US2392646A - Electroplating tin - Google Patents

Description

Patented Jan. 8, 1946 William B. Cooper and more, Md, assignors Company, Inc., Baltimore,

of New York Charles E. Glock, Bamto Crown Cork & Seal Md., a corporation No Drawing. Application July 11, 1942, Serial No. 450,610

1 Claim. (01. 204-54) This invention relates to electroplating with tin and particularly to a bath for plating an indefinite length of black iron or steel strip whereby the same may be continuously passed through the plating bath and provided with a smooth uniform matte 'or satin finish.

The primary object of the invention is to pro vide an alkaline tin plating bath containing an alkali metal stannate, such as sodium stannate. and which will be stable over a wide range of operating conditions.

' tions, due to the use of ioopers so Another object of the invention is to provide a bath in which the sodium stannate is maintained within substantially constant critical limits and 1 without requiring that stannate be added during plating. I

A further object of the invention is to provide a sodium stannate bath in which a balance is constantly maintained between the amount of tin dissolved from the anodes and the amount deposited on the traveling strip by means of a critical alkali content whereby the tin content of the solution does not change and addition of alkali stannate during coating is made unnecessary.

An additional object of the invention is to provide a bath containing a soapy agent which will produce a foam on the surface of the plating so-.

lution, and prevent the solution from"spraying, due to hydrogen being liberated at the cathode and oxygen at the anode.

0i! equal importance with the foregoing, it is an object of the invention to provide an alkaline tin plating bath containing sodium stannate in which the ratio of current density of the anode to the cathode may be maintained substantially equal, 1. e., 121, and with an anode current density of substantially 35 amperes per square foot, without requiring stannate additions during plating.

Another object of the invention is to provide an alkaline sodium stannate bath in which it is unnecessary to maintain a greenish-yellow film in order to produce smooth and uniform electrodeposited coatings of tin.

Another object of the invention is to provide a bath in which an oxidizing agent, such as hydrogen peroxide, need not be employed,

Incarrying out the present invention, we will describe the same in connection with the electrotin plating of 18 inch to 32 inch width steel strip 10! indefinite length. The strip is continuously traveled, first through an acid dip, then through a water rinse, then through the electroplating bath, after which it is spray-washed'and dried.

The operations are carried out in a "line, and the strip never stops during the various operathe ends of the coils of strip can'be welded together and the finished coated coils stripped while the band is in continuous motion.

By strip steel, we mean various grades available and useful for the manufacture of contain ers and container closures, among which maybe mentioned low carbon rimmed steel, low carbon killed steel, and rephosphorized steel, as well as ordinary black iron strip. The strip is of the usual thicknesses employed for making containers and closure caps of sheet metal.

The pickling operation on the traveling strip is preferably carried out in a sulfuric acid solution of 3 to'6% H2304 by weight in water at 170 F., using an inhibitor to prohibit etching of the strip. The sheet as it leaves the acid tank is'covered by a mm of acid since otherwise, a superficial oxide will form on its surface and the sub- The traveling strip is then rinsed free of acid in a simple dip tank having clean water constantly replacing the acid contaminated water resulting from the carry over of acid. The tank is bailled so that fresh water enters as the sheet leaves and is drained ofi at the entry side. The rinse water is removed from the sheet as close to the plating bath as is practical. This is done to eliminate the superficial oxidation that will result whenever the sheet is exposed for a short time to the atmosphere. The faster the strip travels, the less trouble is experienced with this type of oxidation. A squeegee roll is employed to remove most or the rinse water and so constructed that a residual, continuous film of water remains on the sheet whereby the strip is protected until immersion in the plating solution to-which it now passed.

A preferred plating'solution found to give consistently good results has substantially the following formula:

Tin (as sodium stannate) Substantially 30 g./l. Sodium hydroxide Substantially 10/12 g./l.

Hydrogen peroxide (optional)? Substantially pt. every A hr. to 10,500 gallon tank. Substantially Ridosol #14 suflicient amount to obtain a blanket of foam. 1 qt. to 10,500 allons of plating solut on. Temperature About F.

The plating tank has a capacity of about.10,500 gallons, and the tin anodes are disposed vertically therein in spaced relation. The traveling situated that j strip is coated while traveling parallel to the anode surfaces in the above bath.

When the caustic content is allowed to go as low as 9 g./l. (as NaOH), excessive sludging of the bath results, and if raised above 16 g./l., a good smooth coating cannot be obtained. When the tin content drops below 30 g./l., the plating becomes very dull and loses its satin or matte appearance. As the tin content goes above 45 g./l.,

' tin tends to deposit on the current carrying rolls,

forming objectionable tin pimples." These put small dents in the strip and must be scraped from the rolls.

We have discovered that if the caustic content is kept within the critical limits described, a balance is maintained between the amount of tin dissolved from the anodes and the amount deposited on the traveling. strip. In thi manner, the tin content of the solution does not change, as demonstrated by the factthat n additions of sodium stannate were necessary over a severalmonth period with a line operating at speeds up to 370 ft./-min. on 29" wide material 24 hours a day and seven days a week. An anode current density of 35 amp/sq. ft. is used in normal operation, and the anode to cathode ratio is roughly 1:1. This current density allows the anode to corrode uniformly, and no special treatment is necessary to maintain the anodes in good condition.

