US3048497A

US3048497A - Process of coating base metals with aluminum

Info

Publication number: US3048497A
Application number: US716257A
Authority: US
Inventors: Moller Goran August
Original assignee: Individual
Current assignee: Individual
Priority date: 1958-02-18
Filing date: 1958-02-18
Publication date: 1962-08-07
Anticipated expiration: 1979-08-07

Description

Aug. 7, 1962 G. A. MLLl-:R

PROCESS 0F coAIING BASE METALS WITH ALUMINUM 6 Sheets-Sheet 1 Filed Feb.

ILIS.. :nl

INV ENTOR.

llg- 7, 1962 G. A. MLLER 3,048,497

PRocEss oF coATING BASE METALS WITH ALUMINUM Filed Feb. 1s, 1958 e sheets-sheet 2 aus@ BMM f@ vin/orne .s

IN VEN TOR. len/v ,que 0.57- /W Aug. 7, 1962 G. A. MLLER 3,048,497

PROCESS 0F comme BASE METALS WITH ALUMINUM Filed Feb. 1a, 195e 6 Sheets-Sheet 3 INVENTOR. Gamm Hususr MOL/.EQ

Aug. 7, 1962 G. A. MLLER PROCESS 0F COATING BASE METALS WITH ALUMINUM 6 Sheets-Sheet 4 Filed Feb. 18, 1958 K ALUM/NUM Aug. 7, 1962 G. A. MLLER PRocEss oF coATING BASE METALS wITM ALUMINUM Filed Feb. 18, 1958 6 Sheets-Sheet 5 wuzuz. A wwzuz. whc SS Qu 0 O COATDNG POTENTIAL, VOLTS AIQ COOLED INVENTOR.

Go'femv ,Qual/57M OLLEB 3,048,497 PROCESS 0F COMING BASE METALS WITH ALUMINUM G. A. MLLER 6 Sheets-Sheet 6 Aug. 7,1962

Filed Feb.

INVENTOR. 60e/cw ususrMoLLf/a United States Patent Oiilce igil Patented Ang. 7, i952 3,048,497 PROCESS F CATHNG BASE METALS WHH ALUMINUM Gran August Mller, Gotene, Sweden Consuiate General of the United States of America, Cape Town,

Union of South Africa) rues nee. is, 195s, ser. No. 716,257 21 Claims. (Cl. 117-52) The present invention relates generally to the art of metal finishing, and more particularly, to a method of pre-treating metal objects to be coated and thereafter coating them by immersion in a molten metal bath.

Metal coating processes such as galvanizing, i.e., coating with zinc, are well known in the art. Heretofore, however, such coating processes have been confined to relatively few coating metals, among them zinc, which will form a continuous homogeneous protective layer on the metal to be protected and which will wet, that is, adhere rrnly to the parent metal so as not to be removed by subsequent rough handling of the coated articles, and so as not to leave voids, pits, pinholes, and other blemishes in the coating.A It is also axiomatic, of course, that the coating metal be one which has a relatively high resistance to corrosion, wear and the like. Still further, it is desirable that the coating metal be relatively inexpensive.

Aluminum is admirably suited'to the function of a coating metal from the standpoint of its resistance to corrosion and its relatively low cost. Aluminum has the property of forming an loxide layer on its own exposed surface which layer is thereafter highly resistant to further corrosion by the elements and is also quite resistant to abrasion and the like. Aluminum has the further advantage of being highly malleable so that bending or other distortion of the parent metal having an aluminum coating thereon does not tend to crack the coating.

In coating metals such as iron, copper, titanium and nickel with aluminum by hot-dip methods such as disclosed in my prior patent, No. 2,315,725, entitled Process for Metallization, especially Aluminization of Iron Articles, issued April 6, 1943, a separate layer of base metal and aluminum forms at the interface between the outer aluminum coating and the base metal surface. This interfacial layer formation acts as a bonding mediumV between the base metal and the aluminum. It is important, however, that this interfacial base metal-aluminum layer be kept as thin as possible inasmuch as it is found that the ductility of the layer increases as the thickness of the interfactial layer decreases. Hereinafter the term layer is referred to as an alloy or alloy layer inasmuch as it appears that the interface formed between the base metal and aluminum is a mixture of these metals. The term aluminumj as used herein, includes the alloys of aluminum.

In addition to the importance of minimizing the alloy layer, it is highly desirable to produce, in combination therewith, a maximum thickness of an aluminum coating.

The substantial removal-of all the oxides and other contaminants fromv iron and other base metal surfaces just prior to the coating step is essential for adequate formation of the bonding interfacial alloy layer. Hot-dip aluminizing processes, embodying a molten salt pretreatment step just prior to coating for the removal of oxides, as well as other features are disclosed in my prior aboveidentified patent.

The present apparatus and process constitutes in general an improvement over that shown in the aforesaid patent. This 'application is a continuation-impart of my copending application, Serial No. 252,267, entitled Method and Means for Cleaning and Coating Metal Objects, now abandoned.

