- Email: [email protected]
High temperature oxidation of Fe-3wt% Cr alloy
Scripta METALLURGICA
Vol. 17, pp. 121-126, 1983 Printed in the U.S.A.
HIGH TEMPERATURE
OXIDATION
Pergamon Press Ltd. All rights reserved
OF Fe-3wt% Cr ALLOY
A.I. Kahveci and G. Welsch Department of Metallurgy and Materials Science Case Institute of Technology Case Western Reserve University Cleveland, Ohio 44106 (Received September (Revised November
24, 1982) 8, 1982)
Introduction Chromium alloying is used to improve the oxidation resistance of iron at high temperatures (i). The oxide scale, which consists of wustite, magnetite and hematite on pure iron (2), is changed by the alloying: Wustite is suppressed, and instead of magnetite and hematite either Cr- containing spinel (Fe, Cr) 304 or sesquioxide (Fe, Cr) 20 3 or even pure Cr20 3 oxide layers may be obtained (3). The formation of the latter and improved oxide scale adherence provide oxidation resistance. Substantial Cr-coneentrations (>20%Cr) are, however, required for good protection (4, 5). The present investigation focusses on the oxide scale formed at 800°C on a Fe-3% Cr alloy and is part of a wider investigation of the effects of chromium on the morphology and crystallographic nature of oxide scales in iron-chromium alloys. A detailed knowledge of these is a prerequisite for gaining an understanding of the mechanisms of oxidation in the Fe-Cr alloys. Experimental Specimen
Preparation
The Fe-3% Cr alloy was prepared by vacuum induction melting of iron and chromium materials of electrolytic purity. Carbon was present at a concentration of less than 0.01 wt.%. Other elements were detected in trace quantities only. The alloy was rolled into sheet, and rectangular strips approximately 50x9x0.5 mm were cut. They were degreased and annealed for 12 hours at II00°C in an argon atmosphere of 2.7 kPa, then cooled to 800°C and quenched. The specimens were abraded with sand paper, degreased with acetone and then electrolytically polished for 5 minutes in a mixture of glacial acetic acid and perchloric acid (volume ratio 20:1) at a current density of 0.i A/em 2. A final one-minute-long cathodic etch a t 10uA/cm 2 was given in 4 normal HCI, and then the specimens were washed in distilled water and rinsed with methanol. Weight
Gain Measurements
The oxidation experiments were started by inserting the specimen into the hot zone of a reaction tube where the temperature was controlled at 800+ 2°C. All oxidation experiments were carried out in dry air. The weight gain-due to oxidation was continuously recorded with a Cahn 1000 electrobalance. To obtain accurate values of oxidation rate constants, the change in the surface area resulting from the consumption of metal by oxidation was taken into account with the following linear equation: A = A ° -~ AW
121 0036-9748/83/010121-06503.00/0 Copyright (c) 1983 Pergamon Press Ltd.
(i)
122
OXIDATION
OF Fe-Cr ALLOY
Vol.
