US2520658A - Method of galvanizing cylindrical tanks - Google Patents

Description

Aug. 29, 1950 R. s. RHEEM ETAL 2,520,653

METHOD OF GALVANIZING CYLINDRICAL TANKS v Filed Jan. 5, 1948 2 Sheets-Sheet l 5% 8? r A :r; 1 $5 :3 m E g a: Mk 3 3 3 K 3 4 ,3 5

i N H t "i m 3% l w 2 5 A K m X: x Z 0 I1 a (4 w 9 =1 2& Hm Q 11 [9 3 v g Q z 5i .5 6a 0 m r If. k 2 :f' 5 3K1 J 8E INVENTORS K: O RICHARD SRHEBM BY ROBERT DEBT TERSON a ATTORNEYS Aug. 29, 1950 R. s. RHEEM ETAL METHOD OF GALVANIZING CYLINDRICAL TANKS 2 Sheets-Sheet 2v w J S Y Mum. a 4 H T WET? 9 x mm M F 1 HQ M I ww i 1 z 1 Filed Jan. 5, 1948 ATTORNEYS Patented Aug. 29, 1950 METHOD OF GALVANIZIN G CYLINDRICALI TANKS Richard S. Rheem, Orinda, and Robert D. Patterson, Pasadena, Calif., assignors to Rheem Manufacturing Company, San Francisco, Calif., a corporation of California Application January 5, 1948, Serial No. 57 8 2 Claims. (-01. 117 97) This invention relates to a continuous and fully automatic hot dip method of galvanizing ferrous articles. While the invention is applicable to the galvanizing of sheets and other unfabricated articles, or partially, or fully fabricated articles, it is particularly adapted to the galvanizing of partially fabricated tanks, barrels, etc., in which one or both ends of the tank or barrel body still are open.

, Galvanizing long has been known and is used extensively for the protection of ferrous articles of various kinds. However, the methods heretofore employed have been relatively crude. Coatings have been non-uniform and have varied greatly in thickness. Pickling practices and the handling of the work in the zinc kettle have been haphazard and not subject to close control and generally have been wasteful of materials and conducive to the production of inferior coatings. They also have necessitated the use of considerable manual labor for performing the disagree: able and hazardous task of handling the articles being treated in and around the acid tanks and the galvanizing kettle incident to the splashing of corrosive, acids and hot zinc and explosions in and under the surface of the flux and zinc in the galvanizing kettle resulting from the superheating or instantaneous evaporation of moisture carried into the flux and molten zinc.

The present invention provides a method of galvanizing which is continuous, fully automatic, eliminates the hazards heretofore encountered in the handling of the articles, and results in the production of superior coatings of more uniform thickness.

In the present method of galvanizing, the work, or articles to be galvanized, while suspended from a conveyor continuously moving at a constant predetermined rate, successively are immersed and carried through an acid-pickling bath, a water wash tank, a flux-wash bath and the galvanizing kettle. The rate at which the articles are carried along by the continuously-moving conveyor, the time they are permitted to remain in each treating tank, the concentration of the acid in the pickling. tank and its temperature, and the rate at which the articles are withdrawn from the galvanizing tank are accurately controlled and correlated.

The invention will be further described in connection with the galvanizing of tank sub-assemblies, that is, tank bodies, such as hot water storage tank bodies, into one end of which an end cap vor closure having openings to which pipes subsequently are to be attached are secured, but

2 the opposite end of which still is open, and in connection with the accompanying drawings. However, it is to be understood that suchfurther and detailed description is by way of exemplificaticn and the invention is not limited thereby, except to the extent set forth in the appended claims.

In the drawings:

'Fig. 1 is a diagrammatic view illustrating how the method of the present invention is carried out,

Fig. 2. is a cross-sectional view showing the manner in which the tank sub-assemblies are introduced into, carried through and withdrawn from the galvanizing tank,

Fig. 3 is a plan view of the galvanizing tank, and

Fig. 4 is a transverse sectional view through the high-velocity spray chamber.

' Assuming that the articles to'be galvanizedare tank sub-assemblies I of black iron, that is, tank bodies having an end cap or closure welded into one end only of each tank body and the opposite end of each tank body open, they are suspended from a chain 2 which is driven in any suitable manner, as by a variable speed motor 3, along a track 4. The tank sub-assemblies just referred to hereinafter will be referred to as tanks.