Bearing in mind the foregoing facts, it is a major object of the present invention to provide an improved means and a method for coating metal objects by immersing the same in a different molten metal.

Another object of the invention is to provide improved apparatus and methods in which the coating metal is selected from the group consisting of aluminum and aluminum alloys;

it is also an object of the invention to provide a method and means for pretreating, by chemical and electrolytic action, metal objects prior to and in combination with an immersion-coating process and apparatus.

A further object of the invention is to provide a method and means for electrolytically pickling base metal objects in combination with a method and means for immediately thereafter immersion-coating said base metal objects without intervening oxidation, whereby the thickness of the resulting interfacial alloy layer is substantially decreased while at the same time substantially increasing the thickness of the aluminum coating.

It is still another object of the invention to provide a method and means for aluminizing a base metal object which includes the step of employing said base object as an electrode during the actual aluminization step with the molten salt as the other electrode, the direction of current flow between the object and the salt bath producing a maximum aluminum coating with minimum interfacial alloy formation for any given voltage application and for any given type of quenching.

Another object of the invention is to provide a method and means for aluminizing a base metal object by first preparing said object for aluminizing by electrolytic pickling in a molten salt bath, said object acting as one electrode and said salt bath as the other electrode, by

second employing said object as an electrode during the actual aluminization step with the molten salt as the other electrode, the direction and duration of current flow between the object and the salt bath being such as to maximize the thickness of the coating and minimize the formation of the alloy layer for any givenvoltage application, and by third air-cooling the metal object after coating.

Yet another object of the invention is to provide apparatus by which an electric current may be passed between the base metal and the coating metal during the coating process and between the base metal and cleaning bath during the pretreating process.

A further object of the invention is to provide apparatus by which the immersion-coating of certain base metal objects or work pieces is carried on concurrently with the pretreating process of other work pieces, and in which the same electric current flows vthrough all work pieces involved in such concurrent operation.

Still another object of the invention is to provide apparatus of the class described having a novel means for mechanically and electrically connecting work pieces to a work-supporting fixture.

The foregoing objects and advantages of the invention will be apparent from a consideration of the following description of preferred forms thereof and from the attached drawings, in which a' `FIGURE l is a partially sectioned perspective view of an electrically heated salt bath furnace constructed and adapted to carry out one form of the present invention;

FIGURE 2 is an elevational section taken on the line 2 2, in FIGURE 1;

FIGURE 3 is a plan view of the furnace illustrated in FIGURES 1 and 2;

FGURE 4 is an enlarged fragmentary elevational section showing a work piece during the immersion-coating phase of a modified form of my process.

FIGURE 5 is a partially sectioned perspective View of `a modified salt bath furnace showing an alternate form of the invention;

FIGURE 6 is an enlarged fragmentary elevational section taken on the line 6-6 in FIGURE 5 and showing a work-supporting fixture with work pieces thereon;

FGURE 7 is a plan view of the apparatus shown in FEGURE including the work-supporting fixture shown in FIGURE 6;

FIGURE 8 is an enlarged fragmentary elevational seetion of a modified work-supporting fixture showing an alternate modified form of the work-supporting fixture shown in FIGURE 6;

FlGURE 9 is a transverse elevational section taken on the line 9 9 in FIGURE 7, showing a `further modification of the invention;

FIGURE 10 is a similar transverse elevational section showing a still further modification;

FiGURES lla and 11b are photomicrographs of aluminized steel Ydepicting the aluminum coating thickness, and the alloy layer thickness with and without the use of current;

FIGURE 12 is a graph showing the eect of the application of various potentials on aluminum thickness and interfacial alloy thickness when the specimen is air cooled after coating; and

FIGURE 13 is an elevational section of a modified form of salt bath furnace.

In general, the present invention includes many of the yfeatures shown in my above-mentioned prior patent. There, a salt `bath furnace is disclosed in which a layer of molten aluminum is floated on a bath of fused or molten salt and in which the articles to be coated are held in the salt bath until they reach the temperature thereof, and until the pickling action of the molten salt removes impurties from the surface to present a chemically clean surface to be coated by the aluminum. After reaching the desired temperature as just described, and after being cleaned, the work piece is then withdrawn from the salt through the molten aluminum layer, such passage through the molten aluminum causing a coating of the molten metal to adhere to the parent metal.

In the process described in Mller Patent No. 2,315,725, a substantial aluminum-iron interface is 0btained between the surface of the metal and the aluminum coating. It is true that some ferro-aluminum alloy between the parent metal and aluminum coating is helpful since the alloy adheresrmly to both iron and aluminum, thus forming a bond between the parent metal and the coating. The present invention, however, provides a process of aluminization whereby amore uniform and thinner, and therefore more ductile aluminum-iron interface is obtained without sacrificing the thickness or quality of aluminum coating and indeed, increasing the thickness thereof.

