US 2570906 A
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Oct. 9, 1951 M. ALFERIEFF PROCESS FOR COATING METALLIC OBJECTS WITH OTHER METALS Filed Dec. 21, 1946 INVENTOR flit/2d fll/zrz'e f Patented Oct. 9, 1951 PROCESS FOR COATING METALLIC OBJECTS'WITH OTHER METALS Michel Alferiefl, Algorta, Guecho, Spain Application December 21, 1946, Serial No. 717,773 In Spain July 31, 1946 Claims.
The present invention relates in general to a method of coating metals with layers of other metals and has as one of its more special objects the obtention of a hot dipped coating of iron or steel with pure aluminium or with corrosion resisting aluminium alloys or with aluminium-iron compounds.
The fundamental object and nature of my invention is the obtention of metal products of various forms, such as sheets, strip and wire, of highly improved mechanical and surface qualities, provided with continuous, uniform, adherent and ductile, corrosion resisting coatings free of traces of foreign substances and built up of pure homogeneous metal or an alloy or definite metal compounds. These results are obtained in a more economical, efficient and simple way than by any of the heretofore known methods.
The most economical and rational method of metal coating, advanced by modern technical practice, consists in combining in one continuous operation conducted in a reducing atmosphere thelthree principal manufacturing steps of usual hot-dipped metal coating production, which, as it is known, are: the base metal annealing, the deoxidation of the surface to be coated, and the immersion in the molten coating metal bath.
Such a method presents, moreover, the well known economical and technical advantages of continuous production, the possibility of suppressing the conventional pickling in acid which, besides its expensive cost, causes, as it is well known, undesirable phenomena, such as hydrogen absorption by the treated metal and the formation on the surface of the latter of intercrystalline fissures retainin by capillarity salt particles derived from reaction of the acid with the metal treated.
However, heretofore, it has not been possible to obtain by this method an entirely satisfactory product, because of the porosity, poor adherence or brittleness of the coating, which imperfections result in part at least from the insufficiency of the control of the atmosphere maintained in the preparatory heat treatment step and/or from the methods of application of the coating. It also appears that the above and other insuificiencies of the products of the prior coating processes that have omitted the pickling step, such as poor corrosion resistance of the coating, partly result from the presence in the said coating of foreign substance particles and absorbed gases. The low mechanical qualities of the manufactured prodnot appear also in part to result from the presence in the base metal and in the coating of gases absorbed during the previous treatments" and from the diificulties of obtaining a satisfactory heat treatment when using the different temperature ranges heretofore proposed for the various manufacturing steps. For instance, a large scale application of this method for tinning or for alumimum coating has been prevented hitherto because of an excessive porosity and/or poor adherence, brittleness, roughness and exaggerated thickness of the coatings. 7
Its application to the easiest case of the metalcoating art, viz., galvanizing, also resulted in the production of lower quality coatings, as compared to those obtained with the conventional methods usingv the acid pickling step, especially in respect to the poor adherence of the zinc coating. Attempts to overcome this drawback by additions to th coating metal bath of other metals, such as aluminum or silver metal, have allowed of an increase of the adherence of the zinc coating, but lead, on the contrary, to decrease of the corrosion resistance of the latter as a consequence of the structural heterogeneousness of th obtained coating and the resulting formation and action of microscopic electro-V chemical couples. My invention obviates all the aforementione difficulties and procures moreover important advantages, which will be set forth in the follow: ing disclosure. I have discovered that the utilization of a de-: termined heat treatment method in the presence of certain gas mixures, as well as the application of certain treatments during the coating step, makes it possible to obtain, in any one of the aforementioned cases of metal coating, products of a much higher quality than that obtained by any one of the heretofore known methods. In particular, applying my process to the most difii cult case of the metal coating art, the coating of steel with pure aluminium, I am able to obtain a, coating without any porosity and one'that is exceptionally adherent and uniform. Likewise this invention has been applied, with greatly improved result to copper tinning. According to my invention, the bodies that are to undergo the coating treatment are heated to or somewhat below the temperature desired in the subsequent occluded gas removing step in the presence of an atmosphere which accomplishes'a' reducing action. An inert atmosphere is subsequently used, in which the said bodies are consecutively maintained at the desired gas evolving heat treatment temperature, cooled to or below the temperature of the coating bath, and finally especially when using hydrogen as the reducing atmosphere. It also occurs during the initial metal manufacturing steps, suchfas melting and refining, as well as during pickling in acid. after hot rolling or prior to cold rolling or drawing and similar mechanical transformations 'of metals into their conventional shapes,-such. as strips,
sheets or wire. Such absorption causes the'low modified form of one part of the apparatus of mechanical qualities of the article, porosity of the coating and, probably, the poor adherence of the latter.
