Literature Cited Aleshin. E., Roy. R., J. Am. Ceram. SOC.,45, 19 (1962). Bajars, L., U S . Patent 3 978 200 (1976). Cha. D. Y., Parravano, G. J., J. Catal., 11, 228 (1968). Glushkova, V. B., Keier, E. K., Dokl. Akad. Nauk SSSR, 152, 611 (1963). Happel. J., Catal. Rev., 6, 221 (1972). Happel, J.. Hnatow, M. A , Report to Environmental Protection Agency, Grant No. R801321 (1973). Happel, J., Hnatow, M. A., Bajars, L., Kundrath, M., Ind. Eng. Chem., Prod. Res. Dev., 14, 154 (1975). Happel, J., Hnatow, M. A., Rodriguez, A., AlChE J., 19, 1075 (1973). Hollingworth, C. A., J. Chem. fhys., 27, 1346 (1957). Horiuti, J., Ann. N.Y. Acad. Sci., 213, 5 (1973).
Horiuti, J., J. Res. lnst. Cafal. Hokkaido Univ., 1, 8 (1948). Horiuti, J., Nakamura, T., Adv. Catal., 17, l(1987). JANAF Thermochemical Tables, 2nd ed, 1971. Khalafalla, S.E., Haas, L. A., J. Catal., 24, 121 (1972). Leon, A. M., Ph.D. Thesis, New York University, New York, N.Y., 1973. Minachev, Kh. M., Khodakov, Yu. C., Antoshin, G. B., Markov, M. A., "Rare Earths in Catalysis" (in Russian), Isdat. "Nauka", Moscow, 1972.
Receiued f o r review November 18, 1976 Accepted February 16,1977 Presented at the 5th North American Meeting of the Catalysis Society, Pittsburgh, Pa., Apr 25-28, 1977.
Alumina Catalysts in Low-Temperature Claus Process Michael J. Pearson Center for Technology, Kaiser Aluminum & Chemical Corporation, Pleasanton, California 94566
The performance of alumina catalysts was investigated under conditions used in Claus tail gas recovery processes. Greater than 95% sulfur recovery was achieved by the Claus reaction when the elemental sulfur was collected as a liquid in the pores of the alumina catalyst at 135 OC. Over 50 % weight loading was achieved and the catalyst was regenerated with nitrogen at 300 OC. Catalyst aging studies showed that sulfate poisoning combined with surface area loss can cause a considerable decline in catalytic activity; less than 50% sulfur was recovered in extreme cases. High catalytic activity, 90-95% sulfur conversion, can be restored by the use of HPS during regeneration of the catalyst. Alternatively, a promoted alumina catalyst was found to be less susceptible to deactivation by sulfation.
Introduction In the last few years considerable effort has been expended to increase the sulfur recovery from Claus plants. The main driving force has been more stringent pollution regulations aimed at limiting the emission of sulfur compounds. A number of relatively expensive cleanup systems have been developed. However, when very low levels of sulfur emissions are not dictated, an extension of the Claus reaction to low temperatures is more economical. This might also apply where several Claus plants exist in one location so that low temperature Claus treatments can be used to drastically reduce the sulfur levels so that the streams can finally be combined and treated with a single more expensive tail gas unit. Sulfur recovery by the basic Claus reaction between H2S and SO2
2H2S
+ SO2 + 3 / S, ~ + 2H20
(1)
is significantly limited by thermodynamics at typical operating temperatures which are above the sulfur dew point (Gamson and Elkins, 1953). However, when reaction is carried out below the sulfur condensation temperature, then liquid sulfur collects in the pores of the catalyst. Under these conditions high sulfur recovery can be achieved because (1)low temperatures thermodynamically favor high yields of sulfur and (2) one of the products in reaction 1 (sulfur) is condensed in the inlet portion of the bed so that a new equilibrium is established which depends on the approximately constant vapor pressure of sulfur a t the temperature of operation. When the catalyst becomes fully loaded with sulfur, the bed must be heated to drive out the sulfur. After the regeneration is complete, the bed is ready for another sulfur absorption cycle. Typically a two-converter Claus unit can achieve -95% 154
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 2, 1977