Carriers 5 are secured to the chain at spaced As the surface of the material from which the tanks are fabricated carries grease and dirt resulting from handling of the material or contact thereof with the atmosphere, as well as mill scale and rust, it often is desirable first to subject the tanks to a washing or cleaning operation. This may be with a solution of an alkaline.

orv acid compound, a suitable solvent, or any detergent which will remove any grease or dirt from the tank surfaces. first are subjected toa washing or cleaning operation, as, for example, by immersing .them in, and carrying them through a hot alkaline wash 8, which, if desired, may be suitably agitated so that. a. thorough washingofthe tanks and re moval of grease therefrom takes place. If the To that end, the tanks.

'zinc ammonium chloride.

condition of the surfaces of the tanks is such that a cleaning thereof is not necessary, the alkaline wash, or other cleaning operation, may be omitted.

After being properly cleaned, the tanks are car- ,ried through a water-rinse tank 9 where any of the alkali or other cleaning agent adhering to the tanks is removed before introduction of the tanks into the pickling bath, thereby preventing bath, it is to be understood that in commercial; operationthe layout may be such that the tanks are caused to travel the length of the pickling bath two or more timesin order to maintain them in the pickling bath for a suitable length of time, or to avoid the necessity of using an unnecessarily long pickling tank.

After leaving the pickling bath, the tanks are 7 given a thorough washing with water to remove fromtheir surfaces, before further treatment, any of the pickling bath adhering thereto, or any salts formed thereon as a result of the pickling treatment. Such washing may be accomplished by,

passing the tanks through a water-rinse tank, by subjecting them both inside and outside to high velocity water jets or sprays, or by a combination of such treatments. Ordinarily, subjecting the tanks to highvelocity jets. or sprays of water, as

-at'll, will result in a thorough removing from the surfaceslof the tanks the adhering pickling solution and any salts which may be formed thereon asa result of the pickling operation; and such method of washing the tanks prior to further treatment is preferred.

The tanks having been thoroughly washed by the high-velocity water-jets to remove acid or sulfate adhering to their surfaces are then submerged into and carried through a flux wash l3, which may be of hydrochloric acid of suitable temperature and concentration, ora solution of The tanks then are vertically lowered through a layer of flux l4 floating on the surface of a body of molten zinc l5 at the entrance end of a galvanizing tank or kettle l6 andcompletely submerged in the molten zinc.

The tanks then are caused to travel lengthwise of the galvanizing tank and emerge from the discharge orbright end thereof.

From the galvanizing tank, the tanks are passed through a cooling chamber [1, after which they may be removed from the suspending hooks l. The tanks, galvanized both on the inside and out side; then maybe inspected, have end caps secured in their open ends and tested in any suitaretreated is concerned, and in actual operation the several treatingv tanks will'be arranged to best 4 utilize the available space, and so that the position where th tanks are removed from the conveyor chain is located relatively close to the position 7 where the tanks are secured thereto.

As diagrammatically shown in Fig. 1, and as more clearly illustrated in Fig.2, with respect to the galvanizing kettle, the track 4 includes downwardly and upwardly sloping portions over the entrance and discharge ends, respectively, of each of the baths in which the tanks are submerged for treatment, so that as the conveyor chain approaches the respective baths, it gradually takes a downward course and causes the suspended tanks gradually to be submerged into, carried through, and gradually withdrawn from the treating bath, all whilethe tanks are maintained in a vertical position and with their open end at the bottom. 7

As an important factor in obtaining a highquality, uniformly-deposited zinc coating on the tanks is the condition of the surface presented at the zinc kettle for the galvanizing action, the conditions under which the pickling of the tanks takes place, that is, the duration of the immersion of the tanks in the pickling solution, the concentration of the acid therein, and the temperature of the picklingbath should be accurately controlled. Therlength of time to which the tanks should be subjected to the pickling treatment depends upon the condition of the metal from which they were fabricated, primarily the amount of scale and rust which should be removed, the temperature of the pickling bath and the concentration of the acid in the pickling bath, with the 7 temperature of the pickling bath being the predominant factor. 7

As the time the tanks are in the pickling bath I0 is a. function of the linear speed at which the chain conveyor is moving, and, as will appear later, that is determined by the nature of the article being galvanized, the control of the pickling action primarily is obtained by regulating the concentration of the acid and the temperature to which it is heated.