It has been discovered that-the treatment ac-- cdrdingto this invention makesit-possible to accomplish the indispensable perfect deoxidation of the surface of the base metal by the action of a reducing atmosphere while at the same time completely avoiding the pernicious effects of the absorption of this atmosphere by'the base metal and by the coating bath. Moreover, in some cas'es,-the quantity ofgases alreadyoccluded by the base metal before its deoxidation by annealing in a reducing atmosphere may be even considerably lowered.
My invention'furthermore makes it possible to carry out the essential preliminary deoxidation of the metal shapes or bodies with considerably increased-efficiency by mixing with the reducing atmosphere a strictly controlled quantity of water vapor in order to obtain a gaseous mixture exerting the most effective purifying action while avoiding all presence of suchwater vapor in the succeeding ga's removal step-of heating in the inertatmosphere.
The fact that the process according to this invention has been applied with greatlyimproved results to the aluminizing, tinningand galvanizing of steel and to copper and brass tinning, shows clearly that, contrary to the opinion previously held, the coating formationprocess is just the same in all the mentioned cases, provided that the base metal can be put incontact with the coating metal in absence of any occluded gas and of any particle of other foreign substance or chemical combination of gases with metals.
' Now, this invention'permits equally to improve considerably the qualities of the obtained coating by applying determined treatments subsequent to the coating cooling step, and especially in the particular .cases of coating steel or iron with aluminium or aluminium alloys. All these particular cases being completely identical as far as the invention is concerned, I shall henceforth, in order to simplify this disclosure, use the word aluminium not only to designate pure aluminium but equally aluminium alloys, and the word steel to designate either steel oriron.
In fact, it has been discovered, that the ductility and uniformity of the aluminium coating obtained according tothis invention may be greatly improved by subjecting the latter, after its solidification and before cooling it to the tem perature of 300 F., to a controlled light rolling treatment. The ductility and uniformity of the coating may be further improved by retarding the cooling of the treated bodies in a certain determined way during the solidification of the aluminium coating and the subsequent cooling 4 of the latter to a temperature ranging between 750 and 400 F.
Preferred exemplary embodiments of the invention applied to the particular case of coating of cold rolled mild steel strip with aluminium will be now described in detail with reference to the accompanying drawings, such disclosed embodiments constituting only representative examples of the numerous cases and ways which the principle of the invention may be used.
In said accompanying drawings:
Figure l is a diagrammatic sectional View of one form of apparatus adapted to the aluminium coating of steel strip which may be used to perform the 'process'embodying my invention;
Figure :2 isa diagrammatic sectional View of a Figure 1;
Figure "3' is a diagrammatic sectional view of another apparatus for coating a steel strip with an aluminium-iron combination layer.
As'it may be seen by reference to Figure l, the treated strip i, is moved in the direction of the arrow 2 andenters through a conventional gastight entrance device 3 into aconventional gastight furnace l which contains two heating chambers i3 and 6 separated by a gas-tight inner wall '5' provided with the orifice 8 which permits the passage of the strip [from-chamber 5 to chamber E.