The present invention utilizes a fused bath compri-sing a mixture of salts. The molten aluminum layer rests upon this salt mixture. One particularly advantageous salt mixture has been found to `be that comprising approximately 65-85% lbarium chloride, and 15-35% sodium chloride, the latter being added to reduce the fusing temperature of the resulting mixture. Such a salt mixture 'has a specific gravity of approximately 3 to 4, whereas aluminum has a specific gravity of approximately 2.7.I

The fusing temperature of the just-described salt mixture is in the neighborhood of 1200 F., and the melting temperature of aluminum is approximately 1280" F. Thus, when the salt is fused and raised above its fusing temperature to a temperature between 1200o and 1300 F., the aluminum is also maintained in a just-molten state.

Other salts which are useful in the processes herein described are in general the halides of barium, potassium, sodium, and aluminum. For example, sodium aluminum fluoride is useful in dissolving oxides from a Work piece to be coated.

For a more detailed description, reference should be d had now to FIGURE 1 of the drawings, wherein a salt bath furnace is identified generally by the reference character 1d, being provided with heating electrodes 11 which operate in the known manner to pass current through the molten salt to maintain the same in its molten condition. In the present instance, alternating heating current is employed, thus giving rise to a circulation of the molten salt produced by the eddy currents flowing therein.

It will be understood that the means employed to heat the molten salt in the furnace 10 do not form an essential part of the present invention, and thus no detailed description thereof appears herein. It will also be realized that other heating means, such as gas burners, coal burners, oil burners, and the like could be employed to heat the furnace 1t? without altering the characteristics of the invention described herein.

A lower process (as distinguished from heating) electrode f5 is constructed in a U-shape to extend around three sides of the furnace and is supported on a conductor bar 16 near the bottom of the furnace 10, the electrode 1.5 being of metal, for example, iron. The process electrode 15 rnay be completely closed, eg., 4-sided, if desired.

As can also be seen from the phantom line liquid level shown in FIGURE 2, a molten layer of aluminum 20 is 'floated on top of the molten salt 21, a sleeve 19 of ceramic or other insulating material being placed around the upper portion of the conductor bar f6 so as to prevent its contact with the molten aluminum 20.

When it is desired to pretreat a work piece 23, the same is suspended on the hook-shaped lower end of a vertical metal work-supporting rod 23, substantially completely enclosed by a ceramic sleeve 19a or other suitable insulating sleeve and placed near the bottom of the furnace 1@ in close proximity to the electrode 15. The current source is connected directly to the metal rod 29, as shown schematically in FIGURES 1 and 2, so as to make the metal rod 29 positive with respect to the lower electrode f5. Inasmuch as the work-suspending fixture 29, is electrically insulated from the molten aluminum 20, current flows downwardly through the fixture 29 and into the work piece 23, making the latter positive (anodic) with respect to the lower electrode 15 (cathodic).

The time of immersion of piece 28 varies depending upon shape, thickness, material and end use of the piece 23 (whether it should be heat-treated or not), the geometry of the system, aswell as other factors. The period of immersion can thus be generally varied between several seconds to 10 minutes.

During this period `of time, the work piece 23 is thoroughly cleansed and the oxides removed electrolytically. As will be described in more detail hereinafter, it has been found that greater pick-ling voltages (i.e., the Work 28 always being positive with respect to the salt bath 21) during the immersion of the work piece 23 or soaking results in formation of thicker aluminum layers upon the work piece. For example, it has been found that as the EMF. applied during the soaking period is increased from 0-6 volts, (all other conditions being equal) the thickness of the aluminum coating subsequently produced increased lbetween 3 and 4 times. This phenomenon is believed to be due to an increase in roughness of the surface of the parent metal with Aincreasing soaking voltage, `aluminum having greater yadhesion to the roughened surface. On the other hand, the iron-aluminum alloy layer subsequently produced (all other conditions being equal) increased in thickness but in minor amounts in comparison as the was increased from 0-6 volts. In most instances, the workp iece 2S remains positive as it is drawn through the coating aluminum layer 20. The coated steel is then quenched in an oil or other low temperature bath (not shown) which is sufficientlyA large with respect to the coated steel to be retained at 500 F. or lower throughout the quench. This process is found to be much superior to a process employing the same aluminum-molten salt system, `as shown in FIGURES l and 2, but without any direct current input.

Air, or other relatively slow cooling steps, may also be employed resulting in austenitic and pearlitic steel formations if steel is used as the base metal. However, the interfacial steel-aluminum alloy thereby produced has a much greater chance of formation and is in fact, substantially thicker than in oil quenching. A corresponding decrease in the aluminum coating thickness also results. Oil cooling in addition to preventing large interfacial alloy formation, also may bring about the formation of hard martensitic steel if the quench begins at a high temperature, i.e. above its critical of 1333" F., and is quenched rapidly enough i.e. above the critical cooling rate. Thus, the steel may be coated at above l333 F. and quenched below about 550 F. thereby forming 4a substantially martensitic steel core.

