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Corrosion

Corrosion-resistant tube materialsfor extended life o f op ening sin recovery boi lersLarry D . Paul , Joan L. Barna, Michael J . Dan ielson, and Sharon L. Harper

i

ABSTRACT." Th e corrosive condi t ions causing rapid corrosion o fType 3 0 4 L stainless steel in tube openings have been dupl icated in thelaboratory. Al ternate ma terials a/so have been testea~ a nd som e showimp rov ed corrosion resistanceover Type 304L . A l loy 8 25 an d A ll oy62 5 com posite t ub i ng and A l l oy 6 00 an d A ll oy 62 5 w e l d over laymater ia l s a l l show promise as a replacement or Type3 0 4 L i n tu b eopenings. A ll recovery boilers design ed or op erateda t 8 .3 7 5 M P a(120 0p si ) , and above should cons ider us ing these replacementmaterials o r tube openings.KEYWORDS: Allo~ alloy stee~ corrosion resistance, u se d salts,hydroxides, inorgan ic compoun ds, openings, recovery urnace s, re-placement, stainlessstee~ substi tutes, tempera ture, tubes.

p i rece ss recov ery boilers burn sulfur-r ich fuel (b lack l iquor) that re-su lts in h ighly co rrosive condit ions inth e fu rn ace . To r ed u ce co r ro s io n toaccep tab le ra tes , th e fu rn ace wal l tu b esa r e c o m m o n l y c l a d w i t h T y p e 3 0 4 Ls ta in les s s tee l . Wh i le th i s ap p ro achh e lp s p ro tec t th e lo wer fu rn ace f ro macce le r a ted co r ro s io n b y su l f id a t io n ,lo ca lized r eg io n s in th e fu rn ace h av eo th er co r ro s io n co n cerns .Ty p e 3 0 4 L s ta in les s s tee l c lad d in gc a n b e c o r r o d e d i n s m a l l p a t c h e saro u n d b o i le r tu b e o p en in g s ( 1 - 9 ) . F i g -u r e 1 s ho w s a n e x a m p l e o f w a s t a g earo u n d a tu b e o p en in g . Tu b e o p en in g s

th a t can b e a f f ec ted in c lu d e a ll e lev a-t ions of air por ts , l iquor gun and b urn erp o r t s , an d o b serv a t io n p o r t s. Th e r a teso f c o r r o s i o n v a r y w i d e l y a n d h a v eran g ed f ro m 0 to 0 .7 6 2 r am/y ear . Th ecau se o f th i s co r ro s io n h as b ee n a t -t r ib u ted to mo l ten h y d ro x id e sa l t s th a tc o n d e n s e w h e n t h e c o m b u s t io n g a s i sco n tac ted b y th e r e la t iv e ly co o l a i rf r o m a r o u n d t h e c a s in g o f t h e t u b e s( 4 - 5 , 7 - 9 ) . F i g u r e 2 s h ow s c a u s ti c d e -p o s i t s d r ip p in g f ro m aro u n d a tu b eopening .

So d iu m h y d ro x id e a t a i r p o r t o p en -in g s can ex is t o n ly u n d er v e ry sp ec ia l-ized co n d i t io n s ( 1 , 7 - 9 ) . T h e C O 2

P a u l , D a n i e l s o n , a n d H a r p e r w o r k f o r B a b c o c k & W i l c o x C o . , R e s e a r c h & D e v e l o p -m e n t D i v is i o n , 1 5 6 2 B e e s o n S t ., A l li a n c e , O H 4 4 6 0 1 . B a r n a w o r k s f o r B a b c o c k &W i l c o x ' s F o s s i l P o w e r D i v i s i o n , B o x 3 5 1 , B a r b e r to n , O H 4 4 2 0 3 - 0 3 5 1 .