It is to be understood that the stri introduced into'the furnace 4 has .been brought to a dustless and perfectly degreased state by one of the known present day processes such as the electrolytic degreasing process. Degreasing by an imperfect process, such forexample as the out of date method of decomposition by pyrogenationwhich consists inburning the oil when heating the steel in an oxidizing atmosphere .to the temperature of thetempe'r'colours appearance, must be excluded, due to the presence after such a treatment of particles of carbon incrusted in the steel surface, which prevents the formation of a continuous coating.
In the chamber "5 filled with an atmosphere effecting a reducing action, the strip is heated,
according to'the mechanical qualities desired for the finished product, either to the conventional annealing temperature or to the normalizing temperature, viz., from 1470 to 1710" F., or when necessary, to a temperature from 35 to 180 'F. lower than the above mentioned, increasing :in this way, as it has been found, the efiiiciency of the next succeeding treatment designed to bring about evacuation of the absorbed gases. The said reducing atmosphere is continuously supplied to the chamber through the inlet 9 and may be eventually withdrawn and recovered by the outlet 10. This atmosphere may have the usual composition'of one ofthe conventional atmospheres called reducing, which are used in practice to reduce the'superficial oxide layer of metals during their heat treatment. Preferably, however, strictly determined quantity of water vapour is simultaneously andcontinuously'supplied to the inlet 9 in order to increase the purifying action of the said reducing atmosphere, as explained above.
Contrary to the practice of the'known metal coating processes involving the de-oxidation of thebasemetal by aheat treatment in a reducing atmosphere, in which the presence of water vapour in the said atmosphere is rigorously avoided, it has been" found that the voluntary addition'of water vapour to the latter, according a lower temperature range.
to this invention, produces the following desirable results: acceleration of the deoxidizing action of the reducing atmosphere, increase of the efficiency of said action and, in certain cases, a superficial decarburization of the base metal that facilitates the formation of an uninterrupted, uniform and homogeneous coatings. The negative effects of the presence of the water vapour in the reducing atmosphere, in the case of metal coating practice, are the following: (1) absorption by the hot base metal and by the molten coating metal bath of the hydrogen liberated by the water vapour in contact with these metals; (2) reoxidation of the base metal during the cooling step prior to its immersion in the coating bath, which is explained by the fact that the same atmosphere, called reducing, which contains a determined amount of water vapour and which produces a reducing action at a determined temperature, has an oxidizing action at These negative effects are completely suppressed by this invention, firstly, due to the fact that all presence of water vapour is strictly avoided during the steps of the base metal cooling and the immersion in the bath,
and, secondly, due to the subsequent treatment which permits the evolution and the evacuation of the gases absorbed by the base metal.
To perform this last treatment the strip I passes through the heating chamber 6, being maintained during this passage at the normal annealing or normalizing temperature in the presence of an energetically renewed inert atmosphere, then the said strip consecutively passes, always under the said inert atmosphere protection, through the chamber II where it is cooled to, or slightly below, the temperature of the molten aluminium coating bath l2 said cooling chamber ll being sealed at its exit end by said bath [2 and forming with the chamber 6' a gas-tight hole.
The said inert atmosphere must not possess the property of being absorbed by hot steel or by molten aluminium, or at least it should not be absorbed with the same intensity and with the same pernicious effects as all the known reducing atmospheres. On the other hand, it must permit by its nature and by the methods of its application the most complete disengagement of the active components of the reducing atmosphere absorbed by the strip, such as hydrogen. Moreover, the said inert atmosphere must not exercise an oxidizing action on the surface of the treated objects during the cooling step prior to the immersion in the coating bath and it must be, therefore, as free of oxygen and water vapour as possible.
