US1396051A - Process of coating steel sheets with tin - Google Patents

Description

' citizen of the UNITED STATES PATENT DFFICE.

SAMUEL PEACOCK, 0F WHEELING, WEST VIRGINIA, ASSIGNOR, BY MESNE ASSIGN- MENTS, TO WHEELING STEEL A CORPORATION OI WEST VIRGINIA.

& IRON COMPANY, or wnnnmn'o, wrsr VIRGINIA,

PROCESS OF COATING STEEL SHEETS WITH TIN.

No Drawing.

To all whom it may concern e it known that I, SAMUEL PEAcooK, a United States, residing at Wheeling, in the county of Ohio and State of West Virginia, have invented certain new and useful Improvements in Processes of Coating Steel Sheets with Tin; and I 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 a process of coating thin steel sheets with tin, and has for its object to improve the procedures heretofore proposed.

"With these and other objects in view, the invention consists in the novel steps and combinations of steps constituting the process, all as will be more fully hereinafter disclosed and particularly pointed out in the claims.

In order that this invention may be the more clearly understood, it is said: In the present procedure of coating one metal with another, such, for example, as coating thin sheets of mild steel with tin, to form the tin plate of commerce, it is usual to free the steel sheets of oxide, grease, etc., and to then immerse them in a molten bath containing tin, at atmospheric pressure.

The differences in contraction between the steel plates and the tin coatings on cooling, although very small, are, yet, sufiicient to form fissures or cracks in the very thin tin coatings now employed.

These fissures are usually too small to be readily detected by magnifying glasses, but may be readily detected by well known chemical tests, and they offer a sufiicient exposure of the steel through thetin coating to start an electrolytic action when immersed in an electrolytic solution, such, for example, as is afforded by many food products, such as fruits and vegetables, when put up in tin cans.

It therefore follows that .for many industrial uses the present tin plates have a short life due to the inevitable corrosion that follows their use, and the very serious, well known objections that said corrosion entails in producing salts that may be injurious to health.

It has been attempted to avoid these objec- Specification of Letters Patent.

. pheric pressure,

tlon is increased.

latented Nov. 8, 1921.

Application flied November 9, 1920. Serial No. 422,813.

tions by depositing the tin coating electrolytlcally on the steel plate, but such coatings invariably have a crystalline structure, and

are found in practice to be as objectionable provided from a molten bath, and therefore no progress toward a solution of the difli culty is made.-

I have discovered, on the other hand, that if instead of immersing the thin steel sheets in a coating bath of molten tin, under substantially atmospheric pressure, I substitute a solution of a tin salt, such as stannous chlorid SnCI and if I increase the pressure by means of heat to from say 5 to 15 atmospheres or higher, the advantages described below are attained. That is to say, I have discovered it I maintain a low ionic concentration in the tin salt solution, it will deposit out the tin in a colloidal state, insteal of in a crystalline state, and the steel will be covered completely and without fissures with said tin.

On the other hand, a low ionic concentration means a slow rate of coating. But I have observed that by heating the coating solution while maintaining a super-atmosthereon, the rate of deposi- This is especially true, if the solution is agitated and the pressure is maintained at a point sutiicient to prevent boiling of the coating solution. This ma) be readily accomplished by the use of an autoclave.

I have further observed that the degree of ionic concentration governs the colloidal nature of the metal coating, or the lower the ionic concentration, the more colloidal will be the deposited tin. But care must be taken to not employ such a low ionic concentration as will render the rate of coating so slow as to be commercially inoperative.

As a practical operation in carrying out this invention, I may use, say, 5 parts by weight of the stannous chlorid to 100 parts by weight of the diluent to be used, such as an ammonium alum or an alkali bi-tartrate.

I may next make up a 3% solution of said chlorid and alum or bitartrate with water, This said solution together with the sheets to be coated are placed in an autoclave under a steam pressure of say between 5 and 15 atmospheres, or preferably of about 10 atmospheres, when the tin will be found to be coated on the steel sheets in a firmly adhering non-porous, non-fissured condition, much superior to the coatings of the prior processes.

The plates are next removed from the autoclave, and dried, when they are ready for use in making tin cans, or other articles.

The function of the ammonium alum or the potassium or sodium bitartrate is to lower or restrain the natural ionic concentration of the stannous chlorid. But, of course, other restrainers may be employed. The function of the heat is undoubtedly to increase the kinetic energy of the tin ions. Of course, the plates or sheets should be in a clean condition, or free from scale, grease, etc., before being immersed in the solution. The latter should not have a concentration as high as 20% of the stannous chlorid, and from 3% to 5% is preferable.

It is obvious that those skilled in the art may vary the details of the procedure without departing from the spirit of the invention, and therefore, I do' not wish to be limited to the above disclosure except as may be required by the claims.

W'hat I claim is:

1. The process of forming a coherent coating of tin on thin steel sheets which consists in preparing a solution of stannous chlorid mixed with an alum having an alkaline reaction adapted to restrain the ionic concentration of said stannous chlorid; im-

mersing said sheets in a clean condition in said solution; and subjecting the latter to a steam ressure exceeding two atmospheres, substantially as described.

2. The process of forming a coherent coating of tin on thin steel sheets which consists in preparing a solution of stannous chlorid mixed with an alkali alum salt adapted to restrain the ionic concentration of said stannous chlorid; immersing said sheets in a clean condition in said solution; and subjecting the latter to a steam pressure exceeding five atmospheres, substantially as described.

3. The process of forming a coherent coating of tin on thin steel sheets which consists in preparing a solution of stannous chlorid mixed with an ammonium alum adapted to restrain the ionic concentration of said stannous chlorid; immersing said sheets in a clean condition in said solution, and subjecting the latter to a steam pressure exceeding five atmospheres, substantially as described.

4. The process of coating thin steel sheets with tin which consists in preparing a solution containing stannous chlorid, and an ammonium alum; immersing said sheets in a clean condition in said solution; and subjecting said solution to a steam pressure exceeding five atmospheres, substantially as described.

5. The process of coating thin steel sheets with tin which consists in preparing a solution containing less than 20% of stannous chlorid and ammonium alum; immersing said sheets in a clean condition in said solution; and subjecting said solution to a steam pressure exceeding five atmospheres, substantially as described.

Intestimony whereof I allix my signature.

SAMUEL PEACOCK.