Corrosion of Steel by Gaseous Chlorine EFFECT OF TIME AND TEMPERATURE Information is presented on the rate of corrosion and the ignition temperature of sheet steel in commercially dry chlorine gas at temperatures from 77' to 251' C. for exposure periods between 12 and 480 minutes. Gradual increases in the corrosion rate are noted as the temperature is raised to 24.8' C. A t 251' the sheet steel coupons show a greatly accelerated corrosion rate of approximately31 inches penetration per year (170,000 mg. per sq. dm. per day) for exposure periods of less, than 30 minutes. After 30 minuteexposure at 251' C. the sheet steel coupons ignite to give corrosion rates in excess of 170,000 mg. per sq. dm. per day.
T
HE rapidly expanding industrial use of chlorineunder widely
GUSTAVE HEINEMANN, FOSTER G. GARRISON, AND PAUL A. HABER' Southern Alkali Corporation, Corpus Christi, Texas
subdivision of the metal upon this reaction. In the runs on sheet steel, corrosion rates were obtained for various times and temperatures up to 251" C. at which point ignition took place after 30-minute exposure. In the runs on steel wool only the ignition temperatures were determined, and no corrosion rates were obtained. Corrosionmay take many forms, such as direct chemical attack, pitting, galvanic corrosion, concentration or solution cell corrosion, corrosion cracking, and fatigue corrosion (3). However, in this instance all except direct chemical attack are absent or relatively minor in action. Since the most important rate factor in direct chemical action is temperature, it constitutes our chief variable. In this work the corrosive action of dry chlorine on steel was investigated a t various temperatures up to the point where ignition occurred.
varying conditions has indicated the need for securing definiEQUIPMENT tive data on the rate a t which steel is corroded by commercially The equipment is illustrated in Figure 1. The reaction vessel dry chlorine gas a t elevated temperatures. A survey of the consisted of a Pyrex U-tube with a narrow limb and a wide limb literature reveals a wide divergence among values cited as maxiof 2 and 4 em. inside diameter, respectively. The limb of smaller mum safe operating temperatures for equipment used to handle diameter was packed with glass beads through which the air or dry chlorine gas or among data which would be pertinent to the chlorine was passed. The sample was placed a t the bottom of subject. Thus Mellor (4), quoting H. Davy, states that "when the large-diameter limb, and a thermometer was placed so that a piec,e of red-hot iron is introduced into chlorine gas, the iron its bulb touched the sample. The chamber was placed in a burns with a red glow, forming ferric chloride". Presumably the constant-temperature bath which afforded control within 1O C. temperature of "red-hot iron" is in the neighborhood of 500and connected with lines through which the chamber could be 600" C. C. G. Maier (4) recommends heating iron to NO-500' C. evacuated and through which air and chlorine could be passed in an atmosphere of chlorine for the preparation of ferric and vented. The air was first passed through a drying train chloride. Lemarchands and Jacob (a), in a series of experiments consisting of an activated alumina tower and two scrubbers conon the ignition temperatures of various metals, declared that the taining a solution of 15% phosphorus pentoxide dissolved in conignition temperature of iron in dry chlorine was 280" C. Perry centrated sulfuric acid. (6) states that steel is recommended for piping dry chlorine gas For the runs at lower temperatures and for steel wool, a modiat temperatures up to 212" F. (100' C.). A committee of the American Water Works Association (1) states that dry chloTine is very corrosive t o steel above 300 O F. (149' C.) and is somewhat corrosive over 195" F. (91 ' C.). Various trade pamphlets give A - ACTIVATED ALUMINA values from 194' to 212" F. (90' to 100" C.) 8 -SCRUBBERS WITH p24+bSOd C - FLOWMETER. aa upper limits for use of steel with d r y chlorine. D- GLASS BEAD PACKING The discrepancies in these values emphasize the E-CHAMBER need for additional information. F - SAMPLE G - THERMOMETER To obtain such information, corrosion. tests H-ICE-COOLED TRAP on steel were made with chlorine gas obtained J - CHECK VALVE from cylinders of commercial liquid chlorine K - 3-WAY STOPCOCK having a moisture content of approximately L- C L O S E H U B E MANOMETER 0.00570 by weight. In one series of tests 16gage mild steel coupons were used to simulate conditions normally encountered in practice. The composition of these coupons was within the following limits: carbon, 0.08 to 0.15%; manganese, 0.35 to 0.65%; phosphorus and sulfur, up to a maximum of 0.05 % each. r In a second series of tests several grades of I i steel wool, varying in strand thickness, were used to obtain some indication of the effect of