co n ten t o f th e a i r ( ab o u t 33 0 p p m) i ssu f f ic ien t to c o n v er t so d iu m h y d ro x -id e to so d iu m carb o n a te ( 1 ) . H o w e v e r ,th e r eac t io n t imes a r e a fu n c t io n o fC O 2 co n cen t r a t io n an d tem p era tu r e ;a t b o i le r t u b e m e t a l t e m p e r a t u r e s , t h er eac t io n s can tak e sev era l h o u r s toc o m p l e t e ( 1 ) . Th u s , so d iu m h y d ro x id ecan ex is t o n ly n ear tu b e o p en in g s , an drapid co r ro s io n o f s ta in les s s tee l o n lyo ccu r s a t b o i le r o p en in g s.On e ap p ro ach to av o id in g th i s lo -calized corrosion is a substi tu t ion oft h e m a t e r i a ls u s e d f o r t h e t u b e o p e n -in g s. Wh i le we k n o w th a t p u re n ick e lcan r es i s t h y d ro x id e co r ro s io n , o th e rconcerns , such as su lfidation , also needt o b e c o n s i d e re d . T e s t m a t e r i a ls w e r ese lec ted f ro m th o se k n o wn to r es i s tco r ro s io n in th e su l fu r - r ich co mb u s -t i on g a s e s t h a t e x i s t i n t h e r e c o v e r yb o i le r f u rn ace . On ly th o se mate r ia l sth a t co u ld b e o b ta in ed in th e fo rm o f aco mp o s i te tu b e were co n s id ered in thi ss tu d y . Weld o v er lay s a l so were ev a lu -a ted as a me an s to r ep a i r ex is t in g co r -ro s io n s i tes . Ho w ev er , s in ce co r ro s io nten d s to p r ed o min a te o n th e cas in gs i d e a n d d o e s n o t r a p i d l y a t t a c k t h eu n d er lay in g ca rb o n s tee l , we ld r ep a i rm a y b e u n n e c e s s a r y.ExperimentalMaterials screening estC o u p o n s o f s e l e c t e d m a t e r i a l s a n dw e l d o v e r l a y s w e r e e x p o s e d t o m o l t e nh y d ro x id e mix tu res fo r 7 20 h o u r s . Co r -r o s io n r a t e s w e r e d e t e r m i n e d f r o m t h e

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Corrosion~ii!!iiiii! i

1. Example of caust ic corros ion around tube openings 2. Example of caust ic depos i ts dr ipping from around a tube opening

Chemist ry of tes t coupon materialsMaterial C Cr Cu Fe Mn Mo Ni P S Si

C. steel 0.11 NR NR BAL 0.36 NR NR 0.01 0.015 NR304L 0.01 18.20 NR BAL 1.82 NR 8.11 0.031 0.011 0,48310 0.06 24.87 0.11 BAL 1.94 0.16 19.72 0.024 0.001 0.68Alloy 625 0,01 22.10 NR 3.08 0.05 8.81 61.97 0.005 0.001 0.09Alloy 825 0.02 23.34 1.76 30.41 0.41 2.74 40.22 NR 0.001 0.17Weld*309L 0.02 23.5 0.40 BAL 1.5 0.30 13.0 0.015 0.010 0.40312 0.08 30.0 0,40 BAL 1.5 0.30 9.5 0,015 0,010 0.50625 0.008 22.0 2.0 19.5 0,20 7.0 BAL 0.015 0.010 0.30600 0.010 16.0 0,03 7,50 0.10 NR 74,0 NR 0.005 0.10* Nominal compositions of weld wire, NR = not reported.BAL = balance of alloy.

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3 . S c h e m a t i c o f tes t sys tem fo r m ate r ia l s sc r een i n g tes t

Air inletAir ouUet

I

.=s

Specimens

Retort

Furnace

Temperaturerecorder

weight loss of the coupons during thisexposure period. The wrought couponswere 2.54 mm x 12.7 mm x 1.60 mm (1in. x 0.5 in. x 0.063 in.), except thecarbon steel, which was 3.18 mm (0.125in.) thick. The weld overlays were pre-pared using a gas metal arc weld pro-cess to lay down the various weldmaterials over 1018 carbon steel cou-pons. The carbon steel starting blankswere 25.4 mmx 19.05 mmx 4.78 mm (1in. x 0.75 in. x 0.188 in.) with about 15.2mm (0.060 in.) of weld metal build-upon all surfaces.The material chemistries are sum-marized in Table I. Because the weldmetal will be diluted with the base car-bon steel, the composition of the weldon the coupon will be lower in alloyelements than those reported in TableI. The chemistry of the welds was notanalyzed.The test apparatus is shown sche-matically n Fig. 3. All coupons or weldoverlayswere placed in 25.4-mm x 50.8-mm (1 in. x 2 in.) high form aluminacrucibles and covered with the hydrox-ide salt mixture. The crucibles wereplaced in support racks inside a testretort. The re tort was heated using avertical furnace. Air was scrubbed ofresidual CO~ using sodium hydrate-

