US3695919A

US3695919A - Metal coating process

Info

Publication number: US3695919A
Application number: US91387A
Authority: US
Inventors: Kenneth J Brondyke
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1970-11-20
Filing date: 1970-11-20
Publication date: 1972-10-03
Anticipated expiration: 1989-10-03

Abstract

A PROCESS FOR ALUMINIZING AN ARTICLE BY IMMERSION IN A VESSEL CONTAINING A BATH OF MOLTEN ALUMINUM. PROVIDING AT LEAST ONE FORAMINOUS REFRACTORY BRICK IMPREGNATED WITH A FLUX MATEIAL HAVING A DENSITY LESS THAN THE MOLTEN ALUMINUM BATH. INTRODUCING THE BRICK INTO THE VESSEL IN CONTACT WITH THE ALUMINUM TO INITIATE DISCHARGE OF THE FLUX FROM THE REFRACTORY BRICK INTO THE ALUMINUM. RETAINING THE BRICK IN THE VESSEL FOR A SUFFICIENTLY LONG PERIOD OF TIME TO ESTABLISH A THIN FILM OF THE FLUX ON THE SURFACE OF THE MOLTEN ALUMINUM. SUBSEQUENTLY INTRODUCING THE ARTICLE TO BE ALUMINIZED INTO THE MOLTEN ALUMINUM AND RETAINING IT THEREIN FOR A PERIOD OF TIME SUFFICIENT TO COAT THE ARTICLE WITH THE MOLTEN ALUMINUM. THEREAFTER REMOVING THE ARTICLE AT A SUFFICIENTLY LOW RATE OF SPEED TO CAUSE FLUX PICKUP TO PROVIDE A FLUX COATING ON THE COATED ARTICLE AND THEREBY MINIMIZING OXIDE FORMATION ON THE COOLED PART. THE FORAMINOUS REFRACTORY BRICK IS PREFERABLY THERMALLY STABLE AT TGHE MOLTEN ALUMINUM BATH TEMPERATURE AND THE FLUX PREFERABLY HAS A MELTING TEMPERATURE NOT LESS THAN THE TEMPERATURE OF THE MOLTEN ALUMINUM BATH.

Description

ALLOW 7'/ME FOR FLUX F/LM FORMA 7/O/V PREHEAT 7'0 DRY K. J. BRONDYKE METAL comme PROCESS Filed Nov. 2o, 1970 REMOVE A/vo oma/E OFF Mo/s ruRE APPLY ALUM//V/Z- /NG FL UX FLUX /MMERSE CLEAN ART/CLE Oct. 3, 1972 smc/r ART/CLE REMOVE EXCESS METAL REMOVE EXCESS FLUX United States Patent O U.S. Cl. 117-52 9 Claims ABSTRACT OF THE DISCLOSURE A process for aluminizing an article by immersion in a vessel containing a bath of molten aluminum. Provid ing at least one foraminous refractory brick impregnated with a fiux material having a density less than the molten aluminum bath. Introducing the brick into the vessel in contact with the aluminum to initiate discharge of the flux from the refractory brick into the aluminum. Retaining the brick in the vessel for a sufficiently long period of time to establish a thin film of the flux on the surface of the molten aluminum. Subsequently introducing the article to be aluminized into the molten aluminum and retaining it therein for a period of time sufficient to coat the article with the molten aluminum. Thereafter removing the article at a sufficiently low rate of speed to cause ux pickup to provide a flux coating on the coated article and thereby minimizing oxide formation on the cooled part. The foraminous refractory brick is preferably thermally stable at the molten aluminum bath temperature and the flux preferably has a melting temperature not less than the temperature of the molten aluminum bath.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to the aluminizing of articles and more specifically a process for improving the uniformity of the aluminum coating on the article by means of a carefully applied flux film which is established on the surface of the molten aluminum.

Description of the prior art The advantageous use of aluminizing to coat numerous products made of ferrous materials, titanium or other metals has long been known. Such coatings not only provide an attractive exterior appearance, but also provide resistance to corrosion, acid attack and oxidation. ln addition, increased reflectivity and electrical conductivity may be provided. Numerous means of applying such an aluminum coating to an article have been known. Included within the known systems are spraying, cladding, electroplating, casting and immersion of the article in a molten aluminum bath.

One of the principal problems encountered in aluminizing systems is the need to improve the uniformity of coating thickness. This problem is particularly acute with respect to irregularly configurated articles.

