8/3/2019 metalhalide

1/6

JOU RNA L OF MATER IALS SC IENC E 26 (1991) 3693 3698

H i g h - t e m p e r a t u r em e t a l s a n d a l l o y sA r e v i e w

c h l o r i n e c o r r o s i o n o f

Y. - N . C H A N G , F. -I . W E ISteel and Alum inium R and D Department, China Steel Corporation, Hsiao K ang, Kaohsiung,Taiwan

High-temperature ( > 200~C) corrosion of m etals and alloys in atmospheres conta ining CI2an d/o r H CI, w hic h are w ide ly encountered in many industrial environments, is reviewed. Topicsinclude high-temperature corrosion mechanism and kinetics, therm odyn am ic considerations, andeffects of gaseous compo nen ts in the atmosphere (oxygen, a ir, wa ter vapour, sulph ur dioxide, andnitrogen) on the corrosion.

1. I n t r o d u c t i o nC h l o r i n e a n d h y d r o g e n c h l o r i d e g a s es e i t h e r a s r ea c t -a n t s , p r o d u c t s , b y - p r o d u c t s o r a s c o n t a m i n a n t s a r ew i d e l y e n c o u n t e r e d i n c h e m i c a l a n d m e t a l l u rg i c a l i n -dus t r i e s such a s coa l - f i r ed bo i l e r s [1 ] , was t e i nc ine r-a to r s I -2 , 3 ] , p l a s t i c /po ly mer d ecom pos i t i on m i l ls [ 4 ] ,v i n y l c h l o r i d e m o n o m e r m i ll s [ 5 - 9 ] , a n d r e c u p e r a t o r s[10 ] . Fo r example , r ega rd l e s s o f t he mo lec u l a r s t ruc -t u r e o f th e c h l o r i d e - c o n t a i n i n g c o m p o u n d s i n t h e fu e l,a t t h e f l a m e t e m p e r a t u r e i t b e c o m e s m o l e c u l a r C I >F u r t h e r m o r e , d u r i n g c o o l i n g ,C12 m a y a l s o b e c o n -ve r t ed t o HC1 by r eac t i ng w i th e i t he r hyd rogen o rwa te r v apou r. Tab l e I is a li st o f t he se env i ron me n t ss u m m a r i z e d b y E l l i o t te t a t . [11 ] . S t ruc tu ra l ma te r i a l si n s u c h e n v i r o n m e n t s o f t e n c o r r o d e a t a c c e l e r a t e dr a t e s a t h i g h t e m p e r a t u r e s b e c a u s e t h e c o r r o s i o np rod uc t s co ns i s t o f ch lo r ide s wh ich gene ra l l y havelower me l t i ng po in t s and bo i l i ng po in t s t han t he ox -ide s o f t he s ame me ta l s [12 ] . I n 1976 , a r ev i ew pape ro n h a l o g e n c o r r o s i o n o f m e t a l s w a s p u b l is h e d [ 1 2 ]w i t h e m p h a s i s o n t h e t h e r m o d y n a m i c s p r o p e r t i e s .S i n c e t h e n f u n d a m e n t a l s t u d i e s h a v e c o m p l e m e n t e dwo rk on me ta l s and a l l oys. Recen t ly, ano the r r ev i ew

o n c o r r o s i o n i n c h l o ri n e e n v i r o n m e n t s w a s p u b l i s h e d[ 1 3 ] w i t h e m p h a s i s o n p e r f o r m a n c e o f v a r i o u s m a t e r -i a ls a s we ll a s on ma te r i a l s l im i t a t i ons i n such env i ron -men t s . The p re sen t pape r d i s cus se s t he h igh - t emper-a t u r e c o r r o s i o n o f m e t a l s a n d a l lo y s in a t m o s p h e r e scon ta in ing C12 and /o r HC1 . Top ic s i nc lude h igh - t em-p e r a t u r e c o r r o s i o n m e c h a n i s m a n d k i n e t i c s , t h e r m o -dyn am ic cons ide ra t i ons , an d e ff ec ts o f ga seous co m-p o n e n t s i n t h e a t m o s p h e r e ( o x y g e n , a ir , w a t e r v a p o u r,s u l p h u r d i o x i d e , a n d n i t r o g e n ) o n t h e c o r r o s i o n . H i g ht e m p e r a t u r e h e r e m e a n s i n e x c e ss o f 2 0 0 ~ b e l o wwhich t he co r ro s ion r a t e s a r e u sua l l y no t app rec i ab l e .

