US3261733A - Method of dissolving a plug from a bimetallic metal core and etching bath - Google Patents

Description

y 9, 1966 K. P. BELLINGER 7 3,261,733

METHOD OF DISSQLVING A PLUG FROM A BIMETALLIC METAL CORE AND ETCHING BATH Filed July 2, 1963 X y \WM 1 M 1 INVENTOR KENNETH P. BELLINGER a/a/M A T TOR/VE) United States Patent 3,261,733 METHGD 0F DISSOLVING A PLUG FROM A El- METALLIC METAL CORE AND ETCHING BATH Kenneth P. Bellingcr, Ellington, Conn, assignor to Conversion Chemical Corporation, Rockville, onn., a corporation of Connecticut Filed July 2, 1963, Ser. No. 292,325 6 Claims. (Cl. 156-3) The present invention relates to metal fabrication and more particularly to a novel bath and method for removing cores of iron, molybdenum and copper from initial assemblies with other metals.

This application is a continuation-in-part of copending application Serial Number 254,426, filed January 28, 1963, now abandoned.

In the fabrication of various metals such as titanium, zirconium, tungsten, aluminum and their alloys, it is often desirable to employ a core of another metal about which the primary metal is formed. For example, in forming nuclear reactor cores, plugs of iron or copper may be employed to define the desired apertures and titanium or zirconium is formed therearound. Subsequently, the core is dissolved in a bath of nitric acid. In forming filaments for light bulbs, tungsten wire is wound around an iron or molybdenum core and set; thereafter, the core is dissolved in a bath of nitric acid. Generally, such core-removing operations are characterized by a very high degree of exothermic activity, extremely rapid metal removal and copious fuming. The highly exothermic and rapid reaction has created serious problems in control of the operation, and fumes evolved by the operation have presented most significant problems from industrial hygiene and anti-pollution standpoints. Various scrubbers and treatments have been proposed and utilized at great expense and without complete efiicacy in an effort to cope with the problem.

The term core as used herein refers to inserts, mandrels and various shapes of dissolvable metals selected from the group consisting of iron, copper and molybdenum which are readily dissolved by nitric acid baths. Generally, low carbon iron or cold-rolled low carbon steel of low alloy content are the preferred ferrous materials to avoid contamination by alloying elements. Similarly, copper and molybdenum should 'be of low alloy content.

It is an object of the present invention to provide a novel bath for removing metal cores from initial assemblies with other metals formed thereabout and substantially inert to the bath which bath is characterized by rapid but relatively controlled action and substantially freedom from fuming.

Another object is to provide a method for removing metal cores from initial assemblies with other metals which is rapid, relatively controlled and substantially free from fuming.

Other objects and advantages will be readily apparent from the following detailed specification and claims and the attached drawing wherein:

FIGURE 1 is a diagrammatic illustration of apparatus in which a process embodying the present invention is being conducted.

It has now been found that the foregoing and related objects may be readily attained by forming about a dissolvable core of a metal selected from the group consisting of iron, molybdenum and copper a primary metal to produce an initial assembly; and thereafter immersing the initial assembly in a core-removing bath for a period of time sufiicient to dissolve the core, the core-removing bath comprising an aqueous solution consisting essentially of 15.0 to 42.0 percent by weight nitric acid, 2.0 to 28.0 percent by weight of a reaction-controlling agent selected from the group consisting of urea and melamine, and 40.0 to 80.0 percent by weight water. In operation, the

bath should develop a foam blanket for optimum effectiveness in eliminating nitric oxide fumes, but excessive frothing should be avoided to maintain effective core-dissolving action as by a cooling grid above the bath or by ultrasonic energy. This method has significant advantages in the forming of tungsten, zirconium, titanium and aluminum since it effects the speedy removal of the core in a relatively controllable reaction while being substantially innocuous to the formed primary metal thereabout.

The molar ratio of reaction-controlling agent to nitric acid is about 0.1 to 0.75:1, and preferably about 0.25 to 0.60:1 for iron and molybdenum cores and about 0.15 to 0.45 :1 for copper cores. When the nitric acid is in the range of about 35.0 to 42.0 percent by weight of the bath and/or when the iron or molybdenum core metal present in the bath is suflicient to expend substantially the nitric acid, it is generally necessary to use a molar ratio of about 0.3 to 0.621 to eliminate turning and maintain the reaction under control. When treating copper cores, it is generally necessary to use lower weight percentages and molar ratios of reaction-controlling agent since it appears to produce or favor the formation of a barrier coating on the copper core which limits the attack of the nitric acid. Use of the higher molar ratios of reaction-controlling agents may produce a significant increase in viscosity and/ or the development of a pasty condition due to the formation of a precipitate requiring cont-r01 of the frothing and/ or increase in temperature.