In general, the known systems of immersion or hot dip aluminizing have followed the same general pattern. After some sort of preliminary cleaning which may 'oe effected mechanically or in an alkaline solution the article is subsequently pickled. The article is then fluxed, dried and immersed in molten aluminum. After a sufficient period of immersion to accomplish the desired coating thickness while preventing excess formation of the aluminum-fen rous intermetallic compound or-other intermetallic compounds the article is then removed. It is then conventional to attempt to remove excess aluminum adhering to the article either manually or mechanically as `by draining, spinning, vibrating or centrifuging.

3,695,919 Patented Oct. 3, 1972 ICC One of the principal contributing causes of the lack of uniformity of aluminum coating on an aluminized article is the diliiculty in obtaining optimum wetting of the article while it is immersed in the molten aluminum bath and effecting substantially complete removal of any excess material by a subsequent operation. This problem provides both technical and economical obstacles to broader commercial use of hot dip aluminizing.

SUMMARY OF THE INVENTION The process of this invention has solved the above enumerated problems with respect to uniformity of aluminum coating in hot dip alurninizing. In general, this invention contemplates providing a foraminous refractory brick which is mechanically stable under the thermal and metallurgical conditions encountered in the aluminum bath. A flux material which has a density less than the molten aluminum bath is then impregnated into the foraminous refractory brick. The brick, which may have a density greater than, less than or substantially equal to the molten aluminum bath, is then introduced into the vessel of molten aluminum. The brick is retained in the vessel for at least a sufficiently long period of time to establish a thin film of the flux on the surface of the molten aluminum. Subsequently an article to be aluminized is immersed in the molten aluminum bath and retained therein for a sufficiently long period of time to coat the article with molten aluminum. The article is removed at a sufiiciently slow rate of speed as to cause flux pickup and provide a flux coating on the coated article. This flux coating minimizes oxide formation on the cooled part, contributes to improve iiow characteristics of the aluminum and permits increased ability to remove excess aluminum.

The flux material preferably has: a melting temperature approximately the temperature of the molten aluminum and provides a substantially continuous flux film on the surface of the bath. This film preferably has a thickness of less than about %,2 inch.

After removal of the excess aluminum, the article with a solidified aluminum coating may then be cleansed to remove residual flux therefrom.

it is an object of this invention to provide a process of alnminizing an article which will permit ready removal of excess aluminum as a result of a unique iiuxing operation.

It is a further object of this invention to provide a forarninous refractory brick which has been impregnated with a fiux material adapted to be released when the brick is immersed in the molten aluminum bath to establish a thin flux film for coating an article being removed from the molten aluminum bath.

It is a further object of this invention to provide a method of controlling the rate of release of a linx material to provide a thin film thereof on the surface of the molten aluminum bath.

It is yet another object of this invention to provide such a process which is adapted for use with conventional aluminizing equipment and materials and may be economically introduced into such a system.

These and other objects of the invention will be more fully understood from the following description of the invention on reference to the illustrations appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially schematic sectional elevation of a vessel containing molten aluminum and a refractory brick of this invention.

FIG. 2 is a flow chart illustrating one means of practicing the process of this invention.

3 s DESCRIPTION OF THE PREFERRED EMBODIMENTS As used herein, the term aluminum shall refer to aluminum o-f any grade and aluminum base alloys wherein aluminum consists of not less than 80% by weight of the |total composition.

The process of this invention contemplates the hot dip aluminizing of articles made of ferrous materials, titanium, refractory metals and other suitable materials. As will be described in greater detail below, the invention provides a thin film of flux material on the surface of an aluminum bath by a unique controllable method which involves impregnating of a foraminous brick element with a flux material. The brick is adapted to retain its unitary structure and release the tiux material at a predetermined rate when the brick is placed in or on the molten aluminum bath. The impregnated brick may be provided with a density greater than the density of the molten aluminum bath and sinks to the bottom of the bath as shown in FIG. l to discharge the material therefrom. Alternatively, the brick in impregnated form may have a density less than that of the molten aluminum ba-th and may ioat thereon and discharge flux from said position. Finally, a density equal to that of the bath may also be provided.

