US2437919A

US2437919A - Process and means for improving the adherence of aluminum coatings

Info

Publication number: US2437919A
Application number: US605990A
Authority: US
Inventors: Oganowski Kasimir
Original assignee: American Rolling Mill Co
Current assignee: American Rolling Mill Co
Priority date: 1945-07-19
Filing date: 1945-07-19
Publication date: 1948-03-16
Anticipated expiration: 1965-03-16
Also published as: BE478199A; FR957215A

Description

March 16, 1 948. K. OGANOWSKI 2,437,919

PROCESS ES AND MEANS FOR IMPROVING THE ,ADHERENCE OF ALUMINUM COATINGS Filed July 19, 1945 KAI/MIR OGA/VOWIK/ 3% Gaul/9% Patented Mar. 16, 1948 STATE-PATEN PROCESS AND MEANS FOR EMPROVING I ADHERENCE OF ALUMINUM COATING Application July 19,, 1945, Serial No. 605,99@

*2 Claims. i

My invention relates to problems encountered in coating metallic strips, sheets or articles with molten aluminum or similar metals in the type of process in which the products to be coated pass through a hood of protective atmosphere into the bath of coating metal.

A non-limiting example of such a process is to be found in Patent No. 2,110,893 in the name of Sendzimir, wherein the strip metal is drawn first through an oxidizing furnace where oils and other carbonaceous substances are .bumed from its surfaces and Where a uniform and microscopically thin oxide film is formed thereon, and then through a furnace with a reducing atmosphere Where, among other things, the thin oxide film is reduced, leaving the surfaces of the strip completely clean and very receptive to molten coating metal. The strip is led through a hood from the reducing furnace into the bath while still protected by a neutral or reducing atmosphere, and without a flux treatment.

In coating ferrous strip with aluminum by hot dipping, in spite of the protective atmosphere employed, difficulty has frequently been encountered with the formation or deposit upon the strip of films of material which deleteriously affect the adhesion of the molten coating metal. The resulting coating has non-adherent areas, varying from large patches of 50% or more of the total surface, to minute blisters. In these, the coating, though present, is not firmly bound to the base metal and is subject to flaking when the metal is flexed, bent or formed, Pinholes in the coating are also sometimes encountered.

The fundamental object of this invention is the overcoming of the difiiculty which has been outlined. This, and other objects of the invention which will be set forth hereinafter or will .be

apparent to one skilled in the art upon reading these specifications, I accomplish by that pro cedure and in that construction and arrangement of parts of which I shall now describe an exemplary embodiment. Reference is made to the accompanying drawing wherein I have shown in cross section a pot containing molten metal, a portion of the hood, and other appurtenances, illustrative of one form of the invention.

It has hitherto been understood that the protective atmosphere in the hood, which should be reducing, tends under some circumstances to become slightly oxidizing through the effect of temperature, or air which may leak into the hood, or through materials given on by the strip itself; and it has been suggested that the hood atmosvapors of a metal which is more avid for oxygen than the surfaces of the strip itself.

While a scavenged atmosphere prevents oxidation of the strip, the scavenging and other atmosphere-control practices hitherto employed have not solved the problem to which this invention is addressed. I have discovered that, given a satisfactorily clean strip surface, the cause of the difficulty is the formation of substances, largely if not entirely derived from interaction of the bath itself, or vaporized metal from the bath, with constituents of the atmosphere. These sub-= stances adhere to the strip and prevent proper .bond with the coating metal. A portion of these substances may be produced in the atmosphere above the bath and may find their way to the free surfaces of the strip before the strip enters the 4 bath. But the greater quantity of them appear to form a scum or layer floating 0n the bath, which, being contacted bythe strip, adheres to its surfaces and is dragged down into the bath by the strip, with the formation of the abovenoted defects.

Among these substances appear to be nitrides formed by interaction of the bath with nitrogen forming part of the protective atmosphere or en tering it inadvertently, Some protective atmospheres are high in nitrogen, as when a reducing gas containing free hydrogen and formed by the cracking of ammonia is used, Oxides, hydroxides formed by interaction with water vapor, and other substances are also present.

My invention is based upon the discovery that it is possible to form a coating upon the surface of the bath-which is powdery in nature but nonadherent to the strip. This coating appears to prevent to a very large degree at least, interaction of the bath and atmospheric constituents; and in any event neutralizes the effect of and renders non-adhesive to the strip such nitrides, oxides and the like as may otherwise be formed.

By way of example, I have found that it is possible to form upon the surface of an aluminum bath a layer consisting mainly of powdery sodium aluminate which layer does not adhere to the passing strip and which prevents the aluminum from forming a harmful layer of its own.

It is not practicable to attempt to form such a layer by adding metallic sodium to the bath in the hood. Some fused sodium will float on the surface of the bath and, coming into contact with the strip will coat it thinly beneath the aluminum. When the hot coated strip emerges from the bath into the air, a rapid oxidation takes phere be treated with a scavenging agent, e. g. place with destruction of both coatings. At-

tempts to deoxidize the bath with sodium have thus proved ineffective.

