US2918388A - Method and means of coating metals - Google Patents

Description

Dec. 22, 1959 G. A. MoLLER METHOD AND MEANS OF COATING METALS y Filed Sept. 28, 1956 IN V EN TOR. Geen/v ,4. Mau .se

l Arm/New Diteci States Patent O 4METHOD AND MEANS 0F COATING METALS Goran August `Moller, Goteue, Sweden, assigner to American Mollerizing Corporation, `Beverly Hills, Calif., a vfcorporation of :Nevada `)implication September 23, 1956, Serial No. `612,772

10 Claims. (Cl. 117-51) .Thisinvention relates generallyto thecoatingof metals and relates particularly to improvements in an kapparatus and process for the coating of metals.

Generally speaking, this invention constitutes an 1mprovement over the process and apparatus disclosed'in my United States Patent No. 2,315,725, entitled Process for Metallization, Especially 4Aluminization of iron Articlesf? and is `a 'continuationfin-part of my co-pending application, Serial No. 252,267, tiled October 20, 1951, now abandoned.

...In the utilization of my process described in the aboveidentified patent, which has numerous highly advantageous aspects, it is nevertheless sometimes found that .thetalloy .interface between the coating metal and the .base .metalfis very brittle. This brittleness is found to be caused `by an excessive reaction between .the coating metal` and -the base metal.

.Ifthe yinterfacial lloyformed is thick with respect tothe thickness of Vthe product, the entire product may be made brittle. Thus, for example, in ordinarily ductile steel strip-.that .has been coated by theMoller process, it issometirnesiound that the coating metal has alloyed with the base-steel substantially throughout the diameter of `the strip. l, Reaction between iron and molten aluminum proceeds `@assoon-'as the two metalscome in contact. The shorter ,the` time .they .are in contact, the `thinner will be the intermetallic layer and the smaller its effect on the ductility Vchtite coating. n ,Bearing in `mindwthe foregoing facts, 1t is a major .object of :the `present i-nvention to `provide a method and .means `ofacoating metals whereby the `interface formed between .the coatingmetal and base metal is minimized.

.Itis anotherobject of .the present invention to provide means and a method for the coating of metals wherein the coated articleproduced haslow brittleness and high `ductility, and `the coating metal is effectively bonded to the `base metal. ..Still..another.object of the present invention is to provide -a means .and a method for the coating of metals `wherein the coatingmetal-base metal interface formed `is sufficiently `thin-to render the coated product of a highly `ductile nature.

Yetanother object of the present invention is to provide @continuous process for the coating-of metals in which vthetirne of .alloying action between the base` metal and `the .coating metal t is minimized.

alta-isa further object of thetpresent :invention to provide an apparatus wherein the time of contact between the basemetal andthecoatingmetal is greatlyreduced. Itis `stillcanother object of the presentinvention to 4.provide 'means and a method inwhich the continuous .eoatingof metals canbe conducted more rapidly. These `and other` objects and advantages of the present invention will become `apparent frornthe following descriptiona-nd theiaccompanyingudrawings, in which: figure lis la foreshortenedr perspective view` ofthe ap:

t `Inersed in the lower salt bath layer through the molten 2,918,383 Patentednee. z2, 1959 ICC paratus of the present invention, one wall of the apparatus being removed to reveal the interior thereof; and

Figure 2 is a View in cross-section taken along the line 2 2 of the apparatus of Figure 1, the interior thereof containing various materials.

Generally, my improved process and apparatus for the coating of metals makes use of the discovery that the period within which the coating metal and the base metal react at high temperatures largely determines the extent of the alloying action therebetween, which, in turn, mainly determines the brittleness of the resulting coating product.

The process, `illustrated in my above-identified patent, employs a single compartment salt bath furnace in which a pretreating salt bath is placedV and heated to a molten state. The coating metal completely overlies the surface of `the salt bath and is maintained in a molten state .by heat transmitted to it by conduction from the molten salt.

When the base metal (the metal to be coated) is imcoating layer, some localized wetting of the base metal by the coating metal occurs. The coating metal and base metal thus prematurely react to form an alloy, the alloying reaction continuing throughout the entire time that the base metal is submerged within, and heated by, the molten salt bathpreferably to a temperature above .thatof the melting point of the coating metal.

The extended period of alloying action caused by the prewetting of the base metal by the coating metal may result in the production of a relatively thick alloy coating metal-base metal interface, particularly in thin pieces of material, such as wire, strip, or sheet.

