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Rare earths oxide film effect on different metal and alloys at high temperature in oxidizing conditions
Advanced Materials '93, 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.
107
Rare earths oxide film effect on different metal and alloys at high temperature in oxidizing conditions. J.C.Colson, H.Buscail, G.Bonnet, RSotto, J.P.Larpin. Laboratoire de Recherches sur la Réactivité des Solides Fac. Sci. Mirande, BP 138, F.21004 Dijon Cedex Abstract : Different m e t h o d s for rare e a r t h s d e p o s i t i o n o n the s u r f a c e of iron a n d A I S I 3 0 4 , F 1 7 T i s t a i n l e s s s t e e l s l a n d s t e e l s a r e p r o p o s e d . T h e c o a t e d or n o n c o a t e d s a m p l e s in c o r r o s i o n t e s t s u n d e r i s o t h e r m a l c o n d i t i o n s r e v e a l e d a n i d e n t i c a l o x i d a t i o n p r o c e s s but t h e c o r r o s i o n r a t e c o n s t a n t s d e c r e a s e d s i g n i f i c a n t l y for t h e c o a t e d s a m p l e s . In t h e r m a l c y c l i c c o n d i t i o n s , p r o t e c t i v e s c a l e spallation c o m p l e t e l y d i s a p e a r e d for c o a t e d s a m p l e s . It w a s s h o w n that t h e rare e a r t h effect is m o r e e f f e c t i v e w i t h e l e m e n t s l o c a t e d o n t h e left in t h e l a n t h a n i d e period of t h e Periodic T a b l e . D i f f é r e n t s e x p l a n a t i o n s for t h e s e results p r o p o s e d . 1.
INTRODUCTION
oxygen cycling reaction at r o o m
T h e b e n e f i c i a l e f f e c t of r a r e e a r t h o n h i g h t e m p e r a t u r e o x i d a t i o n r a t e of s t e e l a n d p a r t i c u l a r l y o n p r o t e c t i v e s c a l e a d h e r e n c e , is now well e s t a b l i s h e d , as well as by introduction as a minor e l e m e n t (1-3), by i m p l a n t a t i o n (4) or by c h e m i c a l p r o c e s s e s (510). C h e m i c a l p r o c e s s e s (sol/gel and/or electrophoresis, Organometallic Chemical V a p o r D e p o s i t i o n m e t h o d s (11)) to realize rare e a r t h o x i d e c o a t i n g s , w h i c h are e a s i e r s e e m to b e a p r o m i s i n g w a y . M e c h a n i s m s are not yet well understood (1-16).
R a r e e a r t h o x i d e w e r e d e p o s i t e d by v a r i o u s processes : I - P o l y m e r i c c o a t i n g s b y d i p p i n g s a m p l e s in a c o l l o ï d a l s o l u t i o n of c e r i a in a n a q u e o u s solution of p o l y a c r y l a m i d . II - A l c o h o l i c c o a t i n g s by d i p p i n g s a m p l e s in an alcoholic solution of a m m o n i u m c e r i u m nitrate. III - E l e c t r o p h o r e t i c c o a t i n g s r e a l i z e d w i t h a very fine g r a i n e d c e r i u m h y d r o x i d e sol d i s p e r s e d in water or in o r g a n i c s o l v a n t ( D M F ) . IV - M O C V D p r o c e s s w h i c h led us to p e r f o r m a s y s t e m a t i c s t u d y of t h e r a r e e a r t h effect w i t h practically all t h e rare e a r t h e l e m e n t s (17).
In this p a p e r , t h e e f f e c t of rare e a r t h o x i d e deposited by various chemical processes on iron, A I S I 3 0 4 , F 1 7 T i s t a i n l e s s s t e e l s a n d FeMn-AI alloy is s t u d i e d . Fe Electrolytic F e
C
Cr
for all < 0 , 1
at 2 x 1 0 ^ P a , u n d e r i s o t h e r m a l or c o n d i t i o n s ( o n e c y c l e = 2 h r s at t h e t e m p e r a t u r e : 9 5 0 ° C, f o l l o w e d by 1 hr temperature).
W h a t e v e r t h e p r o c e s s t o p r o d u c e t h e rare Ni
Mn
AI
Mo
Cu
e x c e p t i n g for H - N a , K , C a , M o , M n , C o , N i , C u < 1
Si
μg/g
AISI 304
bal
0,042
17,74
8,8
1,4
0,16
0,17
0,48
F17Ti
bal
0,015
17,19
0,12
0,39
0,06
0,07
0,4
Fe-Mn-AI
bal
0,28
0,018
0,01
32
6,6
Fe-Mn-AI-0,5Si
bal
0,27
0,018
0,01
30
6,2
0,55
Fe-Mn-AI-0,8Si
bal
0,29
0,018
0,01
31
6,8
0,8
High
temperature
oxidation
tests
were
p e r f o r m e d in a m i c r o t h e r m o b a l a n c e e i t h e r in air u n d e r a t m o s p h e r i c p r e s s u r e or in pure
earth oxide film, the coated s p e c i m e n s were reheated
following
a
linear
temperature
increase up to 8 0 0 ° C , remaining one
hour,
108 and
slowly
cooled
down
to
the
Isothermal
room
oxidation
kinetics
exhibited
linear m a s s - g a i n c u r v e s ( F i g . 2 ) . T h e c o m p a c t
temperature. The coating characterisation
have
o x i d e s c a l e is c o m p o s e d m a i n l y w u s t i t e . O x i d e
been
grain
r e a l i s e d b y X r a y diffraction a n d S.I.M.S.
growth
occurs
with
a
preferential
orientation. T h e o x i d a t i o n m e c h a n i s m is similar to
that
proposed
by
P. K O F S T A D
who
e m p h a s i z e s a c o n s t a n t n u m b e r of g r o w t h sites d e t e r m i n e d b y t h e t e x t u r e of t h e o x i d e layer. W i t h c o a t e d p u r e iron t h e l i n e a r o x i d a t i o n kinetics a r e c o n s e r v e d for t h e c o a t e d s a m p l e s , but t h e rate c o n s t a n t i n c r e a s e s . T h e o x i d a t i o n m e c h a n i s m is also t h e s a m e , but c e r i u m o x i d e o n t h e s u r f a c e of t h e m e t a l Τ ι me 3 2
O X Y G E N E
p r o b a b l y i n c r e a s e s t h e n u m b e r of n u c l e a t i o n
?COÛ [sec ] N d 2 Ô S ' / 3 0 < 2 h 1 0 0 8
B 0 N E T D ? 3 P f *
P o s i t i v e J5:2I:29
2 2 F e b 19S3
s i t e s a n d , c o n s e q u e n t l y , t h e c o r r o s i o n rate. M o r e o v e r in all c a s e s ( c o a t e d a n d u n c o a t e d
Figure 1 : Concentration profiles at the surface vicinity of N d 2 0 3 coated AISI 304 (5 Minutes
during cooling.
coated and heating 1 h at 800°C and 2h in oxidation conditions).
w a s o b s e r v e d iron o x i d a t i o n b e h a v i o r .
iron)
w e never
observed
scale
spallation
In c o n c l u s i o n , n o p o s i t i v e rare e a r t h effect That
d o e s not m e a n h o w e v e r that t h e r a r e e a r t h Concentration
profiles
show
(Fig. 1 ) a
e l e m e n t d o e s n o t play a role in t h e o x i d a t i o n
mixing with natural oxide produced on the
mechanism.