The bath temperature should be maintained in the range of about to 210 Fahrenheit, and most desirably about to 200 Fahrenheit. When more than about 10 percent by Weight of reaction-controlling agent is used, it is generally necessary to maintain a temperature of about 180 to 200 Fahrenheit to dissolve the agent and to reduce excessive viscosity in the bath. Temperatures of about 170 to 200 Fahrenheit are desirable for maintaining optimum effectiveness of the bath in removing copper cores. Although the bath generates heat during treatment, generally it is necessary to heat the bath initially to obtain full solution of or dispersion of the higher weight percentages of urea. Particularly when the bath volume is small or when continuing use is being conducted with iron or molybdenum cores, cooling coils may be desirably provided to prevent the bath from overheating due to the exothermic reaction and from possibly reaching the boiling point or from developing excessive froth.

The method of the present invention is rapid and highly exothermic but can provide greater control than a conventional nitric acid bath while eliminating substantially the evolution of nitric oxide. Since the reaction is controlled, nitric acid spray is reduced, and the hazards to the operator are minimized. Removal of as much as onethird of an iron core Within a minute in assemblies presenting relatively large surface area is possible with the preferred bath composition operating at the preferred temperature range. Thus, rack treatment of assemblies is possible with the time of immersion required being readily determinable by preliminarily treating a test assembly and visually determining the time required for complete removal of the iron core in the remaining assemblies. Moreover, it has been found that the nitric oxide fuming may be reduced from in excess of 300,000 parts per million to 300 parts per million, and even less. In this manner, high rates of production may be maintained at elevated bath temperatures without expensive fume-scrubbing installations.

The preferred baths of the present invention for iron and molybdenum cores consist essentially of about 28.0 to 38.0 percent by weight nitric acid, 47.0 to 54.0 percent by weight water and 8.0 to 20.0 percent by weight of the reaction-controlling agent with a molar ratio of reactioncontrolling agent to nitric acid of about 0.2 to 0.6: 1, and

most desirably of about 0.35 to 0.55: 1. For copper cores,

the bath preferably contains only about 4.0 to10.0 percent of reaction-controlling agent with a molar ratio of about 0.15 to 0.45:1.

Al-thohgh melemthe y he usedelehe Q p e the tially complete elimination in the treatment tank is de- IOWEI' molar ratios and Wltl'l urea In combination, urea sirable to ensure efiicacy of the bath peration and on. has e found far t e Preferable because of P h trol and to minimize health hazards resulting from poseperetlhg ehareeterlstles, e lower molecule" welght sible defects in the exhaust or scrubbing systems. With and greater eohthlhty t0 PYOVlde the more etteetlve hlghel" the core-dissolving baths of the present invention, scrubrnolar ratios. 0 bing the fume in water alone or water with some alkali Although the urea y he added Increments during is sufficient to remove the nitric acid fumes and any trace the operat n Of the bath, thls tq Preludes? lees amounts of other noxious gases such as nitrogen dioxide ettectlve bath and P e Substantial hazards slhee a since the predominant nitric oxide is apparently converted relatlvely t p reactlon takes Place When urea 18 added to nitrogen within the bath and its foam blanket, analyses t a hot nltfle e 1S0hltl0h- Le amounts of urea indicating the presence of nitrogen and carbon dioxide. y he used With lherelhehtal eddltlen of further amounts The initial assembly should be completely immersed with some loss in effectlveness when a plurality of cycles i hi h b h d b Covered b more h f a d Will fun in a glveh hath e the urea is added Slowly desirably more than six, inches of bath. In practice, the between ye The h 15 Preferably h e by deeper is the immersion, the more efficient is the fume first dissolving the urea in hot water because of 1ts l1rn1ted removal h bli can b k d or i di id ll ylowered into the bath.

