US2405592A - Process of galvanizing - Google Patents

Description

s, 19 6. AJJ. M AUGER- ET AL 2,405,592

PROCESS OF GALVANIZING Filed June 11, 1941 lmem'm's; men/0e J. M4065? afia Patented Aug. 13, 1946 UNITED STATES PATENT OFFICE PROCESS OF GALVANIZING Arthur J. Manger, Gary, Ind., and Alfred H. Ward, Mount Lebanon, Pa.

Application June 11, 1941, Serial No. 397,646

(Cl. 117w51) 4 Claims. 1

In accordance with the present invention, there are provided certain improvements in the C at of ferrous metal shapes such as sheets, strip, or wire, with a layer of another metal, such as zinc,

by a hot coating process. I

Hitherto, it has been the practice to coat iron or steel sheets with zinc by picklin them in an acid solution, passing them through a molten flux prepared from sal ammoniac (ammonium chloride) and dipping them beneath the surface of a bath of molten zinc. It has been the practice to coat iron or steel strip, either hot rolled or cold rolled, in continuous bands by passin the bands through suitable furnaces and chambers which produce oxidation and subsequent reduction of their surfaces and also provide neutral or deoxidizing atmospheres at the point of entry into the molten zinc. It has also been the practice to coat iron or steel wire continuously by pickling it in an acid solution and immersing it in molten zinc with an optional intervening washing operation, or a combined washing and fluxing operation, requirin the use of a liquid flux solution of zinc chloride or zinc-ammonium chloride. The

method of coating as described above for iron or steel sheets has given unsatisfactory results because of insufficient adhesion between the coating metal and the parent metal.

It also has been known in the. art that when certain alloy additions are made to the zinc bath, the molten sal ammoniac flux used at the entrance end of the galvanizing tank and surrounding the rolls at the exit end, may become ineffecive and, i perative.

Thus, specifically, when aluminum, which has long been known in the art to have a beneficial effect enhancing adhesion between the parent metal and the metal of the coating, is introduced into the bath in such quantities as to be useful in this respect, the molten sal ammoniac flux rapidly decomposes, due to the formation of a reaction product, aluminum chloride, which is easily volatile at the temperatures used. Decomposition of this flux makes it impossible to maintain a suitable consistency for satisfactory operation, and none of the practices used hitherto for the continuous coating of wire has been satisfactory when aluminum has been used in suflicient quantities to benefit adhesion propertie because of the decomposition of chloride fluxes as mentioned above. Coatings produced invariably contain dark oxide spots and uncoated areas.

The present invention has, for its general object, the provision of a simple and economical means of producing high quality coated iron or meet satisfactorily, trade requirements, while avoiding the disadvantages of present practices.

In the description of the present invention which follows, it will be shown that the fundamental concept upon which the present invention is based, does not necessarily limit the crosssectional shape nor the width, length, and thickness of the iron or steel articles to be coated; and while the process of the present invention will be described as applied to the coating of com-.

-mercial sizes of steel sheets, it will be understood that simple mechanical changes in the design of apparatus used, makes it possible to adapt the present invention to the coating of continuous flat strips or bands, and also, to the coating of wire of circular or other cross sectional shapes.

The present invention consists, generally speaking, of a procedure by which a scale-free object is passed through a heated chamber which dips at its longer extremity beneath the surface of the molten cOating metal, and which contains a gas produced by heating solid ammonium chloride sufficiently to raiseit above its volatilization temperature, and then immediately dipping the object beneath the surface of molten zinc which may contain aluminum as an alloying ingredient.

It has long been known by those versed in the art, thatwhen ammonium chloride is heated in the presence of some metals such as iron, or in the presence of some metal oxides, such as iron oxide, it breaks down into chlorides of the metal, or metal oxide, ammonia, and hydrogen.

More specifically, in the process of the present invention there is used a scale-free iron or steel sheet of any length, which may have been formed by any process of manufacture, such as hot or cold rolling from bars, hot rolling from slabs, hot rolling from slabs followed by pickling and cold rolling, or any combination of these processes.

