Corrosion bv Chlorine and bv J
d
Hydrogen Chloride at High Temperatures 31. H. RRORX. '6. B. DELONC;, .iND J . R . -4LI.D E. I . clrc P o i i t d e Seinorirs &- Conzpuny. Inc., R i l n i i n p t o n 98. Ut.!.
A
n u n i ~ w r of conimoii engineering materials were e\-po+edto dr) hjdrogen chloride and to dr? chlorine at
ele\ated teniperatnres in short time tests carried out to determine the relative corrosion resiqtance and limiting temperatures of s e n iceabilit) of these materials. 3icLeI and the high nickel allo?s ere indicated t o be most useful under these conditions. Platinum and gold w ere found t o b e reiistaiit to a higher teniperature than niche1 in dr) hjdrogen chloride but not in dry chlorine. For -onie materials the effect on corrosion of dilution of these g a c c - w i t h air, sulfur trioxide, or \+ater \apor \+as al-o in\ citigatcil.
.IT\* industrid oper:itions present tlie prolilem of handling h>-tirogen chloride, chlorine, or both in ga.eoiis forni. Siicli coiiditioni apply t o the m:inufncture of these materials n ~ ! d n1.o t o niimerou; reactions involving their presence either 1 ~ ntltlition or :IS by-product.. If moi-ture i i also present and the temperature is lielox the den- point. H O thnt expwiire to aqueow solution. id nlloys are >everely attacked. In tlie is involved, mo5t metal alisence of moi-ture, lion-ever, hydrogen chloride and chlorine u e nut severely corrosive a t lon- temperatures 2nd are commonly lintidled in cast iron or steel. L-sually n more reiistnnt mnterinlis employed for critical parts. such ns valve.. -4t higher temperntiires a different type of corrorion, concerned principally n-itli volatility, deconipouition, or melting of the metal chlorides, t:ikp.< place. This inrestigatioii of corroFion by hydrogen chloride anti tiy chlorine a t elevntcd temper:itures was undertaken from a t1i.qtiiictly practictil vien-point; tlie objective ~ a ' t: o eitablibh what materials offer most promiqe and n-hat their limiting tempei:itiires for useful service are, rather than t o investigate tlie tlieoretical :ispects of the problem. Some of the data obtained led to speculntion on the mechn~iismsinvolved, but, in general, the res i l t s appear rendily esplninnble on the bwis of knon-n information. TESTISG APP.iR4TL-S
Tlie equipment w e d in this ~ o r isk shon-n dingrnmmntically i i i Figure 1. In the tests using chlorine, the gay i v a i purchased i i i cylinders and K:E metered t o the npp:ir:itua through 1IoneI eontrol valve -1. Tlie ga; \vas cle:ined of scnle :ind other -olid foreign \~-ool-packed column, B , and inaiiitnined at cowtniit pre.slire (approxim:itely 18 iiiche; sulfuric acid, specific gravity 1.84) o n the distributing manifold, I ) , by nlloi\-ing a .light ezce.si to buhble out of leg The flon. t o each of the tubes i n tlie furnace x i s controlled by means of .stol~coclisE and mensured tiy me:ins of orifice meters F . Drying towers G served to ennure the introduction of d r y gas. During vorli on mokturehenring g:i"es the gnh nh*orber< were inserted at this point, and the dryi:ig towers were removed. Chlorine ~ i i saturated s with
moisture by passing it through diitilled !\-atel, ;it room temperature and the hydrogen chloride through 3 7 5 hydrochloric acid. These conditions resulted in the nddition of approximately 0.4' moisture to the chlorine and 0.2rc t o the hydrogen chloi~itie. Empty flask> behind the absorbers prevented tlie c n r r y - c ) ~ ~of~ r liquid by entrainment. The tests n-ere conducted in n three-tuhe carbide re4stariceheated furnace, H , capnlile of maintaining a masimum temperature of 2300" F. The furnace temperature n-ns regulated by means of potentioriieter controllrr I opertited 011 n platinumplatinum-rhodium thermocouple placed Iietxeen two of the tubes i n the furnace proper. Teniperatures of tlie individual t u l w in which the .ample; !\-ere placed ryere mca.~uretlhy nieaiis of chroniel-:ilumel thermocoiipleq, J , n.hich wcre loctited v i t h tlieir hot ends in the center of the 1ie:ited zone. The-e tliermocou~!lcs \yere contnined i n ,~illim:iiiiteprotecting tulles, ivliich :iko ,-rrvrd ns ,stops to poiition the s:imples in tlie tulw-. .-(I tli:it tlie tf,nil tures could b e nccurntely mensui,ed and expowre conditiciiii ( IIplicnted. Teit temperatures were c o n t i ~ i i i o u ~I lcrciicletl y 111 :I c i s point recorder, Ii ~ ~ ( 1 0 1 packed tube, P. t o ensure thorough mising. During the early period of this xvork it was necessary t o genc m t e hydrogen chloride as it n-as uced, tiy tlie dehydration of 37'", citl with '365 sulfuric acid. The gener:itor oper:ited isf:ictorily, h i t its w e was discontiiiiied n. woll as hottied gas became comniercially available. CORROSION TESTING
c.
