US3503775A - Method of preparing metal coated metallic substrates - Google Patents

Description

United States Patent 3,503,775 METHOD OF PREPARING METAL COATED METALLIC SUBSTRATES Lowell W. Austin, Weirton, W. Va., assignor to National Steel Corporation, a corporation of Delaware No Drawing. Filed Apr. 12, 1966, Ser. No. 545,788 Int. Cl. B05b 5/02; B44d 1/092, N44

US. Cl. 117-17 18 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a novel method of eleetrostatically depositing finely divided coating metal on metallic substrates, and to a method of compacting and heat treating or fusing the resulting particulate metal coatings. The invention further relates to the metal coated products thus produced.

Finely divided nonmetallic coating materials which are not good conductors of electricity have been successfully electrostatically deposited heretofore on a variety of substrates. For instance, a mixture of powdered glue of the remoistenable type and a thermoplastic binder has been electrostatically deposited upon paper in producing the gummed paper products of commerce, and refractory oxide annealing separators have been electrostatically deposited on metallic sheet or strip.

When preparing the above mentioned products, it was not necessary to precoat the surface of the substrate with a binder for the purposes of causing the particles of coating material to adhere and assuring that the resulting particulate coating was uniform and had adequate green strength. However, when an effort was made to electrostatically deposit an electrically conductive coating metal under prior art conditions on the surface of an electrically conductive metallic substrate, the resulting particulate metal coating was not uniform and did not have sufficient green strength to allow handling prior to compacting and heat treating or fusing.

The inherently low green strength characteristic of the prior art metallic coatings electrostatically deposited on metallic substrates has been overcome heretofore by precoating the surface with a tacky binder. The tacky binder serves to anchor the finely divided metallic particles on the surface of the substrate and to thereby improve the green strength of the particulate coating sufficiently to allow handling prior to compacting. However, it is neces sary to remove the binder prior to compacting the metal particles and this is a very difiicult task. At least traces of the binder tend to remain on the substrate surface or on the surfaces of the metal particles and the residual binder contaminates the compacted metal coating.

It has also been proposed to increase the green strength of particulate coatings electrostatically deposited on a dry uncoated substrate by using very finely divided metal particles having a mean mass particle size not greater than 15 microns, and preferably not greater than about 5 microns. However, while this method is very effective for producing thin metal coatings and has met with surprising technical success, the present cost of the extremely finely divided metal powders that are required is too high for an economic commercial process which must be com- 3,503,775 Patented Mar. 31, 1970 ice petitive with other coating methods such as electrodeposition. Also, it is difficult to produce such finely divided metal powders in the large quantities required for high volume commercial production.

It has been discovered unexpectedly that the surface of a metallic substrate may be coated with a thin film of water, and thereafter particles of a coating metal may be electrostatically deposited upon the resulting surface, which is free of an oily or tacky binder, in the form of a uniform particulate metal coating which has sufiicient green strength to enable the coated substrate to be handled. It has been further discovered that low cost relatively coarse metal particles which are commercially available in large quantities may be electrostatically deposited on metallic substrates in the absence of a tacky binder, and in the form of heavy or light coatings of a controlled desired thickness. Surprisingly, it has been still further discovered that the water film does not adversely affect the substrate nor the coating metal particles in any way, and that the water film may be completely removed without any difficulty. Thus, for the first time it is not necessary to apply a tacky foreign material which is difiicult to remove from the surface of the substrate, or to use high cost extremely finely divided metal powders.

It is an object of the present invention to provide a novel method of electrostatically depositing a finely divided coating metal on a metallic substrate having a surface which is substantially free of a tacky binder, in the form of a particulate metal coating Which has sulficient green strength to allow handling prior to compacting.

It is a further object to provide a novel method of compacting and heat treating or fusing the particulate metal coatings of the invention to form a substantially continuous coherent and adherent layer of the coating metal on the substrate.

It is still a further object to provide the improved metal coated metallic substrates produced in accordance with the invention.

Still other objects and advantages of the invention will be apparent to those skilled in the art upon reference to the following detailed description and the examples.