1
Figure 1. Equipment for Corrosion Tests
Present address, 1426 Avenue D, Galveston, Texas.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 38, No. 5
priate time, after which the samples xere removed for inspcctiori and determinat,ion of corrosion loss. One of tlvo different procedures was followed, depending upon the degree of corrosion to be expected. At the Ion-er temperatures (from 77" t o 148' C.) the might 10s on the sample was never greater than 0.0009 gram in a 30-minute period. For this reason the reiyht loss was determined by immersing tho corroded specimen in water to remove the ferrous or ferric chloride film. The wash water was then acidified, and the amourit of iron chloride dissolved ~ v mdikermined colorimetrically, using the thioglycolic acid met'hod with a Fisher phot'oelect'ric colorimeter. This method produced accxrate and reproducible results in the rang" used. At tempemtiires above 1-18" C. thiy milthod produced i'rroiicous results; i.e., lower corrosion ratcs KCTC indicated than lisd becn found at lower temperatures. This as apparently due t o a partial vaporizat,ion of ferric chloride under the conditions of tlw t w t . Calculations, based on extrapolated data for the vapor pressure of ferric chloride and upon the volume of gas passing through t,he apparatus, indicated that a significant portion of thc total ferric chloride formed a t this temperature could be volatil60 12C I80 240 300 360 420 480 ized. It, was therefore nrccssary to adopt the direct gravimetric TIME IN M I N U T E S method for determining corrosion loss. I n this method t,lic corFigure 2. Variation of Corrosion Rate with Time r o d d spccimens were immersed briefly in B 0.1 S hydrochloric at Different Trmperaturracid solution, follow.ed by immediate nashing in distilled water and finally in alcohol and ether. The coupons mr(1 then d r i d -fkd Abderhalden apparat,us ~r-asnsed. Temperature control ~ n c under an iiifrared lamp to prevent the rusting which \vas found to obtained by boiling a suitable liquid in a flask connected to ail occur a s the lust tra,ces of etlier evaporated ant3 cooled the metal annular space surrounding the sample chamber, The Abdersurface b c l o n the dew point of the ambient atmosphere. This halden apparatus n-as more difficult t o operati. :ill11 \ r a s considered mcthod of clcnriing removcd ltxs than 0.1 my. of mctal from a clean I t s satisfactory.
3
I
For the scries of test5 on shectbstcel, coupons of 1G-gnge mild steel were used. Each coupon measured 2.92 x 4.04 cm. arid had a total surface area of 0.2596 sq. dm. Tlic coupons were subjected t o a thorough cleaning process which consisted of pickling In dilute hydrochloric acid, n-ashing in a solution containing N trace of sodium hydroxide to inhibit further acid reaction, and finally rinsing with alcohol and ether. The coupons were t,linii immediately dried undvr a n infrrircd lamp :nid stcircd in a di>sicvat or, CORROSIOU 1'KOCEI)UKE
The weighed coupon-n-us placed in the sunipla e l i i ~ ~ nofl ~the ~~r previously dried apparatus. The bath temperature T V ~ Praised, and the apparst,uswas evacuated to an abPolute pressure of less than 1 mm. in order t o remove traces of moisture. Siniultaneuusiy a very small stream of d ir \vas drawn through the chamber. A t the end of an hour the vacuuni pump n-as slnit off. The vent line, containing ZL check valve to prevent ittinospheric iiir from entering, was then opened axid tlie chloriiic)rylinder was (*onnected to the chlorine inlet. Chlorine was then fed. into t h c s .ariil)h c~1i;iniht~r~ ior thc appro-
83 94 10 i
111 115 135 147 I48 151
177
184
158 22: 230 240 247 248 251
3lethod uf Determining corrosion L o i s Colorimetric Colorimetric Colorimetric Colorimetric Colorimetrir Colorimetric Colorimetric Colorimetric Gravimetric Colorimetrii, Gravimetrir Gravimetric G ravi me t ric Gravimetric Gravimetric' Graviinetric Gravimetric Gravimetri 1' Graviinetric
L o b b 111
\+r 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
g.