Referring now to the drawings, the furnace 10 comprises a tank-like, preferably rectangular, Crucible or pot y.12divided by a partition wall 18 into pretreating and coating sections or compartments 14 and 16, respectively. .The crucible 12 is provided with a pair of end walls `20, a pair'of side walls 21 (one of which has been re- Amovedin Figure 1 to reveal the interior of the furnace .10), and a floor 22.

The end and side walls 20, 21, respectively, the floor 22, and the partition wall 18 of the crucible 12 are made of suitable refractory materials capable of withstanding .the corrosive action of chemical salts at high temperatures for long periods of time. For example, the walls 1S, 20 tand 21, and door 22 of the furnace 10 may be composed of a dense, high alumina-containing refractory inner liner Y24aflxed to a layer of standard, high temperature, suitably reinforced, refractory brick 26.

The partition wall 18 is preferably amxed normal to the `side walls 21 of the Crucible 12, is spaced from the end walls 20 and terminates above the oor 22, forming an interconnecting passage 28 to enable the metal to be coated to pass thereunder. It is generally desirable to position the partition wall 1% so that the pretreating section 14 of the Crucible 12 has a considerably greater length than the coating section 15.

For example, a coating furnace has been built having the following inner dimensions: 30 feet in length, 2 feet in depth, and 2 feet in width. The pretreating section of this furnace is approximately 28.5 feet long, 2 feet wide, and 2 feet deep, the coating section being 1 foot in length, and equal to the width and depth of the furnace. The partition wall, approximately 0.5 foot in length, separates the two sections.

If the coating section of the furnace is further reduced, as is preferable in some instances, for example, to a length of 6 inches, the pretreating section is increased in length .of 29 feet, the pretreating section thus 58 times the length and volume `of the coating section.

The crucible 12 is heated electrically by spaced heating electrodes 30 embedded preferably in the lower portion of each of the end walls 20.

The interior of the crucible12 contains a bath of molten salts 31 which completely covers the electrodes 30 in both sections 14 and 16, and is a means of transferring heat from the electrodes 3i) to a material that is placed in the interior of the furnace 10. The coating section 16 also contains a molten coating layer 32 which rests upon the salt layer 31 and is in direct contact therewith. The salt bath 31 and thev coating metal 32 are an integral part of the furnace 10, since these molten materials are replenished as needed, and the level of the materials is essentially constant.

The salt bath 31 is preferably composed of the chloride, bromide, and fluoride salts, especially those of the alkali and alkaline earth groups. Also, aluminum fluoride and cryolite (NA3A1F5) are especially advantageous.

The precise combination of salts used in the salt bath 31 is determined primarily by the composition and density of the coating metal and also by the composition and physical condition of the base metal to be coated. For example, if a pure aluminum metal is the coating mate# rial, the temperature at which it must be maintained is its melting point, 1218 F. The pretreating salt bath 31 must therefore comprise a combination of salts that is stable above 1218 F. On the other hand, if an aluminum alloy is to be used as the coating material, the tem peraturev of the salt bath need not be maintained as high as with the pure aluminum, since the melting point of the aluminum alloys is generally lower than that of pure aluminum. Hence, a factor in the determination of the composition of the salt bath is the composition of the coating metal.

Generally, if the metal to be coated has a large amount of oxides on its surface, it is advisable to include in the salt bath composition a greater amount of uorides than would ordinarily be present for the purpose of dissolving aluminum oxides, the fluorides having also a greater tendency to dissolve the metal surface oxides. The amount of uorides added thus varies from 0.0% to 20% of the total weight of the salt, although it is generally advantageous to employ from 0.1% to 10% of the fluoride salts. Hence, the condition of the base metal is a factor entering into the determination of the salt bath composition.

The composition of the salt bath is also determined, in part, by the density of the coating metal. It is generally desirable to support the coating metal directly on the salt bath, and therefore the composition of the salt bath is such as to have a density that will support a coating layer 32.

Bearing in mind all these factors, a salt bath composition that has been employed with excellent results in the coating of metals with various lightweight metals including pure aluminum or its alloys, at molten salt bath temperatures ranging between 1000 F. and 1650" F. is the following:

I. 70-75% barium chloride, by weight Ztl-25% sodium chloride, by weight 0.1-l% sodium fluoride, by weight 70-75% calcium bromide, by weight 2025% sodium chloride, by weight (ll-% sodium aluminum fluoride, by Weight 70-75 potassium iodide, by weight 20-25% sodium chloride, by weight 0.l-l0% aluminum fluoride, by weight 70-75 barium bromide, by weight 2025% strontium chloride, by weight 1-10% sodium fluoride, by Weight.