17, No. 1
according to the method by Mrowec and Stoklosa (6). Here AW is the weight gain at time (t), 'A' is the true (instantaneous) surface area, 'AJ is the initial metal surface area, and 'p' is a proportionality factor that ~an be calculated from equation (i) after termination of the oxidation test when the final weight gain and the final surface area of the metallic core are known. Microstructural
Investigations
Cross sections of the oxide scale were prepared by spallation and fracturing of the scale at low temperature. The scale morphology was investigated primarily by scanning electron microscopy. Energy dispersive analyses of X-rays (EDAX) were also performed on these cross sections. In addition, X-ray diffraction line scans were obtained on pieces of spalled oxide using Cr Ks radiation. These X-ray diffraction experiments were carried out once with the inner surface (alloy/oxide interface) and once with the outer surface (oxide/air interface) of spalled oxide particles facing the X-ray beam. In this way, information was gained on the oxide phases present in the inner and in the outer layers of the scale. Results Oxidation
Kinetics
The weight gain versus time curves in Figure i describe the kinetics of the oxidation process. The curves represent tests on different specimens carried out under identical conditions. They indicate that the oxidation process is reasonably well reproducible. The curves
can be described by a rate equation of the form: AW A - ktn
(2)
Here 'AW' represents the weight gain over a surface area 'A' in a time 't' The constants 'k' and 'n' are the rate constant and the rate exponent, respectively. They were computed by fitting the measured curves and equation (2) using the logarithmic form of equation (2) and a least squares analysis method. Values of 'k' and 'n' are given in Figure I. Two oxidation stages are evident in Figure i. In the initial period of oxidation (stage I) the oxidation follows a nearly parabolic rate law (n=0.57). This was followed by a slower rate of oxidation in stage II, where the rate exponent 'n' varied between values of 0.18 and 0.13. The rate exponents indicate that two types of deviations from the ideal parabolic behavior (n = 0.5) occurred during the course of oxidation: positive deviation (n>0.5) in stage I due to rapid oxidation was followed by a negative deviation (n<0.5) in stage II due to an oxidation rate slower than parabolic. The value of the rate exponent 'n' decreased continuously with time, beginning near the end of stage I. The low values of 'n' in stage II signify a considerable departure from parabolic behavior. O__xide Phases and Scale Morphology The X-ray diffraction spectra from spalled oxide flakes given in Figure 2 show diffraction lines which are consistent with the cubic M30~ and the rhombohedral M203 oxide phases. In the spinel phase M stands for Fe and Cr whose concentrations vary over the cross-section of the scale (see Figure 3). In the M203 phase M stands for Fe, with possibly a small amount of Cr (see Figure 3). The strong (0006) peak of Me03 in Figure 2a indicates preferred [0001] growth orientation of the outer two hematite layers in the scale.* Figure 2b was taken with the X-ray beam facing the inner surface of an oxide flake (oxide/alloy interface) and shows a dominance of the spinel peaks. More detailed investigations by transmission electron microscopy and electron diffraction (7, 8), and by energy dispersive X-ray analysis, given below, established the oxide phases as being (Fe, Cr) 304 and ~-Fe203 in that sequence from metal/oxide interface to oxide/air interface. *This was confirmed by electron diffraction in transmission electron microscopy on a thin transverse section of the intermediate (columnar) hematite layer.
Vol.
17, No.
1
_~Jl
OXIDATION
OF Fe-Cr ALLOY
..........................
Stag e [ k:l
123
: ...... : ................
•..'''''"'_ ,....
8
....'" ..
.
+
÷
÷
:'+
~
6
Stoge D 4
3.5~k~4.4 O.18>n~OJ3
.+
4 ÷ ÷ 44I. '+
s'0
,6o
,~0
26o TiME
2So
36o
3~o
460
min.
FIGURE 1 Weight gain of Fe-3wt% Cr alloy oxidized at 800°C in dry air. The represent two experiments run at identical conditions. The weight are characterized by a nearly parabolic growth rate in Stage I. A scale develops early in Stage I. Stage II describes the continued a mature multi-layer scale•
two curves gain curves multi-layer growth of
The c r o s s - s e c t i o n of a mature oxide scale (stage II) is shown in Figure 3 The scale exhibits four layers, d i s t i n g u i s h e d by their m o r p h o l o g i e s and by grain sizes. An inner fine-porous and very f i n e - g r a i n e d spinel layer is followed by a c o a r s e - g r a i n e d and relatively dense M203 layer• About 2wt% Cr was found in this layer, so that this oxide can be labelled a sesquioxide (Fe, 2% Cr)z03 Columns of hematite (~-Fe203) have grown out of the sesquioxide layer. Much void space is apparent in this colur~mar layer. The outermost hematite (~-Fe203) layer is c o a r s e - g r a i n e d and relatively dense. However, we cannot preclude the p o s s i b i l i t y of open channels through this layer. C r - c o n c e n t r a t i o n profiles over the c r o s s - s e c t i o n of the oxide scale were obtained by X-ray energy dispersive analyses A C r - c o n c e n t r a t i o n profile is included in Figure 3. The concentration is highest near the o x i d e / a l l o y interface and exhibits a m i n i m u m and then a peak w i t h i n the (Fe, Cr) 30 ~ spinel layer. This curious c o n c e n t r a t i o n profile was m e a s u r e d several times at different locations in the cross-sections and was r e p r o d u c i b l e in each case. As was m e n t i o n e d before, the intermediate (Fe, Cr)203 layer contained a chromium concentration of 1 to 2%, while the two outer ~-Fe203 layers contained no detectable chromium concentration. Discussion The p a r a b o l i c o x i d a t i o n o b s e r v e d in stage I is a well known phenomenon, and has first been e x p l a i n e d by C. W a g n e r (9) as being due to diffusion controlled growth. The m e c h a n i s m is one of either cations diffusing outwards or anions diffusing inwards through the scale.