The method of the P e lhvehtleh 1S rapld and hlghty Referring now to the attached drawing, therein diaeXethermle hh PYOVlde g e e t than a l grammatically shown is a process embodying the present ventional nitric ac1d bath while eliminating substantlally invention The Work tank 2 Contains h b h 4 i hi h the evolution of nrtrieoxlde fumes. The baths of the is immersed the initial assembly ggnerally designated b Present lhvehtloh hlhetloh ethelehtly even when the the numeral 6 and having a core 8 of a dissolvable metal face of the metal has an e t Settle and appear to with the primary metal 10 formed thereabout. As illushanee t aetlOIl 0f the hlttle held on m eores- Y trated, the assembly 6 is disposed well below the surface achlehhg cohtrol of the rate of the reachohg the exces of the bath 4 which has a foam blanket 12 of substantial sive frothing produced by gas bubbles within the bath depth thereoven Any fumes and Spray l i fr can be reduced so that the bath can function more eifi- ,the bath 4 are entrapped in h hood 14 and i d b ythe conduit 16 to the bottom of the scrubber :tank 18 so Although the theehehlem t the Present lhvehtloh 15 that they are washed thereby prior to being discharged hot y o d, it 1s be heved that the urea coopto the atmosphere by the conduti 20. As hereinbefore erates with the nitric acid wrthm the above-described comstated, scrubber may use Water alone, Water i .positlenhl hmlts at temPeraturjcs above Fahrehhelt 'ing urea, or water containing alkali to neutralize nitric by modifying the reaction taklng place at the bath-core acid fumes. inte f The rate of metal attack r rses rapidly W Illustrative of the eifect of varying the molar ratio increase n temperature to about 210 Fahrenheit and (and Weight percentage) of urea in a bath containing the solubility of urea increases (or the viscosity of the 40 50.0 percent by volume f nitric acid B and mixture decreases) to permit increase in its beneficial ac- 500 percent by volume of Water is Table In the tests tron. In contrast with conventional baths Wl'llCll have reported in Table 1, ferrous Specimens were immersed in a deep brown color after contact with the core, the baths 200 of the bath in a 500 graduate f one hour, of e Present lhvehhoh geherahy Tetalh a greehtsh hue 45 the bath having been preheated to the indicated temperaahd, In Place of a heavy froth geherhhy have a blanket ture. The ferrous specimens in these tests are plural of foam over the top of the bath. This foam presumably ieces of Wire of 0.196 inch diameter and about 0.25 inch Presents a large surface e of urea "h f'oam Wah cut on a diagonal. The wire was of 1013 alloy nominally acting to increase the l1kel1hood of reaction with fumes composbd f 14 percent b i h carbon, percent e p g from e y of T team layer 50 by weight manganese with the remainder substantially initially increases in depth with increase 1n urea conceni ith ulf r and phosphorus impurities. The prestration and also appears to increase in viscosity to a point en e or ab n of brown nitri oxide fumes above the of relatively highly concentration after which the foam bath in the graduate and weight loss are noted as evilayer seems to stabilize or decrease in depth. dencing the efficacy of the baths of the invention.

TABLE 1 HNO H2O, Urea, Molar Tempera- Time for Main Re- Wt, Percent Percent Percent Ratio Steel, ture, action to Subside Filming Loss,

by wt by wt. by wt. Urea: Grams Degrees or Steel to Dis- Gram IINOa F. solve, Minutes 39. 2 60.1 0. 7 0. 02; 1 20 180 39. 2 60.1 0. 7 0. 02=1 4 180 39. 0 59. s 1. 2 0. 04; 1 20 180 39. 0 59.8 1. 2 0. 04:1 4 180 33. 7 59.1 2. 2 0. 00; 1 20 180 38.7 59.1 2. 2 0. 05:1 4 180 37.8 57.3 4.4 0.131 20 180 do 15.3. 37. 8 57. 8 4. 4 0.13:1 4 180 10 (c0nsun1ed) Heavy 3 mm, diminished to Total light or none. 36. 3 55. 3 8.4 0.25:1 20 1B0 40 }Heavy 5 min., diminished to 16.4. 36. 3 55. 3 8. 4 0.25:1 4 180 10 tconsumedym. medium, then to light. {Total. 35.0 53.5 11.4 0. 35:1 20 180 Medium, diminishing to light {17.3. 35. 0 53. 6 11. 4 0. 3511 4 180 after 1 minute. Total 33. 6 51. 6 14. s 0. 50:1 20 Slight iorao seconds. 17. 0. 33.6 51.6 1 1.8 0.50:1 4 190 10 (consumed). do Total 39.5 60.5 20 12 (c0nsumcd) 1 Frothed out of graduate.