These sheets, if produced by such a method as to leave them free from scale may be used in accordance with the present improved process without further treatment. If, however, they have been produced by such a method as to leave them covered Wholly or in part with scale, they may be subjected to such subsequent treatments as may be necessary to remove the scale before being used in the present invention. No limitation is to be placed upon the character of the surface of the iron or steel shapes used in the present invention, except that they be free from scale.

With reference to this freedom from scale, it

3 is known to those familiar with the art that iron or steel objects, when exposed to the atmosphere, form on their surfaces a very thin film or coating of an iron oxide compound which is substantially ferric oxide or ferric hydroxide, when such surfaces so coated are brought into contact with molten zinc, which itself contains a slight film of oxide, such surfaces do not become immediately wetted with the molten metal. If conditions are maintained at the point of contact between the parent ferrous metal and the coating metal so that the two metallic surfaces are brought together while free from oxide films, the parent metal will immediately become wetted with the molten coating metal so as to initiate the formation of an inter-metallic alloy.

The present invention is illustrated somewhat diagrammatically, by the accompanying drawing, which shows a diagrammatic side elevation of an apparatus which may be employed in carrying out the improved process.

Referring more particularly to the drawing, there is shown a ferrous body to be coated, which body is indicated at I and which may be a sheet or a coil or a continuous strip, is conveyed by any I suitable means through a chamber 2. This chamber 2 preferably has one end extending into the molten coating metal 3 and beneath the surface thereof so as to be sealed thereby. This is not an essential detail, however, as the ferrous body I is intended to be at such a relatively low temperature as not to be oxidized readily if momentarilyexposed to the atmosphere, which contains a controlled amount of oxygen.

' The chamber 2 contains a current or flow of an atmosphere of ammonium chloride vapors, which are shown as being conveyed from the external retort. 5 containing a quantity of ammonium chloride 4, which isvolatilized therein at a temperature beginning as low as 420 F., and the resulting ammonium chloride vapors are conveyed to the chamber 2 through the conduit 6. As a convenient means of maintaining the conduit 6 at a sufficiently high temperature to prevent condensation of the vapors, such conduit may be passed through the molten metal bath and then be heated thereby.

Preferably, also, a supplemental external heating means I5, which may be of any suitable char acter,.may be provided for the walls of the passage 0r chamber 2 to prevent condensation of the ammonium chloride vapors on the wall of the chamber, and to maintain the ammonium chloride in a thoroughly. vaporized condition, and to maintain a suflicient reserve of thermal energy to compensatefor any temperature lowering effect ofammonium chloride vapors, due to the contact withthe oxide film or the surface of the ferrous object.

In the operation of the process, the relation between the extent of the passage 2 to the rate of speed of the ferrous body I and the initial substantially atmospheric temperature thereof upon entering the chamber 2, and the temperature and rate of flow of the ammonium chloride vapors, are so controlled and related as to maintain the relatively low temperature for the main portion ofthe ferrous body I, so as to minimize or avoid the formation of any oxidized surface on the body afterit passes from the ammonium chloride vapOrs in the chamber 2.

I In order to permit of a practical operating tolerance in the range of operating temperatures above a lower critical subliming temperature, it is desirable to maintain such'operating temperm tures in the chamber 2 and the duct 6 preferably about 700 F. It is, of course, possible and practicable, to operate at lower temperatures but still above the volatilizing temperature of ammonium chloride by proper balancing of other control factors.

A suitable hood I may be provided at the end of the passage 2 at which the ferrous body I enters after its passage between suitable feeder or guide rolls. The hood and the guide rolls serve to seat the chamber 2 against entry of air. A controlled air inlet however, preferably is provided by the valve II which regulates any amount of air introduced into the chamber 2 through inlet port IS, in order to obtain the desired oxidizing reaction for the process, as willbe referred to hereinafter. The excess vapors of ammonium chloride, as they emerge from the upper end of the chamber 2 immediately condense as a dust within the area confind by the hood I, the hood not being heated. The hood I is connected by suitable ducts 8 to a source of suction 9, an intervening bar filter III being utilized to collect and recover the condensed ammonium chloride which may be reused.