124
OXIDATION OF F e - C r ALLOY
Vol.
H203(0006 )
17,
No. 1
~"
M304(3I I ) ,L J _
J.
M30~(~O)
M30~(3I1: M304(220)
Dfffr.c~n A n g l e ~ FIGURE 2 X-ray diffractometer profiles taken with Cr-K radiation on flakes of the oxide scale. (a) Profile obtained with the X-ray beam facing the outer surface (oxide/air) Of the scale. The strong (0006) peak of M203 is due to preferred orientation [0001] of the hematite in the outer two layers of the scale. Only a weak spinel peak is observed due to shielding by the M203 layers. (b)
Profile obtained with the ~ray beam facing the inner surface (alloy/oxide) of the scale. The spinel reflections are most prominent.
The e x p e r i m e n t a l results indicate four oxide layers of different microstructure and varying composition. They imply c o m p l i c a t e d oxidation diffusion mechanisms. A q u a l i t a t i v e description of the diffusion mechanisms is a t t e m p t e d here: Fe-cations diffuse outwards through each of the oxide layers and permit the growth of the outer hematite layers. By comparison, Cr diffuses rather slowly through the inner spinel layer (i0) leading to the C r - e n r i c h m e n t near the oxide/alloy interface. Oxygen anions diffuse inwards through each layer and cause the p r e f e r e n t i a l o x i d a t i o n of Cr at the alloy/oxide interface as well as the growth of the intermediate sesquioxide and the columnar hematite layers. Thus, o x i d a t i o n takes place in the outer as well as in the inner layers of the oxide scale. If one assumes that cation diffusion (Fe and Cr) through the spinel layer is o x i d a t i o n rate controlling, then the change from a parabolic rate law in stage I to a less than parabolic rate law in stage II can be qualitatively rationalized. C o m p a r e d to other diffusion mechanisms, the diffusion of Cr through the spinel layer (i0, ii) is rather slow. This leads to the o b s e r v e d C r - e n r i c h m e n t in the spinel layer at the interface with the alloy. As a consequence of this C r - e n r i c h m e n t the iron diffusivity is also reduced (II), thus lowering the overall oxidation rate. A n o t h e r possible cause for the change in the rate law may be due to a reduction of the number of fast diffusion paths (grain boundaries). This in analogous to the m e c h a n i s m p r o p o s e d by A t k i n s o n et al. (12) for the o x i d a t i o n of nickel. More detailed investigations
Vol.
17, No.
i
OXIDATION
OF Fe-Cr ALLOY
125
Air
\ /
o Fe-3 ~VoCr Aitoy
6
~
Cr Concentration(Wt%) (Excluding oxygen)
FIGURE 3 Horphology and microstructure in a mature multi-layer oxide scale. Also shown is the variation of chromium incorporated into the o×ide. of the oxide on the Fe-3% Cr alloy showed that the outer hematite layer has a very small grain size in stage I and grows to a significantly larger grain size during the latter portion of stage I (13). The resulting decrease of the number of fast diffusion paths can reduce the rate of inward diffusion of oxygen. Both mechanisms, the reduced outward diffusion of iron through the Crenriched spinel layer, and the reduced inward diffusion of oxygen through the outer hematite layer, may serve as explanation for the change in oxidation rate law from stage I to stage II. Summary Oxidation of Fe-3% Cr alloy at 800°C in dry air occurs in two stages. Stage I a p p r o x i m a t e l y follows a parabolic rate law, stage II follows a rate law which is less than parabolic. The mature oxide scale consists of four layers: (Fe, Cr) 304 spinel, (Fe, Cr) 203 sesquioxide, and two ~-Fe203 hematite layers. The layers are d i s t i n g u i s h e d by their chemical compositions and by their microstructures. A qualitative description of the diffusion mechanisms is given. Acknowledgements This research was supported by the National Science Foundation. authors (A.I.K.) acknowledges support from the Government of Turkey. w o u l d like to thank Professor T.E. Mitchell for helpful discussions.
