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On the Scaling of Metals and Alloys
Scheil, E.
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NATIONAL RESEARCH COUNCIL OF CANADA TRANSLATION T T
-
157ON THE SCALING OF METALS AND ALLClYS
(2, M e t a l l k d e . 2 9 , 209, 1 9 3 7 ) B Y Erich S c h e i l T r a n s l a t e d by
Dm
J a W r i g h t O t t a w a S e p t e m b e r 1950NATIONAL RESEARCH C O U N C I L
Ottawa, Canada
TECHNICAL TRAIJSLATION TT
-
157D i v i s i o n of Information S e r v i c e s
On t h e S c a l i n g of Metals and A1Pogs0 (Ueber das Zundern von Metallen und Legf srungen, )
By: E r f c h SehePP
R e f e ~ e n c e 2 Zeitsehrfft fuer MetaPlkunde -J 29 209, 1937, Translated by:
D o
J,
W i g h tOn
the Scal7ng 03%' :IIBX~~T ~ i r ~ i g a d ' l " r ~ : y s ~Vfhile t h e y are being t r e a t e a , many metals are fl-eel?
exposed to a i r duping a n n e a l i n g and Cheip sasfacea thus be-
come oxfdlzed, The noble metals alone a r e an exception,
a h c e t h e decamposf t i o n pp.b@ssum~ 6% t h e i r exidea a r e above
the oxygen p a r t i a l p r e s s u r e of t h e ais. N f t r i d e a a p p a ~ e n t l y do not form when annealing comercia3Xy ueed metals In a l p ,
QP a t l e a s t not i n a p p r e c i a b l e amounts, More exact inves%-
f g a t i o n s have not y e t been made, M ~ r e a v e r , i f one d f s ~ a g a ~ d s
t h e gaseous ( e , g , Moo3) o r lfqufd Ifs,g, V2Q5) ~ e a e t f o n p~od.2
UC$S w h i c i ~ ape only r a p s l y obtained, it appears % h a t s e a l i n g
%n air r e s u l t s f n t h e formation of a c r y s t a l l i z e d or even g l a s s y oxfde la ye^, on p e r m e a b i l i t y and adhesive srrength
of whfeh t h e furthema course of s e a l i n g and t h e deveiopmerit of t h e metal supface under the s c a l e layep depends,
S e a l i n g o f Pups Metals
---
--
-=---The oxfdas are f o ~ m e d on $he m e t a l fn t h e order of
t h e f ~ ~ x f d e content; m a t a l % i ~ s x f d e s may even appe2pau QP one
1
of the st,ab'ae oxfdsa n m y be %ac;k%ng, The f i r s t monaomsleca3.a~ a rafe l a y e r forme v erg. quic%;yo F u ~ t h e ~ thickening t a k e s
p l a c e due t o d i f f u s i o n in t h e s c a l e l a y e r , Craeka o r e p P i t s
In
t h e scale reduce the a f f f u s % o n path, khersby aceals~atfng t h e s c a l l n g oIf t h e seabe Payer c o n s f a t s sf only one type of c q s -
%a%,
t h s n its r a t e of t.hickenfng d n i d z is i n v e ~ s e l y pkopsrt-- f s n a l t o t h e t h i c k n e s s n of t h e scale layer ala"eadv p r e s e n t% Paper p ~ e s e n t e d to t h e Deutsehe G e s s l l s e h a f t f u a r Matall-
a t t h e time z (1, 23:
dn/dz = l\Kn
or, by i n t e g r a t i n g
n z</m = z j f K
Kg r e p r e s e n t s t h e ~ x t e n t t o whieh scaling is checked by %he
~ c a l e l a y e r whfeh a l r e a d y e x f s t s , P i P l i n g and Beawsrth a b b r e v i a t e d from now on t o P o and
B,
-
found t h a t t h i s $%me Taw was v a l i d for t h e m a j o r i t y of metals, even when t h e oxide l a y e r c o n s i s t e d of s a v e p a l oxides of the m e t a l , Byc a r e f u l experiment,
W,,
F e f t k n s e h t ( 3 ) found t h a t t h e s e a l - ing of copper a t f i r s t takes p l a c e more r a p i d l y t h a n t h a q u a d r a t i c law s u g g e s t s , I n t h e i n i t i a l stage F e i t k n e c h t found s t r u e t u r a P ehangee in t h e s c a l e l a y e r $0 which hea t tr i b u t . e d t h e d e v i a t i olns, Within t h e range of o x i d a t i on
t i n t s , i ,e, when t h e s c a l e Payers wepa very t h i n , G o T a m -
ann and
W,
Koester ( 4 ) discovered an exponential t i m e Paw whieh is s t i l l d i f f i c u l t t o e x p l a i n (51, If we d i s r e g a r d t h e s e f n f l 9 . a l phanomsna, the q u a d r a t i c law is w e l l coprob- o r a t s d by t h e experfrnents,A Sew m e t a l s , however, a r e s u b j e c t t o a eompPetely d f f f e r e n t time law o f ssaling, F i g , B shows t h r e e scafa fsothekms of t u n g s t e n , The i n c r e a s e
i n
weight was d e t e r - m i n e d b~ weighing t h e a m p l e i n t h e f u r n a c e , The devfaz- fona i n t h e measurements a s s due t o variatfoxls in tamper- a t u r s ; t h e f u r n a c e was not r e g u l a t e d duping the n i g h t oA t 900' t h e s e a l e no longer adhered t o t h e smp1e but f e n 1 f n t s a small ~ e c e i v % n g pan and was weighed t o g e t h e r w i t h it 161, The m o u n t of s e a l e on the faotherma 500° and 700"
increased in proportion to the time,
P o
and B, f o u n dshe
same linear time Baw in the case of ea1efu.m and magneszum, The author ohecked thelr flndings for magnesium axnd e m corroborate them,
The linear time law means that protection againsn
scaxfng does not become greater as the m a l a layer is pe-
info:~eed, This, however, does not explafn why the exist+
