M. H. BROWN, W. B. DeLONG,
A N DJ. R. AULD
E . I. (lu P o n t de N em o u rs & C o m p an y, In c., W ilm in g to n 98, Del.
A
n u m b e r o f c o m m o n e n g i n e e r i n g m a t e r i a l s w e r e•ex p o se d to d r y h y d r o g e n c h lo r i d e , a n d to d r y c h l o r i n e a t e le v a t e d t e m p e r a t u r e s i n s h o rL t i m e t e s t s c a r r i e d o u t to d e t e r m i n e t h e r e l a t i v e c o r r o s i o n r e s i s t a n c e a n d l i m i t i n g t e m p e r a t u r e s o f s e r v i c e a b i l i t y o f t h e s e m a t e r i a l s . N ic k e l a n d t h e h i g h n i c k e l a llo y s w e r e i n d i c a t e d t o h e m o s t u s e f u l u n d e r t h e s e c o n d i t i o n s . P l a t i n u m a n d g o ld w e r e f o u n d to h e r e s i s t a n t t o a h i g h e r t e m p e r a t u r e t h a n n i c k e l i n d r y h y d r o g e n c h l o r i d e b u t n o t i n d r y c h l o r i n e . F o r s o m e m a t e r i a l s t h e c f fc c t o n c o r r o s i o n o f d i l u t i o n o f t h e s e ,g a s e s w i t h a i r , s u l f u r t r io x i d e , o r w a t e r v a p o r w a s a ls o
i n v e s t i g a t e d .
M
A N Y in d u strial operations p resen t th e problem of handling hydrogen chloride, chlorine, or b o th in gaseous form.Such conditions ap p ly to th e m an u factu re of th ese m aterials and also to num erous reactions involving th e ir presence e ith e r by ad d i
tion or as by-products. I f m oisture is also p re s e n t an d th e tem p era
tu re is below th e dew p o in t, so t h a t exposure to aqueous solutions is involved, m ost m etals an d alloys are severely a tta c k e d . In the absence of m oisture, how ever, h ydrogen chloride an d chlorine are n o t severely corrosive a t low te m p eratu res an d are com m only handled in feast iron or steel. U sually a m ore re s ista n t m aterial is
■employed for critical p a rts, such as valves. A t higher tem p era
tu res a different ty p e of corrosion, concerned principally w ith volatility, decom position, o r m elting of th e m etal chlorides, tak es place. T h is in v estig atio n of corrosion by hydrogen chloride and by chlorine a t elevated te m p e ra tu re s w as u n d e rta k e n from a dis
tin ctly practical view point; th e objective w as to establish w h at m aterials offer m o st prom ise an d w h a t th e ir lim itin g tem p era
tu res for useful service are, ra th e r th a n to in v estig ate th e th eo retical aspects of th e problem . Some of th e d a ta o btained led to speculation on th e m echanism s involved, b u t, in general, th e re
su lts ap p ear readily explainable on th e basis of know n inform a
tion.
T E S T IN G APPARATUS
The equipm ent used in th is w ork is show n diagram m atical]}' in Figure 1. In th e tests using chlorine, th e gas w as p u rchased in
•cylinders and w as m etered to th e a p p a ra tu s th ro u g h M onel con
trol valve A . T h e gas was cleaned of scale an d o th e r solid foreign material in a glass w ool-packed colum n, B , an d m ain tain ed a t constant pressure (approxim ately 18 inches sulfuric acid, specific gravity 1.84) on th e d istrib u tin g m anifold, D , by allowing a slight excess to bubble o u t of leg C . T h e flow to each of th e tubes in the furnace was controlled by m oans of stopcocks E an d m eas
ured by means of orifice m eters F . D ry in g tow ers G served to
■ensure th e introduction of d ry gas. D u rin g w ork on m oisture- bearing gases th e gas absorbers were in serted a t th is point, an d the drying towers were rem oved. C hlorine w as s a tu ra te d w ith
m oisture b y passing it through distilled w ater a t room tem p era
tu re a n d th e hydrogen chloride th ro u g h 37% hydrochloric acid.
