US2330608A - Preparing stock for coating and electroplating - Google Patents

Description

Patented Sept. 28, 1943 PREPARING s'rocx FOR COATING AND nnnoraormrmo John S. Nachtman, Beaver, Pa.

' No Drawing. Application January 25, 1939,

Serial No. 252,78}

This invention relates to the art of coating metallic sheet, strip, wire or similar material, and is concerned with the steps or step of preparing the stock for reception of the coating. The process of the invention will, hereinafter, be described with particular reference to preparation of ferrous metal strip stock for the recep- Claims. (Cl. 204-34) prove the adherence of the electro-deposit with respect tov the base stock.

Another object of the invention is the provision of a mode of preparation of the stock that will serve to improve the bond between the strip and a metal coating, pl and/or electro-deposit tion of electrodeposit's; however, this procedural concept is not restricted to reception of electroplated coating but is applicable to the preparation of surfaces of metallic stock for reception thereon of metallic electrodeposits generally, or for the reception of metallic coatings by a hot dip method, or for the reception of paints, enamels, lacquers, etc. i I In the plating art, particularly, in the art of tin plating, it has been found desirable to skin roll the base metal prior to plating. If it is assumed that preliminary steps could be carried out perfectly in such a manner that the strip is perfectly clean before skinrolling, and assumed that the rolls are perfectly smooth and hard, it would seem that the electroplating could take place without further treatment of the strip. It has, however. been found that where skin rolling is conducted on a commercial scale, there is some lack of uniformity in the physical or chemical condition of the surface of the strip after the skin rolling process, perhaps in part at least, due to rolling into the surface small particles of foreign material which failed to wash off in the preparation of thestrip. To obtain-a perfect bond between the strip and the coating material, the surface condition should be altered before the coating is applied.

Many treatments have been used in preparing metallic stock for the reception of coatings but none have fully attained the desired objects of (1) insuring uniform adherence of the coating to the base stock; (2) bettering the surface smoothness of the final article. This has been particularly true where (as is usual), the strip or stock after being coated, is subjected to manufacturing operations involving manipulation of the same. Coatings applied to strip subjected to ordinary methods of preparation are found to peel and crack or otherwise separate from the base metal, particularly, where the coated metal is subjected toprocesses such as forming, deep drawing, etc. i

It is an object of the present invention to provide a treatment of the metal stock or strip prior to coating that will serve to simultaneously prepare the stock for electroplating and'to imwhereby the latter will not loosen on the former during subsequent alloying, brightening, and/or other manipulative treatments.

These and many other objects will appear to those skilled in the art from the description an the appended claims.

The above, and other objects of the invention are realized by procedure that. involves the steps of: (1) subjecting the stock to a treatment having as its result the formation of a very thin film-like and adherent coating on the surface of the strip or stock; this coating can be an oxygen compound ofthe metal, a phosphate, a sulphide, or other compound, or a salt thatis adherent to the surface metal; and (2) subjecting the stock or strip to a treatment for suitably removin this thin skin or film-like coating.

I have discovered that by the carrying out of such a procedure the stock is admirably prepared for reception of subsequent electro-deposits.

The stock surface is altered in such a way that wise canbe effected with or without electrolytic action.

In accordance with my invention, I preferably positively and controllably induce a suitable uniform, restricted corrosion on the metal surface and then remove the corrosion product. The precise method of inducing corrosion can be chemical, electro-chemical or thermal, and likewise the precise method of removing the corrosion can be chemical, electro-chemical or thermal, as is illustrated by the following outlines of sequence in carrying out the invention.

In accordance with one method, the strip is submerged in a suitable chemical solution until an adherent film-like coating is formed on its surface, it is rinsed and subjected to a suitable chemical solution until the film-like coating is removed, then it is wet mechanically cleaned by subjecting thesurface simultaneously to rotating brushes and hot water sprays. This last action completely removes any residue left by the chemical treatment, and leaves the surface suitable for reception of electroplating. The liquid insuiates and protects the strip from atmospheric gases, etc., prior to the coating application. In some instances (where paint, lacquer, or enamel, etc., is applied to the surface in place of electroplating) it is desirable to dry the strip before the final coating is applied.

