US3027268A - Method and apparatus for coating metals with molten aluminum - Google Patents

Description

March 27, 1962 H. E. LINDEN METHOD AND APPARATUS FOR COATING METALS WITH MOLTEN ALUMINUM Filed Jan. 29, 1960 m wf ,.f, f E0 ,N N VW ml l f f. f r 4 E United States Patent O 3,027,268 METHQD AND APPARATUS FR CATHNG METALS WHTH MGLTEN ALUMENUM Herbert E. Linden, 9459 Dayton Way, Beverly Hills, Calif. Filed dan. 29, 1960, Ser. No. 5,546 Claims. (Cl. 117-51) This invention relates generally tothe coating of metallic materials and relates especially to improvements in the continuous coating of metallic materials with a molten coating metal such as aluminum, magnesium or alloys thereof.

Generally speaking, this invention constitutes an improvement over the process and apparatus disclosed in the United States patent application, Serial Number 612,772, now Patent No. 2,9l8,388, entitled Method and Means of Coating Metals, Goran August Moller, inventor.

The process therein disclosed employs a two-compartment furnace with an interconnecting passage. Both compartments of the furnace are iilled with a molten, predominantly halide, salt and the molten coating metal is usually floated on the salt in one of the two compartments, the salt being heavier than the coating metal.

The molten salt is heated to a temperature above that required to melt the coating metal. Then, the metal to be coated is passed into the uncovered portion of the salt bath, through the interconnecting passage into the second compartment, and then is passed into and through the overlying molten coating metal.

The just-described process has several important advantages over the prior art. Among these are the following: first the metal material to be coated (hereinafter usually termed the base metal) is heated to the proper temperature and may `also be deoxidized and otherwise cleaned by the action of the molten salts. Secondly, the base metal never contacts the atmosphere after the commencement of its heating and cleaning and, therefore, no intervening oxidation of the base metal is possible during its heating or introduction into the coating metal. Thirdly, in the arrangement described, the base metal undergoes an extremely short period of reaction with the coating metal, since a coating metal-base metal alloying reaction can progress only in that period of time from the commencement of the coating operation untilthe coating is cooled by a subsequent cooling or quenching step.

An exceedingly short period of time, of the order of a fraction of a second, is sufficient to cause an effective bond to form, but eliminates, to a great extent, the production of a thick, brittle interface. lt is found that an effective bond is produced, even when the interfacial alloy is exceedingly thin-that is, on the order of molecular dimensions.

While the process described has lbeen employed with much success, certain further improvements have been discovered which, when utilized in combination with ythe just-described process, are highly advantageous.

In the process described, the temperature of the salt bath is preferably maintained well above the melting point of the overlying metal so that the maximum amount of cleaning and other pre-treatment can be accomplished. On the other hand, since the coating material is heated primarily by conduction of heat from the salt bath, a high temperature in the salt bath will result in a high temperature in the coating bath. However, the temperature of the coating bath should be maintained at a temperature as close as possible to and above that of the melting point of the coating material to prevent excessive alloying action. The hotter the molten coating material, eg., aluminum or aluminum alloys, the greater the intermetallic alloying that takes place between the metal being coated `and the icc coating metal. The alloy formed is brittle, and it is found that the greater the alloy thickness produced, the more brittle is the coating. For the reasons just mentioned it is highly advantageous to keep the coating material close to its melting temperature while maintaining the .salt bath at a substantially higher tempera-ture.

Further, when coating metal materials with molten aluminum, aluminum alloys, or other high melting point metals, a layer of extraneous material, such as dirt and various compounds such as are thought to be formed by oxidation of the aluminum and the combination of the aluminum oxides with the molten salt, accumulates at the top of the salt bath just under the aluminum. The base metal must pass through this layer on its way to the coating metal layer. Some of the extraneous material is often picked up by the base met and carried -through the coating metal layer, resulting in spots on the base metal where the coating metal does not bond. This, of course, is most undesirable because the appearance of the coating is poor and the basemetal is not completely protected from corrosion and oxidation in the uncoated spots. Moreover, as the extraneous material accumulates a condition is reached requiring a complete cleaning of the salt bath, which is a very diicult and expensive undertaking.

Bearing in mind the foregoing facts it is a major object of the present invention to provide an improved apparatus and process for coating metals of the type described wherein the formation of a brittle intermetallic alloy layer in the coated metal is inhibited to a substantially great1` er degree than in prior art processes of the type described, the resulting coating thereby having improved ductility.

lt is another object of the present invention to provide an apparatus and process for coating metals of the type described wherein the possibility of reaction between the coating metal and the adjacent salt bath is very substantially decreased.