sulfur recovery and a three-converter unit 97-97.5% recovery with a well-controlled operation and good catalyst. Addition of a low temperature Claus step [commercial designations are Sulfreen (Martin and Guyot, 1971), C. B. A. (Goddin et al., 1974)] increases the overall recovery to >99%. More detailed reports on operational techniques and the economics of the process have been published previously (Krill and Storp, 1974; Cameron, 1974). The particular concern of this paper is the catalyst in the dual role as reaction initiator and product collector. The purpose is to establish the conversion efficiencies and catalyst loadings that can be achieved. Catalysts. The first catalyst used historically in a commercial low-temperature converter was made of activated carbon stabilized with about 6.6% silica (Krill, 1973). The carbon catalyst had a very high surface area (>lo00 m2/g) and a large pore volume. The current trend appears, however, to be toward use of an active alumina catalyst. This material offers several advantages over carbon (Cameron, 1974): (l), a lower regeneration temperature is needed, thereby saving energy; (2) lower plant capital cost results from use of a carbon steel vessel, which is satisfactory at the lower regeneration temperatures; (3) the greater mechanical strength of alumina vs. carbon reduces the consequences of attrition; and (4) alumina exhibits better catalytic stability. Laboratory data on the catalytic activity of active alumina will be presented to illustrate the performance attainable in a low-temperature Claus unit employing this advanced type of catalyst. Experimental Section The system of flowmeters, reaction furnace, and analytical section used was similar to that described previously for other
$
;i-
80
[
I
I
20
0 SULFUR LOADING ( w t
higher temperature Claus studies (Pearson, Feb. 1973). A catalytic reactor with a volume of 265 cc (10 inches high) was used for the current work. Normal operating temperature was 135 "C, although some tests were made a t 175 "C. The gas stream composition was chosen to simulate that of a typical Claus plant tail gas: 0.75% H2S, 0.375% SO2,6%COa, 2.4% Hz, 1.2% CO, 33.8% H20,55% Nz. The gas hourly space velocity was 550. The catalyst used was activated alumina with a specific surface area in excess of 300 m2/g (S-201type catalyst commercially sold by Kaiser Chemicals). Each experiment was continued until a significant decline in HzS/S02 conversion occurred, and sometimes longer in order to study the reactivity fall-off in more detail. The HZS/SOZ conversion efficiency was measured a t least every 30 min and recorded as a function of the weight percent sulfur loading accumulated in the catalyst, i.e., 100 X wt of S/wt of catalyst. The H2S/S02 conversion was usually measured by ratioing the H2S concentration in the exit gas to that in the feed. It was ensured that the H2S/S02 ratio in the exit gas was always 2:l to make this simplification valid. One exception to this was during the startup period of operation with fresh alumina catalyst. This tended to adsorb a certain amount of SO2 very strongly so s that there was an SO2 deficiency in the exit gases. However, this gradually corrected itself so that the SO2 and H2S were in balance in the exit gases after a few hours of operation.
Results The H2S/S02 conversion measured a t 135 "C as a function of sulfur loading for fresh active alumina catalyst is shown in Figure 1. Conversions of 98+% were obtained up to >50% sulfur loading, which corresponds to approximately 2 days of operation. Gradual activity fall-off followed; then a sharp fall-off in activity commenced in the region of 75-80% sulfur loading. The experiment was discontinued when the H2S conversion fell to 31%; this corresponded to a sulfur loading of 92.5%. This alumina catalyst had a pore volume of -0.55 cc/g; therefore, based on a liquid sulfur density of 1.8 g/cc a t 135 "C, it can be calculated that 100 g of alumina has a pore volume capacity for 99 g of sulfur. Thus, a t the end of the experiment described above, the pores were almost completely filled with liquid sulfur. Little, if any, sulfur was found in the interstices between catalyst spheres. Over the important section of the run the conversion data are reproduced on an expanded ordinate scale in Figure 2. For comparison, a run a t a higher temperature (175 "C) is also shown. At 175 "C the maximum conversion obtained was 96.3%,compared to a maximum of 98.5% at 135 "C. The effect of sulfur loading on conversion was similar for the two different temperatures out to 65% loading, after which the conversion fell more rapidly with increased loading at 175 "C than at 135 "C.
80
60
SULFUR LOADING (wt
yo)
Figure 1. H2S/S02conversion over fresh active alumina a t 135 O C .
40
%)
Figure 2. H>S/SO2 conversion over fresh active alumina a t different temperatures. 100
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90
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Surface Area m2/s
a W
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80
70
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148
40
SULFUR LOADING (wt
60
80
%)
Figure 3. H2S/SO? conversion yield for different surface area catalysts a t 135 "C.
