US2407579A

US2407579A - Electrodeposition of tin

Info

Publication number: US2407579A
Application number: US493757A
Authority: US
Inventors: Ernest W Schweikher
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1942-07-04
Filing date: 1943-07-07
Publication date: 1946-09-10
Anticipated expiration: 1963-09-10

Description

Patented Sept. 10, 1946 ELECTRODE?!) SITION OF TIN Ernest W. Schwelkher, Shaker Heights, Ohio, as-

slgnor to E. I. du Pont dc Nemours & Company. Wilmington, DeL, a corporation oi Delaware No Drawing. Application July I, 1943, Serial No. 493,757

12 Claims. 1

This invention relates to the electrodeposltion of tin and is more particularly directed to tinplating compositions, tin-plating baths, and to processes for the electrodeposltion of tin from an alkali fluoride-stannous chloride bath of a character more particularly described hereinafter.

According to present practices tin-plating systems are either strongly acidic or strongly alkaline, and the use of stannous chloride is avoided because it is commonly regarded as causing the formation of loose, crystal-lined deposits. Acid tin-plating baths at present require the use of large amounts of addition agents which influence the character of deposit obtained particularly after such subsequent operations as fusion brightening and prolonged heating. The deposits, moreover, are not as adherent or as dense or as pleasing in appearance as might be desired.

It is an object 01 this invention to provide compositions. baths, and processes by the use of which there may be obtained tin deposits which are dense, adherent and of good appearance. It is a further object to provide platin baths which are easily prepared and easily maintained in use and which have the best possible operating characteristics. It is a still further object to provide tin-plating baths which are only slightly acidic, say a pH above about 1. Another object is to provide acid tin-plating baths which produce deposits that may be readily brightened by fusion to a brilliant appearance and that will not discolor after heating operations. It is a still further object to provide tin-plating baths which are readily amenable to improvement by the use 01 the common addition agents for tin-plating baths. Further objects of this invention will become apparent hereinafter.

The foregoing and other objects of this invention are attained by the use of an alkali fluoridestannous chloride bath in which the ratio of fluoride to stannous chloride is carefully selected and related to the operating pH of the bath and in which the static equilibrium potential of the bath is held within certain limits as more specifically described below. Baths of this invention are readily amenable to improvement by the inclusion of addition agents and it will usually be found desirable to include one or more organic addition agents. There may also be included with advantage 8. triad metal of the iron group.

Fluoride-stannous chloride baths used according to the present invention may be made up with stannous chloride and with an alkali fluoride, say, sodium, potassium, or ammonium fluoride. When reference is made hereinafter to an alkali fluoride it will be understood that sodium, potassium, or

ammonium fluoride or bifluorlde may be used or mixtures of any two or more of these may be emplayed.

The concentration of stannous chloride may vary from about thirty-seven and one-half to one hundred and fifty grams per liter and the alkali fluoride may similarly vary from thirty-seven and one-hall to one hundred and fifty grams per liter. More specifically, it is desired to maintain the concentration of the stannous chloride and the alkali fluoride between about sixty and one hundred grams per liter.

More important, perhaps, than the mere concentration of stannous chloride and soluble fluoride is th molar ratio of these materials as related to the pH at which the bath is operated. An empirical formula which has been found satisfactory is as follows:

M pH mols SnCl;

where MF' is an alkali fluoride and Such is stannous chloride.

The following conditions should be adhered to for successful use of the above formula:

(1) The constituents are used within the pH range of pH 1 to pH 5 under conditions in which they are soluble in the plating bath, the tin content being maintained within the limits of 0.15 mol to 0.6 mol of tin metal per liter of solution.

(2) The ratio of alkali fluoride to tin chloride for any given tin content is controlled by the value oi it. It is then found that k is related to the pH of operation of the bath in such a way that for optimum operation at any given pH, k equals 0.55. However, it will be understood that in commercial operation good results may be obtained with values of it between the broad limits of Ic=0.1 to k=1.0. It is more particularly preerred to stay within the limits, k=0.3 to 10:037.