Composition I is presently preferred because of the relative cheapness of the chlorides.

TheA metal material to be coated, for example, wire or metal sheet 33, is irst immersed in the pretreating section 14, thence passed beneath the partition Wall 18 and upwardly through the coating section 16. The preferred conveying mechanism for this purpose consists of a guide roller 34 rotatably mounted onto the end wall 20 of the pretreatment compartment 14, a submerged guide roller 35 pivotally mounted to the bathe 18 in the coating section 16, and a third roller 36 spaced tangentially above the submerged guide roller, and rotatably mounted preferably substantially above the furnace 10, to allow cooling of the coated material to occur before the metal 33 is bent. The pivotal mounting of the submerged guide roller 35 allows it to be taken out for inspection, and for other purposes, through the salt layer 31 in the pretreating section 14, thus eliminating the need for its withdrawal through the molten aluminum 32. A series of such withdrawals through the molten aluminum results in a high degree of attack on the roller material which is generally made of steel.

The rollers 34, 35, and 36 are preferably of equal length, parallel to each other, and have the ends thereof terminating in the same vertical plane.

The relative arrangement of the pretreating rollers 34, 3S is such that the metal 33 to be coated follows an approximately diagonal path upon passing downwardly through the furnace 10. Further, the tangential arrangement of the submerged and coating rollers 35, 36 is such that the path along which the metal 33 passes upwardly in the coating section 16 is substantially normal to the plane of the coating layer 32, the metal thus following the shortest possible path through the coating material.

The conveying mechanism for coating individual pieces continuously may be modified to consist of an endless chain (not shown) to which the base metal may be hooked and sent downwardly through the pretreating section 14, through the interconnecting passage 28, and upwardly through the coating section 16, the arrangement of the rollers being also modified to allow for the passage of the individual pieces.

Having set forth a preferred embodiment of my coating apparatus, a preferred method of utilizing the apparatus to aluminize materials will now be described.

In this specification and in the appended claims, the term aluminization means a coating of a metal with aluminum or its alloys. Further, whenever the word aluminum is used, it is to be taken as meaning aluminum or its alloys.

The material to be coated, for example, continuous sheet metal 33 is sent over the pretreating roller 34, immersed in the salt bath pretreating section 14, thence passes downwardly beneath the partition 18 through the interconnecting passage 28, around the submerged roller 35, and thence upwardly through the coating section 16 contacting the aluminum coating layer 32 which is one to four inches in thickness, for preferably a fraction of a second, to be thereby alumnized. The aluminized material 33 is then preferably immediately quenched or greatly lowered in temperature to prevent further interreaction between the aluminum and the base metal.

The temperature range of the molten salt bath ranges from 1000 F. to 1650 F., depending on the coating metal 32 used, the degree of cleaning and deoxidation of the base metal 33 desired, and other factors. As mentioned previously, the temperature of the salt bath 31 is maintained at least equal to the melting point of the coating metal 32, so that the salt bath may maintain the coating metal in a molten state by direct conduction of heat therefrom.

The rate of passage of the base metal 33 through the molten salt bath is such as to impart to the base metal a temperature preferably at least equal to the melting point of the coating metal 32 prior to its entry into tbe coating metal 32. It is also particularly effective, in many instances, to impart to the base metal 33 a temperature approximately equal to the temperature of the salt bath 31 prior to its entry into the coating metal 32.

attesta y `It is to be noted that the continuous metal' 33 enters thesalt bath 31 initially and does not lirst enter an aluminurn layer. Thus, no reaction is initiated between the aluminumand the base metal prior to the actual coating step. The pretreated metal material, entering the aluminum coating layer 3`2just prior to its exit from the furnace A1l), has an exceedingly' short period of reaction according to my invention, since the aluminum-base metal alloying action can progress only in that period of time betweenV the actual coating step which occurs as the base metal leaves the furnace" `,"an`d the cooling step.

An exceedingly short'period of time, of the order of a fraction of a second, is sullicient to cause an effective bond to form, but. positively eliminates the production of a thick, brittle interface, which would cause the entire product `to be brittle. It is found that an elective bond is produced, even when the interfacial alloy is exceedingly thinethat is, on the orderof molecular dimensions.

It`is to be understood that the process above-described is` adapted for use with coating metals other than aluminum, these other metals including those lighter than the salt bath 31.