The ferrous bodies entering the coating apparatus are conducted by entrance rolls I2, and through the bath of coating metal by the submerged rolls I3, being finally led out of the metal bath by exit rolls I4. Suitable spaced guides may be provided between the submerged rolls I3 and the entry and exit rolls I2 and I4, such guides also serving to prevent accidental dropping of a body being coated into the bottom of the pot containing the molten bath of coating metal.

The exit rolls I4 may be operated, free from flux, if suitable alloy additions are made to the coating bath to minimize surface oxidation to a point where this is practicable. It is found in practice that satisfactory results can be obtained in a zinc bath containing as little as 0.1 per cent of aluminum.

Instead of generating the ammonium chloride vapors in the chamber 2, it may be found convenient to add solid ammonium chloride at a controlled rate into the chamber 2, and volatilize th ammonium chloride in this chamber.

In addition to the method described above for vaporizing the ammonium chloride, it may be desired to produce such an atmosphere by mixing suitable proportions of gaseous ammonia and gaseous hydrogen chloride.

In providing oxide free surfaces at the point of contact between the iron or steel parent metal surface and that of the molten zinc, there is produced an equivalentv amount of ferric chloride which is volatilized during the process, and is swept out by the current of incoming ammonium chloride vapors, and a certain amount of this ferric chloride may be condensed along with the reclaimed ammonium chloride. When the ferric chloride builds up sufficiently to cause trouble in subsequent revolatilization of ammonium chloride, the impure ammonium chloride may be dis:- solved in water and recrystallized.

There is also formed a slight amount of ferrous chloride which is not volatile at galvanizing temperatures, and would cause difficulty in the zinc bath if produced in any large amounts. Consequently, in order to minimize the amount of such ferrous chloride produced, the temperature of the chamber is controlled and controlled amounts of air are admitted through port I6, an excess of oxygen being maintained in the chamber for preventing substantial formation of ferrous chloride and to maintain the iron compounds in ferric condition.

The sheets entering the coating apparatus are guided into the bath of molten metal 3, by entrance rolls l2, and passed through the molten metal by submerged rolls l3, and finally may be led out by exit rolls I4. The latter rolls may be operated in a molten flux prepared from sal ammoniac, or they may be operated free from flux if suitable alloy additions are made to the zinc bath to minimize surface xidation to a point Where this is practicable. It has been found that satisfactory results are obtained in a zinc bath containing 0.1 per cent of aluminum or more. It is found, also, that it is necessary, first to coat these rolls by immersing them in a molten aluminum-free zinc bath,

We claim:

1. The process of galvanizing ferrous metal shapes, such as sheets, strips, and wire, which comprises passing the shapes through an atmosphere of ammonium chloride vapors maintained within a temperature range of from 420 F. to 824 F., admitting excess air into the ammonium chloride vapor atmosphere, immediately passing the shape from the said atmosphere through a galvanizing bath, and controlling the said temperature and admission of air for obviating formation of substantial amounts of ferrous chloride on the shapes by reactions of the ammonium chloride vapors with the ferrous metal of the said shapes.

2. The process of galvanizing steel shapes, such as sheets, strips, wire, and the like, which comprises initially passing the shapes through an environment containing an atmosphere of ammonium chloride vapors, providing oxidizing conditions in the said atmosphere, and controlling the temperature within the said chamber within a range between 420 F. and 824 whereby the ammonium chloride vapors react with the said shapes to produce volatile ferric chloride to substantial complete exclusion of non-volatile ferrous chloride, passing the ammonium chloride vapors to a condensing environment, condensing the said vapors to solid ammonium chloride, together with volatilized ferric chloride, and passing the thus-treated shapes from the ammonium chloride atmosphere immediately into a galvanizing bath.

3. In galvanizing steel shapes such as sheets, strips, Wire, and the like, the improvement which consists in preparing the shapes for galvanizing by treatingthem with ammonium chloride vapors under oxidizing conditions for forming volatile ferric chloride to the substantially complete exclusion of non-volatile ferrous chloride.

4. The process of galvanizing steel shapes, such as sheets, strip, wire, and the like, which com-- prises initially passing the shapes through an environment containing an atmosphere of ammonium chloride vapors providing oxidizing conditions in the said atmosphere while controlling the temperature within the chamber within a range between 420 F. and 824 F., whereby the

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