(3) The mol ratio of MF SnCl,

MF SnCl,

will be 2.85. Therefore, as will be evident, when K is varied within the narrow limits shown above, and pH is varied within the narrow limits of 2 to 3 4 shown below, the moi ratio may not go below. in round numbers. about 3.

t) The pH of the formula is the maximum at which the bath is stable while the optimum pH for plating is within the range from about 1 to and preferably the pH should be in the range from about 2 to 4. A bath formulation that is stable at a given pH may also be stable at a lower pH but the preferred ranges are those within which addition agents are generally most eilective.

In the practical application or the formula. since in ordinary use the pH of operation will be preselected, lc will be fixed at the desired optimum value. and the amount of tin which it is necessary to have in the bath will be known, it may be best to express the equation in a form as shown below which may be used to calculate the amount of alkali fluoride compound required.

pH (moi): SnCh) Example A Conditions: 0.33 mol stannous chloride to be used at a pH of 4. To calculate the mole of sodium fluoride using the above equation:

(4)(0.33) 2.4 mole of so- 0.55 dium fluoride required Mois sodium fluoride- Ezamnle B MF=sodium fluoride Conditions: 0.33 mol of stannous chloride to be used at a pH of 3.

(3) (0.33) 1.8 mole sodium M018 BOdlllm fluoridew-fluoride requimd Example C MF=sodium fluoride Conditions: 0.33 mol of stannous chloride to beusedatapHofii.

Conditions: 0.5 moi stannous chloride to be used at pH of 4.

(4)(0.5) 3.63 mole sodium 0,55 fluoride required The acidity of the bath may be that which results from the bath constituents, though adiustments of pH may be made as desired. Generally, as mentioned above, the pH will fall within the range of about pH i to 5 while more specifically it is desired to have the pH from about 2 to 4.

In addition to the above characterization of tin-plating baths as set out in my application Serial No. 449,803 filed July 4, 1942 of which the present application is a continuation-in-part. I have found that to obtain the objects of the present invention the bath composition must be so adjusted as to lead to a static solution potential within a narrow range. The solution potential,

Mols sodium fluoride= 4 P, should not fall outside the limits of the following two equations:

In this formula, pH as indicated above, should have a value between i and 5 or, preferably, between about 2 and 4.

After a solution has been prepared with constituents selected according to the methods given above in some detail, then the solution should be examined as to its potential. This can of course be done by making up a small amount of solution first and then adjusting the entire bath composition in accordance with the findings. It the solution potential is below the values given for P in the above formulae, then it may be increased (made more negative) by increasing the MF/SnClz ratio; if the potential is too high it may be lowered by decreasing the MF/SnCl: ratio. In adjusting the potential it will be noted that the potential increases (becomes more negative) with an increase in pH and decreases with a decrease in pH. Ordinarily potential will not be adiusted by changing the DH, though this means of adjustment is available if it is preferred to use some preselected ratio of MP to SnCla.

The potential may be determined in any suitable manner as by connecting a potentiometer across a piece of tin metal and a caiomel half cell in customary manner. The static solution potential, measured in volts, oi tin against the bath is then obtained on the hydrogen scale after correction is made for the calomel half cell on the same scale.

The bath temperature is that customarily used for tin-plating baths and deposits are obtained at room temperature. It is ordinarily preferred, however. to use the bath at a temperature from about to C. in order to secure the optimum quality of deposit over a broad current density range.

Organic addition agents may advantageously be employed to improve the appearance and characteristics of tin deposits produced with baths or this invention. One or more organic addition agents may be used and there may be included in the bath, for instance, suifite cellulose waste, a naphthoi suii'onic acid or a polyether as polyethylene oxide (Carbowax), b-naphthol ethylene oxide, or a poiydioxolane. The aikylene oxides are covered in Hofl'man application, Serial No. 493,755, flied July 7, i943, and the polydioxolanes are covered in Gray, Gresham. and Loder application, Serial No. 493.756, flied July '7, 1943. Gelatin or glue may be used to advantage for producing tin deposits of good appearance for ordinary use but it will be found that glue and certain other agents are not too satisfactory when the deposit is to be heat-fused.