s a m p l e d u r i n g h e a t i n g . S u p e r f i c i a l rare e a r t h
2.2.AISI 304 stainless steel
c o n c e n t r a t i o n is high a n d d e c r e a s e s quickly.
a
2. HIGH TEMPERATURE TESTS 2.1.Pure Iron oxidation
p a r a b o l i c m a s s - g a i n c u r v e s ( F i g . 3 ) . T h e rate
-
Isothermal
OXIDATION
oxidation
Am/S=f(Vt) were respectively kp=5,1.10'
iron. This s y s t e m have been studied ( 2 1 0 "
1 2
g cm"V 2
1
2
3 < P Q < 2 1 0 " ) t o c o n s t i t u t e a r e f e r e n c e with a 4
2
Ε ο
pure metal.
exhibited
1,8.10"
1 2
C
5
I I I I I 1 I 1 > 1 1
Uncoated AISI 304 al
1000'C
Uncoated AISI 304 at
950°C
6 I I 1 1 i I Π
7 I i i,
en
Μ- ι ·)
Pur Iron Iron Coated by Ce 0
1.2 1 0.8
0.8 2
0.8
1 000°C y
0.6 Η
and
at 9 5 0 a n d 1 0 0 0 C . 4
I I I I I I I 1 1 1
1.2
m/s
ε
kinetics
c o n s t a n t s k p m e a s u r e d f r o m t h e linear plots
Firstly w e h a v e s t u d i e d t h e b e h a v i o r of p u r e
I
oxidation
Uncoatied AISI 3 0 4 stainless steel :
0.6
950°C
//
0.4
1OO0°C
0.2
' " ' 950°C
*, ".
.—
ι
1
1
1
—ι
1
1—
ι
10
0
·
12
F i g u r e 2 : Pure Iron uncoated and coated by C e 0 (isothermal oxidation). 2
Figure 3 : AISI 304 isothermal oxidation in air. t(h)
Am/S= f(t) and Am/S = f (VtJ. T h e m o r p h o l o g y a n d c o m p o s i t i o n of t h e oxide
scale
grown
were
determined
as
109 f u n c t i o n of t h e c o r r o s i o n t e s t t i m e . T h e o x i d e w a s initially c o m p o s e d of F e C r 2 0 , t h e n a n
(kp=8,1.10~ and 1 , 8 . 1 0 " g c m " V at 950 and 1000°C respectively). T h e protective o x i d e s c a l e is s t r o n g l y a d h e r e n t t o t h e m e t a l substrate. F r o m a m o r p h o l o g i c a l p o i n t of v i e w o x i d e s c a l e g r o w n w a s practically t h e s a m e a s t o t h e s c a l e o b s e r v e d o n t h e u n c o a t e d alloy. X-ray measurements indicated the following phase : MnFe 0 , Cr 0 but w e also found a 1 3
4
internal c h r o m i a subscale f o r m e d , which w a s c r o v e r e d b y a n e x t e r n a l s c a l e c o m p r i s e d of two spinels : M n C r 0 and F e C r 0 2
4
2
4
containing ( F e C r ^ C ^ nodules. T h e internal g r o w t h of s i l i c a w a s a l s o o b s e r v e d a l o n g t h e alloy grain b o u n d a r i e s .
2
U n d e r t h e r m a l c y c l i n g c o n d i t i o n s (Fig 4)
4
complex
scale spallation a p p e a r e d after 5 or 7 cycles.
1 2
2
oxide
with
a structure 2 1
Λ 1 I I I II II 1 1 1,1 Uncoated AJSI 304 steel
0.6
'Ε
AISI 304 Coated by C e Q
ro.48-^
3 8
close
to
(19).
0.6
Under thermal cycling conditions (Fig 4), t h e s c a l e spallation c o m p l e t e l y d i s a p p e a r e d .
•0.48
The M O C V D m e t h o d lead us to prepare c o a t i n g s of p r a c t i c a l l y all t h e r a r e e a r t h e l e m e n t s . A s s h o w n in F i g 5 , t h e r a r e e a r t h effect is m o r e i m p o r t a n t w i t h e l e m e n t s l o c a t e d o n t h e left in t h e l a n t h a n i d e p e r i o d of t h e Periodic Table.
2
I hO.36 0.360.240.12-
0.12
τ-ρττ
1
3
((Ca,Ce)(Ti,Fe,Cr,Mg)) 0 _
2
Weigth gam lor uncoated AISI 3 0 4 steel
Λιη/s
τ-ρττ 20'
10 20
30
40
60
40
cycles
80
hours
Figure 4 : Thermal cycling oxidation of coated (by C e 0 ) and uncoated samples in air at 950°C.
Η
0.8
h
0.6
I — r
2
0.4
0.4
P r o t e c t i v e o x i d e s c a l e g r o w t h w a s b y cation d i f f u s i o n ( 1 2 , 1 4 , 1 8 , 1 9 ) . T h e c o r r o s i o n rate is m o n i t o r e d b y a d i f f u s i o n p r o c e s s in t h e chromia subscale . Under thermal cycling conditions, thermal s t r e s s e s resulted in s c a l e spallation after 5 or 7 cycles. N e w oxides exhibited a high growth rate. T h e s p a l l i n g p r o c e s s is m a i n l y linked t o t h e a c c u m u l a t i o n of t h e o x i d e g r o w t h s t r a i n c o n n e c t e d w i t h t h e e x p a n s i o n of t h e o x i d e , crystalline d i s t o r t i o n at t h e interfacial z o n e a n d void a c c u m u l a t i o n .
b
- Coated
oxidation
AISI
(Ceria
304 stainless coatings
d i p p i n g or e l e c t r o p h o r e s i s ) : revealed
that
the
processes
are always
steel
produced
by
Corrosion tests
diffusion dominant
oxidation for ceria
U Ce
-ι 56
1
P r Nd r- " Ί
58
60
Sm
Eu
I
I
62
64
Dy
Er
Yb "Τ
66
68
70
72 (Ζ)
Figure 5: Weight gain of coated AISI 3 0 4 steel after 50h oxidation in air at 1000°C.
2.3.F17TÎ a
-
stainless
Uncoated
steel
F17TÎ
oxidation
stainless
steel
oxidation : I s o t h e r m a l o x i d a t i o n k i n e t i c s exhibited parabolic m a s s gain curves with t w o p a r a b o l i c p a r t s ( p a r a b o l i c rate c o n s t a n t s w e r e m e a s u r e d f r o m t h e linear plots A m / S = f ( V t ) ) . C o r r o s i o n p r o d u c t s initially g r e w at t h e b a s e of t h e s u b s t r a t e g r a i n b o u n d a r i e s , t h e n , f o r the next t w o hours, an external M n C r 0 2
4
spinel s u b s c a l e c o n t a i n i n g iron a n d a n internal
coated samples and for the blank ones,and
subscale containing chromia, titanium were
t h a t t h e p a r a b o l i c rate c o n s t a n t s d e c r e a s e d
observed.
110 After a 4hr run, T i 0
2
and M n C r 0 2
4
crystallites
were observed on the scale surface. U n d e r t h e r m a l cycling conditions,scale spalling a p p e a r e d generally after 3 - 5 cycles', d e p e n d i n g u p o n t h e initial m e t a l s u r f a c e roughness.
b
-
Coated
oxidation
F17Ti
(.Ceria
stainless
coatings
steel
produced
by
dipping or electrophoresis) : the ceria coated
Thin M O C V D rare earth oxide films has also have a similar effect on corrosion behaviour both under cyclic or isothermal conditions.
