US2940838A - Chemical milling - Google Patents

Description

7;. QM/7M M, it/@+Mww June 14, 1960 H. B. sNYDER ErAL 2,940,838

Cl-IEMICAL MILLING Filed Aug. 19, 1957 FIG.6

' design is silverware. to form a roughened surface. In order to specifically in- United States Patent O CHEMICAL MILLING Herman Ben Snyder and Ludwig M. Rosenberg, Seattle,

Wash., assignors to Boeing Airplane Company, a corporation of Delaware Filed Aug. 19, 1957, Ser. No. 678,882

8 Claims. (Cl. 4142) This discovery relates to the chemical milling of metal in an acid medium and, more particularly, to the chemical milling of age hardened, precipitation hardened and other heat treatable stainless steels in all their solid phases in an acid medium.

Prior to specifically discussing chemical milling we wish to point out the manner in which it distinguishes from pickling, brightening, decorative design and surface increase. Chemical milling may be considered to be controlled corrosion or controlled metal removal to form Sculptured metal configurations. In chemical milling a relatively large percentage of the metal may be rapidly removed so as to leave a minor amount of the original metal in a new configuration. As contrasted with this is pickling or scale removal whereby as much as possible of the oxide and other coating of the metal are removed but as small amount as possible of the metal is removed. In other words in pickling only the surface coating of the metal is removed. In brightening or surface polishing a minimum amount of the metal is removed to form a reflective surface as the scale has been previously removed. Decorative designs are of a shallow depth with approximately five mils of surface removed by chemical action. In decorative design part of the surface is masked and the design is inscribed through the masked coating. Next, a chemical solution is applied to the masked material and allowed to react with the metal to form the design. An example of decorative In surface increase the object is crease the surface area the same is purposely pitted by chemical action. This is done to make the surface rough enough for bonding purposes such as a metal to metal bond, a base for paint and .the manufacture of condenser plates to name a few of the uses.

The mechanical milling of metal is well-known and extensively used throughout the metal working industry. Even though mechanical milling is extensively used it inherently has certain limitations. One of these limitations is the expensive capital investment for milling machines. Such a machine, capable of making large, complicated configurations, may cost in the hundreds of thousands of dollars. In addition to the capital investment it is necessary to have skilled workmen operating these machines in order to do a creditable job on the milling of metal. Also, because of cost and manhour requirements this type of milling is limited to certain configurations, namely, those configurations which are of a more` simple or open design. However, by the skillful use of welding it is possible to build these limited configurations into an intricate design. Because of the nature of the weld and the inherent weakness therein it is not always advisable to build these limited basic configurations into an intricate design as the weld might rupture or the added weight become excessive. Furthermore, with mechanical milling there usually results a relatively rough surface. Because of excessive cost the tolerance, in a recessed area, is not a close one but must be a rather large one such as in the hundreds of an inch. Also',- on some of the newer metals it is impractical, because of cost due to equipment and to the time of milling involved, to mechanically mill the surfaces as the metal is extremely tough and resistent to mechanical cutting and/or rapid abrasion. For example, only casting and rough grinding can be employed with certain metals.

In recent years, as a supplement to mechanical mill-V ing, there has developed chemical milling. One of the rst patents to be issued for chemical milling and the like is the patent to Wilson et al., United States Patent Number 2,684,291. Wilson et al. describe the emboss ing of steel with an acid medium. Basically, this patent presents the method of taking flat stock, masking certain predetermined areas or portions of this stock with an acid resistant covering, and etching the masked stock with the acidic solution. The acid solution eats away or corrodes away the unmasked areas to form the desired design. Naturally, it is possible to remove the stock from the acid solution, remove the masking cover, add new masking cover, and insert the stock again in the solution. By such means it is possible to form intricate designs in metal. With our discovery for the use of a strong acid solution in chemical milling we find it possible to realize a more rapid action than Wilson et al. realize with a weak acid solution; and to mill steel into predetermined designs and configurations while Wilson et al. are restricted to only surface change. Also, our strong acid milling solution makes it possible to work with equal ease on martensitic steels be they age hardened, precipitation hardened, or heat treated stainless steels.

To further illustrate some of the advantages of chemical milling it is possible to achieve the same output by chemical milling with a lower capital investment than for an equal output with mechanical milling. Also, in chemical milling the application of masking material to the part to be formed requires semi-skilled labor as contrasted with the requirement of skilled labor for mechanical milling. And, more intricate configurations can be formed more rapidly by chemical milling as the metal to be milled can be immersed in the reactant solution so that all parts of the metal can be acted upon simultaneously. In a specic illustration as applied to the manufacture of airplanes it is possible to reduce the weight of certain members of the airplane, as an example, stainless stgel sheet stock may come in a thickness of 0.093 inch; however, from a design standpoint a thickness of 0.086 inch may be adequate. The original stock would not be reduced from the thickness of 0.093 inch to the thickness of 0.086 inch by mechanical milling as the cost would be too great, especially for heat treated stock. However, it is possible by chemical milling to immerse the stock in a chemical reactant solution and reduce it to the thickness desired with ease and economy. In large airplanes this reduction in thickness may mean a considerable saving in the weight of the air frame. More particularly, for every excess pound carried by the airplane there may be required from tive to ten pounds of additional gross air frame weight. Therefore, if in the larger airplanes a thousand pounds of unneeded material can be saved the air frame weight can be reduced by approximately 5,000 pounds to 10,000 pounds.

In another manner the configuration of a chemically milled metal can be more economically achieved than by mechanical means. For example, in the case of a wale pattern, the necessary chemical milling can be relatively easily performed as the masking materials can be placed over the metal and the metal not protected by this masking material can be chemically eaten away or chemically corroded. In a like manner an object can be tapered by removing the same slowly from the chemical milling solution. Also, an object having a large number of irregularly placed recessed areas can be readily manufactured by chemical milling while the preparation of a similar object by mechanical means would be time-consuming and ditiicult to prepare.

With this in mind it is an object of this discovery to provide a method for the chemical milling of metal, examples being intricate configurations, taper, and wallie pattern.

A further object is the provision of a method for the chemical milling of a metal and, particularly, for the chemical milling of age hardened, precipitation hardened and other heat treatable stainless steels in all their solid phases.

A further object is to provide a method for the chemical milling at a relatively uniform and controllable rate.

An additional object is to provide a solution for the chemical milling of metal and which solution can be controlled by standard methods of chemical analysis.

