US2082622A

US2082622A - Daluminum coated metal and process for producing the same

Info

Publication number: US2082622A
Application number: US68814A
Authority: US
Inventors: Colin G Fink
Original assignee: Individual
Current assignee: Individual
Priority date: 1933-02-25
Filing date: 1936-03-14
Publication date: 1937-06-01
Anticipated expiration: 1954-06-01

Description

June 1, 1937.

ALUMINUM COATED METAL AND PROCESS FOR PRODUCING THE SAME c. G. FINK Y 2,082,622

Filed March 14, 19:56

7 9 /0 /2 /4 /6 WIPE/P ans? Msafie 5071c Abe I flympea a C3 Salad Eur F 63 k V dlmnerr/c diva INVENTOR Patented June 1, 1937 UNITED STATES PATENT .oFricE ALUMINUM con'mn METAL AND mocass ron rnonucma 'rna SAME Colin G. Fink, New York, N. Y.

Application March 14, 1936, Serial No.

In Great Britain and Norway 21 Claims. (01. 91-103) This invention relates to aluminum coated metals, and to a process of producing the same, and provides improvements therein. A

(zinc So-called galvanized metal coated metals) and tinned metals have been articles of as an article of commerce, owing to the lack of a satisfactory and commercially practicable process of coating the metal (particularly ferrous metal) with analuminum coating which is tena- 15 ciously adherent, substantially uniform, ductile,

infrangible in the ordinary metal working proccases, and corrosion resistant. Descriptions of processes of coating metals with aluminum are to be found in the technical and scientific litera- 20 ture, and in patents, but these described processes 4 are slow and imperfect, and fail for the production of a commercially satisfactory and serviceable product or article.

Successful coating with other metals, as mag- 25 nesium, barium, calcium, etc. has presented a similar problem.

Aluminum is a metal the oxide of which is not readily reduced by gaseous hydrogen, certainly not in the same way that oxides of metals like 30 tin, copper, lead, and of many other metals, may be reduced. Like aluminum, the oxides of mag. nesium, barium, calcium, and of a number of other metals are not reduced or only difllcultly reduced by gaseous hydrogen. In the descrip- 35 tion which follows the invention will be described with particular reference to its application to aluminum coating, but notwithstanding the particular reference to aluminum, the application of the invention to other coating metals having a behavior toward gaseous hydrogen, similar to aluminum is to be understood aspart of l the invention and matter claimed.

The present invention provides anovel process for coating metals, especially ferrous-metal, with aluminum or aluminum alloy, by means of which tenaciously adherent and substantially uniform smooth, bright coatings may be obtained, suitable and satisfactory for the general uses of commerce and industry. It further provides a proc- 50 ess which may be expeditiously carried on, with a speed and versatility capable of satisfying the manufacturing requirements of modern industry. It provides, in particular, a practical and satisfactory continuouslyworking process .for the 35 manufacture of aluminum-coated wire, rods,

88,814 February 25,

bars, beams, sheets and strips, such as may be used generally in the fabrication of coated metal products. i

The invention further provides a novel product; that is, an aluminum coated metal in va- 5 rious forms, and particularly those forms used in the fabrication of metal products, such as wire,

sheets, bars, etc., on which the coating is tenaciously adherent, smooth, bright, and superficially uniform or continuous, resistant to corrosion at 10' both ordinary and high temperatures, resistant to fruit acids, and-will stand; the stresses. incident to metal- -working. without breaking and exposing the original ferrous metal beneaththe coating, and will maintain its properties after undergoing such metal working processes. The invention further providesa metal article having a coat of the character described which is substantially uniform in'cross-section throughout,its length.

The invention further provides novel apparatus for use in connection with the process herein described. a

An example of an aluminum-coated article according to the present invention, embodiments of apparatus according to the present invention, and diagrams illustrating modes of procedure in practicing the process invention, are illustrated in the accompanying drawing, wherein:

Fig. 1 is a diagrammatic view illustrating said mode of procedure.

Figs. 2 and 3 are respectively a side view and a vertical section of a furnace for containing molten aluminum or aluminum alloy and in which articles are coated, embodying the present invention.