In practicing the present invention, a film of water is applied to the surface of a metallic substrate, and thereafter a particulate coating of a coating metal is electrostatically deposited by introducing a dry gaseous suspension of electrically charged particles of the coating metal adjacent to the surface of the substrate while it is positioned in an electrostatic field which directs the charged particles to the substrate.

The substrate surface should be uniformly coated with a thin film of liquid water at the time of introducing the gaseous suspension of metal particles and electrostatically depositing the particulate coating. The Water should be present in an amount sufficient to form a substantially continuous film of liquid over the surface area to be coated with the metal particles, and in an amount insuflicient to form pools or streams of liquid which collect on the surface and cause uneven or nonuniform coating. The substrate surface should be free of oil, grease and other substances which are not wetted by water in order to form a more uniform film. In accordance with one preferred procedure, the substrate surface is washed free of surface contaminants, the cleaned surface is rinsed with excess fresh water, and then the excess water is removed and a thin film of liquid water is metered onto the surface by passing the substrate between rubber wringer rolls. The water film also may be applied to a substrate surface which is dry initially by contacting it with wet brushes, wet sponges or the like, or by spraying on a controlled amount of water. In some instances, better results may be obtained if the water used in coating the substrate contains a small but effective amount of a prior art surfactant or wetting agent to reduce the surface tension of the water and assure more uniform coating of the surface.

The particles of the coating metal are sufliciently small to be readily suspended in a stream of air or inert gas and deposited on the surface of the substrate in the form of a particulate metal coating having suflicient temporary adhesion and cohesion to allow handling prior to compacting or fusing.

For better results, the particles of the coating metal should have a mean mass particle size not greater than about 50 microns, and preferably not greater than about 30 microns, in the maximum dimension. The lower limit on the particle size is largely practical in nature, and particles having a mean mass particle size of 0.5-1 micron or smaller may be used. Metal particles having a mean mass particle size not greater than about 20 microns, such as about 20 microns, produce a particulate coating which is very uniform and which has excellent adhesion and cohesion in the green state. Metal particles having a mean mass particle size greater than microns and not more than about 30 microns, such as -25 microns, produce good results and are often lower in cost. Regardless of the particle size selected, it is possible to produce more uniform and/or heavier particulate coatings of a desired controlled thickness in accordance with the method of the invention than when using a metallic substrate which is not coated with a film of water. Also, metal particles having a much larger particle size may be used and still obtain adequate green strength in the particulate coating.

A wide variety of coating metals are satisfactory, pro vided they are in the form of small particles within the range set out herein. Examples of metals include alumi num, antimony, cadmium, chromium, cobalt, copper, gold, iridium, iron, lead, magnesium, manganese, molybdenum, nickel, niobium, osmium, palladium, platinum, tantalum, tin, titanium, tungsten, vanadium, zinc and zirconium. Additionally, alloys including one or more of the foregoing metals may be used. The alloys may include brass, bronze, stainless steel, Monel, high chromium ferrous alloys in general such as an alloy containing 70% chromium and 30% iron, zinc-iron alloys such as an alloy containing 70% zinc and 30% iron, and aluminummanganese alloys, such as an alloy containing 10-70% manganese and the remainder aluminum together with incidental impurities. An alloy coating may be formed in situ on the substrate by applying a mixture of two or more of the metal powders in amounts to form the final alloy, preferably followed by heat treating and compacting and heat treating or fusing, or the alloy per se may be preformed, subdivided to the desired particle size, and applied to the substrate to form an alloy coating in particulate form, followed by compacting and heat treating or fusing.

Presently preferred coating metals include aluminum, alloys of aluminum such as the aluminum-manganese alloy mentioned above, copper, nickel, stainless steel, chromium and chromium-iron alloys, zinc and zinciron alloys. Aluminum coatings are especially preferred for some purposes, such as in the manufacture of container stock.

The metal particles may be substantially spheroidal to irregular in shape, such as particles having the appearance of spheres or grains of sand under a microscope, or in the form of finely divided plates. The spheroidal or irregularly shaped particles may be formed by grinding the desired metal in massive form, atomization of molten metal, or by other well known methods. The plate-like particles may be formed by disintegrating thin metal foil, or by rolling spheroidal or irregularly shaped particles.