Another important object is the provision of a process for the chemical milling of metal and which process provides satisfactory surfaces having well defined edges at better than standard machine tolerances.

A still further object is the provision of a process for chemical milling resulting in satisfactory structural and mechanical properties of the milled metal.

A still further and important object is to provide for the chemical milling and which chemical milling results in a satisfactory smooth surface.

Another object is a provision of a process for chemical milling and which process produces a minimum of smut.

A further object is the provision of a process for chemical milling which is less expensive than mechanical milling and makes it possible to produce more intricate configurations than mechanical milling.

In the drawings:

Figure 1 is a plan view of a curved structural member illustrating two rectangular-shaped recesses in the open surface thereof.

Fig. 2 is a longitudinal vertical cross-sectional view taken on line 2 2 of Fig. 1 and shows the recesses in the curved member.

Fig. 3 is a view of a mask which is placed over the non-milled structural member in Fig. l and protects the upper surface of said structural member, except for the two'rectangular shaped openings therein, from being attacked by the milling solution.

Fig. 4 is a longitudinal vertical cross sectional view Vof this curved raw material from which the structural member is prepared and illustrates this raw material as being covered with a protective masking coating except where the rectangular shaped recesses are to be formed.

Fig. 5, on an enlarged scale, shows the formation of a recess in metal with the aid of a masking material coverving part of the metal.

Fig. 6 is an end elevation view of a curved structural member prior to chemical milling.

Fig. 7 is an end elevation view of the member in Figure 6 after being chemically milled to form a section for use in fabricating a fuel tank, and is taken on line 7-7 of Figure 8; and,

Fig. 8 is a plan view looking down on the chemically milled section of Figure 7.

In our chemical milling solution we provide an aqueroded by the chemical milling solution, i.e., all of the alloying elements of the steel are etched at the same rate as the iron or the base material of the steel. The hydrochloric acid appears to play a very active role in the chemical milling process. Also, the combination of the hydrochloric acid and the nitric acid appears to result in the aqua regia effect and plays an active role in the chemical milling The addition of ferrie chloride to hycpfrate. This is espeally so in regard to Bie corrosion of the iron in the steel. However, the ferrie chloride, hydrochloric acid and nitric acid apparently react with the iron in the steel more quickly than they react with the alloying elements such as silicon, nickel and molybdenum. Therefore, to increase the reaction rate of the composition with these alloying elements there is added ammonium chlorostannate or a mixture of stannic chloride and ammonium chloride. The addition of ammonium chlorostannate or the ammonium chloride and stannic chloride mixture makes it possible for the milling solution to substantially uniformly corrode the martensitic steel. Therefore, both the base material, iron, and the alloying elements such as silicon, nickel and molybdenum are evenly eaten or corroded away. The moderator or buter, having an ionization constant in the range of 10-4 ltl-e with respect to the primary ionization constant of the hydrogen ion, appears to play the role of prolonging the active life of the milling solution. More particularly, in a solution not having a moderator present the milling rate decreases almost immediately and continues to decrease at a substantially uniform rate throughout the life of the chemical milling solution. As is readily appreciated, with this decrease in the chemical milling rate the controlling of the milling or corrosion of the metal being reacted upon is difficult. Also, there is an economical loss in the prolonging of the period of time the metal is in the solution as the milling bath is not being advantageously employed. However, with the incorporation of the moderator the reaction proceeds at the same uniform rate for a relatively long period of time. Again, it is seen 4that with -the reaction at a relatively uniform rate for a long period of time that it is possible to more readily control the milling of the metal thereby realizing a more desirable product as well as being economically feasible to mill the metal. In regard to the wetting agent, such as an alkyl aryl sulfonate, i.e., dodecyl benzene sulfonate, this material makes it possible to achieve a better surface. The wetting agent probably functions by reducing the surface tension soas to allow the gases produced by the action of the solution on the metal to more readily escape from the surface and therefore to permit a more uniform concentration of milling solution to react with the surface of the metal. It is realized that if the gases do not readily escape then that portion of the metal covered by them will not be reacted upon the same as that portion not covered by the gase bubbles and therefore there would result an uneven or a less smooth surface.

In the milling solutions we prepared the nitrate was added as hydrogen nitrate or nitric acid, the chloride as hydrogen chloride or hydrochloric acid, the acetate as hydrogen acetate or acetic acid, the citrate as hydrogen citrate or citric acid, the borate as hydrogen borate or boric acid, and the oxalate as oxalic acid or hydrogen oxalate.