Air cooling of the coated metal is nevertheless sometimes used, depending principally upon the type of base metal employed and the end use desired. Even with aircooling, it is still highly desirable to produce the smallest interfacial layer thickness With the largest aluminum coating thickness, and a substantial modication of the basic direct current process just described is employed in conjunction with the air-cooling step to achieve this result.

This modification best shown in FIG. 4 requires that, just as the Work piece 28 enters the aluminum layer, the of the system be reversed. Thus the Work piece 23 becomes cathodic with respect to the molten salt bath during, but not prior to, the coating period. The timing of the reversal of the E.M.F., as just described, is critical in the obtaining of optimum results. Thus, if the voltage is reversed even several seconds prior to the commencement of `the coating period, it is `found that the aluminum does not adhere as Well to the work piece 28. This phenomena is believed to be due to the electrodeposition of metallic sodium or barium on the prematurely negative Work piece 28, which then prevents good adherence of the aluminum unless these metals are dissolved or wiped off in the aluminum float 20.

Referring specifically to FIGURES lla and 1lb, these depict actual photomicrographs of pieces of aircooled aluminum coated steel produced with and Without the use of direct current respectively. The pertinent data for each photomicrograph is listed below:

FIGURE 11a Magnification 1500. v Size of Wire ..080". j Grade of steel Eutectoid composition.

Temperature of Al bath l430 F. Speed of rWire thru bath 60 fpm.

Volts l0 v. D C.

Amperage 2.00 amp.

Remarks Positive pole in Al. layer 20. Cooling ...In ambient air.

Thickness of coating .Outer layer (Al.) 00072 av. Inner layer (Al. alloy) .0007.8 av., Total `layer 00100 av.

FIG URE 11b Magnification 1500. Size of Wire .080". Grade of steel Eutectoid composition. Temperature of Al bath l430 F. Speed of vwire thru bath 60 fpm. Etching solution '1/2 HFT-2% Nital. Remarks Run without direct current. Cooling In ambient air. Thickness of coating Outer layer (Al.) .00038" av., Inner layer (Al. alloy 00039 av., Total layer 00100 av.

The percentage of aluminum of the total layer formed by applying a -ivoltage to the aluminum layer during both soaking and coating periods is 72.0% and is substantially larger than the coating of the same grade of steelwithout the use of current (43.9%). Correspondingly, the total thickness of `the interfacial layer is 50.7% of the total layer without use of current and but 28.0% with the use of current.

Specifically referring now to FTGURE l2, the results of actual tests showing the mean thickness of aluminum deposition and the mean `thickness of steel-aluminum alloy formation respectively on the air-cooled specimens, for various soaking and coating voltages, are presented. The process `followed in obtaining the data for FIGURE l2 included the reversal of EMF. as just described. The mean thickness was determined by photomicrographic survey of `air-cooled insulated stel specimens to which the voltage Was directly applied. The term negative coating potential means that the work 23 is negative With respect to the salt bath 2l. Conversely, a positive soaking or coating potential means that the Work 23 is positive with `respect to the salt bath 2l during the soaking or coating phases respectively.

`In general, FIGURE l2 shows that `a substantially thicker aluminum coating is produced with increasing negative coating voltages. Conversely, the iron-aluminum intermetallic layer becomes thinner as the negative coating voltage increases. If positive coating voltages were employed instead of negative voltages, it can be seen that some decrease in percentage of aluminum results. For example, at +6 volts soaking, and at +6 volts coating, the thicknesses of the various layers is as `follows:

Aluminum, 0.0038 inch-% aluminum of total layer. Aluminum-steel alloy, 0.0016 inch-30% iron-aluminum alloy of total layer.

On the other hand, at +6 volts soaking, `and -6 volts coating, the thickness of the layers is:

Aluminum, 0.0047 inch-84% aluminum of total layer. Aluminum-steel alloy, 0.0009 inch- 16% iron-aluminum alloy of total layer.

Thus, for this particular set of voltage conditions set forth, the actual increase of aluminum deposited is 0.0009 of aluminum, `or

0.0009 in. m 24 Increase Further, the actual decrease in aluminum-steel alloy is 0.0007 inch of alloy or 43 decrease over the amount present in positive soaking and coating phases.

As mentioned, it is found that a thinner intermetallic layer reduces the possibility of rupture of the aluminum layer during bending and forming operations, and the production of air-cooled aluminum coated metals by my method therefore greatly enhances their ductility and forming characteristics.

By way of further contrast, it should he noted that at 0v soaking and 0V coating (that is Without the use of direct current in either the soaking or the coating phases of the process) the aluminum thickness is only 0.0017 inch, and the steel-aluminum thickness is 0.0009 inch. Thus, the percentage of aluminum of the total Ilayer is approximately only 35%, very substantially below the absolute amounts and percentages of aluminum deposited obtained with the use of current. The steel-aluminum thicknesses are also very substantial.

