US2213952A - Process for treating metal surfaces - Google Patents

Description

Patented Sept. 10, 1940 UNITED STATES PROCESS FOR TREATING METAL SURFACES Ralph K. Clifford, Kokomo, Ind., assignor to Continental Steel Corporation, Kokomo, ImL, a corporation of Indiana No Drawing. Application August 26, 1935, Serial No. 37,937. Renewed November 20, 1939 6 Claims.

This invention relates to a process for treating metal surfaces so that a coating applied thereto will adhere firmly to said surface, and the present application is a continuation-in-part of my it earlier application filed April 5, 1933, under Serial No. 664,668.

The invention has for one of its objects the preparation of the surface of iron or steel sheets to provide for film adherence of zinc coatings,

especially coatings of the heavier types, such, for

example, as are used for the manufacture of culverts.

A further object of the invention is to prepare steel surfaces of the highly finished type, such as w are produced in strips, both narrow and wide, and on highly finished sheets produced on mills of either the continuous, semi-continuous or intermittent types, so that after treating said surfaces, zinc coatings will adhere firmly when sub- W jected to fabrication.

Another object of the invention is to eliminate the use of strong acids and/or prolonged immersion for pickling purposes.

For the purposes of illustration only, I will describe my process in connection with zinc-coatin applied to iron or steel, sheets, although it will be obvious that this process may be used in connection with other metallic products or for coatings other than zinc, whether oi a metallic nature such vas lead, or of the paint, enamel, or oxide types. Furtlrierrnore, this process may be used to prepare metallic surfaces for electroplating as well as for hot or molten coatings.

The ordinary practice used for galvanizing (zinc-coating) sheets consists in substantially the following procedure:

1. Removal of scale and etching of the sheet surface by means of acid. This is called pickling. (If the sheet has been oiled or greased, a

de-oiling process may be used before pickling.)

2. Removal of acid from the surface by rinsing in water.

8. Neutralizing the remaining surface acid by rinsing in a neutralizing solution such as a sodium carbonate solution.

4. Soaking in water to eliminate further acid and hydrogen-or 4 Liming and baking-or 50 4 Liming and baking followed by washing and drying-or 4. Washing and drying and eliminating step 4.

5. Immersion in a very weak acid solution, 55 on a fluxing solution, prior to 6. Immersion through a molten flux into a bath of molten zinc.

'7. Removal from the molten zinc, followed by- 8. Cooling, or, some special treatment and 5 then cooling.

The ordinary process possesses a number of disadvantages due to several causes, only a few of which will be mentioned here.

In the first place, the sheets to be pickled are m placed in crates, the sheets standing on edge in said crates. The crates and sheets are then immersed in the acid solution, usually sulphuric acid, the strength of which varies but which may be about six (6) per cent of acid by weight. alloy steels a stronger sulphuric acid solution (about per cent) may be used, followed by nitric acid wash. Sometimes a mixture of acids is used.

The crates are moved mechanically to allow at the acid to move between the separate sheets or else the sheets remain stationary and the acid is caused to move therebetween.

In either event it is diificult to keep the sheets apart during the whole of the pickling opera- 2d tion and consequently. the sheets are unequally pickled. If the acid is too strong or if the sheets are allowed to remain therein for too long a time the sheets become acid burned. Then, too, the black coating formed by pickling is not all thoroughly removed in the subsequent after treatments.

When continuous processes are used, then the crates are not necessary, other means being then employed to support the sheets, strip or Wire passing thru the pickling solution. This process also has a number of disadvantages especially for material which is to be coated.

Furthermore, steels'o-f different types of manufacture and of different analysis do not pickle no as easily nor as uniformly as some others and the results show up later in poor adherence of the galvanized coating when subjected to fabri-' cation.

When it becomes necessary to subject sheets to a severe pickle in order'to clean their surfaces then the hydrogen absorption is increased with a consequential increase in blisters brought out by the subsequent coating process.

Certain types of highly finished surfaces, such for example, as those produced in strip mills, or continuous sheet mills, do not lend themselves to galvanizing by the usual processes used and, so far as I am aware, until the development of my process, no heavy type coatings could be For m applied to these highly finished surfaces which would withstand subsequent fabrication, without flaking and cracking off of the zinc coating.

The use of my process obviates all of these disadvantages.

