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A C O M P R E H E N S I V E S T U D Y O N N A P H T H E N I C A C I D C O R R O S I O N

R . D . K a n e a n d M . S . C a y a r d

Inter orr In ternat ional , Inc .

1 4 5 03 B a m m e l N o r t h H o u s t o n R o a d , S u i te 3 0 0

H o u s t o n , T e x a s U S A 7 7 0 1 4

A B S T R A C T

Thi s s t udy inc luded s t i r red au toc l ave t e s t s t o b roaden the unders t and ing o f t he i n t e rac t i on

be tween su l f i de f i lm s and naph then ic ac id s , and Ho t O i l F low Loop (HOF L) exper im en t s t ha t

p rov ided in fo rm a t ion on t he i n f luence o f f l ow on the co r ros ion p rocess . A se r i e s o f pa ram et r i c

t e s t s were conduc t ed i n an u l t ra l ow su l fu r , hyd ro t rea t ed base o i l w i th add i t i ons o f h igh pu r i t y

reagen t g rade naph then ic ac id . Naph then ic ac id co r ros ion was p roduced a t TAN l eve l s o f 1 .5 m g

K O H / g a n d h i g h e r . T h e l i m i t i n g T A N w a s d e p e n d e n t o n m a g n i t u d e o f t h e m e c h a n i c a l f o r c e s

p roduced by t he f l owing o i l ( i . e . h ighe r l im i t i ng TAN va lues a ssoc i a t ed wi th l ower f l ow

ve loc i t i e s ) . The re l a t i ve naph then ic ac id co r ros ion re s i s t ance o f 5Cr-0 .5Mo (5Cr) and 9Cr- lMo

(9Cr) s t ee l s were on ly m arg ina l l y d i f fe ren t . The s tud i es a l so showed tha t naph then ic ac id

co r ros ion cou ld be success fu l l y i nh ib i t ed on two m a te r i a l s (5Cr and 9Cr s t ee l s ) by t he p resence

o f m odera t e l eve l s o f H2S . However , t h i s phenom enon was l im i t ed by re s i s t ance o f t he m a te r i a l

t o ve loc i t y acce l e ra t ed su l f i d i c co r ros ion a t h ighe r l eve l s o f H2S and wa l l shea r s t re ss . S e l ec t i ve

s tud i es o f t he i n f luence o f o the r su l fu r spec i e s on naph then ic ac id co r ros ion were conduc t ed . I t

was found tha t success fu l i nh ib i t i on o f naph then ic ac id co r ros ion was on ly ach i eved when

in t e rm ed ia t e l eve l s o f H2S were p resen t i n t he o f f -gas e i t he r f rom d i rec t H2S load ing o r f rom

the rm al d ecom pos i t i on o f the su l fu r spec ie s .

K e y w o r d s : N a p h t h e n i c a c i d , p e t r o l e u m , r e f i n i n g , T A N , T o t a l A c i d N u m b e r , h y d r o g e n s u l f i d e ,

inhib i t ion , research , tes t

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

P red ic t i ng c rude o i l co r ros iv i t y i s o f c r i t i ca l im por t ance and a pa r t i cu l a r ly d i f f i cu l t a spec t o f

re f ine ry ope ra t i ons . Corros iv i t y i s one o f m any fac to rs t ha t a re o f t en i nco rpo ra t ed i n to t he

pu rchase p r i ce fo r t he re f ine ry feed s tock a s we l l a s a necessa ry i npu t when assess ing t he

se rv i ceab i l i t y o f re f i ne ry p rocess equ ipm en t . A t the s t a r t o f th i s p rog ram in t he m id -199 0 s , t he re

were o bv ious l im i t a t i ons i n:

• t he unders t and ing o f t he i n t e rac ti ons be twee n co r ros ive spec ie s com m only found in

re f ine ry c rude o i l feeds tocks and p rocess s t ream s ,

1

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Copyright

2002 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACE International,

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.

02555CORROSION2002

Paper No.



• t he unde r s t a nd i ng o f t he i n t e ra c t i on be t we e n c he m i c a l a nd me c h a n i c a l e f fe c t s i n f l owi ng

h i gh t e mpe r a t u r e o i l sy s t e ms , a nd

• t he a va i l a b i li t y o f a nd c on f i de nc e i n p r ove n c o r r o s i v i ty p r e d i c t i on too l s

Suc h knowl e dge i s i mpor t a n t whe n c o r r o s i ve s pe c i e s a r e c ombi ne d w i t h h i gh ve l oc i t y f l ow

e nc oun t e r e d i n c e r t a in r e f i ne r y ope r a t ions . A ma j o r j o i n t i ndus t r y p r og r a m ( J IP ) i nvo l v i ng ove r

20 companies was organized by In te rCorr In te rna t iona l , Inc . to address th i s subjec t . Whereas the

o r i g i na l ly p r opo s e d p r og r a m c onc e p t wa s to u t i li z e s yn t he t ic e nv i r onm e n t s ( i. e. h i gh pu r i t y

ba s e o i l s , r e a ge n t g r a de a c i d s a nd s u l f u r c ompounds ) , t he e mpha s i s o f t he p r og r a m wa s s h i f t e d

t owa r d r e al e nv i r onm e n t s ba s e d on s pons o r i npu t. T he p l a n wa s c ha nge d t o i nc o r po r a t e r e a d i l y

a va i l ab l e l ow s u l f u r hyd r oc a r b on f r a c ti on p r oduc e d in a r e f i ne ry to wh i c h s e l e c te d dos a ge s o f

naphthenic ac ids were to be added . I t was fur ther agreed tha t the naphthenic ac ids used in th i s

s t udy wou l d be de r i ve d f r om c r ude o i l s a nd ob t a i ne d f r om c omme r c i a l s upp l i e r s . Howe ve r , a s

t he p r og r a m p r og r e s s e d , t h i s a pp r oa c h s oon c ompl i c a t e d t he a na l y s i s o f t he da t a ma i n l y due t o

in te r fe rence re la ted to impur i t i es in the base o i l and in the c rude der ived naphthenic ac ids .

However , th i s e f for t d id lead to the in i t i a t ion of severa l focussed s tud ies on naphthenic ac id

charac te r iza t ions (bo i l ing poin t , r ing s t ruc tures ) , d i f fe ren t ia t ion of cor ros iv i ty of naphthenic

a c id s , a nd p r o t e c t i ve be ha v i o r o f s u l f ide f i l ms t ha t ma y no t ha ve be e n pu r s ue d o t he r wi se .

T he f i na l po r t i on o f t he p r og r a m f oc us s e d on u s i ng me t hods c ons i s t e n t w i t h t he o r i g i na l l y

p r opos e d s c ope wh i c h u t i l i z e d a ve r y l ow s u l f u r hyd r o t r e a t e d ba s e o i l a nd r e a ge n t g r a de

na ph t he n i c a c i d t o p r oduc e t e s t e nv i r onme n t s wh i c h ha d u l t r a - l ow l e ve l s o f s u l f u r i mpur i t i e s

(<0 .1 wt .%) . This approach was success fu l in a l lowing d i f fe ren t ia t ion in the cor ros ion behavior

ba s e d on ve l oc i t y , a c i d s t r e ng t h ( T AN) a nd s u l f u r s pe c i e s f o r s e ve r a l c ommon l y u s e d ma t e r i a l s

inc luding: 5Cr , 9Cr and 12Cr s tee l s . This paper descr ibes the resu l t s o f se lec ted s tud ies of th i s

p r og r a m, wh i c h a r e o f pa r t i c u l a r imp or t a nc e a nd r e l e va nc e t o r e f ine r y ope r a t i ons .

B A C K G R O U N D N A P H T H E N I C A C I D C O R R O S I O N

Na ph t he n i c a c i d s a r e p r e s e n t i n ma ny c r ude o i l s i n va r y i ng a moun t s a nd p r ov i de a c l a s s o f

c o r r o s i on - i nduc i ng c on t a mi na n t s . Ar e a s e s pe c i a l l y no t e d f o r p r oduc i ng c r ude o i l s c on t a i n i ng

e l e va t e d c onc e n t r a t i ons o f na ph t he n i c a c i d ha ve h i s t o r i c a l l y i nc l ude d Ca l i f o r n i a , Ve ne z ue l a ,

Ind ia , China , and Russ ia . More recent ly , these a reas have a l so inc luded some reg ions no t

h i s t o r i c a l l y no t e d f o r ha v i ng h i gh l e ve l s o f na ph t he n i c a c i d wh i c h i nc l ude t he Nor t h Se a , We s t

Afr ica , Mexico and of fshore Braz i l .

At the s ta r t o f th i s p rogram, l i t t l e quant i ta t ive in format ion was known regard ing , var ia t ions in

molecula r weight , bo i l ing poin t and r ing s t ruc ture and the i r re la t ionsh ip to cor ros iv i ty in

pe t r o l e um r e f i n i ng s y s t e ms . As w i t h mos t o r ga n i c mo l e c u l e s , t he s e va r i a b l e s c a n i n f l ue nc e bo t h

the i r f rac t iona t ion charac te r i s t i cs and chemica l reac t iv i ty . S impl i s t ica l ly , the naphthenic ac id

conten t in o i l s i s assessed in te rms of to ta l ac id number (TAN) which i s re fe r red to in te rms of

mi l l i g r a ms o f KO H n e e de d t o ne u t r a l iz e a g r a m o f o i l s a mp l e . Va l ue s o f T A N i n the r a nge o f 0. 1

t o 3 . 5 [ T AN un i t s u s e d he r e i n : mg KOH/ g ] a r e c ommon bu t pa r t i c u l a r l y s e ve r e s i t ua t i ons ha ve

be e n f ound f o r pa r t i c u l a r hyd r oc a r bon f r a c t i ons t ha t c a n ha ve T AN va l ue s i n e xc e s s o f 10 . T he

c om pl e x i t y o f t he s i t ua ti on i s e v i de n t c ons i de ri ng t ha t o r ga n i c a c i d c omp one n t s i n c r ude o i ls c a n

va r y c ons i de r a b l y w i t h t he i r s ou r c e . Fu r t he r mor e , t he s e a c i d i c c ompone n t s c a n i nc l ude bo t h

2

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naph then ic ac ids ( t hose typ i ca l ly r e f e r r ed to a s hav ing two o r m ore cyc lopen tane t i ngs ) and

lower m olecu la r w e igh t s t r a igh t cha in o rgan ic ac ids fu rthe r com pl i ca t ing the p i c tu re .

