Corrosion Protection of Kraft Digesters

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C O R R O S I O N P R O T E C T I O N O F K R A F T D I G E S T E R S

n g e l a W e n s l e y

Pulp Paper Cor ros ion Spec ia l is t

D . Ang e la Wen s ley Consul t ing Enginee r ing Se rv ices

15397 Columbia Avenue

W h i t e R o c k , B C , C a n a d a V 4 B 1 K 1

A B S T R A C T

This pape r d i s cus ses nove l appl i ca tions of anodic pro tec t ion , we ld ove r l ay , and the rm a l spray coa t ing for cor ros ion

pro tec t ion of ca rbon s t ee l ba tch and cont inuo us d igest e r s. Ano dic pro tec t ion now of fe rs the pos s ib i l i t y o f p ro tec t ing beh ind

sc reens and b lank p la t e s . S ta in l e ss s t ee l we ld ov e r l ay can be appl i ed beh ind head e rs for p ro tec t ion aga ins t e ros ion-cor ros ion .

Therm al spray coa t ings can be app l i ed ove r c i rcumferen t i a l we ld s eams for p ro tec t ion aga ins t s tre s s cor ros ion c rack ing . A l so

di scus sed a re the advantages and d i s advantages of each pro tec t ion mod e as we l l a s the i r compa t ib i l i t y w i th each o the r.

Keyw ords : ac id c l ean ing , anodic pro tec t ion , c l ad p la t e , d iges te r s , duplex s t a in l e s s s tee l , e ros ion-cor ros ion , i n t e rgranula r

a t t ack , p i t ti ng , pu lp an d pape r , s t res s cor ros ion c rack ing , t he rma l spray , we ld o ve r l ay

I N T R O D U C T I O N

Diges te r ves se l s ( inc lud ing impregna t ion ves se l s ) cons t ruc ted us ing ca rbon s t ee l s o r aus t en i t i c s t a in l e s s s t ee l s

exper i ence cor ros ion problem s tha t ma y inev i t ab ly requi re some form o f p ro tec tion so tha t t h ey can s a fe ly remain in s e rv ice .

Cor ros ion problem s exper i enced in d iges t e r ves se ls inc lude l :

C o r r o s i o n M e c h a n i s m

Caus t i c s t re ss cor ros ion c rack ing o f ca rbon s t ee l .

Cor ros ion th inn ing of ca rbon s t ee l .

P re fe ren t i a l cor ros ion o f ca rbon s t ee l we lds

C o n t i n u o u s D i g e s t e r s

Yes

Acid c l ean ing da mag e of ca rbon and s t a in l e ss s t eel .

Cor ros ion th inn ing o f s t a in l e ss s t ee l .

Intergra nular a t tack o f s ta inless s teel .

Yes

B a t c h D i g e s t er s

Rare

Yes

Yes Yes

Eros ion-cor ros ion of ca rbon s t ee l. Yes Rare

Y e s N o

N o Y e s

Yes

S t ress co r ros ion c rack ing of s t a in le s s s t ee l.

Yes

e s

Yes

M E C H A N I S M S O F C O R R O S I O N IN D IG E S T E R S

S t r es s C o r r o s i o n C r a c k i n g o f C a r b o n Steel

Caus t i c s t re ss cor ros ion c rack ing (SCC ) i s t he mos t s e r ious form of cor ros ion tha t can occ ur in a cont inuous d iges t e r

and has re su l t ed in the ca ta s t rophic fa i lu re of a cont inuous d iges t er . SCC a f fec t s ma in ly we lds in the impregna t ion zone or in

Copyright

©2001 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACEInternational, Publications Division, 1440 South Creek Drive, Houston, Texas 77084-4906. The material presented and the views expressed in this paper are

solely those of the author(s) and not necessarily endorsed by the Association. Printed in U.S.A.

01423

CORROSION2001Paper No.



t h e i m p r e g n a t i o n v e s s e l in t w o - v e s s e l s y st e m s . S C C o c c u r s o n l y w i t h i n a n a r r o w r a n g e o f c o r r o s i o n v o l t a g e p o te n t ia l s . I n

t h e i m p r e g n a t i o n z o n e o f a d i g e s t e r th e c o r r o s i o n p o t e n t i a l c a n re m a i n i n t h e S C C r a n g e l o n g e n o u g h f o r c ra c k s t o i n i t ia t e a n d

p r o p a g a t e . C o n t i n u o u s d i g e s t er s t h a t w e r e n o t p o s t - w e l d h e a t t r e a t e d o r o n l y p a r t ia l l y s t re s s r e l i e v e d h a v e h i g h r e s id u a l

tens i l e s t r e s ses e spec ia l ly a t we lds and in con ica l s ec t ions and a r e m os t suscep t ib le to S C C. S CC i s r a r e in ba tch d iges te r s

d u e t o t h e c y c l i c n a t u r e o f t h e i r o p e r a ti o n . T h e c o r r o s i o n p o t e n t i a l i n b a t c h d i g e s te r s i s n o t i n t h e S C C r a n g e l o n g e n o u g h f o r

S CC to in i t i a te .

R a p i d C o r r o s i o n T h i n n i n g o f C a r b o n S t e e l

R a p i d c o r r o s i o n t h i n n in g c a n r e s u l t i n t h e c o n s u m p t i o n o f t h e c o r r o s i o n a l l o w a n c e o f a d i g e s t e r s u c h t h a t t h e w a l l

th ickness f a l l s be lo w tha t r equ i r ed by the cons t r uc t ion code . S uch ves se l s m us t e i the r be r e s to r ed to a s a t i s fac to r y th ickness

( f o r e x a m p l e , b y c a r b o n s t e e l w e l d b u i l d u p t h a t i t s e l f i s o n l y a t e m p o r a r y m e a s u r e ) o r d e - r a t e d i n o r d e r t o p e r m i t o p e r a t i o n

w i t h a t h i n n e r w a l l. R a p i d t h i n n i n g o f c o n t i n u o u s d i g e s t e rs t y p i c a l l y o c c u r s a ro u n d t h e e x t r a c t i o n s cr e e n s o r i n t h e w a s h z o n e

n e a r t h e b o t t o m o f t h e d i g e s t e r2 '3. I t i s r a r e f o r th inn ing to occu r nea r the top of a con t inu ous d iges te r . Co r r os ion po ten t ia l

m on i to r ing sugges t s tha t the th inn ing o f ba tch d iges te r s occur s d ur ing the l iquo r f i l ling and hea t ing s tages 4 '5 . Co r r os ion

t h i n n in g i s m o s t p r o n o u n c e d w h e r e t h e l i q u o r s p l as h e s o n t h e b a t c h d i g e s te r w a l l d u r in g t h e f i ll i n g p r o c e s s . T h e m o s t

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

pinholes . Cav i t i e s tha t f o r m beh ind o ve r lay p inholes can r e su l t in ex te r na l leaks .

P r e f e r e n t ia l C o r r o s i o n o f C a r b o n S t e e l W e l d s

P r e f e r e n t ia l c o r r o s i o n o f c a r b o n s t e e l w e l d s i s u s u a l l y a c o n s e q u e n c e o f t h e h i g h e r s i l i c o n c o n t e n t o f t h e w e l d a s

c o m p a r e d w i t h t h e p l at e f r o m w h i c h t h e d i g e s t e r w a s c o n st r u c t e d . S t e e ls w i t h a h i g h e r s i li c o n c o n t e n t t e n d t o c o r r o d e m o r e

r ap id ly in k r a f t l iquor s than do s tee l s wi th a lower s i l i con con ten t 6.

