Flux And Method Of Coating Ferrous Article

Abstract

This invention relates to the process of coating ferrous articles by the molten metal immersion method, more particularly to the provision of an aqueous flux applied to said article prior to the metal coating thereof. Specifically, the process includes cleaning the ferrous article, wetting the surfaces with an aqueous flux consisting essentially of potassium fluosilicate, hydrofluoric acid, potassium fluoride and optionally zinc chloride, drying said flux on the surface, immersing said article in a molten metal bath consisting essentially of at least 25% by weight aluminum, balance essentially zinc, and after removal from the bath, shaking off excess metal and cooling to yield a ferrous article having a continuous, adherent and uniform layer of said aluminum-zinc alloy.

Description

CROSS-REFERENCES TO RELATED APPLICATION

This application is related to and represents an improvement over the invention disclosed and claimed in copending application, Ser. No. 275,610, now U.S. Pat. No. 3,806,356 by the inventor herein and assigned to the same assignee.

BACKGROUND OF THE INVENTION

This invention is directed to the concept of coating ferrous metal articles, by the hot metal immersion method, to enhance the article's appearance and resistance to corrosion. To effect such results, a continuous, adherent and uniform layer of a corrosion resistant material on the ferrous article is essential. The prior art using various combinations of salts and acid has met with reasonable success in galvanizing and aluminumizing processes.

Typically, the ferrous article to be coated has to be cleaned of grease and scale (oxides of iron) prior to the application of the metallic coating. In a continuous strand operation, i.e., strip or wire, the material preferably is cleaned and then bathed in an oxide reducing atmosphere, and without exposing it to the atmosphere is immediately immersed in a molten metal bath containing the coating metal. For batch operations, particularly for large structural members, atmospheric exclusion is virtually impossible. Accordingly, fluxes for protecting the cleaned surfaces had to be found. Generally, the procedure is to apply a thin flux coating following the cleaning, and then, after drying, to immerse the flux coated ferrous article into the molten metal bath. Naturally, the nature of the fluxing composition is important to its effect on the ferrous base, for its ability to provide a proper surface for the reception of a metal coating, which is both continuous and adherent. Thus there is a relationship between the flux and coating metal.

While the prior art has made significant contributions in the area of galvanizing and aluminizing, the previous fluxes have not been totally satisfactory for preparing ferrous surfaces for the reception of an aluminum-zinc coating. It was not until the development of the fluosilicic acid containing fluxes, the subject matter of said copending application, that an effective means was found to prepare the surface of a ferrous article for subsequent coating by an aluminum-zinc alloy.

The potassium fluosilicate containing flux of the present invention represents an improvement over said fluosilicic acid flux, which improvements will be detailed in the discussion to follow.

SUMMARY OF THE INVENTION

This invention relates to the process of coating ferrous articles by the molten metal immersion method, more particularly to the provision of a suitable aqueous flux consisting essentially of potassium fluosilicate, hydrofluoric acid, potassium fluoride and optionally zinc chloride, for applying to the surfaces of the ferrous article prior to the metal coating thereon. In its preferred embodiment, said process includes cleaning the ferrous surfaces to remove grease, etc., pickling to remove scale, dipping directly into a flux bath consisting essentially of 3 to 40 gms/100 mls. potassium fluosilicate, 2.6 to 9.0 gms/100 mls. potassium fluoride, 3 to 50 mls/100 mls. (50%) hydrofluoric acid, and up to 5 gms/100 mls. zinc chloride, drying the flux in situ, and immersing the flux coated ferrous article in a molten metal bath of at least 25% by weight aluminum, balance essentially zinc. Upon withdrawal from the molten bath, and removal of excess metal, such as by bumping, the metallic coating solidifies to produce a ferrous article having a continuous, adherent and uniform coating of an aluminum-zinc alloy.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a flow sheet illustrating the preferred sequence of steps followed in carrying out the process of this invention.

FIG. 2 is a plot of the amount of flux deposited on a cleaned and oxide-free ferrous article, at various withdrawal rates from two different fluxes, where the flux solutions are a fluosilicic acid flux and the flux of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Turning now to a more detailed review of the invention, as set forth in the illustrated flow sheet of FIG. 1, it will be seen that the present invention provides an improved hot metal immersion coating procedure yielding a ferrous article having a continuous, adherent and more uniform coating of an aluminum-zinc alloy thereon.

The coating method generally comprising wetting the cleaned scale-free ferrous article with an aqueous solution consisting essentially of potassium fluosilicate, hydrofluoric acid, potassium fluoride and optionally zinc chloride.