2 . H i g h - t e m p e r a t u r e c o r r o s i o nm e c h a n i s m a n d k i n e ti c s

As d i scus sed p rev ious ly [14 ] , i f t he d i f fu s ion p roces s i nthe ox ide i s t he r a t e - con t ro l l i ng s t ep i n h igh - t emper-a t u r e o x i d a t i o n , t h e n t h e o x i d a t i o n r a t e o b e y s a p a r e -

0022-2461/91$03.00 + .12 9 1991 Chapman and Hall Ltd.

bol ic ra te law. In th is case , the ra te o f cha nge of thesca l e t h i cknes s i s p ropo r t i ona l t o t he s ca l e t h i cknes sitself, i.e.

d x / d t = K d / X (1 )

where x i s the sca le th ickness , t the t ime, and Kd iscons t an t c on t ro l l ed b y t he d i ffu s ion p roces s . The p a ra -bo l i c r a t e l aw r e su l t s f rom the pa rabo la so lu t i on o fEq ua t io n 1 . On the o the r han d , i f t he me ta l su r f ace o rthe phase bo un da r y i n t e r f ace r eac t i on i s t he r a t e -c o n t r o l l i n g s t e p , t h e n t h e o x i d a t i o n r a t e d o e s n o tchange wi th t ime, i . e .

d x / d t = K ; (2 )

whe re K 's i s a con s t an t con t ro l l ed by t he me ta l su r f aceo r t he phase i n t e r f ace r eac t i on , and t he ox ida t i onobeys a l i nea r r a t e l aw due t o t he l i nea r l i ne so lu t i on o fEq ua t io n 2 . I f t he d i f fu s ion o f i ons i n t he s ca le and t hevo la t i l iZa t i on ( and /o r vapo r i za t i on ) o f t he s ca l e occu rs i m u l t a n e o u s l y d u r i n g o x i d a t i o n , t h e n o x i d a t i o n i s t h es u p e r p o s i t i o n o f E q u a t i o n s 1 a n d 2 e x c e p t t h a t t h econs t an t K 's i n Eq ua t io n 2 is nega t ive wh ich me anstha t vo l a t i l i z a t i on r e su l ts i n t he r edu c t ion o f s ca le

th ickness , i . e . Equat ion 2 i s modif ied as

d x / d t = - K s (3 )

a n d t h e r a t e l a w e q u a t i o n i s

d x / d t = g d / x - - K s (4 )

T h i s i s t h e f a m o u s Te d m o n e q u a t i o n [ 1 5 ] .H o w e v e r, t hi s t y p e o f ra t e l a w p l a y s a n i m p o r t a n t

ro l e no t i n h igh - t empera tu re ox ida t i on , bu t i n h igh -t e m p e r a t u r e c h l o r i n e c o r r o s i o n ( a n d e v e n i n h a l o g e nco r ros ion ) due t o t he ea s i e r vo l a t i l i z a t i on o f mos tme ta l ch lo r ide s t ha n t he i r own" ox ides a s de sc r ibed

above . I n t h i s t ype o f co r ro s ion , t he s cal e men t ione dabov e i s t he ch lo r ide o f t he a s soc i a t ed me ta l . Thes o l u t i o n o f E q u a t i o n 4 is

t = - K d /K Z s [ x K s / K a +In (1 - x K s / K d ) ] (5 )

pro vid ed the in i t ia l co nd i t io n i s tha t x ---- 0 a t t = 0 .