A. As an example of the formation of an adherent film on the surface of the strip, I form an oxide on a ferrous metal stock by subjecting it to a bath of sodium hydroxide 40 to 50% by weight in water and maintained at a temperature of 220 F., the ferrous metal stock being made anode of a direct current electrolysis procedure employing any suitable (50 to 300 amperes per square foot of surface) current density. The cathode may be of any suitable electrically conducting material such as nickel or iron, which is not attacked by the solution. The treatment is preferably terminated when there has been developed on the strip surface a chemical oxide of iron coating of film thickness.

B. As an example of a solution for removing the film of chemical oxide of iron, I subject the coated stock to a bath of ferric chloride dissolved in water 100 grams per liter, and hydrochloric acid 2% by volume at room temperature. Direct current electrolysis is practiced with the strip as anode and a current density of approximately 10 amperes per square foot of surface being suitable. Any suitable conducting material which will withstand the action of the solution may be used as cathode. By way of example, I have used carbon. This solution may also be used without the aid of electric current by taking a slightly longer time to remove the oxide coating.

C. Another example of a bath for formation of the oxide is sodium hydroxide 40% by weight in water, sodium nitrate 10 grams per liter, potassium nitrate 10 grams per liter.

D. Other examples of baths for removing the oxide coating are hydrochloric acid 5% by volume in water, or sulphuric acid 5% by volume in water.

E. As an example of the formation of oxide coating on ferrous metal stock by thermal means, I heat the ferrous stock in any suitable furnace in the presence of air and moisture. This may be at any suitable temperature, say, IOU-800 F. The period of heating will be determined by the gage of the material being processed and by the depth of oxide of iron coating desired. The strip is then cooled. I have also provided a desired type and/or depth of oxide by controlling the moisture in the heating chamber; the material has also been air-cooled to further desired results.

F. As an example of solution for removing the thermally applied oxide coating, I subject the coated stock or strip to a solution of ferric chloride, 200 grams per liter in water plus hydrochloric acid 5% by volume at room temperature, employing direct current electrolysis, the coated stock being made anode with a current density of approximately 10 amperes per square foot of when the strip emerges from the surface of the bath, while in the case of the fused salt, the oxide coating can be produced directly in the bath in many cases, provided the molten salt contains oxygen.

The oxide formed by the method outlined under G can be removed by any of the solutions as outlined under paragraphs B, D or F.

H. As stated above, an oxide coating can be applied chemically (e. g., without current); as an example of this, I subject the strip to a solution consisting of the following:

Grams Water 600 Sodium hydroxide 400 Potassium nitrate 10 Sodium nitrate 10 The solution temperature should be to centigrade.

consisting of the following:

Grams Water 200 Ferric chloride 10 Potassium ulphide 1 The stock and solution thereon is dried and thereafter after rinsing, the film-like coating can be removed by subjecting the coating to the treatment as outlined under paragraph D. The acid pickle as specified under paragraph D can be used with or without electric current.

J. Similarly, a surface on metallic stock can be prepared for the reception of coatings by developing thereon a very thin phosphate film followed by the removal of this phosphate film. For carrying out this process, I treat the metallic stock in a phosphate bath containing free phosphoric acid or a soluble phosphate, and then remove the phosphate film coating by treating with an aqueous solution of ferric chloride 100 grams per liter containing about 2% by volume hydrochloric acid.

K. A preparation treatment of a different sort than those described can be effected as follows:

Steel strip is immersed in an aqueous solution of sodium thiosulphate and lead acetate of the following proportions:

The solution is used at a temperature up to 212 F. as a chemical dip, and the resulting lead oxide coating is removed by immersing the so-coated steel strip in an aqueous solution of citric acid, 100 grams per liter, and employing direct current electrolysis using 10 amperes per square foot of surface being treated.