One of the The authors
References (i)
(2)
ASM, Oxidation of Metals and Alloys, p. 190, ASM, Ohio (1971). D.A. Voss, M.S. Thesis, 1979, Department of M e t a l l u r g y and Materials Science, Case W e s t e r n Reserve University.
126
(3) (4) (5) (6) (7) (8) (9) (I0) (Ii) (12) (13)
OXIDATION OF Fe-Cr ALLOY
Vol. 17, No. i
D. Mortimer and W.B.A. Sharp, Brit. Corros. Jo, 61 (1968). C.S. Tedmon, Jr., J. Electrochem. Soc., 114, 788 (1967). D°P. Whittle and G.C. Wood, J. Electrochem. Soc., 115, 133 (1968). S. Mrowec and A. Stoklosa, Oxid. of Metals, 8, 379 (1974). P.A. Labun, Ph.D. Thesis, 1982, Department of Metallurgy and Materials Science, Case Western Reserve University. P.A. Labun, J. Covington, K. Kuroda, G.E. Welsch, and T.E. Mitchell, Metall. Transactions, 1982, in press. C. Wagner, J. Electrochem. Soc., 99, 369 (1952). M.G.C. Cox, B. McEnaney, and V.D. Scott, Philos. Mag. 26, 839 (1972). D.P. Whittle, and G.C. Wood, J. Electrochem. Soc., 114, 986 (1967). A. Atkinson, R.I. Taylor and A.E. Hughes, Phil. Mag. A, 45, 823 (1982). A.I. Kahveci, M.S. Thesis, 1982, Department of Metallurgy and Materials Science, Case Western Reserve University.
Vol. 17, pp. 121-126, 1983 Printed in the U.S.A.
HIGH TEMPERATURE
OXIDATION
Pergamon Press Ltd. All rights reserved
OF Fe-3wt% Cr ALLOY
A.I. Kahveci and G. Welsch Department of Metallurgy and Materials Science Case Institute of Technology Case Western Reserve University Cleveland, Ohio 44106 (Received September (Revised November
24, 1982) 8, 1982)
Introduction Chromium alloying is used to improve the oxidation resistance of iron at high temperatures (i). The oxide scale, which consists of wustite, magnetite and hematite on pure iron (2), is changed by the alloying: Wustite is suppressed, and instead of magnetite and hematite either Cr- containing spinel (Fe, Cr) 304 or sesquioxide (Fe, Cr) 20 3 or even pure Cr20 3 oxide layers may be obtained (3). The formation of the latter and improved oxide scale adherence provide oxidation resistance. Substantial Cr-coneentrations (>20%Cr) are, however, required for good protection (4, 5). The present investigation focusses on the oxide scale formed at 800°C on a Fe-3% Cr alloy and is part of a wider investigation of the effects of chromium on the morphology and crystallographic nature of oxide scales in iron-chromium alloys. A detailed knowledge of these is a prerequisite for gaining an understanding of the mechanisms of oxidation in the Fe-Cr alloys. Experimental Specimen
Preparation
The Fe-3% Cr alloy was prepared by vacuum induction melting of iron and chromium materials of electrolytic purity. Carbon was present at a concentration of less than 0.01 wt.%. Other elements were detected in trace quantities only. The alloy was rolled into sheet, and rectangular strips approximately 50x9x0.5 mm were cut. They were degreased and annealed for 12 hours at II00°C in an argon atmosphere of 2.7 kPa, then cooled to 800°C and quenched. The specimens were abraded with sand paper, degreased with acetone and then electrolytically polished for 5 minutes in a mixture of glacial acetic acid and perchloric acid (volume ratio 20:1) at a current density of 0.i A/em 2. A final one-minute-long cathodic etch a t 10uA/cm 2 was given in 4 normal HCI, and then the specimens were washed in distilled water and rinsed with methanol. Weight
Gain Measurements
The oxidation experiments were started by inserting the specimen into the hot zone of a reaction tube where the temperature was controlled at 800+ 2°C. All oxidation experiments were carried out in dry air. The weight gain-due to oxidation was continuously recorded with a Cahn 1000 electrobalance. To obtain accurate values of oxidation rate constants, the change in the surface area resulting from the consumption of metal by oxidation was taken into account with the following linear equation: A = A ° -~ AW
121 0036-9748/83/010121-06503.00/0 Copyright (c) 1983 Pergamon Press Ltd.