asbestos and then metered into thetest retort throughout the test. Thetest temperature was maintained at343C + 5.5C (650F _+ 10F) for theentire test.Salt mixtures were added to cru-cibles containing the weight loss cou-pons. The various salt mixtures usedare given in Table II. After one weekat test temperatures, the retort wasopened, and the salt mixtures werereplenished as needed. Some loss ofsalts was seen due to evaporation andwicking up and out of the crucibles;however, all crucibles still had signifi-cant deposits after the first week. Inaddition, salts were still found in all ofthe crucibles a t the end of the tes t,although some losses occurred.T emperature l imi ts for us eof T yp e 3 04L s tain le s s s tee lThe corrosion rat e of 1020 carbon steeland 304L stainless steel were deter-mined using specially designed resis-tance-type corrosion probes in moltenand frozen NaOH in a temperaturerange of 371C to 323C (700-613F).The NaOH was contained in a nickel200 crucible covered by a lid. The cru-cible was heated by placing the con-tainer into a two-gallon autoclave

(autoclave used as a heat source only).The NaOH was sparged continuouslywith air (air passed through sodiumhydrate-asbestos to remove carbon di-oxide) at the bottom of the melt at aflow rate of 0.0226-0.0283 m3/h (0.8--1.0 standard ft'/h). Care was taken toensure tha t there was no metallic con-tact between the corrosion probes andthe nickel container to prevent gal-vanic effects. Thermocouples im-mersed in the melt were sheathed inAlloy 600.

The t emperature compensation de-vice on the carbon steel corrosion-re-sistant probes did not function asplanned. Consequently, carbon steelcorrosion rate s required evaluation bycooling the probe down after each test.The 304L stainless steel probe workedas designed and could be used to giveon-line corrosion rates (in real time atthe t est temperature). Average corro-sion rates were determined by mea-suring the ambient temperature probereadings at the star t and again at thecompletion of each test. The differencein probe readings was converted into acorrosion rate.RP~ultsMaterials screening tes tThe corrosion rates of the t est materi-als in molten hydroxide salts at 343C(650F) are given in Fig. 4. All mix-tures of hydroxide salts are more cor-rosive to Type 304 stainless steel thanto carbon steel. The most corrosivemixture to stainless steels is NaOH +KOH. Moreover, all complex salts ap-pear to be more corrosive than NaOH;NaC1 and NaeSO4 additions also in-crease corrosion rates above those seenin pure NaOH. At the end of the test,the NaOH + KOH mixture was foundto be nearly totally depleted from thecrucibles. This may be partly due tothe rapid corrosion seen in this mix-ture, but salt also was lost due to evapo-ration and wicking of he salt out of hecrucibles. Significant amounts of all ofthe other salt mixtures were found atthe end of the test.

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Corrosion4. Corros ion rates of weight loss coupons exposed for 720 hours inmo l ten sal t mix tures at 650F wi th ai r cover gas (scrubb ed of carbondiox ide)

2.62.42.22.01.81.61.41.2

mm 100% NaOHr-~ 75% NaOH + 25% KOHmm 90% NaOH 10% Na2SO4

95% NaOH + 5% NaClc:~ Repeat of Test deposit #10 , h i , ,

0.80.6

L,o , , w ''

5. Average cor ros i on ra tes o f p robes exposed to mo l ten sod iumhydrox ide as a f u n c t i o n o f tempera ture . The bo i l e r p ressure cor re -spond ing t o t h e meta l tempera ture a l so is shown (assumes meta ltempera ture i s 50F above sa tura t ion tempe ra ture)BOILER PRESSURE, m P a

