US3259562A - Method of adhering an electrophoretically deposited metal coating to a metal substrate - Google Patents

Description

July 5, 1966 w. SALT T 3,259,562

METHOD OF ADHERING AN ELECTROPHORETICALLY DEPOSITED METAL COATING TO A METAL SUBSTRATE Filed Oct. 12, 1964 4 Sheets-Sheet l \NVENTORS F.W. SALT BY K.G. LEwls wwmmv BAH-Lois ATTORNEY y 1966 F. w. SALT ETAL 3,259,562

METHOD OF ADHERING AN ELECTROPHORETICALLY DEPOSITED METAL COATING TO A METAL SUBSTRATE Filed Oct. 12, 1964 4 Sheets-Sheet 2 ATTORNEYS 'July 5, 1966 F. w. SALT ETAL 3,2 9,562

METHOD OF ADHERING AN ELECTROPHORETICALLY DEPOSITED METAL COATING TO A METAL SUBSTRATE Filed Oct. 12, 1.964 4 Sheets-Sheet 5 lNvEN-roRs F.W. SALT BY K 6. Laws k k Nukmh am swlw ls ATTORNEYS METAL COATING TO A METAL SUBSTRATE 4 Sheets-Sheet 4 Filed Oct. 12, 1964 828mm zGxE $3 3 3 M M 4 r k Q N w W w i w w G. k 5 a E R 1 m o? w w l w SY 3O m 9 United States Patent 3,259,562 METHOD OF ADHERING AN ELECTROPHORETI- CALLY DEPOSITED METAL COATING TO A METAL SUBSTRATE Frederick William Salt, Swansea, Glamorgan, and Kenneth Gill Lewis, Port Talbot, Glamorgan, Wales, assignors to The British Iron and Steel Research Association Filed Oct. 12, 1964, Ser. No. 403,408 Claims priority, application Great Britain, May 22, 1959, 17,501/59 Claims. (Cl. 204-181) This application is a continuation-in-part of our prior application Serial No. 30,316, filed May '19, 1960, now abandoned.

This invention is concerned with a method of forming a metal coating on a metallic substrate and, more particularly, on metallic substrates in elongated form, that is a form which can be subjected to rolling, such as sheet, strip, wire or rod.

By coating a base metal with another metal, it is frequently possible to obtain an advantageous combination of properties, for example various types of steel can be coated with metals which have a better corrosion and/ or heat resistance than steel, while .the composite material retains the advantageous mechanical and metallurgical properties of the steel. Where such a combination of properties can be obtained by the addition of alloying elements to the base steel, it is often the case that the alloy steel is more expensive to produce than a coated steel having the same properties.

Among the coating metals which can impart useful properties to steel substrates are, for example, aluminium, nickel, zinc, copper, brass, stainless steel, cadmium, titanium and zirconium.

Aluminium coated steel is a very useful material for applications where strength coupled with corrosion and heat resistant properties are required. The self-healing oxide film of the aluminium ensures good resistance to corrosion and this coating, under certain conditions, particularly in sulphurous industrial atmospheres, provides superior protection to that obtained with a zinc coating. The oxide film together with the iron-aluminum alloy formed on heating, provide excellent resistance to scaling at high temperatures. The composite material thus provides a perfectly satisfactory substitute for an expensive alloy steel in some applications.

Aluminium may be applied to steel by a variety of methods, the principal methods available being spraying, cladding, vacuum deposition, vapour plating, electro deposition or hot dipping. The processes of spraying and cladding are well known, the former resulting in a rather porous deposit lacking in ductility; vacuum deposition has been used to produce extremely thin and highly refleeting surfaces for particular applications, and thermal deposition of such compounds as tri-isobutyl aluminium has been used to produce deposits 0.005 inch thick. It is not possible to deposit aluminium electrolytically from an aqueous bath and although various organic plating baths have been described, this method is not economic on an industrial scale.

Hot-dipping processes are used on a commercial scale; hot dipped aluminium-coated steel, by reason of the temperature involved in its production, always has a layer of iron-aluminium alloy at the interface between the steel substrate and the coating and while this layer provides 3,259,562 Patented July 5, 1966 the composite material with good heat resistance, its brittleness limits the useful applications of the material, when, for example it is desired to take advantage of the aluminium coating mainly for its corrosion resistance properties. Cladding essentially involves rolling an aluminium foil onto the steel substrate in a plurality of passes so as to obtain a heavy reduction, this being required to obtain good bonding between the substrate and the coating metal.

Such heavy reduction hardens the substrate and makes it unformable and if a formable material is required, the composite material must then be annealed and this may give rise to the formation of an interlaye'r of iron-aluminium alloy as mentioned above.

Of the various coating metals other than aluminium mentioned above, only nickel and copper can be conveniently deposited by electrolysis. In the case of the other coating metals mentioned, the only methods which are suitable for the production of tonnage quantities of coated material having coatings some thousandths of an inch thick are cladding and hot-clipping. Zinc and cadmium coatings can be .formed on steel by bot-dipping and brass, stainless steel, titanium and zirconium coatings can be formed on steel by cladding. In the case of these coating metals, cladding gives rise to the same disadvantages as already mentioned in the case of cladding with aluminium, that is heavy reduction is required which substantially reduces the formability of the substrate. Cladding is also subject to the inherent disadvantage that it necessitates the use of relatively thin toils of the coating metal which may be considerably more expensive than the same metal in ingot or powder form. All hot-dipping processes have the inherent disadvantages that a considerable heat input is required to maintain the bath of coating metal molten and that the molten metal is subject to drossing.

it is an object of the present invention to provide an improved process for the coating of metallic substrates with other metals. It is a further object of the present invention to provide a process for coating metallic substrates with metals which cannot be economically deposited by electrolysis. It is a still further object of the present invention to provide a method of torming a metal coating on a metallic substrate whereby a composite material is obtained which can be subjected to severe deformation without damaging or delamin-ating the coating. Further objects and advantages of the present invention will appear from the following description.