3693

8/3/2019 metalhalide

2/6

T A B L E I H i g h - t e m p e r a t u re c h l o r in e e n v ir o n m e n t s s u m m a r i z e d b y E l li o ttet al. [11]

H i g h t e m p e r a t u r e a p p l i c a ti o n Te m p e r a t u r e r a n g e Ty p e o f c o r r o s io n(~

Miner a l ch lor ina t ion: prec ious meta ls , Ti and Zr product ion ,ch lor ina tors 300-900 Clz

High- tem pera ture ch lor ina t ion , reac tors < 550 CI 2Vinyl ch lor ide monomer product ion , e thylene d ich lor ide c racker

tubes < 650

Produ ct ion of e thylene d ich lor ide 280-480Ti tan ium oxide product ion , hea ter tubes in TiCI4 c i rcu i t 900Alum inium mel t ing , Mg removal by C12 in jec t ion < 850Fibreglass manufa c tur ing , recupera tors < 900Waste inc inera t ion , < 850Plant for H z f rom water < 900Pow er gene ration, boilers . . < 950

Flu id ized bed comb ustor, hea t exchange tubes , 850

In the in i t ia l per iod o f a t tack or i f the vo la t i l iza t ioneffect i s no t s igni f icant , the va lues of x and Ks aresma l l compared wi th Ko , i~e .xKs/Kd ~ 1, andl n ( 1 - x K J K d ) i n Equ a t ion 5 can be expan ded inpo w er series. Th e result is .=_

gt ~ xZ/2Ka (6) ~=

which approaches t he gene ra l s econd-o rde r pa rabo l i cr a t e l aw. In t he g row th pe r iod o f me ta l ch lo r ide andif the vola t i l iza t ion effec t i s s igni f icant enough , obvi-ous ly xKs/Kd~ 1 i s no lo nger va l id . The n the ra te o fchan ge of the chlor ide th icknes s obeys E qu at ion 5 , i .e .t he ch lo r ide p rodu ced f rom the ion d i f fu s ion vo la ti l -izes gradu al ly dur ing the corros ion 9 W he n the th ick-nes s app roaches t he va lue x f =Ka/K~,Equa t ion 4 be -c o m e s d x / d t = 0 and the ch lo r ide does no t g row fu r-ther. Therefore , Eq uat io n 5 i s avai lable only a t x < xf. _oWhen x exceeds xf, the vola t i l iza t ion a t chlor ide /a tmos phe re i n t e rf ace wil l becom e the r a t e - con t ro l l ings t ep and the l i nea r r a te l aw o f co r ros ion , Eq ua t ion 3 , issa t i s fied . Thus , the to ta l we ight of the m eta l decreasesg radu a l ly. F r om the d iscuss iOn above , one can expec ttha t t he t yp ica l we igh t change o f the me ta l w i th t ime i sl ike curve A in F ig . i , wh ere xf (ortf) , which wasdef ined abov e the cr i t ica l th ickness (or t ime) wh en thesca l e no longe r g rows , depends on the t empera tu re ,a tmosph e re , and the ch lo r ide cha rac t e r is t i c s. The t em-pera ture a lways promotes the vola t i l iza t ion effec t . I f

t he g row th o f ch lo r ide is no t f a s t enough to su pp ly theamoun t r equ i r ed fo r vo la t i l i za t ion , t he co r ros ion r a t elaw wi l l obe y Equa t ion 3 (curve B, Fig . 1). Fo r exmple , Feel 3the co r ros ion o f i ron in 1.46 vo l% HC1/N2 (1 a tm) a t FeCI2tempera tu re s be low 500~ obeyed a pa rabo l i c r a t e N iCI2law due to the form at ion of a protec t ive FeC12 f i lm on CoC12the me ta l surface [16] . As the tem per a ture increased CrC13

CrCI2to 550 ~ the rate law was l ike curv e A in Fig. 1. As i t HfCI4increased fur the r to 800 ~ the ra te law becam e l ike PdC12curve B. Simi lar k inet ics a lso resul ted for the car- ThCI4ros io n of i ron in 1 vol % C12/N2 (1 a tm) except tha t CeC13the t r ans i t i on t empera tu re s were d i f f e r en t . O the r i n -

ves t iga t ions [5 -9 , t l , 17 -281, u s ing the rmog rav im e t ryt echn iques , a lso ob ta ined Cor ros ion k ine ti c s t yp i ca l o fthe curves of Fig . 1 .