A preferred and optimum range for each of the flux ingredients is listed below; however, it is contemplated that the preferred range may be modified to the extent as shown in the optimum range, by one or more of the components listed:

Preferred Range Optimum Range per 100 mls. per 100 mls. ______________________________________ K.sub.2 SiF.sub.6 3-40 gms. 7-13 gms. KF 2.60-9.0 gms. 3-7 gms. HF(50%) 3-50 mls. 5-11 mls. ZnCl.sub.2 0-5 gms. 1-3 gms. Water Bal. Bal. ______________________________________

Following the wetting of the ferrous article to be coated, such as by dipping or immersion in the aqueous flux, the ferrous article is dried such as by heating to a low temperature of up to about 400.degree. F. to remove all signs of dampness. This is important, especially from a safety standpoint, to avoid splashing during immersion of the flux coated ferrous article into the molten coating metal. At this juncture note the date illustrated in FIG. 2. Such data indicate one of the advantages of the fluxx of this invention over that disclosed in said copending application.

The function of the flux is to protect the surface of the ferrous article to be coated and to prepare it for the reception of the molten metal coating. However, retention of too much flux on the surface of the ferrous article may result in the appearance of drain lines showing through the metal coating. This is caused by irregular drainage of the flux. Another problem may arise when relatively large amounts of a flux are applied from a thickened flux solution. Excessive surface scum may develop on the surface of the bath and coated part. These were the types of problems observed with the fluosilicic acid fluxes of said copending application. These difficulties have now been overcome with the flux of the present invention.

In any case, with the flux coated ferrous article sufficiently drained and dried, it is then immersed for from 1-5 minutes depending on the thickness of the piece being coated and the coating weight desired, in a molten metal bath containing at least 25% by weight, aluminum, preferably up to about 70%, balance essentially zinc. After removal, excess coating metal is removed, such as bumping or agitating the coated ferrous article, where it may be cooled by air or water quenching. The resulting product exhibits a continuous, adherent and uniform alloy coating on the processed ferrous article, which article is free of flux drain lines.

At this juncture, it should be made clear that as used herein, the term "article" is intended to include strip and wire (treated in a continuous manner), and shapes, such as structural members treated in a batch process. Actually, the greatest benefits from this invention are realized in the batch treatment of ferrous shapes, such as large structural steel members used in the construction industry. Though not limiting, the further description and exemplary showings shall be directed to batch operations.

By way of a specific embodiment, a low-alloy steel was cleaned to reveal a surface free of oxide and grease. Said steel was then immersed for about one-half second in an aqueous flux whose formulation is as follows:

Ingredient per 100 mls. % by weight ______________________________________ K.sub.2 SiF.sub.6 10 gms. 9.1 KF 5 gms. 4.55 HF(50%) 8 mls. 4.2 ZnCl.sub.2 2 gms. 1.8 Water bal. bal. ______________________________________

After drainage and drying, the steel had a thin smooth flux coating on the order of 0.004 gms/in.sup.2 or 0.062 gms/dm.sup.2. The dry flux coated steel was then immersed in a molten metal bath consisting essentially of, by weight, 55% aluminum, 1.5% silicon, balance zinc, for approximately 21/2 minutes. After withdrawal and bumping to remove excessive coating metal, the aluminum-zinc coated steel was cooled in air exhibiting a product having a relatively smooth, continuous and adherent alloy coating. Further, the product was free of a flux drain line pattern.

As indicated previously, one of the most important advantages of the flux of this invention is that it is not as thick as the fluosilicic acid containing flux of said copending application. As a consequence, smaller amounts are deposited and retained on the surface of the ferrous article at comparable withdrawal rates. The difference is even more significant at faster withdrawal rates, see FIG. 2. The respective formulations for the two fluxes illustrated in FIG. 2 are given below in quantity/100 msl.

______________________________________ H.sub.2 SiF.sub.6 Flux Invention Flux ______________________________________ H.sub.2 SiF.sub.6 15 mls. K.sub.2 SiF.sub.6 9.4 gms HF(50%) 7 mls. HF 12 mls. KF 10 gms. KF 5.2 gms. ZnCl.sub.2 2 gms. ZnCl.sub.2 2 gms. Specific Specific Gravity 1.12 Gravity 1.1 ______________________________________

Since the foregoing formulations and the composition ranges noted earlier represent only preferred embodiments of this invention, it is contemplated that variations may be effected herein by those skilled in the art, particularly after reading these specifications. For example, it may be desirable from a cost standpoint to further minimize the amount of potassium fluoride as this is the most expensive ingredient. Further, the potassium fluoride should be present in an amount less than the potassium fluosilicate to minimize bare spots.