Still a further object of the present invention is to provide an apparatus and process for coating metals, of the type described, wherein the amount of accumulation of dirt and other objectionable extraneous compounds in the heating salt bath is substantially reduced, thereby appreciably reducing the amount of cleaning of the salt bath.

Yet another object of the present invention is to provide an apparatus and process for coating metals, of the type described, wherein the amount of objectionable extraneous compounds picked up by the base metal in passing through the heating sait bath is substantially reduced.

Still another object of the present invention is to provide an apparatus `and process for coating metals, of the type described wherein the amount of objectionable extraneous matter carried by the base metal,I into the coating layer, is substantially reduced, the result being the production of a more perfect coating on the base metal.

These and other objects of my present invention will become clearly understood by referring to the following description, and the accompanying drawings in which:

FlGURE l is a perspective view of the apparatus of the present invention, one side wall of the apparatus being removed to reveal the interior thereof;

FIGURE 2 is a side elevational view, in cross section, taken along the line 2 2 of FIGURE 1, the interior thereof containing various materials; and

FGURE 3 is a greatly enlarged view along the line 3-3 of FEGURE 2.

Generally, my improved process and apparatus for the coating of metals makes possible better coatings which are more ductile and which contain fewer spots that are uncoated or imperfectly coated. This is accomplished in an apparatus which stays freer of deleterious substances and requires less cleaning than apparatus heretofore employed.

My invention provides that an interlayer of heat-insulative, high-temperafture-resistant, material, such as quartz cloth, be interposed between the salt bath and the coating bath. A temperature-resistant material positioned as described serves several important functions. First, the temperature in the coating bath will remain somewhat cooler than the salt bath because of the insulating qualities of the interlayer. `In this Way, the rate of chemical interreaction of the coating metal with the base metal is reduced. The alloying reaction appears to be a function of time and temperature. Thus, if the time over which the reaction is allowed to proceed is kept at a minimum, which it |is in my process, and the temperature during coating is also maintained at a minimum level, the chemical interreaotion and consequent brittleness is reduced to a minimum level. Less intermetallic ai-loy is formed and a greater thickness of coating metal is therefore obtained.

Second, because of the insulative interlayer, the temperature of -the predominantly halide salt bath can be maintained at a relatively higher level than before, to 'thereby render the salt bath more efficient in its dissolu- .tion of oxides and other pretreatment operations, while maintaining the coating layer at a temperature just above its melting point. The more efficient dissolution of oxides .and other objectionable compounds existing in the salt reduces the tendency for those compounds to collect at the top ofthe salt bath and reduces the probability of such compounds being picked up on the base metal as it passes out of the salt bath into the coating layer, thus resulting in a better coating, freer from bare spots.

Third, the insulative interlayer prevents dirt, oxides or other compounds of the coating metal, which may settle on or arise within .the coating metal layer, from passing downwardly into `the salt bath wherein such extraneous materials often form a sticky, dirty layer through which the ibase metal must pass on its way to the coating layer. `When such a dirty layer exists, it results in dirt being picked up on the base metal, which dirt prevents proper bonding of the coating metal to the base metal. This is avoided by use of my Iinsulative interIayer.

Fourth, the continuous base metal preferably passes through the insulative interlayer in wiping contact therewith, which results in a final cleaning wipe of the base metal, to rid it of any objectionable substances it may have accidentally picked up in the salt bath, just prior to its entrance into the coating metal.

Referring now to the drawings and to FIGURES 1 and 2 especially, the furnace 10 comprises a tank-like, preferably rectangular, crucible or pot 12 ydivided 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, Ia pair of side walls 21 (one of which has been removed in FiGURE l to reveal the interior of the furnace `and the 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 the chemical salts at high temperatures for long periods of time. For example, the walls 18, 20 and 21, and floor 22 of the furnace 10 may be composed of a dense, high alumina-containing refractory inner liner 24 athxed to an outside layer of standard, high temperature, suitably reinforced refractory brick 26.

The partition wall 18 is preferably affixed normal to the side walls 21 of the crucible 12, and spaced from the end walls 20. The wall 1S terminates above the floor 22, forming an interconnecting passage 28 to enable the metal to be coated to pass thereunder. The passage Z3 may be considerably smaller than shown, if desired. For example, in the case in which the base metal is in the form of Wire, merely an opening to permit the Wire to be passed therethrough from section 14 to section 16 can be employed.