320 330 340 350 360 370TEMPERATURE,C

T h e c o r ro s i o n r a t e o f T y p e 3 0 4 Ls taLr f les s s tee l w as gener a l ly h ighert h a n a n y o f t h e o t h e r t e s t m a t e ri a l s.T y p e 3 1 0 s t a in l e s s s t e e l w a s n e a r l y a sb a d a s , a n d i n s o m e c a s e s w o r s e t h a n ,t h e T y p e 3 0 4 L s t a i n l e s s s t e e l . B o t hA l l o y 6 2 5 a n d A l l o y 8 25 w r o u g h t m a -t e r i a l s r e s i s t e d c o r r o s i o n b y m o l t e nh y d r o x i d e s . T h e T y p e 3 0 9 L a n d 3 1 2w e l d o v e r l a y s p e r f o r m e d b e t t e r t h a nthe w r ought s ta in les s s tee l s . This r e -su l t i s cons i s ten t w i th f ie ld obse r va-t i o n s , w h e r e i n T y p e 3 0 9 L a n d 3 1 2w e l d s i n t h e v i c i n i ty o f c o r r o d e d t u b eo p e n i n g s e x p e r i e n c e w a s t a g e a t a s i g -n i f ican tly s low er r a te than th e 304L-c lad layer . The A l loy 625 w eld over lays h o w e d m a r g i n a l r e s i s ta n c e t o c o r r o -s ion . The A l loy 600 w eld ov er lay w asthe on ly w e ld mate r ia l tha t cons i s ten t lys h o w e d l o w c o r r o si o n r a t e s i n m o l t e nhydr oxide sa l t s .Tem p era tu re l im i t s fo r u seo f T y p e 3 0 4 L s ta i nl es s steelP u r e N a O H h a s a m e l t i n g t e m p e r a -tur e of 318C ( 604F), bu t the p r obe sw e r e c o m p l e t e l y f r o z e n i n p l a ce a t t h e324C (615F) tes t condition. F i gu re 5i s a p l o t o f a v e r a g e c o r r o s i o n r a t e v e r -s u s te m p e r a t u r e . T h e p l o t t e d d a t as h o w t h a t t h e c o r ro s i o n r a t e f o r 3 0 4 L

s t a in l e s s s t e e l i s g r e a t e r t h a n t h a t o fca r bon s tee l by a f ac tor o f a lmos t 10 .T h e c o r r o s i o n r a t e s d e c r e a s e a s t h et e m p e r a t u r e d e c r e a s e s , a n d t h e c a r -bon s tee l cor r os ion es sen t ia l ly s topsa t the f r eez ing poin t .

T h e p o w e r o f th e o n - l in e m e t h o d o fm e a s u r i n g c o r ro s i o n is d e m o n s t r a t e db y t h e d a t a p r e s e n t e d i n F i g . 6 . A t370C (698F) , the cor ros ion rate for304L s ta in les s s tee l i s obse r ved to in -c r e a s e w i t h t i m e ; t h i s w o u l d n o t h a v ebeen a ppar e n t in a conventiona l w e igh tlos s exper iment . C onver se ly , the 324C(616F) dat a for the f roze n sal t condi-t ions show s a cor r os ion r a te tha t de -c r e a s e s w i t h t im e a n d b e c o m e s z e ro ,imply ing tha t some cor r os ive spec iesi s be ing consum ed ( oxygen?) w i th t ime .D i f f us ion of spec ies in a so l id a r e or -der s of mag ni tude s low er in a so l id( f rozen sal t) than in a l iquid (moltensa l t) ; th i s i s l ike ly caus ing the s tead yd r o p o f c o r ro s i o n r a t e s w i t h t i m e a tthe 324 C ( 616 F) te s t t emper a tur e .At 356C (673F) , there is a var iablecor r os ion behavior w i th t ime , w hichw e d o n o t u n d e r s t a n d .