The substrate may be any suitable metal, but it is preferably steel or other ferrous metal. Steel sheet or strip of indefinite length is usually preferred for commercial operations, as it is possible to carry out the invention in a continuous or substantially continuous manner.

The substrate is in an electrostatic field which is applied in a manner to drive the charge particles of coating metal to the surface of the substrate. The substrate may have an electrical charge on its surface which is preferably opposite in polarity to that of the particles of the coating metal at the time of introducing the gaseous suspension thereof adjacent the surface. The specific manner in which the gaseous suspension of electrostatically charged particles is produced, as well as the specific manner in which the electrostatic field is produced, are not of importance to the present invention. Any suitable prior art apparatus and method may be employed, including those disclosed in US. Patent No. 3,090,353.

In operating one type of apparatus, a metallic substrate which may be ferrous metal strip having a thin continuous film of liquid Water on its surface is passed throughan electrostatic deposition zone in a continuous manner. A rapidly flowing stream of air, inert gas, reducing gas, or other suitable gaseous medium containing particles of the coating metal is passed into the electrostatic deposition zone and between the substrate and a plurality of electrodes in the form of wires or sharp points spaced from the strip and extending transversely to the direction of movement of the strip. The strip is electrically grounded, and a high voltage positive or negative potential is applied to the electrodes. A corona discharge having the same polarity as the applied high voltage potential is thereby caused to take place about the electrodes. The gas surrounding the electrodes is ionized, and the spacing of the electrodes is such that in the region between the high voltage electrode and the strip there is a predominance of electrically charged ions of one polarity. The dispersed particles of the coating metal become electrically charged by ion bombardment, and then the electrostatic field acts upon the charged particles to propel them toward the strip. The particles of the coating metal are deposited on the strip surface in the form of a uniform particulate metal coating. The particles lose their electrical charges upon contact with the strip surface, and the particles are then held on the strip surface by molecular or dispersion forces sometimes called Van der Waals forces.

The green particulate metal coating has sufiicient temporary adhesion and cohesion to allow handling. However, it should be treated to form a continuous layer of the coating metal which is permanently coherent and adherent to provide a satisfactory article of commerce.

One method of treating the green particulate coating includes a preheat treatment, compacting the heat treated coating by rolling under pressure to form a continuous layer of the coating metal, and then subjecting the compacted metal coating to an elevated temperature at which a coherent and tightly adherent substantially continuous layer of the coating metal is produced. The exact time and temperature for use in the preheat treatment will vary depending upon the selected coating metal. It is only necessary that the preheat treatment be conducted below the melting point of the coating metal and at a sufficiently elevated temperature and over a sufiiciently long period of time to cause the particles of the coating metal to agglomerate or cohere and form a weak bond with the substrate, and prevent the metal particles from becoming dislodged or sticking to the rolls during the rolling step. When aluminum is the coating metal, a temperature of about 2001050 F. and preferably 400 600 F. may be used for the preheat treatment and the heat treatment may take place in air. The period of preheat treatment may vary over wide ranges, such as from about 15 seconds to 30 minutes or longer, and preferably about 1-5 minutes at 500-600 F. Temperatures and times for the preheat treatment of aluminum-manganese or other aluminum alloys, zinc or zinc-iron alloys, and other nonrefractory metals having similar properties :may be approximately the same as for aluminum. Metals having a high melting point, e. nickel, chromium and stainless steel may require nonoxidizing or reducing atmospheres and higher preheat treatment temperatures and longer periods of time, such as heating at about 800- 1700 F. and preferably about l100-l300 F. over about 15 seconds-2 minutes to 5-60 minutes. As a general rule, within the above ranges, shorter periods of preheat treatment are preferred for the higher temperatures, and longer periods at the lower temperatures.

