ha an appel 91 properties

8/13/2019 Ha an Appel 91 Properties

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Oxidation of M etals Vol. 35 N os . 5/6 1991

The Propert ies o f Protec t ive Oxide Sca les Conta in ingC e r i u m on A l l oy 800H i n O x i d i z i n g an dO xi d i z i n g S u l p h i d i z i n g E n v ir on m e n tsV . A . C . H a a n a p p e l , * T . F r a n s e n , * B . G e e r d i n k , * P . J . G e l l in g s , *a n d M . F . S t r o o s n i j d e r ~Received July 3 1990; revised October 3 1990

T he c or r osi on p r o t e c t i on o f ox i de s c a l e s f o r m e d by e l e c t rophor e t i c de pos it i onin a c e r i u m - c o n t a i n i n g s o l o n A l l o y 8 0 01 4 a 3 2 N i - 2 0 C r s te e l f o l l o w e d b yf i r in g i n a i r a t 1 1 2 3 K w a s s t u d i e d i n o x i d iz i n g a n d m i x e d o x i d i z i n g / s u l p h i d i z i n ge nv i r onm e n t s a t e l e v a t e d t e m pe r a t u r e s . I n par t i c u la r t he i n f lue nc e o f t y pe o fs o l s o l v e n t s o l c onc e n t r a t i on an d t h i c k ne s s o f de po s i t e d l ay e r on t he p r o t e c t i onw e r e s t u d ie d . A n o p t i m i z e d t r e a t m e n t w a s d e d u c e d w h i c h le d to a s i g n if ic a n ti m p r o v e m e n t i n c o r r o s i o n b e h a v i o r i n o x i d i z i n g / s u l p h i d i z i n g e n v i r o n m e n t s .K E Y W O R D S : Alloy 800H; sulphidation; oxidation; sol/gel method; cerium; reactive ele-ments; ceramic coatings; thermogravimetric experiments.

INTRODUCTIONAdvanced coal-conversion processes, such as atmospheric fluidized-bedcombustion and pressurized coal gasification, offer great potential becauseof several advantages, like reduced SO2- and NOx-emissions. However,metallic components to be used in these systems are subjected to severeconditions, resulting in high-temperature corrosion, erosion, andmechanical damage due to stresses. Although the present generation ofhigh-temperature alloys shows generally acceptable mechanical properties,*University of Twente, Department of Chemical Technology,P.O. Box 217, 7500 AE Enschede,The Netherlands.tCommission of the European Communities, Institute of Advanced Materials, Ispra Establish-ment, 21020 Ispra Varese), Italy.:~To whom all correspondence should be addressed.

4050030-770X/91 /0600-0405506 .50 /0 ~ 1991 P lenum Pub l i sh ing Corp ora t ion


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4 6 Haanappei et al

t h e p o o r c o r r o s i o n r e s i st a n c e f o r t h e a b o v e - m e n t i o n e d a p p l i c a t i o n s c a u s e ss i g n i fi c a n t p r o b l e m s . D u e t o t h e l o w o x y g e n a n d h i g l , / s u l p h u r a c t iv i ti e s o ft h e e n v i r o n m e n t s , t h e f o r m a t i o n o f a p r o t e c ti v e o x i d e l a y e r m a y b e d i ff ic u lt ,a n d r a p i d s u l p h i d a t i o n c a n b e e x p e c t e d . E v e n a t t e m p e r a t u r e s a s lo w a s8 23 K , s u l p h i d a t i o n c a n c a u s e c a t a s t r o p h i c f a il u r e o f th e c o n s t r u c t i o nm a t e r i a l s , e v e n a f t e r s h o r t e x p o s u r e t i m e s , w h i l e t h e c o n s t r u c t i o n s h a v er e q u i r e d l i f et i m e s o f s e v e ra l y e a r s .