Part of the hydrofluoric acid can be replaced by hydrochloric acid but at some sacrifice in quality, i.e., coverage. Thus, it is preferred that HCl be present in an amount no greater than 30% of the HF. Finally, some attention should be given to the K.sub.2 SiF.sub.6 as it represents the key ingredient in the successful use of the flux of this invention. Since K.sub.2 SiF.sub.6 is only partially soluble, it must be kept suspended by stirring or agitation of the flux solution. Any attempt to substitute or replace the key ingredient with another alkali metal fluosilicate, such as Na.sub.2 SiF.sub.6 which is much less soluble then K.sub.2 SiF6would result in an unsuitable flux solution. That is, the more insoluble components that are present in the solution, the greater the problems. Thus, as changes or variations are readily contemplated, no limitation is intended to be imposed herein except as set forth in the appended claims.

Claims

I claim:

1. A method of forming a continuous alloy coating consisting of from 25% to about 70%, by weight aluminum, balance essentially zinc, on a ferrous article, comprising the steps of cleaning said ferrous article to remove grease and oxides from the surfaces thereof to be coated, immersing said article in an aqueous flux consisting essentially of potassium fluosilicate, potassium fluoride, zinc chloride, and an acid selected from the group consisting of hydrofluoric acid and a mixture of hydrofluoric acid and hydrochloric acid, removing said article from the aqueous flux, drying said article by heating to a low temperature, and immersing the article in a molten bath containing said aluminum - zinc coating alloy.

2. The method according to claim 1 wherein said aqueous flux, per 100 mls, comprises 3 to 40 gms. potassium fluosilicate, 2.6 to 9.0 gms. potassium fluoride, 3 to 50 mls. hydrofluoric acid, up to 5 gms. zinc chloride, balance water.

3. The method according to claim 2 wherein said acid is a mixture of hydrochloric acid and hydrofluoric acid, the quantity of the hydrochloric acid does not exceed 30% of the hydrofluoric acid.

4. The method according to claim 2 wherein said aqueous flux, per 100 mls., comprises 7 to 13 gms. potassium fluosilicate, 3 to 7 gms. potassium fluoride, 5 to 11 mls. hydrofluoric acid, 1 to 3 gms. zinc chloride, balance water.

5. The method according to claim 4 wherein said acid is a mixture of hydrochloric acid and hydrofluoric acid, the quantity of the hydrochloric acid does not exceed 30% of the hydrofluoric acid.

6. The method according to claim 1 wherein said aqueous flux, per 100 mls, comprises 7 to 13 gms. potassium fluosilicate, 3 to 7 gms. potassium fluoride, 5 to 11 mls. hydrofluoric acid, balance water.

7. A flux for the treatment of a ferrous article prior to its immersion in a molten bath containing an aluminum-zinc alloy, comprising an aqueous acidic solution consisting essentially of potassium fluosilicate, potassium fluoride, fluoride chloride, and an acid selected from the group consisting of hydrofluoric acid and a mixture of hydrofluoric acid and hydrochloric acid.

8. The flux according to claim 7 wherein said aqueous acidic solution, per 100 mls, comprises 3 to 40 gms. potassium fluosilicate, 2.6 to 9.0 gms. potassium fluoride, 3 to 50 mls. hydrofluoric acid, up to 5 jms. zinc chloride, balance water.

9. The flux according to claim 8 wherein said acid is a mixture of hydrochloric acid and hydrofluoric acid, the quantity of the hydrochloric acid does not exceed 30% of the hydrofluoric acid.

10. The flux according to claim 8 wherein said aqueous acidic solution, per 100 mls, comprises 7 to 13 gms. potassium fluosilicate, 3 to 7 gms. potassium fluoride, 5 to 11 mls. hydrofluoric acid, 1 to 3 gms. zinc chloride, balance water.

11. The flux according to claim 10 wherein said acid is a mixture of hydrochloric acid and hydrofluoric acid, the quantity of the hydrochloric acid does not exceed 30% of the hydrofluoric acid.

12. The flux according to claim 14 wherein said aqueous acidic solution, per 100 mls, comprises 7 to 13 gms. potassium fluosilicate, 3 to 17 gms. potassium fluoride, 5 to 11 mls. hydrofluoric acid, balance water.

13. A method of forming a continuous alloy coating consisting of from 25% to about 70%, by weight aluminum, balance essentially zinc, on a ferrous aritcle, comprising the steps of cleaning said ferrous article to remove grease and oxides from the surfaces thereof to be coated, immersing said article in an aqueous flux consisting essentially of potassium fluosilicate, potassium fluoride and an acid selected from the group consisting of hydrofluoric acid and a mixture of hydrofluoric acid and hydrochloric acid removing said article from the aqueous flux, drying said article by heating to a low temperature, and immersing the article in a molten bath containing said aluminum-zinc coating alloy.

14. A flux for the treatment of a ferrous article prior to its immersion in a molten bath containing an aluminum-zinc alloy, comprising an aqueous acidic solution consisting essentially of potassium fluosilicate potassium fluoride and an acid selected from the group consisting of hydrofluoric acid and a mixture of hydrofluoric acid and hydrochloric acid.