Metal coatings are formed, according to the present invention, by a process which essentially comprises the steps of electrophoretically depositing finely divided coating metal on the substrate, heating the coated substrate so that it is thoroughly dried and raised to .a temperature adapted to promote adhesion of the coating to the substrate, rolling the coated substrate while still hot with a pressure suflicient to bring the coating metal particles into intimate contact with one another, and then heating the coated substrate to effect sintering of the coating metal until, but only until, the coating becomes so firmly bonded to the substrate as to prevent delamination upon subsequent deformation of the coated substrate.

It is known to use the phenomenon of electrophoresis to form coatings of various materials on various types of substrate and as a means of applying coatings that can;

of a metallic oxide which is subsequently reduced with hydrogen to the metal, the oxide being deposited alone or simultaneously with a non-reducible metal or nonmetallic compound, such as molybdenum disulphide or silicon carbide. A particular form of the process generically described by Shyne et al. is described by Scheible (one of the co-authors of the Plating article) in US. Patent 2,982,707. Scheible is primarily concerned with the use of prolamines, such as zein, as activators for the electrophoretic deposition; a typical coating process described by Scheible comprises an electrophoretic deposition step (in the presence of a prolamine as just mentioned), air drying to remove the electrophoresis solvents, placing the coated article in a rubber envelope, evacuating the envelope to remove air, compressing the envelope isotatically in a glycerine medium at pressures from 20 to 50 tons/sq. inch to density the coating, and then sintering the coating in a hydrogen'atmosphere at temperatures of from lO0-l200 C. for from 1 to 3 hours. The high sintering temperatures employed by Scheible would certainly lead to grain coarsening of the substrate metal and severely reduce the formability of the latter. Since we are concerned with effecting compaction (i.e. densification) of the coating by rolling, we measure our compaction pressures in terms of rolling load, i.e. tons/inch width, and it is not possible to make a direct correlation between a pressure in tons/ sq. inch and a rolling pressure as the latter will depend upon the diameter of the work rolls which determines the configuration of the nip. It can be said, however, that the pressures of 25 to 50 tons/ sq. inch mentioned by Scheible are much less (by a factor of 2 or 3) than the rolling loads we have found to be necessary in order to obtain adequate compaction of the electrophoretically deposited coating.

The processes described by Shyne et al. and Scheible are essentially suitable only for the treatment of single articles and, in view of the densification step and the hydrogen reduction step which must in most cases be carried out at a very high temperatrue, could not economically be used for the coating of substrates in elongated form in tonnage quantities.

A continuous process for the formation of aluminium oxide or titanium oxide insulating coatings on heater filaments for use in thermionic valves is described by Thomson in US. Patent No. 2,956,937. This is a continuous process suitable for the large-scale production of coated wire in which, following electrophoretic deposition of the coating material, the coated wire is first dried at a relatively low temperature and is then heated to a much higher temperature to effect sintering of the oxide coating, a temperature of from 1300 C. to 1700 C. being mentioned as suitable for the sintering treatment. If such a sintering temperature were used in the case of an aluminium-coated substrate, the aluminium would be com pletely alloyed with the substrate metal and the mechanical properties of the substrate metal would be very deleteriously affected.

As indicated above, the process of the present application is essentially characterised by the use of conditions in the after-treatment of the coating when it has been formed by electrophoresis, which are such that the final product can be deformed without risk of damaging the coating.

Turning now to the drawings:

FIGURE 1 shows diagrammatically apparatus suitable for carrying out the present process on relatively narrow strip substrates, that is having a width of, say, 5 inches;

FIGURES 2 and 3 are photomicrographs of sections through samples of coated strip metal which will be described in detail in a subsequent portion of this application; and

FIGURE 4 shows curves of temperature against time for the sintering of aluminium coatings.

The apparatus comprises an uncoiler for the strip 11, deflector rolls 12 and 13, the latter being a contact roll which serves to polarise the strip as the cathode during operation, a narrow, vertically elongated electrophoresis tank 14, the top of which is surrounded by a header 15 which collectselectrophoresis suspension flowing over the top edge of the tank 14 (which constitutes a weir). The tank 14 contains two sheet anodes (not shown) arranged parallel to and on either side of the path of the strip. A delivery pipe 16 leads from the header 15 to a suspension reservoir 17 from the bottom of which leads a valved pipe 18 which delivers the suspension via a pump 19 to the bottom of the electrophoresis tank 14. Located vertically above the tank 14 is a pair of inclined rolls 20 which are arranged to deflect the strip and pass it downwards through a drying duct 21, the rolls 20 being so arranged that they contact only the edges of the strip. On leaving the duct 21, the strip is passed under a further pair of inclined deflecting rolls 22 and into a pre-heating furnace 23 from which the strip passes directly to a compacting mill 24 and from the latter to a coiler 25.