Any naphthol sulfonic acid may be used with a fluorlde-stannous chloride bath according to the present invention and there may be used, for instance,

The sulionic acids may be used either as the free acid or in the form of any soluble salt such as the sodium, potassium, ammonium, tin or nickel salts. The manner of adding the suli'onic acid is comparatively unimportant since whether the addition agent is present as an acid or as a salt or as .a partial salt will depend upon the 911 oi the bath. For purposes of this description, where reference is made to a sulionic acid it will be understood that this may be a salt or a partial salt as well as the free acid. As a matter of fact the preferred addition agent of the above type is "Schaefl'er's" salt, which is the sodium salt of 2-naphthol-6-sulionic acid. The amount of a naphthol sulfonic acid to use can most easily be determined in any particular instance by trying out various amounts in the specific plating bath and for the specific type of use by routine methods already well-known to the practical plater. It may be indicated that in general from about 2 to 8 grams per liter of Schaefler's salt or of another naphthol sulfonic acid will be suitable.

Various polyethers may be employed as addition agents in the baths oi the present invention. There may be used, for instance, poiyalkylene oxides such as the polyethylene oxides sold under the trade name, Carbowax. There may be used, for instance, polyethylene oxides of molecular weights of 400, 600, 800, 1500, 4000, 5000, and 9000. Substituted polyalkylene oxides may also be used and there may be employed, for instance, compounds such as those shown in the following patents: 1,921.378, 1,922,459, 1,976,678, 2,059,- 273, 2,134,429, 2,134,430, 2,167,073, 2,234.200, 2,- 275.3'78, 2,275,379, 1,970.578, Conrad Schoelier and Max Wittwer; 2,213,477, Adolf Steindoril, Gerhard Balle, Karl Horst. and Richard Michel; 2,144,647, John Burchill, Henry Alfred Piggot and George Stuart James White, patented January 24, 1939, including the patents therein cited: British, 346,550, 432 356, 420,137, 420,518, 380,431. 434,424; German, 575,911.

Another group of polyethers which may be employed are the polydioxolanes prepared by the co-polymerization oi 1,3-dioxolane with materials such as organic acids, alcohols, ethers. nitriles, and the like. Polydioxolanes without substituent groups may similarly be used.

In order that the invention may be better understood reference should be had to the following illustrative examples:

Example I A bath for the electrodeposition of tin was made up as follows:

Gram Grams/l. mom/L Stannous chloride (F'lCig.2H:0) 75 0. 33 Sodium fluoride (HNsF) 75 1.8

Mgram mols/l. MF) gram mols/l. SnCl,

Thus the MF value is 1.8 mols. The pH is 3.0

and the tin chloride is 0.33. Substituting in the formula 3.0== r or k 0.55

the preferred value for k as stated above. Conversely, given the bath composition one could have determined by using the stated preferred value of 0.55 for k that the correct pH at which the bath should be used would be about 3.0.

The static potential, P measured in volts, of the bath of Example I falls within the area prescribed by the formulas P= -0.055 pH-0.265 P= -0.055 PEI-0.870

Substituting in the above formulas using the bath pH oi 3.0

The determined static solution potential of the bath is -0.500 therefore falling within the range shown.

The pH adjustment was effected using the antimony electrode and this method of measuring pH will generally be found most satisfactory for baths of this invention. The quinhydrone or hydrogen electrode is inaccurate because of the reduction in the bath and indicators are also inaccurate because they react with the bath constituents. A glass electrode may be used two or three times with good results but the fluoride soon etches the lass and ruins the electrode.

Example I! A bath was made up as in Example I and to the bath was added 50 grams per liter of sodium chloride and 5 grams per liter of the sodium salt of 2-naphthol-6-sulionic acid added as "Schaefiers salt. The conductivity of the solution was considerably improved. The character of the deposit was improved as to smoothness and adherence by the addition of the "Schaefier's" salt.