It is generally desirable to position the partition wall 18 so that the pretreating section 14 of the crucible 12 has a considerably greater length than the coating section 16. 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 in Width and depth to thepretreating section of the furnace. The partition wall 18, approximately 0.5 foot in length, separates the two sections 14 and 16.

The crucible 12 is heated electrically by spaced heating electrodes 30, embedded preferably in the lower p0rtion of the Walls 21, and suitably energized electrically. Other means well-known in the art may be employed for heating the contents of the crucible 12.

The interior of the crucible 12 contains a bath 31 of molten salts which completely covers the electrodes 30 in both sections 14 and 16, and forms part of means for heating a base metal material 33, shown in the herein illustrated embodiment in the form of a Wire, that passes through the interior of the furnace 10. Electric current passing between the electrodes through the molten salt heats the salt which then conducts the heat to the base metal to be coated. The coating section 16 also contains a. coating layer 32 of molten coating metal which is preferably supported by the salt layer 31, but is not in direct contact therewith because of the interposition of a layer of insulative high-temperatur.,- resistant material 5t), such as quartz cloth, quartz sheet, or other ceramic material.

The salt bath 31 and the 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 maintained essentially constant.

The salt bath 31 is predominantly composed of chloride, bromide, and fluoride salts, especially those of the alkali and alkaline earth groups. Also, aluminum uoride, cryolite (NA3A1F6) and the hydrides of calcium, lithium and barium are especially advantageous in ridding the salt bath and the base metal of oxides.

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 material, the temperature above 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 1218o F. On the other hand, if an aluminum alloy is to be used as the coating ma- `terial, the temperature of the salt bath need not be maintained -as high as with 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 itself.

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 iluorides and hydrides than would ordinarily be employed. The amount of tluorides added 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. The hydrides may be present in amounts up to about 3%. Hence, the condition of the base metal is a factor entering into the determination of the most desirable 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 by means of the salt bath itself, and therefore the composition Ei of the salt bath is such as to have a density that will support a coating layer 32.

Bearing in mind all these factors, salt bath compositions that have 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 l650 F. are the following:

70-80% barium chloride, by weight Ztl-30% sodium chloride, by weight 70-75% calcium bromide, by weight .20-25% sodium chloride, by weight 0.1-10% sodium aluminum fluoride, by weight III 70-75% potassium iodide, by weight Ztl-25% sodium chloride, by weight p 0.l-l0% -aluminum fluoride, by weight 70-75% barium bromide, by weight 20-25% strontium chloride, by weight (ll-10% sodium iiuoride, by weight 0.l-3% calcium hydride Composition I is presently preferred because of the relative cheapness of the chlorides.

Although it is preferable, in my process, to employ a salt bath heavier than the coating metal, the salt bath can be lighter than the coating metal, in which case the salt bath is supported by the coating metal, an interlayer of the resistant material S still being interposed. EX- amples or' such compositions, employed in the coating of various lightweight metals (including pure aluminum, and its alloys) at molten salt bath temperatures ranging between l000 F. and 1650 F. fall within the following limits:

37-57% potassium chloride 2547% sodium chloride 8-20% sodium aluminum uoride 0.542% aluminum uoride The insulative interlayer 50 is preferably made of a ceramic material although other highly temperature-re sistant material, such as asbestos, may also be employed. Examples of ceramic material are magnesia (MgO), beryllia (BeC), and orsterite (MgSiO4). The interlayer 50 may consist of a substantially solid block or blanket of material, having appropriate openings for the passage of wire, strip or sheet, or a uniformly porous ceramic material, such as quartz cloth. ri`he interlayer 50 may also take the form of a non-Woven mat of material of given thickness.

Where either such porous material or a substantially solid insulative mat, block, or blanket is employed, it may sometimes be desirable to support it by attachment to the walls 20 and i3 of the crucible l2, instead of supporting it solely by means of the salt bath i4. When a quartz cloth is supported at its ends (not shown), the salt bath provides a partial support. Where the coating metal is heavier than the salt, the coating metal provides the partial support for the interlayer 50.

A preferred embodiment o-f the interlayer 50 is shown in FIGURES 2 and 3 for use with the coating of wire. The interlayer 50, there shown, is made of quartz cloth or other ceramic material capabie or" being drawn to a fairly tine size. The quartz or other ceramic is then woven, or otherwise fashioned into a cloth of a given mesh size and thickness. The thickness of the interlayer 50 formed from the cloth can vary depending upon the degree of insulation desired, a two-ply cloth blanket being shown, for example, forming interlayer S0. The mesh size of the cloth is preferably smaller than the wire 33 to be passed therethrough. rl`hus, as the wire passes through the cloth, the strands of the cloth are parted and exert a mechanical wiping action on the wire 33, and if dirt or other physical foreign matter is present on the wire surface, the strands of the cloth will thus exert a positive mechanical cleaning action.

ln addition, the presence of the insulative interlayer S0 enables a substantial temperature differential to be maintained between the salt bath and the aluminum or other coating layer 32.