Naph then ic ac id co r ros ion i s d i f f e r en t i a t ed f rom su l f id i c co r ros ion by the co r ros ion m echan i sm

and the m orp ho lo gy o f t he a t t ack . Su l f id ic co r ros ion i s u sua l ly desc r ibed a s be ing a gene ra l m ass

loss or wastage of the exposed sur face wi th the format ion of a sul f ide corros ion sca le . By

com pa r i son , naph then ic ac id co r ros ion i s t yp i ca lly cha rac t e r i zed a s hav ing m ore loca l i zed a t t ack

pa r t i cu la r ly a t a r eas o f h igh ve loc i ty and , in som e cases , whe re condensa t ion o f concen t r a t ed ac id

vapors can occur in crude dis t i l la t ion uni t s . The a t tack i s of tent imes descr ibed as lacking sur face

cor ros ion p roduc t s t he reby expos ing f r e sh m e ta l . Consequen t ly , dam age i s i n t he fo rm o f

unexpe c ted ly h igh co r ros ion r a t e s and loca l ized a t t ack on a ll oys t ha t wo u ld no rm a l ly be expec t ed

to res is t su l f id ic cor ros ion (par t icular ly s tee ls wi th more than 9 Cr) . In some cases , even very

h igh ly a l l oyed m a te r i a l s ( i . e . , 12Cr , 316SS and 317SS , and in som e r a r e ca ses even 6M o

sta in less a l loys) have been repor ted to exhibi t sens i t iv i ty to cor ros ion.

As a ru le-of - thumb, crude oi l s wi th a TAN grea ter than 0 .3 to 0 .5 and ref ined crude oi l f rac t ions

wi th a TA N h ighe r t han 1 .5 have been gene ra l ly cons ide red to be po ten t ia l l y co r ros ive due to t he

p resence o f naph then ic ac id . [1 -4 ] The d i f f e r ence in t hese two va lues com es f rom the

concen t r a t i on o f naph then ic ac id in ce r ta in hydroca rbon f r ac tions du r ing r e f in ing . O ne p rob lem

is t ha t such s im ple ru l e s do no t a lways ind ica t e wh ich hydroca rbon f r ac t ions and in wha t

locat ions in the process the concent ra t ion of the corros ive ac ids wi l l occur . Areas of work in th is

p rogram , bu t no t r epor t ed he re in , were t he cha rac t e r i za t ion and d i f f e r en t i a t i on o f naph then ic

ac ids based on

• the i r boi l ing point range

• TA N ve r sus t r ee bo i l i ng po in t,

• Sul fur speci f ica t ion , and

• C h e m i c a l st ru c tu r e u s i ng F a s t A t o m B o m b a r d m e n t - M a s s S p e c t r o s co p y ( F A B - M S )

These t echn iques showed bo th the spec i f i c t ypes o f ac ids p re sen t , and how these ac ids m ay

concent ra te dur ing crude oi l d is t i l la t ion . This type of informat ion can potent ia l ly resul t in a be t te r

unde r s t and ing o f naph then ic ac id co r ros iv i ty and he lp l oca t e po ten t i a l p rob lem a reas i n t he

refinery.

A m a jo r con t rove r sy in t he p red ic t ion and con t ro l o f naph then ic ac id co r ros ion m en t ioned a t t he

s tar t of th is program was tha t , under cer ta in condi t ions , the presence of sul fur -conta in ing

com pounds i s be l i eved to i nh ib i t naph then ic ac id co r ros ion . Th i s unde r s t and ing has com e

pr im ar i ly f rom expe r i en t i a l i n fo rm a t ion ob ta ined f rom :

• ref in ing high TA N crudes tha t a re a lso h igh in sul fur species , and

• b l end ing o f h igh TA N c rudes wi th l ow TA N c rudes o f va ry ing su lfu r con ten t.

However , the par t icular forms of sul fur tha t can par t ic ipa te in th is process and the threshold

am oun t s have no t been de f ined . The m echan i sm by which th i s e f f ec t can be unde r s tood m ay be

the one tha t uni f ies the unders tanding of c rude oi l cor ros iv i ty . I t involves a rea l iza t ion tha t both

sul fur and ac id species are present to a varying degree in a l l c rude o i l s and f rac t ions . In cer ta in

l im i t ed am oun t s , su l fu r com pounds m ay p rov ide a l im i t ed degree o f p ro t ec t ion f rom cor ros ion

wi th t he fo rm a t ion o f pseudo-pass iv i ty su l f ide f ilm s on the m e ta l su r faces . Ho wev er , i nc reases i n

3




e i ther reac t ive su lfur spec ies or naphthenic ac ids to leve ls beyond the ir th reshold l imits fo r

var ious a l loys may acce le ra te corros ion . The presen t s tudy has looked in to these ques t ions and

provides som e va luable an d prac t ica l ins igh ts .

There is a lso growing ev idence to ind ica te tha t bo th su lf id ic and naphthenic ac id corros ion can

be acce le ra ted by ve loc i ty of the f lowing process environment or by loca l tu rbulence . The wal l

shear s t ress p roduced by the f lowing media contr ibu tes an added mechanica l means to remove

the normal ly pro tec t ive su lf ide f i lms . The wal l shear s t ress is p roport iona l to ve loc i ty bu t a lso

takes in to account the phys ica l p roper t ies o f the f lowing media . These proper t ies inc lude dens i ty

and v iscos i ty of mediu m or media ) which is , in tu rn , af fec ted by the degree of vaporiza t ion and

tempera ture . This p rogram p laced a s t rong emphas is on quant i fy ing the mechanica l fo rces

produced by the f low in te rms of wal l shear s t ress , which can ac t on the surface of opera t ing

e q u ip me n t .

I N F L U E N C E O F N A P H T H E N I C A C I D O N S U L F ID E F I L M S

A ser ies o f te s ts were conduc ted for exposure dura t ions of i to 30 days . Al l te s ts were perform ed

a t 345 C 650 F). These tes ts inc luded the exposures wi th the base o i l wi thou t addi t ion of HzS or

naphthenic ac id . The da ta f rom these tes ts were compared to opera t ing exper ience wi th

equipm ent in se rv ice in the re f inery process ing the Escra vos-C abina E-C) b lend . Tes ts wi th HzS

were a lso conduced and the corros ion versus exposure t ime eva lua ted to assess the ra te o f su l f ide

f i lm format ion and i ts in f luence on corros ion ra tes . S t i r red au toc lave tes ts were conduc ted with

addi t ions of low, medium and h igh bo i l ing po in t naphthenic ac ids . However , a spec ia l s tudy was

conduc ted us ing au toc lave tes ts whereby coupons were pre -su lf ided in a h igh HzS l o w T A N

environment [ i .e . TAN 0 .7 and HzS load ing of 0 .45 ps ia )]. Corros io n ra tes on pre -su lf ided

corros ion coupons were compared with those on bare coupons . Addi t iona l ly , p re -su lf ided

c o u p o n s we re re -e x p o s e d to a n e n v i ro n me n t h a v in g h ig h TAN a n d lo w

HzS.

i .e . TAN 7 and no

HzS load ing) . Su lf ide f i lm th ickness was measured us ing a meta l lographic technique on coupons

both before and a f te r exposure to th is h igh TAN environment .

Cou pons o f 0Cr, 5Cr , 12Cr and 18Cr-8Ni s tee ls were t rea ted by pre -expo sure to a h igh H2S ] oil

environment wi th no added naphthenic ac id for 30 days . This exposure was in tended to produce

a su lf ide corros ion sca le on the surface of the coupons . Contro l spec imens were used to

de te rmine the su lf ide f i lm th ickness and to es tab l ish the s ta r t ing coupon weights for the second

p a r t o f th e t e s t s e q u e n c e in v o lv in g e x p o s u re to th e h ig h TAN e n v i ro n me n t w i th o u t HzS

addi t ions . The pre -su lf ided 12Cr and 18Cr-8Ni coupons exhib i ted much h igher mass loss

corros ion ra tes than d id bare coupons of the same mate r ia ls in the h igh TAN environment . The

coupons made f rom 0Cr and 5 Cr s tee ls a lso showed th is behavior , bu t to a lesse r degree , and

with grea te r sca t te r , than with the h igher a l loy mate r ia ls . This beh avior sugges ted tha t the su lf ide

f i lm a ffec ted the corros ion behavior o f the var ious s tee ls in h igh TAN environment . These

resu l ts p rompted a more de ta i led focus s tudy on the behavior o f su l f ide f i lms in h igh TAN

environments .

M e ta l lo g ra p h ic m e a s u re me n ts o f s u l fid e f ilm th ic k n e ss b e fo re a n d a f t e r e x p o s u re to a h ig h TA N

environment revea led tha t naphthenic ac id had the ab i l i ty to chemica l ly d is so lve su lf ide f i lms on

the meta l surface for a l l a l loys exam ined . See Table 1 .