The preheat treated coated substrate may be passed between pressure rolls and rolled under sufficient pressure to produce a substantially continuous layer of the coating metal. The rolling step may be conducted at normal room temperature, or at an elevated temperature such as the temperature of the preheat or postheat treatment. The rolling pressure is sufficient to compact the particulate metal coating and may be, for example, 2-10 tons per inch of width of substrate. When desired, the rolling step may be conducted under pressure conditions so that there is some reduction in the thickness, such as /2-5%. Also, rolling may be used so as to generate heat for the postheat treatment, as well as to compact the metal coating and/ or cause some reduction in the thickness of the substrate.

The rolled substrate may be subjected to an elevated temperature at which a coherent and highly adherent substantially continuous layer of the coating metal is produced. The temperature and time of postheat treatment will vary somewhat from metal to metal, but in general an elevated temperature and period of time are satisfactory which cause the layer of the coating metal to adhere tightly to the substrate surface, and which also cause the metal particles to bond together and produce a coherent coating. When the coating metal is aluminum, a satisfactory postheat treating temperature and period of heating is about 850-1050 F. for about 15 seconds or longer, and preferably about 900l000 F. over 1-5 minutes, but a longer period of time may be used such as from 30 minutes to 2 hours. Postheat treating conditions similar to aluminum are satisfactory for aluminum-manganese or other aluminum alloys, but often a lower temperature such as 500-600" F. is useful for zinc and zinc alloys. When nickel, chromium, stainless steel, or other coating metals having high melting points are used, then nonoxidizing or reducing atmospheres and higher postheat treatment temperatures are necessary such as about 800-l700 F. and preferably about l1001300 F., and the period of heat treatment may be about 15 seconds-30 minutes to 1-12 hours or longer. As a general rule, the postheat treatment may be conducted at a temperature which is about two-thirds of that of the melting point of the metal, and the period of heat treatment is extended until the desired degree of cohesion and adhesion is obtained.

Another preferred method of treating the particulate metal coating is by fusion of the metal particles to produce a coherent and adherent continuous metal coating. This method is especially effective when the coating metal has a substantially lower melting point than the metal substrate, such as when a ferrous metal substrate is coated with tin, zinc or lead particles. The metal coating on the substrate may be merely heated above its melting point to fuse the particles, and then the resulting molten coating is quenched to solidify the coating and produce a metal substrate having a coherent and adherent substantially continuous layer of the coating metal.

Irregularly shaped metal particles produce a brighter compacted coating and are preferred where brightness in the compacted coating is desirable. The metal particles should be dry and free of dirt, grease and tacky binders, and need not be pretreated to improve adherence or coherence, or the green strength of the particulate coating.

The metal particles are applied to the substrate in an amount to provide a final compacted and heat treated or fused coating of a desired thickness. For example, the final coating thickness may be 0.05-1.5 mils and preferably about 0.1-1 mil. It is also possible to apply more than one metal coating to a given substrate. For instance, a first layer of one of the coating metals disclosed herein may be applied to a ferrous metal substrate, followed by application of a second layer which may be different coating metal. The composite article thus produced may be used as such, or it may be heat treated so as to cause the various layers of coating metals to diffuse and form an alloy layer. It is also possible to apply an organic protective coating of paint, varnish, lacquer, or the like over the compacted and heat treated or fused metal coatings produced in accordance with the invention.

It has also been discovered that the quality of aluminum coatings formed on ferrous metal substrates may be greatly improved by preheating the green particulate coatings at 400-600 F., and preferably by preheating at 500600 F. in an elemental oxygen-containing atrnosphere such as air until a thin layer of a light yellow or straw colored oxide is formed on the surface of the ferrous metal substrate. The temperautre of heating should not be sufficiently high to form an oxide coating having a bluish to black color, and the time of heating should not be extended substantially past the time at which the water film is removed and a very thin light colored oxide layer is formed. Surprisingly, it has been found that preheating the green aluminum coatings at a temperature and time effective to form such light colored oxides also greatly reduces or even prevents picking or removal of the particulate coating by the rolls during compacting by rolling, and aids in forming an adherent and coherent substantially continuous layer of the coating metal. In instances where the rolled continuous aluminum coating tends to blister, the blistering may be controlled by rolling the agglomerated particulate coating at an elevated temperature such as 400 F. or higher, and preferably at about 400-600 F.