Fig. 4 is a vertical sectional view illustrating a second embodiment of a part of the apparatus according to the invention.

Fig. 5 is a reproduction of a photomicrograph of an oblique sectionof a wirecoated with an 40 aluminum rich alloy according "to the present invention, and embedded in a matrix for purposes of photographing.-,

Fig. 6 is a diagrammatic view'illustrating an alternativemode of procedure. 5 Aluminum is very readily oxidizablein the.

molten state,--and the oxide film or layer which forms on the surface of a bath of vmolten aluminum has a high surface-tension, which together with the low'density' of themolten aluminum, 5 resists penetration to a surprising degree, the oxide-film being sharply depressed below the 7 bath level when objects '(a steel rodnr. wire forexample) are depressed on to the surfaceof the;

bath, and the oxide-film can be pressed by anobject to a very considerable depth below the surface of the bath without breaking. Moreover molten aluminum appears to act in a similar way toward articles having an oxid coating or film thereon. It is the interposition of this oxidefilm between the object and the aluminum which has probably been the cause of failure to secure an adherent coating of aluminum in previous attempts 'to coat metal articles by dipping'them into an aluminum bath.

According to the present invention, I proceed in such manner as to remove oxygen, and such oxides as may remain after cleaning, from the article or object to be coated, and provide the article with a charge of a reducing gas which it carries with it into the aluminum bath. This is accomplished by pre-treating or subjecting the article or object to be coated, foran adequate length of time, before passing into the aluminum bath, to the action of a reducing atmosphere under conditions adequate to remove oxygen, reduce such oxides as may have remained after cleaning, and to charge or saturate or pack. the

. article or object with enough of the reducing gas to bring about the complete union of the aluminum coating to the basis metal at the time of introduction into the aluminum bath.

I also proceed in such manner as to avoid passing the article or object through an oxide film on the aluminum coating bath, especially at the point of introduction, so as to eliminate or minimize adhesions on the coated article, of aluminum-oxide from the aluminum coating bath, and this is accomplished by restricting the surface of the'aluminum bath which is exposed at the point of entry of the article or object into the bath, so that the amount of aluminum oxide which may. form on the restricted surface within any given time during which the process is operating is small as regards the aluminum which is taken up by the article or object. Moreover, at-the point of entry through the restricted orifice, of an article which is fed continuously into the bath, there is a vigorous circulation of the molten aluminum, so that contact of the aluminum which may appear at the surface within the orifice at any given moment is of very short duration. Fresh aluminum to replace that taken up by the article comes from the sub-surface portion of the bath. In this way the aluminum-oxide film is virtually eliminated at the surface where the article or object is introduced. A restricted surface'at the point of introduction of the object or article can be obtained by providing a restricted orifice in the side of the aluminum-pot below the surface or in a part extending into the aluminum through the surface of the bath. Oxidation at the surface of the aluminum-bath is advantageously minimized in suitable manner, as by maintaining an atmosphere of a gas or gases such as are ordinarily known as inert or reducing, such as hydrogen gas. However, aluminum-oxide, in contra-distinction to the oxides of metals generally used for coatingv from molten baths, such as tin, copper, lead, etc., is not reduced by gases such as are ordinarily known as reducing gases,

as molecular hydrogen, carbon-monoxide, etc. I have found, however, if the article or object to be coated be subjected to the action of a reducing gas, and the temperature is raised, a point or degree of temperature is, reached where the behavior of the gas-charged or packed article toward the bath of molten aluminum undergoes a definite change. Below said temperature at which the behavior of the article toward-aluminum changes,

the adherence of the aluminum coating to the gas-treated article is imperfect. The coating may be adherent only in spots, or not at all. Coatings which apparently cover the. article are not tenaciously adherent, and easily separate when the coated article is bent, or when the 'coat-' ing is rubbed. On the other hand, when the pretreatment with reducing gas is at the temperature where the aforesaid change in the behavior of the pre-treated article toward molten aluminum occurs, there is obtained, when the article is brought into contact with the molten aluminum, a coating of aluminum which is complete and tenacious and having the properties of withstanding bending, etc. heretofore referred to. Any means or procedure for charging or packing the article with gas by which an equivalent effect is obtained, I regard as within my invention. Such charging or prepacking, I have found also can be done by making the article a cathode in an acid bath and passing current thereto at a current-density at which a large volume of hydrogen is released on the cathode-article. The gas-charge carried bythe article into the molten aluminum is active at the timethe article meets the molten aluminum to reduce oxideof aluminum and combine with oxygen which are dissolved in the aluminum bath,- so that oxidation atthe interface of the metal being coated and the coating metal is prevented.