To obtain the aforementioned results, I prefer to use the inert atmosphere anhydrous nitrogen, adding to the latter a little quantity of anhydrous hydrogen, just in the proportion required to counterbalance any penetrations of atmospheric air into the cooling chamber H, which are practically impossible to avoid completely, and which would promote without this hydrogen addition a reoxidation of the objects cooled in this chamber H. The said inert atmosphere is constantly supplied by the inlet [3 into the cooling chamber l I and passing through it, as well as through the heating chamber 6, is. aspirated through the outlet l4 out of the furnace 4 by a pump not represented in the drawing, carrying away with it the reducing gases released by the treated strip. Once out of the furnace the said atmosphere is separated from the foreign substances, which had contaminatedit, by a con ventional regenerating apparatus l5 and, possessing its initial analysis, is finally sent again through the inlet [3 into the chamber H. Thus the necessary intense circulation of this inert atmosphere is realized at low cost. It can be noted, as an example, that I obtain a perfect re-' sult in treating cold rolled mild steel strip, using as the said inert atmosphere a mixture, practically free of water vapour, composed of 90 to 98% of nitrogen and of 2 to 10% of hydrogen and, as the said reducing atmosphere, a mixture of 75 to 98% of hydrogen and from 2 to 25% of water vapour; the said strip being heated in the chamber 5 to a temperature ranging from 1470 to 1670 F., then maintained in the chamber 6 during a period from to 2 minutes at a temperature ranging from 1510 to 1710 F., and being finally cooled in the chamber H to atemperature ranging from 1110 to 1260 F. The temperature of the pure aluminium bath I2 is maintained at a temperature only slightly-above its melting point and not exceeding 1290 F.
The strip treated in this way and immersed in the coating bath I2 is instantly covered by a continuous and regular layer'of chemical compounds of the two metals, aluminum and iron. As is known, the presence of such a layer is indispensable for obtaining adherent coating. But, as this combination is usually very brittle, its layer has to be as thin and regular as possible, in order to be able to overcome without breaking or flaking the mechanical transformations of a finished product. It is well known that the thickness of this compound layer decreases when the time of immersion of the base metal in the coating bath and the temperature at which occurs the formation of these compounds are decreased. The great speed of formation of the compound layer and the fact that the treated bodies are dipped at a moment when they are cooled to or slightly'below the temperature of the said bath, according to the invention, allows a reduction of the time of immersion in the bath only to a few seconds, and even to a fraction of a second, and make it possible to obtain the formation of compound layers at practically the lowest possible temperature. The chemical compound layer thus obtained is uninterrupted and much thinner and regular, and therefore more ductile, than any one produced by the known methods. The said treated strip is withdrawn from the bath I'Z upwardly with or without the use of the conven-'- tional wiping and/or oxidation preventing exit devices, such as exit rolls, flux exit box or a protective gas fiow.
After the solidification of the coating and be fore it is cooled to the temperature of 300 F. the said coating may be subjected to a light rolling treatment, not affecting the steel base of the product, by passing between the rolls l6; said treatment being particularly eflicient when applied immediately after the moment of coating solidification. To this end the rolls I6 may be placed above the bath 12 at a convenient height corresponding to the place where the said solidification occurs. It was equally found that the efficiency of the said rolling treatment may be further improved when it is performed not by a conventional rolling mill, but when adopting for the rolls l6 as small a diameter as possible, viz., for example, from 2" to 8". The highly positive results of this treatment are probably explained on the ground that the coating, which must be very ductile at such a temperature range,
is *in this way? subjected to a certain. stress o'i compression without affecting the much harder steel base, thus enabling the internal intercrystalline strains to disappear, which otherwise would be present in such a:coating .due to the shrinkage of aluminium during itsisolidification and during the cooling of the product,.the expansion coefficient of aluminium being higher than that of the steel base.
;During the solidification of the coating and the g subsequent cooling of the treated strip, or after the passage of the latter through the rolls [6, the said strip may pass through the heat-insulated mufiie's [1, which must possess a sufiicient heat insulation and a sufiicient length in order to hinder-gradually and during at least from 2 to 5 minutes the cooling of the strip to the temperature comprised between 700and 400 F.; the effect of the said gradual cooling being the improved ductility of the coating, which results probably from the uniform distribution of the forementioned intercrystalline strains in the coating obtained according to this invention. I prefer to perform the. said gradual cooling. step while advancing the strip in vertical planes passing it upwardly'and downwardly around the pulleys 18. The strip coming outfrom themufiles I! may be subjected to a second light rollingtreatment performed by another pair of rolls l9 beforethe stripis cooled to the temperature of 300 F.