Such steps for preparing metallic stock for electroplating may be applied to other metals than ferrous and may be employed regardless of the constitution of the coating thereafter applied to the material. Of course, mechanical cleaning may be added to or combined with chemical removal of the metal oxide, salt, or compound, and other changes may be made in the process without departing from the' spirit and scope of my invention.

' The advantages obtained in the preparation of the base stock by methods covered in this invention is that the adherence between the base stock and the coating is improved to such an extent that heavier coatings may be applied. In the methods of preparation heretofore used, the strip was never considered for the reception of heavier coatings because heavy coatings will crack and peel off during deep drawing and forming operations. In the preparation of metal stock as outlined in this patent application, heavier coatings may be applied without the danger of cracking and peeling off during deep drawing and forming operations.

From the above description, it will appear that l the present invention first deals with forming a suitable oxide or compound, and preferably the latter, on surface portions of a metal shape such as a strip, sheet, plate, etc., that is to be aftercoated or plied with a metal. The metal compound, oxide, etc., is preferably formed from adjacent surface portions of the strip being treated and the depth of the compound is suitably controlled in such a manner that a minimum thickness of the compound is provided while at the same time a sufficient thickness is provided to insure the removal of undesired materials, or in other words, to insure the desired treatment results. In the second place-the metal compound is removed in such a manner that undesired materials and particles are, in effect, "skinned oif the surface portions to be coated without destroying desired characteristics of adjacent surface portions of the metal and while at the same time leaving exposed a surface that is not only clean and free from imbedded particles, but that is also in what may be termed a coating receptive or nascent condition.

The present invention is the result of certain discoveries that I have made. I determined that metal articles, strip, etc., not only have a certain amount of oxide, oil, and other materials on immediate surface portions thereof, but also have small extraneous particles of undesired materials worked into the grain structure adjacent surface portions thereof; that is, the preworking and pretreatment processing of the material tend to introduce extraneous and undesired materials to a certain depth within surface portions of the material which additionally makes the material unsuitable for subsequent coating applications.

In developing the present invention, I also experimented with the idea of removing an oxide such as deliberately produced by a thermal method with an annealing or reducing treatment such that the original metal is retained. However, I determined that such a procedure did not eliminate the extraneous particles imbedded adjacent surface portions of the article, and further, that the crystalline structure or molecular nature of the treated portions of the metal surface has undesirable characteristics imparted to it. The surface thus produced, although cleaned of greases, etc., was somewhat porous-like and/or roughened in nature and did not take a coating as satisfactorily as desired. In other Words. the preworking or conditioning of the crystalline structure of the strip was spoiled at least adjacent its degreased surface portions.

I discovered that a highly suitable type of surface for subsequent coating could be obtained without altering the mechanical or crystalline nature of the surface portions of the sheet to be coated by first forming a metal skin and then peeling oil or removing that skin in such a manner as to remove the oxides, oils, and other extraneous particles, and to expose a bright'coating-sensitive or receptive under surface having all the desired mechanical and/or crystalline characteristics that were previously imparted thereto by pretreatment or preworking, etc. Further, in accordance with the present invention, I prefer to immediately coat the thus-exposed under surface portions of the material before they become exposed to the atmosphere; in this connection, depending upon the type of coating treatment, the sheet after the removal of its skin can be introduced into a treating bath through a conduit containing a non-oxidizing or reducing atmosphere or may be covered with a suitable insulating material such as water. The formed surface skin'is removed, in accordance with myinvention, by chemical, or electro-chemical, and mechanical means. The mechanical means may consist of suitably driven rotating brushes combined with fluid spray means (preferably water or steam) for simultaneously washing and brushing. The latter method is suitable particularly where the strip is to be electroplated and has been successfully employed to complete the removal of a surface skin that' has been preliminarily or substantially removed by chemical, electro-chemical, or other suitable treatment. The water employed will protect the I material from film forming gases such a sulphur dioxide prior to the immersion of the piece in an electroplating bath.