The metal material to be coated, for example, a continuous steel wire 33, is rst immersed in the pretreating section i4, thence passed beneath the partition wall i8 and upwardly through the coating section 16, through the insulative interlayer 50 (which is in contact with both coating metal 32 and salt 31), and through the coating metal 32. 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 mounted to the iloor 22 in the coating section i6, 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 Ibefore the metal '33 is bent. The rollers 34, 35, and 36 are preferably of equal length, and disposed parallel to each other.

The relative arrangement of the pretreating rollers 34, 35 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 vertical, the base metal thus following the shortest possible path through the v coating material and causing any draining of molten coating metal to take place along the wire so as to avoid nonuniformity of coating around the wire.

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 in the specification or claims, it is to be taken as meaning aluminum or its alloys.

The material to be coated, for example, continuous steel wire 33, is sent over the pretreating roller 34, immersed in a predominantly halide 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 i6, through a quartz cloth interlayer 50' of 3/32 inch in thickness. The spacing of strands in the individual plies of the cloth is about 1%.@ inch and the Wire diameter is .106 inch. rthe coating layer 32 is about 4 inches in thickness and is composed of substantially pure aluminum. The wire contacts layer 32 for preferably a fraction of a second, to `be thereby aluminized. The aluminized material 33 is then preferably immediately quenched or greatly lowered in temperature, e.g., to 800 F., 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. In this example, the tempenature of the salt bath 31 is maintained somewhat higher than the temperature of the coating metal 32., specifically at `about 1450 F. The salt bath maintains the coating metal at about l300 F., somewhat above its melting point, by direct conduction of heat through the interlayer 50.

The rate of passage of the base metal 33 through the molten salt bath is such as to impart to the base metal a ademas 7 temperature preferably at least equal to the melting point of th coating metal 32 prior to its exit therefroml The types of base metals that may be successfully aluniinized include iron, nickel, cobalt, manganese, titanium, and copper metals, and alloys of these metals. In addition to aluminum coatings, excellent coatings of mag nesium, on base metals are capable of being produced according to the process described. The advantages of magnesium-coated materials are comparable to aluminized coatings in terms of decreased intermetallic alloy formation, andmore ductile coatings.

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, yI do not intend to be limited by what is shown and described, but intend only to be limited by the appended claims.

I claim:

l. A process `for coating a base metal with a molten aluminum coating metal which comprises: passing said base metal into and through a pretreating molten salt bath; thence immediately passing said base metal through a closely adjacent ceramic insulating inter'lyer with no intervening oxidation; thence passing said base metal into Said coating molten metal, disposed closely adjacent said interlayer, with no intervening oxidation, to be thereby coated; and withdrawing said coated base metal from said coating metal.

2. A process for coating a base metal with a molten aluminum coating metal which comprises: passing said base metal into and through a pretreating molten salt bath; thence passing said base metal through a ceramic heat-resistant insulating interlayer, with no intervening oxidation, said interlayer being in contact with said salt bath; thence passing said base metal into said coating 'metal with no intervening oxidation, said coating metal being in contact with said insulating interlayer; and withdrawing said coated base metal from said coating metal.

3. The process of claim 2, wherein said coating metal is lighter than said salt bath, and said coating metal is supported, at least in part, by said salt bath.

4. The process of claim 2, wherein said coating metal is heavier than said salt bath, and said salt bath is supported, at least in part, by said coating metal bath.

5. A process for coating a base metal with molten aluminum, which comprises: passing said base metal into, and through a pretreating predominantly halide molten 'salt bath; thence immediately passing said base metal through an opening in a heat-resistant ceramic insulating interlayer, with no intervening oxidation; then immediately passing said base metal into said molten aluminum with no intervening oxidation to be thereby coated; and withdrawing said coated base metal `from said coating metal.

6. The process of claim 5, wherein said halide salt `bath is heavier than said aluminum, and said ceramic interlayer is in contact with both said molten salt and said molten aluminum.

7. The process of claim 5, wherein said lhalide salt bath is lighter than said aluminum, and said ceramic interlayer is in contact with both said molten salt and said molten aluminum.