2.4. Fe-Mn-AI alloy oxidation a - Uncoated Fe-Mn-AI alloys: the F e Mn-AI weight-gain curve s h o w s a relatively high oxidation rate (Fig.7). For alloys Fe-Mn-AI0,5Si and F e - M n - A I - 0 , 8 S i , t h e experimental weight-gain decreases significantly (Fig.7).
sample isothermal corrosion tests revealed an identical
oxidation
process with only o n e
FeMnAI
parabolic part in t h e weigth gain curve and parabolic rate constants decreased. T h e p r o t e c t i v e oxide s c a l e adherent to the metal substrate.
is strongly
T E M observations shown that the external sublayer w a s m a d e of M n C r 2 0 4 containing iron and the internal oxide contained only very fine grains of chromia. O n the surface of the oxide scale grown, large C e 0 particles were
FeMnAI0.5Si FeMnAIO.eSi
2
1 I ιιιιjτ t1 ιj 1 1 ι1 j ιι1 1 ;1 1 « « I » • 10 20 30 40 50 60 70 80
observed. In all cases, ceria was localised more in t h e external part of the oxide scale, and X ray m e a s u r e m e n t s
indicated t h e following
p h a s e composition
: MnFe 0 ,
Cr 0 2
3
2
4
Ti0
2
and
and a complex oxide with a structure
close to ( C a , C e ) ( T i , F e , C r , M g ) 0 2 1
3 8
Figure 7: Oxidation kinetics of Fe-Mn-AI, Fe-MnAI-0,5Si, Fe-Mn-AI-0,8Si alloys P = 2 0 0 0 0 P a , Τ = o2
850°C.
(17).
U n d e r t h e r m a l cycling conditions,(Fig 6) scale spallation completely disappeared.
From the slopes of the plot of the specific weight-gain versus the square root of time, the following p a r a b o l i c r a t e c o n s t a n t s w e r e determined :
^(Fe-Mn-AI-O.OSSi)
2
4
1
=1,15 l O ' ^ g ^ m ' V
1
p( Fe-Mn-AI- 0 , 5 S i ) 1,5 10" g cm' s" . O b s e r v a t i o n of t h e oxide s c a l e c r o s s section and the external surface of the F e - M n AI alloy revealed that the oxide layer w a s non adherent to t h e s u b s t r a t e , a n d spallation occured during cooling. T h e w h o l e scale consisted of several subscales containing iron and m a n g a n e s e . T h e different p h a s e s were identified is a - M n 0 , M n F e 0 and a small 11
2
4
1
:
2
F i g u r e 6 : Thermal cycling oxidation of F17Ti coated by C e 0 and uncoated samples in air at 2
950°C.
3
2
4
amount of M n O . T h e oxide scales formed on F e - M n - A I - 0 , 5 Si a n d F e - M n - A I - 0 , 8 S i alloys w e r e also examined . It appears that the outer scale w a s
I ' '
Ill c o m p o s e d o n l y of α - Μ η 0 . U n d e r 2
this o u t e r
3
part, a 2μηη thick dark < * - A I 0 2
3
containing
r e g i o n w a s o b s e r v e d . T h e o x i d e layer w a s also not a d h e r e n t to t h e s u b s t r a t e a n d s p a l l a t i o n occured during cooling,
b - Coated Fe-Mn-AI alloy oxidation T h e w e i g h - g a i n c u r v e s are r e p o r t e d (Fig 8). Fe-MnAI oxidation kinetics followed the classical parabolic rate low with k =1,6 1 0 "
1 1
g cm" s" . 2
4
1
FeMnA10,55Si FeMnAlO.eOSi - FeMnA)
Ί
ΗΓ " 10 Τ
T-T-T-
-T-r-r
40
30
20
70
60
50
O n t h e contrary, t h e c r o s s s e c t i o n of F e - M n - A I -0,8Si alloys o x i d i z e d at t h e s a m e t e m p e r a t u r e exhibited a large difference b e t w e e n coated and uncoated specimens.This scale was d i v i d e d in t w o p a r t s . T h e e x t e r n a l s u b s c a l e w a s n o n a d h e r e n t a n d w a s c o m p o s e d of a M n O g . T h e inner s u b s c a l e w a s c o m p o s e d of 2
Mn 0 3
4
a n d M n O , a n d r e m a i n e d a d h e r e n t to
t h e s u b s t r a t e . N o i r o n w a s d e t e c t e d in b o t h s u b s c a l e s a n d a n ΑΙ-rich z o n e a s s o c i a t e d w i t h S i l i c o n w a s o b s e r v e d at t h e s c a l e a l l o y interface.The observation m a d e on a ten minute experiment s h o w s a thin uniform and a d h e r e n t o x i d e s c a l e w h e r e c e r i u m is m a i n l y i n c o r p o r a t e d in t h e o u t e r p a r t . S i l i c o n w a s o b s e r v e d at t h e inner i n t e r f a c e . In this c a s e t h e s i l i c o n - f r e e alloy c o a t e d by cerium oxide always exhibited a lower mass g a i n c o m p a r e d w i t h b l a n k s p e c i m e n s , but the i n f l u e n c e of s i l i c o n m i n o r a d d i t i o n s w a s q u i t e remarkable.Silicon-containing alloys always gave higher m a s s gain c o m p a r e d with blank s p e c i m e n s after d i p p i n g
t(h)
3. D I S C U S S I O N Figure 8: Oxidation kinetics of coated Fe-Mn-AI, F e - M n - A I - 0 . 5 S Ï , F e - M n - A I - 0, 8 S i - a l l o y s , P = 2 0 0 0 0 P a , Τ = 850°C o2
C o n c e r n i n g t h e s i l i c o n - c o n t a i n i n g alloy, the weight-gain registered was more important : k
p(Fe-Mn-AI-0,08Si)
=
k
p(Fe-Mn-Al-0,5Si)
1
=
>
0
·
2
8
10"
1 1
g cm- s- , 2
4
1
10" g cm"V . 10
2
1
T h e s c a l e of t h e c o a t e d F e - M n - A I alloy w a s c o m p o s e d by t w o 10μΐτι thick s u b s c a l e s c o n t a i n i n g b o t h i r o n a n d m a n g a n e s e in t h e s a m e proportions. A thin aluminium-rich zone w a s f o u n d at t h e s c a l e alloy i n t e r f a c e , but c e r i u m w a s not d e t e c t e d by analytical techniques. Oxide scale had generally the s a m e aspects and composition as that the blank specimen, except a lower thickness o b s e r v e d . H o w e v e r , after t e n m i n u t e s a v e r y t h i n n o n a d h e r e n t C e 0 layer w a s still present 2
a b o v e t h e i r o n - m a n g a n e s e o x i d e scale already grown on the substrate.