If the pickling is carried out in a bath of sulfuric acid, which is preferred, the pickling bath originally should be the equivalent of about a 3% to 10% concentration, by volume, of 66 B. sulfuric acid, suitably inhibited to prevent the acid from unduly attacking the metal of the tanks. In most instances, astarting concentration equivalent to a 6% solution of sulfuric acid heated to a temperature of about F. is sufiicient. A small amount of ferrous sulfate forms during the pickling operation as a result of the sulfuric acid reacting with the iron of the tanks, with resultant gradual depletion of sulfuric acid in the bath. In order that the pickling rate will be maintained substantially constant, additions of sulfuric acid may be made from time to time, as long as it is economical, When the iron sulfate concentration in the pickling bath. reaches a point which renders further additions of acid uneconomical, the temperature to which the acid bath is heated gradually may be raised as the acid in the bath is depleted until a'temperature of about is reached when the acid concentration in the bath has decreased to about 1% by volume. The bath then is discarded.

The above temperature and acid concentrations for, the, pickling bath are based on the tanks being formed from metal containing an average amount of mill'scale and rust and on hydrochloric acid being used in the flux wash. If hydrochloric acid'is not used in the flux wash,

or if the metal of the tanks contains more than an average amount of scale, the concentration of the acid initially may be raised.

While the temperature to which the pickling bath is heated normally will be raised gradually and uniformly as the acid of the bath is depleted, it is to be understood that if at any stage of the operation the metal of the tanks coming through carry a greater or lesser amount of scale than normal, the temperature to which the acid is heated may be raised or lowered so that in all instances the tanks emerging from the pickling bath will have been pickled to just the right extent to properly condition their surfaces for the galvanizing action. However, the temperature of the pickling bath should not be raised appreciably above about 180 F., as above about that temperature too much inhibitor is required to prevent the acid from attacking the metal, and the tanks on emerging from the pickling bath are so hot that the pickling bath carried by the surfaces of the tanks will evaporate before the tanks reach the following water rinse and leave a deposit of sulfate which is difiicult to remove by the water rinse.

The length of the path of the tanks through the pickling bath is so correlated with the linear speed at which the conveyor chain moves that the tanks are subjected to the action of the pickling bath for a period of from about 14 to 66 minutes.

If hydrochloric acid is used in the pickling bath, the original concentration of the bath preferably should be equivalent to about 6% to 15% of B. acid, by volume. In such case, the acid bath need not be heated but the pickling action is speeded at an increased temperature. However, when hydrochloric acid is used, a slow cold pickle is preferred.

While hydrochloric acid is more expensive than sulfuric acid, it more readily dissolves any copper, arsenic and cadmium compounds which are present, as well as adhering scale, produces less adsorption of hydrogen, is not inhibited by iron salts, and is less apt to cause overpickling of parts, thus reducing the amount of inhibitor necessary to prevent excess pickling of the tanks.

Regardless of the nature of the pickling bath, it preferably is subjected to vigorous agitation by introduction of compressed air in such manner as to insure a continuous wiping of the inside and outside surfaces of the tanks, thus removing any gaseous accumulation that might interfere with proper contact of the acid with the tank surfaces.

The washing of the pickled tanks with water before they pass to the flux wash should be thorough and should immediately follow the pickling so that any acid, or iron salt solutions resulting from the pickling action and adhering to the surface of the tanks, will be washed off and any iron salt solution will not be allowed to dry upon them; otherwise such salts would be difficult to remove. Water at room temperature ordinarily is adequate for such washing, and hot water need be used only in case there should be some drying of iron salts on the surfaces of the tanks. When a hot water wash is desirable, water heated to about 100 F. is used and the tanks then immediately should be passed through a cold water wash so that no drying on the surfaces of the hot tanks will take place.