By using the inclined deflector rolls 20 and 22 and by appropriate design of the drying duct 21 and the preheating furnace 23, it is ensured that nothing contacts the deposits on the surface of the strip between the latter leaving the electrophoresis tank 14 and entering the nip of the compacting mill 24. The lower end of the tank 14 is provided with a suitable seal or gland to permit entry of the strip while preventing egress of the electrophoresis suspension. The drying duct 21 and the preheating furnace 23 are provided with heaters capable of raising the coated strip to the desired temperatures (which are more fully described below); any conventional heat ing means can be used for this purpose. The pie-heating furnace 24 is also suitably provided with means for maintaining a reducing atmosphere therein.

The apparatus illustrated and described above is suitable, as stated, for the coating of relatively narrow strip and for the case where the final heat treatment is carried out in coil. If the final heat treatment is to be carried out continuously, that is, in line, the strip is passed from the compacting mill 24 to a sintering furnace arranged in line with the mill and from the latter to the coiler. The use of inclined deflector rolls is not practicable with wide strip, and in plant for the coating of the latter, a suitable arrangement is for a drying and pre-heating furnace to be located vertically above the tank 14 and for the compacting mill to be located vertically above the pre-heating furnace. Once the powder coating has been thoroughly dried, the coated strip can, in fact, be passed over a suitably surfaced roll without damaging the coating, chromium plated and tetrafiuoroethylene surfaced rolls being suitable for this purpose. By using such a suitably surfaced roll as a deflector roll vertically above the drying and pre-heating furnace in the arrangement just described, the compacting mill can be located at ground level instead of vertically above the furnace as described, and this arrangement may be considerably more convenient.

The electrophoretic deposition step of the present process can be carried out in a generally similar manner to electrophoretic deposition processes previously described. Electrophoresis can be effected in a polar organic medium, suitable organic liquids being, for example, methyl alcohol, ethyl alcohol, for example in the form of industrial methylated spirit (denatured ethyl alcohol), isopropanol and acetone. The choice of organic solvent is determined by the requirements of low viscosity, low electrical conductivity, low evaporation rate and high dielectric constant and, of course, the consideration of cost. With these considerations in mind, other suitable organic liquids will be apparent to those skilled in the art.

While a wholly organic medium can be used, it is much preferred to use a medium consisting of a mixture of a major proportion of one or more organic liquids and a minor proportion of water. By using a partially aqueous bath, the electrophoresis yield in terms of grams deposited per coulomb, is considerably increased as compared with deposition under identical conditions but using a wholly organic medium. Suitable proportions of water are from 0.2% to 30% by volume of the mixture, from 2 to 20% being preferred. Within these ranges, it is found that optimum deposits of different coating metals are obtained with different proportions of water in the bath. Thus it is preferred to use a bath containing v./v. of water when depositing nickel, and a bath containing 2% v./v. of water when depositing aluminium.

When using methanol or mixtures of ethanol and methanol as the organic component of the medium, the lower range of water content is best. It has been noticed that with higher Water content (say there is preferential deposition of powder fines. The methanol can in some respects be considered as replacing the water, since it serves to increase conductivity. The advantage of such systems is only in reducing corrosion of the coating metal in the bath. Against this is the fact that there is a tendency for gassing to occur at the substrate surface.

The use of a partially aqueous medium for electrophoresis does result in some degree of gassing occurring at the electrodes, but we have found that such gassing is not necessarily deleterious to the coating. Under any given electrophoresis conditions, if the cathode current density (that is the current density at the metallic surface which is to be coated) is progressively increased, there will come a point at which the coating is deletoriously effected, i.e. by being dislodged, by the evolution of gas, but below this current density value entirely acceptable coatings are obtained even though some degree of gassing takes place and the use of water does enable a much greater yield to be obtained as stated above. The limiting current density for the formation of acceptable coatings is dependent upon other conditions of the electrophoresis and, more particularly, upon the relative velocity between the suspension and the metallic surface. The lower this relative velocity, the lower the limiting current density for the formation of acceptable coatings, and vice versa. In practice this means that if the process is carried out discontinuously, that is the metallic surface is held stationary within the bath and the latter is agitated just sufficiently to maintain the coating metal particles in suspension, the maximum current density which can be used is about 2 amps/ square foot. One the other hand if the process is carried out by passing the elongated substrate continuously through the bath and the latter is itself circulated continuously through the tank, for example by being taken off at the top of the tank and being pumped up from the bottom, current densities greatly in excess of 2 amps/ square foot can be used. For example with a strip speed of 60 feet/ minute and with the suspension pumped at such a rate that the liquid velocity in the electrophoresis cell is 75 feet/minute, a current density of 7 amps/ square foot can be used, and a coating 0.001 inch thick can be deposited in 5 seconds. It will be appreciated that at higher strip speeds and higher pumping rates, greater current densities can be used and that the process according to the invention is, therefore, suited to the high speed continuous production in tonnage quantities of metal coated metallic strip and wire, such as aluminium coated steel strip and wire, which could not previously be produced economically by the methods heretofore available. More particularly while the electrophoretic methods previously described for the formation of meta-l coatings on metallic substrates were suitable for the coating of single objects by discontinuous treatment, they could not be applied to a high speed continuous process because the electrophoresis yield was too low.

Suitable panticle sizes for the coating metal in the suspension are up to 50 microns in diameter, the upper limit of size being governed by the difiiculty of maintaining particles having a diameter greater than 50 microns in suspension. Aluminium powder having particle sizes in the range 5 to 50 microns has been successfully used.

The amount of finely divided coated metal in the electrophoresis bath is not critical, but since the electrophoresis yield has been found to increase approximately linearly with increasing powder content, it is preferred to use as high a powder content as is compatible with obtaining sufiicient wet adhesion for the powder coating to withstand the vibration of the compacting roll. Suitable proportions are, for example, from 10 to 30% by weight.