P r e o x i d a t i o n c a n r e n d e r p r o t e c t i o n t o h i g h - t e m p e r a t u r e a ll o y s e x p o s e dt o o x i d i z i n g - s u l p h i d i z i n g c o n d i t i o n s , 1 2 b u t i n g e n e r a l o n l y f o r a l i m i t e dt im e . T o e x t e n d t h e l i fe t im e o f a sc a le o b t a i n e d b y p r e o x i d a t i o n , t h e a d d i t io no f r e a ct i v e e le m e n t s , s u c h a s c e r i u m a n d y t t r iu m , is o f in t e re s t b e c a u s e o ft h e i r b e n e f i c i a l e f f e c t s o n o x i d e s c a l e p r o p e r t i e s , i n c l u d i n g r e s i s t a n c e t os p a l l a ti o n . 3 A d d i t i o n o f t h e s e e l e m e n t s i s p o s s i b l e e i t h e r b y a l l o y i n g o r b yi n c o r p o r a t i o n i n to a s u r f a c e la Y er (f o r i n s ta n c e , b y i o n i m p l a n t a t i o n o r b yc o a t i n g ) . A r e l a t i v e l y c h e a p a p p r o a c h is th e u s e o f t h e s o l / g e l t e c h n i q u e , 4-7b a s e d o n t h e u s e o f so l s, w h i c h c o n s i s t o f a d i s p e r s i o n o f c o l l o i d a l p a r t i c le so f h y d r o u s o x i d e s o r h y d r o x i d e S . T h e s o l c a n b e a p p l i e d t o t h e s u r f a c e b ye l e c t r o p h o r e s i s o r d i p p i n g f o l l o w e d b y lo w - t e m p e r a t u r e d r y i n g a n d f ir in ga t t e m p e r a t u r e s u p t o a b o u t 1 1 23 K . O n d r y i n g , w a t e r b e t w e e n th e c o l l o i d a lp a r t i c l e s is r e m o v e d , a n d t h e s o l is c o n v e r t e d to a g e l. O n f i r in g , t h e g e l ist r a n s f o r m e d i r re v e r si b l y t o a c e r a m i c m a t e ri a l a n d w i ll b e c o m e p a r t o f t h eo x i d e l a y e r w h i c h i s f o r m e d o n t h e a l lo y . A l t h o u g h t h e i n f lu e n c e o fs u p e r f i c i a ll y a p p l i e d o x i d e s o f r e a c t iv e e l e m e n t s h a s b e e n i n v e s t i g a t e dm a i n l y i n o x i d i z i n g e n v i r o n m e n t s , 4-14 t h e y o f f e r a l so a p o s s i b i l it y f o r u s eo f t h e s o l - g e l te c h n i q u e i n c o m b i n a t i o n w i t h t h e p r e o x i d a t i o n t r e a t m e n td u r i n g f ir in g f o r p r o t e c t i o n i n m i x e d o x i d i z i n g / s u l p h i d i z i n g e n v i r o n m e n t s . 7

I n t h is p a p e r , t h e i n f lu e n c e o f C e O 2 , e l e c t r o p h o r e t i c a l l y d e p o s i t e d f r o ma c e r i u m - c o n t a i n i n g s o l f o l l o w e d b y fi ri n g i n ai r, o n t h e c o r r o s i o n b e h a v i o ro f A l l o y 8 0 0 H i n o x i d iz i n g a n d m i x e d o x i d i z i n g / s u l p h i d i z i n g e n v i r o n m e n t sis r e p o r t e d . T h e i n f l u e n c e o f th e c o a t i n g p a r a m e t e r s a s w e l l a s t h e s o ls t ab i li ty o n t h e p r o t e c t i v e q u a l i t y o f t h e p r e f o r m e d o x i d e l a y er s a re e m p h a -s i z e d . F o r c o m p a r i s o n , s o m e t e s t s w e r e p e r f o r m e d o n m a t e r i a l w h i c h w a sp r e o x i d i z e d u n d e r s i m i la r c o n d i t i o n s a s a p p l i e d d u r i n g f ir in g o f t h e g e l.

E X P E R I M E N T L P R O C E D U R E SM a t e r i a l

T h e c h e m i c a l c o m p o s i t i o n o f th e a u s t e n i t i c st ee l, A l l o y 8 0 0 H , w h i c hw a s i n v e s t i g a t e d , i s g i v e n i n T a b l e I . T h e m a t e r i a l w a s a n n e a l e d i n tw os t ep s : 1 h r a t 1 34 8 K , f o l l o w e d b y w a t e r q u e n c h i n g , a n d a d d i t i o n a l l y 1 h ra t 12 23 K , f o l l o w e d b y w a t e r q u e n c h i n g a g a i n . C y l i n d r i c a l c o r r o s i o n s p e c i-m e n s o f 2 0 m m l e n g th a n d 5 m m d i a m e t e r a n d d i s k -t y p e s am p l e s o f 10 m m


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C e r i u m C o n t a i n i n g S o l o n A l l o y 8 0 0 H

T a b l e I . C h e m i c a l C o m p o s i t i o n o f t h e T e st e d A ll o y 8 0 0 H ( w t . )

4 0 7

F e C r N i C M n S i T iBal . 19.9 31.7 0.08 0.7 0.5 0.4

d i a m e t e r a n d a p p r o x i m a t e l y 3 m m t h i ck n e s s w e r e u se d . T h e f i na l su r f a c ef in is h w a s p e r f o r m e d b y g r i n d i n g w i t h 8 0 0 - gr it c a r b o r u n d u m p a p e r . A l l t e sts p e c i m e n s w e r e u l t r a s o n i c a l l y c l e a n e d w i t h h e x a n e , e t h a n o l , a n d a c e t o n ep r i o r t o c o a t i n g .