%ng scale layer in no way checks further scaling for, d u e to the great rapidity of the cleavage plane reactions, the metals obeying the linear tfme law of sealing would k ~ i v e
to scale completely so quickly that it would no kongex be possible to follow the reaction by weighing, The linear
tlme law, however, is also satisfied when split= occur
periodically in the scale layer, The appearance a% the scale layer, which is to be discussed below, aubstantla%ea this assumption,
Tns acceleration of the scaling which is noticeable in the isotherm 900Q can be attributed to an increase in
t h e temperatwas of the aampla due to the heat of" r,xfdat,fon,
A
much more marked acce9eration of the sealing above a certain temperature was found by P o andB,
in the case ofe a l c i u and magnesium,
In the e a s e of these two metals, P, and B, explain-
ed the linear tfme l a w by the decrease in volume during the formation of oxide, According to
K O
Fisehbeck (79,in the following metals the metal has
a
g ~ e a t e r vsfume than bhe oxfde with the same number of metal atoma:Li, Na, Kg Cs, Rb,
Mg,
Ca, ST,e a s e s almost t h r e e f o l d d u r i n g o x i d a t i o n , A s i s now shown, i t i s even doubtful. I n t h e case of calcium and magnesium whether t h e decrease i n volume i s r e s p o n s i b l e f o r t h e be- havf our of the l i n e a r time law,
The form of t h e s c a l e substance v a r i e s according t o t h e time law of t h e s c a l f n g , F i g , 2 I n d i c a t e s the appear- ance of an oxidized copper sample ( q u a d r a t i c law) and Fig,
3 t h a t of an oxidized tungsten sample ( l i n e a r law), With t h e s c a l f n g of copper, a s c a l e l a y e r forms on the o r i g i n - a l l y sharp-comered sample, t h e dimensions of which ( a p a r t from t h e somewhat rounded c o r n e r s and e d g e s ) a r e proport-
i o n a l t o t h e dfmensfons of t h e o r i g i n a l sample, The t h i c k - ening of t h e s c a l e l a y e r on copper must t h e r e f o r e r e s u l t from t h e formation of l a y e r s of t h e s i z e of t h e s c a l e sur- f a c e s which a t times e x i s t ; f o e , the s c a l e l a y e r develops a t the s c a l e - a i r i n t e r f a c e ( 8 ) ,
I n t h e case of the tungsken sample i n F i g , 3, how- ever, the formerly p r o j e c t i n g edges have shrunk inwards, This type of s c a l i n g i s i l l u s t r a t e d even more c l e a r l y by t h e Wfdia sample shown i n F i g , 4 ( t u n g s t e n carbide i n a c o b a l t groundmass) ( 9 ) i n which the s c a l e l a y e r i s more compact and h a s a smooth s u r f a c e , even where t h e e d g e s have shrunk inwards, The r e c e s s i o n of t h e edges i s ex- p l a i n e d by t h e development of a l a y e r with t h e same dimen-
s i o n s a s t h e metal-scale s u r f a c e , Generally speaking, f n t h e case of t h e type of r e a c t i o n substance i n Fig, 2 t h e thf ckening of t h e r e a c t i o n l a y e r t a k e s p l a c e i n t h e o u t e r s u r f a c e while i n t h e c a s e of a r e a c t 1 on substance such a s
t h a t i n F i g s , 3 and 4 i t does so on t h e i n n e r s u r f a c e , I h S l e t h e t e s t - p i e c e such a s shown i n F i g s , 3 an& 4 i s understandable i n t h e case of tungsten which i n - c r e a s e s i n volume a s it s c a l e s , we canno't e x p l a i n t h e appearance of t h i s t e s t - p i e c e i n the c a s e of calcium 110) and magnesium, Since they de'crease i n volume d u r i n g
s c a l f n g , t h e s u r f a c e s should f a l l inwards i n s t e a d of s t a n d i n g out i n r e l i e f o The d e c r e a s I n volume must t h e r e f o r e be more than balanced by a s u b s t a n t i a l i n c r e a s e i n volume which i s probably due t o t h e f o r m a t i o n of s p l i t s , It t h u s appears t h a t even w i t h calcium and magnesium, t h e r e d u c t i o n i n volume i s n o t t h e main f a c t o r t o have a b e a r - f n g on t h e scalfngo
P o
and Bo have found t h a t aluminum and cadmium a r e i n a s p e c i a l p o s i t f on w i t h r e g a r d t o %he course of s c a l i n g i n r e l a t i o n t o time; a f t e r a s l i g h t , r e l a t i v e l y r a p i ds c a l i n g a t t a c k , i t almost comes t o a complete s t a n d s t i l l ,
C,
Wagner( 1 1 1
s e e s a p o s s i b l e e x p l a n a t i o n f o r t h f s i n t h e f a c t t h a t i n t h e case of aluminum ( 1 2 ) and of cad- mium (13) t h e oxides which a r e formed a r e p r i m a r i l y un-s t a b l e ,
The t h i c k e n i n g of t h e s c a l e l a y e r i s caused by d i f f - u s i o n , Since t h e compounds a r e h e t e r o p o l a r t h e i o n s and
e l e c t r o n s , r a t h e r t h a n t h e n e u t r a l atoms, migrate (141, It i s n e c e s s a r y t o assume t h e m i g r a t i o n of e l e c t r o n s , f o r
otherwf s e h i g h e l e c t r o s t a t i e charges would be h u i l t up a t t h e p o i n t s from o r t o whfch an i o n migrated and such