On the other hand, the bare admixture of vapors of metallic sodium with the atmosphere in the hood is not effective for any purpose except deoxidation or scavenging of the atmosphere,

and will not cure the difficulty to which this invention is addressed. No effective coating of sodium aluminate is formed on the bath under these circumstances.

I have found, however, that the objects of this invention may be attained it, without bringing liquid or solid sodium into contact with the molten coating metal, I produce or introduce sodium vapors adjacent the surface of the aluminum within the hood. Under these circumstances the desired coating will form on the surface of the aluminum bath. and will be constantly renewed as required, without the formation or condensation of any floating layer of liquid sodium on the bath.

Itis necessary that the sodium vapors be formed or introduced in the hood between the surface of the bath and the point of introduction of the protective atmosphere (which is usually located in the lower part of the hood quite near the bell) since otherwise the sodium vapors may be unduly diluted by the protective atmosphere or swept away by it from the surface of the bath. If the sodium vapors are generated elsewhere and introduced into the hood or bell, precautions should be taken to avoid mixing the vapors with the hood atmosphere, and to avoid unnecessary turbulence within the hood.

I have found that the requirements of my process may be very conveniently met by providing in the hood or bell an open top container surrounding the strip or separate containers on each side of it. Into these containers I introduce from time to time metallic sodium which is melted and slowly vaporized by the heat of the bath transmitted to it through the containers. An advantage of a single container is that only one means need be provided for introducing the sodium, the melted metal flowing to cover all parts of the bottom of the container. When handling strip materials it seems requisite to provide a. vaporizer on both sides of the strip since otherwise the sodium vapors are likely to be more or less confined on one side of the strip. The open top of the container or containers is located very close to the surface of the bath exposed within the hood or hell and the generation of sodium vapors at this point serves the purpose of my invention, as above explained. The open top of the container will be located below the inlet into the hood for the protective atmosphere.

Referring to Figure 1, I have indicated at I a pot for molten coating metal containing, say, molten aluminum 2. The strip 9, coming from a reducing furnace in this instance passes through a hood 4 in which it is protected by a reducing atmosphere, e. g. cracked ammonia, enters the bath and passes around a sheave 3 so that it may emerge from the bath outside the hood. The hood terminates in a bell 5, the lips of which extend below the surface of the molten aluminum. An inlet for the protective atmosphere is indicated at 6 near the lower end of the hood.

I build my bell in such a way as to provide the aforesaid open top container surrounding the strip, by carrying inwardly the metal walls of the bell as at 1 and 8 and carrying upwardly within the bell a collar-like portion In. These parts are welded or otherwise fastened together so that the bell walls and parts I, 8 and I0 form an upwardly open container surrounding the strip. The molten aluminum rises within the collar portion l0 and its surface lies very close to the upper end of the collar portion.

Metallic sodium is introduced into the open top container where it melts and flows around. covering the bottom of the container. It is gradually vaporized and it will be noted that the vapors are generated very close to the surface of the molten aluminum, and that in the apparatus disclosed the heavy sodium vapors will flow down upon the surface of the aluminum bath. The rfietallic sodium in the container is indicated at For the introduction of sodium into the container, as is required from time to time, I preferably provide a conduit l2 equipped with a pair of valves or gates l3 and Hi. The employment of two quick opening valves in series permits me to introduce the sodium without permitting the suction of air into the bell. The sodium is preferably introduced in solid form in pieces or pellets of suitable size, although it may be liquefied before introduction, if desired. The liquefaction of the sodium in the apparatus described renders unnecessary its preliquefaction, and the inconvenience of handling liquid sodium in pipes or cond-uits wherein it may freeze and produce stoppage is avoided. It will be understood, of course, that the hood 4 and hell 5 are in communication with each other but are closed to the open air, the bath serving to close the lower end of the bell.

My invention is not limited to the coating of any particular metallic article, but is applicable to wire, sheets and even individual articles, it being understood that in the case of sheets or single articles suitable means are provided for conveying them through the hood and bell, into the bath and out again at the exit portion of the bath.

While I have described my invention in connection with the use of sodium, I have found that the alkali metals all are capable when properly used of forming a protective vapor over the bath and a powdery coating on the bath which exhibits no adhesive properties toward the sheets, strips or articles, and which has the property of preventing the formation of deleterious adhesive substances which interrupt the bonding of the molten metal to the base metal. By way of example, potassium and lithium serve my purpose very well.

While my process is principally used for coating iron or mild steel with molten coating metal, it is not confined to these base metals since the adhesive chemical compounds which affect the bond of aluminum with iron and steel also act with other metals. Again, while I have described my invention in connection with aluminum, it will be understood to be applicable to alloys of aluminum with other metals and also to molten coating metals which do not contain aluminum but which have the property of reacting with nitrogen, oxygen, water vapor and the like to produce adhesive surface scums on the bath which interfere with proper coating.

