US3051587A

US3051587A - Method of treating metallic strip with sodium vapor

Info

Publication number: US3051587A
Application number: US50722A
Authority: US
Inventors: Kenneth G Coburn
Original assignee: Armco Inc
Current assignee: Armco Inc
Priority date: 1960-08-19
Filing date: 1960-08-19
Publication date: 1962-08-28
Anticipated expiration: 1979-08-28

Description

Aug. 28, 1962 K. G. COBURN METHOD OF TREATING METALLIC STRIP WITH SODIUM VAPOR Filed Aug. 19, 1960 E6. 2. ma

INVENTOR. /(/VN7'// 6 Coaue/v,

ATTORN EVS.

hired EJ351537 NIETHOD OF TREATENG NIETALLIC S WITH SQDlUit/l VAPOR This invention relates to a method of treating metallic strip and the surface of the coating bath with sodium vapor in the coating of ferrous metal strips with aluminum or aluminum alloys by the hot dip procedure.

One of the coating procedures now very generally practiced is one in which the metal strip is first passed through an oxidizing furnace to burn oils and other carbonaceous substances from the surface thereof and to form on the strip a microscopically thin oxide film. The strip is then passed through a reducing furnace in which the thin oxide film is reduced so as to leave the surfaces of the strip absolutely clean, in which state they are particularly receptive to the molten coating metal. The strip is then led through a hood under the protection of a neutral or reducing atmosphere and without a flux treatment into the coating bath. The protective hood dips into the coating bath so that the strip is at no time exposed to the atmosphere. The above outlined procedure is taught in the Sendzimir Patent No. 2,110,893.

When the coating metal is aluminum or an aluminum alloy, difficulties have been encountered in that deposits form on the surface of the aluminum bath and are picked up by the strip and these deposits adversely afiectthe adhesion of the molten coating metal. This problem was recognized and an antidote therefor was disclosed and claimed in the Oganowski Patent No. 2,437,919. In accordance with the teachings of Oganowski, a bell was provided at the end of the hood which dipped into the coating bath and the bell was provided with an annular trough surrounding the strip and into this trough metallic sodium was fed. Since the trough was subjected to the heat of the bath, the metallic sodium was therefore melted and vaporized.

Oganowski suggested that various substances such as nitrides were formed by the interaction of the bath with the nitrogen of the dissociated ammonia protective atmosphere in the hood and might also include oxides and hydrides formed by the interaction of various substances present with water vapor. He suggested that these nitrides, oxides and hydrides formed a scum on the bath which was dragged down into the bath on the strip which resulted in surface defects.

Oganowski suggested that it was possible to form on the surface of the bath a powdery coating which would be non-adherent to the strip and which would to a very large degree prevent interaction of the bath and atmospheric constituents and in any event render the strip non-adhesive with respect to such nitrides, oxides and the like which might be formed. Oganowski taught that one could form upon a surface of an aluminum bath a layer which consisted mainly of powdery sodium aluminate, by introducing sodium vapors adjacent the surface of the molten aluminum within the hood.

There are certain disadvantages involved in generating the sodium vapor in the hood by the heat of the bath in that the melting and vaporizing temperature is roughly that of the coating bath which is maintained relatively constant. Therefore, sodium vapor is only generated at a constant rate. If it is attempted to add metallic sodium more rapidly, there is a tendency for liquid sodium to splash directly onto the strip and this also produces defects in the coating. Furthermore, when it is remembered 3,@5l,58? Patented Aug. 28, 1962 that the hood contains hydrogen or dissociated ammonia, it will be understood that sodium reacts rapidly with hydrogen to form sodium hydride and therefore the sodium in the trough is rapidly converted to an inactive product so that there is a substantial waste of sodium.

Oganowski suggested that sodium vapors might be generated elsewhere and introduced into the hood, but indicated that this was diflicult if not impossible because of the dilution of the sodium vapors by the protective atmosphere and because of the likelihood of turbulence within the hood.

With the foregoing considerations in mind, it is an object of the present invention to provide a method of treating a strip, which is to be coated with aluminum or aluminum alloy, with sodium vapor wherein the sodium vapor is generated outside the hood and is conveyed to the hood by means of a carrier gas. It is another object of the invention to provide for a control of the sodium vapor addition. and to provide for greater uniformity in the rate of sodium vapor additions. It is also an object of the invention to eliminate the danger of sodium splash by removing solid and liquid sodium from the immediate vicinity of the strip being coated.