In the milling process there are a number of factors determining the milling rate. Some of these factors are temperature, concentration of components, heat transfer, evaporation, agitation, volume of milling solution to surface area of metal, and so forth. Briefly, it is readily realized that the role of the temperature is very important. Generally speaking a ten degree rise in temperature, on the Kelvin scale, indicates a doubling in the reaction rate. Therefore, it is possible to achieve with a 'relatively dilute or weak milling solution a milling rate at a higher temperature equal t the milling rate of a more concentrated solution at a lower temperature. Also, it is apparent that the concentration of the reactant plays a very important and vital role in the chemical mill- 6 trated ntn'c acid of a specic gravity of approximately 1.42 and of a concentration of 15S-70%; hydrochloric acid of a specific gravity of approximately 1.24 and a concentration of 38%; hydrofluoric acid of a specific ing. In conjunction with the temperature effect it is gravity 0i 1-19 @11d 50% by Weighf gia-Cial acetic mid possible by the use of a concentrated mining solution to 0f s spcltc gravity 0f 1-055.. Also, there were preparqd achieve at a lower temperature the reaction rate equal to a func acld solution an Oxahc acfd solutlon and a bone that of a relatively dilute solution at a higher temperaacldllltwn- These three solutions were. prepared by ture. This is of importance in the fact that a control of heating wat? to s bolhrfg Pqmt and adding an exces the milling can be obtained by control of the tempera- 10 f c1mc amd to one bone ald to another and oxahc ture. Heat transfer is of utmost importance as good heat acid t 'the other' The Solutions were alimfed to cool transfer precludes the possibility of a local buildup of 1 room tempflature and the superman? himd decamed temperature and also is essential for the proper control from th? ,Precllltate Example .of preplpltatlon hardened of the milling rate. The local buildup of temperature lfnlrtenmc Stiuies and femm stainless steels are as means that in that particular area the reaction rate will owi'. prclpnauogoharilned 17"7PH and n fiPH be higher and more metal will be removed or corroded cnlplnsm non cfu n s1 .won nfganesel chromium away. This uneven removal of the metal can result in 211%6 s cpartensmstal. estsh stee s. are hopes uneven surfaces, pitting and tapering. Evaporation is of Smcon maanese an mothr llanrs ae importance in that the evaporation of a vital component, viz., such as a chloride, means that the reaction rate will tsyges 13114 44g43gss"s44octh S'S" I 'CIF change in the life of the milling solution and the uniformo'f art "st ls 'hansAE 4000 er xanlg ity of the reaction rate cannot be as closely controlled and 434% l c ee are e senes Suc as als if evaporation had not taken place. Of great importance in the chemical milling of metal is the agitation of emmgnovfthhna isspetgleemafigis n: the solution. A well agitated solution gives more unithese exam les are b wa of illustration and teach; form results than one which is not agitated. This agitaand are notpmeant to e linitations on the invention I tion may bein the form of mechanical agitation whereby the main these examples were nm in a series of four the metal undergoing the milling action is vibrated or or ve The concentration of the components was varied shaken back and forth in the milling bath. Another form to detrmine the inuence of the different components on of agitation may be that of the milling solution itself the reaction rate In order to arrive at reaction rates for whereby it is agitated by gas bubpling through it' .Anq average temperature it was decided that the base tema still further and d esirable form is that of ultrasonic vipermute should be in the range of 150 1 55 F. In Some Elrtllii ghb'olillltgill'lffgutn'eunldgnrs instances the temperlature of the reaction solution was a roximate 140 In th action so as to break up any localized variations in the ge of 160170 p. Andinegrlstnsola; 3;: solution'concentraion. tllliis alrsfo servetoilivoid gas about 200 F. I t the temperature were approximately cllmu M1011 0n me a SU ace; u emolfe e 140 F. the reaction rate was increased by about 25% um o e vol e f the r acting chemical milling solution in pro to convert n to the rango of about 150.155 E Llkoportion to thebsurfce aref Ofblil material Undelf'oillg 40 wise, if the temperature were in the range of 160-175" F. reaction can c O COUSI era impoftan t e the milling rate was decreased about 25% to brin it volume to surface ratio is rather 10W it iS PQSSibie that in the 15G-155 F. range. It is to be appreciated hat the reactive components available at the milling surface this is an approximation In tho .tblos the milling foto will be rapidly exhausted and therefore there will be an for the iodio-ated temperature is given and the milling uneven reaction rate and possibly an uneven removal of rato for the base .temperature rango is givoo in parenthe metal aieclng the quality 0f the finished Product 45 theses. The metals milled bythe various solutions were Therefore, a large ratio of volume to surface area will AM350 and 17.7PH, The smoothness of .tho mined give a more constant reaction rate and thus permit closer moral is expressed in root mean square terms, R.M.S., tolerances on the finished product. and the unit of measurement is micro inches of amplitude To more particularlyillustrate our discovery and espeof surface variation. cially the effects of the variation of the -concentration of In Table I there are presented four dilerent solutions components and also the effect of the variation of the temand the results of milling with these solutions. These perature we herewith present specific detailed examples. solutions basically comprised water, hydrochloric acid These examples are to be taken for illustrative purposes and ferrie chloride. In solution D there was also nitric only and are not to be taken as limitations on the in- 5r acid. In this table the hydrochloric and nitric acids vention. In these examples the specifically enumerated are expressed in terms of normality and the ferrie chloride components were utilized. More particularly, concenis expressed in molarity. These solutions were used to Table I A B C D Variables Wt. Per- N Wt. Per- N Wt. Per- N Wt. Percent N cent cent cent Hoi as 12.1 1.45 19.8 2.58 18.5 2.58 o ((d) 9.8 0.09 10.0 3.7 22.3 i as n.01 18.2 .2 "air :1:::2 631% P- T? 100.1 100.0 100.3 100.1 ieasss'rr i43350'-" ii350-" trimm dso '"H" Mining Rat'e (ixii's 0.06664'I1 AM350- 00602.-... 0.066`(1&i01'3`50')';"` mi' (150 F (iggoii (150 F uiiiiioaosi (isb-Jim (iggoig, F., R.M.s s0-100 10o-120---. iis-'50...)

mill AM350 at approximately 140 F. Comparison of solutions A, B and C shows that C milled faster than B which in turn milled faster than A. More particularly, C milled at about 0.0002 inch per minute as compared with B at 0.0001 and A at 0.00004 inch per minute. The concentration of the ferric chloride was increased from A through C. And, the concentration of the hydrochloric acid in both C and B was approximately 50% greater than in A. Turning; now .to solution D it is seen that the addition of the nitric acid considerably increased the reaction rate. Solution D may be considered to be very similar to solution B with the replacement of some of the water with nitric acid. The milling rate of D was about 0.0006 inch per minute as compared with approximately 0.0001 inch per minute for B. This increase in the reaction rate may be considered to be partially due to the increased total acid concentration in D. In summary, it may be stated that an increase in the concensolutions .the concentrations of all the components were in the same range, varying from about 2.35 to about 3.36 normal for the hydrochloric acid, Afrom about 0.69 to about 0.75 normal `for the nitric acid, and from about 1.61 to approximately 2.3 molar for the ferric chloride. lFrom this it is possible to state that a moderate variation in the concentration of the components does not appreciably affect the milling rates of the solutions. However, turning to solution D it is seen that the concentration of the nitric acid was increased appreciably to about 1.44 normal. The milling rate for this solution increased to about 0.0005 inch per minute or about 60%. Again, it is seen that an increase in the concentration of the nitric acid within reasonable limits, appreciably increases the milling rate. An alkyl aryl sulfonate was employed to assist in the reaction. More particularly, to permit greater penetration of solution to .the surface of the metal.

Table II A B O D b vm l wa. P6:- N Wsrer- N wsrer- N WsPer- N cent cent cent cent Meta] AM350-... AM350.-. AM350 AM350..-. Temperature, F 140 140 140 un Ml11lngRate(inches/m1n.) 0'0003 o R.M.S 70 70 tration of vthe ferrie ion increases the milling rate. Also, an increase in the 4total acid concentration increases the milling rate. As a sidelight, solutio'n D was used to mill 17-7PH steel as well as AM350. The milling rate of 17-7PH steel was approximately 0.00035 inch per minute as compared with 0.0006 inch per minute for AM350.