Sodium chloride or ammonium chloride are used to increase the conductivity of the bath. It is to be noted that ordinarily the bath constituents should exclude acid radicals other than halides and it is preferable to use chlorides or fluorides, particularly in view of the relatively high cost of iodides or bromides. Of course it will be understood that small amounts of other acid radicals can be included in the bath as, for instance, when sulfonic acids are used in the customary minor amounts as addition agents.

The pH of this bath as made up was 3.0, determined by use of the antimony electrode. The temperature was raised to 160 F. and from the solution tin was electrodeposited using a tin anode, the current efllciency being at both electrodes.

asoasro 7 ftwillbeseenthatthecomoositionofmmplemiswithintherangeoftheformuls Ham moisl]. MF) p gram mois/i. n 1,

Thus the total MI" is 0.00 or 0.88 or a total of 1.48. The pH is 3.0. The amount of stannous chloride is 0.33 mol. Substituting in the formula This compares favorably with the preferred values of 0.55 for k and is within the permissible variation allowed from the preferred formula.

The static potential of the bath of Example III falls within the range prescribed by the formulas:

P=0.055 pH-0.285 and P=-0.055 pH-0.3'i0

Substituting in the above formulas using the bath pH of 3.0

The determined static solution potential of the bath is 0.460 therefore falling within the range of P=-0.430 to -0.530.

Example IV An electroplating solution for plating moving strip steel was made up as follows:

Gram

Grams/i. mom/L Simmons chloride 75 n. 88 Ammonium chloride" 76 Ammonium biiluoridc 1: l. 31 s' k( m mols/l. MF) 9 gram mols/l. Snfil,

which is in the permissible range of variation from the preferred value of k=0.55.

At current densities from about 5 to 200 ampere per square foot smooth, dense, adherent deposits were obtained. Similar results were obtained using baths in which successively diflerent amounts of Schaefl'ers" salt were used from about 2 to 8 grams per liter. Stannous chloride was varied from about 50 to 100 grams per liter.

8 The ammonium bliluoride was also varied. However, the moi ratio of ammonium bifluorlde stannous c o c was desirably maintained within the limits of 4 to 8. Similarly the bath was found to operate very well in a pH range of 1.5 to 4.5. In all cases the potential, P. was maintained within the limits above given.

While excellent results can be obtained with baths such as those shown in this example, it is preferable for certain applications to include a supplementary addition agent such as gelatin, glue. peptone or similar compounds. The amounts of these materials to be used can readily be determined by a few simple tests under the proposed conditions of use though it may be indicated that generally from about one to six Substituting in the formula k( am mols/l. MF) p gram mois/i. Snfii,

At current densities from about 5 to 200 amperes per square foot smooth, uniform, dense. adherent, semi-bright deposits were obtained. Amounts of gelatin from about one to six grams per liter were found suitable in baths of this example and "Schaeifer's" salt in amounts from about 2 to 8 grams per liter were found suitable. The stannous chloride and sodium fluoride were varied from 50 to grams per liter; however the molecular ratio sodium fluoride stannous chloride was maintained in the region about 6 to 'l for best results.

The static potential of the bath of Example V falls within the areas prescribed by the formulas P=-0.055-0.265 and Substituting in the above formulas using the bath pH of 4.0

P=0.220-0.265 or P=-0.483 P= --0.2200.3'70 or P=--0.590

The determined static solution potential of the bath is -0.485 therefore falling within the indicated range.

Deposits of improved character can be obtained by including in the bath of the present invention small amounts of a soluble compound of the triad metals of the iron group. including iron, cobalt and nickel. There may, for instance, be used nickel chloride. nickel sulfate, or cobalt chloride or cobalt sulfate or iron chloride or iron sulfate. The amount of the metal compound to use may readily be determined by a few simple tests and it will generally be found that from about two to ten grams per liter is suitable. The following example illustrates a bath of the invention including a nickel compound.