As previously intimated, the present invention is suitable for conditioning a piece of material such as a metal strip in order to obtain a much improved type of metal coating application thereto. The coating can be applied by hot dipping, electroplating, or other suitable means. Further, this type of conditioningjs also advantageous when the strip or material is to be enameled, painted, etc., with or without a metal coating application thereto. The present invention produces a bright, smooth, and uniform surface; thus, it is apparent that after-treatment cf the surface such as by painting or coating need not necessarily follow, as the invention in general applies to a surface conditioning of metal strips, etc. I

invention, I-have referred to certain specific examples, it will be apparent to those skilled in the 'art that many modifications, alterations,

the coating from the ferrous metal base, fromcommercially cold rolled ferrous metal material having foreign material on and rolled into the surface thereof and having physical and chemical surface conditions of suchlack of uniformity that a uniformly adherent, perfectly bonded, smooth coating of tin whichv will not peel, crack or separate therefrom when formed or deep drawn, normally cannot be applied thereto; thest'eps in the order named of forming onthe surface of said Although for the purpose of illustrating my I commercially cold rolled ferrous metal material a uniform, thin, film-like, adherent coating of a compound of said ferrous metal from said ferrous metal material of the class consisting of iron oxide, iron sulphide, and iron phosphate; then subjecting the film-like coated ferrous metal material to the action of an aqueous solution of an acid of the group consisting of hydrochloric and sulphuric acid to dissolve said film-like coating; then wet mechanically cleaning the same to remove said fihn-like coating completely therefrom; the film-like coating serving as a vehicle, the dissolution and removal of which is accompanied by removal of foreign material on and rolled into the surface of said commercially cold rolled ferrous metal stocktthereby exposing a clean, smooth, bright, uniform ferrous metal surface, free from overlying or imbedded foreign material, and of nascent or coating receptive condition, and having the mechanical and crystalline characteristics previously imparted thereto by cold rolling; and then immediately applying to said coating receptive surface of said ferrous metal material a coating of tin before exposing the coating receptive surface to the atmosphere.

2. In a method of making coated ferrous metal material having a ferrous metal base and a uniformly adhering, perfectly bonded, smooth coating of tin, adapted for being formed or deep drawn without peeling, cracking or separation of the coating from the ferrous metal base, from commercially cold rolled ferrous metal material having foreign material on and rolled into the surface thereof and having physical and chemical surface conditions of such lack of uniformity that a uniformly adherent, perfectly bonded, smooth coating of tin which will not peel, crack or separate therefrom when formed or deep drawn, normally cannot be applied thereto; the steps in the order named of forming on the surface of said commercially cold rolled ferrous metal material a uniform, thin, film-like, adherent coating of a compound of said ferrous metal from said ferrous metal material of the class consisting of iron oxide, iron sulphide, and iron phosphate; then subjecting the film-like coated ferrous metal material to the action of an aqueous hydrochloric acid solution including ferric chloride to dissolve said film-like coating; then wet mechanically cleaning the same to remove said film-like coating completely therefrom; the film-like coating serving as a vehicle, the dissolution and removal of which is accompanied by removal of foreign material on and rolled into the surface of said commercially cold rolled ferrous metal stock, thereby exposing a clean, smooth, bright, uniform ferrous metal surface, free from overlying or imbedded foreign material, and of nascent or coating receptive condition, and having the mechanical and crystalline characteristics previously imparted thereto by cold rolling; and then immediately applying to said coating receptive surface of said ferrous metal material a coating of tin before exposing the coating receptive surface to the atmosphere.