The electrophoresis suspension should also contain a soluble salt of a multivalent metal in order to give good electrophoresis yields (i.e. in terms of weight of deposit per coulomb) and to provide adequate adhesion of the wet powder to the substrate to allow rapid removal from the bath and of the dried powder to permit in-line rolling; rolling in-line is not practical unless the powder stays in position despite inevitable rolling mill vibraton. A variety of multivalent metal salts have been proved to be satisfactory in this respect, for example, salts of nickel, aluminum, manganese, magnesium, zinc, chromium, iron and calcium with monovalent anions, such as nitrate, chloride, iodide or bromide, may be used. All multivalent metals which can form insoluble hydroxides can be recommended as giving good physical properties after heat treatment. There is usually a residual contaminant in the coating from the electrolyte and some electrolytes would be better than others in applications involving the long term corrosion resistance of the product. The minimum amount of multivalent metal salt which should be used is that which will just give an adherent continuous coating from the bath at maximum line speed and pumping velocity of the suspension. The maximum amount is determined by the fact that above a certain concentration there is a reduction in electrolysis yield. Providing the electrolyte concentration is sufficient to produce a uniform layer from the bath, the amount and nature of the electrolyte is not important since it has been shown that a coating with good wet adhesion also has good dry adhesion and rolls well. In general a concentration of electrolyte of about 1 millimole per litre has been found to be very satisfactory.

Following deposition of the coating, the coated substrate is subjected to a drying and pre-heating step which should be effected in such a Way that the substrate is not contacted by any roll or guide prior to completion of this step since the powder coating would otherwise be dislodged to an undesirable extent. Preferably this is effected by passing the coated elongated substrate upwards from the electrophoresis bath through a heating zone, provided for example, with radiant electric heaters, which is located in the path of the upwardly moving substrate, there being no contact between the coated substrate and any part of the heating apparatus.

Particularly at high speeds of withdrawal of the coated substrate from the electrophoresis bath, dragout of the electrophoresis suspension occurs and it is found that the effect of this can be largely neutralised (and therefore the amount of electrophoresis medium requiring removal by evaporation in the heating zone can be considerably reduced) by subjecting the coated substrate, as it leaves the electrophoresis bath, to the action of air knives, that is jets of air or other gas directed onto the coated substrate and having a component of velocity in the direction opposite to that of substrate movement. The effect of such air knives is to blow a proportion of the dragged out liquid back into the electrophoresis bath.

The heating effected in the pre-heating step should in all cases be more than that required to dry the coated substrate thoroughly and we have found that the temperature to which the coated substrate is raised in this step has a profound effect upon the ultimate adhesion of the coating to the substrate. Suitable pre-heating temperatures depend upon the nature of the coating metal and the substrate metal. In the case of steel substrates, suitable pre-heating temperatures are as'follows: for aluminium coatings, from 200 to 350 C.; and for the other coating metals from 100 to 350 C. In the case of these other coating metals, pre-hea-ting should be carried out in a reducing atmosphere, for example hydrogen or the mixture of nitrogen and hydrogen obtained by cracking ammonia, if a temperature above 200 C. is used, but in the case of aluminium, air containing oxygen is preferred. If the minimum pre-heating temperatures indicated above are not obtained, the coating of the final product will, in general, have inferior adhesion, even though the subsequent steps of rolling and sintering are carried out under optimum conditions.

The pre-heated coated substrate is then subjected to rolling. The purpose of this rolling step is to bring the individual particles of the powder coating into intimate contact with one another and for any particular substrate and coating metal, a suitable minimum rolling load is relatively critical, in that unsatisfactory coatings are obtained in the final product if the rolling load is too low. When adhesion of an electrophoretically deposited coating to the substrate is obtained by heating alone (as taught, for example, by Thomson, US, Patent No. 2,956,937), the heat treatment must be longer or at a higher temperature or both, to achieve a coherent and adherent coating. On the other hand, we have found that when adhesion is secured by compaction due to rolling, followed by sintering, the heat treatment can be very much less severe and alloying with the substrate, such as steel, takes place to a far smaller extent. In the case of the aluminium coatings with which the present invention is concerned, any attempt to replace the minimum compaction treatment wholly or partly by heat treatment leads in most cases to the final coating being porous and the heat treatment would certainly have to be so prolonged that brittle alloy would be formed between the substrate metal and the coating metal.

The minimum rolling load required to obtain satisfactory compaction depends upon the hardness of the coating metal and of the substrate metal, and to a lesser extent, as will be more fully described below, on the roll diameter. The greater the hardness of the substrate, the higher will be the rolling load required. As a result of numerous experiments, we have developed a formula for determining the minimum rolling load in tons per inch width required to obtain adequate compaction; this formula is as follows:

Where R=minimum rolling load in tons per inch width for compaction;

B Brinell hardness of bulk coating metal in the annealed state; and

B =Brinell hardness of the substrate.

The Brinell hardness numbers of some of the coating metals which can be used in the process of the present invention, these figures being for the metal in bulk form and in the annealed state, are as follows:

Aluminium Copper 42 Brass 70/30 66 Nickel 85 Zinc 3 8 18/8 stainless steel 180 The Brinell hardness numbers of some of the substrates which can be coated by the process of the present invention, are as follows:

Annealed low carbon steel 82 40% cold reduced low carbon steel 160170 The Brinell hardness of other coating metals and other substrate materials can be readily ascertained from the literature by those skilled in the art.