AND
CONCLUSIONS
To s u m m a r i z e , t h e s e r e s u l t s c o n c e r n i n g iron a n d a l l o y s , w e c a n s a y t h a t in g e n e r a l , r a r e earth-coating produces a decrease in o x i d a t i o n r a t e a n d a n i n c r e a s e in p r o t e c t i v e s c a l e a d h e r e n c e , e x c e p t for p u r e iron. In fact t h e p o s i t i v e e f f e c t is o n l y o b t a i n e d if t h e o x i d a t i o n rate is f i x e d by a d i f f u s i o n p r o c e s s . It is not the c a s e for o x i d a t i o n of p u r iron. N e v e r t h e l e s s t h e p r e s e n c e of a m i n o r e l e m e n t , like silicon, c a n i n v a l i d a t e t h e positive effectof t h e rare e a r t h e l e m e n t s . Different e x p l a n a t i o n s for t h e s e e f f e c t s c a n be proposed : - t h e r a r e e a r t h e f f e c t s in c o n c e r t w i t h t h e r e a c t i o n m e c h a n i s m r e s u l t s of a n i n c r e a s e of t h e p l a s t i c d e f o r m a t i o n c a p a c i t y of t h e r a r e earth-containing oxide scale, - a m o d i f i c a t i o n of the m a t t e r t r a n s p o r t p r o c e s s or/and transport
inhibition along the
oxide
grain boundaries. In the c a s e of u n c o a t e d s a m p l e s , t h e matter transport
occurs
via
cation
vacancies.
A
112 c o n s e q u e n c e c o n s i s t s in a v o i d a c c u m u l a t i o n at t h e o x i d e / m e t a l i n t e r f a c e . T h e s c o u l d b e t h e o r i g i n of t h e o b s e r v e d o x i d e s c a l e d e t a c h m e n t b o t h u n d e r i s o t h e r m a l a n d cyclic c o n d i t i o n s . W h e n a r e a c t i v e e l e m e n t is present over the metal surface, t h e protective layer contains a fine grained chromia scale e x h i b i t i n g a h i g h p l a s t i c i t y [ 2 2 ] . T h e origin of s u c h modification could be attributed to an i n v e r s i o n of m a t t e r t r a n s p o r t m e c h a n i s m , e v o l v i n g f r o m a c a t i o n i c o u t w a r d t o a n anionic i n w a r d d i f f u s i o n p r o c e s s . T h i s i n v e r s i o n will also increase t h e oxide scale adherence, s i n c e t h e r e is n o v o i d a c c u m u l a t i o n at t h e m e t a l / o x i d e i n t e r f a c e . M . J . B E N N E T T [23] d e s c r i b e d similar o b s e r v a t i o n s a n d g a v e similar c o n c l u s i o n s in h i s s t u d y c o n c e r n i n g a n y t t r i u m i m p l a n t e d nickel b a s e d alloy. T h a t is in a g r e e m e n t w i t h r e s u l t s o b s e r v e d in t h e c a s e of p u r e c h r o m i u m a n d h i g h c h r o m i u m c o n t e n t alloys, b y R.J. H U S S E Y a n d M.J. G R A H A M (24). Their c o n c l u s i o n is that for c o a t e d s u b s t r a t e s , t h e r e is a c h a n g e in m e c h a n i s m w h i c h b e c o m e s p r e d o m i n a t e d by anion diffusion. Finally, R . A . R A P P a n d B. P I E R A G G I recently (25) p r o p o s e d that the rare earth effect is a c c o m p l i s h e d by t r a p p i n g t h e v a c a n c i e s at t h e i n t e r n a l i n t e r f a c e leading also t o a n o x i d e g r o w t h b y anion diffusion. In conclusion, h o m o g e n e o u s a n d t h i n r a r e earth oxide films which were realized on stainless steel by chemical p r o c e s s e s or by M O C V D h a v e a positive effect o n the high temperature stainless steel oxidation resistance, but not o n pure iron oxidation r e s i s t a n c e . T h e p o s i t i v e effect is o n l y o b t a i n e d if t h e o x i d a t i o n r a t e is g o v e r n e d b y a diffusion process. In this c a s e : - T h e corrosion rate decreases under i s o t h e r m a l c o n d i t i o n s a s well a s u n d e r c y c l i c conditions. - T h e r m a l c y c l i n g d o e s not c a u s e a n y specific degradation a n d the oxidation rate constants are similar for isothermal a n d cyclic conditions. T h i s r e s u l t is p a r t i c u l a r l y i m p o r t a n t f o r t h e u s e o f s u c h s t a i n l e s s s t e e l s at h i g h t e m p e r a t u r e . It is p o t e n t i a l l y a n i n t e r e s t i n g
p r o c e s s u s e a b l e in d i f f e r e n t h i g h t e m p e r a t u r e corrosion a r e a s . REFERENCES 1) H. NAGAI Mat. Sci. Forum 43 (1989) 75-130. 2) T.A. RAMANARAYANAN, R. A Y E R , R. PETKOVIC, D P . LETA Oxid. Met 29, 5/6 (1988). 3) J. STRINGER Mat. Sci. Eng. A12 (1989) 129. 4) M.J. ΒΕΝΝΕΤΤ,Η.Ε. BISHOP.P.R. CHALKER.A TUSON Mat. Sci. Eng. 9 (1987) 177-190. 5) R.L. NELSON, J.D. RAMSAY, L . WOODHEAD Thin Solid Films 81 (1981) 329-337. 6) M. J. BENNETT J. Vac. Sci. Technol. B2.4, 10-12 (1984) 800-805. 7) P.Y. H O U , J. S T R I N G E R J. Electrochem. Soc. 134-7 (1987)1836 . 8) M R STROOSNIJDER.V. GUTTMANN.T. FRANSEN.J WIT Oxid. Met. 33 (1990) 371-397. 9)D.P. MOON, M.J. BENNETT Mat Sci. Forum 43 (1989) 269. 10) J.P. LARPIN, G. AGUILAR, H. BUSCAIL, J.C. COLSON in "High Temp. Oxid. Sulphid. Processes" Ed J.D. EMBURY Pergamon Press (1990) 337-345. 11) G. BONNET, J.P. LARPIN, J.C. COLSON Solid State Ionics 5 (1992) 125. 12) W.C. HAGEL, U. SEYBOLT J. Electrochem Soc. 12(1961) 1146-52. 13) F. I. WEI, F. H. STOTT Cor. Sci. 29,(1989) 839. 14) C M . COTELL, G.J. YUREK, R.J. HUSSEY, D.F. MITCHELL, M.J. G R A H A M Oxid. Met.,34-3/4 (1990) 173. 15) J.G. SMEGGIL, A.W. F U N K E N B U S H , N.S. BORNSTEIN Met. Trans 17A (1986) 923. 16) J.G. SMEGGIL Mat Sci and Eng. 87 (1987) 261. 17) K.J. EISENTRAUT, R.E. SIEVERS, J. Amer Chem. Soc. 20 (1965)5254. 18) J.G. LANGEVOORT.L.J. HANEKAMP, P. J . GELLINGS Appl.Surf.Sci,28 (1987) 189-203. 19) D.R. BAER, M. D. MERZ Metal. Trans 11A.12 (1980) 1973-80. 20)M. LAMBERTIN, J.C. C O L S O N Oxid. Met. 7,3,(1973) 163-171. 21) B. GATEHOUSE Am. Mineral 63 (1978) 28) 22)K.PRZYBYLSKl,A.GARATT-REED,G.J. YUREK J.Electrochem.Soc,135,2 (1988) 508. 23) M.J. BENNETT, D.P. MOON The role of active elements in the oxidation behaviour of high temperature metals and alloys E. LANG (Ed), ELSEVIER, LONDON (1988)111-129. 24) R.J. HUSSEY, M.GJ. GRAHAM 3rd Int. Symp. High Temp. Cor. May 1992, Les Embiez France. . 25) B. PIERAGGI, R.A. RAPP 3rd Int. Symp. High Temp. Cor. May 1992, Les Embiez.France.