00021 00023 00020 00043 000-14 00048 00071 0008') 0009 00094 0014 0015 0018 0022 0024 0027 0029 0030 9474
- ('orlohion .\lg./sq.
dm./day 41
43
53 78 82 88
130 165 166 173 280 330
108 445 500 s35 080 175,200
-------
Penetration, iii./sear 0.007 0.007 0.010 0.014 0.015 0.016 0.023 0.030 0.030 0.031 0.046 0.050 0.060 0.073 0,080 0.051 0 . 096 0 . 100 31.5
nictsl panel.
hs :t check o n the validity of the t1T-o mcthods For measuring the cstcnt of corrosion, the corrosion rate at, 147-148" C. ivas detc~rminedby both methods. At 147" the rate was 164.9 mg. pcr s q . dm. pcr dag by the indirect Colorimetric method: a ratc: of I(iA.0 mg. pcxr sq. dm. x a s obtained by the gravimetric mc,thod on n rull at 1-18"c. Siiice it is recognized that at low flio1.i- rates clianges in the rato of chlorine flon- affect the rates of corrosion, a series of tests ~ r a s riin at 148-150" C. to detcrmine a minimum flow rstc of chloriiit.
13 30 60 120 480
166
l,5 30 60 1LO 480
t98
1B 30 60
230
120 480 15 30 60 120 480 1;1 30 60 120
240
247
480
12 15 30
251-2
Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimet,ric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetric Gravimetrir Gravimetric GraviinPtric Gravimetric Gravimetric Gravimetric
0.0010 0 0012 0.0016 0.0018 0,0046 0.0016 0.0017 0.0022 0,0025 0.0051 0.0026 0 0024 0.0027 0.0027 0 0047 0 0029 0.0024 0.0028
370
0.066 0.040 0.027 0.015 0.010
550 310 200
0.1U6 0 037 0.037 0 .o m 0.011
220 150 83 53
I20 55 '361 445 250 125 J4
0.172
0.080 0.045 0.023 0 010
0,0030 0.0052
1070 500 260 140 60
0 047 0,055 0. 0 11
0.100 0 . 0x3
0.003'2 0.0025 0.0032 0.0032 0.0050 0,3704 0.4738 0.9474
1180 ,536 296 148 57 171,300 175,000 175.200
0.21 0.OUR 0.034 0.027 0.010 30.83 31,50 31.54
INDUSTRIAL AND ENGINEERING CHEMISTRY
May, 1946
TEMPERATURE FOR VARIOUS GRADIDS OF TABLE 111. IGNIT~ON STEELWOOL Grade of Steel Wool
00 0
2
Temp. of Ignition, 184 189 194
'C .
499
SAMPLES TENDED TO KjNlTE AT 251QC.AFTER 30 MINUTES, GIVING VERY HIGH CORROSION RATES. 1
No. of Detns. 2 2 1
0
POINT COMMON TO ISAND 30-MINUTE CURVES ' required to reach a point where the reaction rate would be largely independent of the gas mass velocity. As the following data show, a flow rate of chlorine in excess of 5.16 grams/sq. cm./hour was sufficient under the conditions of the experiment. I n all other tests flow rates varying between 11.10 and 14.10 grams/ sq. cm./hour were used. Flow of Chlorine
Grams/,%. Cm./Hr.
