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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

-

157

ON 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 1950

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NATIONAL RESEARCH C O U N C I L

Ottawa, Canada

TECHNICAL TRAIJSLATION TT

-

157

D 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 t

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On

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 o

If 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-

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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 , By

c 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 he

a 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 o

A 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"

(6)

increased in proportion to the time,

P o

and B, f o u n d

she

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 and

B,

in the case of

e 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,

(7)

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

(8)

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 d

s 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-

(9)

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 and

F 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 a

(10)

p 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

(11)

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 e

a 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 and

H,

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.+

(12)

low (n ++

<

5

.

loB3)

and was s t i l l w i t h i n t h e margin of c u

e 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 s

t 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

(13)

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 r

r 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 forsms

f 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 y

AG 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,

(14)

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

(15)

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

(16)

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 manganese

s 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 ,

(17)

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 , BO

now 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

(18)

-

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 s

oxygen 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 e

same 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 t

no 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

(19)

-

16

-

been subjected to 8caling, In place of the graphite

seama,

ferrosoferric oxide for the

moat

p a r t has

al-

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

as

to

whether ane

should mix

nobler or baser substanc88,

Both

are enrfehed at the metal-scale interface, and

one might

think

that a compact layer might form which

would

effectively check the

f u r t h e r

development

of seal-

ing, It

must be

observed, however, that the noble

metals can

be

penetrated by oxygen, Froehlieh

(20)

has

shown

that silver plating

does

not protect copper against

scaling, The conpactness of the

l a y e r

plays some part

of

t x v n u f i , but

Pt 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), a

strong nickel layer

mixed

with

s c a l e is

frequently

f o r m a d , Quite

apart from the cost involved

one can hardly expect to achieve any real protection

against scaling by

a13oging a

noble 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

been

qdded, are no exception

f o r

the additive introduced to

ine~ease

the scale resistance is ch~omium

exclusively

( 6 ) -

(20)

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 which

San 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

(21)

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 ,

(22)

Re-renoess

1, G o Tammam,, 2, anorg, d f g , Chem, 111,

-

78, 1920, 2,

N o

B, P i l l i n g and R , E, Bedworth, 3 ,

Inst,

Met, 29,

529, 1923,

3,

Z,

Elektaochem, 35, 142, 1929,

4 , 2, anorg, a l l g , Chem,

-

123, 196, 1923,

5, G o Maeing, Die Korrosion d e r m e t a l l i s c h e n V t e ~ k s t o f f e ,

I,

p p o 97-119, 19360

6 0

Eo

S e h e f l and K o E i a f t , Arch, Eisenhuettenw, 9, 405,

1935136,

7, 2 , Elektroehem, 39, 316, 1933,

8,

L a

G o

.~

Pfeil, J, I r o n S t e e l Inst, 119, 501, 1929,

-

9 , We wf ah to thank C, D~escher, Siemens-Schuekert-We~ke,

for t h e sample,

10. Private eomunication f r o m D k 0

P o

Wfest,

3.1, Z,angew, Chem, -= 49, 935, 1936,

1 2 , A , S t e i l h e i l , A n n , Phys,

-

19, 465, 1934,

13, G o J, Finch and A , G o Q u a r r s l , Nature,

Pal,

877, 1932jo 14, C o Wagner, Z, physo Chemo ,.l2 25, 1933,

1 5 ,

R o

Schenk and T , Dfngmann, Z , anorg, a l l g o Chern, 166 p

.-115, 192Y0

1 6 , J a cham, Physies, ,.3 29, 1933,

17, G , Haegg and

J,

S u c k p ~ d o ~ f , Z a phys, Chem, 22, 444, 1933,

18, G o Haegg and

A0

L,

Klfng;skrpoem,

Z o

phys, Chem,

22,

453, 1933,

1 9 , 2 , phys, Chem, 22,

-

212, 1933, 2 0 , Z, MetaPlkde,

s,

368, 1936,

(23)

21,

H, H,

von Baurnbach and G o Wagner, 2 0 P ~ Y ~ O Chemo

3

23, H, Rerny, Lehrbuch d e r ansrganischen Chemfe, Akademische

Verlagsgesellschaft, Leipzig, 1931, P O 190

25, \If, Guertlep and VJ, Lsitgebel, V o n

Erz

zum metalfischen

Werkstoff, Akad, Verlagsges, Leipzig, 1929, p,297,

28, E o Houdrernont, Sonderstahlkunde, J o Springer, Berlin,

1935, p , 1800

(24)

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

(25)

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.

(26)

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 k

0,

100 H o ",Em 7 2 3 ' 4 5 6 7 1 s Annealing time i n hrs.

(27)

Table 1. Oxidation Sequence of Oxides Element Oxide Tad), A1203 V206 Cr203

1

B . 0 3 T h o 2 Li2O uo2 ZrO* CeO, TIO, Na,O s i o , CaO MgO SrO B o o BaO K*O Rb,O Cs,O MnO Bi,O, MOO, WO, %n 0 Sn O CdO Tl,!O VeO N iO OYO, c o o Cu,O PbO Ru 0, Rh,O JrO, HgO PdO Au,O,

eats

of formation 288.9 269,7 202 267.4 232,9 218.1 206,l 162.1 146.8 142.2 136 133,4 86.8 83,6 82.7 125 137,8 142.8 131.4 83 66,8 65,2 164 162.1 146,8 142.2 136 183.4 130 126 124 123 ' 116 107 98.6 88 66,8 66,2 432 66 98.3 43 I 64.6 62,7 62,6 22,7 40.1 21,6 2Q,4 0.6 62.7 89.4 34.1 80.2 21,6 20.4 9.8 -12,3 -10.2

Figure

Fig.  2  Fig.  3  Fig.  4
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 )
Table  1.  Oxidation  Sequence  of  Oxides  Element  Oxide  Tad),  A1203  V206  Cr203  1  B

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