The water-washing of the pickled tanks is ac complished by passing them through the highyelocity spray chamber I] where they are subjected to jets of water at high velocity. The inside of the tanks are thoroughly washed free of any adhering pickling solution or of any iron salt solutions by jets of water projected from upwardly-directed nozzles 12 at high velocity, while the top and sides of the tanks are thoroughly washed by similar jets'of water projected against them from a series of nozzles l2 positioned at each side of the path of travel of the tanks through the chamber, as clearly shown in Figure 4. The water supplied to the nozzles i2 and i2 may be entirely fresh water, or the water may pass back to a sump and be recirculated. In the latter case fresh water preferably is constantly introduced into the bottom of the sump and overflows therefrom at the top so that a relatively clean body of water always is available for the washing of the tanks.

The subjecting of the tanks to a pressure cleaning operation, as described above, is particularly important if the tanks subsequently are given a flux wash in a zinc ammonium chloride solution which would not react with and remove any sulfates still remaining on the surface of the tanks. Such a pressure cleaning also drives off any hydrogen which may be occluded on the surface of the tanks.

. After the tanks have been pickled and the pickling solution thoroughly rinsed therefrom, they are further prepared for the zinc coat by a chloride rinse or flux wash. This is accomplished in the tank l3 which may contain a solution of hydrochloric acid, or a solution of zinc ammonium chloride, if zinc ammonium chloride is used in the flux.

If hydrochloric acid is used as the flux wash, it will react with and remove any scale or rust which may not have been removed during the pickling of the tanks, and will reacidify any salts remaining as a dried film on the tank surfaces. It also will react with the metal of the tanks to form ferrous chloride which remains on the tanks after leaving the flux wash. This, to a certain extent, is desirable, because as more specifically described below, the chlorine content thereof functions to regenerate the zinc ammonium chic-- ride flux blanket on the entrance end of the zinc kettle.

The concentration of the hydrochloric acid in the flux wash should be between about 2% and '7 by volume, although a concentration of about 6% is preferred. The hydrochloric acid flux wash may be at room temperature, or may be heated to between about and F, so that the tanks on leaving the flux wash will be sufficiently heated that the adhering ferrous chloride solution will dry on them before they are passed through the flux on the molten zinc at the entrance end of the galvanizing kettle.

The tanks need not remain in the flux wash for a period of over 10 minutes. However, in some instances they may remain in the flux wash for a shorter or longer period, for example, between about 8 and 20 minutes.

If zinc ammonium chloride is used in the flux, a solution thereof may be used in the flux wash, in which case it should be :present therein to the extent of about two pounds per gallon of aqueous solution thereof, although in some instances a lesser or greater amount may be used. The solution thereof should have a pH between about 4 and 4.28 and a Baum of approximately 12. These conditions may be maintained by addition of the zinc ammonium chloride to the flux wash from time to time. Thetemperature should be maintained between about 150 to 175 F., and preferably between 1.60 and 170 F. The heating of the flux wash may be obtained by means of steam pipes or otherwise and the bath thereof should be suitably agitated so that thorough contact of the flux-wash solution with the tanks will take place while they are passing therethrough. The zinc ammonium chloride serves to prevent oxidation of the surface of the tanks before their entry into the molten zinc by forming a protective film on the base metal which dries in place. Such film also will serve, to some extent, to regenerate a zinc ammonium chloride flux.

Regardless of whether the flux washconsists of a solution of hydrochloric acid or a solution of zinc ammonium chloride, the tanks should be permitted to thoroughly dry after leaving the flux wash and before immersion in the molten zinc in the galvanizing tank. If necessary, the tanks may be subjected to a drying operation, as by passing them through a heating chamber, or through a gas flame. Regardless of whether the flux wash is a solution of hydrochloric acid, or a solution of zinc ammonium chloride, the

tanks will emerge therefrom carrying a chloride solution on their surfaces which is permitted to dry before the tanks reach the galvanizing tank so that no moisture is introduced into the molten zinc.

When hydrochloric acid is used in the flux wash, reaction thereof with the metal of the tanks or iron oxide on their surfaces results in a gradual increase in the iron content of the flux wash. While the pickling action of hydrochloric acid is accelerated as the dissolved iron content increases, the harmful effects which are noted in the kettle, namely, the carrying of ferrous and ferric chloride into the molten zinc, with attendant accelerated dross formation, more than offsets the loss in discarding the hydrochloric acid flux wash after the iron concentration therein gets too high.