Illustrative of the efficacy of the present invention are the following specific examples:

Example 1 A bath was prepared by admixing 100 cc. of nitric acid (42 Baum) and 100 cc. of water. A slug of low car- TABLE 2 HNOE, H20, Urea, Molar Tempera- Time for Main Re- Wt, Percent Percen Percent Ratio Steel, ture, action to Subside Fuming Loss,

by wt. y wt by wt Urea: Grams Degrees or Steel to Dis- Grams HNOa F. solve, Minutes 1 Froth spilled out of graduate.

In Table 3 are set forth the results of similar tests on copper using as specimens pieces of #12 gauge wire to present a relatively high surface area.

bon steel (No. 1013, containing 0.14 percent by weight carbon and 0. 65 percent by weight manganese) weighing 25.3 grams was immersed in the bath which had been TABLE 3 HNO3, H1O, Urea, Molar percent percent percent Ratio, Cu, Temp, Fuming Wt. Loss,

by wt. by wt. by wt. Urea: G. F. Grams I-INO;

39. 2 60.1 0.7 0. 02:1 33 180 1 18.8 39.0 59. 8 1. 2 0. 04:1 33 180 1 18.3 38. 7 59.1 2. 2 0. 06:1 33 180 1 25.0 37. 8 57. 8 4. 4 0.13:1 33 180 2 17. 9 37. 8 57.8 4. 4 0.13:1 11 180 Light, 5 min Total 36. 3 55. 3 8. 4 0. 25; 1 33 180 Med, 2 min. 15. 2 35.0 53. 6 11. 4 0. :1 33 180 None.-. 3 7. 5 39. 5 60. 5 33 180 Heavy 29. 6 35. 0 53. 6 11. 4 0. 35:1 33 4 190 None... 3 9.0

1 Spilled out of graduate badly. 2 Spilled out to small amount.

3 Green crystals formed in graduate during reaction and copper became coated.

' Heat continuously applied.

Illustrative of the several variables of the bath of the present invention on activity are the data set forth in Table 4. Test specimens of SAE 1010 cold-rolled steel 1 inch x 1.5 inches x 0.030 inch and weighing 5 grams were suspended in aqueous baths of the indicated composition at the temperatures and times indicated. Within the preferred range of bath composition, even initial fuming during the short period of immersion was eliminated. It will be noted that amounts of nitric acid or urea in excess of the aforedescribed compositional limits greatly affected the activity.

place-d in a 500 cc. graduate and preheated to 180 Fahrenheit. The reaction was accompanied by violent pumping and agitation with a froth reaching nearly to the top of the cylinder. Extremely copious brown fumes rose from the bath which had a dark brown color. After about fifteen minutes, the reaction subsided appreciably and after one hour the weight loss was found to be 17.9 grams.

TABLE 4 HNOJ, H 0, Urea, Temp. Time, Weight percent percent percent Increase, See. Loss, Fuming by wt by wt. by wt. F. percent 1 49. 5 24. 2 26. 3 160-180 Paste Paste. 1 42.3 35. 4 22. 3 160-180 13 8 Heavy. 1 42.3 35. 4 22. 3 180-200 16 14 Do. 1 44. 3 44.0 11.7 160-180 12 18 D0. 1 39. 7 39. 5 20. 8 160-180 15 15 D0. 1 40.8 48. 5 10. 7 160-180 12 0 Do.

36. 8 43. 8 19. 4 160-180 20 12 Moderate. 35. 6 42. 5 21. 9 180-190 13 Very Slight. 38. 2 51. 7 10. 1 160-180 10 14 Heavy. 38. 2 51. 7 10. 1 180-200 18 18 Do. 34. 8 46. 9 18.3 160-180 20 21 None. 34. 8 46. 9 18. 3 180-200 12 14 Do. 34. 6 41. 1 24. 3 180-190 60 12 Do. 32. 6 50. 3 17. 1 160-180 12 13 D0. 32. 6 50. 3 17. 1 180-200 18 27 Do. 19.3 70. 5 10.2 160-170 11 Very slight. 19. 3 70. 5 10. 2 180-190 60 13 None. 16. 8 78. 8 4. 4 -170 30 6 Do. 16. 1 75. 5 8. 4 160-170 30 8 D0.