reparation of the SamplesP r e p a r a t i o n o f t h e s a m p l e s b y t h e s o l / g e l t e c h n i q u e w a s p e r f o r m e d

a c c o r d i n g t o t h e f o l l o w i n g p r o c e d u r e s .C e r iu m h y d r o x i d e C e O H ) 4 ) w as p r e p a r e d f r o m a m m o n i u m c e ri u m

n i t r a t e a n d a m m o n i a :( N H 4 ) 2 C e ( N O 3 ) 6 + 4 N H 4 O H ~ C e ( O H ) 4 + 6 N H 4 N O 3 ( 1 )

T h e p r e c i p i t a t e w a s s t i r r e d i n d i l u t e H N O 3 a t 33 3 K , y i e l d i n g a s o l w i t hp H = 0 . 3 :

n C e ( O H ) 4 + x H N O 3 ---* [ C e ( O H ) 4 ] ~ . x H + + x N O 3 - 2 )T o a c h i e v e d i ff e re n t c o n c e n t r a t i o n s o f t h e s o l, a c e t o n e o r w a t e r w a s a d d e d .T h e s o l w as a p p l i e d o n t h e s p e c i m e n s u r f a c es b y e l e c t r o p h o r e t i c d e p o s i ti o n .T h e d e p o s i t i o n c o n d i t i o n s o f t h e m a i n m e t h o d s a r e l is t ed i n T a b l e I I.A c e t o n e , w h i c h , f o r e x a m p l e , w a s u s e d i n m e t h o d A , i s o f i n t e r e s t a s as o l v e n t , s i n c e i t i s k n o w n t h a t t h e h y d r o g e n e v o l u t i o n d u r i n g t h e e l e c -t r o p h o r e t i c d e p o s i t i o n , ~s w h i c h w o u l d h i n d e r h o m o g e n e o u s d e p o s i t i o n , i sr e d u c e d b y t h e r e a c t i o n :

H2 H + + C H 3 - - C - - C H 3 + 2 e - --* C H 3 - - C - - C H 3 3 )

O O HT a b l e I I . E l e c t r o p h o r e t i c C o n d i t i o n s o f t h e M a i n A p p l i e d D e p o s i t i o n M e t h o d s

C o n c . C e - s o l C u r r e n t d e n s i t y E l e c tr . t i m eM e t h o d ( m o l l - l ) S o l v e n t ( A . m - 2 ) ( m i n )A 0 .05 80 ace t o ne 0 .6 20B 0 .40 w a t e r 1 .0 90


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4 8 Haanappel e t a l

A f t e r e l e c t r o p h o r e t i c d e p o s i t i o n , t h e s p e c i m e n s w e r e d r i e d i n a ir f o r 1 6 h ra t 29 3 K , f o l l o w e d b y s i n t e r i n g o f th e c e r i a d e p o s i t s i n a i r b y h e a t i n g , f ir stt o 4 2 3 K a t a r a t e o f 6 K/hr a n d t h e n t o 1 12 3 K a t a r a t e o f 30 K/hr. D i r e c t l ya f t e r r e a c h i n g t h e m a x i m u m t e m p e r a t u r e , t h e s p e c i m e n s w e r e c o o l e d a t ar a t e o f 3 0 K / h r t o 29 3 K . T h i s s i n te r i n g p r o c e d u r e , b e i n g c a r r i e d o u t i n ai r,is a t t h e s a m e ti m e a p r e o x i d a t i o n t r e a t m e n t . I n a p r e v i o u s i n v e s t ig a t io n , *t h e s t a t e o f t h e c e r i u m i n t h e l a y e r a f te r t h is t r e a t m e n t h a s b e e n d i s c u s s e d .

Test ing of Spec imensT e s t in g o f t h e s p e c i m e n s w a s p e r f o r m e d u n d e r p u r e l y o x i d iz i n g

c o n d i t i o n s a n d i n o x i d i z i n g / s u l p h i d i z i n g g a s m i x t u r e s . T h e o x i d a t i o ne x p e r i m e n t s w e r e c a r r i e d o u t i n a S e t a r a m a u t o m a t i c r e c o r d i n g e l e c t ro -b a l a n c e i n p u r e o x y g e n a t a t m o s p h e r i c p r e s s u r e . T h e s e n si ti v it y o f t h eb a l a n c e w a s a p p r o x i m a t e l y 0 .0 1 m g . T h e g a s w a s f l o w in g c o n t i n u o u s l yt h r o u g h t h e s y s t e m a t a r a t e o f a b o u t 0 . 3 1 / h r .