Examples of reducing gases for obtaining the effects hereinbefore stated, are hydrogen, carbon? monoxide, nitrogen, and certain industrial gases containing mixtures .of these, such as blast-furnace gas and blue water gas. -However gases which are unstable and deposit foreign particles on the article to be coated and therebyinterfere with the coating, such as methane, should be eliminated.

According to my investigations to date, the minimum point or degree of temperature where the behavior of a hydrogen-charged or packed. mild-steel, strip. toward molten aluminum occurs, as aforesaid, such as to assurea satisfactory coat-a ing, I have found to be about 732 C. (1350 E), this temperature being that of the steel strip mow ing through a chamber containing hydrogen gasfor charging or pre-packing the strip, at the end thereof adjacent the aluminum-pot, the measurement being made with an optical pyrometer of the hot filament type. in the means of making. high-temperature measurements inclusive .of optical v pyrometers, should be recognized. Measurements obtained bythe use of such instruments are not precise, a certain amount of variability being inherent in the instruments themselves, and in the perceptions of different users.

The aforesaid temperature (732 .C. or thereabout) may be found to vary with different metals and alloys, or different conditions thereof; and also for the particular reducing gas, or gas mixture, used; and therefore said temperature is to However certain limitations be taken as suggestive of an appropriate tempera- Til , the coating of wire, rods, sheets and strips in a continuous manner. The process is also one by which a novel and superior product may be ob tamed. By controlling the temperature of the molten aluminum and the length or timethe .15 article remains in contact with the bath, the ,alloy which is formedat the interface of the aluminum and the basis metal may be made of it may be. greatly limited as to its thickness, and the flow of crystals of the aluminum and of the basis metal may be limited to the alloy intermediate the; basis metal and the aluminum coating, so that i a substantially uniform thickness,

stray crystals of the basis metal are kept out of the aluminum coating.- The alloy which forms at said interface is of a ductile character, and this 1 attribute to the absence of oxygen and of oxidizing conditions, so that .oxygen inthe alloy is absent or inconsequential. I

Now referring to the diagram, Fig. l, a mode of procedure is substantially as follows:

Wire made of steel, designated by letter-X, is taken as an example of an article to be coated.

The wire X, if cleaning or removal of scale is required, is preferably first passed through a 5 pickler or cleaner i0 (which may be a vat containing a solution of hydrochloric acid, or an alkali bath, or any desired combination of cleaners) where its surface is cleaned of scale, oxides, grease, etc. The cleaned wire may then be passed through a solution of boric acid (preferablya concentrated solution-a 7% solution has been used) or a" solution of borax .or other boron salt having a low melting point, contained in a vat i4. Before passing into the boric acid solution, the wire may be and preferably is heated to a red-heat (600 C. for example) asthis evidently drives out the water immediately surrounding the wire and concentrates the solute on the wire. The heating may be effected in an" oven i2 containing heated air.

Pie-packing or charging of the article with the reducing gas (hydrogen for example), to change the behavior of the article toward molten aluminum in the manner aforesaid, may be accomplished in the following manner. The wire X is passed through chamber l6: containing hydrogen. The length of the hydrogen chamber and the temperature to which the wire is'heated therein, are such-that the wire is thoroughly. de- 0 oxidized and packed or charged with hydrogen prior to reaching the aluminum or aluminum alloy bath. A furnace or chamber in which the hydrogen is heated to a temperature of 1000 C. and in which the wire attains a temperature of the order of that of the hydrogen has been used. The wire temperature at the end of the hydrogen furnace. adjacent the aluminum pot measured with optical pyrometer had a temperature in excess of 732 Q. i. e. about 940 C. The hydrogen "furnace was one which was constantly supplied with hydrogenand substantially tight to prevent pollution of the hydrogen-by oxygen or air or other gases active towardthe aluminum, as .it is desirableito'use alpure hydrogen atmosphere,-"-one sufficientlyfreefrom .oxygen and is used; one contact,