In the case of the harder coating metals, that is those having a Brinell hardness of 40 or more in bulk, annealed, form, no variation in the rolling load required to obtain adequate compaction is necessary to take account of the diameter of the Work rolls used to effect compaction and for all work roll diameters, the minimum rolling load given by the above formula gives satisfactory results. In the case of the softer coating metals, however, that is those having a Brinell hardness of less than about 40, the above formula gives the minimum rolling load using 7 inch diameter work rolls and the rolling load should be increased with larger diameter work rolls. Thus, for example, if 12 inch diameter work rolls are used in place of 7 inch diameter rolls, the rolling load given by the above formula should be increased by about 25%. The choice of rolling load to obtain adequate compaction of the softer coating metals using work rolls of other diameters is within the competence of those skilled in the art.

Specific examples of suitable minimum rolling loads are as follows- Annealed and temper rolled low carbon steel strip having a Brinell hardness of 82, 0.025 inch thick, and coated with aluminium:

7 inch diameter work rolls 3-4 tons/ inch width (steel extension 3%). 12 inch diameter work rolls 4-5 tons/inch width (steel extension 3%).

Unannealed low carbon steel strip having a Brinell hardness of and coated with aluminium, where the end uses of the product do not require ductility:

7 inch diameter work rolls 8-10 tons/inch width. 12 inch diameter work rolls 10l2 tons/inch width.

Unannealed low carbon steel strip having a Brinell hardness of 165 and coated with nickel: 1215 tons/inch width.

In the case of annealed and temper rolled low carbon steel strip 0.025 inch thick having a Brinell hardness of 82, the following minimum rolling loads we found to be suitable with the coating metals mentioned:

Copper (Brinell hardness 42) About 6 tons/inch width. Brass (Brinell hardness 66) 810 tons/inch width. Stainless steel (Brinell hardness 15-20 tons/inch width.

It may additionally be said that in all cases, the attainment of the necessary compaction can be observed visually. Under-compacted coatings do not have the characteristic appearance of a solid sample of the coating metal, whereas properly compact coatings do. Thus under-compacted aluminium coatings exhibit characteristic white powdery flecks and under-compacted nickel coatings exhibit an off-white bloom; similar phenomena are observed with the other coating metals. Rolling is preferably effected with a load which is just suflicient to prevent the appearance of the visual characteristics of undercompaction.

In the case of all substrates and coating metals, insufiicient compaction leads to a product with poor corrosion resistance and poor formability, that is the coating will crack and/or delaminate when the product is bent or drawn, since the adhesion of the coating is poor and the relatively high temperatures required to effect sintering of the under-compacted coating lead to the formation of an alloy interlayer.

Rolling loads in excess of the minima described can be used, but when rolling loads greatly in excess of the minimum for a particular substrate and coating metal are used, blisters are occasionally formed in the product during sintering. The effect of rolling loads substantially in excess of the minimum is, in general, to cause deterioration of the mechanical properties of the substrate and, in particular, to reduce its formability. This is of little or no importance where the coating metal is such that the conditions of temperature and time required to sinter it are also such as to effect complete or partial annealing of the substrate since, in these cases, the subsequent sintering step restores all or most of the formability Which was lost in the compaction step. In the case of nickel, copper, brass or stainless steel coatings on steel, therefore, rolling loads substantially in excess of the minima quoted above can be used, since the temperature required to sinter all these coating metals is at or above that at which steel is annealed. In the case of the use of these coating metals on steel, the only reason for limiting the rolling load is economic, that is to limit the size of the compacting mill and the power required.

In the case of coating meta-ls which are sintered at temperatures below the annealing temperature of the substrate, on the other hand, it is preferred to use rolling loads which are equal to or slightly above the minimum values mentioned above in order to minimise deleterious changes in the formability of the substrate as far as possible. In the case of steel substrates, this applies to the use of, for example, aluminium and zinc as coating metals. Thus, for example, if annealed and temper rolled steel strip having a Brinell hardness of 82 and coated with aluminium, is rolled with a load of tons/inch width of strip with 12 inch diameter work rolls, an extension of 10% is obtained and the final product has poor forming qualities.

The final step of the present process consists of a second heat treatment to eflect sintering of the compacted coating; for any coating metal sintering may be carried out at a relatively \low temperature for a relatively long time or at a. relatively high temperature for a relatively short time. In general the least severe conditions compatible with obtaining complete sintering should be employed. If the sintering temperature chosen is such that the time required is more than, say, 30 seconds, the coated substrate is advantageously coiled (if in the form of strip or wire) or stacked (if in the form of sheets) after rolling and sintering effected in the coil or stack as the case may be. If the sintering time is sufiiciently short, on the other hand, sintering can be carried out in line with the sintering furnace located after the compacting rolls.

Suitable sintering conditions for aluminium are as follows:

(i) 600 C. for 2-15 seconds followed by air cooling.

(ii) Heating to 450 C. in 4-5 hours, followed by holding at this temperature for 30 minutes.

(iii) Heating to 300 C. in 2 hours, followed by holding at this temperature for 10-15 hours and then slow cooling.