F o r t es t in g in t h e o x i d i z i n g / s u l p h i d i z i n g e n v i r o n m e n t s , s p r in g b a l a n c e sw e r e u s e d , t h e d e s i g n o f w h i c h is s h o w n i n F ig . 1. T h e b a l a n c e c o n s i s t e do f t w o v e rt ic a l t u b e s . T h e u p p e r p y r e x t u b e d o u b l e - w a l l e d ) w a s w a t e r -c o o l e d 3 03 K ) a n d c o n t r o l l e d b y m e a n s o f a t h e r m o s t a t , t o p r o v i d e au n i f o r m r e s p o n s e a l o n g t h e s p r in g . T h e l o w e r q u a r t z t u b e a n d t h e c y l i n d r ic a lf u r n a c e c o u l d b e m o v e d v e r ti ca ll y . T h e b a l a n c e w a s c o n n e c t e d t o a v a c u u ms y s te m , th e a g g r e s s iv e g a s s o u r c e, a n d a m a n o m e t e r . T h e e n t i re v o l u m e

1 0 1 ) c o u l d t h u s b e e v a c u a t e d , a n d t h e d e s i r e d g a s m i x t u r e w a s i n t r o d u c e d

W ater ou tlet~ 7 - - - ~ Gas in le t--1t?o I---O ou ,,, ~o,,~d pyrex ube~ _S tainless steel spiral

~ ....-Cathetometer-targetGas outletBu ffer vesset~F u ~ n o c e ~=-- ---W ater inlet 20 ~[..---Quartz tube~

Pt wireSpecimenThermocouple

Fig. 1. Testing facility sprin g bala nc e) for the oxidiz-ing/ sulphidizing environm ents.


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C e r i u m C o n t a i n i n g S o l o n Alloy 800H

T a b l e III . Calculated Oxygen Po2) and Sul phu r Psi) Partial Press-ures in atm) as Function of Temperature for the Applied Gas

MixtureTempe ratur e ~ pQ Ps2

550 9 10 .29 I x 10 -9600 6 10 -27 5 10 -9650 2x 10 25 2x 10 8700 7 X 10 24 7 x 10 8

409

i nt o th e t ub e . T e m p e r a t u r e m e a s u r e m e n t s w e r e p e r f o r m e d d u r i n g t h e e x p e r i-m e n t s w i th a c h r o m e l - a l u m e l t h e r m o c o u p l e , p l a c e d c l o s e t o t h e s p e c i m e n .T h e s e n s i t i v i ty o f t h e s p r i n g b a l a n c e s u s e d w a s a b o u t 0 .1 r ag . F o r t e s ti n g ,t h e s p r i n g b a l a n c e , i n w h i c h t h e s a m p l e w a s s u s p e n d e d , w a s f ir st f l u sh e dw i t h a r g o n f o r 1 6 h r w i t h a f l ow r a te o f 1 0 1 / h r . T h e a r g o n w a s s a t u r a t e dw i t h w a t e r a t 2 88 K . A f t e r t h i s, a m i x t u r e o f 5 H 2 S i n H 2 w i t h a f l o w r a t eo f 3 1 / h r w a s i n t r o d u c e d i n t o t h e a r g o n f lo w . A f t e r 2 h r , in o r d e r t o s t a b i li z et h e f l o w , t h e f u r n a c e w a s h e a t e d t o t h e t e s t t e m p e r a t u r e . T h e w h o l e s y s t e mw a s f l u s h e d f o r a n o t h e r 5 h r a n d t h e n c l o s e d . T e s t in g t e m p e r a t u r e s r a n g e df r o m 8 2 3 - 9 7 3 K . T h e c a l c u l a t e d o x y g e n a n d s u l p h u r p a r t ia l p r e s s u r e s o ft h e g a s m i x t u r e s a t t h e v a r i o u s t e m p e r a t u r e s a r e l is te d in T a b l e I I I . F r o mt h e p o s i t i o n o f t h e c a l c u l a t e d o x y g e n a n d s u l p h u r p o t e n t i a l s o f t h e a t m o s -p h e r e i n th e r e l a t e d t h e r m o d y n a m i c s t a b il it y d i a g r a m s , i t f o l lo w s t h a t C r 2 0 3 ,N i3 S2 , a n d F e S a r e t h e r m o d y n a m i c a l l y s t a b le p h a s e s . 1~ A d d i t io n a l l y , s o m ee x p e r i m e n t s w e r e c a r r i e d o u t i n 5 H 2 S i n H 2 , i n w h i c h t h e w a t e r c o n c e n t r a -t io n , p r e s e n t a s a n i m p u r i t y , w a s a p p r o x i m a t e l y 2 0 p p m .