4




T A B L E 1

S u l f i d e F i l m T h i c k n e s s M e a s u r e m e n t s B e f o r e a n d

A f t e r E x p o s u r e t o H i g h T A N / L o w H 2 S E n v i r o n m e n t 0 .0 0 1 i n . = 0 . 02 5 m m )

Film Thickness a f ter

Coupon Pre-sul fided F i lm

M ater i a l T hi ckn ess in ches)

OCr steel A51 6-70) 0 . 0 0 3 3

5Cr steel

0 . 0 0 5 4

12Cr steel 0 . 0 0 2 7

18Cr steel

0 . 0 0 3 5

High TAN~Low H2S Exposure

in ch es/percent chan ge)

0 . 0 0 1 7 / - 4 8 . 5

0 . 0 0 2 1 / - 6 1 . 1

0 .0011 / -59 .3

0 . 0 0 0 9 / - 7 4 . 3

T h e su l f id e f i lm th i c k n e s s w a s m e a su re d fo r 0 C r , 5 C r , 1 2 C r a n d 1 8 C r -8 N i s t e e l c o u p o n s a f t e r a n

in i t i a l e x p o su re i n a h ig h ly su l f id i z in g e n v i ro n m e n t a n d a g a in a f t e r su b se q u e n t e x p o su re t o a n

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

m e t a l l o g r a p h i c s e c t i o n i n g o f c o r r o s io n c o u p o n s . T h i c k n e s s m e a s u r e m e n t s w e r e m a d e o n f o u r

d i f f e r e n t su r f a c e s fo r e a c h c o u p o n , t h e n a v e ra g e d fo r e a c h m a te r i a l .

T h e r e d u c t i o n i n f i l m t h i c k n e s s r e s u l t i n g f r o m t h e h i g h T A N e x p o s u r e a f t e r 3 d a y s w a s i n t h e

ra n g e o f 5 8 to 7 5 p e rc e n t w i th i n c re a s in g p e rc e n ta g e r e d u c t io n s fo r t h e h ig h e r C r a l l o y s . T h e se

d a ta w e re c o n s i s t e n t w i th t h e t r e n d s o b se rv e d in t h e c o r ro s io n e x p e r im e n t s o n p re - su l f id e d a n d

b a re c o u p o n s a l so e x a m in e d in th i s s tu d y . T h i s s e r i e s o f t e s t s id e n t i f i e d c o n c lu s iv e ly t h a t

n a p h th e n ic a c id s c o u ld c h e m ic a l d i s so lv e su l f id e f i lm s o n m e ta l su r f a c e s . I t a l so p ro v id e d

e v i d e n c e o f o n e p o t e n t ia l m e c h a n i s m o f n a p h t h e n i c a c i d c o r r o s io n , w h e r e b y i t c a n c h e m i c a l l y

d i s so lv e n o rm a l ly p ro t e c t iv e su l f id e f i lm s o n th e su r f a c e o f m e ta l s t h u s a l l o w in g n a p h th e n ic a c id

to a t t a c k th e m a te r i a l su b s t r a t e . T h e se r e su l t s a l so i n d i c a t e d th a t l i t t l e o r n o v e lo c i ty o r a g i t a t i o n

w a s n e e d e d to o b ta in t h i s e f f e c t s in c e i t w a s b a s i c a l ly a c h e m ic a l d i s so lu t io n p ro c e s s . H o w e v e r ,

s p e c u l a ti o n w a s t h a t th i s co r r o s i o n m e c h a n i s m c o u l d a l s o b e w o r s e n e d b y c o u p l i n g t h is c h e m i c a l

a t t ac k m e c h a n i s m w i t h m e c h a n i c a l e f f e c ts i n f l o w i n g s y s t e m s .

E N G I N E E R I N G D A T A D E V E L O P M E N T W I T H 5 A N D 9 C R S T E E L S

A se r i e s o f h o t o i l f l o w lo o p (H O F L ) t e s t s w a s in i t i a t e d ( t e s t sy s t e m sh o w n in F ig u re 1 . I t w a s

b a s e d o n t e s t in g i n s y n t h et i c e n v i r o n m e n t s c o m p o s e d o f T u f f l o 1 2 0 0 w i t h a d d i t io n s o f r e a g e n t

g r a d e K o d a k / F l u k a n a p h t h e n i c a c id . B o t h o f t h es e c o n s t i tu e n t s w e r e c h a r a c t e r i z e d a s h a v i n g

u l t r a - lo w l e v e l s o f su l fu r sp e c i e s p r e se n t i n t h e m . T h e se t e s t s h a d th e fo l lo w in g b o u n d a ry

c o n d i t i o n s :

• B a se O il T u ff lo 12 00 ...... ~

~: ~ ~ ~ i I ~ i i ~ ~ i ~ i ~ I i i ~ i ~ i ~ i ~ ~ [ ]

• N ap h th e ni c A c id - K o d a k / F lu k a A~ ,/~iiiil,)i:i i ~ i i ~ d | |

• T A N - 0 to 3 .5 m g K O H /1 0 0 g m . . . . ~,,,:::~i:i:ii::~i~ii~i~iiiiiii,,:~,,i~ii~i~t/iilN |

• H S

l o a d i n g s - 0 , 0 .2 a n d 0 .4 5 p s ia ::::::::::::::::::::::

T e m p e r a t u r e 3 4 3 C ( 6 5 0 F ) ~ c ~ . . . . . . . . . . . . . . . . . ~ - ~ i ~' ; ~ , ,. . . . . . . ~ :~ ;~

V e lo c i ty - 16 to 97 mp s (50 to 350 fps) - . . . . . - =~ ~i"i :~:~~~~%':~ii~

• Ta rg et co up on s - 5C r an d9 C r steels i~::i i iiiii] ii[iii ii i

F ig u re 1 - P h o to o f H o t O i l T e s t L o o p .

5




The t e s t p l an w as t o ob t a i n co r r os i on da t a on t hes e m a t e r i a l s a s a f unc t i on o f f l ow ve l oc i t y and

TA N i n an e f f o r t t o de f i ne t he r eg i on w her e naph t hen i c ac i d co r r os i on d i d and d i d no t occu r .

I n i ti a l ly t hes e t e st s d i d no t con t a i n any H 2S load i ng . Th i s app r oac h a l l ow ed t he co r r os i on

behav i o r o f 5Cr and 9Cr s t ee l s t o be obs e r ved i n a s ys t em w i t hou t t he com pl i ca t i ng i n f l uence o f

s u l f u r s pec i es . O nc e t he bas e l i ne H 2S- f ree te s ts w er e com pl e t e add i t i ona l H O FL t e st s w er e

pe r f o r m ed w i t h a va r i e ty o f H 2S l oad i ngs a t va r i ab l e H 2S pa r ti a l p r e s s u re s . The r e s u l ts o f the

H O FL t e s t s conduc t ed i n t he u l t r a - l ow s u l f u r bas e o i l and naph t hen i c ac i d s how ed cons i s t en t

da t a tr ends w i t h w e l l - de f i ned t h r e s ho l ds f o r the ons e t o f i m p i nge m e n t a t t ack re l a t i ve to TA N

l eve l s and f l ow ve l oc i t y . See F i gu r e s 2 and 3 .

3 5 0

3

2 5 0

~ 2 0 0

~ 1 5 0

~ ' 1 0 0

5 0

I m p i n g e m e n t C o r ro s io n o n C r S t e e l

N o A t t ack R eg i o n o f I m p i n g e m e n t A t ta c k

. . . . . , . . . . . . . . . . .

0 0 . 5 1 1 . 5 2 2 . 5 3 3 . 5 4

TAN mg KOH/g)

Fi gur e 2 - I m p i ngem en t a t t ack f o r 5Cr i n H O FL t e s t s a t 650 F

as a f unc t i on o f ve l oc i t y and T A N w i t h i n f l uence o f H 2S . N o t e : 100 f ps - 30 . 5 m ps

Im p in g em en t Co r r o s io n o n 9Cr S tee l

3 0 0 - - - - - - ~ o - A t t a c k

2 5 0

~ , 2 0 0

Q

v

15o

¢J

i

:~ 1

5 0

• N o A t t a c k

• A t t a c k

O 0 . 2 p s i a H 2 S - i n h i b i t i o n

[ ] 0 . 4 5 p s i a H 2 S - i n h i b i t i o n

~ k ~ R e e j io n -o H m l ~ in g e m e e e k ~

0 ......... .......... . ; . . . . . . ~ . . .. . . .. . .. . : ......................... I . . . . . . . . . I .......... .......... ', .............................

0 0 .5 1 1 .5 2 2 .5 3 3 .5 4

TAN mg KO l - I / g )

Fi gur e 3 - I m p i ngem en t a t t ack f o r 9Cr i n H O FL t e s t s a t 650 F

as a f unc t i on o f ve l oc i t y and T A N w i t h i n f l uence o f H 2S . N o t e : 100 f ps - 30 . 5 m ps

6




The r e su l t s o f t he HOFL t e s t s fo r 5Cr and 9Cr s t ee l s a r e shown in t he i nd i ca t ed f igu res . Bo th

m a te r i a l s showed a s im i l a r t r end o f i nc reas ing im pingem en t a t t ack r a t e wi th i nc reas ing ve loc i ty

and TA N. Tw o we l l -de f ined r eg ions i .e . im p in gem en t a t tack and no im ping em en t a t tack ) were

iden t i f i ed de f in ing the naph then ic ac id co r ros ion behav io r o f t hese m a te r i a l s . Be low spec i f i c

l im it s o f f l ow ve loc i ty and TA N no naph then ic ac id im pinge m en t a t tack was obse rved .