Example VI Grams/l. Stannous chloride 100 Sodium fluoride 126 Nickel sulfate "Schaeifers salt 4 Gela 2 pH 4 Temperature 55 C.

At current densities from about 5 to 200 amperes per square foot dense, adherent, smooth, semi-bright deposits with refined grain structure were obtained. The potential was maintained within the limits above indicated.

Likewise, nickel has been found greatly to improve the character of deposit produced when used in the stannous chloride-sodium fluoride bath with "Schaefiers" salt but without gelatin. The following example illustrates a bath of this type used under the same conditions as the bath described in Example VI.

Example VII A tin-plating bath was made up with the following:

Stannoos chloride H 4. emperatnra 55 0.

used in essentially the same ratios as have been previously described. All of the addition agents and addition agent combinations described in the preceding examples have also been found to operate effectively in baths of this composition,

10 Example V!!! A plating bath was made up for the high speed plating of continuous strip as follows:

Gram Grams/l mob/L Stannous chloride Sodium fluoride Nickel sulfate....

Schaeiiers" salt Gelatin pH 3. Temperature 55' C.

Substituting in the formula ldgram molsfl. MF) p gram mols/l. Snfil,

sodium fluoride The moi ratio is about 6.50

The bath of Example VIII falls within the range expressed by the formulas:

=-0.055 pI-I-0.265

and

'P=-0.055 pH-(LS'IO Substituting in the above formulas using the bath pH of 3.0

P=-0.1650.265 or P=-0.430 P=0.165-0.370 or P=0.535

sodium fluoride stannous chloride being maintained at approximately 6.

Ezample IX As was noted in the equation for bath composition, it is possible to use a lower molecular ratio of alkali metal fluoride to stannous chloride provided the bath is operated at a lower pH.

The following is an example of a bath operating at a lower pH:

Gram Grams/l. mowL Stannoms chloride ii. 444 Sodium fluoride 63 l. B "Schaafler's" salt 6 H 1.7. smperamre F.

The molecuiar ratio sodium fluoride stannous chloride in this bath is 3.3. Thus using the optimum value k=0.55 in the equation -L i LM pH mole/l. SnCl,

11 it may be calculated that the bath should be operated at a pH of about 1.8.

The static solution potential of the bath of Example IX falls within the range of the formulas:

=-0.055 rill-0.265 =0.055 nil-0.370

Substituting in the above formulas using the bath pH of 1.7

P=-0.093-0.265 or P=-0.358 P=-0.093-0.370 or P=0.463

The determined static solution potential of -0.382 falls within this range.

Example I A bath for the electrodeposition of tin on movll'lg strip steel at current densities of from 100 to 1000 amperes per square foot was prepared as follows:

GrW11 Sid.

Sianuous chloride 100 0. 44 Sodium fluoride 45 1.0! Sodium bifluoridem. 45 0. 125 Polydioxoianc 0. 2

H 2.5. emperature 150' F.

Substituting in the formula m mole/l. MF) p gram mols/l. SnCl,

The static potential of the solution above falls within the limits set forth by the formulas:

P=0.055 {iii-0.265 P=0.055 PEI-0.370

Substituting in the above formula The determined static potential of the solution is -0.462.

Example XI A bath for the electrodepcsition of tin on strip steel was prepared as follows:

The static potential of the bath above falls 12 within the limits set forth by the formulas P= -0.055 nil-0.265 and P=-0.055 pH0.870

Substituting in the above formulas:

The determined potential of this bath is 0.472.

Example XII A bath for the electrodeposition of tin on strip steel subsequently to be flow or fusion brightened was prepared as below:

Temperature F.

k( am mole/l. MF) p gram mols/i. Snfil pI-I=2.5

The determined static potential in volts or the above solution is -0.484 falling within the range prescribed by the formulas given above.