Having established that high conversion yields are obtained with fresh active alumina, it becomes important to investigate the catalytic stability. Potential reasons for decline of catalytic activity are (1)surface-area loss, (2) surface sulfate poisoning, and/or (3) carbonaceous deposits. Carbonaceous deposits can coat the catalyst and prevent the reactants reaching the active surface. However, at the relatively low temperatures used in this process, organic cracking or polymerization reactions are unlikely to be initiated by the catalyst; therefore, carbon poisoning would occur only through carry-over onto the bed. This too is unlikely, because reactants entering the low-temperature Claus bed do not need reheating after exiting from the preceding condenser. In any case, such deposition is independent of the catalyst and so is not of major concern here. Stability data obtained were confined to surface area and sulfate poisoning effects. The surface area of the fresh catalyst was approximately 350 m2/g. By heating batches of the catalyst a t progressively higher temperatures, surface areas of 148,82, and 32 m2/g were obtained. The H2S/S02 conversion a t 135 "C as a function of sulfur loading for these catalysts is shown in Figure 3. The conversion does decline with decreasing surface area, but the effect is small relative to the large decreases in surface area. For example, at 35% sulfur loading, the measured conversions were 98.0, 97.0,95.7, and 94.0% for surface areas of 350, 148, 82, and 32 m2/g, respectively. Only a t the lowest surface area of 32 m2/g was there any evidence of a decreased sulfur capacity, Le., an earlier fall-off in activity with loading. For the study on sulfate poisoning of the catalyst, pre-sulfation of the catalysts was achieved by passing S O ~ / a i over r the catalyst for 16 h at 400 "C. This resulted in 8.2% sulfate on the high surface area active alumina (A). The H2S/S02 conversion at 135 "C is shown in Figure 4. Catalyst A maintained 80.4% H2S conversion until -33% S loading, at which Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 2 , 1977
155
100
sl"
\
'0,
I
90
z
N
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Y) 0
2
2
> W
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I
8 x
2
E?
::80
, 0
,
,
I
60
40
20
!2 2
SULFUR LOADING (wt
80
8 X
%)
Figure 4. Conversion activity over sulfated active alumina catalysts at 135 "C. Surface areas: A, 330 m2/g;C, 82 m2/g.
70
00
20
SULFUR
100
60
LOADING (wi X )
Figure 6. Activity of plant aged catalysts after regeneration with N2 plus different additives.
\
S-501
0
20
40
60
Table I. COS Conversion at 135 "C
80
100
SULFUR LOADING (wt %) Figure 5 . HgS/SOa conversion over other types of sulfated catalysts at 135 "C.
point the conversions gradually began to decline. This catalytic activity is considerably below that of fresh catalyst. To illustrate the effect of sulfation superimposed on surface area loss, data are also shown in Figure 4 for catalyst C. The surface area was 82 m2/g; however, after sulfation under the same conditions as for the previous catalyst, it contained only 2.1% sulfate. The H2S conversion of catalyst C showed a gradual increase with time, reaching a maximum of only 55% a t -16% S loading; Le., sulfation together with considerable surface loss had markedly reduced the activity and capacity of the catalyst. Finally, two other catalysts were tested after sulfation under the same conditions: (1)gel alumina, and (2) promoted active alumina S-501 (commercially sold by Kaiser Chemicals). The H2S/S02 conversions at 135 "C are shown in Figure 5. The sulfated-gel alumina (9.5% sulfate, surface area 275 m2/g) gave inferior performance to the corresponding active alumina; it reached a maximum conversion of 67% a t 35% S loading. However, the sulfate promoted active alumina (12.5%sulfate, 200 m2/g) gave a remarkably high H2S conversion level: 96.5 to 97.0% was obtained until over 40% S loading, after which the activity declined slowly to -65% S loading, then declined fairly sharply. Therefore, under equivalent sulfation conditions, the promoted active alumina showed considerably improved sulfur conversion efficiency as compared with the unmodified active alumina. This finding is quite in accord with the increased sulfation tolerance of this modified catalyst in conventional Claus converters, as has been reported previously (Pearson, May/June 1973). Note also the good conversions obtained with the promoted alumina which was calcined and sulfated (surface area 75 m2/g, sulfate 4.3%);this suggests that the promoted alumina also possesses good stability. 156