The speed at which continuous materials and discontinuous materials may be continuously coated is limited mainly by the fact that the base metal 33 is preferably brought to coating temperature, thatr is, a temperature above the melting` point of the coating material 32, before the actual Coating can occur. In order to increase the speed at which metals may be coated, the pretreating section 14 is greatly elongated with respect to the coating sectionl, aslpreviously mentioned'. Thus, as the metal 33 travels through the salt bath 31 at great speed, it will nevertheless readily attain the temperature of the salt bath prior to its entry into the coating layer 32. In this connection, it is preferable also that the base metal material 33 pass diagonally downwardly through the furnace 10. Continuous materials have been run through the elongated furnace 10 in this manner at speeds of several hundred feet a minute without any sacrifice in the quality of the coating or the ductility of the product produced.

It is also desirable, in some instances, to pass the material to be coated vertically downwardly into the pretreating section 14, around a submerged roller, along the Hoor 22 of the furnace 10, to the coating section 16,

' and thence vertically upwardly through the coating section in order to obtain a yet longer run through the furnace.

It is to be noted that, in any case, the continuous material 33 travels in one direction through the furnace 10 in order to avoid any points of tension and stress that might otherwise arise if reversals of direction of the material were provided for.

The types of metals that have been aluminized with especially excellent results include iron, nickel, cobalt, manganese, titanium and copper metals and alloys thereof. These metals fall in the transition group of elements.

While a preferred embodiment of my apparatus and method has been described, it is apparent that many changes and modifications may be made which lie within the scope of the invention. Therefore, I do not intend to be limited by what l have shown and described, but only to be limited by the appended claims.

I claim:

1. A process for continuously coating base metals with a coating metal selected from the group consisting of aluminum and aluminum alloys, which comprises the steps of: immersing the base metal to be coated in a preheating molten salt bath having a temperature of between l000 F. and l650 F., said base metal being immersed in said salt bath without previously contacting said coating metal. said salt bath comprising at least one halide salt; withdrawing said pretreated base metal, after it has reached substantially the temperature of said salt bath, directly into a molten coating metal layer, over lyinga'portion ofsaid' salt bath, without intervening'i" im oxidation; andfwithdrawingl said coated base metal me'diately from saidV coating metal.

2.Y A process of""continuou'sly` coating metals-with 7a? coating metalselectedi from the `group consisting of alumilf num" andthe aluminum alloys, which comprises the steps-l of: immersing saidl metal in a pretreating moltensalti bath; said base metal being immersed in` said salt bath;

wthoutpreviously contacting" said coating metal, saidlV salt bath being'composed of at least 90% of halide salts',y

otherthan` fluoride salts; andup to 10%, by weight, ofi

fluoride salts; passing said pretreated metal from said salt bath, after it has reached the temperature` of` said salt bath, directlyiintoa salt bath of a coatingr section, `said coating section having an aluminum layer from two to four inchesin thickness, yoverlyingY said salt bath in said coating" section; `and passing said metal ver?? tically through said-coating section salt bath and normal coating metall selected from the group consistingV of aluminum! and` aluminum; all;iys',wl`1ich` comprises `the` steps of: immersingtthe base metal material in a pre-` treating molten salt bathA maintained at a temperaturev above the melting point of the coating metal, said base-V t metal being immersed'in saidsalt bath without previously contacting `said coating metalypassing said material from,` said salt bath directly into an` interconnected salt. bath. of a coating-section which supports an overlying aluminum layer one to four inches in thickness thereon, each of said salt baths being maintained at a temperature above the melting point of said aluminum layer and being composed of halide salts; and passing said material, after it has attained substantially the temperature of said salt bath, vertically through said molten aluminum layer to be coated thereby.

4. The process of coating continuous metal materials as dened in claim 3, characterized in that the salt bath comprises halide salts selected from a group consisting of alkali and alkaline earth tluorides, chlorides, iodides, and bromides.

5. A process for coating a base metal with a coating metal selected from the group consisting of aluminum and aluminum alloys, which comprises the steps of: immersing the base metal to be coated in a salt bath having a temperature of between 1000o F. and 1650 F., said base metal being immersed in said salt lbath without previously contacting said coating metal, said salt bath comprising at least one halide salt; withdrawing said base metal, after it has reached at least the melting point temperature of said coating metal, into said coating metal, without intervening oxidation, said coating metal overlying a portion of said salt bath and being maintained in its molten state, and withdrawing said coated base metal immediately from said molten coating metal.