We have carried out many experiments to determine suitable sintering conditions for the case of aluminium coatings on steel and the results of these experiments are shown graphically in FIGURE 4 of the drawings which shows two curves, AB and AC, obtained by plotting sintering temperature, in C., against log sintering time in seconds. Any combination of temperature and time lying between the curves AB and AC gives a satisfactory, i.e. tightly adherent, coating, whereas any combination of temperature and time lying above the curve AB will give an unsatisfactory coating due to the formation of a deleterious quantity of brittle alloy between the coating and substrate metals, and any combination of temperature and time lying below the curve AC will also give an unsatisfactory coating due to the aluminium being unsintered or insufiiciently sintered. Suitable sintering conditions for aluminium coatings on steel can, therefore, be readily read off from FIGURE 4.

Suitable sintering conditions for a variety of other coating metals are as follows:

Nickel:

(i) 1000 C. for 2 seconds followed by cooling in static gas. (ii) 700 C. for 5 hours. (iii) Heating to 680 C. in 3 hours and holding for 6 hours. Copper: 600700 C. for 30 minutes. Brass: 600700 C. for 30 minutes. Stainless steel: 10001100 C. for 60 minutes. Aluminum containing 1%, 5% or 10% zinc: temperature up to 400 C.

The following examples of the present process are given by way of illustration and not limitation:

Example I Annealed and temper rolled steel strip was coated with aluminium in apparatus of the kind illustrated in FIG URE 1.

The strip had a width of 5 inches and thickness of 0.025 inch and had been degreased by cathodic treatment in hot alkali, lightly pickled in cold 50% v./v. hydrochloric acid, and then washed. The strip was passed continuou'sly into the electrophoresis cell through the bottom of the cell and passed vertically upwards through the bath contained therein, the path length of the strip in the bath being 30 inches. The bath was continuously circulated through the cell, being pumped in at the bottom of the cell and allowed to flow over a weir at the top before being recirculated to the pump. A steel anode 30 inches long and 5 inches wide was arranged parallel to each side of the strip, the spacing between the surface of the strip, which was connected as the cathode, and the surface of the anodes being 1 inch.

The bath consisted of a 10% W./v. suspension of aluminium powder of which passed through a 300 mesh sieve) in a mixture of 80% by weight of industrial methylated spirit and 20% by weight of water and contained 1.0 millimole/ litre of aluminium nitrate.

The strip speed was 10 feet/minute, the bath velocity in the cell was 75 feet/ minute, the applied voltage 75 volts and the cathode current density 2.4 amps./ square foot.

After passing out of the electrophoresis cell, the coated strip was dried by passing it through an electrically heated drying chamber which raised the temperature of the coated strip to 200 C. and then passed through a rolling mill having 7 inch diameter work rolls and giving a load of 4 tons/inch width of strip. The thickness of the coating after rolling was 0.001 inch.

The rolled strip was then coiled and heated in the coil to 500 C. over a period of 3 hours, the rate of heating being such that its temperature was below 300 for 2 hours. The coiled strip was then allowed to cool in air.

The resulting coating was tightly bonded to the strip and the coated strip could be bent round a mandrel having a diameter equal to the thickness of the coated strip without delamination occurring.

Example 2 Coating of annealed and temper rolled mild steel strip similar to that of Example 1 was carried out substantially as described in that example, except that the anodes were formed of pure nickel and were 36 inches long, their spacing from the strip being as before.

The suspension contained 10% w./v. of aluminium powder having a particle size range of 5 to 50 microns in a mixture of 98% by weight of industrial methylated spirit and 2% by weight of water which also contained 1 millimole/ litre of nickel chloride.

The strip was passed through the electrophoresis cell at 10 feet/minute, a current of 1.5 amps was applied to each side of the strip and the applied voltage was volts; the finished thickness of the coating was 25 microns.

In a number of runs, the coated strip was subjected to different combinations of after-treatments, rolling being effected in each case with 12 inch diameter work rolls. The results obtained are summarised below; in these results, satisfactory coating means that the coated strip The suspension consisted of 30% by weight of nickel powder in a mixture of 90% by volume of industrial methylated spirit and 10% by volume of water which also contained 1 millimole of nickel chloride per litre.

The strip was passed through the electrophoresis cell could be bent round a inch mandrel without damagat feet/mlnute, a current of 1.5 amps was applied to ing or delamlnatmg the coating and unsatisfactory coateach side of the strip and the applied voltage was 60 mg means that the coating was damaged and/ or devolts. The finished thickness of the coating was laminated on being sub ected to this test. microns.

Pre-heating Rolling load, sintering temperature, Run temperatons/in. 0. Product ture, 0. width 250 4 450 in 4-5 hours and Satisfactory coating (see hold for minutes. Figure 2). 350 5 do Satisfactory coating. 250 10 300 for 15 hours Unsatisfactory coating (overcompacted). 250 2 450 in 4-5 hours and Unsatisfactory coating hold for 30 minutes. (undereompactcd). 4 600 in 15 seconds Unsatisfactory coating.

250 4 u-.. o Satisfactory coating. 250 4 520 for2hours... Unsatisfactory coating (brittle alloy). 250 4 620 for 10 minutes Unsatisfactory coating (see Figure 3 (a)). 250 4 620 for 15 minutes Unsatisfactory coating (see Figure 3 (12)). 250 4 620 for 60 minutes Unsatisfactory coating (see Figure 3 (0)).

1 Omitted, coating dried at 60 0.

FIGURE 2 of the accompanying drawings comprises photomicrographs at a magnification of X500 of a section through (a) an undistorted sample of the product obtained in run 2a and (b) a sample of the same product which had been bent round a inch mandrel, the latter photomicrograph being of a portion of the sample near the apex of the bend. In both these photomicrographs 26 is the steel substrate and 27 the compacted and sintered aluminium coating. It will be seen from. FIGURE In a number of runs, the coated strip was subjected to different combinations of after-treatments, rolling being effected in each case with 12 inch diameter work rolls. The results obtained are summarised below; in these results, satisfactory coating means that the coated strip could be bent around a inch mandrel without damaging or delaminating the coating and unsatisfactory coating means that the coating was damaged and/or delaminated on being subjected to this test.