R E S U L T SP r e l i m i n a r y r e s u l ts s h o w e d t h a t th e u s e o f a c e t o n e a s a so l v e n t w a s

l es s fa v o r a b l e d u e t o s o l ag i n g , r es u l ti n g i n a l a y e r o f p o o r p e r f o r m a n c e .A n e x a m p l e i s s h o w n i n F i g . 2 a ; t h e f o r m e d o x i d e s h o w e d a f l a k e - l i k em o r p h o l o g y . T h e m e t h o d s w h e r e w a t e r w a s u s e d a s a s o l v e n t g a v e si g-n i f i c an t l y b e t t e r r e s u lt s a n d n o e f f e ct o f so l a g i n g w a s o b s e r v e d a f t e r 4 w e e k s( F i g . 2 b ) . T o f i n d t h e o p t i m u m t r e a t m e n t , t h e t e s t p a r a m e t e r s w e r e v a r i e do v e r a w i d e r r a n g e . T h e r e s u l ts s h o w e d t h a t m e t h o d B y i e l d e d t h e b e s tr e su l ts u p o n e x p o s u r e t o t h e o x i d i z i n g / s u l p h i d i z i n g e n v i r o n m e n t s . T h i n n e rc e r i u m o x i d e d e p o s i t s , o b t a i n e d b y d e c r e a s in g t h e s o l c o n c e n t r a t i o n o r t i m eo f e l e c t r o p h o r e s i s , g a v e i n s u f fi c ie n t p r o t e c t i o n . W i t h th i c k e r c e r i u m o x i d ed e p o s i t s , t h e p r o t e c t i v e n e s s d e c r e a s e d a l s o . T h i s i s o b v i o u s f r o m F i g . 3 ,w h e r e a n o x i d e s c al e o f 0 .2 5 m g / c m 2, e q u a l t o a n o x i d e t h i c k n e s s o f


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41 Haanappel e t a l

a bFig. 2. SEM-micrographs of cer ium oxide surface-treated Alloy 800H with cer ium oxideapplic at ion using sols aged for 4 weeks, fol lowing a) method A and b) method B.

approximate ly 0 .5 /zm, appl ied , fo l lowing method B, gave the best protec-t ion. The opt im um proce dure leads to an oxide layer of ra ther c lose-packedcer ium oxide par t ic les posi t ioned on a layer mainly consis t ing of C r 2 03 .14How ever , i f a th icker layer of cer ium oxide was deposi ted , severe agglomer-ation of cerium oxide part icles occurred, which in turn led to a f lake-l ikem orp hol og y after f ir ing Fig. 4) . The opt im um C e O 2 t reatment gave asignificant improvement in corrosion resistance in oxidizing/sulphidizingenvironments in compar ison to the untrea ted a l loy, as can be seen f romFig. 5.Also in oxidizing condit ions the cerium-oxide-surface-treated materialbehaved bet ter in compar ison to specimens that were only preoxidized

1.21.0

~E 0.80.6

&o .40.2

0

I I I f

Alloy 800HH2 50 H2S / /550 ~ o

. jz ~ 0.10 mg /cm ~.

.15 mg /c m 2~ 8 ~ , - - b v ' - ' - - - o - - ~ , ' - : _ -I I I 025 mg/c mZ

50 100 150 200Time h

Fig. 3. Mass g ain vs. t ime for corrosion of Alloy 800H inan oxid iz ing/su lphid iz ing envi ronm ent a t 550~ for b lankpreoxidized and cerium oxide surface-treated condit ions.The lat ter with varying thickness of the oxide layer .


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Cerium Containing Sol on Alloy 800H 411

a bFig. 4. SEM micrographs of the scale surface of Alloy 800H after cerium oxide surfacetreatment with a) optimum cerium oxide deposition and b) too thick cerium oxidedeposition.

un der s imi l a r cond i t ions a s app l i ed du r ing f i ri ng a s dep ic t ed in F ig. 6. Bo thma t e r ia l s s h o w e d a p a r a b o l i c c o r ro s i o n b e h a v i o r w i t h p a r a b o l ic r e a c t i o ncons tan t s o f 1 10 6 and 2 x 10 6 mg -2 cm 4 s -a re spec t ive ly . SEM obse rva -t io n s r e v e a l e d s e l e ct iv e g r a i n -b o u n d a ry o x i d a t i o n o n t h e u n t r e a t e d p r e o x i d -i zed ma te r i a l wh ich was absen t on the ce r ium -ox ide - t rea t ed ma te r i a l a sshow n in F ig . 7. Add i t iona l ly the CeO2-su r face - t rea t ed ma te r i a l show ed ag o o d r e s i s t an c e t o t h e rma l s h o c k. U p o n c o o l in g t h e u n t r e a t e d a l l o y a f t e rp r e o x i d a t i o n e x h i b i te d c o n s i d e ra b l e s p a ll in g w h i c h w a s a b s e n t i n t h e c a s eo f t he C eO2-su r face - t rea t ed a l loy .