Turning now to Table II another series of solutions was prepared comprising water, ferrie chloride, hydrochloric acid and nitric acid. The concentrations of the components in 4these solutions were varied to bring forth the effects of the components on the milling rate. In this table the concentration of hydrochloric and nitric acids is indicated by normality while the concentration of the ferrie chloride is expressed by molar-ity. A comparison of the milling rates of solutions A, B and C on AM350 at about 140 F. shows that they were substantially the Turning now to Table III another series of milling solutions was prepared. In this series the concentration of the nitric acid was varied over a Wide range. Again the components were water, ferrie chloride, hydrochloric acid and nitric acid. The hydrochloric and nitric acids were expressed in terms of normality and the ferrie chloride in terms of molarity. The metal milled was AM350 and at a temperature of about 140 F. In this series of solutions solution A was solution D of Table I. The concentrations of the hydrochloric acid and the ferrie chloride were maintained in about the same range, i.e., the hydrochloric acid varied from about 1.98 normal to about 2.35 normal and the ferric chloride varied from about` 4.42 to 1.68 molar. However, the concentration of the nitric acid was increased from 1.44 normal in solution A to 3.64 normal in solution E. The milling rate of solusame, i.e., about 0.0003 inch per minute. In these three tion A was the lowest, 0.0005 inch per minute. In solu- Table III .4 B o D E Variables Wt. Per- N Wt. Per- N Wt. Per- N Wt. Per- N Wt. Per- N cent 0011i; 0B Cent Dent nc1(as%) 16.7 2. 35 15.9 2.24 15.2 2.16 14.6 2.07 14.0 1.98 tgcl). (g1g .76% 1.22 1.64 17.6 1.48 16.9 1.42 11a/fm1 maar.; 1: :1 :1 M3 mi if?. 24:? L 11,0 53.6 51.1 48.9 46.9 44.9 l-f 100.1 99.9 100.0 100.0 99.9

Metal AM350. A141360... AM350 AM 5 Temperature GF.) 140 140 140 140 3 O iiiom Mining Bate (inches/mm.) 0. 0005.--.. 0.00067 0.00082 000053-... 0.00066 an... 90s-.1. 2 150 RMR m. M. gooo) 6{151.00070} ,action rate.

tion B the milling rate was 0.00057 inch per minute, a slight increase over solution A. In solution C, with a concentration of nitric acid at 2.63 normal the milling rate was the highest for the series, 0.00082 inch per minute. With a further increase in the concentration of the nitric acid, see solutions D and E, the milling rate decreased to approximately that for solutions A and B. This indicates that the increase in the concentration of nitric acid is beneficial to a certain concentration and about 1Z0-140 F. Solution A was the fundamental so lution. Solution B was similar to solution A except that the concentration of the ferrie chloride was increased approximately by one third. Solution C was similar to solution A except that there was added ferric nitrate so as to increase the concentration of the fern'c salt. And, solution D was also similar to A except that there was added ferric sulfate to increase Athe concentration of the ferrie salt. The milling rate of all of these solutions was then it is of no further value and actually retards the re- 10 in the same range, i.e., 0.0003 to 0.0006 inch per minute.

TableIV .1. B c D Variables Wt. Per- N Wt. Percent N Wt. Per- N Wt. Per- N 0811i; Cent cent Home?) 160 2.34 155 2.34 15.6 2.34 15.6 2.34 F0011( 202 1.65 252 2.24 18.9 1.65 18.9 1.65 HNO| 6870 l,) 95 1.43 90 1.43 8.9 1.43 8.9 1.43 0:1(M 6.3 1.24 Feo (M 6.3 0.75y H 53.0 50.3 50.3 50.3

Metal.. M350 M350 I A11/1350.... AM350 Temperature F.) 12o-140.--. {fffll 12o-140..- x20-140.-. Mnnngmte(mehes/m1n.).. 00005.... O'ggg-: 0.0006,... 00003-..-

(160F (150F.. (12o-140F, (150216., (150 F., 0. 00075.) 0. 090g; 0. 0009.) 0. 00045.)

0000525 R.M.s---

More particularly, it may be surmised that too high a concentration of the nitric acid passivates the surface of the metal being milled. Possibly there is formed an oxide layer on the surface which acts as a membrane to decrease the ow of active components in the solution to the metal. Again, an alkyl aryl sulfonate such as the sodium salt of dodecyl benzene sulfonic acid was incorporated into the aqueous reaction solution. This salt appears to have decreased the surface tension of the solution so as to allow the same to penetrate to the surface of the metal for greater ease of reaction.

There was prepared another series of milling solutions, see Table IV, in which -the concentration of the ferrie salt was varied and in which diierent ferrie salts were employed. The fundamental solution comprised water,

In Table V there is illustrated another series of milling solutions. Again, the fundamental solution comprised water, ferrie chloride, hydrochloric and nitric acids. The concentrations of the hydrochloric and nitric acids are expressed in terms of normality and the concentration of the ferric chloride is in terms of molarity. These solutions were employed to mill AM350 in the temperature range of about l60-170 F. Solution A was the funda mental solution. Solution B was similar to solution A but comprised about 3.2% by weight of acetic acid. Solution C was similar to A except that it comprised about 3.2% by weight of glycerine. The milling rates of A and B were substantially the same, 0.0008 and 0.0007 inch per minute. The milling rate of solution C was considerably less than these two as it was 0.0002 inch ferrie chloride, hydrochloric acid and nitric acid. This per minute.

Table V .A B C Variables Wt. per- N Wt. per N Wt. per- N cent cent cent Metal AM350-.- AM350.. AM350... Temperature, F 1GO-170 i60-170... 160-170 Milling Rate (inches/mln.) 0.0008 0. 0007...-. 0.0002

(150 F., (150 F., (150 F.,

0. 0006.) 0. 00053) 0. 00015) R M n 65 100 Another series of milling solutions was prepared com prising water, ferrie chloride, hydrochloric and nitric acids. The results of this series are presented in Table VI. To this fundamental solution there was added ammonium chloride, stannic chloride and ammonium chloroemployed to mill AM350 at a temperature inthe range of stannate. In this table the ferrie chloride, ammonium 11 chloride, stannic chloride and ammonium chlorostannate were expressed in terms of molarity and the nitric acid and hydrochloric acid in terms of normality. These solutions were employed to mill AM350 in the temperature range of about 1GO-170 F. The fundamental or control solution was solution A. 'Ihe milling rate of this solution was about 0.0005 inch per minute. Solution B was similar to A except that there was added ammonium chloride. The milling rate of solution B was 0.0008 inch per minute, an increase of about 35% over the 412 of E was slightly lower than D, due to the lower milling temperature. Solution E milled AM350 at 0.0011 inch per minute, faster than it milled 17-7PH, 0.00025 inch per minute. In summary, the addition of ammonium chloride to the fundamental solution increased the reaction rate. .The addition of stannic chloride increased the reaction rate almost three fold over that of the fundamental solution. And, the addition of both ammonium chloride and stannic chloride increased the reaction over that of the fundamental solution and also gave a smoother millmg rate of A. Solution C was also similar to A surface than the control solution gave.