In cases where it is not possible to avoid atmospheric air infiltrations into the cooling chamber I I, the proportion of hydrogen in the atmosphere of the last chamber may be increased correspondingly, for example, up to in order. to prevent the risk of re-oxidation of the strip in the said chamber; this, by adopting the methods represented in the Figure 2. According to the modifications of this latter embodiment of my invention, an atmosphere composed, for example, a of 80% nitrogen'and 20% hydrogen, is introduced at-a low rate into the cooling chamber I through the inlet 20. In order to avoid the 'contactof this last atmosphere relatively rich in hydrogen with molten aluminium and thus to avoid hydrogen absorption by the latter, the endof the chamber H is'sealed by the molten lead bath 2|, in which the treated strip is immersed and from which it is withdrawn passing throughta .layer of moltenaluminium'z'z floating abovethesaid lead bath in a container 23. It has been found that aluminium being much lighter than-.lead, these two metals do not mix and donot react together. Likewise, the steel strip is not affected in any way by this passage through thelead bath. In order to diminish the danger of reoxidation of the strip, the timenecessary forthe cooling of the strip in the chamber ll may be'reduced and the latter may be provided at this end with water jacketing 24. In this case the inert atmosphere formed by pure nitrogen is introduced directly at a high rate into the'heating chamber 6 by multiple pipes 25, which distribute'the fiow of this last gas upontlie whole surface of the treated strip in order that the hydrogen partialv pressure may be constantly vmaintained very :low. on the surface of the strip,:all hydrogen released by the latter, as well as the hydrogen diffusing 'fromthe chamber I I, being immediately evacuated-by this how of nitrogen.
In case when the speed of advancement of the strip has -to be slow, the bathrepresenteii in the Figure 2 may housed in order to decrease the time of contact o'f' the strip with themelten aluminium; the layer- Z2 "of this latter-metal be ingsin :this case aslithiniaspdesired. In View of speciali' applications of the. product, the -corrosi'onxresistance athigh temperatures and the surfaceelectronic emission powerof the aluminium coating. may be :greatly improved :by an additional.treatment-which is represented in the embodimentof invention shown by this Figure 2. .LHGIGIthS strip, after its aluminium coating solidification andbefore beingcooled to the tem perature of 930.F; is subjected to an additional treatment, passing through a furnace 26 where theistrip surface is heated to a temperature from 1290 to 1830 F. during a few seconds until a blackish lustreless scoa'ting is'obtained.
.Referring to Figure' ii' Ihave indicated amodified. apparatus'for coating steel strip with an aluminiumironchemical compound layer. ,Here the coating bathis formed by the molten. lead 21, wherein is submerged the bell:28, formedlby the. exit. end of the cooling chamber ,1 I, con-'- ta-ining amolten aluminium layer 29 floating on thesaidlead. .Both aluminium and lead are maintained at a temperature slightly higher than that of the aluminium melting point. The treated strip is passed .through the molten aluminium layer '29, where itiscoated by a layer of iron and aluminium compounds, into the lead bath 2], from which it is withdrawncoated with a layer of the said compounds free of lead or offree aluminium. The formation of a pure aluminium layer is preventedby this method, which makes it possible apart from the economy of aluminium thus obtained, to produce a coating formed exclusively by iron-aluminium compounds possessing'a high melting point and thus furnishing an efficient protection against corrosion at temperatures which may reach 1920 F. Such-a coating has a surface showing a very uniform structure of simple crystals presenting an agreeable pattern. The micrographic and X-ray study of this compound layer shows that it is formed by: (l) a solid solution of aluminium in iron; (2) an overstructure of FesAl and of FeAl; (3) a nonidentified phase, probably AlzFe; (4:) and finally by the phase AliFe; the total thickness of such a coating being of about 1-5 microns.