C o m p a r i s o n o f 5 a n d 9 C r S t e e ls The nap h then ic ac id co r ros ion behav io r o f the 5C r s tee l and

tha t o f t he 9C r s t ee l were s im i l a r i n na tu re . Bo th exh ib i t ed the onse t o f im p ingem en t a t tack a t

abou t a TA N va lue o f 1 .5 o r h ighe r wh ich depend ed on the f low ve loc i ty in t he HO FL appa ra tus .

Loca l i zed im pinge m en t a t t ack was obse rve d to occur a t TA N l eve ls o f abou t 1 .5 and g reat e r. Fo r

h igh ve loc it i es [64 m ps >200 fps ) ], t he onse t o f im pin gem en t a t t ack occur r ed a t TA N 1 .5 .

However , a s t he TAN leve l s i n t he HOFL t e s t s were i nc reased , t he t h r e sho ld f l ow ve loc i ty fo r

a t t ack was r educed . At TA N 3 .5 , t he lowes t ve loc i ty em ploy ed in these t e st s 50 fps ) r e su l ted in

im pingem en t a t t ack . The behav io r o f t he 9Cr s t ee l i n t he HOFL t e s t s was ve ry s im i l a r t o t ha t

no ted fo r t he 5Cr s tee l. O n ly a s l i gh t i nc rease i n re s i s tance to im pinge m en t a t t ack was obse rved

a t TA N 3 .5 and a f l ow ve loc i ty o f 16 m ps 50 fps ) was no ted . The re fo re, i nc reas ing the

chrom ium con ten t o f t he s t ee l f rom 5 to 9 wt . pe rcen t d id no t appea r t o have bene f i t s i n t e rm s o f

res is tance to naphthenic ac id corros ion.

L o w e r T A N L i m i t s f o r N a p h t h e n i c A c i d C o r r o s i o n The lower l im i t fo r naph then ic ac id

cor ros ion in t he HOFL t e s t s was a t a TAN va lue o f abou t 1 .5 . Th i s f i nd ing was in c lose

agreem en t wi th r e f ine ry expe r i ence fo r t he onse t o f naph then ic ac id co r ros ion o f s t ee l s wi th

in terme dia te a l loy content . In ref inery vacuu m dis t i lla t ion , genera l guidel ines are tha t

naph then ic ac id co r ros ion i s a conce rn when the TAN of t he hydroca rbon f r ac t ion i s 1 .5 o r

g rea t e r . These HOFL t e s t s p rov ided good co r r e l a t i on wi th p l an t expe r i ence . However , t he da t a

a lso sugges t tha t th is threshold may be var iable depending on the f low condi t ions . I t i s poss ib le

tha t the resul t s of H O FL tes ts cond ucted a t TA N 1.5 at ve loc i t ies grea ter than 200 fps e .g .

h ighe r wa l l shea r s t re s s ) cou ld r e su l t in im p ingem en t a t tack o f 5C r and 9Cr s tee ls .

F igure 4 shows the r e l a t i onsh ip be tween f low ra t e and wa l l shea r s t r e s s fo r t he HOFL appa ra tus

us ing the Tuff lo 1200 base o i l . I t indica tes tha t in the range of 200 to 300 fps , the maximum wal l

shea r s t r e s s deve loped by the HOFL was in t he r ange o f 600 to 1200 Pa . To a sce r t a in t he

re l a t i onsh ip o f t h i s cond i t i on to r e f ine ry cond i t i ons , f l ow-m ode l ing ana lyses wou ld have to be

conduc ted on the spec i f ic f l ow cond i t i ons p re sen t i n t he p lan t . These w i l l va ry wi th t he geom et ry

of the sys tems, physica l proper t ies of the o i l and percentages of the phases present , p lant

th roughpu t , and f low reg im e . I t sho u ld be cau t ioned tha t d i r ec t use o f the r e su lt s presen ted in

Figure s 2 and 3 on the bas is of l ine ar f lo w ve loci ty wi l l not usua l ly provide accurate predic t ion

o f corros ion behavior

W al l shea r s tr e ss i s u sua lly a m ore r e l evan t pa ram e te r wh en a t t em pt ing to

predic t the inf luence o f f lu id f low par t icular ly mu l t iphase f low) on the corros ion behavior .

7




; . ~ o o . o

._ ,: 2000.0

i m l . . . . .

~ . :

. , , ,

.~ 1500,0

. i . - , .

: ¢ 1 ' ]

= i l ooo~o :

: ¢ 1 ~

0:.0

o

~r~=:2 ~

~: :;--::r/r° 0 ~ ~

~ r / t o = , 3 : 5

, . . ~ . . : : : : : |

. : ~ . ~ .

50 100 150:: 20 0 250 300 :350: ,400

4 . 5 0

F low Ve loc i ty : ' ( fp$ )

Figure 4 - W a l l shea r st re s s ve rsus HO FL f low ve loc i ty wi th Tuf f io 1200 o i l. [ 5]

Note :100 fps - 30 .5 m ps

I n f l u e n c e o f H 2 S. A d d i ti o n s o f

H2S

t o a t e s t cond i t i on tha t p roduced seve re naph then ic ac id

im pingem en t a t t ack (TAN 3 .5 and 200 fps ) s ign i f i can t ly i n f luenced the co r ros ion behav io r o f

bo th 5C r and 9Cr s t ee l s. Low l eve ls o f H2S load ing in the sys t em w ere found to i nh ib i t

im pingem en t a t t ack . However , h ighe r l eve l s o f H2S load ing p rov ided e i the r i nh ib i t i on o r no

inhibi t ion depending on the res is tance of the mater ia l to sul f id ic cor ros ion. See Figures 2 and 3 .

For 5Cr s tee l , addi t ion of 0 .2 ps ia H2S to the tes t envi ronment resul ted in a comple te inhibi t ion

of im pingem en t a t t ack on th i s m a ter i a l. H oweve r , w hen a h ighe r l eve l o f H2S was im posed (0 .45

psia H2S) , loca l ized, f low-induced corros ion was in i t ia ted . I t was suspected tha t th is onse t of

loca l i zed im pingem en t a t t ack a t t he h ighe r H2S load ing was r e l a t ed to ve loc i ty acce l e r a t ed

su l f id i c co r ros ion . Th i s phenom enon cou ld occur f rom the com bined in f luence o f t he m echan ica l

forces of the f lu id f low wi th the normal sul f id ic cor ros ion process .

Tes t s were pe r fo rm ed to eva lua t e t he abovem er i ti oned theory a t a cond i ti on tha t d id no t p roduce

naph then ic ac id co r ros ion . Th i s cond i t i on cons i s t ed o f t he Tuf f lo 1200 o i l w i th no naph then ic

ac id add i t i on and a HOFL f low ve loc i ty o f 200 fps . Tes t s were conduc ted on 5Cr s t ee l t a rge t

coupons both wi th and wi thout a h igh H2S loading (0 .45 ps ia H2S) . The tes t wi thout H2S (and

wi th no naph then ic ac id add i t i on ) d id no t p roduce im pingem en t a t t ack . However , a t t h i s s am e

cond i t i on wi th an H2S load ing o f 0 .45 ps i a , l oca l i zed im pingem en t co r ros ion was obse rved . Due

to the lack of naphthenic ac id in these tes t envi ronments , the only chemical species wi th

substant ia l potent ia l for cor ros iv i ty was the sul fur or ig ina t ing f rom the H2S loading. Therefore , i t

appea r s t ha t f low induced wa l l shea r s t r es s can sepa ra t e ly i n f luence the co r ros ion by naph then ic

ac id and by sul fur species as wel l .

For compar ison, another ser ies of tes ts wi th var iable

H2S

l oad ing was conduc ted , t h i s t im e wi th

9Cr s t ee l t a rge t coupons a t TAN 3 .5 and 200 fps . I n t he ca se wi thou t

HzS

ex tens ive naph then ic

ac id co rros ion w as obse rved . Th i s was s im i l a r t o tha t obse rved fo r t he 5Cr s t ee l. How ever , when

te s t s were conduc ted wi th 0 .2 ps i a

HzS

and 0 .45 ps ia

HzS no

i m p i n g e m e n t a t t a ck w a s o b s e r v e d

to occur . In these cases , both the lower and higher levels of

HzS were

successful in inhibi t ing

naph then ic ac id im pingem en t a t t ack on th i s m a te r i a l . The d i f f e r ences i n t he co r ros ion behav io r

be tween the 5Cr and 9Cr s t ee l t a rge t coupons in t he env i ronm en t wi th 0 .45 ps i a

HzS

is l ikely the

resul t of the h igher sul f id ic cor ros ion res is tance of the 9Cr s tee l . This mater ia l mainta ined i t s

co r ros ion r e s i s t ance even unde r t he worse case exam ined in t h i s s tudy con ta in ing bo th h igh

8




l eve l s o f n ap h th en ic ac id an d H 2S a t 200 fp s in th e H O FL tes t. W h i l e th e ac tu a l corros ion

b e h a v i o r o f t h e s e m a t e r i a ls i n s e r v i c e m a y v a r y t h e b e h a v i o r o f t h e 9 C r s te e l c o u l d b e e x p e c t e d

to b e b e t ter th an th a t o f th e 5Cr s tee l w i th resp ec t to n ap h th en ic ac id corros ion res i s tan ce . Th i s

wou ld b e on th e b as i s th a t i t migh t h ave a b road er ran ge o f p ro tec t ion from reac t ive su l fu r

sp ec i es su ch as H 2S in en v iron men ts con ta in in g n ap h th en ic ac id .

