp values of the as-diffusion-annealed foil w a s lower than that of the as-rolled foil.
Table 2 Parabolic rate constants at 1474K for Fe-17Cr7A1 and Fe-17Cr-9Al foils. Alloy Fe-17Cr-7AI
Condition as rolled
as diffusion a n n e a l e d Fe-17Cr-9AI as rolled a s diffusion a n n e a l e d
Kp(g%rrrVs) 4.6 χ 10-12 3.9 χ 10 4.7 χ 10-12 1 2
3.2 χ 10 -
1 2
5 100
200
300
O x i d a t i o n t i m e (hr)
Figure 2 Effect of foil thickness on oxidation behavior at 1473K for Fe-17Cr-llAl alloy. 2.0 ι Ε ο
£r
1.5 -
>v
Fe-17Cr-AI foil 0.05mm thick oxidized at 1423K
CN
C
1.0 -
Q.
0.5
10 Al content
15 (%)
Figure 3 Effect of Al o n parabolic rate constants at 1423K for Al-diffused Fe-Cr-Al foils.
147
O x i d a t i o n t i m e (hr)
Figure 4 Oxidation of as-rolled and as-diffu sion-annealed Fe-17Cr-9Al foils at 1473K. 3-4 Oxide morphology FigureS s h o w s SEM images of fracture cross sections of AI2O3 scales formed on the as-rolled and the as-diffusion-annealed Fe-17Cr-7Al foils. In both foils, equiaxed grains were ob served at the outer surface of the scale and columnar grains at the inner. Columnar grains of the as-rolled foil w e r e larger than those of the as-diffusion annealed foil. AI2O3 scales formed on Fe-17Cr-13Al foil had n o equiaxed grains and significantly larger columnar grains as compared with Fe-17Cr-7Al foil as s h o w n in Fig. 6. 4.
Figure 5 Fracture cross sections of AI2O3 scales formed on (a) as-rolled and (b) as-diffusion-an nealed Fe-17Cr-7Al foils oxidized at 1473K for 24hr.
DISCUSSION
The oxidation process for the Fe-Cr-AI foil pro ceed in three s t a g e s . (7)
1st. stage : the g r o w t h of AI2O3 scale governed by the parabolic rate law. 2nd. stage : the growth of C n 0 3 scale beneath AI2O3 layers according to the li near oxidation law after the ex haustion of Al in the foil. 3rd. stage : breakaway oxidation In the second stage, the linear rate constant is said to be proportional to the parabolic rate con stant in the first s t a g e . Therefore, the parabolic rate constant for the growth of AI2O3 scale is one of the most important factors in de termining oxidation resistance of Fe-Cr-AI foil. (7)
Figurée Fracture cross sections of AI2O3 scales formed on (a) Fe-17Cr-7Al foil and (b) Fe-17Cr13A1 foil oxidized at 1423K for 96hr. The foils were as diffusion-annealed before oxidation.