below the surface having a restricted passage or orifice for the wire,

other gases which are active toward aluminum to avoid producing any detrimental effect on the bond between the aluminum coating and the basis metal of the article coated. In the example given, the temperature caused a pre-packing or charging of the steel wire with hydrogen sumcient toproduce a coating of the aluminum on steel, which was complete and tenaciously adhercut, and capable of bending with the steel without separation.

Instead of heating the article by the heated hydrogen in chamber 16, the article itself, in, many cases, may be heated to the desired temperature. As shown inv Fig. 6, an electric-current maybe the current passing passed through the article, by way of contact. rollers c, c' or the like through the wire or article. Where the solution tank it as the contact 0 may be placed ahead oi. tank It, so as to heat the article before entering said tank. When the solution of boron compound (oran equivalent) is used, the

.article is provided with a glaze which at temperatures-above its melting point, permits hydrogen torreadily pass through to the wire, and re- .tarclsthezpassing off-of the hydrogen during any exposure and any cooling between the time the articles leave the hydrogen furnace and enter the aluminum bath.

From the furnace IS the wire passes into the molten aluminum, without undesirable cooling, the hydrogen furnace having an extended nozzle which surrounds the wire as it passes from the hydrogen furnace it to the melting pot 22, and so prevents excessive cooling. The wire is passed into the molten aluminum or aluminum alloy in a manner to avoid passing through an-aluminum oxide film. This has been accomplished by passing it into the bath of aluminum below the surface, as by passing it through a restricted orifice or opening 20 in the pot 22 containing molten aluminum, as shown in Figs. 2 and 3, the orifice 20 opening into the aluminum pot below the level 2% of the molten aluminum contained therein. The

ess as to be negligible, and so that aluminum' oxide adhesions on avoided.

The wire also has been passed into the molten the coated metal may be aluminum in a manner to-avoid passing through an aluminum oxide film by passing it into a bath of aluminum through a part 36 extending oftife bath from above, and

as illustrated in Fig. 4.

The surface of the molten aluminum may be kept free of oxide other than aluminum oxide by covering it with anatmosphere of an inert or reducing gas substantially free from oxygen etc., as for example, hydrogen gas, and the amount of aluminum oxide formed may be greatly minimixed by restricting the exposed area of the bath. As shown in Fig; 4, the pot 22 is providedwith'a hood or cover 30, provided with

openings

32, 33, through which the wire passes in and out. A guide or roller 35 may be provided for guiding the wire in and outrof the aluminum bath." The hydrogen or other'gaswhich is inert toward, the

aluminum may be supplied beneath the hood I by means of pipes 31.

' Means, as a reel 39, may be provided for drawing the wire or other article continuously through the bath 'of aluminum, and for collecting the -.-coated wire. Drawing speeds of -50 feet per minute have been used in coating #28 gauge wire, in the work forming the basis of the mode of procedure herein given, and higher speeds have 10 been used since.

The aluminum-coated article may be wiped after leaving the aluminum bath and while the aluminum-coating is still plastic, to remove surplus aluminum and to regulate the thickness of the aluminum coating. A wiper is shown at B5.

The aluminum may be kept molten in the pct 22 by gas-burners 40 or by other suitable means, and the pot may be surrounded by walls 42. In most cases it is desirable not to exceed greatly the melting point of the aluminum, or of the aluminum alloy, as there is a tendency for crystals of the basis metal tomigrate into the aluminum coating at very high temperatures.

- Sheets, bars, rods, etc. may be similarly coated 25 with aluminum. Shaped articles such as metal hollow-ware, nails, etc.-in fact a countless variety of such articles may be coated with aluminum or aluminum alloys according to the present process.

contact with the molten aluminum or alloy, an alloy bond of aluminum and iron is formed at the, surface of the ferrous article, and in order that the thickness of this bond may be substantially unform over individual articles, the articles are passed into and out of the aluminum bath in such manner that all parts are exposed to the bath for substantially the same length of time. This is accomplished by drawing wire, sheets, bars and '40 the like through at a substantially uniform rate so that each part or spot enters and leaves the bath in the same time interval. Small articles are plunged into the bath was to be submerged or immersed substantially all at once, and removed 45 after a suitable interval. Moreover the time during which the articles remain exposed to the aluminum bath is preferably short.