107
Rare earths oxide film effect on different metal and alloys at high temperature in oxidizing conditions. J.C.Colson, H.Buscail, G.Bonnet, RSotto, J.P.Larpin. Laboratoire de Recherches sur la Réactivité des Solides Fac. Sci. Mirande, BP 138, F.21004 Dijon Cedex Abstract : Different m e t h o d s for rare e a r t h s d e p o s i t i o n o n the s u r f a c e of iron a n d A I S I 3 0 4 , F 1 7 T i s t a i n l e s s s t e e l s l a n d s t e e l s a r e p r o p o s e d . T h e c o a t e d or n o n c o a t e d s a m p l e s in c o r r o s i o n t e s t s u n d e r i s o t h e r m a l c o n d i t i o n s r e v e a l e d a n i d e n t i c a l o x i d a t i o n p r o c e s s but t h e c o r r o s i o n r a t e c o n s t a n t s d e c r e a s e d s i g n i f i c a n t l y for t h e c o a t e d s a m p l e s . In t h e r m a l c y c l i c c o n d i t i o n s , p r o t e c t i v e s c a l e spallation c o m p l e t e l y d i s a p e a r e d for c o a t e d s a m p l e s . It w a s s h o w n that t h e rare e a r t h effect is m o r e e f f e c t i v e w i t h e l e m e n t s l o c a t e d o n t h e left in t h e l a n t h a n i d e period of t h e Periodic T a b l e . D i f f é r e n t s e x p l a n a t i o n s for t h e s e results p r o p o s e d . 1.
INTRODUCTION
oxygen cycling reaction at r o o m
T h e b e n e f i c i a l e f f e c t of r a r e e a r t h o n h i g h t e m p e r a t u r e o x i d a t i o n r a t e of s t e e l a n d p a r t i c u l a r l y o n p r o t e c t i v e s c a l e a d h e r e n c e , is now well e s t a b l i s h e d , as well as by introduction as a minor e l e m e n t (1-3), by i m p l a n t a t i o n (4) or by c h e m i c a l p r o c e s s e s (510). C h e m i c a l p r o c e s s e s (sol/gel and/or electrophoresis, Organometallic Chemical V a p o r D e p o s i t i o n m e t h o d s (11)) to realize rare e a r t h o x i d e c o a t i n g s , w h i c h are e a s i e r s e e m to b e a p r o m i s i n g w a y . M e c h a n i s m s are not yet well understood (1-16).
R a r e e a r t h o x i d e w e r e d e p o s i t e d by v a r i o u s processes : I - P o l y m e r i c c o a t i n g s b y d i p p i n g s a m p l e s in a c o l l o ï d a l s o l u t i o n of c e r i a in a n a q u e o u s solution of p o l y a c r y l a m i d . II - A l c o h o l i c c o a t i n g s by d i p p i n g s a m p l e s in an alcoholic solution of a m m o n i u m c e r i u m nitrate. III - E l e c t r o p h o r e t i c c o a t i n g s r e a l i z e d w i t h a very fine g r a i n e d c e r i u m h y d r o x i d e sol d i s p e r s e d in water or in o r g a n i c s o l v a n t ( D M F ) . IV - M O C V D p r o c e s s w h i c h led us to p e r f o r m a s y s t e m a t i c s t u d y of t h e r a r e e a r t h effect w i t h practically all t h e rare e a r t h e l e m e n t s (17).
In this p a p e r , t h e e f f e c t of rare e a r t h o x i d e deposited by various chemical processes on iron, A I S I 3 0 4 , F 1 7 T i s t a i n l e s s s t e e l s a n d FeMn-AI alloy is s t u d i e d . Fe Electrolytic F e
C
Cr
for all < 0 , 1
at 2 x 1 0 ^ P a , u n d e r i s o t h e r m a l or c o n d i t i o n s ( o n e c y c l e = 2 h r s at t h e t e m p e r a t u r e : 9 5 0 ° C, f o l l o w e d by 1 hr temperature).
W h a t e v e r t h e p r o c e s s t o p r o d u c e t h e rare Ni
Mn
AI
Mo
Cu
e x c e p t i n g for H - N a , K , C a , M o , M n , C o , N i , C u < 1
Si
μg/g
AISI 304
bal
0,042
17,74
8,8
1,4
0,16
0,17
0,48
F17Ti
bal
0,015
17,19
0,12
0,39
0,06
0,07
0,4
Fe-Mn-AI
bal
0,28
0,018
0,01
32
6,6
Fe-Mn-AI-0,5Si
bal
0,27
0,018
0,01
30
6,2
0,55
Fe-Mn-AI-0,8Si
bal
0,29
0,018
0,01
31
6,8
0,8
High
temperature
oxidation
tests
were
p e r f o r m e d in a m i c r o t h e r m o b a l a n c e e i t h e r in air u n d e r a t m o s p h e r i c p r e s s u r e or in pure
earth oxide film, the coated s p e c i m e n s were reheated
following
a
linear
temperature
increase up to 8 0 0 ° C , remaining one
hour,
108 and
slowly
cooled
down
to
the
Isothermal
room
oxidation
kinetics
exhibited
linear m a s s - g a i n c u r v e s ( F i g . 2 ) . T h e c o m p a c t
temperature. The coating characterisation
have
o x i d e s c a l e is c o m p o s e d m a i n l y w u s t i t e . O x i d e
been
grain
r e a l i s e d b y X r a y diffraction a n d S.I.M.S.
growth
occurs
with
a
preferential
orientation. T h e o x i d a t i o n m e c h a n i s m is similar to
that
proposed
by
P. K O F S T A D
who
e m p h a s i z e s a c o n s t a n t n u m b e r of g r o w t h sites d e t e r m i n e d b y t h e t e x t u r e of t h e o x i d e layer. W i t h c o a t e d p u r e iron t h e l i n e a r o x i d a t i o n kinetics a r e c o n s e r v e d for t h e c o a t e d s a m p l e s , but t h e rate c o n s t a n t i n c r e a s e s . T h e o x i d a t i o n m e c h a n i s m is also t h e s a m e , but c e r i u m o x i d e o n t h e s u r f a c e of t h e m e t a l Τ ι me 3 2
O X Y G E N E
p r o b a b l y i n c r e a s e s t h e n u m b e r of n u c l e a t i o n
?COÛ [sec ] N d 2 Ô S ' / 3 0 < 2 h 1 0 0 8
B 0 N E T D ? 3 P f *
P o s i t i v e J5:2I:29
2 2 F e b 19S3
s i t e s a n d , c o n s e q u e n t l y , t h e c o r r o s i o n rate. M o r e o v e r in all c a s e s ( c o a t e d a n d u n c o a t e d
Figure 1 : Concentration profiles at the surface vicinity of N d 2 0 3 coated AISI 304 (5 Minutes
during cooling.
coated and heating 1 h at 800°C and 2h in oxidation conditions).
w a s o b s e r v e d iron o x i d a t i o n b e h a v i o r .
iron)
w e never
observed
scale
spallation
In c o n c l u s i o n , n o p o s i t i v e rare e a r t h effect That
d o e s not m e a n h o w e v e r that t h e r a r e e a r t h Concentration
profiles
show
(Fig. 1 ) a
e l e m e n t d o e s n o t play a role in t h e o x i d a t i o n
mixing with natural oxide produced on the
mechanism.
