US1979996A - Electroplating process - Google Patents

Description

Patented Nov. 6, 1934 ELECTROPLATING PROCESS William M. Phillips,

Birmingham, and Guy M. Cole, Detroit, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Application April 3, 1931,

Serial No. 5275618 '1 Claims.

The invention relates to the electrodeposition of tin particularly upon surfaces of ferrous metal, i. e., metal consisting largely or chiefly of iron. More specifically, it is designed to provide a method whereby the coating metal may be deposited and the surface of the base metal may be cleaned or pickled, both in a single operation. Tin possesses marked advantages as a coating for ferrous metalsurfaces. It provides a smooth bright surface of pleasing appearance and of considerable protective value in resisting oxidation and corrosion of the base metal. More recently also it has been found that a thin deposit of tin on engaging surfaces of parts subjected to friction, such as pistons and gears, imparts extraordinary antifriction properties to the surfaces. The electrodeposition of tin is therefore assuming increased industrial importance.

It is necessary, of course, to clean the surface by removal of scale and other matter, such as oil and grease, before the plating operation and we have discovered that it is possible to perform both the cleaning operation and the plating operation in a single bath. by suitable regulation of the bath ingredients and proper coordination of the treatments.

It is known that removal of scale and other materials adhering to ferrous metal can be accomplished by treatment with a pickling acid, which may be, for example, sulphuric, hydrochloric, phosphoric, etc. The cleaning operation is also facilitated by electrolytic action by passing an electric current through the bath and thus evolving hydrogen at the surface of the metal to be cleaned. Since the electrodeposition of tin requires a direct current through the bath to the article as cathode, we utilize the same current to facilitate the pickling operation and to deposit tin on the cleaned surface.

Tin may be deposited from acid solutions of its salts, such as the sulphates, chlorides, phosphates, etc., and we have found that the pickling acids, for example, sulphuric acid, may be added to the bath in considerable excess without interfering with the deposition of a smooth, dense and adherent coating of tin.

This fact makes it possible to increase the acid content to such extent that it will remove scale and, atthe same time, by passing direct current through the bath, deposit tin. In order to secure the advantages of electrolytic pickling, the current is supplied at such strength as to cause copious evolution of hydrogen at the cathode. The deposition is sufficiently selective in action as the tin does not deposit on the scale but is deposited only on the unoxidized or cleaned surfaces. Since the plated surface is also resistant to corrosion by the pickling acid, the deposition of the tin also serves to arrest further attack by the acid upon the cleaned surfaces.

The proportions of tin salt and of acid may be varied over quite a wide range in accordance with the required conditions of cleaning and plating. The same is true of the amount of current required and the optimum temperature range. Since tin is a relatively expensive metal, it is desirable, for economic reasons, to maintain as low a tin concentration as is compatible with the production of a satisfactory deposit. The time required for the cleaning operation is also an important factor. The throwing power of the tin bath varies, roughly, directly with the concentration of the tin salt and inversely with the strength of the acid. It will be understood therefore that the solution must be so constituted, with reference to the condition and character of the article, as to effect the desired cleaning operation in the predetermined time and with the selected conditions of temperature and current. These factors having been determined, it then remains to so adjust the contact of tin salt as to enable the article to be completely covered in the time allowed, the solution being made more concentrated for shorter periods of treatment since deposition takes place more rapidly from more concentrated baths. Solutions for commercial use have been prepared, using, for example, tin sulphate and sulphuric acid, with the tin salt concentration varying from 1 gram per liter to 100 grams per liter and the acid concentration varying from 5 to 20% (by volume). With these bath concentrations, the current was varied from 25450 amperes per sq. ft. of cathode surface and the temperature from 65 F. to 212 F. The effect of increasing the current and raising the temperature is to increase the rate of cleaning with a given acid concentration and increase of current will, of course, increase the rate of deposition. The tin salt preferably used is determined chiefly by economic considerations. The sulphate is cheap and is entirely satisfactory for most purposes. Tin may also be supplied in the form of various other salts, for example, sodium stannate. Sulphuric acid is generally preferred as the pickling acid because of its availability and other, advantages. With the sulphuric acid bath, anodes of pure lead or antimonial lead may be used as such anodes are sufliciently inert in the sulphate bath. Tin anodes. might be emp y d. as a source of metal, but the use of tin salts renders it easier to control the bath concentration. If hydrochloric acid is used as apickling acid it is desirable to use anodes relatively inert in that acid, for example, carbon anodes.