charges would n o t permit t h e movements which have taken p l a c e , We do n o t wish t o d e a l w i t h t h e s e a t t e n d a n t phen-
omena h e r e b u t w i l l l i m i t o u r s e l v e s t o t h e i o n i c m i g r a t i o n s , One n u s t d i f f e r e n t i a t e between t h e d i f f u s i o n of t h e a n i o n s and t h e c a t i o n s , s i n c e t h e r a t e o f t h e two r e a c t i o n s g e n e r - a l l y d i f f e r s s o g r e a t l y t h a t o n l y t h e d i f f u s i o n of t h e one t y p e of i o n i s t o be c o n s i d e r e d ,
T h i s i s p a r t i c u l a r l y t r u e when one of t h e two t y p e s o f i o n i s p r e s e n t i n l e s s q u a n t i t y t h a n i s r e q u i r e d t o ach- i e v e t h e i d e a l l a t t i c e s t r u c t u r e of t h e o x i d e , T h i s i s t h e c a s e , f o r example, w i t h f e r r o u s o x i d e , According t o
R ,
Schenk ( 1 5 ) , f e r r o u s o x i d e h a s a lower i r o n c o n t e n t t h a n t h e s t o i c h i o m e t r i c c o m b i n a t i o n o f FeO demands, By comparing t h e volume o b t a i n e d by a d e t e r m i n a t i o n of t h e d e n s i t y w i t h t h a t c a l c u l a t e d f r o n X-ray d e t e r r n i n a t i o r ; ~ , E ,
R,
J e t t e andF o
F o o t e ( 1 6 ) have shown t h a t t h e f e r r o u s o x i d e l a t t i c e l a c k s s i t e s i n t h e ~ e + * p a r t i a l l a t t i c e , According t o G u Haegg t h e same t h l n g i s t r u e of FeS ( 1 7 ) and FeSe ( 1 6 ) ,
I n a l a t t i c e o f t h i s s t r u c t u r e , of c o u r s e , t h e i r o n i o n s n e a r an u n p o p u l a t e d p l a c e c a n e a s i l y change o v e r t o i t , The d i f f u ~ i c n of t h e i r o n i o n s w i l l t h e r e f o r e be e x t e n s i v e i n cornparison w i t h t h e d i f f u s i o n of t h e a n i o n s ,
S i m i l a r l y ,
H,
Duenwald and C , Wagner ( 1 9 ) a l s o assume t h a t t h e r e a r e u n p o p u l a t e d s i t e s i n t h e c o p p e r i o n p a r t i a l l a t t i c e of c u p r o u s o x i d e , because o f t h e f o l l o w i n g o b s e r v a t -*
i o n s , I n e l e c t r o l y t i c t r a n s p o r t e x p e r i m e n t s , t h e Cu i o n s m i g r a t e d , Moreover, t h e conductance of t h e c u p r o u s o x i d e i n c r e a s e d a s t h e oxygen p r e s s u r e r o s e , i . e , a s t h e oxygen c o n t e n t of t h e cuprous o x i d e i n c r e a s e d and t h e number of d e f e c t s t h e r e f o r e i n c r e a s e d , From measurements made f o r ap r e s s u r e of 30 mm H g a t 1 0 0 0 ~ , Duenwald and Wagner c a l c i l l -
-3
a t e d t h a t t h e r e was a co;nper d e f i c i t of a b o u t 10 g atoms of Cu, The d e v i a t i o n s from t h e s t o i c h i o m e t r i c combination a r e t h e r e f o r e s e v e r a l powers l e s s t h a n i n t h e c a s e of f e r r o u s o x i d e , I n c o n t r a s t t o t h i s r e s u l t , we have K O W, F r o e h l i c h s s
( 2 0 ) a n a l y t i c a l d e t e r m i n a t i o n s of t h e copper c o n t e n t of cup- r o u s o x i d e , a c c o r d i n g t o which a copper e x c e s s of about 0,2$ Cu ( a v e r a g e of t h e a n a l y s e s g i v e n ) was supposed t o e x i s t . No e x p l a n a t i o n f o r t h e d i f f e r e n c e i n t h e r e s u l t s has y e t been
found ,
The a u t h o r r e g a p d s Viagner P s e x p e r i m e n t a l method a s more c e r t a i n t h a n t h e a n a l y t i c a l o b s e r v a t i o n s and assumes
a s Viagner does t h a t t h e r e i s a copper d e f i c i t i n t h e cuprous o x i d e ,
With r e g a r d t o t h e o x i d e s of o t h e r m e t a l s , i t i s a known f a c t t h a t i n t h e c a s e of z i n c oxide ( 2 1 ) , cadmium oxide (211, aluminum o x i d e ( 2 2 ) and t a n t a l u m o x i d e (221, t h e conductance d e c r e a s e s w i t h t h e oxygen p r e s s u r e , i , e , t h e r e i s a n e x c e s s of t h e m e t a l , On a c c o u n t of t h e s m a l l
0
++
-
0 's i z e of the m e t a l l i c i o n s ( ~ n + + = 0.85 A, Cd
-
0 , 0 5 A ,~ 1 + + + = 0 , 5 5 A 0 and Ta '&'++ = 0 0 7 A ) (231, von Baumbach and 0
Wagner assumed t h e a r r a n ~ e m e n t of t h e e x c e s s m e t a l l i c i o n s on i n t e r m e d i a t e l a t t i c e s p a c e s ,
No s t u d y h a s y e t been made, however, of t n e m a j o r i t y of t h e m e t a l l i c o x i d e s , The l e n g t h s of t h e i o n i c r a d i f l e a d one t o b e l i e v e t h a t o n l y m e t a l l i c i o n s m i g r a t e i n t h e o x i d e s f o r t h e i o n i c r a d i u s of oxygen, which i n c r e a s e d t o 1.3
X