8. The process of claim 5, wherein said base metal is shaped in the form of wire, and said ceramic insulating interlayer has a plurality of openings of normal meshsize smaller than the diameter of said wire.

9. A process for coating a base metal with molten aluminum, which comprises: passing said base metal into and through a pretreating predominantly halide molten salt bath, said halide salt bath being heavier than said aluminum coating metal; passing said base metal through an opening in a heat-resistant ceramic interlayer, said interlayer being in direct contact with said salt bath; thence passing said base metal into said molten aluminum, said aluminum being in contact with said interlayer, said aluminum having an average temperature substantially lower than that of said salt bath, but having a temperature above its melting point; and withdrawing said coated base metal from said coating metal.

10. The process of claim 9, wherein the ceramic interlayer is made of quartz strands, forming quartz cloth, having an average mesh-size such that the strands are parted by the base metal in its passage therethrough, the strands thereby contacting the surface of said base metal, and exerting a wiping action on the surface of the base metal.

11. An apparatus for coating metals which comprises: a salt bath furnace having a molten salt contained therein, said salt bath being adjacent a coating metal; a ceramic insulating cloth interlayer interposed between said salt bath and said coating metal, and means for initially immersing said metal to be coated in said salt bath, and conveying said metal through said salt bath and ceramic interlayer, and thence through said coating bath to be coated.

12. An apparatus for coating metals which comprises: a salt bath furnace having a molten salt bath contained therein, said salt bath having a density higher than the density of a molten coating bath, said salt bath supporting said molten coating bath which overlies a portion only of said salt bath; a heat-insulative, high-temperatureresistant ceramic cloth interlayer interposed between said salt bath and said coating metal; and means for initially immersing said metal to be coated in said salt, and conveying said metal through said salt bath and said interlayer, and thence through said coating bath to be coated.

13. The apparatus of claim l2 wherein said heat-insulative, high-temperature-resistant interlayer is quartz cloth.

14. An apparatus for coating a continuous base metal material which comprises: a refractory pot having a door, a pair of end walls, and a pair of elongated side Walls; a partition wall affixed to said side walls and spaced from said end walls to form a pretreating and a coating section, said partition wall defining an interconnecting passage between said sections; means for conveying said Continuous material through said pot and through said passage, and approximately vertically upwardly through said coating section; and means for heating said base metal material comprising molten salt in said preheating and said coating sections, heating electrodes embedded in a wall of said pot and in contact with said molten salt, a ceramic insulating cloth layer, said cloth having a finer mesh size than the diameter of the base metal material, contacting said molten salt in said coating section, and a molten coating metal contacting said ceramic insulating layer, said base metal material in passing upwardly through said coating section passing through said molten salt bath, said ceramic insulating layer and said coating metal.

15. An apparatus for coating continuous base metal materials which comprises: a refractory pot having a door, a pair of end walls, and a pair of elongated side walls; a partition wall anxed to said side Walls and spaced from said end walls to form a pretreating and a coating section, said partition wall dening an interconnecting passage between said sections; means for conveying said continuous material through said pot and through said passage, and approximately vertically upwardly through said coating section; and means for heating said base metal material comprising molten salt in said preheating and said coating sections, heating electrodes ernbedded in a wall of said pot and in contact with said molten salt, a quartz cloth layer overlying and in contact with said molten salt in said coating section, and a molten coating metal overlying and in contact with said quartz cloth layer, said base metal material in passing upwardly through said coating section passing through said molten salt bath, said quartz cloth layer and said coating metal.

(References on foiiowing page) 9 References Cited in the le of this patent 2,918,388 UNITED STATES PATENTS 935,420

1,719,512 Krembs Iuly 2, 1929 1,776,852 Finkbone Sept. 30, 1930 1,907,890 Steckel May 9, 1933 521,668

10 Moller Dec. 22, 1959 Lenden May 3, 1960 FOREIGN PATENTS Canada Feb` 7, 1956

Claims (1)

1. A PROCESS FOR COATING A BASE METAL WITH A MOLTEN ALUMINUM COATING METAL WHICH COMPRISES: PASSING SAID BASE METAL INTO AND THROUGH A PRETREATING MOLTEN SALT BATH; THENCE IMMEDIATELY PASSING SAID BASE METAL THROUGH A CLOSELY ADJACENT CERAMIC INSULATING INTERLAYER WITH NO INTERVENING OXIDATION; THENCE PASSING SAID BASE METAL INTO SAID COATING MOLTEN METAL, DISPOSED CLOSELY ADJACENT SAID INTERLAYER, WITH NO INTERVENING OXIDATION, TO THEREBY

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