The obtaining of a proper galvanizing surface on articles depends upon such factors as the pickling, that is, whether or not the surfaces were clean, or roughened by overpickling; upon the product itself, that is, its chemical composition, physical structure, size, shape, thickness, and weight; whether its surface was dense or porous, or rough or smooth; on the purity of the zinc, its temperature and fluidity; upon the length of time the article is immersed in the molten zinc; and the rate at which it is withdrawn from the molten zinc, the latter being very important.

When ferrous articles are permitted to remain immersed in molten zinc sufficiently longto bring them to the galvanizing temperature, the zinc will alloy with the iron of the articles to form surface layers of alloys containing gradually less and less iron toward the surfaces, and finally outer coatings of zinc. The thickness of the iron-zinc alloy layers will depend primarily .upon the temperature of the zinc and the length of time the articles are immersed therein. The relative thickness of the outer coatings of zinc is not necessarily an indication of the quality of the coating. Although the corrosive resistance of a relatively thick coating of zinc is greater than that of a thin coating, the flexibility of a thick coating is a great deal less than a relatively thin coating, and, in time, may crack and :flake 01f. Hence, a relatively thin coating not only requires the use of less zinc, and consequently is less expensive, but, is more satisfactory. Ac.-

tually, all that is necessary is that there be a skin coatin of pure zinc on the outside of a layer of an iron-zinc alloy of minimum thickness.

In all galvanizing operations, some dross formation takes place in the galvanizing kettle. Dross is a compound of zinc and iron, roughly 96% zinc and 4% iron. It represents a direct loss in the galvanizing operation, Its formation results from the introduction of iron into the molten zinc through a variety of ways, such as from insoluble compounds remaining on the surface of the work due to improper pickling practices, from improper rinsin operations, from the carrying of iron chlorides into the galvanizing bath from the flux wash, from iron entering the bath from the kettle walls, and from iron entering the bath from the work itself. Dross frequently is the cause of imperfect zinc coatings being formed. Containing about 4% iron, it has a specific gravity slightly higher than the molten zinc and tends to settle to the bottom of the body thereof. However, as the specific gravity of the dross is not much higher than that, of the molten zinc, it is readily stirred up and maintained suspended in the molten zinc by agitation. As a result, it is not unusual in present galvanizing methods for small particles of the dross to adhere to the surface of articles being galvanized and thereby form impurities in the protective coating. In practicing the present method of galvanizing, impurities in the protective zinc coating applied to the tank are practically completely avoided.

The treatments previously referred to cause the surfaces of the tanks to be thoroughly cleaned of any mill scale, rust, or dirt, and generally to be in the best condition for receiving the zinc coat.

From the flux Wash 53 the conveyor chain carries the tanks, properly conditioned for galvanizing, to the galvanizing tank or kettle it, As the tanks approach the kettle IS, the conveyor chain from which they are suspended follows the downwardly-inclined portion l8 of the track 4 so that the tanks gradually are lowered downwardly in a vertical direction and open bottom-end first through the flux blanket or layer M which floats on the molten zinc at the entrance end of the galvanizing kettle. The tanks pass downwardly through the flux blanket until they are complete- 1y submerged within the molten zinc.

As the tanks descend open bottom-end first downwardly through the flux blanket, the flux floating on that portion of the molten zinc within the confines of the tank walls will be entrapped within the tanks, but as the tanks pass complete- 1y into the body of molten zinc, such entrapped flux will flow out of the tanks through the openings in the closed upper ends of them. The angle of inclination of the portion E8 of the track, and consequently the rate at which the tanks are submerged into the molten zinc, are not important as long as the rate of introduction is not such as to cause appreciable agitation of the molten zinc in the kettle. I

The flux layer or blanket M preferably is of ammonium chloride or zinc ammonium chloride or a mixture of the two. Either, in contact with the molten zinc, gradually will liberate chlorine. The chlorine itself, or hydrogen chloride resultin from reaction of the chloride, or both, will attack the surfaces of the tanks as they are lowered through the flux blanket and provide the final cleaning action upon the surfaces of the tanks, just as they pass into the molten zinc. The chlorine of the ferrous or other chloride carried by the tank leaving the flux wash will serve, at least in part, to regenerate the flux blanket The flux blanket I4 is confined at the entrance end of the galvanizing kettle by a pair of dams I9 which extend inwardly from the opposite edges of the kettle to a point adjacent the longitudinal center of the kettle, beneath the path of the conveyor chain over the molten zinc. The adjacent ends of the dams are spaced from one another a distance only suificient to permit the shanks of the hooks 1 which support the tanks I to freely pass betweenthem.