For aluminum-coated articles which are to be subjected to metal working processes usual in the 5,0 fabrication of coated metal articles, bending,

drawing, etc., the time of exposure to the aluminum bath is preferably such thatonly a thin alloy bond is formed betweenthe basis metal of the article and the aluminum coating, and the bond is therefore kept thin so as to favor bending thereof with the basis metal without cracking or breaking. A thickness of 2 to 3 ten-thousandths of an inch of the alloy bond has been found to withstand bending without showing corrosion at 0'0 the bend when subjected to corrosion tests.

Wire and flat strips coated with aluminum according to the present invention are smooth and bright; the aluminum coating is adherent and substantially uniform over the metal over which it is coated. The aluminum coating, inclusive of the alloy-bond between the aluminum coating and base, are substantially uniform in thickness. Coated pieces of metal may be sharply bent without the aluminum coating being cracked and becoming detached at the bond. The aluminum coating 50, Fig. 5, is integral with the base or foundation metal 52, the bond'54 being an alloy of aluminum with the metal on which the coating of aluminum is applied. 56 is the matrix sur- 75 rounding the coated article and applied for the when ferrous articles for example, come into purpose of photographing the enlarged section through the coated wire.

Tests show that metal coated with aluminum according to the present invention have an exceptionally high resistance to corrosion, or oxidation, both at ordinary temperatures and at'high tern-- peratures. Tests also show that such aluminumcoated articles are exceptionally resistant to fruit and vegetable acids or compounds which attack most metals.

Aluminum'alloys and especially the aluminumbase alloys, as well as commercial aluminum, may be applied as a tenaciously adherent coating according to the present invention, and the word aluminum as used herein and in the claims is to be understood as covering aluminum 1 as well as such alloys.

The present process as hereinbefore stated, is also applicable to coating with other metals likealuminum, the oxides of which are not reduced by gaseous (molecular) hydrogen, as equivalents of aluminum.

The process may be carried out by modes of procedure otherthan as herein specifically described.

The present application is a continuation in part of my application, Serial No. 595,494, filed February 27, 1932.

What is claimed is:

1. A process of coating metals with aluminum. which comprises packing or charging the metal to be coated with a reducing gas to an extent where the behavior of the gas-charged metal toward molten aluminum undergoes a definitechange in its susceptibility of forming a complete and tenacious union with molten aluminum. and thereafter introducing the gas-charged metal into molten aluminum.

2. A process of coating metals with aluminum, which comprises pretreating the surface of the introduced metal with gas to remove oxygen and prevent formation of oxide at the line of contact with the molten aluminum, and then introducing the metal into molten aluminum at a point where the exposed area is restricted.

3. A process of coating metals with aluminum, which comprises pretreating the surface of the introduced metal with gas to remove oxygen and prevent formation of oxide at the line of contact with the molten aluminum, and then introducing the metal into molten aluminum at a point where the exposed area is restricted, and at a point below the surface level of the bath.

1 4. A process of coating metals with aluminum, which comprises pretreating the surface of the introduced metal with gas to remove oxygen and prevent formation of oxide at the line of contact with the molten aluminum, and then introducing the metal into molten aluminum at a point where the exposed area is restricted and protected by an atmosphere inert toward aluminum.

5. A process of coating metals with aluminum, which comprises pretreating the surface of the introduced metal with gas to remove oxygen and prevent formation of oxide at the line of contact with the molten aluminum, and then introducing the metal into molten aluminum at a point where the exposed area is restricted and at a rate such that the amount of aluminum oxide which forms at said area is small as regards the aluminum which .is taken up by the metal to be coated.

6. A process of coating metals with aluminum according to claim 1, further including introducv ing the gas-charged metal into the molten aluminum in such manner as to avoid passing through an oxide film.