For best results, the temperature of the rolled continuous coating should not be raised rapidly during the final heat treatment. Usually, it is preferred that the rolled coating be conditioned by heat treating at about 400-600 F. for a short period of time such as l-15 minutes, or until the occluded gases have been expelled and temperature equilibrium is reached, and then the substrate may be heat treated at a much higher temperature such as 800-1700" F. without damage to the substantially continuous coating or loss of adherency,

Surprisingly, the film of liquid water does not adversely affect the substrate surface nor the coating metal particals which are deposited thereon. This is true even when the substrate is ferrous metal and the substrate is heat treated in air. The oxide which forms on a ferrous metal surface when it is treated with water and/or heated to an elevated temperature in air usually adversely affects the bond between the coating metal and the surface. Nevertheless, this does not occur when practicing the present invention. In fact, the oxide often has an advantageous effect, such as when the water film is removed during a preheat treatment of a ferrous metal substrate at 400-600 F. prior to rolling or compacting the green coating. The water film may be completely removed during this preheat treatment and a residue of water does not remain on the strip surface or in the final coating.

The present invention is especially useful for coating unannealed cold rolled ferrous metal substrates as the annealing step which is normally practiced may be incorporated with the heat treatment steps of the invention. For example, large coils of unannealed cold rolled steel may be treated by a continuous prior art method to remove oil, grease and other contaminants from the moving steel surface, the cleaned steel surface is washed with fresh water and a uniform thin film of liquid water is continuously applied or metered onto the surface, and thereafter a desired coating metal is continuously electrostatically deposited on the moving wet, clean steel surface. The resulting green particulate coating may be given a preheat treatment as described herein to agglomerate the metal particles, remove the water film and form a thin light colored oxide, and is then compacted by passing the coated steel between pressure rolls where, if desired, a reduction in the substrate thickness may take place and a continuous layer of the coating metal is formed. The resulting unannealed, coated steel having a continuous layer of the coating metal thereon is then annealed in a reducing atmosphere following prior art practice, such as by box annealing or by passing the coated steel in the form of strip through a prior art continuous annealing line. Thus, the coherency and adherency of the coating layer may be improved and the steel substrate may be annealed in a single heat treatment. This combined coating and annealing process is especially useful in the application of coating metals having high melting points to unannealed cold rolled steel strip, such as aluminum, chromium, manganese, molybdenum, nickel, titanium, vanadium, and alloys thereof including, for example, stainless steel and high chromium-iron alloys. High melting point coating metals usually require a reducing atmosphere for the final heat treatment of the compacted coating, and as a reducing atmosphere is present in the prior art annealing processes, the heat treatment and atmosphere of the annealing step are performing two functions simultaneously and Without added cost.

Still further improved results may be obtained by application of the coating metal by electrostatic deposition on a continuously moving substrate of indefinite length, such as ferrous metal strip, followed by continuous preheat treatment of the particulate coating immediately thereafter, and continuous compacting of the heat treated metal coating by passing the strip between pressure rolls. Preferably, the foregoing steps are performed in line and without passing the strip around a guide roll to prevent any possibility of damage to the coating. The resulting-coating is more uniform, and is more adherent and free from defects than when the steps are not performed continuously and in sequence. The coating metal may be applied to one side of the substrate surface only or on both sides, as desired. This is of importance in preparing certain products of commerce, such as ferrous metal sheets coated on one side only with zinc, where the opposite or uncoated side of the sheets is to be provided with a different type of protective or decorative finish.

The foregoing detailed description and the following specific example are for purposes of illustration only, and are not intended as being limiting to the spirit or scope of the appended claims.

EXAMPLE i This example illustrates the present invention when using a continuous electrostatic coating line for coating steel strip.

An annealed blackplate strip of tinplate gauge and quality is electrolytically cleaned in an aqueous alkaline solution of known type following prior art practices, rinsed in fresh water to remove the cleaning solution, pickled in aqueous sulfuric acid, rinsed in fresh Water to remove the excess pickle liquor, dried and coiled.