E r o s i o n C o r r o s i o n o f C a r b o n S t e e l

E r o s i o n - c o r r o s i o n o c c u r s i n c o n t i n u o u s d i g e s t e r s a t l o c a t i o n s w h e r e a h i g h v e l o c i t y l i q u o r f l o w i s p o s s i b l e , e . g . ,

a b o v e t h e i n t e r n a l c o n e o r b e h i n d s c r e en s a n d h e a d e r p l a te s . E r o s i o n - c o r r o s i o n is t y p i c a l l y l o c a l i z e d b u t c a n b e v e r y d e e p .

T h e d i g e s t e r w a l l a re a b e h i n d h e a d e r s w h e r e a v e r y h i g h v e l o c i t y f l o w c a n o c c u r t h r o u g h t h e o r i f i c e h o le s i n t h e

c i r c u m f e r e n t i a l b a c k i n g b a r s b e t w e e n t h e s c r e e n s a n d t h e h e a d e r m a y b e e s p e c i a l l y b a d ( F i g u r e 1 ).

A c i d C l e a n in g D a m a g e

A c i d c l e a n i n g i s d o n e m a i n l y i n N o r t h A m e r i c a n i n o r d e r t o r e m o v e t h i c k d e p o s i t s o f c a r b o n a t e s c a le .

U n f o r t u n a t e l y , t h e a c i d m o s t c o m m o n l y u s e d i n t h is c l e a n i n g i s m u r i a t i c a c id , a t e c h n i c a l g r a d e o f h y d r o c h l o r i c a c i d. A c i d

c lean ing a t t em per a tur es above 70°C can r e su l t in p i t t ing cor r os ion dam age to bo th ca r bon and s ta in le s s s tee l s r ega r d le s s a s to

wh e the r the cor r ec t inh ib i to r i s used . End - gr a in cor r os ion of st a in le s s s tee l can occur a t the s lo t s in the s c r eens . Th e f e r ri t e

phase in s ta in le s s s tee l we lds i s pa r t i cu la r ly suscep t ib le to cor r os ion by ho t m ur ia t i c ac id c lean ing so lu t ions . Gi r th we lds in

t h e c e n t r a l p i p e a r e a t h i g h r i sk s i n c e t h e w e l d r o o t s m a y b e a t t a c k e d b y t h e h o t m u r i a t i c a c i d

C o r r o s i o n o f S t a i n l e s s S t e e l s

C o r r o s i o n t h i n n i n g o f c o n v e n t i o n a l a u s t e n it i c s t a in l e ss s t e e ls , s u c h a s t y p e 3 0 4 L , i s n o t a p r o b l e m i n c o n t i n u o u s

d iges te r s . Ty pe 304 L s ta in le s s s tee l is su i t ab le a s a m a te r ia l o f cons t r uc t ion f o r s c r eens , b lank p la te s , cen t r a l p ipe , in te r na l

c o n e , a n d n o z z l e s i n c o n t i n u o u s d i g e s te r s . I n b a t c h d i g e s te r s , h o w e v e r , c o r r o s i o n t h i n n i n g o f s t a in l e s s s t e el s c a n o c c u r a t a

r ap id ra te . S ta in le s s s tee l s hav in g a low chr om ium conten t s such a s type 316L ( 17 Cr ) and o lde r type 309 s ta in le s s s tee l

we ld ov e r lays ( wi th l e ss than 20 Cr ) a r e m o s t suscep t ib le to cor r os ion th inn ing 7 . W eld ove r la ys in ba tch d iges te r s a r e

s u s c e p t ib l e t o l o c a l i z e d r a p i d c o r r o s i o n o f i n d i v i d u a l w e l d b e a d s t h a t a r e l o w e r i n c h r o m i u m c o n t e n t t h a n t h e r e m a i n d e r o f

t h e o v e r l a y . L o c a l i z e d r a p i d c o r r o s i o n c a n a l s o o c c u r a t l o c a ti o n s o f h o t c r a c k i n g i n f u l l y - a u s t e n i ti c o v e r l a y s .

I n t e r g r a n u l a r A t t a c k o f S t a i n l e s s S t e e l s

I n t e rg r a n u l a r a t ta c k ( I G A ) o f c o n v e n t i o n a l a u s te n i t ic s t a i n l es s s t e e l s c a n b e p r o b l e m i f th e s t a i n l e ss s t e e l b e c o m e s

s e n s i ti z e d d u r i n g p o s t - w e l d h e a t t r e a t m e n t s T h i s c a n o c c u r i n c l a d d i g e st e r s th a t a r e c o n s t r u c t e d u s i n g r o l l -b o n d e d c o m p o u n d

p l a t e w h e r e t h e c a r b o n s t e e l t h i c k n e ss i s g r ea t e n o u g h t o r e q u i r e m a n d a t o r y h e a t t r e a t m e n t a c c o r d i n g t o t h e c o n s t r u c t i o n c o d e .

T h e e x p o s u r e o f e v e n l o w - c a r b o n t y p e 3 0 4 L s t ai n l es s s t e e l to h e a t t r e a t m e n t t e m p e r a t u r e s r e s u lt s i n t h e p r e c i p it a t io n o f

c h r o m i u m c a r b i d e s a t t h e g ra i n b o u n d a r i e s i n t h e a u s t e n i ti c m i c r o s t ru c t u r e . T h e s u r r o u n d i n g g r a i n s a r e d e p l e t e d i n c h r o m i u m

c o n t e n t t o t h e e x t e n t th a t t h e y e x p e r i e n c e c o r r o s i o n b y IG A . R o l l - b o n d e d c l a d p l a t e a l s o h a s a z o n e o f in t e n s e s e n s it i z a ti o n



a d j a c e n t t o th e c a r b o n s t e e l s u b s t r at e ( t h e c a r b i d e a f f e c t e d z o n e ) th a t r e s u lt s f r o m t h e d i f f u s i o n o f c a r b o n i n t o t h e c la d d i n g

d u r i n g h o t r o l l in g . I f t hi s h i g h l y s e n s i ti z e d l a y e r b e c o m e s e x p o s e d t o t h e p r o c e s s i t w i l l r a p i d l y c o r r o d e b y I G A ( F i g u r e 2 ) .

Str e ss Cor r os ion Cr ac king o f Sta in l e ss Ste e l s

S C C o f s t a in l e ss s t e el w e l d s , n o z z l e s , a n d d o m e l i n e r p l a t e s i n c o n t i n u o u s d i g e s t e r s i s a ls o a c o n s e q u e n c e o f

s e n s i ti z a t io n o c c u r r i n g d u r i n g t h e p o s t - w e l d h e a t t re a t m e n t . T h e S C C i s c h a r a c t e ri s t i c a ll y i n t e r g r a n u l a r i n n a t u r e. S C C o f

sens i t i zed s ta in le ss s t ee l s can oc cur a t any e lev a t ion in the d iges te r , f rom the top to the bo t tom . S CC of s t a in le s s s t ee ls in

b a t c h d i g e s t e r s is a r a r e p h e n o m e n o n b u t h a s b e e n r e p o r t e d i n a n o v e r l a y t h a t w a s b e h i n d a l i n e r th a t l ea k e d . S C C o f t y p e

3 0 4 L s t ai n l es s s t e e l c e n t r a l p i p e s h a v e b e e n o b s e r v e d i n s o m e c o n t i n u o u s d i g e s t e r s, m o s t l y i n v e s s e ls t h a t h a d a n o l d s t y l e ( n o

l o n g e r u s e d ) t y p e o f c e n t r a l p i p e c a t h o d e . T h i s S C C m a y b e r e l a te d t o th e h i g h a n o d i c p r o t e c t i o n ( A P ) c u r r e n t s t ha t w e r e

a s s o c i a t e d w i t h t h is o l d d e s i g n t h a t s h i ft e d t h e p o t e n t i a l o f th e c e n t r a l p i p e i n t o t h e r a n g e w h e r e S C C c o u l d o c c u r .