T hese conditions resulted in th e a d d itio n of approxim ately 0.4%
m oisture to th e chlorine and 0 .2 % to th e hydrogen chloride.
E m p ty flasks behind th e absorbers p rev en ted th e carry-over of liquid b y en tra in m e n t.
T h e te s ts were conducted in a th ree-tu b e carbide resistan ce- h e a te d furnace, H , capable of m ain tain in g a m axim um tem p era
tu re of 2500° F . T h e furnace te m p e ra tu re w as regulated by m eans of p o ten tio m eter controller I o p erated on a p la tin u m - p latin u m -rh o d iu m therm ocouple placed betw een tw o of th e tu b es in th e furnace proper. T em p eratu res of th e individual tu b es in.
w hich th e sam ples w ere placed w ere m easured by m eans of ehrom el-alum el therm ocouples, J , w hich were located w ith th e ir h o t ends in the cen te r of th e heated zone. T hese therm ocouples were contained in sillim anite p ro tectin g tu b es, w hich also served as sto p s to position th e sam ples in th e tubes so t h a t th e te m p e ra tu res could be accu rately m easured and exposure conditions d u plicated. T e s t te m p e ra tu re s were continuously recorded in a six- p o in t recorder, K . T h e com bustion tu b e s em ployed were 1‘A inches in inside diam eter an d 30 inches long, an d were norm ally of sillim anite, alth o u g h some ru n s w ere m ade in refractories of o th er types.
A fter passing th ro u g h th e refracto ry tubes, th e gas w as bubbled th ro u g h bubble tow ers, L , containing stro n g sulfuric acid, w hich served to in d icate flow o u t of th e tu b es an d also to seal ag ain st diffusion of w a te r vapor.
I n th e te s ts involving m ixtures of hydrogen ch lo rid e-air and hydrogen ch loride-sulfur trioxide, th e a p p a ra tu s w as altered to allow th e mixing of these gases. W hen m ixtures of hydrogen chloride and sulfur trioxide were m ade, hydrogen chloride w as fed a t c o n sta n t te m p e ra tu re th ro u g h pressure leg M , cleaned of sus
pended m aterial in tu b e N , an d s a tu ra te d w ith sulfur trioxide by passage th ro u g h oleum to introduce approxim ately 10% sulfur trioxide. A ir for m ix tu re w ith hydrogen chloride w as ta k e n from th e p la n t a ir supply, passed th ro u g h reducing valve an d co n stan t- pressure leg Q , cleaned in tu b e R , an d passed th ro u g h a glass w ool- packed tu b e, P , to ensure thorough mixing.
D u rin g th e early period of th is w ork it w as necessary to gen
e ra te hydrogen chloride as it w as used, by th e d eh y d ratio n of 37%
hydrochloric acid w ith 96% sulfuric acid. T h e gen erato r op erated reasonably satisfactorily, b u t its use w as discontinued as soon as b o ttled gas becam e com m ercially available.
C O R RO SIO N T E S T IN G
W ith th e exception of th e noble m etals, th e specim ens used in th is s tu d y were approxim ately 1 '/< X 3A X V< inch in size (ab o u t 18.5 square cm. surface area) and w ere uniform ly polished to a 120-grit finish before te s t. T h e gold an d th e p la tin u m sam ples were c u t from 0.010-inch sh eet an d th e silver sam ples from 1 / inch sheet, th e specim ens being p rep ared b y careful cleaning w ith 839
840 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 39, No. 7
l ’i g u r c I. A p p a r a t u s f o r C o r r o s i o n T e s t s a t l l i " h T e m p e r a t u r e
a rough eraser. T h e fu rn ace w as allowed to com e to th e testin g te m p e ra tu re an d th e tu b es w ere flushed w ith th e p u re o r th e m ixed gas to be used, before th e te s t pieces w ere in tro d u ced into th e tu b es. T h e specim ens were placed in th e c e n te r of th e 13- inch h eate d len g th im m ediately ad jac en t to th e therm ocouple p ro te c tio n tu b es, w hich acte d as a sto p to ensure uniform positioning.