Table VI A B C D E variables Wt. Per- N Wt. Per- N Wt. Per- N Wt. Per- N Wt. Per N cent cent cent cent cent Hmmm 15.0 2.34 147 2.34 14.7 2.34 147 234 14.7 2.34 Fem. (M) 25.2 2.23 23.0 2.23 230 2.a 23.5 2z; 23.0 2.23 HNo.s-70%) 8.9 1.44 a4 1.44 8.4 1.44 8.4 144 9.4 1.44 13,0 50.3 47.3 47.3 47.3 47. 3 NBtol (M) 5.9 1.7 snc), (M) 5.9 0.4

(NH 0'4 (Nmiil) 0'4 SChzNHClm- 4.4 0.25 4.4 "6.'25 (SnCh) (SnCl Metal AM350.. M350 M350 AM350 411143725,H Temperature F...) 10o-170 16o-170 5%14 agp-1Z0 152%?) Milling Ratetlnches/min.) 0.0005 0. 0008 0:00 0:0602) 0.00112 (150 F., (150 F., (150 F., (150 F., 17-7PH,

0.00033.) 0.00068.) 'R M Ft 60 45 m except that there was incorporated stannic chloride. The milling rate for C, 0.0014 inch per minute, was almost three times the milling rate of A, 0.0005 inch per minute. As is seen, the addition of the stannic chloride considerably increased the milling rate. Solution D combined all the components so that it was similar to A but with the addition of ammonium chloride and stannic chloride. The ammonium chloride and stannic chloride can be considered to be ammonium chlorostannate. The milling rate of D was comparable to that of C, 0.0013 inch per minute. In 4the table the ammonium chloride and the stannic chloride in solution D were expressed as being separate but it is to be realized that in actuality they were in the form of ammonium chlorostannate. Solution E was substantially the same as solution D. However, the milling temperature of E was approximately lower than for D. Also, solution E was used to mill both AM350 and 17-7PH steels. The milling rate In Table VII there is presented another set of milling solutions having Water, ferrie chloride, nitric acid and" hydrochloric acid. This fundamental solution was varied by the addition of boric acid, phosphoric acid or oxalic acid. In this table the hydrochloric and nitric acids are expressed in terms of normality and the ferrie chloride, boric acid, phosphoric and oxalic acids are expressed in terms of molarity. 'Ihese solutions were used to mill AM350 in the temperature range of about 160-170 F. Solution A was the control solution and milled at the rate of about 0.0007 inch per minute. Solution B was similar to A except that there was added boric acid and it milled at the rate of about 0.0008 inch per miuute. VSolution C was similar to A except that it contained phosphoric acid and milled at the rate of approximately 0.0007 inch per minute. Solution D was similar to A but contained oxalic acid and milled at the rate of about 0.0006 inch per minute. Also,'solution D was employed Table VII AA B C D Variables Wt. Per N Wt. Per- N Wt. Per- N Wt. Per- N cent cent cent cent HCl (38?? 15. 6 2. 34 14. 7 2. 34 14. 7 2. 34 14. 7 2. 34 FeClg (Lf 25. 2 2. 23 23.6 2.23 23.6 2.23 23.6 2.23 HNO; (S8-70% 8.9 1.44 8.4 1.44 8.4 1. 44 8.4 1.44 H,O 50. 3 47. 3 47. 3 47. 3 Boric Acid (M) 5. 9 1. 47 Phosphoric Acid (M)' 5. 9 9 Oxalic Acid (M) 5. 9 1.0

Metal AM350 AM350.... M350 Allviiggh Temperature, F 160-170 160-170 i60-170-... i60-170 Milling Rate (inches/min.). 0.0007 0.0008 0.0007

( r., (150 F., (150 F., (150 F.,

17-7PH 0.0005 3353. n M s 60 50 50 M 13 to mill 17-7PH steel with a milling rate of about 0.0005 inch per minute.

Table VIII presents the summary of the results with two milling solutions. These solutions comprised water, nitric acid, citric acid, hydrochloric acid, hydrolluoric acid, acetic acid and disodium monohydrogen phosphate. Solution B4 dilered from A in that B contained a ferrie salt, ferric nitrate. In this table the ferrie salt is expressed in terms of molarity and the concentration of the nitric acid, acetic acid, hydrochloric and hydrolluoric acids are expressed in terms of normality. These solutions were employed to mill 177PH steel at approximately room temperature. Solution A, without the beneiit of the ferrie salt, milled at the rate of 0.0001 inch per minute. Solution B, with the benefit of the ferrie salt, milled at the rate of 0.0003 inch per minute. From these results it is possible to state that the addition of a ferric salt increases the milling rate of the solution on 17-7PH metal.

Table VIII I A B Variables Wt. Per- N Wt. Per N cent cent HNO. (B8-70%) 20.9 2.58 20. Aoetic Acid (glacial) 2. 3 2. HCl (38%) 9.2 9. BF (60% 13.1 13. Citric Acid (sat 0.6 0. NaiHPO (satd). 0.6 0. H10 53. 5 62. Fe(Nx)| (Ml 1. 8 0.07

Metal 17-7PH 17-7PH Temperature, F Room Room Milling Rate (inches/min.) 0. 000 0.0003 R.M.S 80-100 i90-250 Taxle IX presents the results of milling with two other milling solutions. These solutions comprised water, nitric acid, acetic acid, hydrochloric acid, hydrouoric acid, hydrobromic acid, citric acid and disodium monohydrogen phosphate. In this table the concentrations of the ferrie salt is expressed in terms of molarity and the concentrations of the nitric acid, acetic acid, hydrochloric acid, hydrouoric acid and hydrobromic acid are in terms of normality. These solutions were used to mill l7-7PH steel at approximately room temperature. Solution B van'ed from solution A in that the former contained a ferric salt, ferrie nitrate. Solution A, without the benefit of ferrie salt, milled this steel at the rate of 0.0001 inch per minute. Solution B, with the benefit of the ferrie salt, milled this metal at the rate of 0.00022 .inch per minute. Again, it is possible to state that the addition of a ferrie salt to a milling solution increases the milling rate on martensitic steel.