P R O T E C T I O N

G I N S T S T R E S S

C O R R O S I O N C R A C K I N G

I n t h e e a r l y 1 9 8 0 ' s , a D i g e s t e r C r a c k i n g R e s e a r c h C o m m i t t e e w a s f o r m e d t o e v a l u a t e t h e v a r i o u s a l t e r n a t i v e s f o r

p r o t e c t i o n a g a i n st S C C o f c a r b o n s t e e l co n t i n u o u s d i g e s te r s . T h e t h r e e p r o t e c t i v e m e a s u r e s t h a t w e r e r e c o m m e n d e d b y t h is

c o m m i t t e e ( a n o d i c p r o t e c t i o n , w e l d o v e r l a y , t h e r m a l s p r a y c o a t i n g ) a r e s t il l v a l i d t o d a y 9, a l t h o u g h m u c h g r e a t e r e x p e r i e n c e

has been ga ined on the va r ious l im i ta t ions o f these a l t e rna t ives .

nodic P r ote c t ion for SCC P r ote c t ion

A P h a s b e e n f o u n d t o b e t h e m o s t e f f e c t i v e p r o t e c t i v e m e a s u r e a g a i n s t S C C i n c o n t i n u o u s d i g e s t e rs a n d

im pre gna t ion ves se l s 1°-15. Th e pr inc ip le o f AP i s to ins tal l ca thode s ins ide the ves se l , e i the r o n the w a l l o r a lo ngs ide the

c e n t r a l p i p e , a n d t o s u p p l y c u r r e n t f r o m a n e x t e r n a l r e c t i f ie r o r r e c t i fi e r s s u c h t h a t t h e c o r r o s i o n p o t e n t i a l o f t h e d i g e s t e r w a l l

i s s h if t e d a b o v e t h e r a n g e w h e r e S C C i s p o s s i b l e. A P c a n b e i n s t a l le d w i t h n e w c a r b o n s t e e l b a t c h d i g e s t er s b u t i s ty p i c a l l y

i n s ta l l ed i n r e s p o n s e t o t h e d i s c o v e r y o f a n S C C p r o b l e m .

I t is i m p o r t a n t t h a t a l l p r e - e x i s t in g c r a c k s b e r e m o v e d p r i o r t o e n e r g i z i n g a n A P s y s te m . T h e r e m a y b e s o m e

t h r o w i n g p o w e r o f A P d o w n e x i s t i n g c ra c k s o f s h a l lo w d e p t h ( s u c h a s l es s t h a t 3 r a m ) b u t t h is i s n o t a t a l l w e l l - e s ta b l i s h e d .

E x i s t i n g c r ac k s o f d e p t h g r e a t e r t h a n 3 m m a r e l i a b le t o c o n t i n u e t o g r o w e v e n w i t h A P . I t is p a r t ic u l a r l y im p o r t a n t t o

e n e r g i z e a n A P s y s t e m a s s o o n a s p o s s i b le i n v e s s e ls w i t h a s e r i o u s c r a c k i n g p r o b l e m s i n c e s t re s s c o r r o s i o n c r a c k s h a v e b e e n

o b s e r v e d t o g r o w t o o v e r 2 0 m m i n d e p t h i n o n l y a c o u p l e o f m o n t h s.

W e l d O v e r l a y f o r S C C P r o t e c ti o n

A t o n e t i m e i t w a s t h o u g h t t h a t b a n d i n g o f c ir c u m f e r e n t i a l w e l d s e a m s i n c o n t i n u o u s d i g e s t e r s w i t h s ta i n le s s s te e l

o r n ic k e l -b a s e w e l d o v e r l a y w o u l d p r o v i d e p r o t e c ti o n f r o m S C C . S u c h b a n d i ng o n l y r e s u l te d in m o v i n g t he S C C p r o b l e m

t o th e c a r b o n s t e e l h e a t a f f e c t e d z o n e s t o p a n d b o t t o m e d g e s o f th e o v e r l a y b a n d s. W e l d o v e r l a y b a n d i n g i s n o t

r e c o m m e n d e d s i n c e it d o e s n o t s o l v e t h e S C C p r o b l e m . N o n d e s t r u c t i v e t e s ti n g o f th e o v e r l a y w e l d t o e s i s o f t e n c o m p l i c a t e d

b y t h e p r e s e n c e o f f i n g e r n a i l c o r r o s i o n o f t h e a d j a c e n t c a r b o n s te e l th a t n e e d s to b e r e m o v e d b y g r i n d i n g b e f o r e e f f e c t i v e

c r a c k i n s p e c t io n c a n b e d o n e . D u e t o t h i s d i f f i c u lt y i n i n s p e c ti o n t h e o v e r l a y b a n d s i n m a n y d i g e s t e r s h a v e b e e n r e m o v e d .

I n t h o s e m o r e r e c e n t c a s e s w h e r e w e l d o v e r l a y h a s b e e n a p p l i e d i n th e i m p r e g n a t i o n z o n e i t is r e c o m m e n d e d t h a t

t e m p e r b e a d w e l d i n g b e u s e d . T h i s i n v o l v e s g r i n d i n g a w a y t h e to p h a l f o f t h e fi n a l f o u r o r f iv e b e a d s ( o f h o r i z o n t a l w e l d

o v e r l a y ) a n d t h e n r e - a p p l y i n g a n a d d i t i o n a l f o u r o r f i v e b e a d s o n t o p . T h e t e m p e r i n g e f f e c t o n t h e h e a t - a f f e c t e d z o n e o f t h e

c a r b o n s t e e l m a y i m p r o v e S C C r e s is t a n ce .

W e l d o v e r l a y f o r S C C p r o t e c t i o n i n t h e i m p r e g n a t i o n z o n e ( o r i n a n i m p r e g n a t i o n v e s s e l ) c a n b e e f f e c t i v e i n

c o m b i n a t i o n w i t h o t h e r f o r m s o f p r o t e c t i o n s u ch a s A P o r t h e rm a l s p r a y co a t i n g . A c o m p l e t e w e l d o v e r l a y o f t h e

i m p r e g n a t i o n z o n e w h e r e n o o v e r l a y e d g e s e x i s t c o u l d a l s o p r e s u m a b l y b e e f f e c t i v e i n S C C p r o t e c t i o n .

T h e r m a l S p r a y C o a t i n g f o r S C C P r o t e c ti o n

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

a n d t h e d i g e s t e r w a l l. T h e c o a t i n g a l s o l o c a l l y a l te r t h e c o r r o s i o n p o t e n t ia l o f t h e c a r b o n s t e e l s o i t i s p o s si b l e t h a t t h e rm a l

s p r a y c o a t in g s m a y a l s o s e r v e t o s h i ft t h e c o r r o s i o n p o t e n t i a l o u t o f th e r a n g e w h e r e S C C c o u l d o c c u r . E v e n i f a c o a ti n g i s

p o r o u s i t c o u l d s ti ll p r o v i d e S C C p r o t e c t i o n . S e a l i n g o f c o a t in g s w a s o n c e r e c o m m e n d e d b u t i s n o l o n g e r c o m m o n p r a c ti c e .

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



t w i n w i r e a r c s p r a y c o a t in g s e l i m i n a t e d t h e n e c e s s i ty f o r c o a t se a l in g . N o h e a t - a f f e c t e d z o n e i s c r e a t e d w h e n a t h e r m a l s p r a y

coa t ing i s app l ied .

T h e m o s t p r o m i s i n g u s e o f th e r m a l s p r a y c o a t i n g s f o r S C C p r o t e c t i o n i s a s b a n d s o n s u s c e p t i b l e w e l d s e a m s s u c h a s

s t a in l e s s s te e l to c a r b o n s t e e l t ra n s i ti o n w e l d s e a m s ( F i g u r e 3 ) . S u c h b a n d s o n l y n e e d t o b e w i d e e n o u g h t o c o v e r t h e w e l d

a n d h e a t - a f f e c t e d z o n e o f th e w e l d . S i n c e s o m e t h e r m a l s p r a y c o a t i n g s a r e a p p l ie d t h i n a n d e x p e r i e n c e t h in n i n g i n s e rv i c e ,

a n n u a l m e a s u r e m e n t o f th e c o a t i n g t h i c k n e ss u s i n g a m a g n e t i c l i f t - o f f g a u g e i s r e c o m m e n d e d . M a i n t e n a n c e o f th e c o a t i n g s i s

r e q u i r e d f o r l o n g - t e r m p r o t e c t i o n .