These and other objects of the invention which will be apparent to one skilled in the art are accomplished by that method of which an exemplary embodiment will now be. described.

Reference is made to the drawings forming a part hereof and which will illustrate more or less diagrammatically an apparatus by means of which the method may be carried out.

In the drawings,

FIGURE 1 is a diagrammatic view of a vaporizer and coating bath showing the relationship between the two.

FIGURE 2 is a cross-sectional view through an exemplary vaporizer; and

FIGURE 3 is a fragmentary cross-sectional view on the line 3-3 of FIGURE 1.

Briefly, in the practice of the invention there is provided a coating pot which may be conventional with a conventional hood according to the Sendzimir patent, dipping into the molten coating metal and providing a protective passage for the strip from the reducing furnace into the coating bath. A vaporizer is provided with means for heating the vaporizing chamber and means are provided for adding metallic sodium to the vaporizer. A carrier gas is caused to flow into the vaporizer and through the vaporizer, whereby to entrain vaporized sodium and a conduit .is provided to carry the carrier gas and its entrained sodium vapor from the vaporizer to the hood.

Referring in greater detail to the drawings, a coating pot is generally indicated at 10 and contains a body of molten coating metal 11. Within the pot there is mounted the usual pot roll 12 aroundwhich the strip 13 passes. The hood 14 extends from the exit of they reducing furnace down into the bath 11 and means are provided (not shown) for feeding into the hood a protective atmosphere which will usually be dissociated ammonia. It will be understood that the strip, in passing through the hood 14, has been pretreated so that it is absolutely clean and it then passes into the coating bath, around the .pot roll 12 and issues from the bath and is permitted to cool.

In FIGURE 1 a vaporizer is indicated generally at 15. A carrier gas is fed into the vaporizer through a conduit 16 from a suitable source (not shown) and the carrier gas with its entrained sodium Vapor passes through a conduit 17 from the vaporizer 15 to the hood 14. Metallic sodium is introduced into the vaporizer through the pipe 18 which is provided with a pair of single action, wide opening valves 19a and 19b. By means of these requirements.

J resides -in the, particular carrier gas which is used.

valves it is possible. to introduce sodium into the gvaporizer without sucking air into it. Referring now to FIGURE 2, the vaporizer comprises sodium '22. .,Under the-influence of the heat provided by itheelectric heating elements 21,-.the liquidsodium is :vaporized and'the balance of the vessel .29 is filled with sodium-vapor as indicated at 243. A bafile'24is provided the sodium ,vapor passes into the hood and build up of frozen metal or high melting point reaction products within the hood has been very greatly reduced. Elimination of the sodium trough of the Qganowski patent permits better heat transfer from the aluminum bath outside thehood, through the metal hood, so as to. pre- 1 vent freezing of the coating metal within the hood. The

1o cause the carrier gas entering through the conduit 16- to'flow downwardly around the baflle, whereby .to pick up and entrain-vaporized sodium. The carrier gas with the entrained sodium vaponthen passes through the con- Iduit .17 which-may be heated asby means of heating:

elements .17, -,and/or.insulated asat 17d, to prevent condensationof the vapor before it reaches the hood. The vessel.20;is enclosed within an insulating container so as to minimizeheatloss to economize on heat As seen in FIGURE3 where a' strip is beingicoated, V

the conduit ,17 :is provided ,with two branches 17a and 17b preferably entering from opposite sides .of the 'hood and "disposed adjacent the surfaceof the coating bath at a small angleofjincidence withrespect tothe strip so as .to cause the sodium vapor topass across thesurface of thebath andthe strip with a ofturbulence to achievethe results outlinedabove.

invention It might vbe :thought thatbecause the hood contains ,dissociated ammonia it would be desirable to use dissociated One very important aspect of the present lammonia as the carrier for the sodium'yapor. This,

however, is not so because. the use of dissociated ammonia results in theformation of sodium -hydrides which wouldtendto plug up the. conduit 17.

