US1527577A - Electroplating bath - Google Patents

Description

Patented Feb. 24, 1925.

UNITED STATES PATENT OFFICE.

ISAAC M. SCOTT AND SAMUEL PEACOCK, OF WHEELING, WEST VIRGINIA, WIL- LIAM EARL ARMSTRONG, OF MARTINS FERRY, OHIO, ASSIGNORS' T WHEELING STEEL & IRON COMPANY, 0F WHEELING,

WEST VIRGINIA.

WEST VIRGINIA, A CORPORATION OF ELECTROPLATING BATH.

No Drawing.

To all whom it may concern Be it known that we, IsAAo M. Soo'rT, SAMUEL PEACOCK, and WILLIAM EAR ARM- STRONG, all citizens of the United States, the

said IsAAo M.- Soom and SAMUEL PEACOCK residing at Wheeling, in the county of Ohio and State of West Virginia, and the said lVILLIAM EARL ARMSTRONG residing at Martins Ferry, in the county of Belmont and State of Ohio, have invented certain new and useful Improvements in Electroplating Baths; and we do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make and use the same.

This invention relates to improvements in electro-tinning and more particularly to the solution of electrolyte, whereby said electrotinning can be carried out on a commercial scale and in a manner more economical than has been heretofore attained.

With these and other objects in view the invention consists in the novel composition of matter constituting the electrolyte, all as will be more fully hereinafter disclosed and particularly pointed out in the claims.

In order that the precise invention may be the more clearly understood it is said :-In the electro-deposition oftin, great difficulty has been heretofore experienced in producing a dense and coherent deposit. This is especially emphasized when the objects to be electro-tinned are constructed of metals higher in the electrochemical series than is tin, such for example, as steel or iron. Here-' tofore, articles constructed of metals such as iron, were first electroplated with a metal lower in the electro-chemical series such for example as copper, and upon this last named metal the tin was deposited. But, solutions that are available for depositing the tin directly on iron or steel deteriorate rapidly, and cannot be operated on a commercial scale, economically, unless associated with a suitable restoring solution. These difficulties are all well known in this art.

On the other hand, this invention involves a solution and a method by which electro- 59 tinning can be carried out without difiiculty and much more efliciently than heretofore; and this solution can be used for plating Application filed February 19, 1923. Serial No. 620,010.

directly on steel and other metals higher in the 'electro-chemical series. Further, this method may be used for large scale operations with difl'erent concentrations and without causing a formation of colloidal or spongy deposits as in the prior procedures. The solutions contain a lower percentage of tin than those heretofore employed, they do not deteriorate or hydrolize to an objectionable extent, and they operate with almost 100% efliciency. This bath does not require a periodical addition of tin salts as heretofore, 01' in other words, it is self sustaining.

It will be understood by those skilled in the art that the thickness of deposit or amount of tin plated per' unit of surface will depend upon the length of time and the number of amperes used, irrespective of the voltage, but of course, the lower the voltage employed the more economical will be the deposition.

It has been found in practice that with say to 1% volts good deposits are obtained with this solution. In carrying out the invention inits preferred form, one may take, for example, 3 parts by weight of a commercial hydrofluoric acid (HF) and dissolve therein 1.6 parts by weight of boric acid H BO These acids react with an evolution of heat and form an acid known as fluoboric acid or borofluohydric acid, the simplest formula of which is HBF In this acid, one dissolves stannous oxide SnO until the acid has taken up the equivalent of 1.35 parts by weight of metallic tin, leaving about 31% of the acid present in the free state.

By this means, there is produced a solution of stannous fluoborate Sn(BF,) containing an excess of free fluobori-c acid HBIQ. To increase the conductivity of this solution one adds a metallic salt of a metal higher in the electro-cliemical series than iron, and prefers for this purpose the addition of 1.3.parts by weight of aluminum sulphate Al (SO 3 or 1.3 parts by weight of magnesium sulphate MgSO,,. This solution is then diluted with water to such a concentration that it will contain about 1 ounces of metallic tin to one gallon. In this diluted solution there is suspended metallic tin as an anode on a positive bus bar-and one suspends the articles to be lated such as sheet steel in the solution an connects them to the negative bus bar whereby on the passage of a suitable electric current, the steel articles become plated with metallic tin.

It has beenfound it is also desirable to add so-called addition agents or brightening agents, such as mollasses, gelatine, etc., to the solution for the purpose of obtaining somewhat improved and bright deposits. It is preferred to use ordinary glue for this purpose and in a quantity of about .03 ounce per gallon of solution, but a larger amount will not interfere with the success of the process. Instead of using stannous oxide $110 for introducing the tin into this solution, one may add other tin salts, such as stannous hydrate Sn(OH) or may dissolve in the solution metallic tin with or without the aid of the electric current. It has been found that the economical addition of tin may also be obtained by suspending metallic tin as anodes in the solution, connected with a positive bus bar and suspending metal strips, surrounded by porous cups in the solution, the metal strips being connected to a negative bus bar. By the passage of a suitable electric current, such as that above disclosed, the tin is dissolved from the anodes into the solution and being unable to pass into the porous cups, it is taken up by the acid. When suflicient tin has been dissolved the cups are removed and the plating continued.