Theoretically, it appears that the reason for the advantageous effects of current reversal is that an extremely spaans? thin lm of sodium and/or barium forms on the work piece which inhibits subsequent interfacial alloy formation thereby decreasing the thickness of this layer and correspondingly increasing the aluminum layer.

It can also be seen that increasing the posiitve applied to the work piece 2S during the soaking period (at constant coating potential) results in a 20G-400% increase in the aluminum ylayer thickness, while the steelaluminum alloy layer either decreases or, if increasing, increases substantially less -rapidly in thickness. Thus, taking 3v as the fixed coating potential, the percentage decrease in the alurninum-steel alloy in going from soaking to 6v soaking was On the other hand, the percentage of `aluminum increase was approximately f tical lmatter, the increased quality of oil cooled product,

due to the reversal, usually does not warrant the increased expenditure of providing for a current reversal ste ign `summary then, my oil cooled coated steel product is generally produced without an reversal step while my air cooled coated steel is generally manufactured with an reversal step. More specifically, one preferred process includes l) a soaking phase Wherein the work piece Z8 is made anodic and then electrolytically pickled until it reaches the approximate temperature of the salt bath 21, (2) a coating phase wherein the direction of current is -reversed just as the work piece 28 is drawn into the overlying aluminum layer 26 to be thereby coated. (3) an air cooling of the coated Work piece 28.

Another preferred form of my process includes (l) a soaking and `coating phase wherein the work piece 28 is made and maintained anodic with respect to the negative salt bath (2) a severe cooling, such as in oil or water.

v Either of the preferred forms of my process has successfully employed the following metals as base metals: titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper and alloys thereof. v

A modification of the apparatus of lFIGURES 1 to 3 has also been used wherein the aluminum float 20 becomes the electrode by insertion of a conducting bar 17 as best shown in FIGURE 13. In this modification, the metal Work piece 28 is not insulated so that the current flows during soaking Afrom the aluminum 20 to the -metal support 29a and thence to the work piece 28. [In this modification, While useful process-wise in eliminating the need for insulated work supports 29, the advantageous effects of applied voltage are not as great as with the process referred to in FIGURES 1 to 4. It is thought that the large area of aluminum 20 to salt 21 contact compared to that of Work piece 28 to salt contact permits some shortcircuiting of the current otherwise flowing from the aluminum to the work piece. This short-circuiting phenomenon would reduce the magnitude of the current and the advantageous effects, above-described, due to it.

While inthe examples noted, the applied E.M.F. ranged 'from 6V to +6", the applied E.M.F. has been increased considerably where necessary to get sufficient current iiow in any particular system.

In FIGURES 5 through 7, a modified salt bath furnace 35 is provided with a central partition 36 which does not extend clear to the bottom of the furnace 35, whereby to permit the substantially uniform heating of the entire bath due to the interconnection of the salt bath under the lower edge of the partition 36.

In this modified form of the invention, a shroud-like electrode 37 is suspended by a conductor bar 38 in the right-hand section 39 of the furnace 35, whi-le upper electrode bars 40, similar to the conducting bar 17 of the embodiment described with reference to FIGURE l3 are suspended in the left-hand furnace section 41. Heating electrodes 42, similar to those employed in the previous embodiment, are mounted in the two sections of the furnace 35, although it will Ibe realized that in this embodiment as before, other means for heating the salt bath may be employed.

As can be seen in FIGURE 6, an aluminum layer 43 is iioated on top of the salt 44 but only in the left-hand section 41 of the furnace 35.

A bifurcated work-supporting xture 50 having a Vertical shank S4 is positioned to straddle the partition 36, one leg 51 of the xture extending downwardly into the left-hand furnace section 41, and the other leg 52 extending into the right-hand section 39. Work pieces 28 are suspended on hooks 53 extending laterally from both legs of the iixture Sti. The work pieces positioned in the righthand furnace section 39 are in proximity to the shroud electrode 37 and are electrolytically pickled in the molten salt, as previously described with reference to FIG- URE 4.

It will be realized that FIGURE 6 illustrates the process in media res, that is, the work pieces in the section 41 have already been pretreated in the section 39.

'During the soaking phase, in the form of the device shown in FIGURES 5 and 6, the current ilow is from the bar 40 to the aluminum layer 43, up through lthe transverse bar of the fixture 50, down the leg 52, and into the Work pieces 28 and thence to the negative shroud electrode 37. The direction of the current between the aluminum 43 and the salt 44 may be reversed just prior to the immersion of a work piece 28 into the molten aluminum 43 by the interconnection of appropriate independent DsC. circuits between the molten salt 43 and each of the work pieces (the D.C. circuits not being shown). The advantages of current reversal with air-cooling may be thus obtained in this modification, as well as the advantages of direct current without reversal and with rapid cooling.