1 These formulations were very pasty even at elevated temperatures due to a precipitate.

A similar test was conducted with a bath additionally containing 42.0 grams of urea and heated to 190 Fahrenheit initially. In this instance, a foam blanket was generated which extended to about four inches from the top of the graduate. The bath retained a greenish hue throughout and only very light fuming occurred for the first two minutes during the development of the foam blanket and thereafter there was no evidence of fuming. The reaction subsided substantially after twenty-tfive minutes and, after one hour, the weight loss was determined at 16.1 grams.

Example 2 A bath was prepared by admixing 200 cc. of 42 Baum nitric acid and 200 cc. Water. A test specimen of SAE 1010 cold-rolled steel 1 inch x 4 inches x 0.030 inch and weighing 18 grams was immersed in the resultant bath at an initial temperature of 180 Fahrenheit. The specimen was withdrawn after the bath had reached a temperature of 200 Fahrenheit, a time period of about thirteen seconds. The test specimen was weighed and determined to have incurred a 26 percent weight loss. During the treatment operation, there was extremely copious fuming.

To a bath of the same composition was added 50 grams of urea and a similar test specimen was immersed therein at a temperature of 180 Fahrenheit and withdrawn after the bath temperature had reached 200 Fahrenheit, a time period of sixteen seconds. The test specimen was weighed and found to have incurred a 23 percent weight loss. During the treatment, there was but a slight evolution of fume.

Another bath was prepared by adding 100 grams urea, and the test was repeated. The time for the bath temperature to rise from 180 to 200 Fahrenheit was twenty-eight seconds, and the weight loss of the specimen was 29 percent. No fuming occurred during the treatment.

Example 3 A bath was prepared by admixing 500 cc. of 42 Baum nitric acid, 500 cc. water and 125 grams melamine. A test specimen of cold-rolled steel weighing 18 grams was immersed in the bath at an initial temperature of 180 Fahrenheit and withdrawn therefrom after the bath had reached a temperature of 200 Fahrenheit, a time period of about forty seconds, during which there was extensive fume evolution.

The test was repeated with a bath containing 250 grams of melamine with the time period for the temperature to rise from 180 to 200 Fahrenheit being about fifty seconds. The treated specimen was weighed and found to have undergone a weight loss of 17.5 percent. During the period of immersion, there was but a very slight amount of fuming.

Example 4 Tungsten wire was wound about a core of molybdenum of variable diameter tapering to a point to form an initial assembly. The resulting assembly was immersed in a bath containing 50.0 percent by volume 42 Baum nitric acid, 50.0 percent by volume water and 250 grams urea per liter and operated at a temperature of about 160 to 180 Fahrenheit.

The molybdenum core was dissolved out with substantially no fume evolution and the tungsten wire was not attacked.

Example 5 A nuclear reactor component was produced by forming Zircalloy (nominal composition1.5 percent tin, 0.15 percent iron, 0.10 percent chromium, 0.05 percent nickel, and zirconium) about low carbon steel cores to define apertures therein.

The component was then immersed in a bath essentially the same as that in Example 4 and operated at a temperature of about 160 to 180 Fahrenheit. The steel cores were dissolved out with substantially no fume evolution. l

a the reaction was less violent.

8 Example 6 A bath was prepared by admixing 100 cc. of nitric acid (42 Baum) and 100 cc. of water. Apparatus was provided to collect fumes evolved from the bath and wash them in a water scrubber provided by an Erlenmeyer flask. The bath was preheated to 180 Fahrenheit and a low carbon steel plug of 25.3 grams was placed therein. The reaction was violent and heavy brown fumes flowed from the bath and through the scrubber. After about twelve minutes, the reaction slowed appreciably and after one hour, the Weight loss was found to be 17.9 grams.

A similar test was conducted using a bath additionally containing 22 grams of urea. Although the bath frothed, Moderate fuming for the first five minutes diminishing to light was evident in the bath vessel, but little or no fumes were evolved from the water scrubber. After one hour, the weight loss was found to be 16.4 grams.

Example 7 A bath was formulated by admixing 75 cc. of nitric acid (42 Baum), 75 cc. of water and 12.5 grams of urea. A specimen of molybdenum weighing 1.6 grams was completely dissolved in eighteen minutes in a rapid but controlled reaction. In a similar test bath containing no urea, heavy brown fumes were evolved.

Example 8 A bath Was formulated containing 50 percent by volume nitric acid (42 Baum) and 50 percent by volume Water. To 300 cc. of this formulation which had been preheated to 180 Fahrenheit was added pieces of #12 gauge copper wire. Copious brown fumes were evolved and the reaction was violent.