"RidosoP #14 is an agent which is of soapy character and is added to the alkaline tin plating bath to produce a foam on the surface of the plating solution. This foam prevents the solution from spraying, which is caused by hydrogen being liberated at the cathode and oxy en at the anode. These gases, rising out of the solution as very fine bubbles, carry some of the caustic plating solution with them and thereby causes discomfort to the people workingin the vicinity. In addition, parts of the nearby machinery are coated with this caustic compound. 'I'he'foarri collects these gases and subsequently releases them in larger masses and at a slower velocity.

The RidosoY' alsoreduces the surface tension of the solution, thus increasing its detergent properties. The solution has a greater tendency to plate more uniformly even on fairly greasy surfaces.

If desired, sodium acetate can be used in the bath but is not necessary after the bath has been in operation for a time. Its primary action seems to be in builering the solution.

In order to eliminate the use of hydrogen peroxide, we employ one iron anode 15" wide by St" long, which appears to guard against formation of spongy deposits.

ing as by a fusing operation, the brightening appears to be facilitated. Sodium cyanide is not preferred due to its creation of a health hazard. Sodium orthosilicate in amount of about up to 13 grams per liter may be used to replace caustic coda with good results when the bath is fresh, but on aging for a few days, a silicious precipitats forms. This precipitate canbe redissolved by additions of caustic soda. This bath is not considered to be as practical in operation as one using caustic. By way of example, the composition of an orthosilicate bath is:

Tin (as sodium stannate) g./l. 30 Sodium orthosilicate g./l-- 13 Sodium acetate g'./i it Sodium phosphate to replace sodium hydroxide may be added to the stannate bath in amount of up to about 25 g./l.' and a good plating obtained.

Also, an alkaline'bath made up of sodi suiflde replacing sodium hydroxide and sodium stan= mate as the principal constituents will produce satisfactory tin plating. An example of the com position of a sodium sulfide bath is:

Tin (as sodium stannate) -g./i 5t Sodium sulfide (Na-2S) g./i it The solution was filtered and operated at 1T0 F. giving very good results.

Sodium orthosilicate, sodium phosphate and sodium sulfide have all been used to replace sodium hydroxide, all or in part.

In the use of the alkali, such as the caustic or the equivalent alkaline materials, the amount present in the bath is always efiective to main tain a constant balanc between the quantities of anode dissolved tin and deposited tin, whereby plating on a strip of indefinite length may be carried out continuously without necessity of adding stannateto the bath during the coating.

The strip leaves the plating tank through two sets of wringer rolls. The first set removes most of the plating solution. The second set is supplied with water sprays which wash most of the remainder of the solution back into the plating tank. A balance is maintained between the rinse water entering the tank and water lost by evaporation so that the'amount of solution is con stant. The solution is constantly filtered to re move any coarserparticles.

The coated strip, after leaving the plating bath. is subjected to high pressure hot water sprays in a rinse tank. These sprays thoroughly clean the strip without scratching or otherwise changing the surface of the coating. It then passes through wringer rolls to a drying tunnel consisting of a blast of hot air and then the strip is continuously coiled and is ready for the next operation. This step can consist of shearing to sheets or a reflowing or brightening operation by fusing the tin surface or cold rolling. v

A tabulation of line speed and coating weights is here shown for illustrating the advantageous results obtained by employing the critical ranges of caustic andstannate specified:

[Speed vs. coating on 29" wide (average) strip] Solution composition Ti FtJmiu. Am e... mtg;

G. Sn/l G. NaOH/l LbJbb. 40, 000 42. 41 i0. 51 32 40,000 36. 94 10.03 40 40, 000 36. 94 l0. l2 50 40, 000- 39. 52 i1. 11 56 40, 000 39. 27 ll. 35 40, 000 40. 22 10. 95 40, 000 38. 70 10. 03 81 While we have referred herein to continuous strip metal as being successfully coated, it is to be understood that other continuous materials, such as wire, as well as individual objects, are

susceptible of being electroplated with equally satisfactory results.

Rodosol 14 is a product the composition of which usual y is of complex organic nature, that is, sulphonated hydrocarbons. It is oi. a sea forming material and can be replaced by soaps such as sodium palmitate, oleate, etc.

We claim:

A process for the electrodeposition 0! metal- I .10 anode to cathode ratio of about 1:1.

lic tin comprising conducting an electrolizing current from a tin anode to the article to be coated as a cathode through an aqueous solution having a temperature oi about 190 F. and comprising essentially sodium stannate to give substan- 1 tially 30 to 45 grams per liter of tin and sodium hydroxide substantially 10 to 12 grams per liter 'to produce a substantially constant balancebe-f tween the quantities of anode dissolved tin and deposited tin, maintaining an anode current density of substantially thirty five amperes per square foot with a ratio of current densities'ot anode to cathode or substantially 1:1 andan wnmm a. 000mm. CHARLES E. cnocx