.Referring again to Figure 3 the inert atmosphere isico'nstantly introduced firstly through the inlet 20 of-the chamber II, in a small quantity which is just suflicient to prevent the re-oXidathan of the treated strip in this chamber II, and secondly, in a great quantity, by-multiple pipes 25 in theheating chamber 6, exactly in thesame way as it was-described with reference to Figure 2. The wh'oleof the inert atmosphere flows continuously into the heating chamber 5, where a reducing gas, such as hydrogen, optionally mixed with water vapour in the required proportion and continuously introduced through the inlet 30,.transforms this inert atmosphere in one exerting a reducing action previously described. The whole of the used gases may escape to the atmosphere by the entrance orifice 3| of the furnace 4.
The treated strip may be advanced during all =the'previously described treatments by uncoiling it at the entrance of the furnace 4 andcoiling it at the end of the treatments, avoiding inthis way the useof expensive conveyors. t is clear that wi're may be treated exactly in the same way. Discontinuousobjects, such as sheetsand tubes, for example, may be advanced during the aforementioned treatments by suitable conveying means. I
lt is evident -that'the said coating of iron and aluminium chemical compounds, which consti+ tutes a particularly efficient protection against the corrosion at high temperature, may be obtained in the'same way on ferrous objects such as strip, wire, :sheets or any other objects, which may be used, for example, for manufacturing heating apparatus, pipes and containers for hot gases or liquids.
The coating of the said chemical compounds, with or without a final aluminium layer, manufactured according to the invention, has the properties of being particularly adherent and ductile, and may endure the mechanical deformations to which the coated objects may be submitted, such as deep drawing, folding and bending, without scaling nor flaking. 'Moreover it has "a'valuable quality of presenting an exceptionally efficient underlayer for subsequent application of adherent layers of paints, varnishes, I enamels, lacs and glasses for example, steel sheets thus coated may be used for the manufacture of automobile bodies, being painted or varnished after forming and drawing steps, or for the production of elements for refrigerators and stoves enamelled after their forming. The steel wire thus coated may be used, for example, as a stainless and undetachable core in the manufacturing of reinforced glass.
The. aluminium coated steel strip or sheet manufactured according to the invention, may be advantageously used, for the aforementioned reasons and in view of the physical and chemical properties of the aluminium, in manufacturing of canning boxes, receptacles for products to be preserved dry, boxes for paints, varnishes, lacs, waxes, boot polish, cans for gasoline, alcohol and oil, kitchen implements, roofing, protectors for electric wires and cables, light reflectors and infra-red ray reflectors.
Modifications may be made in my invention, without departing from the spirit of it.
Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:
1. A process of coating metallic bodies with a metal selected from the group consisting of aluminum, zinc, lead, cadmium and tin which comprises heating the said bodies to a temperature within the range l4'70-16'70 F. in the presence of a reducing atmosphere consisting predominantly of hydrogen and containing from 2 to 30% water vapor, then expelling occluded gases from said bodies by heating them to at least the temperature attained in said first heating step while in direct contact with an inert atmosphere consisting predominantly of nitrogen, and while maintaining said bodies wholly protected by an inert atmosphere consisting predominantly of nitrogen and substantially free from water vapor, cooling them to a temperature not exceeding the temperature of the molten bath of the coating metal, and then passing them into said coating metal bath.
2. A process as set forth in claim 1 wherein the metallic bodies in the gas expelling heating step are heated to a substantially higher temperature, within the range 15l0-1700 F., than was attained during the initial heating step to thereby enhance the gas expulsion action.
3. A process as set forth in claim 1 wherein the reducing atmosphere maintained during the first heating step consists essentially of 75-98% of hydrogen and 2-25% of water vapor.