P R O T E C T I O N A G A I N S T C O R R O S IO N T H I N N I N G

A l t h o u g h t h e r e w e r e a f e w c a s e s o f r a p i d t h i n n in g o f c o n t in u o u s d i g e s t er s r e p o r t e d a s e a r l y a s th e 1 9 6 0 's , t h e

p r o b l e m o f r a p id c o r r o s i o n th i n n i n g d i d n o t b e c o m e w i d e s p r e a d u n t i l th e 1 9 9 0 's w i t h t h e a d o p t i o n o f s u c h e x t e n d e d

d e l i g n i f ic a t i o n p r a c t i c e s as e x t e n d e d m o d i f i e d c o n t in u o u s c o o k i n g a n d i s o t h e r m a l c o o k i n g . E v e n d i g e s t e r s i n c o n v e n t i o n a l

o p e r a t i o n e x p e r i e n c e d a r a p i d t h i n n i n g i n t h e s o - ca l l e d w a s h z o n e i f t h e w a s h i n g w a s m o v e d o u t s i d e t h e d i g e st e r . I t i s n o w

u n d e r s t o o d t h a t t h e r a p i d c o r r o s i o n i s re l a t e d t o t h e o p e r a t i o n a t h i g h e r t e m p e r a t u r e s ( e s p e c i a l l y t e m p e r a t u r e s g r e a t e r t h a n

1 6 0 ° C ) a n d p o s s i b l y w i t h l o w e r h y d r o x i d e c o n t e n t i n t h e e x t r a c t i o n l i q u o r ( l e ss t h a n 8 g / L a s N a O H ) t6 .

P for Protection gainst Thinning

C o r r o s i o n t h i n n i n g o f d i g e st e r s i s a f o r m o f a c t i v e c o r r o s i o n th a t o c c u r s i n a w e l l - d e f i n e d r a n g e o f c o r r o s i o n

p o t e n t ia l s . W i t h A P , t h e p o t e n t i a l o f th e d i g e s t e r w a l l i s s h i ft e d a b o v e t h e r a n g e w h e r e a c t i v e c o r r o s i o n o c c u r s a n d i n t o t h e

z o n e w h e r e t h e d i g e s t e r i s p a s s i v e. P a s s i v e c o r r o s i o n r a te s f o r c a r b o n s t e e l a r e t y p i c a l l y v e r y l o w ( t e n t o o n e h u n d r e d t i m e s

l o w e r t h a n t h e a c t i v e c o r r o s i o n r a te s ). A t h i g h t e m p e r a t u r e s a s h i g h a s 1 6 5 ° C t h e p a s s i v e c o r r o s i o n r a t e m a y b e a p p r e c i ab l e .

I n s u c h c a s e s f i n e - t u n i n g o f t h e A P p r o t e c t i o n p o t e n t i a l i s e s se n t i al i n o r d e r t o m i n i m i z e t h e c o r r o s i o n r a t e d u r i n g o p e r a t i o n .

T h e A P o f c o n t i n u o u s d i g e s t e r s f r o m c o r r o s i o n th i n n i n g i s t y p i c a l ly a c c o m p l i s h e d u s i n g e i t h e r w a l l - m o u n t e d o r

c e n t r al p i p e - m o u n t e d c a t h o d e s a n d e x t e r n a l r e c ti f ie r s . I n s ta l l at i o n i s r e a d i l y a c c o m p l i s h e d w e l l w i t h i n t h e t i m e s p a n o f a

n o r m a l s h u t d o w n . T h e w a l l - m o u n t e d c a t h o d e s c a n e i th e r b e o f c i r c u m f e r e n t ia l o r v e r t i c a l o r i e n t a ti o n . T h e s p a c i n g b e t w e e n

s u c h c a t h o d e s i s d e s i g n e d s u c h t h a t t h e p r o t e c t iv e c u r r e n t c a n re a c h t h e d i g e s t e r w a l l a t t h e m o s t r e m o t e p o i n t s . W a l l -

m o u n t e d c a t h o d e s c a n a l s o b e i n s t al l e d o n b l a n k p l a te s ( F i g u r e 4 ) o r b e h i n d b l a n k p l a t e s ( F i g u r e 5 ) a n d s c r e e n s. C e n t r a l

p i p e - m o u n t e d c a t h o d e s c a n b e e i t h e r l in e a r o r t e e - ty p e .

A P o f b a t c h d i g e s t e rs h e l p s t h e n a t u r al p a s s i v a t io n p r o c e s s t h a t o c c u r s d u r i n g a b a t c h c o o k . P a s s i v a t i o n w i t h o u t A P

m a y t a k e 1 h o u r . W i t h A P p a s s i v a t i o n i s t y p i c a l l y a c c o m p l i s h e d w i t h i n m i n u te s . A P h a s b e e n u s e d i n m a n y b a t c h d ig e s t e rs

i n S c a n d i n a v i a b u t h a s y e t t o b e t r i e d i n N o r t h A m e r i c a .

Weld Overlay for Protection gainst Thinning

W e l d o v e r l a y c a n b e u s e d f o r p r o t e c t i o n o f la r g e a r ea s i n th e w a s h z o n e s o r b e l o w t h e c o o k i n g a n d e x t r a c t i o n

sc r eens in con t inu ous d iges te r s . Bo th s ta in le s s s tee l and n icke l - base ove r lay s have be en ap p l ied ~7-2°. Be low the im pr egna t ion

z o n e S C C a t o v e r l a y e d g e s d o e s n o t se e m t o b e a p r o b l e m . A m i n i m u m a s - d e p o s i t e d c h r o m i u m c o n t e n t o f 2 0 i s

r e c o m m e n d e d f o r s t a in l e s s s t e e l o v e r l a y in c o n t i n u o u s d i g e s te r s a s a m e a s u r e o f c o m f o r t a g a i n st c o r r o s i o n b y t h e p r o c e s s .

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

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

w a l l, e . g ., i n th e w a s h z o n e . T h e r e i s c o n s i d e r a b l e g o o d e x p e r i e n c e w i t h c o n v e n t i o n a l h o r i z o n t a l l y - a p p l i e d w e l d o v e r l a y s .

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

m a c h i n e s o r b y a p p l y i n g t h e o v e r l a y i n t h e v e rt i c a l m o d e . V e r t i c a l o v e r l a y s ar e m o r e s u s c e p t i b l e t o d e f e c ts w h o s e r e m o v a l

m a y c o n s u m e t h e t i m e s a v e d d u r i n g a p p l i c a t io n .

W e l d o v e r l a y i n b a t c h d i g e s t er s i s t y p i c a l l y a p p li e d o v e r t h e e n t i re i n t e r n a l s u r f ac e ( s o m e t i m e s e x c e p t i n g t h e t o p

d o m e ) i n a p e r i o d o f 2 t o 3 w e e k s . C o r r o s i o n t e s ti n g h a s r e v e a l e d t h a t th e c o r r o s i o n r a t e o f s t ai n l e ss s t e el w e l d o v e r l a y s c a n

b e h i g h i f t h e a s - d e p o s i t e d c h r o m i u m c o n t e n t i s l es s t h a n 2 5 TM T y p e 3 1 2 s t a i n l e s s s t e e l w e l d o v e r l a y d e p o s i t s c a n h a v e

g r e a t e r t h a n 2 5 C h r o m i u m a n d a r e c u r r e n t l y t h e m a t e r i a l o f c h o i c e fo r w e l d o v e r l a y o f b a t c h d i g e st e r s o p e r a t i n g u n d e r

a g g r e s s iv e c o n d i t io n s . T h e c h e m i s t r y o f t y p e 3 0 9 s t a i n le s s s t e e l s u b m e r g e d a r c w e l d i n g w i r e c a n b e m o d i f i e d t o y i e l d

depos i t s hav ing 25 Cr , bu t th i s i s no t pos s ib le f o r type 309 ba r e wi r e used in the gas m e ta l a r c we ld p r oces s . Nick e l - base

w e l d o v e r l a y s m a y e x p e r i e n c e r a p i d c o r r o s i o n in b a t c h d i g e s te r s a n d a r e n o t r e c o m m e n d e d .