(i)
122
OXIDATION
OF Fe-Cr ALLOY
Vol.
17, No. 1
according to the method by Mrowec and Stoklosa (6). Here AW is the weight gain at time (t), 'A' is the true (instantaneous) surface area, 'AJ is the initial metal surface area, and 'p' is a proportionality factor that ~an be calculated from equation (i) after termination of the oxidation test when the final weight gain and the final surface area of the metallic core are known. Microstructural
Investigations
Cross sections of the oxide scale were prepared by spallation and fracturing of the scale at low temperature. The scale morphology was investigated primarily by scanning electron microscopy. Energy dispersive analyses of X-rays (EDAX) were also performed on these cross sections. In addition, X-ray diffraction line scans were obtained on pieces of spalled oxide using Cr Ks radiation. These X-ray diffraction experiments were carried out once with the inner surface (alloy/oxide interface) and once with the outer surface (oxide/air interface) of spalled oxide particles facing the X-ray beam. In this way, information was gained on the oxide phases present in the inner and in the outer layers of the scale. Results Oxidation
Kinetics
The weight gain versus time curves in Figure i describe the kinetics of the oxidation process. The curves represent tests on different specimens carried out under identical conditions. They indicate that the oxidation process is reasonably well reproducible. The curves
can be described by a rate equation of the form: AW A - ktn
(2)
Here 'AW' represents the weight gain over a surface area 'A' in a time 't' The constants 'k' and 'n' are the rate constant and the rate exponent, respectively. They were computed by fitting the measured curves and equation (2) using the logarithmic form of equation (2) and a least squares analysis method. Values of 'k' and 'n' are given in Figure I. Two oxidation stages are evident in Figure i. In the initial period of oxidation (stage I) the oxidation follows a nearly parabolic rate law (n=0.57). This was followed by a slower rate of oxidation in stage II, where the rate exponent 'n' varied between values of 0.18 and 0.13. The rate exponents indicate that two types of deviations from the ideal parabolic behavior (n = 0.5) occurred during the course of oxidation: positive deviation (n>0.5) in stage I due to rapid oxidation was followed by a negative deviation (n<0.5) in stage II due to an oxidation rate slower than parabolic. The value of the rate exponent 'n' decreased continuously with time, beginning near the end of stage I. The low values of 'n' in stage II signify a considerable departure from parabolic behavior. O__xide Phases and Scale Morphology The X-ray diffraction spectra from spalled oxide flakes given in Figure 2 show diffraction lines which are consistent with the cubic M30~ and the rhombohedral M203 oxide phases. In the spinel phase M stands for Fe and Cr whose concentrations vary over the cross-section of the scale (see Figure 3). In the M203 phase M stands for Fe, with possibly a small amount of Cr (see Figure 3). The strong (0006) peak of Me03 in Figure 2a indicates preferred [0001] growth orientation of the outer two hematite layers in the scale.* Figure 2b was taken with the X-ray beam facing the inner surface of an oxide flake (oxide/alloy interface) and shows a dominance of the spinel peaks. More detailed investigations by transmission electron microscopy and electron diffraction (7, 8), and by energy dispersive X-ray analysis, given below, established the oxide phases as being (Fe, Cr) 304 and ~-Fe203 in that sequence from metal/oxide interface to oxide/air interface. *This was confirmed by electron diffraction in transmission electron microscopy on a thin transverse section of the intermediate (columnar) hematite layer.