4. A process as set forth in claim 1 wherein the inert atmosphere maintained during the gas expulsion heating stage consists essentially of -98% nitrogen and 2-10% hydrogen and is substantially free from water vapor.
5. A continuous process of coating mild steel strip with a metal selected from the group consisting of aluminum, zinc, lead, cadmium and tin, which includes the steps of heating the said strip to a temperature ranging from 1470" to 1670 F. in contact with an atmosphere consisting. predominantly of hydrogen and containing water vapor in an amount not exceeding 30%, then expelling occluded hydrogen from the said strip by heating it during at least of a min; ute ata higher temperature than maintained in the first heating step within the range from 15l0- to 1700 F, while subjecting the whole surface of said strip to a continuous flow of an inert atmosphere containing at least 90% of nitrogen to carry away occ1uded gases given ofl by said strip to said atmosphere, and while the said strip is wholly protected by. an inert'at mosphere consisting predominantly of nitrogen, cooling it to a temperature not exceeding the temperature of the molten bath of the coating metal and then passing it into said bath.
6. A process of coating ferrous bodies with aluminum which consists in heating the said bodies to a temperature within the range 1470"- 1670 F. in the presence of a reducing atmosphere consisting predominantly of hydrogen and containing 2-30% of water vapor, then expelling occluded active gases from said bodies by heating them to at least the temperature at-. tained in said first heating step while in direct contact with an inert atmosphere consisting predominantly of nitrogen, and while maintaining said bodies wholly protected by an inert atmosphere consisting predominantly of nitrogen and substantially free from water vapor, cooling them to a temperature not exceeding the temperature of the molten aluminum coating bath and then passing them into said bath.
'7. A process according to claim 6 in which said bodies, after they have been passed into the molten aluminum bath, are withdrawn therefrom and cooled gradually for at least 2 to 5 minutes to a temperature within the range 400-700 F., and thereafter cooled to room temperature.
8. A process according to claim 6 in which said bodies, upon cooling to a temperature not exceeding the temperature of the aluminum coating bath and while protected by the inert atmosphere provided during said cooling step, are passed beneath the surface of a molten aluminum coating bath which floats upon the surface and at the entrance side of a molten lead bath, are then passed through said aluminum coating bath into said molten lead bath, and finally are withdrawn from said lead bath at a. point thereof which is free of aluminum.
9. A continuous process of coating ferrous metals in strip form with aluminum which comprises passing a ferrous metal strip into and through a first heating zone wherein it is maintained in direct contact with an atmosphere consisting predominantly of hydrogen and containing from 2 to 30% water vapor, and. during the travel of said strip through said first heating zone, heating it to a temperature within the range 1470-1670 F., then leading said strip to and through a second heating zone wherein the strip is maintained at a temperature at least as high as that attained by it in the first heating zone, and during the course of its travel through said-second heating'zone subjecting saidstrip to' directcontact with an inert atmosphere consisting predominantly of nitrogen; causing said ine'rt atmosphere to flow'over the surfaces of said strip and carry away occluded gases given off from said strip in its passage through-said second heating zone, and while said strip remains wholly protected by an inert atmosphere con-'- sistin-g predominantly of nitrogen and substantially free from Water vapor, passing-it through a cooling zoneto a molten metal bath comprising-a bodyof molten'aluminum', and so regulating the temperature and the time of travel of saidstrip through said cooling zone as to reduce the temperature of said strip to'a value not exceeding the-temperature of the molten aluminum at'the'point where it enters t e body of molten aluminum.
l0. The-process as setjorth in claim 9 wherein the-strip, after'itis passed into the-molten aluminum coating bath is-withdrawn there- 1'2 fromiand cooled, and whilesitlisvcooling within the temperature range betweenthe solidification temperature of'aluminum and 300 R, is isubjected to a light pass between: rolls. suffioient to moderately densify the aluminum acoating' but insuflficient 'to efiect any'reduction of the thickness of the ferrous base strip.
The following references are of record in the file of this patent:
UNITED, STATES" PATENTS