~ 6

t~o

=~2

0

)550oC AIIoy800HAT00~ Ar-19H2-1H20-IH2So~/a o blank650 A,O,~, O Ce02 treated

A [3A a 600oc~ v~A ~ o / ~ ~ 550~I I I I

200 400 600 800 1000Time, hFig. 5. Mass gain versus time for corrosion under oxidiz-ing/sulphidizing conditions for blank and cerium oxidesurface trea ted method B) Alloy 800H at various tem-peratures.


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08 I I I

pre-oxidized o ~1760.6 _~ --~.o A ~d 0.4 o A

0 ~ ~ Ce02 treated:~ 0.2 AIIoy800H/ / 950~ 020 I I i I i20 Z,0 60 80Time, h

Fig. 6. Mass gain versus time for corrosion of Alloy 800Hin oxygen at 950~ for the cerium oxide surface treated(method B) and blank pre-oxidized conditions.DISCUSSION

The results obtained have shown that the application of cerium oxideby the sol/gel technique to the surface followed by firing in air can lead toa significant improvement in corrosion resistance in oxidizing, as well asin oxidizing/sulphidizing conditions. Thermogravimetric measurementsregarding the corrosion resistance und er oxidizing/sulphidizing conditions(Fig. 5) showed a clear temperature dependence: whereas no corrosioncould be observed after 900 hr at 823 K, it was clearly present after 400 hrat 873 K, 240 hr at 923 K, and after 120 hr at 973 K. Three effects maycontribute to this observed change with temperature:

9 Increasing diffusion coefficients in the protective oxide layer withincreasing temperature.

9 An increase of the sulphur partial pressure (Table III) .

a bFig. 7. SEM-micrographsof the scale surface after exposure in oxygen at 950~ for 80 hrof Alloy 800H: (a) blank preoxidized; (b) CeO2 surface-treated (method B).


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Cerium Containing Sol on A lloy 800H 413

9 T h e f o r m a t i o n o f a l o w - m e l t i n g N i / N i 3 S 2 e u t e c t ic at t e m p e r a t u r e sa b o v e 9 1 8 K .

H o w e v e r , a v e ry c a r ef u l tr e a t m e n t d u r i n g c e r i u m - o x i d e - s u r f a c e a p p l ic a -t i o n i s n e c e s s a r y i n o r d e r t o a c h i e v e a n o p t i m i z e d c o r r o s i o n r e s i s t a n c e . F o re x a m p l e , a t o o - l o w c e r i a c o n c e n t r a t i o n o f t h e s o l d u r in g e l e c t r o p h o r e t i cd e p o s i t i o n l e a d s t o a n i n s u f fi c ie n t e f fe c t o n t h e o x i d e s c a l e q u a l i ty . U s i n ga c e t o n e a s a s o l v e n t l e a d s t o t h e f o r m a t i o n o f a l e s s - p r o t e c t iv e o x i d e s c a l ew h i c h i s p r o b a b l y d u e t o i n s u f f ic i e n t s t a b i li t y o f t h e s o l. T h i s m a y b e d u et o a d e c r e a s e d t h i c k n e s s o f t h e d i f fu s e d o u b l e l a y e r , c a u s e d b y a lo w e rd i e l e c t r i c c o n s t a n t f o r a c e t o n e c o m p a r e d w i t h w a t e r , r e s u l t i n g i n r e d u c e ds t a b i l i t y o f t h e s o l a n d r a p i d c o a g u l a t i o n . 16"17 I n t h i s w a y , e l e c t r o p h o r e t i cd e p o s i t i o n l e a d s t o c l u s t e rs o f c o l l o i d a l p a r t i c le s o n t h e s u b s t r a t e , w i t h t h ec o n s e q u e n c e o f a re l a ti v e ly p o o r a d h e r e n c e b e t w e e n t h e s u b s tr a t e a n d t h es u p e r f i c i a l l y - a p p l i e d c e r i a . A f t e r f i r i n g , a f l a k e - l i k e s u r f a c e m o r p h o l o g y i so b s e r v e d ( F ig . 2 ), r a t h e r s i m i l a r t o th e a p p e a r a n c e a f t e r f ir in g o f a to o - t h i c kc e r ia l a y e r (F ig . 4) . S u c h a m o r p h o l o g y l e a d s o b v i o u s l y t o a r e d u c e dp r o t e c t i v e a c t i o n i n c o r r o s i v e e n v i r o n m e n t s , a s h a s a l s o b e e n o b s e r v e d i nt h e l i t e ra t u r e . 13 A l s o , o t h e r p a r a m e t e r s m a y b e r e l a t e d t o t h e c o r r o s i o nr e s i s te n c e o f t h e c e r i a - s u r f a c e - t r e a t e d a l lo y . F o r i n s t a n c e , it w a s f o u n d t h a tp o l i s h i n g w i t h 0 .0 5 ~xm A 1 2 03 , a n d a d d i t i o n a l t r e a t m e n t f o l l o w i n g m e t h o dB , g a v e p o o r c o r r o s i o n r e s i s t a n c e i n c o m p a r i s o n w i t h g r i n d i n g o n 8 0 0 - g r i tc a r b o r u n d u m p a p e r . T h i s m a y b e d u e to a d e c r e a s e d a d h e r e n c e o f t h e c er i ap a r ti c le s t o t h e s m o o t h e r s u r f a c e o r al s o to c o l d d e f o r m a t i o n i n t h e s u r f a c er e g i o n o f t h e m e t a l, l e a d i n g t o e n h a n c e d C r d i f fu s i o n a n d i m p r o v e d C r 2 0 3f o r m a t i o n .