In carrying out my process, I may first subject a metal sheet to a light pickling to remove the major portion of the scale and thereafter, as the next step, I treat one or both surfaces of the sheet, from edge to edge, -to a pelting process that I will call mechanical etching, to produce a substantially uniform matte surface which is characterized by a multitude of alternate points and valleys or pits, the points being of about even elevation and spacing. This mechanical etching is preferably carried out through the use of a suitable granular cutting agent of predetermined size, directed with force against the surface of the sheet as by a blast of air. The cutting material or etchants, while in a loose condition, may be applied otherwise to the surface of the sheet, as, for example, by the use of some holding material, or by centrifugal force. The etchants may be sand, metallic particles of a hard nature, refractory materials, or the like, depending upon the material and character of the surface to be treated, so as to produce a mechanical etching of the kind above defined. It is also important that the mesh of the etchants be suitably chosen for the work in hand, in order to produce uniform results. For example, a large or coarse mesh etchant will produce a matte surface which is deeper and rougher than will a small or fine mesh etchant.

The amount or extent of such mechanical etching by pelting, to produce the required height and depth of the adjacent points and valleys which characterizes my uniform matte surface, depends somewhat on the character of the surface to be treated, as well as the weight of coating which is to be placed thereon. It is desirable, for example, that a greater amount of mechanical etching should be applied for a coating which is heavier in weight. I am not at present aware of the exact ratio necessary between the amount of treatment to produce the required relief texture and the weights of coating to be applied, but I do know that some such relationship exists. If the surface of the sheet to be coated is such that little or no scale is present, or if a heavy coating is desired, I may dispense altogether with a pickling operation, and employ only the mechanical etching treatment, as the scale need not be completely, but

should be substantially, removed, to permit of the production of a uniform relief texture.

Following a mechanical etching as above described, it is important that the sheet be either Washed and dried; that its treated surface receive a blast of air, steam, or some other gaseous medium; that a light pickling solution be applied; that-the sheet be subjected to a fluxing solution; or that combinations of two or more of these treatments be employed to thoroughly remove any and all fine particles of etching material or finely divided steel or scale which may remain as a powdery substance within the multitude of valleys, or as a coating on the multitude of high points, which characterize the relief matte surface of the sheet. This step of removing the powdery substance from the mechanically etched surface is important in order to realize fully the advantages of this invention, otherwise the coating may not firmly adhere to the base in the manner described. By following,

the directions herein given, it will be found that the coating will become firmly bonded to the mechanically etched surface, and will remain adhered thereto even under conditions of bending or fabrication.

This process may be carried out as a step by step process, or it may be carried out continuously. In the latter event, the sheet passes first through the pickling solution (if necessary), then through a washing and drying apparatus, then through a mechanical etching apparatus, then through a wash tank, from which it passes into the molten galvanizing bath. A fluxing solution may be used following the wash tank and before entering the molten fiux on the ordinarymolten galvanizing bath.

It will be obvious to those skilled in the art that one or more of the above steps, either prior to, or subsequent to, the mechanical etching may be obviated depending on the condition of the material being coated and the kind of coating desired.

I have found that by the use of my process I am able to attach heavy malleable zinc coatings to sheets which-will withstand fabricating operations heretofore considered impossible.

This process may be used also for continuous wire galvanizing, in which case I may eliminate acid pickling entirely prior to coating, in a manner similar to that described for sheets.

The process ascarried out by my invention contemplates mechanical etching, per se, as a tool for use in preparing surfaces as a part of my process for forming adherent, malleable coatings, particularly those of zinc, or lead, on metallic bodies where heretofore such has not been accomplished either practically, economically or commercially.

I claim:

1. In the manufacture of coated sheet metal, the steps including pickling base metal in sheet form, centrifugally blasting one side only of the pickled base metal, and coating both sides of the base with a protective metal.

2. As a new manufacture, coated sheet metal comprising a ferrous base having one side pickled and the other side blasted, and a protective metal coating covering both sides.

3. In the manufacture of sheet metal, the process which consists in rolling a ferrous base metal, abrading the rolled surface with a centrifugal blast to provide a matte surface free from hydrogen inclusions and moisture, and then passing the sheet metal through a molten bath of coating metal.

4. The method defined by claim 3 wherein only one side of the base is subjected to abrasion prior to coating.

5. The method defined by claim 3 wherein the base metal is subjected to abrasion with the scale or oxide resulting from rolling, remaining thereon.

6. The method defined by claim 3 wherein the base metal is pickled and then subjected to abrasion on one side only before coating.

RALPH K. CLIFFORD.