US1841978A - Tin plating - Google Patents

Description

Jan. 19, 1932. F. F. OPLINGER TIN PLATING Filed Feb. 61929 ,Ufff/70W ous/259 INVENTOR A TTORNEY Patented Jan. 19, 1932 UNITEDSTATES PATENT orrlcs FLOYD F. OIPLINGER, 0F PERTH AMBOY, NEW JERSEY, ASSIGNOB, BY MESNE ASSIGN- MENTS, T0 THE ROESSLER & HASSLACHEIB, CHEMICAL COMPANY, A. CORPORATION OF DELAWARE TIN PLATIN G- Application med February 6, 1929. Serial No. 837,833.

- This invention has for its object the continuous production of white, smooth and n onporous tin deposits over long periods of time and at a high rate of speed, with practically no production of waste materials.

I-Ieretofore alkaline tin baths have been characterized by inability to continuously produce the bright, smooth deposits desired in commercial work. The majority are uneconomical in that heavy sludges, probably consisting in the main of small particles of metallic tin miXed with tinoxides or other tin compounds, are formed in the bottom of the bath after continued use. Such sludges must eventually be discarded.

I have now found, that highly satisfactory deposits of tin may be produced for extended periods of time from alkaline tin baths in which the analytically determined concentration of alkali metal stannate bears a definite relationship to the analytically determined alkalinity. All of my baths possess this definite relationship within certain limits of alkalinity and alkali 'metal stannate concentrations, described more fully below, and will therefore plate tin satisfactorily. I have further found that continuous plating will occur if the above mentioned deinite relationship is maintained in my baths.-

One way of maintaining this relationship is to add caustic alkali when analysis shows the alkalinity of the bath to be low and to add alkali metal stannate when it shows the alkalinity of the bath to be high.

Above, I have specifiedalkalinity as determined by analysis, since any attempt at determination of alkalinity by other methods will be affected by various factors, such as -impurities usually present in commercial alkali metal stannate, among which are free caustic, the natural alkalinity of an aqueous The minedv analytically for similar reasons. v In determining the alkalinity, I use thymol phthalein as an indicator, since its blue color in the presence of alkali and alkali metal stannate mixtures disappears upon the acldition `of acid, for example .of hydrochloric or sulfuric acid, at the same time that nsoluble tin compounds begin to precipitate.

Of course, other indicators can be used instead of thymol phthalein provided one corrects the alkalinity so determined to give it the same value as that which would be 0btained by the use of thymol phthalein. Such corrections could be made on the basis of pH values of the solution.

The alkali metal stannate content of the solution may be determined by any satisfactory known method, such as reduction of the acidified solution and titration against standard iodine solution. v

Because of the above stated reasons and for the purpose of simplifying further explanations of my baths, I wish wherever referring to concentrations of caustic alkali or, specically, sodium hydroxide, whether stated in terms of normality, molality, ounces per gallon, or the like, to be understood as referring, not to concentrations of caustic added as such to the bath, but to total alkalinity, as determined by titration, using thymol phthalein as an indicator or using an equivalent procedure. Likewise, whenever referring to concentrations of alkali metal stannate I wish it to be understood as referring to those concentrations determined either by an analysis of the solution itself or by making up the bath with a material the stannate content of which is accurately known. Obviously, for a given grade of caustic alkali and of alkali metal stannate, whether these are chemically pure or ofcoinmercial quality, the actual amounts which' one will use in making up my baths can be easily calculated. by using the ratios obtained by dissolving weighed quantities of the two materials to a convenient volume and comparing their concentrations thus produced with the concentrations of caustic and stannate asvdetermined from this solution by analysis, as given above..

The accompanying graph shows the relationship between the caustic alkali and alkali metal stannate concentrations in my baths. The concentration of caustic for optimum plating results, as compared with the concentration of alkali metal stannate, is represented by the 'line B of the graph and may also be represented by the equation:

Caustic alkali (normalit )=0.4 times the alkali metal stannate normality) +0.3.

In this equation "a normal solution of alkali metal stannate is considered as containing 1A of a mole of stannate per liter. The above equation may therefore be expressed as,

Caustic molality=1.6 times the stannate molality 0.3. Y

' caustic which is always present mmy baths at optimum plating compositions, regardless of the stannate concentration.

My preferred solution and method of operation'are as follows:

Example I Sodium stannate 0.4375 mol/1. or 12:43 oz/gal. Sodium hydroxide 1.0 mol/1. or 5.34 oz/gal. (Concentrations ot the above 2 ingredients by analysis of solution.)

Anodes-Straits tin Ratio ot anode to cathode area 3 to 1 Cathode current denslty 20 to 60 A/SF E. M. F .0 to 6.0 volts f Temperature ot ao1ution 70 to 80 C. (15B to 176 F.)

A white smooth and non-porous tin deposit will be produced in a short time on a metal surface, such as clean sheet steel, by using a bath of this kind. The anode and cathode efficiencies in the above example are approximately 50% each, calculated on the basis of divalent tin. A

Results comparable with the above may also be obtainedwith the following-bath; A

E example II s Sodium stannate 0.634 mol/1. or 18.0 oz/g'al. Sodium hydroxide 1.3 mo 1. or 6.94 oz/gaL (Concentrations ot the above 2 ing ents by analysis of solution.)

modes-Straits tin Ratio of anode to cathode area- 3 to 1 Cathode current density 20 to 60 A/SF n. M. r 4.o to 6.o vom Temperature of solution -80 C. (158 to 176 F.)