6. A process for coating a base metal with a coating metal selected from the group consisting of aluminum and aluminum alloys, which comprises the steps of: immersing a metal to be coated in a salt bath having a temperature above the melting point of said coating metal, said base metal being immersed in said salt bath without previously contacting said coating metal, said salt bath comprising at least one halide salt; withdrawing said base metal, after it has reached substantially the temperature of said salt bath, into said coating metal, without intervening oxidation, said coating metal overlying a portion of said salt bath and being maintained in its molten state; and withdrawing said coated base metal immediately from said molten coating metal.

7. A process of continuously coating a continuous base metal material with a coating metal selected from the group consisting of aluminum and aluminum alloys,

which comprises the steps of: immcrsing the continuous base metal material in a molten salt bath maintained at a temperature above the melting point of the coating metal, said base metal being immersed in said salt bath without previously contacting said coating metal; passing said base metal material from said salt bath into an interconnected salt bath of a coating section which supports an overlying molten aluminum-containing layer one to four inches in thickness thereon, each of said salt baths being maintained at a temperature above the melting point of said aluminum layer and being composed of at least one halide salt; and passingisaid material, after it has attained at least the melting point temperature of s aid coating metal, through said aluminum layer to be coated thereby.

8. The process of coating continuous base metal materials as defined in claim 7, characterized in that the salt bath comprises a halide salt selected from a group consisting of alkali and alkaline earth uorides, chlorides, iodides and bromides.

9. A process for coating base metals with a coating metal selected from the group consisting of aluminum and aluminum alloys, which comprises the steps of: immersing said base metal in a molten salt bath having a temperature at least equal to the melting point of said coating metal, said base metal being immersed in said salt bath without previously contacting said coating metal, said salt bath comprising at least one halide salt and being heavier than said coating metal; immersing said base metal, after it has reached at least approximately the melting point temperature of said coating metal, into said molten coating metal, overlying a portion of said Salt bath, without intervening oxidation, any salt, transferred to said coating metal, by means of said base metal, sinking below said coating metal; and withdrawing said coated base metal immediately from said coating metal.

10. An apparatus for coating continuous base metal materials which comprises: a rectangular refractory pot having a oor, a pair of end walls, and a pair of elongated side walls; a transverse partition wall affixed to said side walls and spaced from said end walls to form a pretreating and a coating section, said partition wall terminating above said ioor to form a lower interconnecting passage between said sections; means for conveying said continuous material diagonally downwardly through Said pot through said passage, and approximately vertically upwardly through said coating section, said conveying means comprising roller means axed onto one of said walls of said pot; heating electrodes embedded in the lower portions of the end wall of each compartment; and means for transferring heat from said electrodes to said material passing through said compartments, said means comprising a molten salt.

References Cited in the tile of this patent UNITED STATES PATENTS 406,356 Midgley July 2, 1889 2,315,725 Moller Apr. 6, 1943 2,544,671 Grange et al Mar. 3l, 1951 2,557,764 Renkin June 19, 1951 2,686,135 Butler Aug. 10, 1954 2,751,311 Rosseau June 19, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIUN Patent No.. 2,918,888 December 22, 1959 Goran August Moller.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should readas corrected below.

Column 5, line 69, for "preheating" read pretreatng column 6, line 57, for "state," read state;

Signed and sealed this 28th day of June 1960.

(SEAL) Attest:

KARLl H. AXLINE ROBERT C. WATSON CommiseionerI of Patents Attesting Ofcer

Claims (1)

1. A PROCESS FOR CONTINUOUSLY COATING BASE METALS WITH A COATING METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ALUMINUM ALLOYS, WHICH COMPRISES THE STEPS OF: IMMERSING THE BASE METAL TO BE COATED IN A PREHEATING MOLTEN SALT BATH HAVING A TEMPERATURE OF BETWEEN 1000*F. AND 1650*F., SAID BASE METAL BEING IMMERSED IN SAID SALT BATH WITHOUT PREVIOUSLY CONTACTING SAID COATING METAL, SAID SALT BATH COMPRISING AT LEAST ONE HALIDE SALT, WITHDRAWING SAID PRETREATED BASE METAL, AFTER IT HAS REACHED SUBSTANTIALLY THE TEMPERATURE OF SAID SALT BATH, DIRECTLY INTO A MOLTEN COATING METAL LAYER, OVERLYING A PORTION OF SAID SALT BATH, WITHOUT INTERVENING OXIDATION: AND WITHDRAWING SAID COATED BASE METAL IMMEDIATELY FROM SAID COATING METAL.

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