due t o t h e a b s o r p t i o n of two e l e c t r o n s , e x c e e d s t h e i:1-~,4.c r a d i u s o f e o m n ~ e r c i a l l y used m e t a l s whose l e n g t h has been r e d u c e d by t h e r e j e c t i c n cf e l e c t r o n s (23), A c c o r d i n g l y , t h e new 1 a t t i c . e p l a n e s of t h e s c a l e l a y e r a r e formed a t t h e s c s l e c r a i r i n t e r f a c e ,. %hen t h e s c a l e l a y e r i s c o r r e c t l y Termed it i s t h e r e f o r e l i k e t h a t shown i n F i g , . 2 , )
If t h e a n i o n s were t o m i g r a t e , however, t h e nevr l a t t - i c e p l a n e s would then be f'ormsd a. t t h e s c a l e - m e t a l i n t e r f a c e , g i v i n g , a s c a l e s u b s t a n c e l i k e t h a t shown i n F i g s , 3 and 4,, One d a r e n o t c o n e l u d e , h o v ~ e ~ ; e r , on the b a s i s of t h e a p p e a r - a ~ c e of t h i s t y p e of s c a l e s u b s t a n c e a l o n e , t h a t t h e a n i o n s m i g ~ a t e , Due t o t h e p e r 3 d i . s f o r m a t i o n of c r a c k s , t h e new f o r m :ion ~ ? f tile la?. Lice 91a.nes
-
e v e n when f t t a k e s p l a c ea t %he s c a l e - a i r i n t e r f a c e
-
can always be t ~ a n s f ' e r ~ e d t o t h e nefghbourhood of t h e s c a l e - m e t a l i n t e r f a c e , When t h e m e t a l l i c i o n s m i g r t i t c , b o t h t y p e s o r s c a l e s u b s t a n c e mag t h e r e f o r e a p p e a r , , 9ne can c o n c l u d e from t h e f o r e g o i n g t h a t ,i n t h e c a s a of the o x i d e s o,F t u n g e t e n , magnesium and c ~ l . c i u m t o o , t h e r n e t a l l f o i o n s m l g ~ a t , e ,
I n preqricv.s I n v e s t i g a t i o n s , no c o n s i d e p a t i o n w a s
g i v e n t o t h e f a o t t h a t some m e t a l s f o r m s c a l e S a y e r s of' sev- e ~ a l o x i d e s , Of t h e m e t a l s which a r e of comrriercial import-
a n c e , i r o n (Fs3;. Fe,O and Fag03) and c o p p e r (CuZO and CuO)
L C 4
are i n c l u d e d i n t h i s c a t , e g o r y . According t o t h e i n v e s t i g a t - i o n s m a d e by
H, H,
von Bau.mbach andH,
Duenwald and C , Wagner, t h e conductance b e a r s h a r d l y any r e l a t i o n t o t h e oxygen p r e s s -.t.+
low (n ++
<
5.
loB3)
and was s t i l l w i t h i n t h e margin of c ue x p e r i m e n t a l e r r o r , The c u p r l c o x i d e was t h e r e f o r e supp- osed t o l i m i t t h e s c a l i n g of t h e copper t o a c o n s i d e r a b l e e x t e n t , According t o F e i t k n e c h t D s measurements ( 3 ) , t h e r a t e of s c a l i n e , a t low oxygen prpessures below t h e e q u i l f b - r i u m p r e s s u r e of Cu20 + CuO i n c r e a s e s a s t h e oxygen p r e s s - u r e of t h e g a s r s i e s , and becomes c o n s t a n t when t h e e q u i l -
ibrpium p r e s s u r e i s a c h i e v e d , A s CuO forms ,' t h e s c a l i n g
r a t e of c o p p e r t h e r e f o r e d i m i n i s h e s b u t i s n o t c o r r ~ p l e t e l g a r r e s t e d ; f u r t h e r s c a l i n g i s r e r e l g r e s t r f c t e d , P r o b a b l y t h e CuO l a y e r does n o t forn? a c o n p a c t c r u s t , T h i s i s con- f i r m e d by t h e f a c t t h a t t h e volume d e c r e a s e s a s t h e Cu 0
2
c h a n ~ e s i n t o CuOo The phenomena have s t i l l n o t been t h o r - c u g h l g e x p l a i n e d ,
Both in c o p r > e r and i n i r o n , t h e m e t a l l i c i o n s move most e a s i l y i n t h e o x i d e which i s p o o r e s t
In
oxygen; i t I st h i s l a y e r , t h e r e f o r e , which has t h e g r e a t e s t b r e a d t h , S c a l i n g o f A l l o y s
-
-
The o r d e r of o x i d e f o ~ r n a t i o r i , - I n t h e c a s e of a l l a y s t h e---
o x i d e s of t h e v z r i o u s a l l o y i n g e l e m e n t s , and s o m e t i n ~ e s even c o i r b i n a t i o n s of t h e s e o x i d e s ( s , . g o F e 2 S i 0 4 ) . a r e f ' ~ r r o a r 1 .Fig, 5 r e p r e s e n t s t h e sinipLes5 c a s e o f a n a l l c y of t w o corn-
p o n e n t s , i n whSzh o z l y t h e twc ~ e t a l s and t h c l r o x f d s s asp- e a r a s t y p e s of c ~ y s t a l , L e t u s c o n s i d e r t h e changes in a t h i n boundary l a g s ? a f a m i x t u r e , Due t o o x i d a t % i o n th4
s t r u c k a r e c f t h e Lsyer s h i f t s a l o n g t h e l i n e XO from X t o 0 ; 4,n d o f n g t h i s , t h e s t r u c - t i l r e of t h e l a y e r f i r s t e n t e r s tihe
f i e l d A + l3
+
BOO A t f i r s t , t h e r e f o r e , o n l y t h e c r y s t a l t y p e S i s o x i d i z e d s o that when t h e l i n e XO b i s e c t s t h e l i n e A+
BO a l l t h e B of t h e l a v e r i n q u e s t i o n i s a x f d f z e d , A s t he o x i d a t i o n p r o g r e s s e s fur the^, t h e boundary l a y e rr e a c h e s the f i e l d A + RC + BO and t h e o x i d a t i o n of k b e g f n s , It i s complete whabl t h e l i n e XO h a s peached t h e l i n e A 0 + BOO
F i g o 6 r e p r s s e n L s a sornewt-lat n o r e c o m p l i c a t e d c a s e : i n a d d i t i o n ta t h e o x i d 2 s AO and 30, we a l s o have t h e corn- b i n a t t o n of t h e m , A 0
.