Other examples of my baths may be obtained by consulting the accompanying graph wherein symbol (D denotes compositions from which excellent deposits were obtained; the anodes remaining clean and the anode and cathode eiliciencies being equal to within about 10%. The symbol a: on the graph above line A denotes bath compositions from which spongy deposits were obtained.

Instead of making up In tin baths with alkali metal stannate itself, may use any tin salt which does not 'put objectionable impurities in the bath in the presence of alkali, to form the stannate in situ.

The sodium stannate concentration of my baths may vary from practically zero to the value at saturation of the solution with the same; good results being obtained between 0.07 and 0.70 moles per liter or between about 2.0 and 20 oz. per gallon, provided the caustic soda content always bears a close relation to the stannate content as shown by the graph. My preferred range, however, ies between about 0.25 and 0.65 moles per liter, that is, about a 1 normal to 2.6 normal solution or, between about 7.0 oz/gal. and 18.5 ozs. per gallon, since experience has demonstrated that below this range of stannate concentration, too much or too little caustic, to fall in the range represented between lines A and C, is much more likely to be produced during plating as the result of accidental variations in conditions, such as changes in current density, tem erature, introduced impurities, etc. Below? oz/gal. of sodium stannate, or its equivalent in other alkali stannate, the current eiiiciencies are usually lower with an attendant poor quality of deposit. On the other hand, when the stannate concentration is greater than about 18.5 oz/ al., the losses produced by the adhering og some of the solution to the plated articles as they are removed from the bath is too great for economical operation. Crystallization of tin salts from solutions at these higher concentrations has also beennoticed.

Good results may be obtained by keeping the plating solution below 85 the range' of 60 to 80 C. Above 85 C. there is danger of precipitation of solids within the bath.

I have found that my tin baths operate satisfactorily with cathode current densities above about 10 amperes per square foot, good deposits having been produced at 100 A/SF. Satisfactory anode current densities lie below 20 A/SF.

My tin plating baths may be used for lating many kinds of metal and articu arly steel, copper, brass, lead, zinc, ca ium, cast iron and the like. s

What I claim is 1. An aqueous electrolyte for the electro- `deposition of tin, containing 1.6 moles of caustic alkali per mole of alkali metal stannate plus an additional concentration, not dependent upon the amount of said stannate, of about 0.2 to 0.4 moles of caustic alkali per liter.

- 2. An aqueous electrolyte for the electrodeposition of tin, containing 1.6 moles of caustic alkali per mole. of alkali metal stannate plus an additional concentration, not dependent upon the amount of said stannate, of

C., but I prefer about 0.2 to 0.4 moles of caustic alkali per nate being 0.25 to 0.65 moles per liter.

3. An aqueous electrolyte for the electrodeposition of tin,containing about 1 mole of caustic alkali and about 0.4375 moles of al: kali metal stannate per liter of solution.

4. An electrolyte for the electrodepositionl of tin, comprising an aqueous solution containing 1.6 moles of caustic soda per mole of sodium stannate plus an additional concentration, not dependent upon the amount of stannate, of about 0.2 to 0.4 moles of caus- .tic soda .per liter.

5. An electrolyte for the electrodeposition of tin, comprising an aqueous solution containing 1.6 moles of caustic soda per mole of sodium stannate plus an additional concentration, not dependent upon the amount of stannate, of about 0.2 to 0.4 molesof caustic soda per liter, the concentration of sodium stannate being 0.25 to 0.65 moles per liter.

6. An electrolyte for the electrodeposition of tin, which comprises 1 mole of caustic soda and 0.4375 moles of sodium stannate per liter of solution.

7. A process for the electrodeposition of metallic tin, comprising conducting an electrolyzing current from a tin anode to the article to be coatedv as a cathode through an aqueous solution of alkali metal stannate and caustic alkali whose composition is limited by a caustic normality which increases at auniform rate from 0.7 when the stannate .normality is 1, to 1.34 when the stannate lsodium hydroxide whose composition is limited by a sodium hydroxide normality which increases at a uniform rate from 0.7 when the stannate normality is 1, to 1.34 when the stannate normality is 2.6, the range of sodium hydroxide normality at each normality of stannate being 0.1 less to 0.1 more than the values thus defined.

9. In a process for the continuous electrodeposition of tin from an aqueous solution of caustic alkali and alkali metal'stannate, the ste which comprises regulating the compostxon of said solution so asA to maintain 1.6 moles of caustic alkali per mole of said stannate plus an additional concentration, not dependent upon the amount of stannate, of about 0.2 to 0.4 moles of caustic alkali per liter.

10. In a proccssfor the continuous electrodeposition of tin from an aqueous solution of caustic soda and sodium stannate, the step which comprises regulating the composition of said solution so as to maintain 1.6 moles of caustic soda per mole of sodium stannate plus an additional concentration, not dependent upon the amount of stannate, ot

about 0.2 to 0.4 moles of caustic soda per liter.

Signed at Perth Ambo in the county of Middlesex and State of ew Jersey this 5th day of January A. D. 1929.

FLOYD F. OPLINGER.

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