D i s c u s s i o nN a O H c o r r o d e s T y p e 3 0 4 L s t a i n l e s ss t e el , a n d t h e r e f o r e 3 0 4 L m a y n o t b et h e o p t i m u m m a t e r i a l f o r t u b e o p e n -ings , pa r t i cu la r ly in h igher - pr es sur eboi le r s w her e h igh ly caus t ic condi t ionspr eva i l . The h igh cor r os ion r a tes ofT y p e 3 0 4 L s e e n i n t h e f i e ld h a v e b e e nd u p l i c a te d i n t h e l a b o r a t o r y , m a k i n gth i s t e s t a r easo nable bas~s f or a l t e r -h a t e m a t e r i a l s e l ec t io n . F u r t h e r m o r e ,t h e h y d r o x i d e s a l t s a r e m o r e c o r r o -s ive to the s ta in les s s tee l s than to ca r -b o n s t e e l . T h e h i g h e r c o r ro s i o n r a t e sof s ta in les s s tee l dupl ica te the f ie ldcondit ions .

F r o m t h i s s t u d y , t w o r e p l a c e m e n tm a t e r i a l s a n d t w o w e l d o v e r l a y s f o rt h e p u r p o s e o f r e p a i r s e m e r g e a s t h em o s t r e s i s t a n t t o m o l t e n h y d r o x i d ecorrosion: A l loy 625 b imeta l l i c tub ing A l loy 825 b imeta l l i c tub ing A l loy 600 w eld over lays A l loy 625 w eld over lays .

B e c a u s e t h e c o r r o si o n r e a c t io n i nmo l ten hyd r oxide sa l t s involves chr o-m i u m , h i g h e r c h r o m i u m l e v e ls in t h ea l loy a r e de t r imenta l . H ig her n icke l

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i i i6 ' Co r ros ion ra tes o f 304L s ta i n l ess s tee l co r ros ion p rob es as afunct ion o f t ime a t s e le c t e d t e s t t e m p e r a t u r e s 7 . Co r ros ion ra tes o f va r i ous ma te r i a l s as a func t i on o f the i r N i /Crra t io . A l l o f the res i s tan t ma te r i a l s have N i /Cr r a t i os ove r 1 .5

!zffl0,,0o

4321

I0 0

3 5 6 C

3 3 2 C=_ . _ . - - - - -

3 2 4 C2 0 4 0 6 0 8 0

T I M E , h

2 .82 .62 .4Z2

- ~'.0Wt - - 1 . 8Q : 1 . 6Z 1 . 4O 1 .20 1 .00 .8O 0 . 6O 0 .40 .20

z z z=

= , . j . . j ~ ~ = N

" ~ 1 I I I I , II v W e l d m e t a li a W r o u g h tm e t a l

. . . . . . l rY _- .1 2 3 4N i / C r R A T I O O F M E T A L

I I . Sa l t m i x tu res used fo r we igh t l oss co r ro -s ion re tor t t e s tWeigh t %

NaOH KOH Na2SO NaClDepo s i t I 100 0 0 0Depos i t 2 75 25 0 0Depos i t 3 90 0 10 0Depos i t 4 95 0 0 5

l e v el s i n t h e m a t e r ia l s e e m t o b e b e n -e ficial du e to th e f or m at ion o f a p r o-tec t ive n icke l ox ide . Bec ause o f theb e n e f i t o f n i ck e l a n d t h e d e t r i m e n t o fchr omium, i t i s r easona ble to e xpec t ar e la t ionsh ip be tw een cor r os ion r a tesa n d a p a r a m e t e r s u c h a s a n i ck e l- to -c h r o m i u m r a t i o ( N i / C r ) . F i g u r e 7s h o w s t h e c o r ro s i o n r a t e s o f t h e v a r i -ous te s t mate r ia l s p lo t ted as a f unc-t ion of the i r N i /Cr r a t ios . A l l o f ther e s i s t a n t m a t e r i a l s h a v e N i / C r r a t io sa b o v e 1 . 5. H o w e v e r , w h e n c o n s i d e r-ing r ep lacement mate r ia l s addi t iona lc o n s i d e r a t i o n s s h o u l d b e i n c l u d e d ,such as r es i s tance to su l f ida t ion andoxidation. Therefore, a mater ial withadeq ua te chr omium leve ls a l so i s r ec -o m m e n d e d .B e c a u s e K O H m i x t u r e s a r e m o r ecor r os ive , w e be l ieve tha t do sed mi ll so r h i g h - p o t a ss i u m m i ll s m a y s e e m o r e

sever e cor r os ion . L ikew ise , sod iumchlor ide appear s to acce le r a te cor r o-s i on o f T y p e 3 0 4 L i n m o l t e n h y d r o x -ides . H ow ev er , r ap id cor r os ion of Ty pe304L s ta in les s s tee l i s s een ev en w henn o p o t a s s i u m i s p re s e n t .