B O O A s o x i d a t i o n o c c u r s , BO forsmsf i r s t , t h e n AC
.
130 f o r m s a s A o x i d i z e s and t h e r e s u l t a n t A0 r e a c t a w i t h t h e BO whj.c;ti i s a l r e a d y p r e s e n t , and f i n e l l yAG a l s o forrms, The more c o m p l i c a t e d c a s e s c a n be s i m i l a r l y
desc1-ibed . I n b o t h t h e c a s e s under c o n s i - d e r a t i o n , , one w i l l . s a y t h a t A i s n o b l e r t h a n B , B y l i m i t i n g o u r s e l v e s t o the l o w e s t s t a g e c f o x i d a t i o n o f e a c h m e t a l and t o a l l o y s of t w o m e t a l s , , t h e n l e t a l s c a n be a r r a n g e d i n a s e ~ i e s i n s u c h a way t h a t e a c h m e t a l o x i d i z e s s o o n e r t h a n t h e one f e l l o w -
2ng i t .In t h e s e - i s s (251, For* t h e c c r r e s p o n d i n g sulphi,Ae s e r i e s , G o Taimann and H o von Samson-Hfmmelstjerna ( 2 6 )
have d e t e r m i n e d t h e sequence of t.b.e m o t s l s e x p e r i m e n t a l l y , Although suitable e x p e r i m e n t a l d a t a a r e l a c k i n g f o . ~ t h e o x l . d a t i o n s e r i e s , one c a n a p r a n g e t h e m e t a l s by o r d e r of o x i d a t i o n by : ; I a s s i f y i n g t h e e l & m e n t s a c c o r d i n g t o d i u n h -
ishEng h e a t s of f o r m a t i o n cf t h e f r l o w e s t o x i d e s , c o n v e r t e d t o t h e h e a t o f forrr~at;ion o f one o x i d e , MeO,
c o p r e c t -to base i n v e s t i g a t i o n s on t h e h e a t s of f o r m a t i o n c o n v e r t e d t o MeO. T h i s q u e s t i o n may be d i s c u s s e d i n t h e l i g h t of F i g , 7 , It i s assuxed t h a t o n l y t h e h e a t s of f o r m a t i o n p r o v i d e any starldard f o r t h e f r e e e n e r g y , The o x i d e B 0 i s l e s s noble t h a n A @ because t h e s t r a i g h t l i n e
2
A + B "O l i e s beyond t h e d o t t e d l i n e B-A09, I n o r d e r f o r
2
t h i s t o be v a l i d f o r o t h e ~ o x i d e s B 0 a s w e l l ( e , g , f o r x Y
B O ) , t h e s t r a i g h t l i n e BO" must l i e beyond B O Y , One can
now e i t h e r use,, a s G u e r t l e r ( 2 5 ) d i d , t h e a n g l e bet.-t~een
BO and BO" o r t h e ~ a l u e which t h e l f n e BO" assumes a t a c e r t a i n p o i n t , e,g,. f o r t h e c<:~riposi.tTon BO, a s a measure of t h e a f f i n i t y o The second of t h e s e measures was u.sed i n Table 1, I n column 1 s f t h i s Table we g i v e t h e chem- f c a l symbol, i n c o l ~ ~ m n 3 t h e f'ormtlla of t h e o x l d e which i s p a o r e s t i n oxygen, i n col~.imn 3 i t s h e a t of f o r r l ~ t i o n
and i n column 4 t h e heat. cf formatl..on c o n v e r t e d t o t h e oxide ItIeO,
The sequenca of' t h e T a b l s c o r r e s p o n d s t o t h e oxfd- a t i o n s e r i e s a t room t e n p e r a t u r e , assuming t h a t t h e d ~ -
t e r m i n a t i o n of t h e f r e e e n e r g y i s based p r i m a r i l y on tb
h e a t of f o ~ m a t i o n , When t h e v a l u e s a r e v e r y c l o s e t o one a n o t h e r d e v i a t i o n s may appear because, on t h e one h a n d ,
o t h e r f a c t o r s p l a y some p a r t i n t h e d e t e r m i n a t i o n o.f t h e f r e e energy and, on t h e o t h e r hand, i t would be most. f i t t i n g ' t o use t h e h e a t s of' f o r m a t i o n a t s c a l i n g tempzr- n t u r e f o r t h e b a s i s of t h e s t u d y , For t h i s r e a s o n , f o r e x m p l e , copper does n a t appeala i n t h e s e r i e s i n tile order
made durfng s c a l i n g .