1 Omitted, coating simple dried at 60 0.

thicker from FIGURE 3(a) to FIGURE 3(a), showing the increasing formation of this alloy on holding at a high sintering temperature. The products of runs 211, 2i and 2j all failed the deformation test because of the presence of the layer of brittle iron-aluminium alloy shown in FIGURE 3, the formation of which was avoided by the process followed with runs 2a, 2b and 2f.

Example 3 Coating of hard unannealed mild steel strip 5 inches wide and 0.025 inch thick with nickel was carried out in apparatus as described in Example 2.

2 Reducing atmosphere.

Example 4 Steel strip of the type used in Example 1 was coated with zinc by a procedure substantially as described in that example, except that the aluminium powder in the suspension was replaced by zinc powder (10% w./v.) and the aluminium nitrate electrolyte was replaced by Zinc nitrate (1 millirnole per litre).

On leaving the electrophoresis cell, the coated substrate was heated to 200 0., passed through a rolling mill having 7 inch diameter work rolls and giving a load of 4 tons/inch width of strip and coiled. The coiled strip was heated for 1 hour at 250 C. to give a tightly adherent zinc coating.

The metallic substrate should, of course, be free of grease and other contaminants prior to effecting electrophoretic deposition thereon. Suitable metal cleaning procedures are well known to those skilled in the art and one suitable cleaning procedure has been described in the above examples. Degreasing by cathodic treatment in hot alkali followed by washing in water is in many cases quite sufficient. A more thorough cleaning schedule comprises (a) immersion of the substrate in a aqueous caustic soda solution at 60 C., (b) cathodic electrocleaning in an aqueous solution containing caustic soda and 5% sodium carbonate using a current density of 100 amps/square foot, and (c) fiash pickling in hydrochloric acid. In a typical process treatment (a) is effected for 20 seconds, and the substrate is then washed in water, treatment (b) is effected for 30 seconds and the substrate is then washed in water, and treatment (c) is effected for seconds and the susbtrate is again washed before being passed into the electrophoresis tank.

Another suitable cleaning procedure comprises subjecting the substrate to vapour blasting; this gives a clean matt surface.

While in the foregoing exam'ples, we have described application of the process to the coating of strip, it is equally applicable to Wire. In this case, a hollow-cylindrical anode is preferably employed, the wire to be coated being aligned along its axis. Rolling of the coated wire after the preheating step is preferably carried out by two passes between shaped rolls which are designed so that 60% of the wire surface is compacted at each pass, the axes of the rolls for the second pass being at right angles to the axes of the rolls used for the first pass. In this way the coating is uniformly compacted around the wire. The degree of pressure applied by the rolls is preferably such as to cause a 1% reduction in cross-sectional area in the case of a mild steel wire; a suitable rolling load is, for example, approximately 0.2 ton per stand for hard drawn 01120 inch mild steel wire. In the case of harder, i.e. less deformable, wires the reduction in area can be less, but should be at least 0.2%. The second stand may, if desired, be rotated with respect to the first stand to allow for any twisting of the wire as .a result of induced stresses. The rolls should be kept clean, i.e. free from metal powder, suitably by means of rotating brushes.

Instead of two passes through shaped rolls, two fourroll Turks head rolling units can be used for compacting the powder coating, but this offers no advantages over the simpler and cheaper double roll units already described. The powder can also be compacted by rolls describing a helical path around the wire.

Whichever arrangement of rolls is used to compact the coating, they will be heated by the coated wire which is still hot from the first heat treatment and it may be desirable to provide means for cooling the compacting rolls.

What is claimed is:

1. The method of electrophoretically depositing a metal coating on a metal substrate in elongated form, which method comprises the step of continuously passing the substrate through an electrophoresis cell containing a suspension of finely divided coating metal in its metallic form in a polar organic solvent said suspension also con taining from 2% to 30% water by volume and a minor proportion of a soluble rnultivalent metal salt, said substrate being connected as cathode and said suspension being continuously agitated, whereby said coating metal is electrophoretically deposited on said substrate.

2. The method claimed in claim 1, in which said metal coating is aluminium, and comprising the additional steps of heating said coated substrate to a temperature between 200 and 350 C., rolling said coated and heated substrate with a rolling load at least equal to that given by the formula Where R is the minimum rolling load in tons per inch width, B is the Brinell hardness of the bulk coating metal in the annealed state, and B is the Brinell hardness of the substrate, and then sintering said coated substrate at a temperature insuflicient to cause the formation of a brittle alloy layer at the interface between the coating and substrate.

3. The method claimed in claim 1, in which said metal coating is aluminium, and comprising the additional steps 14 of heating said coated substrate to a temperature between 200 and 350 C., rolling said coated and heated substrate with a rolling load at least equal to that given by the formula where R is the minimum rolling load in tons per inch width, B is the Brinell hardness of the bulk coating metal in the annealed state, and B is the Brinell hardness of the substrate, and then sintering said coated substrate under conditions of temperature and time insufiicient to cause the formation of an alloy layer at the interface between the coating and substrate, said time and temperature lying between the curves AB and AC of FIGURE 4 of the drawings.