T h e e x p e r i m e n t s U n d e r p u r e l y o x i d i z i n g c o n d i t i o n s s h o w e d t h a t f o rt h e C e O 2 - s u r fa c e - tr e a te d m a t e r i a l th e o x i d a t i o n w a s m o r e u n i f o r m , a n d t h a ts e le c ti v e g r a i n - b o u n d a r y a t ta c k , a s o b s e r v e d f o r th e u n t r e a t e d p r e o x i d i z e dm a t e r i a l , w a s a b s e n t . T h i s c a n b e e x p l a i n e d b y t h e f a c t t h a t t h e c e r i u mo x i d e p a r t i c l e s a c t a s n u c l e i f o r o x i d a t i o n , t h u s d e c r e a s i n g t h e i m p o r t a n c eo f g ra i n b o u n d a r i e s a s s e le c t i v e n u c l e a t i o n s it es , as s h o w n i n t h e w o r k o fR h y s - J o n e s a n d G r a b k e . 18'19 A d d i t i o n a l l y , i t w a s f o u n d t h a t t h e C e O 2 -c o n t a i n i n g o x i d e s c a l e s h o w e d a g o o d a d h e r e n c e u p o n t h e r m a l s h o c k . Ap o s s i b le e x p l a n a t i o n m i g h t b e a n i m p r o v e d o x i d e - s c al e p la s t ic i ty i n c o m b i -n a t i o n w i t h a d e c r e a s e d o x i d e g r a i n s i z e c a u s e d b y C e O 2 p a r t i c l e s a c t i n ga s n u c l e i f o r o x i d a t i o n . A m o r e e x t e n s i v e d i s c u s s i o n o n t h e i m p r o v e dc o r r o s i o n b e h a v i o r d u e t o C e O 2 c a n b e f o u n d e l s e w h e r e . 9"14

S U M M A R Y A N D C O N C L U D I N G R E M A R K ST h e a p p l i c a t i o n o f c e r iu m o x i d e to t h e s u r f a c e o f A l l o y 8 0 0 H b y t h e

s o l / g e l t e c h n i q u e f o l l o w e d b y f i r i n g i n a i r c a n l e a d t o a n i m p r o v e m e n t i n


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c o r r o s i o n r es is t an c e b o t h u n d e r o x i d i z in g a n d o x i d i z i n g / s u l p h i d i z i n g c o n d i -t i o n s . H o w e v e r t h e i m p r o v e m e n t i n c o r r o s i o n r e s i st a n c e d e p e n d s s t r o n g l yo n c o a t i n g p a r a m e t e rs s u c h a s t y p e o f s o l v e n t a n d s o l c o n c e n t r a t i o n .A c e t o n e w a s f o u n d t o b e l e s s f a v o r a b l e d u e t o t h e s e n s it i vi t y fo r so l a g i n g .S o l s b a s e d o n w a t e r a p p e a r e d n o t to s uf fe r f r o m s o l a g in g . A n o p t i m u mp r o t e c t io n w a s o b t a i n e d w i t h a n o x i d e l a ye r o f a p p r o x im a t e l y 0 . 5 / x mt h i c k n e s s . T h e e x t e n t t o w h i c h a n i m p r o v e m e n t i n c o r r o s i o n p r o t e c t i o n a l s oh o l d s f or s u p e r i m p o s e d m e c h a n i c a l d e f o r m a t i on w h i c h c a n l e ad to c r a ck in go f t h e p r o t e c t i v e o x i d e l a y e r i s t h e s u b j e c t o f o t h e r i n v e s t i g a t i o n s . 2~

A C K N O W L E D G M E N T ST h e a u t h o r s t h a n k P r o f. J . H . W . d e W i t D e l f t U n i v e r s i t y o f T e c h n o l o g y

f o r u s e f u l d i s c u s s i o n s .