The clean and pickled backplate strip is passed between a series of fresh water sprays arranged to apply a small controlled amount of fresh water to the strip surfaces, and the strip is passed between rubber wringer rolls which uniformly distribute the water over the surface area of the strip in the form of a thin, uniform film. The rubber wringer rolls also serve to remove any excess water. The resulting strip is immediately passed through a horizontal electrostatic deposition zone and a particulate metal coating is electrostatically deposited on the strip surface in the presence of the film of water.

A plurality of charging wires are arranged in the electrostatic deposition zone to extend across the width of the strip. The charging wires are spaced four inches apart with respect to the direction of the strip movement, and two inches above the strip surface. A total of eight stainless steel charging wires having a diameter of 0.005 inch are used. The ends of the charging wires are attached to end supports, and the wires are electrically connected to a high voltage source.

An air-blower and powder feeder assembly is used to generate a cloud or gaseous suspension of the metal powder for feeding to the electrostatic deposition zone. The powder feeder is used to meter the metal powder into the blower, and the powder is suspended in a stream of air and the gaseous suspension is blown through a conduit into the electrostatic deposition zone. The terminal end of the conduit has a nozzle thereon provided with a slotted outlet arranged to distribute the suspension of powder across the width of the strip, and to introduce the suspension between the charging wires and the upper surface of the horizontally moving strip. The nozzle is arranged to feed the suspension longitudinally along the strip and in the same direction as the strip is moving. The strip is provided with an electrical ground. A negative potential of 14-15 kilovolts is applied to the charging wires to produce ions for charging the particles of the metal powder.

The speed of the blower is regulated to produce an air suspension from the metal powder fed thereto, and the air suspension is then introduced between the charging wires and the upper surface of the moving strip where the suspended metal particles are charged electrically. The charged metal particles are directed downward toward the moving strip surface by the lines of force of the electrostatic field, and deposited thereon in the form of a uniform particulate metal coating. r

The coated strip emerging from the electrostatic deposition zone is passed horizontally through an infrared oven and preheat treated as noted in the table. The preheat treatment removes the water film by evaporation and agglomerates the metal particles sufliciently to cause the particles to cohere and form a weak bond with the strip surface. The infrared oven is provided with an air atmosphere, or a reducing atmosphere (H and N as noted in the table.

The strip having the agglomerated particulate metal coating on its upper surface is passed horizontally to a to a rolling mill and immediately rolled under a pressure of approximately 4 tons per inch of width to compact the particulate coating and effect a reduction of the strip of about 1-2%. The strip emerging from the rolling mill has a substantially continuous uniform layer of the coating metal thereon, and it is then postheat treated at the temperature and time noted in the table to assure that a coherent and tightly adherent coating is produced. An air atmosphere or reducing atmosphere (H and N is maintained during the postheat treatment, as noted in the table.

The metal coating on the resultant strip is tightly adherent as shown by the Erichsen cup test. Also, the coating is substantially continuous and nonporous and provides good corrosion resistance when the coated side of the strip is subjected to corrosion tests.

A number of runs compared the various metal powders noted in the table. The metal powders have mean mass particle sizes varying between about 5 microns and about 29 microns, and uniform coatings of the particulate metal could be obtained in each instance which had sufiicient green strength to allow subsequent handling. However, when the water film is omitted, with all other conditions remaining the same, it is impossible to produce uniform coatings having adequate green strength when using powders having a mean mass particle size greater than 15 microns. The water fihn also aids greatly in the electrostatic deposition of the metal powders having mean mass particle sizes less than 15 microns as there is less shifting of the powder and the yield of the metal powder in the resulting coating is much greater. It is also possible to control the thickness of the applied coating more effectively, and heavier coatings may be produced.

The metal powders and the treating conditions are given below in the table. Very satisfactory coatings were 6. The method of claim 1 wherein the coating metal comprises an alloy selected from the group consisting of brass, bronze, Monel, stainless steel, zinc-iron alloys, aluminum-manganese alloys and chromium-iron alloys.