4. The method claimed in claim 1 according to which said substrate is steel, the coating metal is selected from the group consisting of copper, brass, stainless steel, zinc and nickle, and said water comprises from 2 to 20% by volume of the bath, said method comprising the further steps of continuously passing said coated substrate to a station at which it is heated to a temperature between and 350 C., rolling'said coated and heated substrate with a rolling load at least equal to that given by the formula where R is the minimum rolling load in tons per inch width, B is the Brinell hardness of the bulk coating metal in the annealed state, and B is the Brinell hardness of the substrate, and then sintering the coated substrate.

5. The method of causing an electrophoretically deposited coating of aluminium to adhere to a steel substrate sufficiently to prevent delamination when said coated substrate is subsequently bent about a A inch mandrel, which method comprises the steps of preheating said coated substrate to a temperature between 200 and 350 C., compacting said coated substrate while still hot with a rolling load of at least three tons per inch of subtrate width, and then sintering the coated substrate under conditions of temperature and time insufiicient to cause the formation of an alloy layer at the interface between the coating and substrate, said time and temperature lying between curves AB and AC of FIGURE 4 of the drawings.

6. The method of causing an electrophoretically deposited coating of aluminium to adhere to a steel wire which method comprises the steps of preheating said coated wire to a temperature between 200 and 350 C., compacting said coating by compressing said heated and coated wire along at least two diameters positioned at right angles to each other sufficiently to reduce the crosssectional diameter of the wire between 0.2% and 1%, and then heating said coated wire to a temperature and for a length of time suflicient to sinter the coating but insufiicient to cause the formation of a layer of alloy at the interface between the coating and substrate said time and temperature lying between curves AB and AC of FIG. 4 of the drawings.

7. The method of causing an electrophoretically deposited coating of copper to adhere to a steel substrate sufficiently to prevent delamination when said coated subtrate is subsequently bent about a inch mandrel which method comprises the steps of preheating said coated substrate to a temperature between 100 C. and 350 C., compacting said coated substrate while still hot with a rolling load of about 6 tons per inch of substrate width and then sintering the coated substrate at between 600 and 700 C.

8. The method of causing an electrophoretically deposited coating of brass to adhere to a steel substrate sufficiently to prevent delamination when said coated substrate is subsequently bent about a inch mandrel which method comprises the steps of preheating said 15 coated substrate to a temperature between 100 and 350 C., compacting said coated substrate while still hot with a rolling load of from 8 to 10 tons per inch of substrate width and then sintering the coated substrate at between 600 and 700 C.

9. The method of causing an electrophoretically deposited coating of stainless steel to adhere to a steel substrate sufiiciently to prevent delamination when said coated substrate is subsequently bent about a inch mandrel which method comprises the steps of preheating said coated substrate to a temperature between 100 and 350 C., compacting said coated substrate while still hot with a rolling load of from 15 to 20 tons per inch of substrate width and then sintering the coated substrate at between 1000 and 1100 C.

10. The method of causing an electrophoretically deposited coating of nickel to adhere to a steel substrate sufiiciently to prevent delamination when said coated substrate is subsequently bent about a inch mandrel which method comprises the steps of preheating said coated substrate to a temperature between 100 and 350 C., compacting said coated substrate while still hot with a rolling load of from 12 to 15 tons per inch of substrate width and then sintering the coated substrate at no more than 1000 C.

References Cited by the Examiner UNITED STATES PATENTS 2,37 2,607 3 1945 Schwarzkopf 75208 2,442,863 6/ 1948 Schneider 204-181 2,878,140 3/1959 Barr 204181 2,935,402 5/1960 Trotter et al. 75208 2,982,707 5/1961 Scheible 204181 3,142,560 7/1964 Storchheim 75-208 ALLEN B. CURTIS, Examiner.

Claims (2)

1. THE METHOD FO ELECTROPHORETICALLY DEPOSITING A METAL COATING ON A METAL SUBSTRATE IN ELONGATED FORM, WHICH METHOD COMPRISES THE STEP OF CONTINUOUSLY PASSING THE SUBSTRATE THROUGH AN ELECTROPHORESIS CELL CONTAINING A SUSPENSION OF FINELY DIVIDED COATING METAL IN ITS METALLIC FORM IN A POLAR ORGANIC SLVENT SAID SUSPENSION ALSO CONTAINING FROM 2% TO 30% WATER BY VOLUME AND A MINOR PORPORTION OF A SOLUBLE MUTIVALENT METAL SALT, SAID SUBSTRATE BEING CONNECTED TO CATHODE AND SAID SUSPENSION BEING CONTINUOUSLY AGITATED, WHEREBY SAID COATING METAL IS ELECTROPHORETICALLY DEPOSITED ON SAID SUBSTRATE.

4. THE METHOD CLLAIMED IN CLAIM 1 ACCORDING TO WHICH SAID SUBSTRATE IS STEEL, THE COATING METAL IS SELECTED FROM THE GROUP CONSISTING OF COPPER, BRASS, STAINLESS STEEL, ZINC AND NICKEL, AND SAID WATER COMPRISES FROM 2 TO 20% BY VOLUME OF THE PATH, SAID METHOD COMPRISING THE FURTHER STEPS OF CONTINUOUSLY PASSING SAID COATED SUBSTRATE STATION AT WHICH IT IS HEATED TO A TEMPERATURE BETWEEN 100 AND 350*C., ROLLING SAID COATED AND HEATED SUBSTRATE WITH A ROOLING LOAD AT LEAST EQUAL TO THAT GIVEN BY THE FORMULA

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