Table IX v A B Variables Wt. Per- N Wt. Per- N cent cent HNO: (G8-707) 19. 8 2. 48 19. 5 2. 48 acetic Acid (glacial) 2. 2 o. 4 2.1 o. 4 l (387) 8.8 1. 02 8. 8 1. 02 12. 4 4.2 12.2 4.2 5. 1 0. 3 5. 0 0. 3 0. 0.5 0. 5 0. 5 H2O 50. 6 49. 8 Fe(N0t)| (M)- l. 1 0.7

Metal 177PH 17-7PH Temperature, F Room Milling Rate (inches/m 0.

................ ISO-230 Table X presents the summary of milling with two other milling solutions. These solutions comprised water, nitric acid, acetic acid, hydrouoric acid, citric acid, and disodium monohydrogen phosphate. In this table the concentration of the ferric salt is expressed in terms of molarity and the concentrations of the nitric acid, acetic acid and hydroiluoric acid are in terms of normality. These soltuions were employed to mill 17-7PH metal at approximately room temperature. Solution B differed from A in that the former comprised ferrie nitrate while the latter did not. The milling rate of solution A, without benefit ofthe ferric salt, was 0.00002 inch per minute. The milling rate of solution B, with benefit of the ferrie salt, was 0.00008 inch per minute. Again it is possible to state that the addition of a ferrie salt to a milling solution increases the milling rate of the solution on martensitic steel.

Table X Variables HNO; (0S-70%) Ateticoacid (glacial).

Metal 17-7PH At this time it is proper to compare the results of the milling solutions in Tables VIII, IX and X. The milling solutions in Tables VIII and IX comprised hydrochloric acid. The milling solutions in Table X did not have the benefit of the hydrochloric acid. It is seen that the milling solution comprising hydrochloric acid milled at an increased rate over the milling solutions not having hydrochloric acid. More particularly, in Tables VIII and IX the milling solutions having hydrochloric acid but not having a ferric salt milled at the rate of 0.0001 inch per minute. A corresponding solution in Table X not having hydrochloric acid and not having the benefit of a ferrie salt milled at the rate of 0.00002 inch per minute. In other words, the milling solution without benefit of hydrochloric acid milled at a rate of one-sixth of those solutions having the benet of hydrochloric acid. Furthermore, for those milling solutions having both hydrochloric acid and the benefit of ferrie ion the milling rate was greater than for the milling solutions not having either hydrochloric acid or ferric ion. Solution B in Table IX, having both hydrochloric acid and ferrie ion, had a milling rate of 0.00022 inch per minute. Solution B in Table VIII, having both hydrochloric acid and ferrie ion, had a milling rate of about 0.0003 inch per minute. Solution B in Table X, having neither hydrochloric acid nor ferrie salt, had a milling rate of about 0.00008 inch per minute. It is seen that the milling rates of solutions B in Tables VIII and IX are at least ten times the milling rate of solution B in Table X. These two series of data illustrate the value of hydrochloric acid in a milling solution.

A further series of milling solution was prepared. This series comprised water, nitric acid, acetic acid, hydrouoric acid, citric acid, disodium monohydrogen phosphate and fernc nitrate. In this table the nitric acid, hydrouoric acid and acetic acid are expressed in terms of normality and the ferrie salt in terms of molarity. These solutions were used to mill l7-7PH steel at various temperatures ranging from room temperature to about 200 F. The fundamental solution was A and 15 comprised water, nitric acid, acetic acid, hydrouoric acid, citric acid, disodium monohydrogen phosphate and ferric chloride. This solution milled at the rate of 0.00014 inch per minute. To a solution such as A there was to be chemically milled and is dipped or submerged in the milling solution for a predetermined time. With the passing of the time the milling solution eats away or corrodes away the metal so as to form the two recesses 11.