Usual ly the d i ffus ion of ions in chlor ide i s fas teno ug h only a t tempe ra ture s abo ve 0 .5Tr, , where Tm isthe me l t i ng po in t o f t he ch lo r ide . Fu r th e rmo re , vo l a t -

3 6 9 4

Hal ide gas

HCl, air, ethyleneC12 and oxidationFlue gases with CI=, HCI, sulphur, etc.F lue gases inc luding ha l ides and su lphidesFlue gase s including C12, and sulphur , etc.HC 1Com bust ion gases inc luding HC1, fue l ash ,

su lphate /ch lor ideH C I f r o m c o a l w i t h C a S 0 4 , C a O

. . ." . ' ' ' ' ' "

9 Parabolic9 r a t e l a w. . - ' "

Ti m e

, i

Figure 1 Typica l curves of weight change of meta l wi th t ime inhigh- tempera ture ch lor ine cor ros ion .

TA B L E I I C o m m o n b i n a r y m e ta l c h l o ri d e m e l t in g p o i n t s ( T= )and tempera tures (T4) a t which the ch lor ide vap our pressure i s10 -4 a tm

Chlo ride Tm (~ T4 (~ Reference

303 167 [12 ]676 536 [12]

1030 607 [12]740 587 [12]

1150 61~ [12]820 741 [12 ]434 132 [12]678 474 [12]770 564 [29]800 911 [29]

i l iza t ion is s ign i f ican t on ly a t vapo ur p re s su res above10 -4 a tm. T able I I l is t s the T= an d T4 (the temper-a tu re a t w h ich the vapou r p re s su re i s 10 -4 a tm) va lueso f so m e c o m m o n m e t a l c h lo r id e s s u m m a r i z ed p r e-vious ly [12, 29] . However, we bel ieve tha t T4 plays a

8/3/2019 metalhalide

3/6

more im por t a n t ro l e t han Tm in h igh - t empera tu rech lo r ine co r ros ion .

3 . T h e r m o d y n a m i c c o n s i d e r a t i o n sIn h igh - t empera tu re ox ida t ion , t he t empera tu re de -pendence o f t he f r ee ene rg i e s o f fo rma t ion o f ox ides(E l l i ngham d iag ram) p l ay an impor t an t ro l e because

one can predic t the s table oxides a t a par t icular tem-pe ra tu re . S imi l a r ly, in h igh - t empera tu re ch lo r ine co r-ros ion, s table chlor ides can a lso be predic ted f romthei r f ree energies of for ma t ion as d iscussed ear l ie r[12, 13 ] . However, i n a mix tu re o f more tha n one gascomponen t , t he s i t ua t ion i s more complex . Fo rexample , cons ide r ing pu re i ron in t he mix tu re o f hy -d rogen ch lo r ide and oxygen , the poss ib l e r eac t ions a r eas fo l lows.

n F e + m /2 O 2 = Fe,O, , (7)

Fe + 2HC1 = FeC12 +H2 (8)

FeC I2 + HC1 = FeCI3 + 1/2 H2 (9)Fe + 2H CI + 1 /2 O2 = FeC12 + H 20 (10)

FeCI2 + HC1 + 1/4 02 = FeC13 + 1/2 H 2 0 (11)

1/3 Fe 203 + 2HC1 = 2/3 FeC13 + H 2 0 (12)

FeCI2 + 3/4 O2 = 1/2 Fe 20 3 + CI2 (13)

2 /3 FeC13 + 1 /2 0 z = 1 /3 FezO3 +C12 (14)

F ig . 2 shows the t emp era tu re dependence o f t hes t anda rd f r ee ene rg ie s o f Reac t ions 7 -14 ca l cu l a tedf r o m th e t h e r m o d y n a m i c d a t a [ 2 9 ]. F r o m t h e d ia g r a m

one can expect tha t , in pu re H C1, FeC12 is form ed byReac t ion 8 , bu t fu r the r ch lo r ina t ion (Reac t ion 9 ) isthe rm ody nam ica l ly imposs ib l e . In con t r a s t , when thereac t ion gases con ta in bo th HC1 and 02 , t he oxy-

2 0

I 0

E

- ~ o

v

~ = - I 0o

-2 0

,s

&

IZ l

-3 0

-4 0

-5 0

-6 0

-7 05 0 0 4 0 0

I z t ~

11

I

500 60 0 700 800 900 1000Temperature (K)

Figure 2 Changes n standard free energies of Reactions 8-14 withtemperature calculated from [29], in wh ich sublimation, meltingand vaporization of reactants and products are not take n intoaccount.

ch lo r ina t ion l eads t yp ica l ly to t he fo rm a t ion o f FeC13by R eac t ion 11 a s we l l a s t he fo rm a t ion o f FeC12 byReac t ion 10 , a s demons t r a t ed by Iha rae t a l . [17].