Vol.
17, No.
1
_~Jl
OXIDATION
OF Fe-Cr ALLOY
..........................
Stag e [ k:l
123
: ...... : ................
•..'''''"'_ ,....
8
....'" ..
.
+
÷
÷
:'+
~
6
Stoge D 4
3.5~k~4.4 O.18>n~OJ3
.+
4 ÷ ÷ 44I. '+
s'0
,6o
,~0
26o TiME
2So
36o
3~o
460
min.
FIGURE 1 Weight gain of Fe-3wt% Cr alloy oxidized at 800°C in dry air. The represent two experiments run at identical conditions. The weight are characterized by a nearly parabolic growth rate in Stage I. A scale develops early in Stage I. Stage II describes the continued a mature multi-layer scale•
two curves gain curves multi-layer growth of
The c r o s s - s e c t i o n of a mature oxide scale (stage II) is shown in Figure 3 The scale exhibits four layers, d i s t i n g u i s h e d by their m o r p h o l o g i e s and by grain sizes. An inner fine-porous and very f i n e - g r a i n e d spinel layer is followed by a c o a r s e - g r a i n e d and relatively dense M203 layer• About 2wt% Cr was found in this layer, so that this oxide can be labelled a sesquioxide (Fe, 2% Cr)z03 Columns of hematite (~-Fe203) have grown out of the sesquioxide layer. Much void space is apparent in this colur~mar layer. The outermost hematite (~-Fe203) layer is c o a r s e - g r a i n e d and relatively dense. However, we cannot preclude the p o s s i b i l i t y of open channels through this layer. C r - c o n c e n t r a t i o n profiles over the c r o s s - s e c t i o n of the oxide scale were obtained by X-ray energy dispersive analyses A C r - c o n c e n t r a t i o n profile is included in Figure 3. The concentration is highest near the o x i d e / a l l o y interface and exhibits a m i n i m u m and then a peak w i t h i n the (Fe, Cr) 30 ~ spinel layer. This curious c o n c e n t r a t i o n profile was m e a s u r e d several times at different locations in the cross-sections and was r e p r o d u c i b l e in each case. As was m e n t i o n e d before, the intermediate (Fe, Cr)203 layer contained a chromium concentration of 1 to 2%, while the two outer ~-Fe203 layers contained no detectable chromium concentration. Discussion The p a r a b o l i c o x i d a t i o n o b s e r v e d in stage I is a well known phenomenon, and has first been e x p l a i n e d by C. W a g n e r (9) as being due to diffusion controlled growth. The m e c h a n i s m is one of either cations diffusing outwards or anions diffusing inwards through the scale.
124
OXIDATION OF F e - C r ALLOY
Vol.
H203(0006 )
17,
No. 1
~"
M304(3I I ) ,L J _
J.
M30~(~O)
M30~(3I1: M304(220)
Dfffr.c~n A n g l e ~ FIGURE 2 X-ray diffractometer profiles taken with Cr-K radiation on flakes of the oxide scale. (a) Profile obtained with the X-ray beam facing the outer surface (oxide/air) Of the scale. The strong (0006) peak of M203 is due to preferred orientation [0001] of the hematite in the outer two layers of the scale. Only a weak spinel peak is observed due to shielding by the M203 layers. (b)
Profile obtained with the ~ray beam facing the inner surface (alloy/oxide) of the scale. The spinel reflections are most prominent.