s a m p l e d u r i n g h e a t i n g . S u p e r f i c i a l rare e a r t h
2.2.AISI 304 stainless steel
c o n c e n t r a t i o n is high a n d d e c r e a s e s quickly.
a
2. HIGH TEMPERATURE TESTS 2.1.Pure Iron oxidation
p a r a b o l i c m a s s - g a i n c u r v e s ( F i g . 3 ) . T h e rate
-
Isothermal
OXIDATION
oxidation
Am/S=f(Vt) were respectively kp=5,1.10'
iron. This s y s t e m have been studied ( 2 1 0 "
1 2
g cm"V 2
1
2
3 < P Q < 2 1 0 " ) t o c o n s t i t u t e a r e f e r e n c e with a 4
2
Ε ο
pure metal.
exhibited
1,8.10"
1 2
C
5
I I I I I 1 I 1 > 1 1
Uncoated AISI 304 al
1000'C
Uncoated AISI 304 at
950°C
6 I I 1 1 i I Π
7 I i i,
en
Μ- ι ·)
Pur Iron Iron Coated by Ce 0
1.2 1 0.8
0.8 2
0.8
1 000°C y
0.6 Η
and
at 9 5 0 a n d 1 0 0 0 C . 4
I I I I I I I 1 1 1
1.2
m/s
ε
kinetics
c o n s t a n t s k p m e a s u r e d f r o m t h e linear plots
Firstly w e h a v e s t u d i e d t h e b e h a v i o r of p u r e
I
oxidation
Uncoatied AISI 3 0 4 stainless steel :
0.6
950°C
//
0.4
1OO0°C
0.2
' " ' 950°C
*, ".
.—
ι
1
1
1
—ι
1
1—
ι
10
0
·
12
F i g u r e 2 : Pure Iron uncoated and coated by C e 0 (isothermal oxidation). 2
Figure 3 : AISI 304 isothermal oxidation in air. t(h)
Am/S= f(t) and Am/S = f (VtJ. T h e m o r p h o l o g y a n d c o m p o s i t i o n of t h e oxide
scale
grown
were
determined
as
109 f u n c t i o n of t h e c o r r o s i o n t e s t t i m e . T h e o x i d e w a s initially c o m p o s e d of F e C r 2 0 , t h e n a n
(kp=8,1.10~ and 1 , 8 . 1 0 " g c m " V at 950 and 1000°C respectively). T h e protective o x i d e s c a l e is s t r o n g l y a d h e r e n t t o t h e m e t a l substrate. F r o m a m o r p h o l o g i c a l p o i n t of v i e w o x i d e s c a l e g r o w n w a s practically t h e s a m e a s t o t h e s c a l e o b s e r v e d o n t h e u n c o a t e d alloy. X-ray measurements indicated the following phase : MnFe 0 , Cr 0 but w e also found a 1 3
4
internal c h r o m i a subscale f o r m e d , which w a s c r o v e r e d b y a n e x t e r n a l s c a l e c o m p r i s e d of two spinels : M n C r 0 and F e C r 0 2
4
2
4
containing ( F e C r ^ C ^ nodules. T h e internal g r o w t h of s i l i c a w a s a l s o o b s e r v e d a l o n g t h e alloy grain b o u n d a r i e s .
2
U n d e r t h e r m a l c y c l i n g c o n d i t i o n s (Fig 4)
4
complex
scale spallation a p p e a r e d after 5 or 7 cycles.
1 2
2
oxide
with
a structure 2 1
Λ 1 I I I II II 1 1 1,1 Uncoated AJSI 304 steel
0.6
'Ε
AISI 304 Coated by C e Q
ro.48-^
3 8
close
to
(19).
0.6
Under thermal cycling conditions (Fig 4), t h e s c a l e spallation c o m p l e t e l y d i s a p p e a r e d .
•0.48
The M O C V D m e t h o d lead us to prepare c o a t i n g s of p r a c t i c a l l y all t h e r a r e e a r t h e l e m e n t s . A s s h o w n in F i g 5 , t h e r a r e e a r t h effect is m o r e i m p o r t a n t w i t h e l e m e n t s l o c a t e d o n t h e left in t h e l a n t h a n i d e p e r i o d of t h e Periodic Table.
2
I hO.36 0.360.240.12-
0.12
τ-ρττ
1
3
((Ca,Ce)(Ti,Fe,Cr,Mg)) 0 _
2
Weigth gam lor uncoated AISI 3 0 4 steel
Λιη/s
τ-ρττ 20'
10 20
30
40
60
40
cycles
80
hours
Figure 4 : Thermal cycling oxidation of coated (by C e 0 ) and uncoated samples in air at 950°C.
Η
0.8
h
0.6
I — r
2
0.4
0.4
P r o t e c t i v e o x i d e s c a l e g r o w t h w a s b y cation d i f f u s i o n ( 1 2 , 1 4 , 1 8 , 1 9 ) . T h e c o r r o s i o n rate is m o n i t o r e d b y a d i f f u s i o n p r o c e s s in t h e chromia subscale . Under thermal cycling conditions, thermal s t r e s s e s resulted in s c a l e spallation after 5 or 7 cycles. N e w oxides exhibited a high growth rate. T h e s p a l l i n g p r o c e s s is m a i n l y linked t o t h e a c c u m u l a t i o n of t h e o x i d e g r o w t h s t r a i n c o n n e c t e d w i t h t h e e x p a n s i o n of t h e o x i d e , crystalline d i s t o r t i o n at t h e interfacial z o n e a n d void a c c u m u l a t i o n .
b
- Coated
oxidation
AISI
(Ceria
304 stainless coatings
d i p p i n g or e l e c t r o p h o r e s i s ) : revealed
that
the
processes
are always
steel
produced
by
Corrosion tests
diffusion dominant
oxidation for ceria
U Ce
-ι 56
1
P r Nd r- " Ί
58
60
Sm
Eu
I
I
62
64
Dy
Er
Yb "Τ
66
68
70
72 (Ζ)
Figure 5: Weight gain of coated AISI 3 0 4 steel after 50h oxidation in air at 1000°C.
2.3.F17TÎ a
-
stainless
Uncoated
steel
F17TÎ
oxidation
stainless
steel
oxidation : I s o t h e r m a l o x i d a t i o n k i n e t i c s exhibited parabolic m a s s gain curves with t w o p a r a b o l i c p a r t s ( p a r a b o l i c rate c o n s t a n t s w e r e m e a s u r e d f r o m t h e linear plots A m / S = f ( V t ) ) . C o r r o s i o n p r o d u c t s initially g r e w at t h e b a s e of t h e s u b s t r a t e g r a i n b o u n d a r i e s , t h e n , f o r the next t w o hours, an external M n C r 0 2
4
spinel s u b s c a l e c o n t a i n i n g iron a n d a n internal
coated samples and for the blank ones,and
subscale containing chromia, titanium were
t h a t t h e p a r a b o l i c rate c o n s t a n t s d e c r e a s e d
observed.
110 After a 4hr run, T i 0
2
and M n C r 0 2
4
crystallites
were observed on the scale surface. U n d e r t h e r m a l cycling conditions,scale spalling a p p e a r e d generally after 3 - 5 cycles', d e p e n d i n g u p o n t h e initial m e t a l s u r f a c e roughness.
b
-
Coated
oxidation
F17Ti
(.Ceria
stainless
coatings
steel
produced
by
dipping or electrophoresis) : the ceria coated
Thin M O C V D rare earth oxide films has also have a similar effect on corrosion behaviour both under cyclic or isothermal conditions.