From the entrance end of the zinc kettle, the tanks gradually pass beneath a section of the kettle which is overlain by a body 20 which is heavier and thicker than the flux layer M. This heavier, thicker layer may be generally similar to the flux blanket [4 but made heavier and thicker by the addition of some inert material thereto, or it may be of some other material which is inert with respect to the molten zinc. The heavier, thicker layer 20 is maintained between the dams l9 and a pair of similar dams 2| spaced therefrom in the direction in which the tanks travel through the molten zinc. The dams l9 and 2| extend below the surface of the molten zinc but not a distance sufficient to interfere with the free passage of the tanks beneath them.

The body of heavier, thicker flux 20, or other material which is maintained on the surface of the molten zinc between the dams l9 and 2! prevents the shanks of the hooks from which the tanks are suspended from dragging any appreciable amount of the more fluid flux l4 through the opening between the adjacent ends of the dams l9 and, itself being relatively heavy, is not readily dragged by the shanks of the hooks through the opening between the adjacent ends of the dams 21. Hence, the discharge or bright end of the kettle is maintained relatively free of flux which otherwise would be dragged thereto by the shanks of the suspending hooks.

In the present process, zinc of high purity is used and is maintained in the galvanizing tank at a temperature from about 830 F. to 870 F. The zinc will reach its maximum, and therefore its optimum fluidity at about 850 F. Hence, a temperature from about that down to about 840 F. is preferred. At temperatures appreciably below about 830 F. the zinc becomes so viscous that the zinc pick-up by the tanks is too great. At temperatures appreciably above 850 F, the layers of iron-zinc alloys on the surfaces of the tanks "beneath the coating of zinc become too thick. If desired, up to about 0.1% of aluminum may be added to the molten zinc to increase its fluidity, and thereby permit the galvanizing to take place at a slightly lower temperature. The temperature of the zinc in the kettle may be maintained by heating the kettle electrically, or by oil, gas, or coal burners, ordinarily arranged Within the walls of the refractory setting for the kettle.

The chain conveyor travels at a predetermined constant linear speed of from about 6 to 13 feet per minute. At such speeds the tanks Will be carried through the molten zinc without appreciable agitation thereof which, in turn, would stir up the dross from the bottom of the galvanizing tank. As a result, the coatings of zinc which are obtained are continuous and free of imperfections such as would occur if particles of dross contacted and adhered to surfaces of the tanks as they were carried through the body of molten zinc.

Not only should the tanks be carried through the molten zinc at a speed sufficiently slow as not to appreciably agitate the body thereof and stir up dross from the bottom, but care should be taken to see that they are maintained immersed in the molten zinc for a length of time sufficient to bring them to the galvanizing temperature and sufficient to let the desired alloying of the zinc and the iron of their surfaces proceed to just the right extent, This will vary with the thickness and nature of the articles. With galvanizing tanks of about 12 gauge metal, the rate of travel of the tanks through the molten zinc is such that they are caused to remain immersed in the molten zinc for a period of from about one and three-fourths to three minutes and preferably from about two to two and one-half minutes. This is suflicient to form an iron-zinc alloy and an over-laying zinc coating of the desired thickness on the surfaces of the tanks.

The bright or discharge end of the zinc kettle should be kept free of zinc oxide which forms on the surface of the molten zinc so that particles thereof will not adhere to the surfaces of the tanks as they are raised from the molten zinc. This may be done by skimming or the like.