148 Parabolic r a t e c o n s t a n t s for Al-diffused foils in this s t u d y are g i v e n in Table 3 c o m p a r e d w i t h d a t a in t h e literature for oxidation of c o m m e r cially p r o d u c e d Fe-20Cr-5Al alloy foils and A l - d e p o s i t e d stainless steel f o i l . Kp v a l u e s for t h e Al-diffused foils are l o w e r t h a n t h o s e of Fe-20Cr-5Al foils a n d a g r e e well w i t h t h a t for A l - d e p o s i t e d stainless steel foil. A s s h o w n in Fig. 5 a n d 6, AI2O3 scales w i t h l o w oxide g r o w t h rates h a d coarse columnar grains. By c o m p a r i n g t h e activation energies ob t a i n e d in this s t u d y ( s e e Figure 7) w i t h t h e d a t a of ref.10,11 a n d 12, b o t h Al o u t w a r d diffusion t h r o u g h t h e b u l k oxide a n d o x y g e n i n w a r d dif fusion along grain b o u n d a r i e s could control a AI2O3 scale g r o w t h . C o a r s e 01-AI2O3 g r a i n s h a v e fewer p a t h s for o x y g e n diffusion, leading to a l o w o x i d e g r o w t h rate. It is n o t clear w h a t d e t e r m i n e s the grain size of C1-AI2O3 scales. T h i n film of a l u m i n u m oxide w a s detected b y AES in the as-diffusion-an n e a l e d foil prior t o oxidation test. It is consid ered t h a t this film could p r e v e n t transitional o x i d e f o r m a t i o n at t h e early s t a g e of oxidation a n d p r o m o t e coarse c o l u m n a r a - A b C b s c a l e s . Increasing Al concentration in t h e foil m a y m o d ify the thin film a n d reduce the s u b s e q u e n t oxide g r o w t h rate in air while forming coarse oxide grains. (9)
(10)
4. C O N C L U S I O N S H i g h t e m p e r a t u r e o x i d a t i o n b e h a v i o r of Aldiffused Fe-Cr-Al alloy foils containing u p to 13% Al w a s i n v e s t i g a t e d . ( l ) D u r a t i o n s t o b r e a k a w a y o x i d a t i o n of t h e foil w e r e p r o l o n g e d b y increasing t h e Al concentra tion a n d also b y thickening the foil. (2)The p a r a b o l i c r a t e c o n s t a n t for t h e AI2O3 g r o w t h d e c r e a s e d w i t h increasing Al content. (3)The foil in t h e as-rolled c o n d i t i o n exhibited p o o r e r o x i d a t i o n resistance t h a n t h a t in t h e asdiffusion-annealed condition. ( 4 ) A b 0 3 scales f o r m e d o n t h e foil w i t h a l o w e r g r o w t h r a t e h a d coarser oxide grains, indicat ing t h a t o x y g e n i n w a r d diffusion along grain boundaries were retarded.
Table 3 Parabolic r a t e c o n s t a n t s in air at 1423K. Alloy
Kp(10 - i 2 g 2 / 4 / s e c ) c m
Fe-17Cr-7to13AI:
1.0 to 1.8
Fe-20Cr-5AI(ref.8):
5.2 to 6.2
AI
1.1
10 •11,
ο Ε υ
10 -12
OU 10"
ο
•
17Cr-7AI 1 7 C r - 1 1 Al
activation energy 17Cr-7AI :351kJ/mol 17Cr-11AI:323kJ/mol
10* 6.5
7.0 1 /Temperature
8.0
7.5 (10 /K) 4
Figure 7 A plot of logKp vs. t h e reciprocal of the absolute t e m p e r a t u r e for 0.05mm thick Fe17Cr-7Al a n d F e - 1 7 C r - U A l foils. REFERENCES 1. N . N o n n e n m a n n : SAE p a p e r 850131(1985) 2. T. K a w a s a k i a n d K. Ishii: P r o c e e d i n g s of International Conference o n Stainless Steel, ISIJ, (1991), 1205 3. K. O h m u r a et al: ibid, (1991), 1212 4. N . H i r a m a t s u et al: ibid,(1991),1227 5. Y. I k e g a m i et al: ibid,(1991),1235 6. Y. I k e g a m i et al: N i p p o n Yakin Technical Report, 2(1993), 71 7. K. Ishii a n d K. K a w a s a k i : J. J a p a n Inst. Metals, 56(1992), 854 8. D. R. Sigler: Oxid. Met., 36(1991), 57 9. A. A n d o et al: N i s s h i n Steel Technical Report, 65(1992), 1 10. Y. Oishi a n d W. D. Kingery: J. C h e m . Phys., 33(1960), 480 11. A. E. P a l a d i n o a n d W. D . Kingery: ibid, 37 (1962), 957 12. H . A. Wang a n d F. A. Kroger: J. A m . C e r a m . S o c , 63(1980)613