Referring now to FIG. 1, it is seen that a vessel 2 has a lining 4 which may be of refractory or other suitable material, an outer steel material 8 and an interposed insulating material 6. The vessel 2 contains a charge of molten aluminum 10. Submerged within the vessel is a foraminous brick 12 from which is being discharged tlux 14 which migrates upwardly to form flux film 16 on the surface of the molten aluminum 10. (A brick 12a having a density less than that of the molten aluminum is illustrated in dotted form in FIG. l.)

In fabricating the brick 12, it is preferred that it be made from a refractory material which is foraminous in nature and has a bulk density range from about 30 lbs./ ft.3 to 145 lbs./ft.3. The brick must have such thermal and mechanical properties as to resist excessive fragmentation or other forms of destruction of its integrity. The size of brick selected and the number inserted within any vessel will depend in part upon the porosity or permeability of the material, the area of the aluminum surface upon which the film will be established, the film thickness desired and the relationship between the temperature of the molten aluminum and the melting point of the ux material. The porosity of the brick will generally range from about 20 to 85%.

In selecting an appropriate tlux material for use in this process in general it is desirable to select a material which will have a melting point approximating that of the temperature of the molten aluminum bath and preferably equal to or greater than the bath temperature. A flux melting temperature from about 10 degrees Fahrenheit below the temperature of the molten aluminum bath to about l degrees Fahrenheit above the bath temperature may be used. It is preferred to select a material having a melting point of about live degrees Fahrenheit below the bath temperature to about five degrees above the bath temperature. The molten aluminum bath depending upon alloy and weight characteristics sought and the material of the article to be coated will frequently be operated at a temperature of about 1275 to 1400 F. If an increase in the rate of discharge of flux from the refractory brick is desired, a melting point about 5 to 25 F. above the temperature of the molten aluminum bath is preferred. Numerous types of uxes may be employed in the practice of this invention. One suitable form of flux is of the type employing sodium chloride, potassium chloride and potassium aluminum lluoride. Other suitable compositions are shown in U.S. Pats. 2,569,097, 2,544,670 and 2,544,671.

In order to establish the desired flux film position the ux must have a density less than that of the molten aluminum bath. `It is desired tov provide a very thin film of the ux material on the surface of the bath. This film may be thought of as being preferably less than 1&2 inch in thickness and adapted to coat an emerging aluminum coated article in order to provide for eicient mechanical removal of excess aluminum and thereby establish a coating of uniform thickness.

As is shown in the ow diagram of FIG. 2, one convenient means of impregnating the brick with the flux is to provide for immersion of the brick in a molten flux bath and retention therein for the period necessary to accomplish the absorption. This has frequently been found to require one-half hour or more. The brick may then be removed. Before immersion in the molten aluminum bath, the impregnated brick is preferably subjected to heat in order to drive off any sorbed moisture.

Example A flux formulation consisting of 62.7% sodium chloride, 32.3% potassium chloride and 5% potassium aluminum fluoride was provided in a vessel and maintained at a temperature of 1400io F. A number of refractory bricks were immersed in the molten salt mixture and held therein for a one hour period. The bricks Were removed and heated to drive off any sorbed moisture. They were found to have increased in weight by approximately one pound.

Referring once again to FIG. 2 and the flow diagram shown therein, the article to be aluminized will lirst be subjected to a conventional cleaning operationwhich may include mechanical cleaning, alkaline cleaning and pickling. The article is then coated with an aluminizing flux material as by immersion in an aqueous solution and is subsequently dried. After the refractory brick has been in the molten aluminum bath for a sufficient period of time to provide the desired ux lm on the bath surface, the article is immersed in the bath sufficiently far as to be entirely beneath the aluminum surface. The article is retained in the bath for a sufficient period of time to provide the optimum desired coating thickness and coating characteristics in terms of control over the' quantity of intermetallic compounds formed between the aluminum coating material and the metal material of which the article is composed. The article is then withdrawn from the bath at a sufficiently slow rate of speed to permit ux pickup and provide a flux coating on the article. The presence of the ux coating on the article provides a means of reducing harmful oxidation on the aluminum surface 0f the emerging article and reduces influence of the oxide film or surface tension of the aluminum to provide for greater iiowability and more eflicient mechanical removal of the excess aluminum. This results in a more uniform coating thickness and appearance.

After removal from the molten aluminum bath, the article is subjected to a manual or mechanical process stage which serves to remove excess aluminum. This may be accomplished by draining, spinning, centrifuging or other desired means. It is the thin, preferably substantially continuous, film of iiux material which serves to protectively surround the aluminum coating and establishes the desired free flow of the aluminum to provide discharge of the excess material. In addition, the presence of an oxide inhibiting ux also serves to prevent undesired discoloration which tends to impair the desired uniform surface appearance of the article.