T h ree sam ples w ere te ste d sim ultaneously (one in each of th e th re e tubes) a t slig h tly v ary in g tem p eratu res. E ach tu b e h ad its in d iv id u al therm ocouple, a n d th e tem p eratu res w ere m easured to
± 1 0 ° F .
E a rly in th e w ork th e effect o f th e gas velocity th ro u g h th e tu b e on th e corrosion of th e sam ples w as checked. T h e r a t e of corro
sion did n o t v a ry to a n im p o rta n t degree w ith th e gas velocity ov er th e range 125 to 1200 cc. p er m inute. T o ensure uniform ity, how ever, a flow of approxim ately 460 cc. p er m in u te w as chosen as th e s ta n d a rd te s t velocity. T h is corresponded to a gas velocity in th e tu b es of ap p ro x im ately 1.3 feet p e r m inute, an d a flow ra te of 1.47 gram s p er m in u te for chlorine a n d 0.75 gram p e r m inute for hydrogen chloride.
A t th e en d o f th e te s t th e sam ples were w ith d raw n to th e en
tra n c e en d of th e tu b es a n d allow ed to cool in a n atm osphere of th e gas being used in th e te s t. T h ey were th e n rem oved to th e air, scrubbed w ith a ru b b e r sto p p er u n d er ru n n in g w ater, rinsed in acetone, d ried , a n d w eighed. I t w as som etim es necessary to soak specim ens in h o t w a te r (for silver specim ens a b rie f exposure to d ilu te am m onium h ydroxide w as em ployed) before scrubbing in ord er to rem ove tig h tly a d h e re n t coatings. I n th e case of cast iron specim ens exposed in th e te m p e ra tu re range 900-1100° F . to m ixtures of hydrogen chloride a n d air, tig h tly ad h eren t oxide coatings w ere form ed w hich w ere v e ry difficult to rem ove, an d m echanical m ethods w ere reso rted to . I n such cases th e indicated corrosion ra te s m u st be considered on ly as approxim ations.
In order to o b tain d a ta w ith in a reasonable length of tim e and to m ake th e freq u en t observ atio n s required to assure uniform con
dition^, testing periods of 2 to 20 hours w ere used.
CORKOSIOiN IN DUY G A SES
In m ost cases th e corrosion ra te of m etals an d alloys in hydro
gen chloride a n d in chlorine ten d s to increase relativ ely slowly as th e tem p e ra tu re is increased, up to a critical p o in t v arying w ith th e individual m aterial. A bove th is p o in t fu rth e r increase in tem p eratu re rap id ly accelerates a tta c k . In p lo ttin g corrosion ra te against te m p e ra tu re on arith m e tic paper, th is resu lts in sh arp change in th e slope of th e curve w ith in a lim ited tem p era
tu re range. T he sam e d a ta p lo tte d on sem ilogarithm ic p a p e r re
su lt in a nearly s tra ig h t line. T he range of corrosion ra te s in
volved is so wide th a t p lo ttin g th e d a ta on a rith m e tic p a p e r is n o t practical in som e cases.
T he behavior of in d iv id u al m aterials in d icates th a t, in general, th e degree of corrosion is roughly pro p o rtio n al to th e v ap o r pres
sure of th e p a rtic u la r chlorides involved. I n som e cases, a n ex
am ple of w hich is show n in F ig u re 2, th is relatio n appears to be q u a n tita tiv e over th e te m p e ra tu re ran g e in v estig ated . T h e re
sistance of a m aterial to hydrogen chloride o r to chlorine a t high tem p eratu res m ay n o t be safely p red icted b y th is m eans alone, however, for th e following reasons: (a) V apor pressure d a ta are available for only a p a r t of th e chlorides involved. (6) Some chlorides m elt o r decom pose a t a te m p e ra tu re a t w hich th e v ap o r pressure is still relativ ely low, an d th e reactio n can th e n proceed ra p id ly because th e p ro te c tiv e effect of th e coating is lost. (<-) Som e m aterials are in h eren tly m ore re sista n t to th e in itial for
m atio n of a surface film of c h lo rid e .. (<1) S teel, cast iron, copper, a n d alum inum w ere found to ignite in chlorine above a certain
July 1947 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 841 temperature with a violent evolution of heat, so that the surface
temperature was raised and the rate of reaction increased. This phenomenon was not observed in hydrogen chloride.