3. In a method of making coated ferrous metal material having a ferrous metal base and a uniformly adhering, perfectly bonded, smooth coating of tin, adapted for being formed or deep drawn without peeling, cracking or separation of the coating from the ferrous metal base, from commercially cold rolled ferrous metal material having foreign material on and rolled into the surface thereof and having physical and chemical surface conditions of such lack of uniformity that a uniformly adherent, perfectly bonded, smooth coating of tin which will not peel, crack or separate therefrom when formed or deep drawn, normally cannot be applied thereto; the steps in the order named of forming on the surface of said commercially cold rolled ferrous metal material a uniform, thin, film-like, adherent coating of iron oxide from said ferrous metal material by subjecting the ferrous metal material to the action of a hot aqueous solution of sodium hydroxide; then subjecting the film-like,

iron oxide coated ferrous metal material to the action of any aqueous solution of an acid of the group consisting of hydrochloric and sulphuric acid to dissolve said film-like coating; then wet mechanically cleaning the same to remove said film-like coating completely therefrom; the filmlike coating serving as a vehicle, the dissolution and removal of which is accompanied by removal of foreign material on and rolled into the surface of said commercially cold rolled ferrous metal stock, thereby exposing a clean, smooth, bright, uniform ferrous metal surface, free from overlying or imbedded foreign material, and of nascent or coating receptive condition, and having the mechanical and crystalline characteristics previously imparted thereto by cold rolling; and then immediately applying to said coating receptive surface of said ferrous metal material a coating of tin before exposing the coating receptive surface to the atmosphere.

4. In a method of making coated ferrous metal material having a ferrous metal base and a uniformly adhering, perfectly bonded, smooth coating of tin, adapted for being formed or deep drawn without peeling, cracking or separation of the coating from the ferrous metal base, from commercially cold rolled ferrous metal material having foreign material on and rolled into the surface thereof and having physical'and chemical surface conditions of such lack of uniformity that a uniformly adherent, perfectly bonded, smooth coating of tin which will not peel, crack or separate therefrom when formed or deep drawn, normally cannot be applied thereto; the steps in the order named of forming on the surface of said commercially cold rolled ferrous metal material a uniform, thin, film-like, adherent coating of iron sulphide from said ferrous metal material by subjecting the ferrous metal material to the action of a solution consisting of ferric chloride, potassium sulphide and water; then subjecting the film-like iron sulphide coated ferrous metal material to the action of an aqueous solution of an acid of the group consisting of hydrochloric and sulphuric acid to dissolve said film-like coating; then wet mechanically cleaning the same to remove said film-like coating completely therefrom; the film-like coating serving as a vehicle, the dissolution and removal of which is accompanied by removal of foreign material on and rolled into the surface of said commercially cold rolled ferrous metal stock, thereby exposing a clean, smooth, bright, uniform ferrous metal surface, free from overlying or imbedded foreign material, and of nascent or coating receptive condition, and having the mechanical and crystalline characteristics previously imparted thereto by cold rolling; and then immediately applying to said coating receptive surface of said ferrous metal material a coating of tin before exposing the coating receptive surface to the atmosphere.

5. In a method of making coated ferrous metal material having a ferrous metal base smooth coating of tin,

formed or deep drawn without peeling, crack- V and a uniformly adhering," perfectly bonded,

adapted for being ing or separation of the coating from the ferrous metal base, from commercially cold rolled ferrous metal material having foreign material on and rolled into the surface thereof and having physical and chemical surphosphoric acid; then subjecting the film-like iron phosphate coated ferrous metal material to the action of an aqueous hydrochloric acid solution to dissolve said film-like coating; then wet mechanically cleaning the same to remove said film-like coating completely therefrom; the filme like coating serving as a vehicle, the dissolution and removal of which is accompanied by removal of foreign material on and. rolled into the surface of said commercially cold rolled ferrous metal stock, thereby exposing a clean, smooth, bright, uniform ferrous metal surface, free from overlying or imbedded foreign material, and of nascent or coating receptive condition, and having the mechanical and crystalline characteristics previously imparted thereto by cold rolling; and then immediately applying to said coating receptive surface of said ferrous metal material a coating of tin before exposing the coating receptive surface to the atmosphere, 1

JOHN S. NACHTMAN.