The aluminum coating on the article may then be permitted to solidify. After solidification has occurred, the article may be cleaned to remove the residual flux present on the surface. This is preferably accomplished by means of an aqueous solution which may be a quench in water having a temperature of about 50 F. to 180 F. The higher temperatures within this range are preferred as increased solubility of the flux in the aqueous solution results.

It will therefore be appreciated that the process of this invention provides a means of effectively controlling the surface tension of the aluminum on the emerging coated product and restricting oxidation thereof. This contributes to ready removal of excess material in order to provide uniform coating thickness and aesthetically pleasing surface characteristics. All of this is accomplished by impregnating a refractory brick with a suitable flux material which is adapted to migrate to the surface of the molten aluminum bath and establish a thin lm thereon. The process may be employed with conventional hot dip aluminizing equipment and is readily adaptable to use with various size molten metal bath vessels as bricks in various sizes and numbers may be employed therein.

It will also be appreciated that the impregnated bricks may be prepared in advance of the aluminizing operation and provide a means of accurately controlling the amount of uX charged into a vessel and thereby controlling the lm thickness and amount of flux dissipation as the process is performed on a larger number of articles.

Whereas particular embodiments of the invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details may be made without departing from the invention as defined in the appended claims.

Having thus described my invention and certain embodiments thereof, I claim:

1. A process of aluminizing a metal article, comprising providing a vessel containing a molten aluminum bath,

providing at least one foraminous refractory brick impregnated with a flux material, introducing said brick into said vessel in contact with said aluminum to initiate discharge of said impregnated ux from said refractory brick into said aluminum without appreciable physical destruction of said foraminous refractory brick, retaining said brick in said Vessel for a su'iciently long period of time to establish a substantially continuous thin lm of said flux on the surface of said molten aluminum while avoiding appreciable physical destruction of said foraminous refractory brick,

subsequently introducing said metal article to be aluminized into said molten aluminum and retaining it therein for a suicient period of time to coat said article with said molten aluminum, and

thereafter removing said article at a suciently slow rate of speed as to cause flux pickup to provide a ux coating on said coated article.

2. The aluminizing process of claim 1 wherein said refractory brick has a lower density than said molten aluminum, and

said brick is floating on the surface of said molten aluminum bath during discharge of flux therefrom.

3. The aluminizing process of claim 1 including providing said impregnated foraminous refractory brick by immersing said foraminous brick in a molten flux bath for a suicient period of time to effect impregnation of a predetermined quantity of said ux and subsequently heating said brick to drive off any sorbed moisture.

4. The aluminizing process of claim 1 wherein said refractory brick has a porosity of about 20 to 85%,

said ux has a melting temperature of about 10 degrees Fahrenheit below the temperature of said molten aluminum to about 15 degrees Fahrenheit above the temperature of said molten aluminum, and

said refractory brick is physically stable and remains unitary at the temperature of said molten aluminum.

5. The aluminizing process of claim 4 wherein said refractory brick has a higher density than said molten aluminum bath, and

said brick is positioned below the surface of said molten aluminum bath during discharge of flux therefrom.

6. The `aluminizing process of claim 1 wherein said flux has a melting temperature of about 5 to 25 F. abovel the temperature of said molten aluminum bath and said refractory brick has a density of about 30 lbs/ft.3

.to lbs./ft.3.

7. The aluminizing process of claim 6 wherein after withdrawal of said coated article from said molten aluminum bath but prior to full solidication of said aluminum coating, mechanically removing the excess of said aluminum coating to establish a substantially uniform coating on said article.

8. The aluminizing process of claim 7 wherein said tilm of said flux has a thickness of not greater than 1/32 of an inch, and

subsequent to full solidication of said aluminum coating cleansing said coated article by means of an aqueous solution to remove residual flux therefrom.

9. The Ialuminizing process of claim 8 wherein said residual flux removal is accomplished by immersing the coated article in water having a temperature of about 50 F. to 180 F.

References Cited UNITED STATES PATENTS 3,037,880 6/1962 Hanink 117-52 3,486,884 12/1969 Baille 75-68 R ALFRED L. LEAVITT, Primary Examiner J. A. BELL, Assistant Examiner