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 2, 1977
S loading,
COS convn,
Catalyst
%
%
Fresh active alumina (S-201) Sulfated active alumina (S-201)
25.0 48.0 20.7 41.6 60.0 24.0 57.0
87 71
Sulfated promoted active alumina (S-501)
19.5
18 1 5 53 30 9
In Claus systems considerable COS is frequently present. Usually this is decomposed in the high-temperature (Le., # 1) Claus converter where reaction rates are rapid. Some data on COS conversion at the present low temperature (135 "C) are shown in Table I. For fresh active alumina catalyst, decomposition was good at 87% for 25% S loading, but sulfated catalyst gave very poor conversions with only 18%at 20.7% S loading. Hence, COS conversions in low-temperature converters may not remain high for long periods. Considerable improvement was shown by the sulfated promoted active alumina catalyst, which gave 53%COS decomposition at 24.0% S loading. Regeneration. After the loading cycle has been completed, the catalyst is regenerated with an inert gas stream at 300 "C. The liquid sulfur is driven out over a period of several hours. Catalysts which have been in service for 1-2 years have been through many cycles. These catalysts show symptoms of aging, e.g., surface areas around 160 compared to over 300 m2/g initially, and sulfate contents of 2-3%. The activity of such an aged catalyst after a regeneration at 300 "C in nitrogen is shown in Figure 6. The maximum H*S/S02 conversion achieved was 82% which was way below that attained with fresh catalyst; also the activity started to decline much sooner, i.e., a t a lower sulfur loading. It should be noted that there is a small increase in activity during the first few hours of operation. This leads one to suspect some catalyst reactivation is occurring. Therefore, it was interesting to find that, when the regeneration was carried out with 2% H2S in the nitrogen stream, then the activity of the catalyst showed a remarkable increase in the subsequent sorption cycle (see Figure 6). The maximum conversion exceeded 94% and the effective sulfur capacity of the catalyst was much im-
proved. When a similar test was run with 2% H2 instead of 2% H2S in the regeneration gas, the subsequent activity was barely improved over that obtained with nitrogen alone. Similarly, a test run with nitrogen containing water vapor gave only a modest increase in activity. It is apparent that H2S either removes a surface poison or activates the alumina surface in some manner. Analytical measurements were made to determine whether there was any difference in free elemental sulfur between the HzS and NP regenerated catalysts. No difference was detected. The test with water vapor indicated that protonation or hydrolysis was only playing a very minor role in improving the catalytic activity of the surface. It was thought that the most likely reactivation mechanism was sulfate reduction. Sulfate reduction-activity measurements have been made and averaged over six different catalyst samples which were given a milder HyStreatment. The sulfate declined 0.23% while activities increased from 81.3% to 90.5% H2S/SOz conversion. This sulfate decline seems small, but it corresponds to 1.4 X l O I 9 molecules of S04/g of alumina. I t is believed that sulfate forms on the active catalyst sites by occupying two 0 sites on the alumina surface in the following manner:
Assuming that such a sulfate molecule takes up an area of 29 A2, the 1.4 X 1019molecules would occupy an area of 4 X l O 2 O A2/g of alumina. For the aged catalysts with a surface area of 150 m2/g this represents 2.7% of the total surface. This suggests that the most active sites for the low temperature Claus reaction may represent only a modest fraction of the total alumina surface. Regardless of the mechanism involved, it has been shown that the use of H2S in regeneration can have a dramatic effect upon restoring the activity of partially deactivated alumina catalyst.