K i t h the exception of the noble metals, the specinienc t i m i i n this study were approximately 1 x X inch in size (about 18.5 square cm. surface area) and were uniformly polished t o a 120-grit finish before test. The gold and the platinum %miples were cut from 0.010-inch sheet and the silver samples from '8inch sheet, the specimens being prepared by careful cleaning with ;,lq
839
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INDUSTRIAL AND ENGINEERING CHEMISTRY
VOl. 39, No. 7
P RECORDER
sioii did not vnIy to a11 important degree ivitli tlic' g:i- vr.locity nvri' the range 125 to 1200 cc. per minute. Tii P I I S ~ I Iu~ tI r~ ~ l ' ~ ~ i ~ n i i t ~ , IiinvcJver, a f l u x of approximately 460 cc. ])pi' niiiiiitc. \\-:I> clio.i,ii a.: tlie standard test velocity. This corre~poii(!c~tl t o :I g i . veliicity in the tubes of appi-osiniately 1.3feet per initiutt~,:urd :I HIJ\Y 1:itc of 1.47 grnni. per minute for rhloi,irir : t i i t 1 0.7: yi.:im 1 i t ' i miiiiitc, CIII. Iiycli,ogen cliloridr. At the end of the tebt tlie i:iniple. \ v e i ~ \\i ~it111li~:i~~ti t o t l i t a PItrance end of tlie tulles and nlion-et1 t o roo1 in : i t ] :itmocphcrc~(if tlie gas being used in t h e test. Thry \\-ere then renic~rrtlto tlic, nir, scrubbed n-ith a rubher stopper under runiiiiig n-ate],,riiisrtl in acetone, dried, and weighed. It was sometime, nee( soak specimens in hot 7%-ater(for silver specimens n brief espo3ure to dilute a.mmonium hydroxide \vas employed) before scrubbing in order t o remove tightly adherent coatings. In tlie cn?e of cact iron specimens esposed in the temperature r m g e 900-1100 F. to mixtures of hydrogen chloride and air, tightly ndlierent oxide coatings Tvere formed which n-ere very difficult t o remove, nncl meclinnical methods n-ere resorted to. In such e w e s the indicated corrosion rate? muat, he con~ideredonly n i npprnsimations.
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1947
temper;iture \yith a violent evolution of heat, so t h a t the aui,f:ice temperature v a s raised and the rate o f renction iricre:tsed. This phenomenon mas not, observed in hydrogen chloiide. Figures 3 and 4 illustrate the type and degree of uniformity of the data obtained. These Phon the behavior of Inconel in vhlorine and hydrogen chloride, reapectivelj-. Figure 3 c80tnpares the results of 2-6 hour and 10-20 hour Iuns; Figure 1 is based on 2-hour runs. With the exception of rervice at temper:iture;. hufficiently high t o result in a high vapor pre.-but'e for the particular chloride involved, or in its melting or derotnpnsition, the cmatings formed :ire protective t o some extent, and no indication of pitting \vasob-rived except in \\-et gases a t Ion- temperatures. The corrosion ~ t e ohtained s in short time tests in dry gas are helieved, therehire, t o be somewhat higher than the rate that would apply for mntinuoua esposure up to the temperature (varying with indi1.idual materials) where the coating ceases t o he protective. T h e u*e of short testing periods also tends t o make the data morr variable than they would have been if longer exposure times had heen employed, especially where the loss in weight was small. For this reason the corrosion rates listed should be interpreted only as tieing indicative of the limitations of the material?, sinre they are not sufficiently accurate for other than it rough estimate o f equipment life. The hehavior of platinum arid gold iri chlorine (Figurea 5 and 6 ) is interesting from the theoretiral viewpoint. The chlorides of both platinum and gold are unstable a t high temperatures and tend t o break down either t o another chloride or, a t higher temperatures, t o the metal and chlorine. PtCI, tlecoinpo 698' F., and P t C l z a t 1078' F . The behavior of rold chlot less firmly estahlished, and disagreenient appears i n the literature :LS t o the mechanism involved; in any event, none of the three vhlorideP commonly listed (.iuCl, AuCld,arid Au7C14)exist above ahout 500" F. This suggests the possibility that, if t h e metals \yere exposed t o temperatures above the decomposition point of tlie c~hloritlesa n d the nidal c~liloridt~ c1t~c~oin~)~icition products \vere
84 1