It should also be understood that the work supports of FIGURES 5 to 7 can be insulated from the aluminum bath 43 and the current introduced along the work support 50 itself, as described with reference to FIGURES l to 4.

In a further modification of the two-compartment furnace of FIGURES 5 through 7, the partition 36 is extended clear to the bottom of the furnace so as to completely separate the -two sections 39 and `41. In this way, different salts may be used in the different sections, that in the coating section 41 being used solely for keeping the yaluminum molten and preheating the work piece.

As soon as one set of work pieces 28 has been pretreated and the other set coated by drawing the same up through the molten aluminum layer 43, the latter set of Work pieces is removed from the hooks 53 and the iixture 50 rotated about the axis of the central supporting shank 54 so as to place the leg 52 in position to be immersed in the aluminum bath section 41 of the furnace 35. Before lowering the ixture 50 to immerse the

legs

51 and 52, a set of untreated work pieces 28 is placed on the hooks 53 of the leg 5I whereby to be immersed in the pre-treatingsection 39 of the furnace 35.

Thus it will be seen-that the process performed by the apparatus as shown in FIGURES through 7 is practically continuous, and that the `most efficient use of the processing current is employed since it is used both in the coating and the pretreatment phases of the process.

Still further it should be noted that in the embodiments of the invention thus far discussed, such power as is absorbed by either the mol-ten aluminum bath or by the salt bath is dissipated as heat therein, and thus is not wasted.

In some instances, such as `where the parts to 'be coated are constructed of sheet metal, it is diiiicult to effect a secure, low-resistance connection between the work piece and the work-supporting fixture 29 or Si). This is particularly diiiicult when the work-supporting xture is at the elevated temperature produced by immersion in the salt bath. To obviate this difficulty, the modication .illustrated in FIGURE 8 is provided.

Here it will be seen that a metal cylindrical container 60, having an open upper end, is welded to the vertical shank of a work-supporting fixture 51a which may be either one leg of the fixture Sil or the vert-ical shank of the nxture 29 employed in the first embodiment. In the container o@ is placed a bath of lead 61 or other metal having a low melting temperature and a relatively high specic gravity as compared to either the salt bath or the aluminum. Upon immersion in the salt bath furnace, the lead 61 is melted and is thus adapted to receive, immersed therein, an arm 62 of the clamping hook 63 which is securely fastened to a work piece 28 by suitable means such, for example, as a bolt 64 and wing nut 65. Alternatively, the hook connector 63 may be Hash-welded to the work piece 28 and removed after the coating process is completed.

A substantial length of the hook arm 62. being immersed in the lead bath 61, a low resistance electric connection is effected between the xture arm Sla and the work piece Z3. Also it will be realized that the work pieces Z8 are quickly and easily removed and replaced on the fixture including the arm 51a. The metal, such as the lead 6l, in the cylindrical container 60, must be relatively non-soluble in aluminum, and vice Versa. Lead is such a metal.

Also it will be noted that when the fixture 50, modified as shown in FIGURE 8, is removed from a furnace and permitted to cool, the lead 61 or other Similar metal in the container 60, solidies and thus is not lost or separated from the fixture, as would bevapt to be the case with moving clamping parts and the like.

A further development of the lead bath electrical connection is illustrated in FIGURE 10. Here is shown a two-section furnace, similar to that previously discussed and shown in FIGURE 7, similar parts being identified with the same reference characters as in FIG- URE 7. The furnace in FIGURE l0 is modified, however, to include a molten lead pool 70 in the bottom of the pretreating section 39, and a relatively deep well 71 formed in the partition 36, also containing molten lead. The two `baths 70 and 71 of molten lead are interconnected by abus bar 72 which may, if desired, be embedded in the material of which the partition 36 is constructed.

The pool 70 forms an electrical connection for long work pieces or fixtures, such as the bar 73, the upper end of the bar being supported, if necessary, in an external frame 74 so as to prevent shorting of the bar 73 against the shroud electrode 37. Electrical interconnection between the molten aluminum i3 and the lead bath 71 may be accomplished by a hook-shaped fixture such as that indicated at 75, and the latter Velectrode may also serve t'o support a work piece 76 for the coating operation. The shank portion 77 of the hook 75 is of sufficient length and the lead bath well 7l is of suiiicient depth so It) that the work piece 76 can be drawn entirely through the molten aluminum 43 `before the shank 77 is withdrawn from the lead bath 71.

When it is desired to coat the lower end only of a long bar, such as that indicated at 73, the same may be first pretreated by inserting it in the pretreating section 39 with the lower end thereof dipping into the lead bath 7d whereby to perform the electrolytic action in the manner previously described, the circuit being completed by a suitable hook in the well 7i extending into the aluminum 43. The bar 73 may thereafter be removed and dipped in the molten aluminum d3, external electrical potential being supplied if desired.

