US2211251A

US2211251A - Tin plating

Info

Publication number: US2211251A
Application number: US242042A
Authority: US
Inventors: Harold J Barrett; Christian J Wernlund
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1936-07-13
Filing date: 1938-11-23
Publication date: 1940-08-13
Anticipated expiration: 1957-08-13
Also published as: FR863403A; DE882505C; US2171842A; FR824277A; CH201630A; BE422634A; NL48191C; DE687136C

Description

Patented Aug. 13, 1940 UNITED STATES 2,211,251 I TIN PLATING Harold J. Barrett;

com-hams.

Niagara Falls, Wernlund, North Tonawanda, N. Y., assignors to E. I. du Pont de Nemours& Company, Wilmington, Del., a corporation of Delaware No Drawing. Application November 23, 1938, stl'lll N0. 242,.

140mm; wiser-11) r depositing tin from stannite-stannate baths of v This invention relates to the art of electroplating metals and more particularly to a novel addition agent for electroplating baths containing bivalent tin, more particularly alkaline electroplating baths, to improve the qualities of the electroplated deposits.

This applicaiionis a continuation-impart of our copendingapplication Ser. No. 90,382, filed July 13, 1936.

In the art of electroplating metals with tin from alkaline baths it is known that thepresence of bivalent tin ions tends to lead to the formal tion of spongy or porous deposits. Heretofore,

therefore, tin plating baths were almm exclusively prepared from tin in its quadrivalent form. The deposits obtained from such stannate baths possess a very fine grain structure, the fineness of which still can be further enhanced by the addition of corn sugar, starch or the like addition agents to the plating bath, This electroplating process is also characterized by its good throwing power, i. e. the tin deposited from such stannate baths is evenly deposited on articles of any kind, both on the inner and outer surfaces. The current efllciency of these known processes is about based on the deposition of Sn++++ ions. Although this process leads to highly satisfactory results as to the quality of the products obtained, it leaves much to be desired with respect to the 'current efliciency and to the stability ofthe stannate during the electroplating process.

The weight of tin deposited from stannate baths at a given current density is only half'as great as that from stannate baths. Besides this, usually a slight amount of the stannate is reduced to stannite and it is known that even small amounts of stannite in a stannate plating bath may lead to the formation of spongy and porous deposits. To overcome this condition hydrogen peroxide or other oxidizing agents may be added from time to time to such stannate plating baths to oxidize any stannites formed. Although by this expedient the formation of stannite is prevented the cost of the plating baths is increased thereby. This disadvantage as well as the rela-'- tivelylow rate of metal deposition is not very serious where only thin deposits are required. However, for the formation of heavy deposits a higher speed of metal deposition is urgently needed to decrease the time of plating.

The object. of the present invention is an improved electroplating process for depositing metals from plating baths containing tin in the bivalent form. Another object of the invention is varying proportions. A iurther object of the invention is an improved process for depositing tin from straight stannite plating baths. A further object of the invention is a new addition agent for electroplating baths containing bivalent tin ions. Further objects :of the invention will be hereinafter apparent.

The above objects of the invention are accomplished by adding to and dissolving in a plating bath containing bivalent tin ions or tending to the formation of such bivalent tin ions a hydrophilic' polyvinyl compound selected from the group consisting of polyvinyl alcohol, polyvinyl esters, polyvinyl ethers and polyvinyl acetals. By dissolving such ahydrophilic polyvinyl compound in a plating bath containing bivalent tin ions, the formation 01' spongy or porous deposits is inhibited or substantially prevented. Fbr example, when adding a small amount of such a hydrophilic polyvinyl compound to an alkaline stannite plating'bath, deposits of a substantially increased whiteness and a pleasing matte luster are obtained. Thus it is made possible to deposit tin from straight stannite plating baths and thereby to obtain twice the thickness of plate under a given set of conditions in a given time as when using the stannate bath. Although the throwing power of such stannite baths is somewhat inferior to the throwing power of stannate baths, such stannite baths will be preferable wherever articles of simple shape such as metal sheets are coated or where low costs or high power eiliciencies, respectively, or thick coatings are essential.

When the polyvinyl addition agents are added to a stannate bath the deleterious efiect on the deposits of small amounts of stannite present therein or formed during the plating process is eliminated and the plating bath will retain substantially the characteristics of a pure stannate bath. By this expedient therefore the addition of hydrogen peroxide or other oxidizing agent to the stannate bath is made unnecessary.

Our invention makes it further possible to deposit tin from plating baths containing any ratio of stannate and stannite. The properties of the resulting deposits will be governed by the ratio of the stannate to stannite used in the bath, the

Our invention is accordingly applicable wherever bivalent tin ions are present or are formed 55 'taining bivalent tin ions.

regardless of the specific composition of the plating bath. Our invention accordingly is not restricted to any specific composition of the plating bath but in its broad aspect is directed to a plating process or plating bath, respectively, where bivalent tin ions are present either in small or in large amounts. Preferably, our invention is utilized for alkaline plating baths con- A large number of tin plating baths are known which are satisfactory but for the presence of stannite and it will be self-evident for anyone skilled in the art of electroplating where to apply our invention. Our invention is likewise applicable for plating tin alloys where the plating bath contains one or more metals other than tin but tends to lead to the formation of spongy or porous deposits due to the presence of bivalent tin ions, and where the herein described polyvinyl addition compounds are suitable for electroplating such other metals. Also our method obviously is suitable for plating from stannite plating baths which contain minor .amounts of any other metal.