The e x p e r i m e n t a l results indicate four oxide layers of different microstructure and varying composition. They imply c o m p l i c a t e d oxidation diffusion mechanisms. A q u a l i t a t i v e description of the diffusion mechanisms is a t t e m p t e d here: Fe-cations diffuse outwards through each of the oxide layers and permit the growth of the outer hematite layers. By comparison, Cr diffuses rather slowly through the inner spinel layer (i0) leading to the C r - e n r i c h m e n t near the oxide/alloy interface. Oxygen anions diffuse inwards through each layer and cause the p r e f e r e n t i a l o x i d a t i o n of Cr at the alloy/oxide interface as well as the growth of the intermediate sesquioxide and the columnar hematite layers. Thus, o x i d a t i o n takes place in the outer as well as in the inner layers of the oxide scale. If one assumes that cation diffusion (Fe and Cr) through the spinel layer is o x i d a t i o n rate controlling, then the change from a parabolic rate law in stage I to a less than parabolic rate law in stage II can be qualitatively rationalized. C o m p a r e d to other diffusion mechanisms, the diffusion of Cr through the spinel layer (i0, ii) is rather slow. This leads to the o b s e r v e d C r - e n r i c h m e n t in the spinel layer at the interface with the alloy. As a consequence of this C r - e n r i c h m e n t the iron diffusivity is also reduced (II), thus lowering the overall oxidation rate. A n o t h e r possible cause for the change in the rate law may be due to a reduction of the number of fast diffusion paths (grain boundaries). This in analogous to the m e c h a n i s m p r o p o s e d by A t k i n s o n et al. (12) for the o x i d a t i o n of nickel. More detailed investigations
Vol.
17, No.
i
OXIDATION
OF Fe-Cr ALLOY
125
Air
\ /
o Fe-3 ~VoCr Aitoy
6
~
Cr Concentration(Wt%) (Excluding oxygen)
FIGURE 3 Horphology and microstructure in a mature multi-layer oxide scale. Also shown is the variation of chromium incorporated into the o×ide. of the oxide on the Fe-3% Cr alloy showed that the outer hematite layer has a very small grain size in stage I and grows to a significantly larger grain size during the latter portion of stage I (13). The resulting decrease of the number of fast diffusion paths can reduce the rate of inward diffusion of oxygen. Both mechanisms, the reduced outward diffusion of iron through the Crenriched spinel layer, and the reduced inward diffusion of oxygen through the outer hematite layer, may serve as explanation for the change in oxidation rate law from stage I to stage II. Summary Oxidation of Fe-3% Cr alloy at 800°C in dry air occurs in two stages. Stage I a p p r o x i m a t e l y follows a parabolic rate law, stage II follows a rate law which is less than parabolic. The mature oxide scale consists of four layers: (Fe, Cr) 304 spinel, (Fe, Cr) 203 sesquioxide, and two ~-Fe203 hematite layers. The layers are d i s t i n g u i s h e d by their chemical compositions and by their microstructures. A qualitative description of the diffusion mechanisms is given. Acknowledgements This research was supported by the National Science Foundation. authors (A.I.K.) acknowledges support from the Government of Turkey. w o u l d like to thank Professor T.E. Mitchell for helpful discussions.
One of the The authors
References (i)
(2)
ASM, Oxidation of Metals and Alloys, p. 190, ASM, Ohio (1971). D.A. Voss, M.S. Thesis, 1979, Department of M e t a l l u r g y and Materials Science, Case W e s t e r n Reserve University.
126
(3) (4) (5) (6) (7) (8) (9) (I0) (Ii) (12) (13)
OXIDATION OF Fe-Cr ALLOY
Vol. 17, No. i
D. Mortimer and W.B.A. Sharp, Brit. Corros. Jo, 61 (1968). C.S. Tedmon, Jr., J. Electrochem. Soc., 114, 788 (1967). D°P. Whittle and G.C. Wood, J. Electrochem. Soc., 115, 133 (1968). S. Mrowec and A. Stoklosa, Oxid. of Metals, 8, 379 (1974). P.A. Labun, Ph.D. Thesis, 1982, Department of Metallurgy and Materials Science, Case Western Reserve University. P.A. Labun, J. Covington, K. Kuroda, G.E. Welsch, and T.E. Mitchell, Metall. Transactions, 1982, in press. C. Wagner, J. Electrochem. Soc., 99, 369 (1952). M.G.C. Cox, B. McEnaney, and V.D. Scott, Philos. Mag. 26, 839 (1972). D.P. Whittle, and G.C. Wood, J. Electrochem. Soc., 114, 986 (1967). A. Atkinson, R.I. Taylor and A.E. Hughes, Phil. Mag. A, 45, 823 (1982). A.I. Kahveci, M.S. Thesis, 1982, Department of Metallurgy and Materials Science, Case Western Reserve University.