A still further modified form of the two-section furnace of FIGURE 7 is shown in FIGURE 9, the same reference characters being employed to indicate similar parts. A pair of rollers Si? are mounted on the upper edge of the end walls of the furnace 3S, and a similar roller 81 immersed in the salt bath is mounted on the lower edge of the intermediate partition 36. Thus a continuous strip or wire 82 may pass over the rollers Si) and under the roller Si, and be moved in the direction of the arrows whereby to be continuously pretreated and coated by passing first downwardly through the shroud electrode 37, and thereafter upwardly through the electrically positive aluminum bath 43. The wire titi is then preferably oilquenched. If desired, the strip 82 may be constructed in the form of an endless chain (not shown) having intermittent carriers or hooks thereon, whereby individual articles may be carried continuously down through the pretreating section 39 and upwardly through the aluminum 43.

While the forms of the device and the processes shown and described herein are fully capable of achieving the objects and providing the advantages hereinbefore stated, it will be realized that they are capable of some modification without departure from the spirit of the invention. For this reason, I do not mean to be limited to the forms shown and described, but rather to the scope of the appended claims.

I claim:

1. The processof coating base metals with a metal selected from the group consisting of aluminum and aluminum alloys which comprises: electrolytically pickling said base metal in a molten salt bath wherein said base metal is made anodic and said salt bath is made cathodic; `and immersing said base metal into the coating metal, without atmospheric contact, said base metal being coated thereby.

2. The process of coating `base metals with a metal selected from the group consisting of aluminum and aluminum alloys which comprises: electrolytically pickling said base metal in a molten salt bath wherein said hase metal is made anodic and said salt bath is made cathodic; immersing said pickled base metal into said coating metal, without intervening atmospheric contact, the base metal being maintained positive ywith respect to the salt bath throughout the coating; and substantially immediately quenching said coated base metal.

3. The process of coating -a base metal with a coating metal, selected from the group consisting of aluminum and aluminum alloys, which comprises: floating a molten bath of said coating metal on a heavier fused salt bath; passing a direct electric current between said base metal and said salt bath, the direction of the current passage being such as to make said base metal positive with respect to said salt bath; immersing said base metal in said salt bath; moving said base metal directly from saidV salt bath, without any atmospheric contact, into said molten metal; and thereafter removing said base metal from said coating metal whereby a l-ayer of molten metal adheres to said base metal.

4. The process of coating a base metal with a coating metal, selected from the group consisting of aluminum and aluminum alloys, which comprises: floating a molten bath of said coating metal on a portion of a heavier fused salt bath; passing a direct electric current between said base metal and said salt bath, the direction of the current passage being such as to make said base metal positive with respect to said salt bath; immersing said base metal in said salt bath; moving said base metal directly from said salt bath, without any atmospheric contact, into said molten metal; thereafter removing said base metal from said coating metal whereby a layer of molten metal adheres to said base metal; and immediately quenching said coated base metal.

5. The method of claim 4 wherein said quenching is performed in an oil bath sufficiently large to rapidly lower the temperature of the base metal and prevent excessive interfacial alloy formation.

6. The method of claim 4 wherein said quenching is performed in ambient air.

7. The method of claim 4 wherein said base metal is selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper.

8. The method of claim 4 wherein said base metal is electrically insulated from said coating metal except on its withdrawal into said coating metal for the coating of said base metal.

9. The process of coating a base metal selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper, with a coating metal selected from the group consisting of aluminum and aluminum alloys, which comprises: iioating a molten bath of said aluminum coating metal on a portion of a heavier fused halide salt bath; immersing said base metal in said salt bath; passing a direct electric current between said base metal and said salt bath, the direction of the current passage being such as to make said base metal positive with respect to said salt bath; moving said base metal directly from said salt bath without any atmospheric contact, into said molten metal after said base metal has approximately attained the salt bath temperature; removing said base metal from said coating metal bath whereby a layer of molten metal adheres to said base metal; and immediately quenching said coated base metal in `an oil-bath which remains at a temperature of below about 500 F. throughout the quench.

l0. The process of coating a base metal with a coating metal selected from the group consisting of aluminum and aluminum alloys which comprises: floating a molten bath of said coating metal on a heavier fused halide salt bath; passing said base metal into said molten salt bath; maintaining a potential between said base metal and said salt bath whereby said base metal is made anodic and said salt bath cathodic; commencing withdrawal of said base metal from said salt bath and just prior to its withdrawal reversing the direction of current iiow between said base metal and salt bath whereby said base metal becomes cathodic with respect to said salt bath; completing the withdrawal of said base metal from said salt bath; moving said base metal Idirectly into said coating metal; thereafter removing said base metal from said coating metal bath', and substantially immediately thereafter cooling said coated base metal in ambient air whereby a thin base metal-coating metal interfacial alloy is formed in conjunction with a thickened coating metal layer.

l1. The method of claim l wherein said base metal isV selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel and cooper.

l2. The process of coating a base metal with a coating metal selected from the group consisting of aluminum and aluminum alloys, which comprises: iioating a molten bath of said aluminum coating metal on a portion of a heavier fused halide salt bath; immersing said base metal in said salt bath; passing a direct electric current between said base metal and said salt bath, the direction of the current passage being such as to make 4said base metal positive with respect to said salt bath; moving said base metal directly from said salt bath without any atmospheric contact, into said molten metal after said base metal has approximately `attained the salt bath temperature; removing said base metal from said coating metal bath whereby a layer of molten metal adheres to said base metal; and slowly cooling said coated base metal.