2.42 2.82 5.16 6.84 9.78
Temzerature, C. 150 148 149 149 149
CL
a w
Corrosion Mg./Sq. Dm.)Day 78 111 167 165 164
I t was found that a t 251 C. the coupons tended to ignite after about 30-minute exposure periods unless the flow was materially reduced. Since a curtailment of flow would have meant limiting the speed of the reaction, no corrosion rates could be obtained on the highly exothermic reaction which took place after 30 minutes. . In the series of tests using steel wool in place of the sheet metal coupons, three grades of steel wool were used (American Nos. 00, 0, and 2, representing three common grades from fine strands to coarse). Each specimen was thoroughly degreased by extraction with carbon tetrachloride in a Soxhleb extractor t o prevent the possibility of premature ignition resulting from chlorination of a grease film which might have been present on the surface. The drying procedure for equipment and sample was the same as previously described for steel panels. Chlorine gas was then admitted to the dry chamber. The ignition temperature was clearly evidenced by the propagation of a bright red glow throughout the mass of the wool. Experimental data are given in Tables I, 11, and 111. DISCUSSION 'AND CONCLUSION
The data show that there is an increasein corrosion rate with temperature and that the rate of increase diminishes as the exposure time increases. The rates of corrosion increase regularly and steadily up to 247-248" C. Beyond that temperature there is an extremely rapid increase in corrosion rate, rising abruptly from 560 mg./sq. dm./day a t 247-248' C. for the 30-minute curve to 175,200 mg. a t 251 ' C. The solid, vertical line in Figure 2 indicates a merging of the five separate corrosion curve8 a t 247-248' C. (showing relatively low corrosion rates) to the point a t 251 C. of excessively high corrosion rate. The region between 248" tmd 251" C. covers what is possibly a range of metastable conditions. The dotted continuation line ending in an arrow indicates that at 251 " C. enough heat was generated after 30 minutes for the panel t o ignite. With ignition, very rapid and unreproducible temperature increases were encountered which made determination of corrosion rates impossible. Our experience is corroborated by Lemarchands and Jacob (g) who claim that iron will ignite (a rapid reaction takes place) in dry chlorine a t 280' C. The high rate of attack a t 251 O C. for periodspf less than 30 minutes was marked by the deposition throughout the tube of long needlelike crystals of a deep purple-black or reddish color. Figure 3 illustrates the variation of corrosion rates with time a t different temperatures which may be ascribed to a protective film of ferric chloride. At a low temperature (77' C.) the initial and final corrosion rates do not differ greatly. This indicates that the quantity or condition of the ferric chloride formed a t 77" C. does not appreciably retard the already low corrosion rates, at least within 8 hours. At intermediate temperatures (between 166" and 247" C.) the initial corrosion rate is very high but rapidly
t
IO 1
75
I
100
I
I
125
I
175 200 TEMPERATURE, 'C. 150
225
250
J
IO 275
Figure 3. Variation of Corrosion Rate with Temperature at Different Times
diminishes as the protective film is formed. At the ignition temperature of 251 ' C. the corrosion rate does not fall off over a period of 30 minutes. The extremely rapid increase in corrosion rate occurring a t 251 C. is probably due to the fact that the rate of vaporization of the ferric chloride scale becomes greater than its tendency to form a protective film. This hypothesis is in accord with the finding&of Lemarchands and Jacob (8) who maintain that the ignition temperature of a metal in chlorine is dependent upon the boiling point of the metal and the boiling point of the metal ahloride. After about 30 minutes enough heat has been generated by the exothermic reaction t o ignite the metal panel. As Table I11 shows, the ignition temperature is lowered markedly when the metal is relatively finely divided. There is also some elevation of ignition temperature as the thickness of the steel wool strands increases. These data emphasize the important effect of the physical state of the metal upon the rate of corrosion. ACKNOWLEDGMENT
The authors take this opportunity to express their gratitude to Dwight Means, Alphonse Pechukas, Stanley J. Hultman, and Henry W. Rahn for suggestions and assistance, and to Harold F. Pitcairn and 0. N. Stevens for encouragement and for permission to publish the resolts of this investigation. LITERATURE CITED
(1) Comm. on Chemical Hazards in Water Works Plant, A.W.W.A., J. Am. Water Works Assoc., 27, 1226 (1935). (2) Lemarchands, M., and Jacob, M., Bull. SOC. chim., 53, 1139-44 (1933). (3) McKay, R. J., and Worthington, R., "Corrosion Resistance of Metals and Alloys", p. 17, New York, Reinhold Pub. Corp., 1936. (4) Mellor, J. W., "Comprehensive Treatise on Inorganic and Theoretical Chemistry", Vol. XIV, p. 40, New York, Longmans, Green and Co.,1935. (5) Perry, Chemical Engineers Handbook, p. 1732,New York, McGraw-Hill Book Co., 1934.