Modifications may be made in my invention without departing from the spirit of it. Having thus described my invention in an exemplary embodiment, what I claim as new and desire to secure by Letters Patent is:

1. In'apparatus for the purpose described, in combination with a container for a bath of molten coating metal, a hood terminating in a bell,

an end portion of said bell dipping beneath said bath, said hood and said bell serving to protect metal to be coated from the external atmosphere while it is carried into said bath, and an open top container for alkali metal located within said bell and formed in part of the bell walls, including inward and upward extensions thereof, said container extending about said bell on the inside and serving to maintain alkali metal in heat exchange relationship to the metal of said bath, the top of said container being open and located in close proximity to the surface of the bath within said bell, and means for introducing alkali metal into said container through the bell wall without opening up said bell to the surrounding atmosphere, said means comprising means in connection with said bell for accepting said alkali metal, and means operable sequentially to close said last mentioned means to the atmosphere in said bell and open it to the external atmosphere for receiving said metal, and to close it to the external atmosphere and open it to said bell for admitting said metal to said bell.

2. In apparatus for the purpose described, in combination with a container for a .bath of molten coating metal, a hood terminating in a bell, an end portion of said bell dipping beneath said bath, said hood and said bell serving to protect metal to be coated from the external atmosphere while it is carried into said bath, and an open top container for alkali metal located within said bell and formed in part of the bell walls, including inward and upward extensions thereof, said container extending about said hell on the inside and serving to maintain alkali metal in heat exchange relationship to the metal of said bath, the top of said container being open and located in close proximity to the surface of the bath Within said bell, and means for introducing alkali metal into said container through the bell wall without opening up said bell to the surrounding atmosphere, said means comprising means in connection with said bell for accepting said alkali metal, and means operable sequentially to close said last mentioned means to the atmosphere in said bell and open it to the external atmosphere for receiving said metal, and to close it to the external atmosphere and open it to said bell for admitting said metal to said bell, and means for introducing into said hood adjacent to but above said bell a protective nonoxidizing atmosphere containing hydrogen,

3. In a process of coating ferrous metal articles with molten coating metal of a class consisting of aluminum and aluminum alloys in which the metal articles are passed through a hood into a bath of said molten coating metal, the steps of maintaining a protectiv: atmosphere in the hood substantially non-oxidizing to ferrous metals and introducing an atmosphere containing alkali metal vapor between the surface of said bath of molten coating metal and said protective atmosphere.

4. In a process of coating ferrous metal articles with molten coating metal of a class consisting of aluminum and aluminum alloys in which the metal articles are passed through a hood into a bath of said molten coating metal, the steps of maintaining a protective atmosphere in the hood substantially non-oxidizing to ferrous metals and introducing an atmosphere containing alkali metal vapor between the surface of said bath of molten coating metal and said protective atmosphere, by maintaining a quantity of alkali metal within said hood in heat exchange relationship with said bath of molten coating metal whereby said alkali metal is liquified and slowly vaporized, said alkali metal in liquid form being maintained out of contact with said bath of molten coating metal.

5. In a process of coating ferrous metal articles with molten coating metal of a class consisting of aluminum and aluminum alloys in which the clean metal articles are passed through a protective atmosphere into a bath of said molten coating metal, the steps of maintaining above the surface of said bath an atmosphere of reducing as containing nitrogen, and providing between said atmosphere and the said surfaces of the bath an atmosphere containing alkali metal vapor.

6. In apparatus of the class described in combination a container for a bath of molten coating metal, a hood through which metal to be coated may be passed into said bath, said hood having a portion extending into the bath of molten metal whereby the end of said hood is sealed, inlet means for introducing a protective atmosphere into said hood at a point spaced from the surface of said bath, whereby said atmosphere moves into said hood in counter-current to the metal to be coated, and is substantially quiescent at the surface of said bath, a container having walls in heat exchange relation with said bath of molten metal, said container having an open top located wholly within said hood portion, and near but above the surface of said molten metal, and means for introducing alkali metal into said container without opening up said hood to the external atmosphere, said means comprising means in connection with said ball for accepting said alkali metal, and means operable sequentially to close said last mentioned means to the atmosphere in said bell and open it to the external atmosphere for receiving said metal, and to close it to the external atmosphere and open it to said bell for admitting said metal to said bell, whereby a substantially quiescent atmosphere of the vapor of alkali metal may be formed overlying the immediate surface of said molten metal within said hood portion.

7. In a process of coating ferrous metal articles with molten coating metal of a class consisting of aluminum and aluminum alloys, the steps of preventing the formation of a scum of compounds on the surface of a bath of said molten coating metal which compounds are adhesive to ferrous metal and interfere with the bond thereto or the metal of said molten bath, said formation being prevented by forming upon the surfaces of said bath through which the metal articles to be coated enter, a powdery layer comprising an aluminate of an alkali metal which is non-adhesive 'to ferrous metal, the formation of said layer being accomplished by providing immediately above the said surface of the bath a gaseous mass of the vapor of an alkali metal.

KASIMIR OGANOWSKII.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,378,052 Peacock May 17, 1921 2,046,036 Rodriguez June 3, 1938 2,166,251 Herman July 18, 1939