It will be understood that baths of the present invention may be made up at the place of use by admixing the various ingredients in the manner indicated, but preferably premred compositions will be made up including some or all of the bath constituents. There may. for instance. be made up a mixture of an alkali fluoride, stannous chloride, Schaeilers salt. and if desired. glue or gelatin, and a nickel compound. The prepared compositions will ordinarily contain such ingredients that when they are dissolved in the amount required to give the desired concentration of plating ingredients the bath produced will correspond to the formulae above discussed. For instance the compositions will satisfy the formula when a sui'iicient amount has been added to give a stannous chloride concentration equivalent to from about 37.5 to 135 grams per liter as more particularly set out above. It will be understood that it may sometimes be found desirable to make pH adJustments by the use of suitable acids or bases after the solution has been prepared, but this is an obvious equivalent when so employed.

I claim:

1. A tin electrodepositing composition comprising an alkali fluoride and stannous chloride, the composition upon being dissolved in water to give a stannous chloride concentration of between about 37.5 and grams per liter satisfying the equation:

wherein the following conditions are simultaneously true; the pH is equal to about 1 to 5, I: has

13 a value from 0.1 to 1.0, MP is alkali fluoride, and the mol ratio M F SnCl, is about from 2 to 12, the static solution potential of tin in the bath being equal in volts to from --0.055 pH-O.265 to 0.055 PHI-0.370

2. A tin electrodepositing composition comprising an alkali fluoride and stannous chloride, the composition upon being dissolved in water to give a stannous chloride concentration of between about 37.5 and 150 grams per liter satisfying the equation:

k(mols M F) mois SnCl,

wherein the following conditions are simultaneously true; pH is equal to about 2 to 4, It has a value 01' from 0.3 to 0.7, MI is alkali fluoride, and the mol ratio SnCl; is about from 3 to 12, the static solution potential of tin in the bath being equal in volts to from --0.055 pH-0.265 to -0.055 pH-0.370

3. A tin electrodepositing composition comprising an alkali fluoride, and stannous chloride, the composition upon being dissolved in water to give a stannous chloride concentration of between about 37.5 and 150 grams per liter satisfying the equation:

k(mols MF) pH mols SnCi,

wherein the following conditions are simultaneously true; the pH is equal to about 2 to 4, I: has a value of about 0.5, MF is an alkali fluoride, and the mol ratio is about 6, the static solution potential of tin in the bath being equal in volts to i'rom -0.055 pH0.265 to 0.055 pH0.370

4. A tin electrodepositing composition comprising an alkali fluoride, stannous chloride, and 2 to 8 grams per liter oi a naphthol sulfonic acid, the composition upon being dissolved in water to give a stannous chloride concentration of between about 37.5 and 150 grams per liter satisying the equation:

k(mols MF) pH mols SnCl,

wherein the following conditions are simultaneously true; the pH is equal to about 1 to 5, k has a value from 0.1 to 1.0, MP is alkali fluoride, and the mol ratio M F SIlClg is about from 2 to 12, the static solution potential of tin in the bath being equal in volts to from -0.055 pH--0.265 to --0.055 pH-0.370

5. An aqueous tin eleetrodepositing bath comprising from about 37.5 to 150 grams per liter each of an alkali fluoride and stannous chloride, the bath satisfying the equation:

k (riols MF) pH mols SnCi,

wherein the following conditions are simultane- 14 ously true; the pH is equal to about 1 to 5, It has a value from 0.1 to 1.0, MI is alkali fluoride. and the mol ratio MF Such is about from 2 to 12, the static solution potential of tin in the bath being equal in volts to trom -0.055 pH-0.265 to 0.055 pill-0.370

6. An aqueous tin electrodepositing bath comprising from about 37.5 to grams per liter each of an alkali fluoride and stannous chloride, the bath satisfying the equation:

k(mols MF) mois sncl,

wherein the following conditions are simultaneously true; the pH is equal to about 2 to 4, It has a value of about 0.3 to 0.7, MP is alkali fluoride. and the mol ratio SnCl,

is about from 3 to 12, the static solution potential of tin in the bath being equal in volts to from -0.055 pH-0.265 to ---0.055 pill-0.370