The group of addition agents of our invention includes a variety of hydrophilic polyvinyl compounds selected from the group of polyvinyl alcohol, polyvinyl esters, polyvinyl ethers and polyvinyl alcohol acetals which all are more or less soluble in water. As used in the specification and the appended claims the terms "soluble in water and "in active solution and similar expressions are meant to indicate that the hydrophilic polyvinyl compounds are capable of forming true solutions in water or colloidal suspensions (hydrosols) in water. Polyvinyl alcohol itself is a resinlike material which may be obtained by hydrolysis of polyvinyl acetate which in turn is made by polymerizing vinyl acetate. Polyvinyl alcohol may be made in a number of grades or modifications which differ one from another by their degree of polymerization. All of these modifications are fairly soluble in water, for example, 10% by weight aqueous solution of polyvinyl alcohol may readily be made although the actual solubility will vary somewhat depending upon the degree of polymerization. Various derivatives of polyvinyl alcohol may be made by different chemical reactions, which derivatives may be either water soluble or capable of being swelled by contact with water. We use the term hydrophilic polyvinyl compounds selected from the group consisting of polyvinyl alcohol, polyvinyl esters, polyvinyl ethers, and polyvinyl alcohol aicetals" hereinafter and in the appended claimsfigto designate such materials made from or derived from polyvinyl alcohol which are soluble in water in the eifective amount or are capable of forming colloidal suspensions in water. One example of such hydrophilic polyvinyl compounds of the above speci fled group is the so-called partial ester of polyvinyl alcohol which may be made by only partially hydrolyzing a polyvinyl ester. If the hydrolysis of the ester is carried out to a sumcient extent, the resulting compound which contains both ester groups and free hydroxy groups in the same molecule will be hydrophilic in nature. Other derivatives of polyvinyl alcohol having similar properties are the partial polyvinyl alcohol acetals which may be made by incompletely condensing polyvinyl alcohol with aldehydes such as formaldehyde or acetaldehyde and the partial ethers of polyvinyl alcohol which may be made by partially etherifying polyvinyl alcohol with some other alcohol such as glycerine, methanol or the like. Also certain esters of polyvinyl alcO- hol may be made which are sufficiently watersoluble for the purposes of the present invention, for example, the boric acid ester and the sulfuric acid ester; the latter ester may be made by reacting polyvinyl alcohol with sulfuric acid.

The amount of hydrophilic polyvinyl compounds to be added to the electroplating bath in general will vary, depending upon the composition of the plating bath, and the degree of whiteness required. Very small amounts of these materials bring about the desired effect and as a matter of fact almost any amount dissolved in the plating bath will bring about an improvement in the deposits. However, additions of such hydrophilic polyvinyl compounds greater than the equivalent of one gram per liter of polyvinyl alcohol usually will not bring about any further improvement of the desired effect and therefore will not be utilized in commercial plating work. As a rule, the upper limit will be determined by the solubility of the polyvinyl compound used and additions in amounts which lead to precipitation should be avoided since precipitates in plating baths generally are undesirable.

We have found that for the application of our invention for plating baths which consist preponderantly of stannite solutions, optimum results can be obtained when observing specific conditions. Thus, for electroplating metals in plating baths substantially composed of alkali metal stannite containing from about 13 grams per liter to 104 grams per liter of sodium stannite and from about 10 grams per liter to 65 grams per liter of sodium hydroxide and from about 0.1 to 1 gram per liter of polyvinyl alcohol are especially suitable. Equivalent amounts of other soluble stannites and other alkaline agents and other hydrophilic polyvinyl compounds can be used. Cathode current densities up to 30 amperes per sq. ft. can be advantageously utilized when keeping the baths at about C. Further increase of current density may lead to the formation of dark deposits.

The sodium stannite solutions, for example, can be readily prepared by adding a stannous chloride solution slowly to a caustic soda solution with thorough stirring. The temperature should not exceed 20 C. If desired, the slight amount of precipitate which is formed may be filtered off. Especially suitable solutions can be prepared by mixing stannous chloride and caustic soda within the following range:

Grams per liter SIJCh-2Hz0 15 to NaOH- 15 to Polyvinyl alcohol 0.1 to 1 Preferably we use 110 grams per liter of Such-211:0 and 150 grams per liter of NaOH and 0.2 gram per liter of polyvinyl alcohol. The cathodic current density for different temperatures preferably is maintained as follows:

30 C.--not more than 7 amp/sq. ft. 65 C.-not more than 15 amp/sq. ft. 80 C.--not more than 30 amp/sq. ft.

tic soda on the above concentrations to 50 grams per liter lowers the current density range to about up to 10 amperes per sq. ft. The invention is further illustrated by the following examples:

Example 1 One liter of a solution containing 220 grams of stannous chloride per liter of water was slowly added to one liter 01' a cooled solution of sodium hydroxide containing 300 grams per liter and the temperature was carefully held below 20 0. Small amounts of precipitate were filtered off. The sodium chloride formed by this reaction had no influence whatsoever on the subsequent electroplating process and therefore was not removed from the plating bath. To this solution 0.2 to 4 grams per liter of polyvinyl alcohol was added and dissolved therein.