7. The method of claim 1 wherein the coating metal is prepared in each instance. 5 aluminum, the substrate is heat treated at a temperature of TABLE Preheat treatment Postheat treatment Particle size Temperature, Temperature, Metal (microns) F. Time (min.) Atmosphere F. Time (min) Atmosphere 5.3 500-000 1-3 oo-1,000 30 Air. 6.4 5004100 1-3 oo-1,000 30 Air. 7.0 500-000 1-3 oo-1,000 30 Air. 13.9 500-000 1-3 900-1,000 30 Air. 14.3 500-000 1-3 900-1,000 30 Air. 18.0 500-000 1.3 900-1,000 50 Air. 20.0 500000 1-3 900-1,000 30 Air. 21.0 500-000 1-3 900-1,000 30 Air. 25.0 500000 1-3 900-1,000 30 Air. 29.0 500-000 1-3 goo-1,000 30 Air.

9.0 1,300 1-2 1, 1-2 Hz-i-N: 13.5 1, 300 1-2 1, 000-1, 700 1 12 Hz+Nz 30.0 1, 300 1-2 1, 000-1, 700 12 H2+NZ 13.0 1, 300 1-2 Hr+N2 1, 600-1, 700 12 H2+N2 Hours.

What is claimed is: about 400-1050 F. to cause the aluminum particles in 1. A method of coating a ferrous metal substrate with the particulate coating to agglomerate, and the substrate acoating metal comprising the steps of is heat treated at a temperature of about 400-1050 F. applying a film of water on the surface area of the after compacting the agglomerated particulate coating to ferrous metal substrate to be coated with the coating produce a coherent and adherent substantially continuous metal, the surface of the ferrous metal substrate belayer of the coating metal. ing free of an applied coating of a tacky binder and 8. The method of claim 1 wherein the coating metal is the water being present in an amount suflicient to selected from the group consisting of nickel, chromium wet the surface area and form a thin substantially and stainless steel, the substrate is heat treated at a temif film of ter, perature of about 800-1700" F. in a nonoxidizing atmosintroducing a gaseous suspension of dry electrically phere to cause the metal particles in the particulate coatcharged particles of at least one coating metal ading to agglomerate, and the substrate is heat treated at jacent to the surface of the ferrous metal substrate a temperature of about 800-1700 F. in a nonoxidizing having the film of water thereon, the particles of atmosphere after compacting the agglomerated particulate the coating metal having a mean mass particle size coating to produce a coherent and adherent substantially not greater than about 50 microns, continuous layer of the coating metal. the ferrous metal substrate being in an electrostatic 9. The method of claim 1 wherein the ferrous metal field applied in a manner whereby the electrically substrate initially is cold reduced unannealed blackplate, charged coating metal particles are driven toward the and subsequent to rolling the particulate metal coating the wet surface of the substrate and are deposited th blackplate having the substantially continuous layer of on in the form of a particulate metal coating, coating metal thereon is heat treated at an elevated anheatihg the ferrous metal Substrate having the Water fi ncaling temperature to anneal the blackplate and produce and particulate metal coating thereon at an elevated a tightly adherent layer of the coating metal. temperature to evaporate the Water film and ProduCe 10. The method of claim 1 wherein the ferrous metal a dried particulate metal coated subst ate, substrate is ferrous metal strip, the strip is passed conagglomerathlg the coating metal Particles y subjecting tinuously through an electrostatic deposition zone and the the dried particulate metal coated su s t an gparticulate metal coating is electrostatically deposited slornerating temperature of at least but less thereon, the coated strip is passed through a heat treating than the melting Point of the coating metal until zone to agglomerate the metal particles, and the heat t Coating metal PertieleS have agglomerated, treated strip having the agglomerated particulate metal thereafter Compacting the agglomerated Particulate coating thereon is rolled under pressure, the foregoing metal coating y rolling the Coated Substrate under steps of electrostatic deposition, heat treating to agglomerpressure to produce a substantial continuous layer ate the metal particles, and ll being rfo ed of the coating metal, and tinuously and sequentially on portions of the continuously subjecting the rolled substrate to an elevated temperamoving t i ture at which the substantially continuous layer of 11. The method of claim 10 wherein the ferrous metal coating metal adheres to the substrate but less than strip is initially cold reduced unannealed blackplate strip, the melting point of the coating metal until a coand subsequent to rolling the particulate metal coating the herent and adherent substantially continuous layer of llnannealed bla kplate strip having the substantially conthe coating metal is produced, tinuous layer of coating metal thereon is passed through The method of Claim 1 wherein the coating metal acontinuous annealing line to thereby continuously anneal comprises at least one metal selected from the group conthe blaFkplate strlP and Produce a tlghtly adherent layer sisting of aluminum, chromium, copper, nickel and zinc. o of coatmg metal- 3. The method of claim 1 wherein the coating metal is The method of clalm 1 Wherem the coatmg P 15 aluminum. alurtnifipm, and the ferrous mlftal subsgat: thavtmdg tthe par 1c a e a uminum coa ing ereon 1s ea rea e a a method of clalm 1 wherem h parades of the temperature of about 400-600" F. in an oxidizing atmoscoatmg metal have a mean mass particle size of about Phere containing elemental oxygen to agglomerate the 1040' mlcronsaluminum particles and produce a thin layer of light The method of dam 1 Wherelh the coatlhg metal 15 colored iron oxide on the surface of the substrate under aluminum, and the particles of the coating metal have a the agglomerated l i i 1 mean mass particle size greater than 15 microns but not 13, Th th d f l i 12 h i th b t t i more than 30 microns. heat treated at a temperature of about 400-600 F. after rolling to condition the compacted aluminum coating, and thereafter the substrate is heat treated at a temperature of at least 800 F. to produce a tightly adherent layer of aluminum.