I t i s pa r t i cu la r ly impor tan t tha t we ld ove r l ays in ba tch d iges t e r s be depos i t ed wi thout de fec t s such a s p inholes o r

c racks tha t can expo se the u nder ly ing ca rbon s t eel subs t ra te 21. Com ple te pene t ran t t e s t ing i s recomm ende d a f t e r t he ove r l ay

appl i ca t ion i s comp le te . In t e rna l ho t c racks in fu l ly -aus ten i t ic type 309 we ld ov e r l ays a re pa r t i cu la r ly de t r imenta l a s these

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

provide a fa s t t r ack for the l iquor to ga in acces s to the subs t ra t e . The cyc l i c ope ra t ion of ba tch d iges t e r s causes the l iquor

ins ide pores or exposed h o t c racks to be re f re shed wi th each cook , re su l t ing in rap id cor ro s ion of the under ly in g ca rbon s t ee l

and the fo rma t ion of cav i t i e s benea th the ove r l ay .

I f we ld o ve r l ay procedures a re qua l i f i ed as g roove w e lds (bu i ldup) i t i s pos s ib le to t ake some c red i t fo r the ove r l ay

th icknes s in the to ta l p res sure wa l l ca l cu la t ion for the d iges t e r. Such qua l i f i ca t ion may be use fu l i f the d iges t e r wa l l was

c lose to or a t t he min im um va lue be fore the ov e r l ay was appl i ed . I t is re l a t ive ly easy to qua l i fy type 309 s t a in l e s s s tee l we lds

but i t i s no t easy to qua l i fy type 312 s t a in l e s s st ee l. Wh ere the d iges t e r wa l l is s ign i f i can t ly be low the co de m in imu m (which

ma y be the re su l t o f a rc goug ing to remove a rough sur face ) i t may be neces sa ry to apply a l aye r o f ca rbon s t ee l we ld bu i ldup

prior to o verlay ing with s ta inless s teel.

Over l ay o f cor roded s t a in l e s s st ee l c l adding in ba tch d iges te r s should no t be done on top of the ex i s t ing c l adding

because o f the danger o f d i sbonding o f the c l adding due to the h igh re sidua l s t res ses in the we ld ove r l ay . The c l add ing

should f i r s t be rem oved b y a rc gou ging so tha t t he ove r l ay can be appl i ed d i rec t ly on the ca rbon s t ee l subst ra te .

Therm al Spray Coa ting for Protection gainst Thinning

Therm al spray coa t ings have been appl i ed for p ro tec t ion of l a rge sur face a reas in the wash zone o f cont inuous

diges t e rs and in ba tch d iges t e r s 22. Coa t ings have predom inant ly been appl i ed u s ing the tw in w i re a rc spray proces s where the

coa t ing is depos i t ed up to 2 mm th ick . Sea le rs were prev ious ly used bu t a re no longer comm onplace . H igh ve loc i ty oxyg en

fue l and enh anced a rc spray coa t ings have been appl i ed th inne r than 1 ram.

Auto mated appl i ca t ion is h igh ly recom men ded for coa t ings such as h igh ve loc i ty oxyg en fue l. Arc spray and

enhanced a rc spray coa t ings can be appl i ed man ua l ly a l though au tomated appl i ca t ion g ives a supe r ior coa t ing . Ext raord ina ry

sur face prepa ra t ion i s an abso lu te p re requi s i t e fo r a succes s fu l t he rma l spray coa t ing s ince the bond wi th the subs t ra t e i s

pure ly mechanica l .

Therm al spray coa t ings have prov ided a t l eas t 8 yea rs o f p ro tec t ion in some d iges t e r s . The coa t ings can exper ience

in-se rv ice problems such as cor ros ion , b l i s te r ing , and a t t ack of the coa t ing edges . Ma in tenanc e i s requi red for such coa t ings

to be succes s fu l ove r the long t e rm.

Lining for Protection gainst Thinning

Some o lde r cont inuous d iges t e r s had type 304L s t a in l e s s s t ee l l i ne rs in the top and bo t tom domes tha t , apa r t f rom

SCC problems , h ave surv ived for many yea rs . Leakag e beh ind such line rs d id no t p roduc e s ign i f i can t cor ros ion of the

ca rbon s t ee l beh ind . In some cont inuous d iges t e r s l in ings of type 2205 dup lex s t a in l e ss s t ee l have been succe s s fu l ly ins t a ll ed

in orde r to p ro tec t t he wa l l f rom fur the r cor ros ion . The s t ab le ope ra t ion of cont inuous d iges t e rs mak es l in ing a feas ib le

a l t erna t ive . A l thou gh som e ba tch d iges t e r s have had l iners surv ive nea r ly 50 yea rs , l i n ing of ba tch d iges t e r s is p rob lemat i c

owing to the cyc l i c p res sur i za t ion tha t causes o i l -canning of the l ine r p l a t e s and can cause fa i lu res a t the we lds . Leaking of

l iquor beh ind l ine r p l a t e s in ba tch d iges t e rs can re su l t i n h igh ra t e s o f cor ros ion o f the wa l l beh ind .

Replacement of Carbon Steel with Stainless S teel

Carbo n s t ee l b l ank p la t e s in cont inuous d iges t er s o f t en exper i ence rap id cor ros ion th inn ing . Replace men t in k ind

wi th new ca rbon s t ee l b l ank p la t e s does no t so lve the cor ros ion problem. Rep lacemen t o f b l ank p la t e s us ing type 304L

s ta in le s s s tee l (o r even type 2205 duplex s t a in l e s s s tee l ) i s a more pe rm anent so lu t ion .

P R O T E C T I O N A G A I N S T E R O S I O N - C O R R O S I O N

Eros ion-cor ros ion o f ca rbon s t ee l occurs because i t s pas s iv i ty i s no t s t ab le where the re i s h igh-ve loc i ty l iquor f low.

Eros ion-cor ros ion problems h ave been succes s fu l ly ove rcom e us ing AP , we ld ove r l ay , o r rep lacem ent o f a f fec t ed pa r t s us ing

s ta in l e s s s tee l . AP i s pa r t icu la r ly e f fec t ive aga ins t e ros ion-cor ros ion problem s tha t a f fec t con t inuous d iges t e r s above the

in te rna l cone . AP s t rengthens the pas s iv i ty of the ca rbon s t ee l mak ing i t r e s is t an t to e ros ion-cor ros ion .



Sta in l e ss s t e e l s a re r e s i s t an t t o e ros ion-cor ros ion p rob l ems in d ige s t e r s due t o t he i r much supe r io r pa ss iv i t y .

S t a in le ss s t e e l we ld ove r l ay i s an e f fec t i ve p ro t ec t i ve measu re aga ins t e ros ion-cor ros ion pa r t i cu l a r l y in l oca t i ons t ha t a re

d i f f i cu l t t o i nspec t such a s a rea s beh ind heade r s (F igure 6 ). Rep l acem ent o f c a rbon s t ee l i n te rna l cones wi th new in t e rna l

cones co ns t ruc t ed us ing t ype 304L s t a in le ss s t e e l e lim ina t e s e ros ion-cor ros ion p rob l ems on t he cone .