We are unable to satisfactorily explain the exact scientific reasons why this solution produces such surprising efiiciencies in the coating of tin onto iron, but a plausible explanation resides in the following facts It is well known that the usual bath for electro-depositing tin contains the tin in the form of a sodium stannite or a sodium stannate. It is doubtful if a sodium stannate is in any way useful for tin deposition on iron. It is also well recognized that potash salts are equally inetlicient with sodium salts in the electrodeposition of tin on iron. But while these alkali metal tin baths give good initial deposits, they very rapidly l1y-.

drolize in practical use so that after a very small percentage of tin has been deposited a satisfactory coating is no longer obtained. The tin content of these prior baths hydrolizes in various forms but it is generally accepted that said content separates out in the form of an insoluble meta stannic acid. There is, however, some question as to what is the real form this said insoluble tin actually takes, but there is no dispute as to the fact that it separates out of the solution in a formthat makes it no longer available for elcctro deposition purposes.

Another fact which makes the alkali tin baths unsatisfactory is that the anode efficiency is very low.

That is, the tin is removed from the solution at an efficiency which is nowhere near a theoretical efiiciency if we consider the quantity dissolved from the anodes. It results that for any considerable quantity of tin deposited by the old procedures, fresh additions of alkali tin salts must constantly be added. 'It is therefore evident that it is only a question of time when the concentration of caustic alkali in the solution will be so great as to preclude any further deposition of tin and this fact alone constitutes a serious objection to the prior procedures.

It is a further recognized fact that with tin depositing salts, in which the tin exists as a base, such for example, as the chlorides or the sulphates, there is also experienced a hydrolytic separation of the tin in forms that are insoluble, and therefore not ionized. It thus again follows that these salts are not utilizable for a satisfactory electrodeposition of metallic tin. Unless these salts are maintained in a very acid solution, this hydrolytic decomposition is so rapid as to preclude the use of such baths for deposition purposes. But on the other hand, if the solution is maintained rather strongly acid, it is impossible to deposit satisfactory coherent tin coatings on metallic iron or steel, because of the action of the acid upon the iron or steel articles. That is, although the deposit may be made, it is very spongy and does not adhere.

There have also been tin baths proposed in which cyanides are employed, and while these baths serve somewhat better than the alkali tin salts for depositing on iron, yet, they are dangerous in practical operations, because stannic acid is capable of setting free prussic acid which is very poisonous and highly dangerous to the operatives. But even if this danger were obviated for which no means are known at this time, the bath is still unsatisfactory in practice due to an excessive hydrolytic decomposition of the tin.

It has further been proposed to use baths of tin containing alkali fluorides. While these baths have a somewhat higher anode efficiency than the baths in which tin is present as a base, yet, they still fail to deposit in large scale operations on iron or steel a satisfactory firmly adherent reguline deposit and are also subject to a very objectionable bydrolytic dissociation.

The bath composed of stannous oxide dissolved in fluoboric acid on the other hand is found to be largely free from the above objections, due probably to the presence of both the boric and the fluoric ions, and so far as is known seems to be the only bath which is not accompanied by an objectionable hydrolytic dissociation. This feature enables this bath to deposit tin on iron and steel in a firmly adherent, reguline coating, which practice proves to be very satistory indeed.

In the foregoing description there is discloseda tin solution in a somewhat diluted form with an acid reaction, but the solution can be used in more concentrated forms such for example as in those having a plurality of times the concentration of that above mentioned. The solution can also be used in the neutral state and for certain purposes one prefers a neutral solution, and such last named solution is especially desirable when chemically pure tin is being deposited.

The above solution has proved satisfactory for the deposition of tin on various metals such as iron, steel and copper, and therefore, it is not desired to be limited to the procedure set forth in the description. This solution and rocess may also be used with various modifications for numerous purposes that will readily suggest themselves to those skilled in the art without departing from the spirit of our invention.

For example, a conducting metal salt, carbon or other electric conductor may be deposited byvwell known means in the pores of a fabric or other material and tin may be deposited thereon by following the foregoing procedure. Therefore, it is not desired to be limited to the above disclosure except as may be required by the claims.

What is claimed is 1. The herein described electro-plating bath for depositing tin on electric conductors the same consisting essentially of a solution of stannous fluoborate.

2. The herein described electro-plating bath for depositing tin on electric conductors the same consisting essentially of a solution of stannous fluoborate admixed with the salt of a metal electro-positive to iron.

3. The herein described electro-plating bath for depositing tin on iron or steel the same consistin of a solution of stannous .fluoborate admixed wlth a salt of a metal