attached other protective neoprene iilms. A crosssectional view of the structural panel before chemical milling takes place is shown in Figure 4 whereby the panel is covered by the neoprene having the two cutouts 14 added ferrie nitrate to form B. Solution B milled at 5 It is to be noted that that portion of the metal covered by the rate of 0.00033 inch per minute. The milling rate the protective neoprene tilm is not attacked and only that of B was over twice the rate of A. From this itis possiportion of the metal not covered by the protective lm is ble to state that the addition of ferrie nitrate to a millcorroded away. After the passage of a predetermined ing solution, or the increase ofthe ferrie ion in. the millof time and the recesses are of a sut'licient depth and ing solution, increases the milling rate. To a solution size the structural panel is removed from the milling such as B there was addedn powdered titanium to form solution. The adherent solution is washed away and C. Solution C milled 177PH steel at temperatures of the neoprene protective lm stripped otf the panel to pro- 80 and 145 F. As the milling temperatures of A and duce that shown in Figure l. Other teachings for the B were room temperatures, it is not possible to compare masking of metal may be found in United States Patents the milling rate of C with these. However, it is seen Numbers 2,739,047 to Sanz and 2,684,291 to Wilsonet al. that at 80 F. the milling rate was lower than for A In Figure 5 there is illustrated the action of the milling and B and at 145 F. the milling rate was greater than solution on the metal at the junction of the protective for A and B. To a solution such as C there was added lm and the metal. IIt is seen that milling solution 16 eats aluminum nitrate to form solution D. This solution away metal 17 so as to undercut protective film 18. One milled at a temperature of 135 F. and the milling rate 20 of the particular advantages of the use of a wetting agent was 0.0011 inch per minute. It is seen that the addiis to induce better contact between the milling solution tion of aluminum nitrate to the milling solution increased and the metal. With a better contact or a lowering of the milling rate. To a solution such as D there was addthe effective surface tension there is less possibility of ed a powdered aluminum alloy, 75S, to make solution E. gases collecting on the surface or near masked edges un- The milling temperature of this solution was 200 F. and 25 derneath the protective lm. For this function there are the rate was 0.00026 inch per minute. These results are a number of suitable wetting agents such as pine oil. tabulated inTable XI. One of the common wetting agents is an alkyl aryl sul- Table XI A B O D E Variables Wt.Per N Wt. Per- N Wt.Pe1-nt N Wt.Per N wmer- N cent cent cent cent HNO. (6s-70%)---" 20.2 2.48 19.2 2. 4s 19.7 2.48 19.5 2.48 19.3 ats Aeetic acid (glacial)- 2.2 0.4 2.2 0.4 2.1 0.4 2.1 0.4 2.1 0.4 HF (00 o 12.6 4.2 12.4 4.2 12.3 4.2 12.2 4.2 12.1 4.2 citric Acid (sand) 0.6 0.6 0.6 0.5 0.5 NaiHPo. (sand). 0.0 0.6 0.0 0.5 0.5 11,0 52.1 61.0 00.6 00.0 59.4 Feci. (M) 1.2 0.12 1.a 0.12 1.7 0.12 1.7 0.12 1.7 FetNom (M) 1.8 0.07 1.7 0.07 1.1 0.07 1.7 0.7 0.7 0.7 AnNon. 1.0 1.0 75 s 0. 8 Metal. 17-7PH 17-7PH 17-7PH 17-7PH 17-7PH Temperature F.) Room Room 80, 5 235 200 Mining nate (inches/ming--. 0. 000144 0. 00033 ljjmg 0.0011 0.00025 R.M.s 00-110 13o-180 j 10o-150 90-130 Having described our discovery and illustrated the same fonate. Specific sulfonates which are useful are the with reference to various compositions of matter for the sodium salts of dodecyl benzene sulfonic acid and use of chemical milling solution, reference is hereby made pentadecyl benzene sulfonic acid. to the practical application of these solutions. Referring Another instance where our solution is of value is in to the drawings, and especially Figure l, it is seen that the chemical milling of components for aircraft. |In Figthere is illustrated a structural panel 10 having two chemi- 55 ure 6 is illustrated a curved structural plate having a thickcally milled recesses 11 therein. In Figure 2, a vertical ness of 0.750 inch. This plate was transformed into a cross-sectional view of Figure l. it is seen that this struestructural member, for use in a fuel tank, having a thicktural panel is curved and that these two recesses are ness of 0.100 inch. The finished Product, See Figures separated from each other by a rib 12. In the manufac- 7 and 3, W28 Prepared from the Original Structural Plate ture of this type of structural panel the material to be 60 by masking the two central portions so as to form the boss saved is covered by an adhering and strippable protective upon the corrosion of the surrounding metal. As is ilm or coating, there are numerous such coatings or tilms seen, one of these bosses is 0.500 inch in thickness and examples being neoprene, vinyl etch-proof film, and the other boss is 0.750 in thickness. vilach of these is others. More particularly, for shallow milling there may tapped. The outside perimeter of this section is formed be used various acid resistant paints. To pictorially llusinto a thickness of 0.250 inch, approximately two and trate the making of this structural panel reference is made one-half times as thick as the main part of the section.

. to Figures 3 and 4. In Figure 3 there is illustrated a neolll'he increased thickness of the perimeter is for added prene mask 13 having two recesses 14 therein. This neostrength upon the welding of this section into the main prene film 13 is placed on the concave side of the strucpart ofthe fuel tank. Normally such a section would be tural panel 10 and on the edges and the convex side are 70 formed by fabrication using a welding technique. However, the welding process causes grain growth which in turn causes a weakening of the metal. With our method there is no grain growth.

The control for determining the degree of chemical illustrated therein. This structural panel now ready '(0 may be carried out in a number of ditferent man- 17 ners. One of these is to introduce a material of a predetermined thickness into the chemical milling solution along with the member to be chemically milled and to periodically weigh or measure this member. When this member has decreased a certain amount in weight or thickness it is possible to decide Whether the structural member undergoing chemical milling has been corroded away suiciently.

Returning now to the metals and the examples presented therein it is noticed that the smoothness of R.M.S. values are mainly in the range of approximately 50-125 R.M.S. This is of a special importance as this value represents a surface that is relatively smooth. In fact, the milled metal is often smoother than the metal when machine milled. In machine milling the average smoothness value is approximately 125 R.M.S. Of course, this value of 125 R.M.S. is subject to variation as with soft alloys it is possible to machine mill the metal somewhat smoother than 125 R.M.S., but with heat treated, work hardened and tempered alloys which may have a tough surface skin it is difficult to achieve a smoothness value of 125 R.M.S. on machine milling. However, with this chemical milling process it is possible to achieve a smoothness value which approaches that used for mirrors and reflective surfaces, and it is possible to achieve this smoothness on all types of alloys be they hard, heat treated, work hardened, or tempered.

The smoothness values were determined by a protilometer. The profilometer used possessed a diamond needle which rode on the surface of the metal and measured the profile of the surface in millionths of an inch in amplitude. These measurements are expressed in R.M.S. or root means square values. Such an instrument is commercially obtainable from Physicists Research Company, Ann Arbor, Michigan,

In this chemical milling process itis noticed that there is little if any smut. Smut is defined as that material which clings to the metal itself. Smut is usually an oxide of the alloy or chemical compounds of the alloy or unreacted finely divided alloying materials and is attracted to the -bulk metal by electrochemical and electrostatic forces. l

With our process in chemical milling it is possible to achieve tolerances in some usages which have not been obtainable in commercial practice before. These tolerances are in the range of approximately plus or minus 0.0005 inch, making it possible to achieve a high quality and/r a precision product. Also, in our process it has been observed that there is no intergranular corrosion due to the chemical action of the solutions. This success in not having intergranular corrosion makes it possible to achieve higher strength components for less weight of the component as compared with the presently machine milled and welded components.

Our process is also adaptable for a continuous method or a batch method. In the continuous process it is possible tocontact the structural item to be made with the milling solution and to continuously recirculate the milling solution so as to maintain uniformity of solution in contact with all reacting surfaces. Chemical processing equipment, i.e., tanks, pipe fittings, valves, and pumps lined with the suitable resistant material such as polyethylene, polyvinyl chloride, and polyvinyl esters, capable of withstanding the chemical action of these solutions are available. Also filter screens and heat exchange equipment to withstand the chemical action of these solutions are available. Therefore it is possible to maintain close Ysolution control necessary for continuous operation.

Examples of other ways and means for chemically milling metals are herewith presented. A metal may be removed by spraying the solution onto it so as to have a fine stream of the reactant solution contact the metal. By this technique deep grooving can be realized by rotating a part. In another manner a pipe may be made lighterin-weight by running the solution through it so as to de- 18 crease the wall thickness. As is realized the interior wall of the pipe is eaten away and the inside diameter thereby enlarged.