C o r r o s i o n M e c h a n i s m s . V i s u a l e x a m i n a ti o n o f 5 C r a nd 9C r s t ee l ta r ge t s p e c im e n s i n d ic a t ed

th at th e mech an i sm o f n ap h th en ic ac id corros ion was re la ted to l o ss o f th e p ro tec t ive su l f id e f i lm

on th e meta l su r face . Th i s l o ss o f p ro tec t ion cou ld ar i se from e i th er ch emica l d i sso lu t ion or

m e c h a n i c a l a c t io n o f t h e f l o w i n g f lu i d o r s o m e c o m b i n a t i o n o f t h e s e p h e n o m e n a . S e e F i g u r e s 5

th rou gh 7 .

- . ~ ~ : : ~ - : ~ : ~ ~:~..".:~..~:.~...i~' ~ .... ::.::~I~V.....

~:ii~ :~i?:il i:iS iii::ii))i~:,' , i : ) ~ t ~ ~

~:: i~i::::i~i~: ~i;::i:i~;.'-~i: " i~,i':~ ~i:::::~' ~ j . ~ :~;~ '~i~:~:~ :~

i i : : ~ , ~ : : : : i : ~ : ~ p : . - ~ : ~ i:-.~ ,.. .. . ~:" .~i:~:.:~z :.::~:~,~,~,:' '~ ii~i~i~. . . . .

. . . . . . . ~ . . . . . . ~ . . . . . ~ "~ ? i :~: " '

~ . ~ : M : ~ ; i : ~ . ~;~ z~';.~i:~-:~C~;~-":ii:::~

.::, izSi-'.:i ::::Z::ii~.~:i:i:iz) ::zi:i)::5~:~:~)iii i:~:?5::z'z~::i~U :::.

F i g u r e 5 - L o c a l c h e m i c a l d i s s o lu t i o n

o f s u l fi d e f il m b y h i g h T A N

e n v i r o n m e n t .

. ~ : : ~ :: . :~ . i . :~ ~ : ; ;~ : ~ " ~ : : . ~ : : ~ i - " ~ : . ; : : , : : : . ~ ~ ( ~ - ~ . ~ : . : ; ~ ~ ; . : . ~ . ' . t : , i : . . . . . . . . : - ~ . : - . ~ ~ . ~ " ~ , i . : :: ~ : ~ . : , . ~ ~ . : ~ ; M . . " , : - ": , : : : - - ~ " . ~ i ? ~ ~ . ~ . ~ : ' : . ~ : : : : : : ~ . ~ / : : :~ .

' g : : ~ ~ i ~ : : . ~ ~ : . ~ : : ~ . " , ~ , : ~ : ~ . . . . . ~ : . ~ ; ~ , : ~ : . ~ : ~ . . ; - ~ :. . : .~ : ~ . . : :~ ; . . : . ~ . ~ : : ~ - ~ . . ~ .. : , ,: ~ : ~ - ~ , ~ . , . : - : . . : ~ , ~ : . ~ : : . ~ ~ ~ . L ~ i i . ~ • ~ ~ : . : . : i: . e : : ' : . . ~

~ : ~ i ~ ' ~ ?. ~ . . . ~ .~ ? . ~ . ~: ~ ; ; . ~. . . Z . ~ . ~ . c ~ : ` i .. . ~ . ~ .. ~ . ~ ? ~. . ~ : ~ :~ . . ` ~ t ~ ' . ~ .. ` . ~ . .~ . . . . ~. i ~ ` ~ :~ . ~ . ` ~:~ ....~::.:;:~.-~."::..:o.::~i~:.s*.::~'-.: .:"::~. .~::..,

~ ~ % ... i~ ~:~;~i.. ~::i. ,~,~:~.:-:.~~.::L?:~5~:.~i.:~,5 ~.V~?:~iii..:i~~'.?'~'::~:::.~..U~ z::"~:":.~.....~::::i~,:-:-:i::i;i-:?:::: . S :::,i> :..-::<.. , , ~ :

F i g u r e 6 - M e c h a n i c a l d a m a g e o f

s u l f id i c f i l m b y h i g h v e l o c i t y j e t

i m p i n g e m e n t .

9




Fi gure 7a - L oca l i zed impingement

a t tack p roduced by combina t i on o f

mechanica l and chemical a t tack of

sulf ide f i lm: target coupon.

: . . . ~ - * ~ . . . . ~ o : , ~ . ~ , ~ : : : : : . ~ . ~ : ~ : : : :: : : :: : : :: : : :: : : :: ~ ; . : . . . . . . ~

. . ~ : . : , . , . , . ~ . . . . . . . . . .. . . . : . . : : : ~ : . : ~ : ~ . : : : , : ~ : ~ . : : : : : : ~ . : ~ : ~ . : ~ , ~ - ~ , ~ . ~ . . . . . . . . .. . . : , : ~ : : ~

: . . : : . . . .. ~ . . . i; - • . ~ ~ : : : ~ : ~ : . ~ . . . : ; , ~ : ~ i ~ : ~ : ~ ~ , ~ : ~ . ~ : ~ :~ : ~ . & , ~ : ~ •

~ : 9 : : : .~.~i~?;~i~.:~i~::~::~::~ ~i~~.~ i~i:.;:...~::::~:. :::~~ : ~

i ~ i l ; : i l l ~ : : :4 ~ : ~ :~ , ~ . .~ : . . . . . . . . . . . .

Fi gure 7b - L oca l i zed impi ngement

a t tack p roduced by combina t i on o f

mechanica l and chemical a t tack of

sulf ide f i lm: target coupon at higher

magni f ica t ion.

As di scussed previous ly , the sul f ide f i lming s tudy revealed tha t naphthenic ac ids could

chemical ly di ssolve i ron sul f ide f i lms on the sur face of a l l s tee l s ranging in chromium content

f rom 0 to 18 percent . These losses were subs tant ia l and the percentage loss of sca le weight

increased wi th increas ing chromium content of the mater ia l . The data in tha t s tudy were

genera ted in autoclave test s involving low veloci ty envi ronm ents <3 fps) where mechanica l ,

f low- induced forces were minimal . Therefore , chemical d i ssolut ion of the i ron sul f ide f i lms was

the l ikely cause of this at tack.

Visual examinat ion of the ta rget coupons f rom thi s por t ion of the program also showed evidence

of chemical d i ssolut ion of the sul f ide f i lm. At h igh TAN and lower f low veloci ty , there was

visual evidence of loca l ized pi t t ing and per fora t ion of the sul f ide f i lm around the edges of the

impingement a t tack. The local loss of th i s protec t ive sul f ide f i lm resul ted in regions of the

subs t ra te mater ia l be ing exposed to the naphthenic ac id fur ther resul t ing in in i t i a t ion of

impingement a t tack. By compar i son, t es t s conducted a t lower TAN and higher f low ra tes ,

show ed visual evidence of mechanica l f rac ture of the sul f ide f i lms on the sur face of the materia l.

The cracks produced in the sul f ide f i lm by the f lowing media produced spal l ing of the sul f ide

f i lm tha t resul ted in areas of the subs t ra te mater ia l be ing exposed to the cor ros ive media . These

locat ions w ere wh ere the imp ingem ent a t tack in it ia ted .

10




I N F L U E N C E O F H S A N D S U L F U R S P E C IE S O N N A P H T H E N I C A C I D C O R R O S I O N

T h e f i n a l e x p e r i m e n t a l e f f o r t o f t h e p r o g r a m i n v o l v e d a n e x a m i n a t i o n o f t h e e f f e c t o f H 2 S a n d

se lec ted su l fu r spec ies on the cor ros iv i ty o f ho t o i l under f lowing condi t ions on 9Cr s tee l . A

s e r ie s o f H O F L t e s ts w a s c h o s e n t h a t in v e s t i g a t e d t h e f o ll o w i n g :

• O n e h i g h e r H 2S lo a d i n g ( f r o m t h o s e u s e d in S e c t io n 1 0 . 1 ) a n d it s e f f e c t o n i m p i n g e m e n t

c o r r o s i o n i n 9 C r s t e e l

• Tw o l iqu id su l fu r spec ies tha t were separa te ly adde d to the Tuf f io 1200 o il ins tead of H2S

on th i s mate r ia l .

T h e r e s u l t s o f t h e s e t e st s w e r e c o m p a r e d w i t h t h e r e su l t s f r o m t h o s e i n v o l v i n g e i t h e r n o H 2S o r a

lower H2S load ing to be t te r de f ine the ro le o f su l fu r spec ies on inh ib i t ion of naphthen ic ac id

c o r r o s io n . T h e s e t e s t s h a d t h e f o l lo w i n g b o u n d a r y c o n d i t io n s :

• Base Oi l - Tuf f lo 1200

• N a p h t h e n i c A c i d - K o d a k / F l u k a

• T A N - 3 .5

• S u l f u r l o a d i n g s - ( S e e b e l o w )

• T e m p e r a t u r e - 3 43 C ( 65 0 F )

• Ve loc i ty - 65 mp s (200 fps )

• T a r g e t c o u p o n s - 9 C r s t ee l

T h e t e s t p l a n i n v o l v e d o b t a i n i n g c o r r o s i o n d a t a o n 9 C r s t ee l t a rg e t s p e c i m e n s a t a c o n d i t i o n t h a t

h a s b e e n s h o w n t o p r o v i d e s u b s t a n t ia l s u s c e p t i b i li t y t o n a p h t h e n ic a c i d i m p i n g e m e n t a t t a c k. T h e

base l ine tes t fo r th i s s tudy were a l ready run and repor ted in th i s sec t ion . They cons i s ted o f t e s t s

i n i t i a l l y c o n d u c t e d a t T A N 3 . 5 a n d 6 5 m p s ( 2 0 0 f p s ) w i t h n o H 2 S , t h e n a g a i n a t t h e s a m e

condi t ion , bu t th i s t ime wi th 0 .2 and 0 .45 ps ia H2S loaded to the rese rvo i r o f the HOFL. As

d e s c r i b e d p r e v i o u s l y i n t h is r e p o r t , s u c c e s s f u l i n h ib i t io n o f n a p h t h e n i c a c i d c o r r o s i o n o f 9 C r s t e e l

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

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

was used as a measure o f the i r the rmal s tab i l i ty and po ten t ia l to fo rm H2S a t the t es t t empera ture .