Figures 3 and 4 illustrate the type and degree of uniformity of the data obtained. These show the behavior of Inconel in chlorine and hydrogen chloride, respectively. Figure 3 com
pares the results of 2-6 hour and 10-20 hour runs; Figure 4 is based on
2
-hour runs.With the exception of service at temperatures sufficiently high to result in a high vapor pressure for the particular chloride in
volved, or in its melting or decomposition, the coatings formed are protective to some extent, and no indication of pitting was ob
served except in wet gases at low temperatures. The corrosion rates obtained in short time tests in dry gas are believed, there
fore, to be somewhat higher than the rate that would apply for continuous exposure up to the temperature (varying with indi
vidual materials) where the coating ceases to be protective. The use of short testing periods also tends to make the data more variable than they would have been if longer exposure times had been employed, especially where the loss in weight was small.
For this reason the corrosion rates listed should be interpreted only as being indicative of the limitations of the materials, since they are not sufficiently accurate for other than a rough estimate of equipment life.
The behavior of platinum and gold in chlorine (Figures 5 and
6
) is interesting from the theoretical viewpoint. The chlorides of both platinum and gold are unstable at high temperatures and tend to break down either to another chloride or, at higher temperatures, to the metal and chlorine. PtCl
4
decomposes at 098° F., and P tC l2
at 1078° F. The behavior of gold chlorides is less firmly established, and disagreement appears in the literature¡us to the mechanism involved; in any event, none of the three chlorides commonly listed (AuCl, AuC13, and Au
2
Cb) exist above about 500° F. This suggests the possibility that, if the metals were exposed to temperatures above the decomposition point of the chlorides and the metal chloride decomposition products wereTEMPERATURE - °F.
Figure 2. Corrosion o f Nickel in Dry C hlorine and Dry H ydrogen Chloride
metal and chlorine, no reaction would occur. Tests in anhydrous hydrogen chloride in the vicinity of the decomposition point re
sulted in no attack and no appreciable weight loss until approxi
mately 2200° F. was exceeded for platinum and 1600° F. for gold.
In chlorine, however, a sharp maximum in the corrosion rate- temperature curve was observed for platinum a t approximately 1070° F., with some indication of a minor peak a t 700° F. and a minimum at about 1220° F. Above this point corrosion in
creased regularly at a comparatively gradual rate. In the case of gold a less sharply defined maximum occurred a t about 510° F., followed by a minimum at approximately 880° F. and then by a regularly increasing rate with rising temperature. The increase in corrosion rates at the higher temperatures is probably ex
plained by the rate of reaction to form chloride exceeding that for chloride decomposition. In the case of hydrogen chloride the metals are apparently sufficiently resistant so th at there is little tendency for the chloride to form.
A summary of the results obtained in anhydrous chlorine and in anhydrous hydrogen chloride on the various metals and alloys investigated is presented in Table I. The temperatures at which given corrosion rates were exceeded in short time tests were ob
tained by plotting data as in Figures 3 and 4, drawing in appro
priate curves, and rounding off indicated temperatures to the nearest 50 ° F.
Suggested values for upper temperature limits are intended as a rough guide of maximum temperature at which given materials can be used without serious attack in an atmosphere of dry hydro
gen chloride 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 muriatic furnace operation) rates of attack up to
0.1
inch per month or even higher may be considered satisfactory. Also w'ith a low cost material, such as cast iron, replacement in a relatively short time is permissible, whereas with a high cost material, such as platinum any appreciable attack would preclude its use. In interpretation of the data in these tables, the following facts mustbe.remem-TEM PERATU RE-°E
Figure 3. Corrosion of Inconel in Dry C hlorine Gas
842 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 39, No. 7
bered: (a) The surface coating of chloride tends to provide some protection for most materials up to a point where melting, vaporization, or decomposition removes it as it 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. (
6
) Dilution of hydrogen chloride or of chlorine with other materials may not only change the degree of attack but also the relation between different materials. I t is believed that the values given are conservative rather than op
timistic.