S imi l ar ly, t he poss ib le r eac t ions o f ch rom ium in themix tu re o f HC1 and 0 2 a re a s fo l lows .

2Cr + 3 /2 02 = C r 2 0 3 (15)

Cr + 2HC1 = CrC1 2 + H2 (16)

CrC12 + HC1 = CrC1 3 + 1/2 H2 (17)

Cr + 2HC1 + 1/2 O2 = CrC12 + H 2 0 (18)

CrC12 + HC1 + 1/4 02 = CrC13 + 1/2 H 2 0 (19)

1/3 Cr2 03 + 2HC1 = 2/3 CrCI3 + H 2 0 (20)

CrC12 + 3/4 02 = 1/2 Cr 20 3 + CIz (21)

2/3 CrC1 3 + 1/2 02 = 1/3 Cr 20 3 + C12 (22)

G r e e n s u b l i m a t e c o r r o s i o n p r o d u c t C r C I 3 - 6 H 2 0af t e r t he co r ros ion o f ch rom ium in HC1/20-75 vo l %02 mix tu re a t 550 -800 ~ obse rved [9] migh t come

f rom the oxy-ch lo r ina t ion Reac t ion 19. Oxy-ch lo r ina -t i on phen om ena were a lso obse rved in o the r i nves tiga -tion s [-4, 5, 7, 8, 11, 18-23, 30 -3 3] .

4 . E f f e c t s o f g a s e o u s c o m p o n e n t s o n t h ecor ros ion

Because many indus t r i a l a tmosphe res may a l so con -t a in o the r f l ue gases in add i t i on to C12 an d / o r HC1 , i ti s necessary to v isual ize the effec ts of these gaseouscomponen t s on the ch lo r ine co r ros ion . The ro l e o fchlor ine i t se l f in f lue gas corros io n ha s been reviewed

recent ly [13] .

4 .1 . O x y g e n a n d a irAs descr ibed abov e, the react ion k inet ics of i ron in1 .46vo1% HC1/N2 (1 a tm) be low 500~ obeyeda parabol ic ra te law due to the protec t ive FeCI2 f i lm.I h a r a et a l . [17 ] a l so found tha t i n pu re HC1 gas a t300-800 ~ the co r ros ion behav iou r was de t e rminedby the fo rm a t ion and sub l ima t ion o f FeC12. Add i t i onof 20 -50 vo l % 02 to H CI g rea t ly acce l e ra ted theco r ros ion because o f t he fo rm a t ion and sub l ima t ion o f

a l ow me l t i ng po in t vo la t il e FeC13 v i a oxy-ch lo r ina -t ion such as React ion 11 descr ibed above. Simi larresul ts were a lso observed by the same inves t iga tors[7 ] on Fe -Ni a l l oys i n t he s ame gas mix tu re a t t em-pe ra tu re s g rea te r t han 500 ~ and by Jacobson [18 ]on r eac t ion o f i ron w i th 1 vo l % HC 1/0 -50 vo l %O2/Ar mix tu re a t 550~ Above 50 vo l % 02 , t heacce l e ra t ing e ff ec t became l e s s p ronounced and i tcould even be reversed as shown in Fig . 3 [17] due tothe form at io n o f surface oxide f ilms. In C12, howev er,oxygen h ad l i tt l e ef fec t on the ch lo r ina t ion r a t e o f i ronbe low abo u t 300~ a l thou gh i t p r even ted vapor i z -

a t ion o f FeC13 above th is t em pera tu re due to t hecompe t i t i ve fo rma t ion o f an i ron o x ide f ilm too [34 ,35].

In HC1 gas t he co r ros ion r a t e o f ch ro miu m a t400-800 ~ is de t e rmined by the fo rm a t ion and vapor-iza t ion o f CrC12 sca le to som e exten t up to 600 ~

3 6 9 5

8/3/2019 metalhalide

4/6

i 0 1 0 2

PE

10 3

= n 1 0 2 "

0L

c0

go 101

1o

i 0 I