1-4. The method of claim 12 wherein the substrate having the particulate aluminum coating thereon is rolled at a temperature of at least 400 F. to prevent blistering of the resulting substantially continuous layer of aluminum.

15. The method of claim 12 wherein the substrate is rolled at a temperature of about 400-600 F., the substrate having the compacted aluminum coating thereon is conditioned by heat treating it at a temperature of about 400 600 F. after rolling, and then the substrate is heat treated at a temperature of about 8501050 F. to tightly adhere the substantially continuous layer of aluminum to the substrate.

16. The method of claim 12 wherein the ferrous metal substrate having the particulate aluminum coating thereon is cold reduced unannealed blackplate, and subsequent to rolling the particulate aluminum coating the blackplate having the substantially continuous layer of --aluminum thereon is heat treated at an elevated annealing temperature to anneal the blackplate and produce a tightly adherent layer of aluminum.

17. The method of claim 12 wherein the substrate is frerous metal strip, the strip is passed continuously through an electrostatic deposition zone and the particulate aluminum coating is electrostatically deposited thereon, the coated stripis passed through a heat treating zone to heat treat the strip at the said temperature of about 400600 F., and the heat treated strip is rolled under pressure, the foregoing steps of electrostatic deposition, heat treating at 400-600 F. and rolling being performed continuously and sequentially on portions of the continuously moving strip.

18. The method of claim 17 wherein the ferrous metal strip is cold reduced unannealed black-plate strip, and subsequent to rolling the particulate aluminum coating the unannealed blackplate strip having the substantially continuous layer of aluminum thereon is passed through a continuous annealing line to thereby continuously anneal the blackplate strip and produce a tightly adherent layer of aluminum.

References Cited UNITED STATES PATENTS 1,197,695" 9/1916 Watkins 117-22X 1,922,254 8/1933 McCulloch 117-22 X 2,990,293 6/1961 Toulmin 117-93.4 X 3,019,126 1/1962 Bartholomew 117-17 3,197,324 7/1965 Brooks 117-21 3,248,253 4/1966 Bardford et a1. 117-17 3,323,933 6/1967 Bardford et a1. l17-17 3,327,948 6/1967 Gignoux 11793.4 X 3,336,903 8/1967 Point 11717 X 3,382,085 5/1968 Wren et al.- 11793.4X

WILLIAM D. MARTIN, Primary Examiner EDWARD J. CABIC, Assistant Examiner US. (:1. X.R. 117 22, 31, 50, 65.2, 131