According to the present invention, the carrier gas used is nitrogen which .is completely inert with respect to 7 sodium, and which has the advantage ofbeing heavier. than hydrogen .so that the hydrogen-which is present in V the dissociated ammonia, atmosphere in the hood is displaced upward by the heavier-nitrogen}carrier gas. "Fhus,

:while-asmall amount of hydrides maybe formed, this hydride formation will be'negligible.

- It willnow be nnderstoodthatthe uniformity of sodium .vapor. addition may. be .very accurately controlled by a 1 controlv of the vaporizing heat-appliedthrough theheating elementsll, by therateof addition ofmetallic sodium and-by the rate of flow of the carrier gas.

.Without limitation but by waytof. example, the above -disclosedprocedure' has been practiced using a tempera, ,ture between11000 F. and 1100 F. and a flow of nitroj gen offlSOcubic-feet per hour. It be understood I that the temperature must be .suflieient to heat the carrier gas as well as toiprovidefonthe melting and vaporiza- Q tionof the sodium. When the carrier gas is .used in large volume, preheating thereof-ahead of the sodium.-\' aporizing chamber is advisable, as by'means of heating elements ,16a. Y e 7 It alsobe clear thatby removing the. source of the sodium vapor from .thehood andthe strip passing 'therethrough, the problem of .sodium splash has been entirely eliminated. The use of nitrogenas a carrier gas has eliminated the plugging up of the linesithrough which sive that it is the carrier of choice.

It will be understood that various modifications may be made without departing from the spirit of the invention and therefore no limitation not specifically set forth in the claims is intended.

7 What is claimed is: V V V '1. In a process of coating a ferrous metal article with molten coating metal of a class consisting of aluminum and aluminum alloys, in whichthe metal article is passed through a hood into a bath of said moltencoating metal, and in'which' a protective atmosphere which issubstantially non-oxidizing to ferrous metals is maintained in said hood; the steps of generating sodium vapor outside said hood, conveying said sodium vapor, by means of a carrier stream of nitrogen gas, intosaid hood adjacent the surface of said molten coating metal, whereby to subject the surface of the molten coating metal and the surfaces of the metal article being coate d,'to the action of sodium vapor. V 7

2.. .A process according to claim 1, wherein the protective atmosphere in said hood is dissociated ammonia.

3. A process according to claim 1, whereinthe article being coated is a strip of ferrous metal, and said sodium vapor, inits nitrogen gas carrier, is directed at both sides of said strip, adjacent the surface of the molten coating metal, at a small angle of incidence.

4. .In a process of coating a ferrous metal articlewith molten coating metal of a class consistingof aluminum and aluminumalloys, in which the metal :article is passed through'a hood into a bath of said molten coating metal, and in which a protective atmosphere which issubstantiallyhon-oxidizing to ferrous metals is-maintained. in said hood; the'step of continuously subjecting the surfaces of said article and of the moltencoating metal within said hood,'to the action of sodium vapor entrained ina stream of nitrogen gas. e j r 5; YA process according toclaim 4, wherein said article being coated is a strip of ferrous metal, and said sodium vapor, in its nitrogen gas carrier, is'dire cted at both sides of said strip adjacent thesurface of the'molten coating metal, .ata small angle of incidence.

tive atmosphere in said hood is dissociated ammonia.

V 1 References Cited in the file of this patent 7 UNITED sT TEsPATEnT s 2,437,919 Qrganowski Mar. 16,1948 2,503,571 Wehe Apr. 11, 1950 2,785,651 Pawlyk" Mar. 19, 1957 2,914,419

Organowski Nov. 24, 1959

Claims

1. IN A PROCESS OF COATING A FERROUS METAL ARTICLE WITH MOLTEN COATING METAL OF A CLASS CONSISTING OF ALUMINUM AND ALUMINUM ALLOYS, IN WHICH THE METAL ARTICLE IS PASSED THROUGH A HOOD INTO A BATH OF SAID MOLTEN COATING METAL, AND IN WHICH A PROTECTIVE ATMOSPHERE WHICH IS SUBSTANTIALLY NON-OXIDIZING TO FERROUS IS MAINTAINED IN SAID HOOD; THE STEPS OF GENERATING SODIUM VAPOR OUTSIDE SAID HOOD, CONVEYING SAID SODIUM VAPOR, BY MEANS OF A CARRIER STREAM OF NITROGEN GAS, INTO SAID HOOD ADJACENT THE