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

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

b e l o w :

• 1 ,9 - Non aned i th io l (NE T) - 95 ; HS(CH 2)9SH; fo rm ula we igh t - 192 .39 ; bo i l ing po in t

- 284 C (543 F) . 200 ppm and 1000 ppm.

• D i b e n z o t h i o p h e n e ( D B T ) - 9 9 + ; C ~ 2H 8 S; f o r m u l a w e i g h t - 1 8 4 .1 8 ; b o i l i n g p o i n t - 3 3 3

C ( 6 3 1 F ) . 20 p p m a n d 2 0 0 p p m .

A t t h e t e s t t e m p e r a t u r e o f 3 4 3 C ( 6 5 0 F ) , i t w a s e x p e c t e d t h a t t h e c o m p o u n d w i t h t h e l o w e r

b o i l i n g p o i n t ( N E T ) w o u l d b e l i k e l y t o p r o d u c e m o r e H 2 S t h a n w o u l d t h e m o r e t h e r m a l l y s t a b l e

DBT. I t was a l so suspec ted tha t , due to the h igh reac t iv i ty o f H2S, the re might be a poss ib le t rend

w i t h H 2 S i n t h e o f f - g a s t h a t w o u l d d o m i n a t e n a p h t h e n i c c o r r o s i o n a n d i t s i n h i b i t i o n i n t h e s e

e n v i r o n m e n t s . H o w e v e r , a d i f f e r e n t t r e n d w a s a l s o l i k e l y t o b e o b s e r v e d w h e r e b y t h e c o r r o s i v i t y

and inh ib i t ion does no t fo l low the H2S leve l in the o f f -gas . In th i s even t , a more complex

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

l iqu id phase en te r s ign i f ican t ly in the cor ros ion process .

11




The da ta f rom the 9Cr ta rge t coupons in the tes ts involv ing var ious H2S loadings a re shown in

Figure 2 . They ind ica te tha t impingement a t tack was produced in the tes t where no H2S was

added , and s ign if ican t reduc t ions in the a t tack ra te wi th addi t ions of 0 .2 and 0 .45 ps ia H2S to the

tes t environment . Afte r the conc lus ion of these tes ts , another tes t was conduc ted a t a s t i l l h igher

H2S loading in an a t tempt to de te rmine the l imi t to the range of condi t ions where HeS offe rs

pro tec t ion f rom na phthenic ac id corros ion . With a st i ll h igher H2S par t ia l p ressure addi t ion 1 .8

ps ia ) , the imp ingem ent a t tack ra te on 9C r s tee l inc reased dram at ica l ly to leve ls in excess of those

tha t involved no H2S loading . .

V elo c i ty A cce lera ted A t ta ck

Th e im p in g e me n t a t t a c k res u l tin g f ro m n a p h th e n ic a c id c o r ro s io n

was v isua l ly d is t inc t ive f rom tha t p roduced by su lf id ic a t tack . The a rea of impingement produced

by naphthenic ac id a t tack was f ree of su l f ide corros ion produc ts . The a rea of ve loc i ty acce le ra ted

su lf id ic corros ion c onta ined a th in , du l l g ray surface f ilm. V isua l observa t ion s m ade in these tes ts

ind ica ted tha t the corros ion in the test wi th on ly naphthenic ac id addi t ion without H2S) involved

both loca l d is so lu t ion and/or mechanica l damage of the su lf ide f i lm in the a rea of impingement .

This le f t the corroded a rea under the impingement nozz le essen t ia l ly f ree of corros ion produc ts .

The two tes ts wi th in te rmedia te leve ls o f H2S revea led dynamic f i lm format ion charac te r ized by

the presence of p ro tec t ive su lf ide f i lms . These f i lms were charac te r ized as cons is t ing of loca l

shee ts o f th in su lf ide sca le tha t appeared to in te rfe re wi th the normal naphthenic ac id corros ion

process the reby produc ing an inh ib i tive e ffec t on imping em ent see F igures 8a and b) . F ina l ly , a t

the h ighes t leve l o f H2S loading , the impingement a t tack reappeared , bu t i t was d i f fe ren t ia ted

from tha t in the tes t wi th no H2S addi t ion by havin g a th in gray f i lm in the a t tack a rea . This la t te r

type of corros ion appeared to be a formed by ve loc i ty acce le ra ted su lf id ic a t tack as descr ibed

prev ious ly . I t involved removal o f the pro tec t ive su lf ide f i lm on the coupon surface in the

impingement zone and reac t ion of the bare mate r ia l in th is loca t ion with the environment

conta in ing a h igh H2S concentra t ion . This p roduced an impingement zone with du l l g ray

colora t ion .

i ~ i ~ i ~ i i ~ i i i ~ i i i : ; . . ~ ; ~ k ~ i ~ i i ~ i i ~ i i I i i ~ i ~ ; ~ k i : i ~ i ~ i i J : ~ ' . . . k ~ i ~ ~ i ~ i . ~ i ~ i i ~ i i ~ i ~ . ` . , ~ } . . . r . . : : ~ i ~ i ~ i i ~ i i ~ i i i i : : i i i i i : : : : i : : i i : : i i : : ~ : : i : : : : i ~ : : i i : : i ~ : : ~ i ~ : : : : ~ i : : i : : : : : : i i i : : i : : i ~ i : : i : : i i ~ i i : : i i : : : : i i i i i : : i : : i i

: : i i ~ i ~ i i i i i ~ i i i ~ i ~ i ~ i i i ~ i i ~ 3 i ~ : : ~ i i : : i : : ~ i : : g i i ~ : : i i ~ i i ~ i ~ i ~ i ~ ~ : i i i i ~ i i : : i i ~ : : i ~ i : : i : : i ~ i : : i : : ~ i i ~ : : i : : ~ i i i ~ : : i ~ : : i ~ i : : i : : i ~ i i i ~ i : : ~ : : i : : ~ : : ~ : : i : : ~ : : ~ : : i ~ i : : ~ : : ~ : : ~ : : ~ : : ~ : : i ~ i : : i : : i ~ i : : i : : i i : : ~ : : i ~ i : : ~ i : : i ~ i : : i :

: : : : : : : : : : : : : : : : : : : : : :' .. ~ . ~ 5 ~ : : ~ i i k : : : ~ i : : :: i I ~ i : : i ~ i i ~ ~ i g ~ i i ~ i i I j j i ~ i ~ : : i ~ : : ~ ~ ~ i ~ i ~ i : : i : :i i ~ i i ~ i i i: . ~ : : . : . . . :~ i~ :: i i~ i : : i i ik i i~ i i~ . . . { i i~ i : : : : :: : : : :~ i : : i~ i~ i~ i: : i : :i~ : :~ i~ i~ ~ i~ i~ i ~ i i~ i ii~ i i : : i i~ i i : :~ : : ~ i~ : :~ :~ : . i : : i : : : : i : :~ : : i : :~ : : ~ : :~ : : i : :g :~ ~ ::~ ::~ ::~ ::~ :## :~ ::~ :~ :~ :~ :~ :~ :~ :~

: : i : : ii i i ii i i i i i 0 i ii i i f l ® i / i ii i i i i { i @ i i i i i i ® i i i i I i i ii i i i i i i i i ii i i ii { {{ { i ii i

Figure 8a - Con di t ion of 9Cr s tee l

ta rge t spec imen exposed to naphthenic

corros ion condi t ions wi th in ten t iona l

H2S loading before c lean ing) .

12




))))6)):)))))+)i)+i)))+))))):):i:))~);))+)~++))~+)))ij)))):(+)))~))))))))~+))~i))~))))~ )))))):~))~:))~):)::~)j):))~))))))jl)I++)))+:@ ))~))))+)):+))):::::);)~::))+:)):))+):)+))i:)I)l)~)+~®~giP~))) ))))~+)+))j)))))~)l{)

: ~ : ' . - . . . . . . . > - ~ .. " : : ?~ : . r 4 ~ : ' ) : . ' : : : : : - ' . ' . ~,'.~ ,. '.".i : : ~ : > . : g + . ' . , ' - ~' . ' .. < . : : : : ~ - ~ i ~ . > . : > : ~: , i : : . . ' . ' 1 6 N ' 6 : ~ : : I ~ . " ~ N ~ * . > . ~ . ~ : I :~ . ~ : : . ~)i~-i . ' . * ~ i > . ~ i : : > . ' : ; 6 : ~ ; : ~ : : i * i : i .~ i * . ~ ' . . 6 ~ . ' :. ' : ' , . ' ~ : ~ ( ~ . : ~ : i . '. : ' . < : ~ * : :; : : : -) . . ' ' ~ ' , < ~ : ; > . ~ . ~ , : :~* : : ' ) ~ * ' ) i : ~ : " . t & ~ : : : ~ g ~ : : ~ 5 ~ s : ~ : : : : ' . : ~ . ' . - ' .~ :->~.':*::.~ :. '-. '.:::.~: : : ~ . : , : : " { ~ : ~ : : : ~ : :

: : . > . : ~ . . : ~ . : .'~$,:::~::6 ::.>.: ?. ,~ .:.>-~:P.::~ :::): < .".. l:E : : ~ :~ :...( '>;:Y.):$>-:i~ .:::'~6 ~ ; ~ : ~ I : : Q . ~ t . . . 5 " ~ < . 5 2 , ~ i ~ 6 . ~ ) . 1 " . i ~ ) . , : : i 6 1 6 6 g f ; ; : ~ 6 ) ~ : );::'6.)..~.~?."6:111: : j : 6 : i)f.~:':..(.'~:~) i ~ ~ ~ ~ . ~ . > . : ) : : . ' . ~ . : ~ . J . ~ . . : : 6 . ': .:ii :.J: . " . ) i : : : ~ . :~,.'..)6)))~{)~i1~6f.i~)J))~ lE:..~::..~)~.~~))1{6.:~:~'~61~:

. . . . . . ~ . ~ . . ) " , " ) ~ ) . . . . . . ~ . . . . . . . . .. . . . ~ { ~ . . . . . . . , > ~ : . ~ > < ~ , .. ~ .~ : . ,+ . , .: : . . . . .. . ,, .) j :. :) )~ : ., .~ : ., :~ < ', ,. .) ,' .) ) , ~ :. ,, :, .~ . ,. :. ,.