Changes i n t h e sequence may a l s o be produced, moreover, i n t h e c a s e of a l l o y s of t h r e e o r more m e t a l s
due t o t h e f o r m a t i o n of combinations of t h e i n d i v i d u a l o x i d e s , Thus, f o r example, t h e sequence f o r s i l i c a t e s w i l l n o t always a g r e e w i t h t h a t f o r oxides. To a c h i e v e a complete s y n o p s i s , t h e oxide s e r i e s would have t o be extended t o i n c l u d e f u r t h e r s e r i e s of combining compon- e n t s , b u t t h i s i s n o t p o s s i b l e a t t h e p r e s e n t t i m e , The d i s t r i b u t i o n of t h e o x i d e s i n t h e s c a l e l a y e r , - I n c o n n e c t i o n w i t h t h e development of t h e s c a l e s t r u c t u r e i t i s s i g n i f i c a n t t h a t t h e o x i d e s of the added m e t a l s a r e g e n e r a l l y e n r i c h e d a t p a r t i c u l a r p o i n t s i n t h e s c a l e l a y e r , namely a t t h e s c a l e - m e t a l i n t e r f a c e , I n d e r i v i n g t h i s r u l e we assume t h a t t h e oxide A 0
does not d i s s o l v e B and l i k e w i s e BO does n o t d i s s o l v e A, and f u r t h e r m o r e t h e n o b l e r A should be p r e s e n t i n consfd- e r a b l e e x c e s s , Then t h e l e s s noble oxide BO Forms on t h e s u r f a c e f i r s t , The c a s e of acornpletely ccunpact c o a t - f n g of t h e s u r f a c e by t h e oxide BO, which i s p a r t f c u l a r l y i m p o r t a n t i n t h e p r o d u c t i o n of a c a l e r e s i s t a n t a l l o y s , w i l l be mentioned below, It i s assumed i n t h e Following
d i s c u s s i o n t h a t the BO l a y e r has gaps i n which A 0 can
form, I n b o t h t y p e s of c r y s t a l , f u r t h e r development t a k e s p l a c e due t o d i f f u s i o n through t h e s c a l e c r y s t a l s , i , e , because of t h e l a c k of d i s s o l v i n g power of t h e two o x i d e s , t h e A i o n s m i g r a t e o n l y t h r o u g h t h e A 0 l a y e r and t h e B
i o n s only through t h e 30 l a y e r and form new l a t t i c e p l a n e s of t h e s c a l e - a i r i n t e r f a c e , Since t h e concen- t r a t i o n of B or A plays an e s s e n t i a l p a r t i n t h e speed of t h i s p r o c e s s , more A 0 i s formed i n a c e r t a i n u n i t of time than BO. The A 0 would e v e n t u a l l y outgrow t h e BO
because it alwags develops a t t h e scale-metal i n t e r f a c e , !;loreover, t h e A atoms w i l l p a r t i a l l y m i i r a t e around t h e
BO p a r t i c l e s s o t h a t BO t a k e s some p a r t i n t h e s h i f t i n g of t h e surface towards t h e c e n t r e of t h e metal sample,
This r e s u l t s i n an enrichment of t h e BO a t t h e s c a l e i n t e r f a c e . This could be proved i n t h e case of i r o n a l l o y s by a n a l y s i s of t h e i n d i v i d u a l s c a l e l a y e r s which could be s e p a r a t e l y detached ( 6 ) . F i g . 8 g i v e s a s an example t h e measurements of iron-aluminum a l l ~ y s , The d i s t a n c e from t h e l i n e s r e p r e s e n t i n g equal d i s t r i b u t i o n g i v e s t h e i n d i c a t i o n of t h e e x t e n t
t o
which they had been enriched or impoverished. A t temperatures of over 1000° t h e enrichment of t h e alumina i n t h e s c a l e - a l l o y i n t e r - f a c e l a y e r i s p a r t i c u l a r l y g r e a t . The enrichment of t h e l e s s noble admixture a t t h e s c a l e - a e t a l i n t e r f a c e occurs with a l l i r o n a l l o y s w i t h t h e exception of manganeses t e e l ( 6). According t o K. X , F r o e h l i c h ( 2 0 )
,
t h i s law has a l s o proved t r u e r e p e a t e d l y f o r copper a l l o y s ,If t h e metal A which i s p r e s e n t i n excess i s l e s s noble t h a n B, t h e P;b forms f i r s t and B i s enriched a t the
scale-metal i n t e r f a c e . It cannot oxidize h e r e , however, because t h e more e a s i l y o x i d i z a b l e A i s alwags p r e s e n t ,
S i n a e t h e %hf ckening o f the l a y e r t a k e 8 plaee at the
s c a l e - a f p i n t e r f a c e , n o t h i n g 1 9 changed u n t i l t h e con-
c e n t r a t i o n of
B