There are two classes, chemically, of the tin salts, viz., the stannous and stannic salts. The stannous salts, being of lower valence, deposit at greater efliciency than the stannic salts. However, the production of smooth, bright, adherent deposits of tin from solutions of stannous salts, requires generally the presence of an addition agent, frequently of colloidal nature, such as glue (or related substances, as gelatin, dextrin, etc.), phenol, aloin, sulphonated castor oil, etc. Without the use of one of these agents, which we term for convenience adhesion-promoting addition agents, nothing more than spongy and loose deposits are ordinarily obtained from stannous salts.

The stannic salts show much less tendency to form spongy or loose deposits and do not generally require the addition agent. The rate of deposition and throwing power of such baths, however, are inferior to those of the stannous salts, and, although some of the well-known oxidizing agents may be employed to convert the stannous to stannic salts and thus reduce the tendency to form loose deposits, the use of the stannous salts with an adhesion-promoting addition agent is considered preferable.

As a specific example of successful commercial operations, we have treated forged heat-treated gears having on their surfaces a heavy scale resulting from heat treatment. These gears were subjected to pickling and plating treatment in baths containing between 10 and 15% of sulphuric acid (by volume) and about 1 /2 to 2 /2 grams per liter of tin. The latter was added in the form of some convenient salt of tin, for example, sodium stannate, the tin of which appears in the bath in the stannic form, or stannous chloride, in which latter case one of the adhesionpromoting addition agents is employed e. g. about a half gram of glue per liter. The current strength supplied was about 70 amps. per sq. ft. of cathode area and the temperature of the bath was about 150 F. The treatment was continued for from 3 to '7 minutes depending upon the character of the scale. The gears come from the bath with scale entirely removed and with a smooth bright deposit of tin over the entire surface.

The use of our method enables a considerable saving to be made in time, equipment and expense over those involved in the usual tin plating methods in which separate baths are required for cleaning and plating. Furthermore, the usual methods generally require one or more additional operations to protect the pickled surfaces and/or to clean them again preliminary to the plating operation. The coating obtained is dense, smooth and strongly adherent and may be obtained in any desired thickness by suitable adjustment of the tin content and current density with relation to the other factors. For economic reasons, however, a thickness of not over one half a thousandth of an inch is recommended as thicker deposits can be more cheaply produced in separate plating baths. Even when thicker deposits are required, however, it is desirable in some cases to first treat the articles by the method herein set forth and increase the thickness of the deposit by a subsequent plating operation in a more concentrated bath. Other protective metal coatings adherent to tin, such as copper, may also be deposited directly on the tin coating.

We claim: v

1. The process of electrolytically removing scale from ferrous metal surfaces and simultaneously electrodepositing tin thereon comprising subjecting said surface to the action of a bath containing a scale-removing concentration of a pickling acid while passing current through the bath to the surface as cathode, to thereby evolve hydrogen at said surface, the bath also containing such concentration of tin salt that the current employed will deposit a continuous adherent coating of tin on said surface in substantially the time required for the removal of scale therefrom.

2. The process as set forth in claim 1, said tin salt being a stannous salt and the bath containing an addition agent adapted to promote adhesion of tin deposited from such salt.

3. The process as set forth in claim 1, the bath containing sulphuric acid to the amount of approximately 5 to 20% by volume.

4. The process set forth in claim 1, the bath containing between 5 and 20% of sulphuric acid, by volume, and a salt containing tin in amount sufficient to provide 1 to 2 grams of tin per liter.

5. The process of cleaning and coating an article having ferrous metal surfaces comprising immersing the article in'a bath containing a pickling acid in such concentration as to remove scale and also a tin salt from which tin may be electrolytically deposited, supplying electric current to the article as cathode in the bath in quantities between 25 and 150 amperes per square foot of cathode surface, the acid concentration being maintained such as to effect electrolytic pickling with the current employed and the concentration of tin being maintained such as to effect deposits of tin upon the cleaned surfaces sufiicient to protect the cleaned surface from further attack by the acid.

6. Process as set forth in claim 5, the content of acid being 5 to 20% by volume of sulphuric acid and the tin being present as a stannous salt in quantity sufficient to provide 1 to 2 /2 grams of tin per liter of bath.

7. The process of treating articles of ferrous metal to remove scale comprising placing the article in a bath containing a sufficient concentration of pickling acid to effect scale removal, passing through the bath to the article as cathode an electric current of such strength as to cause copious evolution of hydrogen at the cathode to assist in removal of scale, maintaining in the bath a concentration of tin salts of such character that tin can be deposited therefrom by the said current, and so adjusting the concentration of tin salts, the acidity, current density and temperature that a thin deposit of tin forms upon the scale-free surfaces of the article in substantially the time required to remove the scale and of substantially only the thickness required to protect the cleaned surface from attack by the acid.

WILLIAM M. PHILLIPS. GUY M. COLE.