T h e t e m p e r a t u r e e f f e c t o n c o r ro -s ion of Ty pe 304L s ta in les s s tee l sug-ges t s tha t bo i le r s oper a t ing a t 8 .375M P a ( 12 0 0 p s i) a n d a b o v e m a y b e m o r ep r o n e t o a t t a c k b y m o l t e n h y d r o x i d e s .T h e r e m a y b e s o m e m i x t u re s o f h y -d r o x i d e s t h a t m a y b e m o l t e n a t l o w e rt e m p e r a t u r e s , b u t m a n y c o n s t i t u e n t sa l so could r educe the cor r os iv i ty orr a i s e t h e m e l t i n g t e m p e r a t u r e .Conclusions andrecommendationsB a s e d o n t h i s s t u d y , w e c a m e t o t h efollowing conclusions: T y p e 3 0 4L is ra p i d l y c o r ro d e d b ym o l t e n h y d r o x i d e m i x t u r e s . H ig h n ickel , low chr omium a l loys

a r e t h e b e s t r e p l a c e m e n t s f o r T y p e304L s ta in les s s tee l . A l loys 825 and625 b imeta ll i c tub ing and A l loys 600a n d 6 2 5 w e l d o v e r l a y s h a v e b e e nd e m o n s t r a t e d t o r e s i s t c o r ro s i o n .

A l t e r n a t e m a t e r ia l s s h o u l d b e c o n -s ide r ed on a ll un i t s oper a t ing abov e8.375 M Pa (1200 psi). [ ]

Literature cited1. Odelstam, T., Tran, H. N., Barham, D., e t ~ ,TA PPI 1987 Engineering C onference Pro-ceedings, TA PPI PRE SS, Atlanta , p . 585.2 . Barna, J . L. and Rogan, J . B. , TA PP I 1986E n g i n e e r i n g C o n f e r e n c e P r o c e e d i n g s ,TA PPI PRE SS, A t lan ta , p . 1 29 6.3 . Barna, J . L . , Rogan, J . B. , Ma tt ie , R. J . , e tal . , TAPP11989 Kra ft Recovery Boiler Op-e ra t io n s Semin a r No te s , TAPPI PRESS,Atla nta, p. 1350.4 . McGu rn , J . F . , TAPP I 1 9 8 8 En g in ee r in gCo n fe ren ce Pro ceed in g s , TA PPI PRE SS,Atla nta, p. 147.5. M etals Handbook, V ol. 13, Corrosion inth e Pu lp an d P ap e r In d u s t ry (A . Ga rn e r ,ed .) , ASM In te rn a t io n a l , Me ta l s Pa rk , OH,1987, pp. 11 87-1220.6 . Wensley , D. A. , TA PP I 1986 Kra ft Recov-e r y B o i l e r O p e r a t i o n s S e m i n a r N o t e s ,TA PPI P RES S, At lan ta , p . 2 3 1.7 . Tran , H. N, Barham , D. , and Hupa, M. ,C O R R O S I O N / 8 8 P r o c e e d i n g s , N A C E ,Houston, P ape r 437, 1988.8 . Tran , H. N. , Barbara , D. , and Hupa, M. ,Ma teria ls Perform ance 27(7): 40-45(1988).9 . Tran , H. N. , Barham, D. , and Hupa, M. ,TAPP1 1 98 9 Kra f t Reco v e ry Bo i le r Op e ra -t io n s S e m i n a r N o t e s, T A P P I P R E S S , A t -lanta, p. 283.Received for review July 1,1992.Acc epted No v. 16, 1992.Presented at the TAP PI 1992 Engineering Con-ference.

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