Discussion Total conversion of HzS/S02 in the Claus reaction is always limited by the thermodynamic constraints on reaction 1.Apart from the favorable effects which lower temperatures have in increasing conversions, there is a further advantage in the low temperature system through condensation of one of the products-sulfur. The gas phase concentration of sulfur is its vapor pressure which is only about 0.08 mmHg at 135 "C. A thermodynamic calculation, assuming all sulfur is present as Sa,indicated that an equilibrium conversion of -96% can be attained for the gas composition used in the current tests. The experimental conversions over fresh catalyst were slightly higher than this, and possible reasons for this are (1) sulfur adsorption on the alumina surface; (2) capillary effects which cause a reduction in the sulfur vapor pressure; (3) effect of dissolved gases, e.g., H2S on sulfur vapor pressure. It is clear that removal of the water vapor could lead to even higher equilibrium conversions. The concentration ratio of HzO to H2S in the reacted gas is greater than 1000 to 1 and therefore has a large effect on the equilibrium. Also, water must compete strongly with H2S in adsorbing on the active surface. Some evidence for the retarding effect of water has been reported though in a slightly modified system (oxygen was also present) (Struck et al., 1971). One experimental test was run here with fresh catalyst in which no water was added but otherwise the feed gas composition was the same; no un-
reacted H2S or SO2 were detected within the limits of our GC analytical system (Le., >99.7% conversion). Unfortunately, water removal has proven impractical up to now in Claus units, although it would certainly mean much higher conversions could be attained. The higher temperature, 170 "C vs. the standard 135 "C, gave lower conversions because of the greater sulfur vapor pressure in addition to the less favorable thermodynamics. Temperatures lower than 135 "C would similarly be more favorable. However, it is probably undesirable to operate below the solidification temperature of sulfur (-120 "C). Therefore, it does not appear practical to operate a t a temperature significantly below 135 "C since a safety margin is advisable. It is important to discuss a number of factors relating to the system and the catalyst necessary for maintaining high conversion of HzS/S02. (1)Sulfur mist: conventional Claus condensers are not fully efficient in trapping sulfur vapor. However, the low-temperature Claus catalyst bed acts as an efficient sorbent for sulfur mist, and in the laboratory no sulfur mist could be detected leaving the bed. (2) HzS/SOz ratio: to maintain the optimum conversion it is imperative to maintain the H2S/SO2 ratio in the reactor feed gas at 2.0. In practice this is often not easy to do, because adjustments are made at the front end where sulfur concentrations are much higher. It is therefore suggested that the ratio could be adjusted to 2.0 with auxiliary streams of H2S or SO2 just before entering the lowtemperature Claus reactor. (3) Catalyst stability: loss of surface area by itself was found to have a remarkably small effect upon the activity. Sulfate poisoning has been shown to have a far greater depressing effect on the catalyst activity. Though the data indicate that declining surface area in combination with sulfation is particularly damaging (see Figure 4), this is probably because the lower surface area catalyst has fewer active sites and hence needs only a little sulfate to poison a significant percentage of them. Sulfate poisoning has been demonstrated in relation to higher temperature Claus activity previously (Pearson, 1973),so its effect is not surprising. Only minimal amounts of sulfate are likely to form during the H2S/S02 reaction step, because temperatures are low; there is no pre-reheat step whereby SO3 might be formed in advance, and any SO3 or free 0 2 from the original combustion step should have reacted in the high-temperature Claus beds. Sulfate formation can occur in the regeneration step. Therefore, it is imperative to exclude oxygen during regeneration and, indeed, preferable to have H2S present as described previously. This should minimize sulfation and even reduce some sulfate if any is already present on the catalyst. Nevertheless, it is unlikely that sulfation can be completely eliminated. Therefore, the promoted catalyst designated S-501 seems to offer a very interesting potential. This catalyst sulfates but maintains higher activity despite the sulfate. The performance of this catalyst has not yet been tested in largescale, low-temperature reactors, and may depend on the nature of the regeneration procedure used. In conclusion, the use of active alumina in a low-temperature Claus reactor can reduce the sulfur tail gas concentration by more than 20-fold in a well-controlled operation, when the catalyst is in reasonably fresh condition. A low-temperature reactor is a simple, logical extension to a Claus plant that appears to offer economic advantages whether it is used as the only cleanup unit or as a means of reducing the sulfur load fed to a more exhaustive tail gas treatment system.
Literature Cited Cameron, L. C.. Oil Gas J., 72, 110 (June 24, 1974). Gamson, E.W., and Elkins, R. W.. Cbem. Eng. Prog., 49, 203 (1953). Goddin. C. S.,Hunt, E. B., and Palm, J. W., HydrocarbonProcess., 53, 122 (Oct.
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157
1974). Krill, H., Control of Gaseous Sulfur Compound Emission Conference, Salford. April 1973. Krill, H., and Storp, K.,Chem. Eng., 84 (July 23, 1974). Martin, J. E., and Guyot, G., Can. Gas J., 64, 26 (Nov/Dec 1971). Pearson, M. J., Hydrocarbon Process., 52, 81 (Feb. 1973). Pearson. M. J:, Can. Gas J., 65, 22 (May/June 1973).
Struck, I?. S., Kulek, M. D., and Gavin, E., Environ. Sci. Techno/., 5, 622 (1971).