7. A process of coating metals with aluminum according to claim 1, further including passing the metal to be coated into and out of the aluminum bath so that all parts are exposed to the aluminum for substantially the same length of time.

8. A process or coating metals with aluminum according to claim 1, further'including passing the metal to be coated into and out of the aluminum bath so that all parts are exposed to the aluminum for substantially the same length of time, said metal being passed into and out of said aluminum bath at such rate that the base forms a relatively thin alloy with the aluminum of the bath.

9. A process of coating metals with aluminum according to claim 1, wherein the metal is charged or packed with the reducing gas at a minimum temperature on the order of 732 C. v

10. A process of coating metals with aluminum according to claim 1, further comprising passing the metal into an aluminum bath through an entrance orifice of restricted area whereby passing of the metal to be coated through an oxidefilm is avoided or the film greatly minimized.

11. A process of coating metals with aluminum according to claim 1, wherein the reducing gas is hydrogen.

12. A process of treating metals with aluminum, according to claim 1, further comprising covering the metal to be coated with a thin film of boric acid or a borate prior to the gas-treatment.

13. A process of coating metals with aluminum, which comprises introducing the metal to be coated into a molten aluminum bath after it has been pretreated by separately heating in the presence of a reducing gas at a temperature and for a time sufficient to charge or pack it with the reducing gas.

14. A process of coating metals with aluminum, which comprises introducing the metal tobe coated into a molten aluminum bath after it has been pretreated by separately heating in the presence of hydrogen at a temperature and for a time suilicient to charge or pack it with hydrogen.

15. A process of coating metals with aluminum according to claim 1, wherein the metal to be coated is treated with the reducing gas at a temperature of the order of 1000 C.

16. In a method or coating metal, pre-treating it in an atmosphere or a reducing gas at a mimmum temperature on the order 01' 732 C.

17. A process of coating metals with aluminum which comprises introducing the metal to be coated into a molten aluminum bath after it has been pre-treated by separately heating at a minimum temperature on the order of 732 C. in' the presence a reducing gas and for a time sumcient temperature on the order of 732 oxygen and to prevent basis metal.

a 5 to charge or pack it with the reducing gas, the metal being introduced into a bare part of the molten bath, and emerging from the bath as a continuation or its introduction-movement, and the exposed area of the aluminum bath at the point of introduction being so restricted and the rate of movement of the metal into the bath being such that the amount of forms or may form at said restricted area is so small as regards the aluminum whichis taken up by the metal, as to be inconsequential in its effect upon the union of the aluminum with the metal.

18.- A process of coating metals with aluminum which comprises introducing the metal to be coated into a molten aluminum bath after it hasbeen pre-treated by separately heating at a minimum 0. in the presence of hydrogen and for a time sufficient to charge or pack it with the hydrogen, the metal being introduced into a bare part of the molten bath, and emerging from the bath as a continuation of its introduction-movement, and the exposed area of the aluminum bath at the point of introduction being so restricted and the rate of movement of the metal into the bath being such that the amount of aluminum oxide which forms or may form at said restricted area is so small as regards the aluminum which is taken up by the metal, as to be inconsequential in its eifect upon the union of the aluminum with the metal.

19. A process of coating metals with aluminum, which comprises pretreating the surface of the introduced metal with gas to remove oxygen and to prevent the formation of oxide at the line of contact with the molten aluminum, and then introducing the pretreated metal into molten aluminum.

20. A ductile and malleable metal product comprising a basis metal, a coating of aluminum of substantially uniform thickness and an intermediate layer or bond of aluminum with the basis metal also of substantially unirorm thickness and which is produced by passing through the molten aluminum the basis metal pretreatedwith gas to remove oxygen and to prevent the formation of oxide at the contact of the molten aluminum and the basis metal.

21. A ductile and malleable metal product comprising an iron or steel basis metal, a coating of aluminum of substantially uniform thickness, and an intermediate layer or bond of aluminum with the iron or steel basis metal also of substantially uniform thickness and which is produced by passing through the molten aluminum the basis metal pretreated with gas to remove the formation of oxide molten aluminum and the com G. mm.

at the contact of th aluminum oxide which