-%:< ~f~ '.:*:: '::, '::.:.:.:.:,.:-. '-::.:.::~ .: :::::,,.>..:,~> ..~ ::,...................... ~ i``~:~>.>.:~::~:~ .>..>...~ .~ .>)~..`6.~ .~ .`:.~ .:~ .g:~::~:~>:~i~:~:~;:i~ ;;..iy:.,~ i. . . . . . . : ' . " . > . i : * . : 6 . . . . . . . ' i . . . . . . . ~ . . . . ~ . ~ . . . ~ . ' ~ . . . ~ ) : ` ' . : ~ ) [ g ~ ) ~ y 2 : ) t . : ~ j : ~ . . . . ~ . > . ~ : ` ~ . ~ 4 ~ . > . * ~ > : ~ . ` . . : s < * ~ : ~ ` : : * ` . . : : * ~ r . . : ; ~ : . > . . . > . . ~ t ~ : ~ ` ~ , ~ . > . ~ .. , .. . ,. . . .~ . . ., . g , .. .

. . ~ . . ~ : ~ : ~ { ~ . ` . ~ . ~ + ¢ . ~ : : : : ~ : ~ ) ~ ` > . : . . ` ? ~ . * . . ~ : ~ . ~ : ~ : : . ~ : 6 . : < ~ . . 2 . ) . . . ~ . ~ . ~ : ~ . : ~ : ~ : ~ + ~ . . ~ . + ~ ) : ~ : ~ . ~ : ~ : . ~ : : ~ : ~ ~ : ' . ' .: * ~ . . . . ~ . , . . ~ . . . . . : ~ . ' . . . . . ' . " , , ' , ' : . ' . . ~ : . " . . .~ : : * : ~ ¢ , . ~ , ' . ~ . < ~ . . . ` . ~ . ~ . ea . ~ . ~ a . ~ . . < . ~ : ~ ' : * ` . ~ : ~ ` ~ . < . < ' . ' : . > ` : I ~ . ` ` . ` ` ` ~ ` . . ~ . . ` . ` ~ ~<..>.'>..~ '~ .. '."

::::::::::::::::::::::::::::::::::::::::::::: • . : : ~ ] . : . ~ : . : . , . . . - . . . , - . : , : ~ > . . . ® , ¢ = = = = = = == = = = = = == = = = = = == = = = = = == = = = = = == = = = = = =

F i g u r e 8 b - S a m e s p e c i m e n s h o w n in

Figu re 8a (a f ter c l ean in g) . No

i m p i n g e m e n t a t t a c k - o n l y s l i g h t

s ta in in g at im p in g em en t s i te .

E f f e c t o f S u l f u r S p e c i e s . T h e c o m b i n e d r e s u l t s o f t h e H O F L t e s t s i n v o l v i n g H 2 S l o a d i n g s a n d

t h o s e f r o m t e s ts w i t h l i q u i d s u lf u r s p e c ie s ( N E T o r D B T ) s h o w e d c o n s i s t e n t r e s u lt s . T h e t e st s

w h e r e o n l y a l i m i t e d a m o u n t o f H 2 S w a s i n v o l v e d i n th e t es t ( e it h e r fr o m H 2 S l o a d i n g o r fr o m

th e l iq u id su l fu r sp ec i e s ad d ed ) , imp in gemen t a t tack was ob served . H owever , in th e te s ts wh ere

i n t er m e d i a t e l e v e l s o f H 2 S w e r e i n v o l v e d , i n h i b i ti o n o f i m p i n g e m e n t a t ta c k w a s o b s e r v e d . F o r

th e te s ts wh ere a su b stan t ia l amou n t o f H 2S was in vo lved , imp in gemen t a t tack reap p eared . S ee

Figu re 9 .

Th e d ata from b oth th e in i t i a l te s ts w i th H 2S load in g an d th ose in vo lv in g ad d i t ion s o f DBT an d

NET are p lo t ted versu s th e amou n t o f H 2S in th e o f f -gas a t th e en d o f th e te s t . I t can b e seen th a t

wh en a l l o f th e te s ts are tak en in to accou n t , th ey sh ow th e in h ib i t ive e f fec t on n ap h th en ic ac id

corros ion d escr ib ed p rev iou s ly for in termed ia te H 2S l eve l s . I t a l so sh ows th e in f lu en ce o f very

h igh l eve l s o f H 2S in th e b reak d own o f th i s p ro tec t ion an d th e in i t i a t ion o f ve loc i ty acce l era ted

sulf id ic attack.

: / n f l u e n o e o f H ZS o n N a p A o i d C o r r o s i o n

: T A N 3 . 5 , 2 0 O f p s , 6 5 0 :: F

•~. ,.~c .,~,.~

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

: l ~ + m ~ m : ~ m ~ _ iii)i)i)i:i)i)i)

. . . .

-~;:iiiiiiiiii ii~,--

~ ~ ~ ~ ~ ~ ~ ~

i : { i i { i

: ~: ~: ~: ~: ~: ~: ~: ~

::) ) ) ) i:))

................ ~):)::)):):::::i"

-:;:~ :~:~:~:;:~ :~ ~;~;~;;~;~;~

:~:~:~:~:~:~:~

. . . . . . . . . . . . . . .

6 6 ~ 6 ~

. . . . : : : ~ + : ~ i + : ~ : . . . . . .

i { i{ i{{i{ i l i { i{ - : i ~ i i i ~ i : i L. . . . . . . . . . . . . .

~ ~ : ,~{i:iiiiiii +iii jii :iiiiiiii :..

iiiii i i+ig ~ : :ii}iiiii+iiii~ i - : i i i i i i i i i i i i i i - -

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : : 2 : : : : : : : : : : :

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.................. ................

. . . . . . . . . . . : : : : : : . .

..............

• i)i{ {)}iliiii i :iiii)iiiiiiiiiiii: ;.~;ii:iiiii;i~ . . . . . . . . . . . . . . . .~ : : : : : : :

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

: : : i : i i i i i : i i i : i i i :

. . . . . . . . . . . . . . . . . . . . : . . .. . .. . .. . .. .

mi): ' :': ,,,,,,,,,,,,,,,

e :m ++ +::+:.,+,:

i oo : i ~ o 7 0 0 a n o l e o o + o n o ~ a o o : : ~ o ~ o =

H 2 S i n O f f . G i l ( l~ m l t} ~

Figu re 9 - G en era l an d

i m p i n g e m e n t c o r r o s i o n

rates on 9Cr steel target

sp ec imen s in te s ts w i th

a n d w i t h o u t H 2 S , D B T

a n d N E T . N o t e : 1 0 0 f p s

= 3 0 . 5 m p s

Th e in f lu en ce o f su l fu r sp ec i e s on n ap h th en ic ac id corros ion ap p ears re la ted to th e i r ab i l i ty to

d e c o m p o s e i n t o H 2S a t t h e e x p o s u r e t e m p e r at u re . H O F L t e s ts c o n d u c t e d w i t h i n t er m e d i a t e l e v e l s

o f H 2S from d irec t l oad in g o f H 2S gas resu l ted in in h ib i t ion o f imp in gemen t a t tack in th e H O FL

tests . Th e ran ge o f H 2S o f f gas con cen tra t ion s th a t p rod u ced th i s in h ib i t ion in th ese te s ts was

1 , 6 0 0 t o 1 4 , 0 0 0 p p m ( 0 . 0 2 t o 0 . 2 p s i a ) . T h e t e s t s p e r f o r m e d w i t h a d d i t i o n s o f D B T d i d n o t

13




prod uce h igh leve ls of H2S in the of f -gas [100 to 700 ppm 0 .0015 to 0 .01 ps ia ) ]. This was l ike ly

re la ted to i t s re la t ive ly h igh boi l ing poin t re la t ive to the tes t t empera ture . Consequent ly , h igh

i mp i n ge m e n t a t t a c k r a t es a bou t 150 t o 320 mpy) we r e obs e r ve d i n t he s e te s ts . T he t es t s

c ondu c t e d w i t h a dd i t ion o f NE T wi t h a l owe r bo i li ng po i n t t ha n DBT ) s ho we d a t r a n s it i on fr om

high a t tack ra te a t an H2S leve l o f 1 ,000 ppm 0 .01 ps ia ) in the of f -gas to very low co r ros ion ra te

a t an H2S of f -gas leve l o f 1 ,600 ppm 0 .02 ps ia ) .