Since stress-corrosion cracking has been observed in stainless steel exposed to aqueous chloride solutions, stress-corrosion cracking may possibly occur in service involving gaseous hydro
gen chloride or chlorine. This factor was not investigated in the present study, and no instance is known in which alloys of the 18-8 type have been used in high temperature service involving these gases. One experience is known where exposure at atmospheric temperature to wet mixed gases containing hydrogen chloride re
sulted in stress-corrosion cracking of Type 347 stainless steel.
A cast alloy designated as SHA-1, with the composition 37%
nickel, 27% chromium, 3% molybdenum, 2% copper, 0.25%
carbon (maximum), and the balance iron, is employed for certain
N ickel
muriatic furnace parts; no difficulty has been experienced with cracking of this material.
Suggested limiting temperatures for service in hydrogen chlo
ride and chlorine were listed by Friend and Knapp (1). Only some of the values given were based on experimental data, and fewer materials were represented than in Tables I and II. Where comparisons are possible, agreement is, in general, quite good;
however, there are a few notable exceptions, such as silver in both hydrogen chloride and chlorine, and platinum in chlorine, in which actual tests indicate much poorer resistance than that pre
dicted by Friend and Knapp. A limited number of tests were also made by Pershke and Pecherkin (
4
) in chlorine and hydrogen chloride gas of unknown moisture content. The theoretical aspects of the reaction of chlorine with metals at elevated tempera
tures have been considered by Lemarcbands and Jacob (3).
The results of a careful study of the behavior of carbon steel in chlorine, employing test periods up to
8
hours in duration, were recently reported by Heinemann, Garrison, and Haber (2). In their work, in which the maximum temperature necessary was less than 500° F., specimens were exposed in all-glass equipment with the heating chamber consisting of a U-tube immersed in a constant temperature bath controlled within 1° C. They reported ignition of carbon steel in chlorine to have taken place in 30 minutes or less at 251
0
C. (4840 F.). Although no attempt wasJuly 1947 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 843
TEM PERATURE-°F.
F ig u r e 6. C orrosion o f G old in D ry C h lo rin e G as
C O R R O S IO N IN ¡M O IST G A S E S
The effect of a small amount of moisture in the gas on the cor
rosion rate of several materials was investigated. The methods used resulted in a moisture content of about 0.4% in the “wet chlorine” and of about
0
.2
% in the “wet hydrogen chloride.”With chlorine of this moisture content there is some increase in the rate of attack as compared to dry gas, even above the dew point. This effect disappears at about 700 °F .; at higher tem- pçratures the presence of 0.4% moisture docs not appear to influ
ence corrosion. Hydrogen chloride with 0.2% moisture did not show appreciably higher rates above the dew point than did dry gas.
The behavior of 18-8 stainless steel in dry and in moist chlorine is shown in Figure 7. As has been mentioned, one example of stress-corrosion cracking of stainless steel in moist hydrogen chloride gas is known.
D R Y H Y D R O G E N C II L O R I D E -A IR M IX T U R E S
In muriatic furnace operation the hydrogen chloride gas gen
erated is considerably diluted because of air leakage into the fur
nace. It has been noted that variation in life of corresponding comparable operating temperatures, and that the shortest life appeared to be obtained in the furnaces with lowest gas strength.
The dilution of hydrogen chloride With air could conceivably either decrease corrosion by limiting the amount of the corroding agent in contact with the material, or increase corrosion of some materials because of the presence of oxygen, as sometimes occurs
The dilution of hydrogen chloride With air could conceivably either decrease corrosion by limiting the amount of the corroding agent in contact with the material, or increase corrosion of some materials because of the presence of oxygen, as sometimes occurs