After the tanks have traversed substantially the full length of the zinc kettle, the conveying chains reach the upwardly-inclined portion 22 of the track and take an upward course which causes the tanks gradually to be lifted vertically from the molten zinc in the kettle. The angle of inclination of the portion 22 of the track should be so correlated with the linear speed at which the conveyor chain is driven that the tanks should be withdrawn from the zinc kettle at a rate of about forty to eighty inches per minute. In other .words, at such a rate that it will require from about 45 to seconds to withdraw a range boiler or hot water heater tank about five feet long. If the rate of withdrawal is faster than that just indicated, the molten zinc carried by the tank surfaces in excess of that necessary to form. an adequate coating on them will not have time to flow downwardly along the tank surfaces and back into the molten zinc in the kettle before it freezes, thus resulting in zinc pick-up beyond and in excess of that necessary adequately to protect the surfaces of the tanks, and consequently greatly increasing the cost of coating the tanks with the zinc. If the rate of withdrawal is not faster than that indicated, heat from the molten zinc in the kettle will be conducted upwardly through the tank walls and maintain them at a temperature sufiiciently high to prevent excess zinc freezing to them before it can run back into the body of molten zinc in the kettle. Zinc tends to build up on zinc. Therefore, if the rate of withdrawal is slower than that indicated, the coating of zinc on the tanks, although of uniform thickness, will be greater than that desired and greater than that necessary adequately to protect the surfaces of the tanks.

As the lower ends of the tanks are open, no difficulty is encountered in getting all of the molten zinc out of the tanks, such as is encountered when the inside of closed tanks are galvanized. The tanks merely are lifted gradually from about that portion of the body of molten zinc which their side walls surround so that there is no s-purting back of molten zinc from openings in the tanks, with resulting splashing and agitation and greater formation of zinc oxide because of greater contact of the molten zinc with the air.

' or otherwise.

From the zinc kettle, the tanks pass through the'cooling chamber I! where their temperature is lowered sufiiciently to permit them being handied, after which they are removed from the conveyor chain and inspected inside as well as outside, which readily may be done in view of the fact that the bottom of each tank still is open. Each tank then is completed by forcing a flanged end cap, galvanized on both sides, into its open end so that the flange is on the outside and forms a tight fit with the end of the tank body, and finally resistance seam welding the flange of the end cap into the end of the tank body, as more fully described in the aforesaid application.

thickness, and coatings which are free from impurities caused by adhering particles of dross As the tanks are subjected to the preliminary treatments and finally galvanized before the bottom and caps or closures are inserted into the tank bodies, no difficulty is encountered in causing them to submerge in the molten zinc or the preliminary treating baths; there is no danger of flux being trapped and remaining in the tanks; the tanks readily are submerged into and withdrawn from the molten zinc without causing agitation of the body thereof; and the tanks, after being galvanized, may have their informed cylindrical ferrous tank having one end open and the other end closed by a head having an opening therein, which consists in suspend ing the partially-formed tank from above, passing the partially-formed tank downwardly, openend first, into a bath of molten zinc having a fluidity substantially that of molten zinc of a temperature between 830 F. and 870 F. while at least in part venting the partially-formed tank through the opening in the upper closed end, maintaining the partially-formed tank in the molten zinc for a length of time suficient to form a protective zinc coating on its inside and outside surfaces while substantially continuously moving it forwardly along a path through the molten zinc at a rate not substantially exceeding 13 feet per minute and sufficiently slow that it passes through the molten zinc bath without such agitation of the bath as will stir up dross from the bottom thereof, and withdrawing the coated partially-formed tank from the bath, closed-end first, at a substantially constant rate not substantially exceeding inches per minute, whereby a uniform thin coating of zinc is obtained on both the inside and outside of the partially-formed tank.

2'. The method of galvanizing a partiallyformed cylindrical ferrous tank set forth in claim 1 in which the coated partially-formed tank is withdrawn from the bath at a rate between about 40 and 80 inches per minute.

RICHARD S. RHEEM. ROBERT D. PATTERSON.

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

UNITED STATES PATENTS Number Name Date 1,740,639 Austin Dec. 24, 1929 2,234,586 Bertalan Mar. 11, 1941 2,304,069 Beckwith Dec. 8, 1942 2,307,613 Wilson Jan. 5, 1943 2,477,684 Borg Aug. 2, 1949

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