From the above description of our discovery it is seen that the same is applicable to the chemical milling of metals and alloys. From our discovery it is possible to save considerable waste in the manufacture of metal products; to produce the parts in batches or continuously; the capital investment for production of the components may be as low as approximately 5 to 10 percent of the capital investment for equivalent machine milling. Also, it is possible to produce highly complex shapes and congurations with chemical milling which are not possible with machine milling, to use non-symmetrical patterns with chemical milling which are dicult to use With machine4 milling, to produce integrally stilened structures whereby one unit serves as a structure instead of an integrally fabricated structure depending upon welds, which are weaker than the metal itself, to produce a structure, and to make tapered structural materials which are diflicult to make with machine milling. Furthermore, from an engineering design standpoint it is seen that it is possible to etch after forming the structure. In many operations it is economically feasible to obtain closer tolerances, to have a number of various steps to cut, and to mill all surfaces of the part simultaneously. In this regard, it is possible to chemically mill with equal ease all types of martensitic steel. From a labor standpoint and skilled artisan viewpoint it is not necessary in chemical milling to use as highly skilled operators as in machine milling.

Having described our discovery, what we claim and wish to protect is as follows:

1. An aqueous acidic composition of matter adapted for chemically milling metal which contains chloride, nitrate and a ferric ion therein, the chloride concentration being at least about 1.0 normal, the nitrate concentration being at least about 0.75 normal, the ferrie concentration being at least about .07 molar, the available hydrogen ion concentration being however, at least 1.8 normal, said composition further including a minor amount of a member selected from the group consisting of ammonium chloride and stannic chloride, ammonium chlorostannate and mixture thereof.

2. The composition of claim 1 containing also a weak acid having a primary ionization constant in the range of 10-4 to 10-5.

3. An aqueous acidic composition of matter adapted for chemically milling metal and which contains a chloride, a nitrate and a ferrie ion therein, the chloride concentration computed as 38% HCl being at least about 9% by weight, the nitrate concentration computed as 70% nitric acid being at least about 5%, the ferrie concentration computed as ferric chloride being at least about 1%, all percentages being by weight, the available hydrogen ion concentration being, however, at least 1.8 normal, said composition further including a minor amount of a member selected from the group consisting of ammonium chloride and stannic chloride, ammonium chlorostannate and mixtures thereof.

4. An aqueous acidic composition of matter adapted for chemically milling metal which contains a chloride, a nitrate and a ferric ion therein, the chloride concentration computed as 38% HCl being in the range of about 9-25%, the nitrate concentration computed as 70% nitric acid being in the range of about 5-25%, the ferrie concentration computed as ferrie chloride being about 1-26%, all percentages being by weight, said composition further including a minor amout of a member selected from the group consisting of ammonium chloride and stannic chloride, ammonium chlorostannate and mixtures thereof.

5. 'Ihe composition of claim 4 containing also from about 16% of a weak acid having a primary ionization constant in the range of 10* to 1.0".

6. A process for manufacturing a metal structure from steel, said process including covering portions of the steel with a protective covering, subjecting the uncovered steel to the action of an aqueous acidic composition of matter adapted for chemically milling metal which contains a chloride, aA nitrate and a ferric ion therein, the chloride concentration being at least about 1.0 normal, the nitrate concentration being at least about 0.75 normal, the ferric concentration being at least about .07 molar, the available hydrogen ion concentration being, however, at least 1.8 normal, said composition 'further including a minor amount of a member selected from the group consisting of ammonium chloride and stannic chloride, ammonium chlorostannate, and mixtures thereof.

7. A process for manufacturing a metal structure from steel, said process including covering portions of the steel with a protective covering, subjecting the uncovered steel to the -action of an aqueous acidic composition of matter adapted for chemically milling metal, which contains a chloride, a nitrate and ferric ion therein, the chloride concentration computed as 38% HCl being at least about 9% by weight, the nitrate concentration computed as 70% nitric acid being at least about 5%, the ferric concentration computed as ferric chloride being at least about 1%, the available hydrogen ion concentration being, however, at least 1.8 normal, said composition further including a minor amount of a member selected from the group consisting of ammonium chloride and stannic chloride, ammonium chlorostannate and mixtures thereof.

8. A process for manufacturing a metal structure from martensitic steel, said process including covering portions of the raw material metal with a protective covering and subjecting the uncovered raw material metal to the action of an aqueous acidic composition of matter adapted for chemically milling metal, which contains chloride, nitrate and ferric moities therein, the chloride concentration computed as 38% HC1 being in the range of about 925%, the nitrate concentration computed as 70% nitric acid being in the range of about 5-25%, the ferric concentration computed as ferric chloride being about 11-25 said composition further including a minor amount of a member selected from the group consisting of ammonium chloride and stannic chloride, ammonium chlorostannate and mixtures thereof, and from about 16% of a weak acid having a primary ionization constant n the range of 10-4 to 10-6, all percentages being by Weight.

References Cited in the file of this patent UNITED STATES PATENTS 469,704 Knidermann Mar. l, 1892 2,429,107 Petren Oct. 14, 1947 2,446,060 Pray July 27, 1948 OTHER REFERENCES Thum: Book of Stainless Steel, 2nd ed., Amer. Soc. of Metals, Cleveland, Ohio, Jan. 1935, p. 151.

Monypenny: V. 2., 3rd ed., rev. 1954, Chapman & Hall, London, p. 253.

Claims (1)

1. AN AQUEOUS ACIDIC COMPOSITION OF MATTER ADAPTED FOR CHEMICALLY MILLING METAL WHICH CONTAINS CHLORIDE, NITRATE AND A FERRIC ION THEREIN, THE CHLORIDE CONCENTRATION BEING AT LEAST ABOUT 1.0 NORMAL, THE NITRATE CONCENTRATION BEING AT LEAST ABOUT 0.75 NORMAL, THE FERRIC CONCENTRATION BEING AT LEAST ABOUT .07 MOLAR, THE AVAILABLE HYDROGEN ION CONCENTRATION BEING HOWEVER, AT LEAST 1.8 NORMAL, SAID COMPOSITION FURTHER INCLUDING A MINOR AMOUNT OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF AMMONIUM CHLORIDE AND STANNIC CHLORIDE, AMMONIUM CHLOROSTANNATE AND MIXTURE THEREOF.

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