P R O T E C T I O N A G A I N S T A C I D C L E A N I N G D A M A G E

I t i s be t te r t o p ro t ec t aga ins t t he e f fec t s o f mur i a t i c a c id c l ean ing by ch ang ing t he c l ean ing p rocess i ns t ead o f

chang ing t he ma te r i a l o f cons t ruc ti on . Ac id c l ean ing damage m ay a f fec t c a rbon s t ee l s , aus t en i t ic s t a in l ess s t e el s , and even

dup lex s t a in l e ss s te e l s. Ac id c l ean ing sho u ld p re fe rab ly be don e co ld ( a t a t empe ra tu re o f approx im a te ly 40°C once t he hea t

o f m ix ing o f the ac id wi th t he wa te r is t aken i n to accoun t ) . P re fe rab ly ac id c l ean ing shou ld be done w i th l ess -aggre ss ive

ac ids such a s su l f amic o r fo rmic23, o r r ep l aced a l t oge the r wi th h igh p re ssu re w a te r b l a s t i ng .

N E W D I G E S T E R S

I t is obv iou s ly be t te r t o have co r ros ion p ro t ec t i on i n mind w hen co ns t ruc t i ng a new d ige s t e r t han to have t o r eac t to

cor ros ion p rob l ems w hen they sudd en ly occur in se rv ice . The t h ree ma te r i a ls a l t e rna t ive s fo r new con t inuous d ige s t e r s are

carbon stee l , c lad sta inless s tee l , and sol id duplex sta inless s tee l .

N e w a r b o n S t e e l D i g e s t er s

Desp i t e t he se r i ous SCC and t h inn ing p rob l ems expe r i enced i n numerous con t inuous d ige s t e r s c a rbon s t ee l ha s

se rved we l l fo r ove r 40 yea r s i n ma ny vesse l s. I f c a rbon s t ee l we re t o be se l ec t ed a s the ma te r i a l o f cons t ruc t i on fo r a new

diges t e r it shou ld o f course be w e lded us ing l ow s i l i con con t en t we ld ing consumable s . The ve sse l shou ld be fu l l y pos t we ld

hea t t r e a ted a f t e r cons t ruc t i on . On e op t ion fo r p ro t ec t i on is t o ins t a l l an AP sys t em in t he new con t inuo us d ige s t e r and

ene rg i ze t he AP sys t em a s soo n a s t he d ige s t e r en t e rs se rv i ce .

Ca rbo n s t ee l i s a su i t ab l e op t i on fo r new ba t ch d ige s t e r cons t ruc t i on on ly i f a gene rous co r ros ion a l l owan ce ( e .g . , 25

mm ) i s used . Even t hen t he p rob l em of p ro t ec t i on o r r ep l acement is on ly pos tponed . Lo w s i l i con con t en t c a rbon s tee l s such

as SA285 -Grade C M odi f i ed a re no longe r r ead i ly ava i l ab l e a l t hough such s t ee ls a re p re fe rab l e t o mo de rn p re ssu re ve sse l

g rades such a s SA516 -Grade 70 t ha t have a subs t an t i a l l y h ighe r s i l icon con t en t. Ne w ca rbo n s t ee l ba t ch d ige s t e rs cou ld

pre sum ably be p ro t ec t ed w i th an AP sys t em tha t cou ld be ene rg i zed a t d ige s t e r s t a r t-up .

Ne w lad Sta in l e ss Ste e l Dige s te r s

Cons t ruc t i on o f con t inuous d ige s t e r s us ing ro l l - c l ad t ype 304L s t a in l e ss s t e e l compound p l a t e wi l l o f fe r p ro t ec t i on

aga ins t t he ma jo r co r ros ion p rob l ems tha t a f f ec t c a rbon s t ee l (SCC, co r ros ion t h inn ing , e ros ion-cor ros ion) bu t wi l l no t

necessa r i l y p rov ide p ro t ec t i on aga ins t a c id c l ean ing damage , IGA, o r SC C of t he s ta in l e ss s te e l c l ad p l at e . The c l add ing is

t yp i ca l l y t he co r ros ion a l l owance and i s no t i nc luded i n t he p re ssu re she l l th i ckness ca l cu l a t i ons . Som e c l ad p l a t e we ld

procedure s t ha t do n o t r equ i re pee l -back o f t he c l add ing a re l e ss expens ive t o i ns t al l bu t c an r e su l t in we ld c rack ing i f u sed

ca re l ess ly . W here pos t -we ld hea t t r e a tmen t i s requ i red by t he cons t ruc ti on code (whe re t he und e r ly ing ca rbon s t ee l wa l l

t h i ckness i s g rea t e r than a c e r t a in min imu m va lue ) the t ime a t t empe ra tu re mus t be kep t t o t he min im um requ i red . Pro lon ged

hea t i ng a t pos t -we ld hea t t r e a tmen t t empe ra tu re s c an r e su l t i n t he sens i t i z a t i on o f t ype 304L s t a in le ss s t e e l hav ing l e ss t han

0 .02% ca rbon .

Cons t ruc t i on o f ba t ch d ige s t e r s f rom ro l l - c l ad t ype 304L and 316L s t a in l e ss s t e e l i s no t r e commended s ince t he se

ma te r i a l s c an expe r i ence r ap id co r ros ion t h inn ing i n ba t ch d ige s t e r se rv i ce . Type 304L s t a in le ss s t e e l ha s a s l i gh t l y h ighe r

chrom ium con ten t (18% to 19% Cr ) and i s supe r io r t o t ype 316L s t a in l e ss s te e l (16% to 17% Cr ) . In ac tua l serv i ce , howeve r ,

bo th m a te r i al s c an expe r i ence r ap id co r ros ion t h inn ing t ha t r e su l ts i n exposure o f t he und e r ly ing ca rbon s t ee l . I t i s poss ib l e

tha t a new ba t ch d ige s t e r cou ld be cons t ruc t ed hav ing a new s t a in l e ss s t e e l we ld o ve r l ay l i n ing , fo r example , o f type 312

sta inless s tee l tha t would be preferable for this service appl ica t ion.

Ne w Duple x Sta in l e ss Ste e l Dige s te r s

Sol id t ype 2205 du p lex s t a in l e ss s te e l (UN S $32205) i s cu r ren t l y the be s t ma te r i a l op t i on ava i l ab l e fo r new d ige s t e r

5 6

cons t ruc t i on ' . Due t o t he h ighe r t ens i l e s t r eng th o f dup l ex s t a in l e ss s te e l s t he r equ i red wa l l t h i ckness is s i gn i f i c an t ly l e ss

than i s t he case fo r c a rbon s t ee l o r c l ad ve sse l s . Th i s c an r e su lt i n apprec i ab l e we igh t and cos t sav ings . For con t inuous

d iges t e r s cons t ruc t i on us ing dup l ex s t a in l e ss s t ee l shou ld e l im ina t e a l l cu r ren t l y know n cor ros ion p rob l ems wi th t he poss ib l e



e x c e p t i o n o f d a m a g e d u r i n g h o t m u r i a t ic a c i d c l e a n in g . W h i l e c o r r o s i o n t e st i n g s u g g e s ts t h e p o s s i b i li t y o f c o r r o s i o n o f ty p e

2 2 0 5 d u p l e x s t a i n le s s s t e e l in b a t c h d i g e s t e r l i q u o rs , e x p e r i e n c e t o d a t e h a s s h o w n t h a t c o r r o s i o n i n s e r v i c e is v e r y l o w . A 3

m m c o r r o s i o n a l l o w a n c e i s p r o b a b l y s u f f i c i e n t f o r a l o n g s e r v i c e l i f e o f g r e a t e r t h a n 2 0 y e a r s .