2.4. Fe-Mn-AI alloy oxidation a - Uncoated Fe-Mn-AI alloys: the F e Mn-AI weight-gain curve s h o w s a relatively high oxidation rate (Fig.7). For alloys Fe-Mn-AI0,5Si and F e - M n - A I - 0 , 8 S i , t h e experimental weight-gain decreases significantly (Fig.7).
sample isothermal corrosion tests revealed an identical
oxidation
process with only o n e
FeMnAI
parabolic part in t h e weigth gain curve and parabolic rate constants decreased. T h e p r o t e c t i v e oxide s c a l e adherent to the metal substrate.
is strongly
T E M observations shown that the external sublayer w a s m a d e of M n C r 2 0 4 containing iron and the internal oxide contained only very fine grains of chromia. O n the surface of the oxide scale grown, large C e 0 particles were
FeMnAI0.5Si FeMnAIO.eSi
2
1 I ιιιιjτ t1 ιj 1 1 ι1 j ιι1 1 ;1 1 « « I » • 10 20 30 40 50 60 70 80
observed. In all cases, ceria was localised more in t h e external part of the oxide scale, and X ray m e a s u r e m e n t s
indicated t h e following
p h a s e composition
: MnFe 0 ,
Cr 0 2
3
2
4
Ti0
2
and
and a complex oxide with a structure
close to ( C a , C e ) ( T i , F e , C r , M g ) 0 2 1
3 8
Figure 7: Oxidation kinetics of Fe-Mn-AI, Fe-MnAI-0,5Si, Fe-Mn-AI-0,8Si alloys P = 2 0 0 0 0 P a , Τ = o2
850°C.
(17).
U n d e r t h e r m a l cycling conditions,(Fig 6) scale spallation completely disappeared.
From the slopes of the plot of the specific weight-gain versus the square root of time, the following p a r a b o l i c r a t e c o n s t a n t s w e r e determined :
^(Fe-Mn-AI-O.OSSi)
2
4
1
=1,15 l O ' ^ g ^ m ' V
1
p( Fe-Mn-AI- 0 , 5 S i ) 1,5 10" g cm' s" . O b s e r v a t i o n of t h e oxide s c a l e c r o s s section and the external surface of the F e - M n AI alloy revealed that the oxide layer w a s non adherent to t h e s u b s t r a t e , a n d spallation occured during cooling. T h e w h o l e scale consisted of several subscales containing iron and m a n g a n e s e . T h e different p h a s e s were identified is a - M n 0 , M n F e 0 and a small 11
2
4
1
:
2
F i g u r e 6 : Thermal cycling oxidation of F17Ti coated by C e 0 and uncoated samples in air at 2
950°C.
3
2
4
amount of M n O . T h e oxide scales formed on F e - M n - A I - 0 , 5 Si a n d F e - M n - A I - 0 , 8 S i alloys w e r e also examined . It appears that the outer scale w a s
I ' '
Ill c o m p o s e d o n l y of α - Μ η 0 . U n d e r 2
this o u t e r
3
part, a 2μηη thick dark < * - A I 0 2
3
containing
r e g i o n w a s o b s e r v e d . T h e o x i d e layer w a s also not a d h e r e n t to t h e s u b s t r a t e a n d s p a l l a t i o n occured during cooling,
b - Coated Fe-Mn-AI alloy oxidation T h e w e i g h - g a i n c u r v e s are r e p o r t e d (Fig 8). Fe-MnAI oxidation kinetics followed the classical parabolic rate low with k =1,6 1 0 "
1 1
g cm" s" . 2
4
1
FeMnA10,55Si FeMnAlO.eOSi - FeMnA)
Ί
ΗΓ " 10 Τ
T-T-T-
-T-r-r
40
30
20
70
60
50
O n t h e contrary, t h e c r o s s s e c t i o n of F e - M n - A I -0,8Si alloys o x i d i z e d at t h e s a m e t e m p e r a t u r e exhibited a large difference b e t w e e n coated and uncoated specimens.This scale was d i v i d e d in t w o p a r t s . T h e e x t e r n a l s u b s c a l e w a s n o n a d h e r e n t a n d w a s c o m p o s e d of a M n O g . T h e inner s u b s c a l e w a s c o m p o s e d of 2
Mn 0 3
4
a n d M n O , a n d r e m a i n e d a d h e r e n t to
t h e s u b s t r a t e . N o i r o n w a s d e t e c t e d in b o t h s u b s c a l e s a n d a n ΑΙ-rich z o n e a s s o c i a t e d w i t h S i l i c o n w a s o b s e r v e d at t h e s c a l e a l l o y interface.The observation m a d e on a ten minute experiment s h o w s a thin uniform and a d h e r e n t o x i d e s c a l e w h e r e c e r i u m is m a i n l y i n c o r p o r a t e d in t h e o u t e r p a r t . S i l i c o n w a s o b s e r v e d at t h e inner i n t e r f a c e . In this c a s e t h e s i l i c o n - f r e e alloy c o a t e d by cerium oxide always exhibited a lower mass g a i n c o m p a r e d w i t h b l a n k s p e c i m e n s , but the i n f l u e n c e of s i l i c o n m i n o r a d d i t i o n s w a s q u i t e remarkable.Silicon-containing alloys always gave higher m a s s gain c o m p a r e d with blank s p e c i m e n s after d i p p i n g
t(h)
3. D I S C U S S I O N Figure 8: Oxidation kinetics of coated Fe-Mn-AI, F e - M n - A I - 0 . 5 S Ï , F e - M n - A I - 0, 8 S i - a l l o y s , P = 2 0 0 0 0 P a , Τ = 850°C o2
C o n c e r n i n g t h e s i l i c o n - c o n t a i n i n g alloy, the weight-gain registered was more important : k
p(Fe-Mn-AI-0,08Si)
=
k
p(Fe-Mn-Al-0,5Si)
1
=
>
0
·
2
8
10"
1 1
g cm- s- , 2
4
1
10" g cm"V . 10
2
1
T h e s c a l e of t h e c o a t e d F e - M n - A I alloy w a s c o m p o s e d by t w o 10μΐτι thick s u b s c a l e s c o n t a i n i n g b o t h i r o n a n d m a n g a n e s e in t h e s a m e proportions. A thin aluminium-rich zone w a s f o u n d at t h e s c a l e alloy i n t e r f a c e , but c e r i u m w a s not d e t e c t e d by analytical techniques. Oxide scale had generally the s a m e aspects and composition as that the blank specimen, except a lower thickness o b s e r v e d . H o w e v e r , after t e n m i n u t e s a v e r y t h i n n o n a d h e r e n t C e 0 layer w a s still present 2
a b o v e t h e i r o n - m a n g a n e s e o x i d e scale already grown on the substrate.