a t t h e s c a l e boundary f o ~ m s a l a y e r o e e l u d l n g t h e A and t h e f u r t h e r o x i d a t i o n t h e n t a k e s p l a c e ars if t h e n o b l e r I3 wepe p r e s e n t in e x c e s s , BOnow a p p e a r s t o o u n t i l t h e e x c e s s has a g a i n been reduced t o a c e r t a i n amount, A h 1 i n a l l , enrichment of the noblezc s u b s t a n c e a t t h e s c a l e - m e t a l interoface i s t h e r e s u l t of t h e s e changes i n c o n e e n t r a t f o n , I t s exist- ence was d e t e c t e d i n t h e scale of s e v e r a l i r o n a l l o y s
( 6 ) and in c o p p e r - s i l v e r a l l o y s which were r i c h in copper q20),
It i s t h e r e f o r e e s t a b l i s h e d t h a t a s a r u l e t h e elements which aye present. i r i s l i g h t e r amounts ape e n r f c h a d a t t h e s c a l e - m e t a l i n t e r f a c e , a s oxlde when t h e admixed substance is 19ss noble and I n m e t a l l i c
form when f t i s noble^ t h a n t h e m e t a l present f a r ic
The p i c t u r e g i v e n of t h e s c a l i n g p r o c e s s f s
v e r y sketchy, b u t i t ! shows t h e main f e a t u ~ e s , , F u ~ t h e s
%nnigh% 8 s a f f o ~ d s d by a g l a n c e a t t h e structure of the s c a l e b o u n d a ~ y , F i g , 9 gives a s an example t h e s t ~ u e t u r e of a s t e e l w i t h 4% A i which was subjected t o acaling f o r 6 hours a t 1 0 0 0 ' ~ The alumina, e a s i l y d f s t f n g u f s h a b h e from t h e i r o n oxldea by i t s eoloup, p r o j e c t e d i n t o t h e i r o n l a y e r , These advancing o x i d e s
-
15-
its a l f g h t d i f f u e a b i l i t y In i r o n , p e n e t r a t i n g t h e
i r o n and t h e r e o x i d i z i n g t h e aluminum, It
i s
p r o b a b l y n o t a case of tkue d i f f u s i o n b u t of an advance along cleavage p l a n e s o r g r a f n boundaries, A s soon as t h i soxygen e n c o u n t e r s an aluminum atom, i t w i l l r e a c t w i t h
I t and be added t o a l r e a d y e x i s t i n g alumina c r y s t a l s as alumina OP w i l l form new alumina n u c l e i ,
I n
t h i s wag, t h r e a d s of alumina would g r a d u a l l y form i n s i d e t h e s t e e l , u n l e a s t h e metal boundary grew inwards a t t h esame t h e due t o t h e f o r m a t i o n of i r o n s c a l e , Because of t h e f a c t t h a t t h e i r o n changes i n t o s c a l e , t h e i r o n between t h e t h r e a d s of alumina d i s a p p e a r s and t h e alum- i n a i s e n r i c h e d n e a r t h e s c a l e boundary,
The advance o f t h e alumina t h r e a d s from t h e
s c a l e boundary w f l l c o n t i n u e l o n g e r s t h e l e s s t h e o x i d i z - i n g s t r e n g t h of t h e gas, I n t h e case of weakly oxfdiz- i n g g a s e s , t h e r e f o p e , t h e s p e c i a l s c a l i n g can p r o j e c t d e e p l y i n t o t h e m a t e r i a l , c a u s i n g c o n s i d e r a b l e harm because af the l o n g a t t a c k , This has occurred i n t h e
c a s e of c a s t %POD, in which the g r a p h i t e seams p r o v i d e
p a t h s f o r t h e p e n e t r a t i o n of t h e oxygen i n t o i t s i n s i d e l a y e r s , The o x f d a t i o n s t a r t s i n t h e g r a p h i t e seams, The oxygen e o n e a n t ~ a t i o n , however,
i s
s o s l i g h t t h a t a t f i r s tno i r o n o x i d e s b u t o n l y i r o n s i l i c a t e s are formed f r o m
t h e s f l i e o n e o q t e n t of t h e c a s t i r o n (27)- Only a f t e r t h i s has m a d e eoxlaiderable progress and t h e g r a p h i t e seam is mostly g a s i f f e d , do i r o n o x i d e s form i n p l a c e of
-
16-
been subjected to 8caling, In place of the graphite
seama,
ferrosoferric oxide for the
moat
p a r t hasal-
ready f
ormed. Emanating from the graphite
seams,a
very finely dfspersed amount of iron and iron silicate
has formed,
Seale
resistant alloys,- The question now arises
asto
whether ane
should mixnobler or baser substanc88,
Both
are enrfehed at the metal-scale interface, and
one might
thinkthat a compact layer might form which
would
effectively check the
f u r t h e rdevelopment
of seal-ing, It
must beobserved, however, that the noble
metals can
bepenetrated by oxygen, Froehlieh
(20)has
shown
that silver plating
doesnot protect copper against
scaling, The conpactness of the
l a y e rplays some part
of
t x v n u f i , butPt can hardly be expected thst scaling
would produce a more oompact layer than silver plating.