B e c a u s e m o s t s h o p s a r e n o t f a m i l i a r w i t h w e l d i n g o f d u p l e x s t ai n l es s s t e e l i t is a b s o l u t e l y n e c e s s a r y t o h a v e g o o d

q u a l i t y a s s u r an c e b y a p e r s o n e x p e r i e n c e d i n w o r k i n g w i t h d u p l e x s t a in l e s s s t e e ls d u r i n g t h e c o n s t r u c t i o n o f d u p l e x s t a in l e s s

s t e e l d i g e s te r s . I n a d d i t i o n t o t h e r e q u i r e m e n t s o f t h e c o n s t r u c ti o n c o d e , s u c h a s t h e i m p a c t e n e r g y r e q u i r e m e n t s o f s e c t i o n

U H A - 5 1 , i t is a l s o d e s ir a b l e t o t e st t h e w e l d s a n d h e a t a f f e c t e d z o n e s a c c o r d i n g t o a l l t h r e e m e t h o d s o f A S T M A 9 2 3 t o e n s u r e

t h e w e l d s d o n o t c o n t a i n b r i t t l e s e c o n d a r y p h a s e s .

D I S C U S S I O N

T h e s e l e c t io n o f th e b e s t p r o t e c t i v e m e a s u r e f o r a g i v e n d i g e s t e r d e p e n d s o n t h e m a n y f a c t o r s . N o t o n l y sh o u l d

c u r r e n t c o r r o s i o n c o n d i t i o n s b e t a k e n i n t o c o n s i d e r a t i o n b u t a l so t h e p o s s i b i l i ty a n d c o n s e q u e n c e s o f th e s u d d e n o n s e t o f

c o r r o s i o n p r o b le m s . M a n y c o n t i n u o u s d i g e st e r s w i t h r a p i d th i n n i n g o p e r a t e d f o r y e a r s w i t h o u t a p p r e c i a b l e c o r r o s i o n

p r o b l e m s a n d t h e n s u d d e n l y e x p e r i e n c e d c o r r o s i o n t h a t th r e a t e n e d t h e c o r r o s i o n a l lo w a n c e .

T h e l e n g t h o f a s h u t d o w n m a y d e t e r m i n e t h e m o s t p r a c ti c a l c o r r o s i o n p r o t e c t i o n a l t e r n a ti v e . I n a ty p i c a l c o n t in u o u s

d i g e s t e r s h u t d o w n o f 7 t o 8 d a y s d u r a t i o n t h e r e m a y o n l y b e 4 t o 5 d a y s f o r w o r k t o b e d o n e i n s i d e th e d i g e s te r . A l t h o u g h a n

A P s y s t e m c a n b e i n s t a l l e d w i t h in t h i s ti m e f r a m e i t is n o t p o s s i b l e t o a p p l y w e l d o v e r l a y o r t h e r m a l s p r a y c o a t i n g o v e r m o r e

than abou t 100 squa re m e t re s (110 0 squa re fee t ) in th is t im e pe r iod . I f i t i s in tended to cove r la rge r a reas wi th e i the r we ld

o v e r l a y o r t h e r m a l s p r a y t h e n t h e a p p l ic a t i o n s m u s t b e d o n e o v e r a n u m b e r o f s h u td o w n s , o r a n e x t e n d e d s h u t d o w n m u s t b e

t a k en . I f m a j o r w o r k i n a n o t h e r a r e a ( e .g . , th e r e c o v e r y b o i l e r) n e c e s s it a t e s a n e x t e n d e d o u t a g e t h i s w o u l d b e a g o o d

o p p o r t u n i t y t o a p p l y a s m u c h o v e r l a y o r t h e r m a l s p r a y c o a t i n g a s p o s s ib l e .

P r o t e c t i v e m e a s u r e s c a n b e m u t u a l l y c o m p a t i b l e . A s h a s a l r e a d y b e e n m e n t i o n e d A P c a n b e u s e d t o p r o t e c t t h e

e d g e s o f w e l d o v e r l a y b a n d s f r o m S C C . I n t h i n n in g d i g e s t e rs w h e r e w e l d o v e r l a y is a p p l i e d o v e r s e v e r a l y e a r s A P c a n b e

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

s c h e d u l e d t o b e o v e r l a i d . B a n d s o f t h e r m a l s p r a y c o a t i n g a r e a ls o u s e f u l i n p r o t e c t i n g t h e e d g e s o f s t a in l e s s s te e l w e l d

o v e r l a y f r o m b o t h S C C a n d f i n g e r n a i l c o r r o s i o n . C a u t i o n s h o u l d b e ta k e n w h e n c o m b i n i n g A P w i t h la r g e s u r f a c e a r e a s o f

t h e r m a l s p r a y c o a t i n g . I f th e c o r r o s i o n p o t e n t i a l o f t h e t h e r m a l s p r a y c o a t i n g i s s i g n i f ic a n t l y l o w e r t h a n t h a t o f t h e c a r b o n

s t ee l d i g e s t e r w a l l t h e r e c t i f ie r s m a y p r o v i d e c u r r e n t i n o r d e r t o r a i s e t h e p o t e n t i a l o f t h e t h e r m a l s p r a y c o a t i n g .

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

t h a t th e w o r k m e e t s e x p e c t a t io n s . A n d w e l l b e f o r e t h e w o r k i s t o b e d o n e i t is r e c o m m e n d e d t h a t c o r r o s i o n te s t in g b e

p e r f o r m e d i n t h e a c tu a l l i q u o r s f r o m t h e d i g e s t er . F o r A P c o r r o s i o n t e s t in g t h is w i l l i n v o l v e t h e d e t e r m i n a t i o n o f th e

o p t i m u m c o n t r o l p o t e n t i a l ( t h e p o t e n t ia l a t w h i c h t h e l o w e s t c o r r o s i o n r a t e o c c u r s ) o r t h e p o t e n t i a l w h e r e p a s s i v a t i o n i s t h e

m o s t p r o f o u n d . F o r w e l d o v e r l a y c o r r o s i o n t e st i n g th i s w i l l i n v o l v e t h e d e t e r m i n a t i o n o f t h e m i n i m u m a s - d e p o s i t e d

c h r o m i u m c o n t e n t t o b e a p p l i e d i n o r d e r to a t t ai n t h e m i n i m u m c o r r o s i o n r a te i n t h e d ig e s t e r . F o r t h e r m a l s p r a y c o a t in g

cor ros ion t e s t ing th i s wi l l invo lve wha t i s the bes t a l loy to app ly , de te rm ine the coa t ing p roces s , and coa t ing th icknes s

n e c e s s a r y i n o r d e r t o o b t a i n t h e m i n i m u m c o r r o s i o n r a t e i n t h e d i g e s te r .

C O N C L U S I O N S

T h e r e a r e t h r e e p r i m a r y m e t h o d s f o r c o r r o s i o n p r o t e c t i o n in b a t c h a n d c o n t i n u o u s d i g e s t e r s : a n o d i c p r o t e c t i o n , s ta i n le s s

s t e e l w e l d o v e r l a y , a n d t h e r m a l s p r a y c o a t i n g .

2 . A n o d i c p r o t e c t i o n s y st e m s c a n b e d e s i g n e d to p r o t e c t a g a i n s t c o r r o s i o n b e h i n d s c r e e n s a n d b l a n k p l at e s .

.

A u t o m a t i c s t a in l e s s s t ee l w e l d o v e r l a y c a n b e a p p l i e d b e h i n d h e a d e r s t o p r o t e c t a g a i n s t e r o s i o n - c o r r o s i o n o f t h e d i g e s t e r

w a l l d u e t o h i g h v e l o c i t y l i q u o r f l o w .

4 . T h e r m a l sp r a y c o a t i n g s ca n b e a p p l i e d in b a n d s o v e r w e l d s e a m s to p r o t e c t t h e m f r o m s t re s s c o r r o s i o n c ra c k i n g .

.