These da ta sugges t tha t su l fur spec ies in the l iqu id phase tha t do not conver t to reasonable leve ls

of H2S may not d i rec t ly cont r ibu te to the inh ib i t ion of naphthenic ac id cor ros ion . H2S

concent ra t ion in the of f -gas a re shown as a func t ion of t es t dura t ion for two tes t s involv ing

a dd i t i on o f DBT a nd NE T i n F i gu r e 9 . I n bo t h t e s t s , t o t a l s u l f u r l e ve l s we r e s i mi l a r a nd ne a r l y

unc ha nge d a s a f unc t i on o f t i me . T he ma j o r d i f f e r e nc e i n t he c ompos i t i on o f t he t wo t e s t

env i ronm ents was in the conc ent ra t ion of H2S in the of f-gas . The con cent ra t ion o f of f -gas H2S

de c r e a s e d du r i ng t he t e s t w i t h NE T f r om a bou t 6 , 000 t o 1 , 000 ppm. By c omp a r i s on , t he t e st w i t h

DBT ha d a s i gn i f i c a n t l y l owe r H2S o f f - ga s c onc e n t r a t i on de c r e a s i ng f r om a bou t 1 , 000 t o 100

ppm dur ing the tes t dura t ion . Hence , the ava i lab i l i ty of H2S gas was not iceably lower in the tes t

c on t a i n i ng t he DBT t ha n w i t h t he l owe r bo i l i ng po i n t a nd mor e uns t a b l e NE T .

T he da t a p r e s e n t e d f o r 9Cr s t e e l i n t he s u l f u r s t udy c ombi ne d w i t h t he t e s t s w i t h a nd w i t hou t

in ten t iona l H2S loadings show three reg imes of o i l cor ros iv i ty :

1 . Na phthe nic ac id cor ros ion w here the ava i lab i l i ty to H2S i s l imi ted .

2 . Re du c t i on o r i nh i b i ti on ) o f na ph t he n i c a c i d c o r ro s i on be c a us e o f the p r e s e nc e o f

modera te leve ls of H2S e i ther evolved f rom the su l fur spec ies in the o i l upon hea t ing or

by d i rec t load ing of H2S.

3 . Ac ce le ra t ion of cor ros ion a t h igh H2S leve ls prod uc ing v e loc i ty acce le ra ted su l f id ic

cor ros ion .

T he da t a f r om t h i s s t udy a l s o i nd i c a t e t ha t t he r e i s a c on t i nuum be t we e n na ph t he n i c a c i d

c o r r o s i on a nd s u l f i d i c c o r r o s i on i n h i gh t e mpe r a t u r e f l owi ng o i l s y s t e ms . T he da t a f r om t h i s

p r og r a m i nd i c a t e t ha t t he s e t wo c o r r o s i on phe nome na a r e l i nke d by t he a b i l i t y t o t he ma t e r i a l t o

form a pro tec t ive su l f ide f i lm tha t can res i s t naphthenic ac id a t tack . However , i f the

concent ra t ion of the reac t ive su l fur spec ies i s too h igh , then su l f id ic cor ros ion can occur . These

is a l so some sugges t ion tha t the suscept ib i l i ty to ve loc i ty acce le ra ted su l f id ic cor ros ion may be

e nha nc e d by t he p r e s e nc e o f the na ph t he n i c a c i d in t he s y s t em. W he r e a s t he da t a i n th i s s t udy a r e

s ome wha t l i m i te d , t he y i nd i c a t e de f i n i t ive p r oo f t ha t t he r e is a l ink be t we e n na ph t he n i c a c i d a nd

sul f id ic cor ros ion . They a l so ind ica te tha t the resu l tan t behavior a t a par t icu la r t empera ture i s

p r oduc e d by t he a va i l a b i l i t y a nd i n t e r p l a y o f na ph t he n i c a c i d s a nd s u l f u r s pe c i e s c ombi ne d w i t h

me c h a n i c a l f o r c e s c r e a t e d by t he fl owi ng me d i um.

C O N C L U S I O N S

Ba s e d on t he r e s u lt s de s c r i be d he r e i n , t he fo l l owi ng c onc l u s i ons we r e m a de :

1 . M e t a l l og r a ph i c me a s u r e m e n t s o f su l f ide f il m t h i ckne s s be f o r e a nd a f t e r e xpos u r e t o a h i gh

T A N e nv i r onm e n t r e ve a l e d t ha t na ph t he n i c a c i d ha s t he a b i l it y to c he m i c a l l y d i s s o l ve s u l fi de

f i lms on the meta l sur face for a l l a l loys examined thus lead ing to increas ing cor ros ive a t tack .

2 . The resu l t s o f the H O FL tes t s cond uc ted in the u l t ra - low su l fur base o i l Tuf f lo 1200) and

r e a ge n t g r a de na ph t he n i c a c i d F l uke / Kod a k ) s ho we d c ons i s t e n t da t a tr e nds w i t h we ll -

de f i ne d t h re s ho l d s ve l oc i t y v s . T A N) f o r the ons e t o f i mp i nge m e n t a t t a ck .

14




3. The naphthen ic ac id corros ion behavio r o f the 5Cr s tee l and tha t o f the 9Cr s tee l were s im ila r

in na ture wi th respec t to TAN and f low ve loc i ty l imi ts . Both exhib i ted the onse t o f

impingement a t tack a t about a TAN va lue of 1 .5 or h igher , depending on the f low ve loc i ty in

the HOFL appara tus . Resu l ts fo r 12Cr s ta in less s tee l ind ica ted tha t th is mate r ia l was

subs tan t ia l ly more res is tan t to naphthenic ac id impingement a t tack than e i the r 5Cr or 9 Cr

steels.

4 . The low er l imi t fo r naphthenic ac id corros ion of 5Cr and 9Cr s tee l in the HO FL tes ts was

observed a t about TAN 1 .5 , which was in c lose agreement wi th re f inery exper ience for the

onse t o f naphthenic ac id corros ion of s tee ls wi th in te rmed ia te a l loy conten t .

5 . Add i t ions of H2S to a tes t condi t ion tha t p rodu ced severe naphthenic ac id impingem ent

a t tack (TAN 3 .5 and 200 fps ) s ign if ican t ly in f luenced the corros ion behavior o f bo th 5Cr and

9Cr s tee ls. Lo w leve ls o f H2S loading in the sys tem were found to inh ib it imp ingem ent

a t tack for bo th mate r ia ls . However , h igher leve ls o f H2S loading provided inh ib i t ion of

naphthenic ac id corros ion for 9Cr s tee l bu t acce le ra ted corros ion on 5 Cr s tee l.

6 . Visua l exam ina t ion of targe t spec imens ind ica ted tha t the m echa nism of naphthenic ac id

corros ion was re la ted to loss o f the pro tec t ive su lf ide f i lm on the meta l surface . This loss o f

pro tec t ion could a r ise f rom e i ther (a ) chemica l d is so lu t ion by naphthenic ac id or

(b) mechanica l ac t ion of the f lowing f lu id , o r some combina t ion of these phenomena .

7 . The da ta f rom the tes ts wi th and without H2S loading ind ica ted tha t the corros iv i ty of the ho t

o i l va r ied as a func t ion of the H2S off-gas in the HOFL. Loca l impingement a t tack ra tes were

grea tes t under condi t ions wi thout H2S loading ( i .e . naphthenic ac id corros ion) and where the

H2S loading exceed ed the res is tance of the mate r ia l fo r ve loc i ty acce le ra ted su lf id ic corros ion

8 . The co mb ined resu l ts o f the HO FL tes ts involv ing H2S loading and those f rom tests wi th

l iqu id su lfur spec ies (NET or DBT) showed cons is ten t resu l ts . Impingement a t tack was

observed in tes ts when l i t t le o r no H2S gas was evolved dur ing the tes t . When in te rmedia te

leve ls o f H2S were evolved , inh ib i t ion of impingement a t tack was observed . For the tes ts

where a subs tan t ia l amount of H2S was involved , impingement a t tack reappeared .

9 . These resu l ts show ed tha t the in f luence of su l fur spec ies on naphthen ic ac id corros ion was

re la ted to the i r ab i l i ty to decompose in to H2S a t the exposure tempera ture combined with the

res is tance of the m eta l to su lf id ic corros ion .

A C K N O W L E D G E M E N T

The au thors wish to thank the sponsors of th is p rogram tha t p rovided bo th f inanc ia l and technica l

ass is tance wi thout which th is technica l e ffor t would no t have been poss ib le . The l is t o f sponsors

inc ludes the fo l lowing companies :

Arc o P ro d u c t s C o .

Baker Pe tro l i te

B P A m o c o

C O M I M S A

Ch e v ro n Re s e a rc h & Te c h n o lo g y Co .

C O N O C O I nc .

Ec o p e t ro l

Eq u i lo n En te rp r is e s LLC

J a p a n En e rg y Co rp o ra t io n

J GC Co rp o ra t io n

K o c h P e t r o l e um G r o u p

M o b i l R e s e a rc h & D e v e l o p m e n t

Na lc o Ex x o n En e rg y Ch e mic a l s

P D V S A I n t e v e p

Pe t ro b ra s CENPES

Phi l l ips Pe tro leum

Sun Oil Co .

Su n c o r En e rg y

Te x a c o G .E .D .

Sy n c ru d e Ca n a d a L td .

Tota l Raff inage Dis t r ibu t ion

15




R E F E R E N E

1. R.L. Piehl , Co rrosion, 16 (1960): p.139.

2. J . Gu tzei t et . al ., in A SM H and boo k, Vol . 13, Corros ion, Materials Park, OH : ASM ). P. 1270

3. R.L. P iehl , Naph thenic Acid Corros ion in Crude Oi l Dis t i ll a t ion Uni t s , Corros ion/87, Paper

No. 196 (Houston, Texas, 1987).

4. R.D. Kane, M aterials Perform ance , July (1999): p. 48.

5 . R.D. Kane e t. a l. , Assessm ent of F low Induced Co rros ivi ty in Pet roleum Envi ronm ents ,

Proceedings E uroco rr '98, EF C, Ut recht , The N ether lands , Sept. 1998.

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