13. The process of coating a continuous base metal with a coating metal selected from the group consisting of aluminum and alumimun alloys, which comprises: floating a molten bath of said aluminum coating metal on `a portion of a heavier fused halide salt bath; immersing said base metal in said salt bath; passing a direct electric current between said base metal and said salt bath for at least -a portion of the travel of said base metal through said salt bath, the direction of the current passage being such as to make said base metal positive with respect to said salt bath; moving said base metal directly from said salt bath without any atmospheric contact, into said molten metal after said base metal has approximately attained the salt bath temperature; removing said base metal from said coating metal bath whereby a layer of molten metal adheres to said base metal; and substantially immediately cooling said coated base metal.

14. 'Ihe process of coating a base metal with a coating metal, selected from the group consisting of aluminum and aluminum alloys, which comprises: iioating a molten bath of said aluminum coating metal on a portion of a heavier fused halide salt bath; passing said base metal into said molten salt bath; commencing the withdrawal of said base metal from said salt bath; maintaining lan electric potential between said base metal and said salt bath whereby said base metal is anodic and current iiow is directed from the base metal to said salt bath during immersion of said base metal `in said salt bath, but just prior to the commencement of said withdrawal of said base metal reversing the polarities whereby said base metal becomes cathodic with respect to said salt bath; completing the withdrawal of said base metal from said salt bath; moving said base metal directly into said coating metal; thereafter removing said base metal from said coating metal bath; and substantially immediately cooling said coated metal.

15. The process of claim 3 wherein said base metal is maintained electrically positive with respect to said salt bath during its passage through .said coating metal.

16. The process of claim 3 wherein said base metal ismaintained electrically negative with respect to said salt bath during its passage through said coating metal.

17. Apparatus for pre-treating and coating metal objects which includes: -a salt bath furnace having a crucible and a bath of fused salt therein adapted to support a layer of molten metal thereon; a first process electrode disposed in said salt bath; a second process electrode disposed adjacent the upper face of said bath to contact a layer of molten metal thereon; means for connecting said electrodes to a source of direct current whereby to make said second electrode anodic, the direction of current flow being from said second electrode to said first electrode; and means for supporting a work piece immersed in said salt bath and establishing electrical contact between said work piece and said molten metal.

18. Apparatus for pre-treating and coating metal objects which includes: a salt bath furnace having a nonconductive crucible and a bath of fused salt therein adapted to support a layer of molten metal thereon; a first process electrode affixed to, and within, said Crucible disposed in said salt bath; a second process electrode afiXed to, and within, said Crucible disposed 4adjacent the upper face of said bath to contact a layer of molten metal thereon; means for connecting said electrodes to a source of direct current in a manner to make said second electrode anodic; and means for supporting a work piece immersed in said salt bath and establishing electrical contact between said work piece and said molten metal, whereby current may ow from said second electrode through said work piece and salt bath to said rst electrode.

19. An apparatus according to claim 18 wherein said supporting means includes a container portion having a body of molten metal therein, said container being adapted to receive a conductor member electrically connected to said work piece.

20. An apparatus according to claim 18 wherein said supporting means comprises a ixture supported for movenient to reverse its position, whereby to place the portion previously in one section into the other sections, and vice versa.

21. An apparatus according to claim 18, in which said Crucible is provided with `a transverse partition dividing the same into two sections; said first electrode being disposed in one of said sections, and said supporting means being bifurcated and having portions extending into each section.

References Cited in the 'Eile of this patent UNITED STATES PATENTS Betts Aug. 1, Chance Oct. 3, Rosseau Nov. 9, Dellgren Nov. 1, Moller Apr. 6, Tainton May 18, Clenny May 25, Webster Apr. 19, Spence et al. Ian. 16, Burkhardt Apr. 2,

OTHER REFERENCES ser. No. 369,610, Helling et ai. (APC), May is, 1943.

Claims

1. THE PROCESS OF COATING BASE METALS WITH A METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ALUMINUM ALLOYS WHICH COMPRISES; ELECTROLYTICALLY PICKING SAID BASE METAL IN A MOLTEN SALT BATH WHEREIN SAID BASE METAL IS MADE ANODIC AND SAID SALT BATH IS MADE CATHODIC;