A n o d i c p r o t e c t i o n o r t h e r m a l s p r a y c o a t i n g m a y b e u s e d t o p r o t e c t t h e a d j a c e n t c a r b o n s t e e l a t s t a i n l e s s s t e e l w e l d

o v e r l a y e d g e s f r o m s t re s s c o r r o s i o n c r a c k i n g .



R E F E R E N E S

1 . A. Wens ley , Corros ion o f Carbon and S tainless S tee l s in Kraf t Digeste rs , Paper No . 589 p resen ted a t the NA CE

Corrosio n 2000 Conferenc e, Orlando, F L (2000).

2 . E. Ma spers , Gen eral Corrosion In Continuous Digesters , Proc. Eighth Int l , Symp . on Cor rosion in the Pulp and Pape r

Ind., S tockholm, pp. 1-11 (1995).

3 . L . Keiss ling , A S tudy o f the In f luence o f Modi f ied Con t inuous Cook ing Processes on the Corros ion o f Con t inuous

Digester Shel ls, Proc. Eighth Int l . Syrup. on Corro sion in the Pulp and Pape r Ind., Stockholm , pp. 12-19 (1995).

4 . S. Ost lund, Em erfeldt , B. Sandberg, and M. Linder, Invest igat ions Regard ing Corrosion in Batc h Digesters , Proc. Sixth

Intl . Symp. o n Corros ion in the Pulp and P aper Ind. , Hels inki , pp. 48 -59 (1989).

5 . H. Le inonen , Corros ive Env i ronmen ts in Di f fe ren t Process S tages o f M ode m Kraf t Batch Cook ing , Paper No . 590

presen ted a t the NA CE Corros ion 2000 Conference , Or lando, FL (2000) .

6 . A. Wens ley , Effec t o f Su l f id i ty on the Corros iv i ty o f Whi te , Green , and Black Liquors , Paper . No . 281 p resen ted a t the

NACE Corrosion 99 conference, San Antonio, TX (1999).

7 . A. Wens ley , Corros ion o f W eld Over lay in Batch Digeste rs , Proc . TA PPI Eng . Conf . , pp . 1193-1200 (1998) .

8 . A. W ensley, In tergranular Attac k of Stainless Steels in Kraft Digester Liquors , Pap er No . 465, NA CE Corro sion 96

Conference , (1996) .

9 . R. Yesk e, Stress Corro sion Cracking of Continuous D igesters for Kraft Pulping, a Final report to the Digester Cracking

Research C omm it tee , T he Ins t i tu te o f Paper C hemis t ry p ro jec t 3544 , ( January 1983) .

10. J .I. Munro, Anod ic Protec t ion of Continuous Digesters to Preven t Corro sion and Cracking, Proc. TA PP I Eng. Conf. , pp.

181-185 (1983).

11. H. Savisalo, M. Pull ianen, T. Kero la, Corro sion of Kraft Continuous Digesting: The Resu l ts of In-Si tu Electrochem ical

Test ing, Proc, Fifth Intl . Syrup. on Corrosion in the Pulp and Pape r Ind., V ancouv er, (1986).

12. D. Singbei l and A. Garner, Ano dic Protect ion to Preve nt the Stress Corros ion Cracking o f Pressure Ve ssel Steels in

Alkaline Sulfide Solutions, Materials Perform ance, V ol , 26 No. 4 pp. 31-36 (1987).

13. D. Singbeil , D oes Anodic Prote ct ion Stop Digeste r Cracking?, Proc. Sixth Intl . Symp . on Corrosio n in the Pulp and

Pape r Ind. , Hels inki , pp. 109-116 (1989).

14. L. Kiessling., App licat ion of an Anodic Protect ion System to a Preim pregnat ion V essel , Proc. TAPP I Eng, Conf., pp.

659-666 (1989) .

15. M. Pu l l i anen and H. Sav isa lo Exper iences o f Corros ion Con t ro l in Con t inuous Hydrau l ic and Tw o Vesse l Digeste rs by

An odic Protect ion, P roc. Sixth Int l. Syrup. on Corrosion in the Pulp and Pa per Ind. , Hels inki , (1989).

16. A. Wens ley, Corr osion of Batch and Continuous Digesters , Proc. Ninth Int l. Sym posiu m on Corrosion in the Pulp

Paper Industry, Ottawa, pp. 27-37 (1998).

17. T. Erikkson, E. Borjesson , and L. Johansson, Ex perience from Overlay W elded Lining in Digesters , Proc. Eighth Int l.

Symp. on Corros ion in the Pulp and Pape r Ind., Stockholm, pp. 58-64 (1995 ).

18. R . Workman , Adv ances in Weld Over lay Techno logy Prov ide Corros ion Pro tec t ion in Kraf t Diges te rs, Proc . TA PPI

Eng. Conf. , pp. 173-176 (1995).

19. P. Huls izer, Exp edited Weld Overla y of Continuous Digesters , Proc. TAP PI Eng. Conf. , pp. 193-202 (1995).

20. G. Lai and P. Huls izer, Applicat ion of Typ e 312 Stainless S teel W eld Ove rlay for Corro sion Protect ion of Batch

Digesters , Proc. TAPPI Eng. Conf. , pp. 399-404 (1999).

21 . Gu ide l ines fo r Corros ion Res i s ting Weld Over lay in Su lphate and Soda Diges te r Vesse l s , TAPPI T echn ica l In fo rmat ion

Paper TIP 0403-02 , TA PPI (1999) .

22 . J . Croopn ick , Per fo rmance o f HVO F Coatings in Diges te r Env ironmen ts , Paper No . 279 p resen ted a t the NA CE

Corros ion 99 Conference in San Anton io , TX (1999)

23. D.C. Crow e, Corrosion in Ac id Cleaning Solut ions for Kraft Digesters , Proc. Sev enth Intl . Syrup. on Corro sion in the

Pulp and Pa per Ind. , Orlando, pp. 33-40 (1992).

24. J .-P. Aud ouard, F. Dupoiron, and D. Jobard, Stainless S teels for Kraft Digesters in N ew Pollut ion Free Mil ls , Proc.

Sixth Int l . Symp. on C orrosion in the Pulp and Pa per Ind., H els inki (1989).

25 . R .M. D av ison and M. D . Moskal , Qual if i ca t ion o f Weld ing Procedures fo r Dup lex S tain less S teel s, Proc . TA PPI Eng .

Conf. , pp. 379 -398 (1999).



F i g u r e 1 . E r o s i o n - c o r r o s i o n o f a c o n t i n u o u s d i g e s t e r w a l l

b e h i n d a w a s h h ea d e r . E r o s i o n - c o r r o s i o n d e p t h s a r e

ind ica ted in m i l l im e te rs and inches .

F i g u r e 4 . B l a n k p l a te c a t h o d e f o r a n o d i c p r o t e c t i o n

sys tem in a con t inuous d iges te r .

F i g u r e 2 . C o r r o s i o n o f t y p e 3 2 1 s t a i n l es s s t ee l c la d p l a te

i n a b a t c h d i g e s t e r t h a t h a s e x p o s e d th e c a r b i d e a f f e c t e d

zone in the c ladd ing ad jacen t to the ca rbon s tee l subs t ra te .

C ladd ing th icknes ses a re ind ica ted in m i l l im e te rs .

F i g u r e 5 . C a t h o d e f o r a n o d ic p r o t e c ti o n lo c a t e d b e h i n d

b l a n k p l a t e s i n a n i m p r e g n a t i o n v e s s e l .

F ig ur e 3 . Band of the rm a l spray app l ied to p ro tec t a

s ta inless s teel to carbon s teel weld in the imp regnat ion zone

of a cont inu ous diges ter . Coa t ing thicknesses are indicated

in mils.

F i g u r e 6 . W e l d o v e r l a y i n a h e a d e r f o r p r o t e c t i o n a g ai n s t

e r o s i o n - c o r r o s i o n .

Corrosion Protection of Kraft Digesters

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