7. An aqueous tin electrodepositing bath comprising irom about 37.5 to 150 grams per liter each of an alkali fluoride and stannous chloride, the bath satisfying the equation:

k(mols MF) PH- mols SnCl,

wherein the iollowing conditions are simultaneously true; the pH is equal to about 1 to 5, It has a value from 0.1 to 1.0, MP is alkali fluoride, and the mol ratio is about from 2 to 12, the static solution potential of tin in the bath being equal in volts to from 0.055 pI-I0.265 to 0.055 pH-0.370

and the bath containing from 2 to :3 grams per liter of a naphthol sulionic acid.

8. In a process for the eiectrodepositlon oi tin the step comprising eflecting electrodepositlon from an aqueous bath comprising from about 37.5 to 150 grams per liter of an alkali fluoride, and from about 37.5 to 150 grams per liter of stannous chloride and satisfying the equation:

is about from 2 to 12, the static solution potential of tin in the bath being equal in volts to from -0.055 pH-O.265 to 0.055 PEI-0.370

9. In a process for the electrodenosition oi tin the step comprising effecting electrodeposition from an aqueous bath comprising from about 37.5 to 150 grains per liter of an alkali fluoride and from about 37.5 to 150 grams per liter of stannous chloride and satisfying the equation:

ktmois MF) P mols 3E1,

wherein the following conditions are simultaneously true: the pH is equal to about 2 to 4, in has a value of about 0.3 to 0.7, MP is alkali fluoride, and the moi ratio MF SD Cl is about from 8 to 12. the static solution potential of tin in the bath being equal in volts to from --0.055 phi-0.265 to -0.055 pH--0.37O

10. In a process for the eiectrodeposition of tin, the step comprising eiiectlng electrodeposition from an aqueous bath comprising from about 37.5 to 150 grams per liter of an alkali fluoride and from about 37.5 to 150 grams per liter of stannom chloride and satisfying the equation:

k(mols MF) H1018 SnCl,

wherein the following conditions are simultaneouslytrue; pHisequaltoaboutlto5,lchas a value from 0.1 to 1.0, M1 is alkali fluoride, and the moi ratio MF Sn Cl,

is about (mm 2 to 12, the static solution potential oi tin in the bath being equal in volts to from -0.055 {iii-0.265 to -0.055 xiii-0.370

.i:(mols MF) mois Snci,

16 wherein the following conditions are simultaneously true: the pH is equal to about 2 to 4, I: has a value 0! about 0.5, MI is alkali fluoride, and the moi ratio MF Sn Cl,

is about from 3 to 12. the static solution potential oi tin in the bath beins equal in volts to irom -0.055 pH-O.265 to -0.055 PHI-0.370

and the bath containing from 2 to 0 grams per liter of "Bchaeii'er's" salt.

12. In a process for the electrodeposition of tin. the step comprising eilectina electrodeposition from an aqueous bath comprisins from about 87.5 to grams per liter of sodium fluoride and from about 37.5 to 150 Items per liter of stannous chloride. the bath composition satistying the equation:

k(mols MF! pH mole Sn 1.

wherein the following conditions are simultaneously true; the pH is equal to about 2 to 4, It has a value of about 0.5, MF is alkali fluoride, and the mol ratio MF BnCl:

is about from 3 to 12,.the static solution potential of tin in the bath being equal in volts to from 0.055 pH-0.265 to 0.055 PEI-0.370

and the bath containing from 2 to 8 grams per liter of "Schaefl'er's" salt.

ERNEST w. B0.

Certificate of Correction Patent No. 2,407,579.

September 10, 1946.

ERNEST W. SCHWEIKHER It is hereby certified that error appears in the printed specification of the above numbered patent re uiring correction as follows: Column 5, line 59, Example I, for Sodium fluoride NaF) read Sodium fluoride (NaF); and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 19th day of November, A. D. 1946.

LESLIE FRAZER,

First Assistant Commissioner of Patents.