R E F E R E N C E S1 . F . H . S t o tt , F . M . C . C h o n g , a n d C . A . S t i rl i n g , Proe. 9th lnternat. Congress Metall. Corros.2, 1 (1984) .2 . T . F r a n s e n , M . M . A . P e r i k , H . F i k k e r t , M . A . d e J o n g h , a n d P . J . G e l l i n g s , Proc. 8th

European Congress Corr. 53-1 (1985) .3 . D . P . M o o n , Mater. Sei. Tech. 5 , 7 5 4 (1 9 8 9 ) ; D . P . W h i t t l e a n d J . S t r i n g e r , Phil. Trans. R.Soc. London A 295 , 309 (1980) .4 . U . K . P a t e n t A p p l i c a t i o n , G B 2 0 3 3 4 5 3 A ( 1 9 7 8 ) .5 . R . L . N e l s o n , J . D . F . R a m s a y , J . L . W o o d h e a d , J . A . C a i r n s , a n d J . A . A . C r o s s l e y , ThinSolid Films 81 , 329 (1981) .6 . M . J . B e n n e t t , J. Vac. Sci. Tech. 2B, 800 (1984) .7 . T . F r a n s e n , M . A . d e J o n g h , M . M . A . P e r i k , a n d P . J . G e l l i n g s , Proc. 9th Int. Congr.Metall. Corr. Toronto 4, 311 (1984) .8 . T . F r a n s e n , P . J . G e l l i n g s , J . C . F u g g l e , G . v a n d e r L a a n , J . M . E s t e v a , a n d R . C . K a r n a t a k ,Appl. Surf Sci. 20, 257 (1985) .9 . T . F r a n s e n , E . P o l m a n , M . M . A . P e r i k , a n d P . J . G e l l i n g s , i n High Temperature Alloys

for Gas Turbines and Other Applications V ol . 2 , W . Be t z et aL e ds . ( D . Re i de l , 1986) , p . 1173 .1 0. T . F r a n s e n , B . G e e r d i n k , P . J . G e l l i n g s , a n d M . E . L a n g , Materialen 9, 10 (1987) .1 1 . G . M . E c e r , R . B . S i n g h , a n d G . H . M e i e r , Oxid. Met. 18, 55 (1982).1 2. M . L a n d h o f , A . V . L e v y , D . H . B o o n e , R . G r a y , a n d E . Y a n i v , Corrosion 41 , 344 (1985) .1 3. P . Y . H o u a n d J . S t r in g e r , Mater. Sci. Eng. 87, 295 (1987) .1 4. M . F . S t r o o s n i j d e r , V . G u t t m a n n , T . F r a n s e n , a n d J . H . W . d e W i t , Oxid. Met. 33, 371 (1990) .1 5. P . F . G r o s s o , R . E . R u t h e r f o ~ J r . , a n d D . E . S a r g e n t , J . Electrochem. Soc. 117, 1456 (1970).1 6. H . R . K r u y t , Colloid Science Vol. I Irreversible Systems (E l se v i e r , N e w Y o rk , 1952), p . 128.1 7. W . J . M o o r e , Physical Chemistry (L on gm a n , 1972), p . 513 .1 8. T . N . R h y s - J o n e s , H . J . G r a b k e , a n d H . K u d i e l k a , Corros. Sci. 27, 49 (1987) .1 9. T . N . R h y s - J o n e s a n d H . J. G r a b k e , Mat. Sci. Tech. 4, 446 (1988) .2 0 . V . A . C . H a a n a p p e l , T . F r a n s e n , B . G e e r d i n k , a n d P . J . G e l l i n g s , Oxid. Met. 30, 201 (1988) .2 1. M . F . S t r o o s n i j d e r , V. G u t t m a n n , R . J . N . G o m m a n s , a n d J . H . W . d e W i t , Mater. Sci.and Eng. A 121 , 581 (1989) .2 2 . M . F . S t r o o s n i j d e r , V . G u t t m a n n , a n d J . H . W . d e W i t , Proc. 9th European Congress onCorrosion ( U t r e c h t , O c t o b e r 2 - 6 , 1 98 9, V o l . I I , E G 2 5 7 ) .

ha an appel 91 properties

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