A similar test was conducted with a bath additionally containing 24 grams of urea. Slight fuming occurred only initially. After one hour, 37.8 grams of copper was dissolved; after continuing immersion overnight, 39.8 grams was dissolved.

Thus, it can be seen that the method and bath of the present invention permit speedy but controlled removal of core metals of an initial assembly. Bath activity in some instances may be enhanced and in other instances is not extensively aifected, but bath action is more controlled and fume evolution is substantially eliminated to reduce hygiene and fume-disposal problems.

Having thus described the invention, I claim:

1. The method of forming metals substantially inert to concentrated nitric acid comprising forming about a core of a dissolvable metal selected from the group consisting of iron, molybdenum and copper a primary metal substan tially inert to nitric acid to produce an initial assembly; and immersing said initial assembly in a core-removing bath for a period of time suificient to dissolve said core, said bath being at a temperature of about 140 to 210 Fahrenheit and comprising an aqueous solution consisting essentially of about 15.0 to 42.0 percent by weight nitric acid, 2.0 to 28.0 percent by weight of a reactioncontrolling agent selected from the group consisting of urea and melamine, and 40.0 to 80.0 percent by weight water, said bath having a molar ratio of reaction-controlling agent to nitric acid of 0.1 to 075210, the immersion of said initial assembly in said bath producing a blanket of foam on the top of said bath through which fumes must pass.

2. The method in accordance with claim 1 wherein said substantially inert metal is selected from the group consisting of titanium, Zirconium, tungsten and alloys thereof.

3. The method of forming metals substantially inert to nitric acid comprising forming about a core of dissolvable metal selected from the group consisting of iron, molybdenum and copper a primary metal substantially inert to nitric acid to produce an initial assembly; and immersing said initial assembly in a core-removing bath for a period of time sufiicient to dissolve said core, said bath having a temperature of about to 200 Fahrenheit and comprising an aqueous solution consisting essentially of about 28.0 to 38.0 percent by weight nitric acid, 8.0 to 20.0 percent by weight of a reaction-controlling agent selected from the group consisting of urea and melamine, and 47.0 to 54.0 percent by weight water, the molar ratio of reaction-controlling agent to nitric acid being selected from the group consisting of (A) 0.35 to 0.55:1.0 when said dissolvable metal is selected from the group consisting of iron and molybdenum; and (B) 0.15 to 0.45:1.0 when said dissolvable metal is copper, the immersion of said initial assembly in said bath producing a blanket of foam on the top of said bath through which fumes must pass.

4. The method in accordance with claim 3 wherein said substantially inert metal is selected from the group consisting of titanium, zirconium, tungsten and alloys thereof.

5. The method in accordance with claim 1 wherein UNITED STATES PATENTS 2,177,751 10/1939 Sikorski 15618 X 3,192,084 6/1965 Vaughen et al 1563 ALEXANDER WYMAN, Primary Examiner.

JACOB STEINBERG, Examiner.

Claims (1)

1. THE METHOD OF FORMING METALS SUBSTANTIALLY INERT TO CONCENTRATED NITRIC ACID COMPRISING FORMING ABOUT A CORE OF A DISSOLVABLE METAL SELECTED FROM THE GROUP CONSISTING OF IRON, MOLYBDENUM AND COPPER A PRIMARY METAL SUBSTANTIALLY INERT TO NITRIC ACID TO PRODUCE AN INITIAL ASSEMBLY; AND IMMERSING SAID INITIAL ASSEMBLY IN A CORE-REMOVING BATH FOR A PERIOD OF TIME SUFFICIENT TO DISSOLVE SAID CORE, SAID BATH BEING AT TEMPERATURE OF ABOUT 140 TO 210* FAHRENHEIT AND COMPRISING AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF ABOUT 15.0 TO 42.0 PERCENT BY WEIGHT NITRIC ACID, 2.0 TO 28.0 PERCENT BY WEIGHT OF A REACTIONCONTROLLING AGENT SELECTED FROM THE GROUP CONSISTING OF UREA AND MELAMINE, AND 40.0 TO 80.0 PERCENT BY WEIGHT WATER, SAID BATH HAVING A MOLAR RATIO OF REACTION-CONTROL LING AGENT TO NITRIC ACID OF 0.1 TO 0.75:1.0, THE IMMERSION OF SAID INITIAL ASSEMBLY IN SAID BATH PRODUCING A BLANKET OF FOAM ON THE TOP OF SAID BATH THROUGH WHICH FUMES MUST PASS.

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