This bath was then utilized for the deposition of tin on a steel surface, utilizing pure tin as anode and clean sheet steel as cathode. Equally good results were obtained with varying amounts of polyvinyl alcohol in the above specified range.

For the first 8 hours a current density of about 30 amperes per sq. it. at 80 C. was applied and during the second 8 hours the current density was lowered to 15 amperes per sq. ft. The plated articles had a dense tin coating of a pleasing white color and a matte luster. The surface of the deposits as compared with the surface of deposits obtained from a stannate bath had a somewhat coarser grain.

Example 2 Equal volumes of the following solutions were prepared and mixed together.

The resulting bath was operated in the temperature range of 70 to 80 C. to plate tin onto a steel cathode. For the first hour 30 amperes per sq. It. was utilized and thereafter about 20 amperes per sq. ft. A small amount of tin oxide precipitated out of this bath but this did not aifect the quality of the tin plate produced. The electroplated articles were of similar appearance to those described in Example 1.

Example 3 The following plating baths were prepared and operated at '70 to 80 C.

#3 #4 Plating bath Grams per Gra ns pcr Grams per Grains per liter liter liter liter SnCl1-2H20 30 l5 15 15 NaOH 30 15 20 15 Polyvinyl alcohol 0.2 0.3 0. 2 0.2

Bath #1 5 to 20 Bath #2 5 to 15 Bath #3 5 to 15 Bath #4 5 to 10 Tin salts other than tin chloride and soluble stannites other than sodium stannite can be used for the process of our invention. As stated above the process of our invention is applicable for any electroplating bath containing stannite vinyl compound selected from the group consisting of polyvinyl alcohol, polyvinyl esters, polyvinyl ethers, and polyvinyl acetals.

3. An electroplating process which comprises electroplating metals with tin in an alkaline plating bath containing a soluble stannite and having dissolved therein a hydrophilic compound selected from the group consisting of polyvinyl alcohol, polyvinyl esters, polyvinyl ethers, and polyvinyl acetals.

4. An electroplating process which comprises electroplating metals with tin in an alkaline plating bath consisting preponderantly of a soluble stannite and having dissolved therein a hydrophilic compound selected from the group consisting of polyvinyl alcohol, polyvinyl esters, polyvinyl ethers, and polyvinyl acetals.

5. An electroplating process which comprises electroplating in an alkaline plating bath containing a soluble stannite and polyvinyl alcohol.

6. An electroplating process which comprises electroplating metals with tin in an alkaline plating bath containing an alkali metal stannite and having dissolved therein polyvinyl alcohol.

'7. An electroplating process which comprises electroplating metals with tin in an alkaline plating bath consisting preponderantly of alkali metal stannite and having dissolved therein polyvinyl alcohol.

8. An electroplating bath containing a soluble stannite and a hydrophilic compound selected from the group consisting of polyvinyl alcohol. polyvinyl esters, polyvinyl ethers, and polyvinyl acetals.

9. An electroplating bath comprising alkali metal stannite and alkali metal hydroxide and containing a hydrophilic polyvinyl compound selected from the group consisting of polyvinyl alcohol, polyvinyl esters, polyvinyl ethers and polyvinyl acetals.

10. An electroplating bath comprising alkali metal stannite and alkali metal hydroxide and containing polyvinyl alcohol.

11. A tin plating bath comprising sodium .0

stannite, sodium hydroxide and a hydrophilic polyvinyl alcohol derivative, prepared from a mixture of 15 to grams per liter stannous chloride, 15 to grams per liter sodium hydroxide and 0.1 to 1 gram per liter polyvinyl alcohol.

12. A tin plating bath containing alkali metal stannite and stannate and a hydrophilic'polyvinyl compound selected from the group consisting of polyvinyl alcohol, polyvinyl esters, polyvinyl ethers, and polyvinyl acetals.

13. The process comprising preparing alkaline tin plating bath by dissolving together in water a stannous salt, alkali metal hydroxide and a hydrophilic polyvinyl compound selected from water stannous chloride, sodium hydroxide and the group consisting oi polyvinyl alcohol, polypolyvinyl alcohol and thereafter electrolyzing vinyl ester, polyvinyl ethers and polyvinyl acetals said bath. and thereafter electrolyzing said bath. HAROLD J. BARREIT.

5 14. The process comprising preparing an al- CHRISTIAN J. WERNLUND. 5

kaline tin plating bath by dissolving together in