Received for review November 23,1976 Accepted January 24, 1977
Chemical Profile of Polychloroethanes and Polychloroalkenes Wesley L. Archer' and Elbert L. Simpson Inorganic Product Department, Dow Center, 2020, Dow Chemical U.S.A.,Midland, Michigan 48640
A summary is presented of the chemical reactivity of several chloromethanes and chloroethanes as compared with the unsaturated species, trichloroethylene and perchloroethylene. A total of eleven chlorinated solvents
were investigated. Metal reactivity-corrosion was accelerated in all the solvents when a water phase was present. Although solvent attack on aluminum, iron, or zinc is negligible in dry trichloroethylene or perchloroethylene, aluminum and zinc currosion generally occurs in dry systems containing the saturated chlorinated solvents. Iron corrosion is no great problem in these dry solvents. The reactivity of trichloroethylene and perchloroethylene with amines is minor as contrasted to the C, and saturated Cz solvents. The presence of the olefinic bond in trichloro or perchloroethylene renders the chlorine atoms less labile in metal or amine reactions, but this point of unsaturation affords easy oxidative degradation. lI1,2-Trichloroethane is the only saturated solvent that shows appreciable oxidative breakdown.
Introduction Polychloroethanes and ethylenes are large volume chemical products used in a wide variety of industrial applications, such as cold cleaning, vapor degreasing, dry cleaning, extraction solvents, reaction medium, etc. The purpose of this paper is to provide a comparison of some of the chemical properties of these two solvent groups. The saturated polychloroethanes covered in this paper include I,1,l-trichloroethane (methylchloroform), 1,1,2-trichloroethane, 1,l-dichloroethane, and 1,2-dichloroethane. The three unsaturated polychlorinated solvents are trichloroethylene, perchloroethylene, and cis,trans-1,2-dichloroethylene. References are also made to three of the chlorinated methanes: methylene chloride, chloroform, and carbon tetrachloride. The reactivity of the chlorine atom is greatly influenced by the olefinic bond. The unsaturation affords a point for oxidation, but the double bond also renders the chlorine atom less labile in reactions involving metals. The initial reactivities of carbon tetrachloride and l,l,l-trichloroethane (unstabilized) with metals are very similar and thus carbon tetrachloride will be used in many cases to illustrate the metal reactivity of the saturated polychloroethane structure, Le., l,l,l-trichloroethane. I t should be borne in mind that once the solvent-metal reaction occurs, other competing reactions from the resultant metal chloride reaction product can, and often do, dominate the reactive system. Experimental Section I. Corrosion Data (Table I). Standard coupon cleaning procedures and corrosion data evaluation were used throughout the reported work. 158
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 2, 1977
A. Metal Coupon Cleaning Procedures Prior to Corrosion Studies. (a) Metal: 2024 aluminum, coupon size 2.0 cm X 15.0 cm X 0.2 cm. Cleaning procedure: Metals Handbook, 1961. The cleaning solution contained 95 ml(42 Be) of nitric acid, 11ml of 48% hydrofluoric acid, and 151ml of water. Immersion time was 1-5 min at room temperature. (b) Metal: pure zinc, coupon size 2.5 cm X 15.0 cm X 0.1 cm. Cleaning procedure: clean with acetone. (c) Metal: 1010 iron, coupon size 2.5 cm X 15.0 cm X 0.1 cm. Cleaning procedure: cleaning solution contained 10-15% hydrochloric acid; immersion of coupon for a few minutes in the acid, followed by dipping it in carbonate solution, rinsing, scrubbing with soap and a bristle brush, rinsing, and drying with acetone completed the coupon cleaning procedure. B. Metal Coupon Cleaning Procedures Used after Corrosion Tests (Uhlig, 1948). Note: As there is some attack of the base metal by these cleaning solutions, a weight loss should be predetermined on a number of clean unused metal coupons. These cleaning losses or gains were taken into consideration (subtracted from weight loss encountered during test) when calculating corrosion weights. Cleaning weight corrections were determined for coupons after treatment for both light and heavy corrosion. C. Reporting of Corrosion Data. The weight loss is first calculated as mg/sq in./day = coupon wt loss in mg/(area of coupon) X 7 where the coupon weight loss considers the cleaning loss correction. The above corrosion rate is converted to mils of penetration per year by the following formula. MPY = mg/sq in./day (22.3/metal density) The metal densities used were 2.79 g/cc for aluminum, 7.88