metal and chlorine, no reaction \voultl occur. Tests in anhydrouh hydrogen chloride in the vicinity of the decomposition point resulted in no attack and no appreciahle 1veight.loss until spproximately 2200' F. was exceeded for platinum and 1600 O F. for golcl. In chlorine, however, a sharp maximum in the corrosion ratetemperature curve n-as obqerved for platinum a t approximately 1070" F., with some indication of a minor peak a t 700" F. anti ZI niininium a t ahout 1220" F. -Above this point corrosion increased regulsrly a t a comparatively gradual rate. In the ('aw of gold a less sharply defined maximum occurred a t about 510" F., followed by a minimum a t approximately 880" F. and then by :t regularly increasing rate Kith rising temperature. The increahe in corrosion rates a t t h e higher temperatures is probably explained by the rate of react,ion t o form chloride exceeding t'hat for chloride decomposition. In the case of hydrogen chloride the metals are apparently sufficiently resistant so t h a t there is little tendency for the chloride t o form. -4summary of t h e results obtained in anhydrous chlorine and in anhydrous hydrogen chloride on the various metals and alloys investigated is presented in Table I. T h e temperatures a t d i i c h given cwrosion rates were exceeded in short time tests were obtained hy plotting data as in Figures 3 and 4,drawing in appropriate curves, and rounding off indicated temperatures t o the nearest 50" F. Suggested values for upper temperature limits are intended as a rough guide of maximum temperature a t which given materials can he used without serious attack in a n atmosphere of dry hydrogen rhloride or dry chlorine. Such a value has obvious limitations, since in a particular application permissible corrosion may be essentially nil, whereas in another (such as muriat,ic furnace operation) rate!: of attack u p t o 0.1 inch per month or even higher may he considered satisfactory. illso with a low cost material, such as rant iron, replacement in a relat,ively short time is permissible, whereas with a high cost material, rurh as platinum any appreciable attack irould preclude its use. I n interpretation c i f the data in t h w e tahles, the follon-ing facts must, be :rememI
r1.00
z
0
\
5
z
>
z
I
W
I
I-
2 0.10-
w
5
0.01
a
z 0 v) 0
a a
"0 0.001
1 0.1 800 1000 1200 1400 1600 I800
0.001
I
I
I
I
1
-
'
1
TEMPERATURE-
TEMPERATURE O F . Figlire 2. Corrosion of \ickel i n Dr? ('hlorine and Dr, II?clrogeti Chloride
I
I
I
0.0001 I 400 600 800 1000 1200 1400 Figure 3.
OF.
Corrosion of Iiironel in Dr? C h l o r i n e Gas
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INDUSTRIAL AND ENGINEERING CHEMISTRY
%TO 950
1000 1000 1000 1000
550
900 900
500
300 500 700 900 1100 1300 1500 1700 TEMPERATURE-OF Figure 1. Corrosion of Inconel in Dr! Chloride Gas
IIydrogen
Carbon areel Cast iron 25 aluiriinuiii (;old >il~er
ionn
950 900 900
(no
s50
600 550 900 400 350 250
650
450 350
250 250 100
250 300 300 150
"no
600 950 450
Vol. 39, No. 7'
1100 1050 1100 1050 1100 B50 1000
000 750 650 1000 550 500 400 350 300 350 250
1'00 1200 1'00 1200 1200
loon
1180 1000 8.50 750 1050 500 450 450 350 400 450
12RO
1250 1250 1050
loon 900 850
1050 550b
4.50 450' 350 1 400
son
Corrorion in D r y Hydrogeu Chloride
bered: (a)T h e surface coating of chloride tends to provide some protection for most materials u p to a point ivhere melting, vaporization, or decomposition removes i t as i t is formed (about the point listed as the upper temperature limit) ; in this case the corrosion rate for extended exposure may be considerably lower than in a short time test. ( b ) Dilution of hydrogen chloride or of chlorine with ot,her materials may not only change the degree of attack but also the relation betiyeen different materials. It is believed t h a t the values given are conservative. rather than opt iniistic. Since stress-corrosion cracking has been observed in stainlecs steel exposed t o aqueous chloride solutions, stress-corrosion cracking may possibly occur in service involving gaseous hydrogen chloride or chlorine. This factor was not investigated in the present study, and no instance is kno\vn in xhich alloys of the 18-8 type have been used in high temperature service involving these g3.e~. One experience is knonm nhere exposure a t atmospheric temperature to n-et mixed gases containing hydrogen chloride resulted in stress-corrosion cracking of Type 347 stainless steel. A cast alloy designated as SHA-1, xvith the composition 37% nickel, 27% chromium, 37, molybdenum, 2% copper, 0 . 2 c ? 7 ~ carbon (maximum), and the balance iron, is employed for certain
I'latinuni Guld Sickel Inconel Ha,celioy B YH.\-1 Hastelloy C Hastelloy A Hastelloy D 18-8-MO 25-12-Cb 18-8 C a r b o n steel Si-Re.ist ( T y p e 1) 3Ionel Silver Cabt i r o n
Durichliir I>urir