AND
CONCLUSIONS
To s u m m a r i z e , t h e s e r e s u l t s c o n c e r n i n g iron a n d a l l o y s , w e c a n s a y t h a t in g e n e r a l , r a r e earth-coating produces a decrease in o x i d a t i o n r a t e a n d a n i n c r e a s e in p r o t e c t i v e s c a l e a d h e r e n c e , e x c e p t for p u r e iron. In fact t h e p o s i t i v e e f f e c t is o n l y o b t a i n e d if t h e o x i d a t i o n rate is f i x e d by a d i f f u s i o n p r o c e s s . It is not the c a s e for o x i d a t i o n of p u r iron. N e v e r t h e l e s s t h e p r e s e n c e of a m i n o r e l e m e n t , like silicon, c a n i n v a l i d a t e t h e positive effectof t h e rare e a r t h e l e m e n t s . Different e x p l a n a t i o n s for t h e s e e f f e c t s c a n be proposed : - t h e r a r e e a r t h e f f e c t s in c o n c e r t w i t h t h e r e a c t i o n m e c h a n i s m r e s u l t s of a n i n c r e a s e of t h e p l a s t i c d e f o r m a t i o n c a p a c i t y of t h e r a r e earth-containing oxide scale, - a m o d i f i c a t i o n of the m a t t e r t r a n s p o r t p r o c e s s or/and transport
inhibition along the
oxide
grain boundaries. In the c a s e of u n c o a t e d s a m p l e s , t h e matter transport
occurs
via
cation
vacancies.
A
112 c o n s e q u e n c e c o n s i s t s in a v o i d a c c u m u l a t i o n at t h e o x i d e / m e t a l i n t e r f a c e . T h e s c o u l d b e t h e o r i g i n of t h e o b s e r v e d o x i d e s c a l e d e t a c h m e n t b o t h u n d e r i s o t h e r m a l a n d cyclic c o n d i t i o n s . W h e n a r e a c t i v e e l e m e n t is present over the metal surface, t h e protective layer contains a fine grained chromia scale e x h i b i t i n g a h i g h p l a s t i c i t y [ 2 2 ] . T h e origin of s u c h modification could be attributed to an i n v e r s i o n of m a t t e r t r a n s p o r t m e c h a n i s m , e v o l v i n g f r o m a c a t i o n i c o u t w a r d t o a n anionic i n w a r d d i f f u s i o n p r o c e s s . T h i s i n v e r s i o n will also increase t h e oxide scale adherence, s i n c e t h e r e is n o v o i d a c c u m u l a t i o n at t h e m e t a l / o x i d e i n t e r f a c e . M . J . B E N N E T T [23] d e s c r i b e d similar o b s e r v a t i o n s a n d g a v e similar c o n c l u s i o n s in h i s s t u d y c o n c e r n i n g a n y t t r i u m i m p l a n t e d nickel b a s e d alloy. T h a t is in a g r e e m e n t w i t h r e s u l t s o b s e r v e d in t h e c a s e of p u r e c h r o m i u m a n d h i g h c h r o m i u m c o n t e n t alloys, b y R.J. H U S S E Y a n d M.J. G R A H A M (24). Their c o n c l u s i o n is that for c o a t e d s u b s t r a t e s , t h e r e is a c h a n g e in m e c h a n i s m w h i c h b e c o m e s p r e d o m i n a t e d by anion diffusion. Finally, R . A . R A P P a n d B. P I E R A G G I recently (25) p r o p o s e d that the rare earth effect is a c c o m p l i s h e d by t r a p p i n g t h e v a c a n c i e s at t h e i n t e r n a l i n t e r f a c e leading also t o a n o x i d e g r o w t h b y anion diffusion. In conclusion, h o m o g e n e o u s a n d t h i n r a r e earth oxide films which were realized on stainless steel by chemical p r o c e s s e s or by M O C V D h a v e a positive effect o n the high temperature stainless steel oxidation resistance, but not o n pure iron oxidation r e s i s t a n c e . T h e p o s i t i v e effect is o n l y o b t a i n e d if t h e o x i d a t i o n r a t e is g o v e r n e d b y a diffusion process. In this c a s e : - T h e corrosion rate decreases under i s o t h e r m a l c o n d i t i o n s a s well a s u n d e r c y c l i c conditions. - T h e r m a l c y c l i n g d o e s not c a u s e a n y specific degradation a n d the oxidation rate constants are similar for isothermal a n d cyclic conditions. T h i s r e s u l t is p a r t i c u l a r l y i m p o r t a n t f o r t h e u s e o f s u c h s t a i n l e s s s t e e l s at h i g h t e m p e r a t u r e . It is p o t e n t i a l l y a n i n t e r e s t i n g
p r o c e s s u s e a b l e in d i f f e r e n t h i g h t e m p e r a t u r e corrosion a r e a s . REFERENCES 1) H. NAGAI Mat. Sci. Forum 43 (1989) 75-130. 2) T.A. RAMANARAYANAN, R. A Y E R , R. PETKOVIC, D P . LETA Oxid. Met 29, 5/6 (1988). 3) J. STRINGER Mat. Sci. Eng. A12 (1989) 129. 4) M.J. ΒΕΝΝΕΤΤ,Η.Ε. BISHOP.P.R. CHALKER.A TUSON Mat. Sci. Eng. 9 (1987) 177-190. 5) R.L. NELSON, J.D. RAMSAY, L . WOODHEAD Thin Solid Films 81 (1981) 329-337. 6) M. J. BENNETT J. Vac. Sci. Technol. B2.4, 10-12 (1984) 800-805. 7) P.Y. H O U , J. S T R I N G E R J. Electrochem. Soc. 134-7 (1987)1836 . 8) M R STROOSNIJDER.V. GUTTMANN.T. FRANSEN.J WIT Oxid. Met. 33 (1990) 371-397. 9)D.P. MOON, M.J. BENNETT Mat Sci. Forum 43 (1989) 269. 10) J.P. LARPIN, G. AGUILAR, H. BUSCAIL, J.C. COLSON in "High Temp. Oxid. Sulphid. Processes" Ed J.D. EMBURY Pergamon Press (1990) 337-345. 11) G. BONNET, J.P. LARPIN, J.C. COLSON Solid State Ionics 5 (1992) 125. 12) W.C. HAGEL, U. SEYBOLT J. Electrochem Soc. 12(1961) 1146-52. 13) F. I. WEI, F. H. STOTT Cor. Sci. 29,(1989) 839. 14) C M . COTELL, G.J. YUREK, R.J. HUSSEY, D.F. MITCHELL, M.J. G R A H A M Oxid. Met.,34-3/4 (1990) 173. 15) J.G. SMEGGIL, A.W. F U N K E N B U S H , N.S. BORNSTEIN Met. Trans 17A (1986) 923. 16) J.G. SMEGGIL Mat Sci and Eng. 87 (1987) 261. 17) K.J. EISENTRAUT, R.E. SIEVERS, J. Amer Chem. Soc. 20 (1965)5254. 18) J.G. LANGEVOORT.L.J. HANEKAMP, P. J . GELLINGS Appl.Surf.Sci,28 (1987) 189-203. 19) D.R. BAER, M. D. MERZ Metal. Trans 11A.12 (1980) 1973-80. 20)M. LAMBERTIN, J.C. C O L S O N Oxid. Met. 7,3,(1973) 163-171. 21) B. GATEHOUSE Am. Mineral 63 (1978) 28) 22)K.PRZYBYLSKl,A.GARATT-REED,G.J. YUREK J.Electrochem.Soc,135,2 (1988) 508. 23) M.J. BENNETT, D.P. MOON The role of active elements in the oxidation behaviour of high temperature metals and alloys E. LANG (Ed), ELSEVIER, LONDON (1988)111-129. 24) R.J. HUSSEY, M.GJ. GRAHAM 3rd Int. Symp. High Temp. Cor. May 1992, Les Embiez France. . 25) B. PIERAGGI, R.A. RAPP 3rd Int. Symp. High Temp. Cor. May 1992, Les Embiez.France.