In maLy casss, partfcgularly in the scaling of nickel
steels
(28), astrong nickel layer
mixedwith
s c a l e isfrequently
f o r m a d , Quiteapart from the cost involved
one can hardly expect to achieve any real protection
against scaling by
a13oging anoble metal to the basfo
metal,
All scale-resistant alloys contain a baser
eiem-ent
intended to reduce the scaling, The scale-resistant
chromim-nickel-steels, made of iron to which a nobler
element, nickel, and a baser element, chromium, have
beenqdded, are no exception
f o rthe additive introduced to
ine~ease
the scale resistance is ch~omium
exclusively
( 6 ) -I n o r d e r %o achieve r e a l s c a l e r e s i s t a n c e , a t i g h t l y
sealed l a y e r must be formed on the alloy by t h e oxide of a
b a s e r secondary me",l. The r a t e of m i g r a t i o n of t h e m e t a l l -
fe i o n s i n thfs l a g e r must be very bow because t h e thfcken- i n g of t h e l a y e r dependa on f t ,
The e x t e n t of t h e d i f f e r e n c e i n s e a l f n g a t t a c k i s shown i n F9go 11, taken from a paper by A, Portevin,
E,
P r e t e t and
H,
JoPivet (29). As long a s t h e aluminum s t e e l w i t h 8,5$ AP was covered only by t h e white alumina s c a l e ,t h e i n c r e a s e in weight due t o s c a l i n g was very s l i g h t , From
a c e r t a i n moment on, whfch v a ~ f e d f o r t h e i n d i v i d u a l samples, a considerably s t r o n g e r s c a l f n g a t t a c k cunbined wfth t h e formation of i r o n oxide s e t i n because somehow t h e alumina
l a y e r was d e f e c t i v e , It i s t h e r e f o r e a l s o necessary t h a t t h e s p e c i a l oxide f o ~ m a t l g h t l y s e a l e d c r u s t ,
M o r e o ~ ~ e r , t h e melting p o i n t of t h e s p e c i a l oxide
layer, a s well a s tha melting p o i n t of i t s mixture with t h e oxide of t h e b a s i c metal and with o t h e r secondary substances
i n t h e alEcy, must I f e walk above t h s s e a l i n g t e m p e r a t w e ,
Thua, f o r example, fha e x t e n t of t h e s c a l e k e s i s t a n e e 0%
s i l i c o n s t e e l s f a l i m i t e d by the m e l t i n g p o i n t of t h e eut-
e c t i ~ mixture of f e r r o u s oxide and f a y a l i t e (Fe2S10 ) which
8
i s 1240'. Even at 12006, s i l i c o n s t e e l s a r e no l o n g e r rea- i s t a n t
ta
s c a l i n g , Likewise, e o n t a e t wfth substances whichSan e n t e r i n t o a r e a c t i o n wlth the special oxfde may r e s u l t i n t h e change from p ~ o t e c t f o n a g a i n s t s c a l i n g t o v i o l e n t
f ' r ~ m t h e decreased ascalfng of the m a t e r i a l t o be annealed,
may entep I n t o such a reactLon with the special. oxide l a y e r s
o f t h e s e a l e r e s i s t a n t a l l o y s ,
Conditions f o r t h e formation of good p r o t e c t i v e 0x1 de l a y e r s %re b e s t f u l f i l l e d i n t h e ease of substances whfch
a r e used 8 9 ~ e f ~ . a c t o r f e s I n the eerarnfe i n d u s t r y , e , g ,
alumina, s i l f ~ i ~ a c i d , e n ~ o m l e oxide, magnesia and beryl.llum
axf de,
Summary
The a c a l f n g of metals i s due t o d f f f u a i o n of the m e t a l l i c i o n s through the s c a l e l a y e r , The new l a t t i - e e p l a n e s a r e t h e r e f o r e formed on t h e s u r f a c e of the s c a l e ,
If s p l i t s oecar, t h e p o i n t a t which s e a l e forms s h f f t s t o t h e i n n e r s u r f a c e adjacent t o t h e m e t a l l i c s u s f a c e , Due
t o p e r i c d f c spl*ftf;fngs, the a t t a c k p r o g r e s s e s l i n e a r l y i n r e l a t i o n t o time,
In a l l o y s , t h e oxides of the various a l l o y f n g elem-
ents form i n t h e f i r s t ~ p p ~ o x i m a t l o n i n o ~ d e r of t h e heats of f o ~ m a t f o n , Fop %he arrangement of t h e oxides i n the s c a l e layer, t h e p r i n c i p l e t h a t the a l l o y f n g a d d i t i v e s are enriched a t the s c a l e - m e t a l i n t e r f a c e h o l d s t ~ u e , ~
r e g a r d l e s s of wnetbsr t h e y m e nobler o r b a s e r t h a n t h e metal w h , f c i s prasefit fn excess, The c o n d i t i o n s for t h e formation of s c a l e - k s s f s t a n t l a y e r s on a l l o y s a r e s s a t e d ,
Re-renoess
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Qw S c a l i n g ternps .V
//# --
0 'OW {W
m Time in minutes
F i g . 1
-
S c a l e Isotherms o f Tungsten.Fig. 2 Fig. 3 Fig. 4
Fig. 5
Order of Oxide Forma- t i o n . i3 baser t h a n A.
Fig. 6
Order of Oxide Forma- t i o n . Combination of oxides, 3 baser t h m A.
Fig. 8
0 7 2 3 4 5 6
-=Aluminum c o n t e n t i n Ip'
Aluminum Contents of Various Scale Layers of S t e e l . Aluminum Fig. 9
-
S t r u c t u r e of t h e Scale Boundaries of a S t e e l with 4% Al a f t e r Annealirlg f o r 5 Hours a t 1 0 0 0 ~ . Fig. 10,-
Scale S t r u c t u r e ( I r o n S i l i c a t e s ) i n t h e Cast Iron.Fig. 11
-
Scaling Isotherms of an Aluminum S t e e l with 8.5% Al a t 1 0 0 0 ~ (according t o Portevin, P r e t e t and ~ o l i v e t ).
a, n cd P) ZOO k0,
100 H o ",Em 7 2 3 ' 4 5 6 7 1 s Annealing time i n hrs.Table 1. Oxidation Sequence of Oxides Element Oxide Tad), A1203 V206 Cr203