US3331762A - Process of forming metal coatings on metal strip by electrophoretic deposition - Google Patents

Description

July 18, 1967 A. E. JACKSON 3,331,762 7 PROCESS OF FORMING METAL COATINGS ON METAL STRIP BY ELECTROPHORETIC DEPOSITION Filed April 18, 1963 INVENTOR A. E. Jackson .ATIORNEYS United States Patent PROCESS 6F FORMING METAL COATINGS 0N METAL STRIP BY ELECTROPHORETIC DEPOSITION Albert Edward Jackson, Swansea, Wales, assignor to The British Iron and Steel Research Association Filed Apr. 18, 1963, Ser. No. 273,969 2 Claims. (Cl. 204-181) This invention relates to the formation of metal coatings on metal strip by electrophoretic deposition.

In British Patent No. 884,797 to Salt and Lewis there is described and claimed a process for the formation of metal coatings on, inter alia, metal strip which comprises electrophoretically depositing the coating metal on to the strip from a suspension of finely divided coating metal, drying the coated strip to remove any electrophesis medium removed from the electrophoresis bath, rolling the coated strip to compact the coating, and then heating the coated strip to obtain a tightly bonded coating.

As the coated substrate is removed from the electrophoresis bath, the coating is in a non-coherent and nonadherent form and if the coated substrate, in this condition, is passed over a roll in order to change its direction of travel, i.e. so as to pass it to the drying station, a part of the coating becomes transferred to the roll with consequent impairment of the final coating after the rolling and heating treatments.

According to a first aspect of the present invention, therefore, the coated strip is passed in a straight pass from the electrophoresis bath through the drying station to the compacting rollsand the coated strip is not contacted by any other roll until it reaches the compacting rolls. Once the coated strip has been compacted there is substantially no tendency for the coatingto adhere to rolls and the coated strip may be passed around a roll in order to change its direction of travel.

The compacting pressure applied by the compacting rolls is preferably from 1 /2 to 10 tons per inch width of the coated strip, the optimum pressure for any particular coated strip within this range being determined by the hardness of the strip and the diameter of the compacting rolls.

In carrying out this procedure it is preferred to pass the strip vertically upwards through the electrophoresis cell and the drying station to the compacting rolls. In some cases it may not be convenient to locate a rolling mill capable of applying pressures within the above range, at the required distance above ground level, that is at a height such that the electrophoresis bath and the drying station can be accommodated vertically below the rolling mill, and in these cases a rolling mill capable of applying a relatively low pressure can be used instead. Such a rolling mill can be used, as we have found that compacting pressures below those required to ensure adequate adhesion of the coating in the final product, are sufficient to prevent the coating adhering to rolls. Thus while it is preferred to use compacting pressures of from 1 /2 to 10 tons per inch width in order to develop good adhesion of the coating in the final product, we have found that a preliminary compaction with a pressure of from /2 to 1 ton per inch Width (which, it will be apparent, requires a much lighter and smaller rolling mill than that required for full compaction) substantially removes the tendency of the coating to adhere to rolls. When preliminary compaction is used, a light rolling mill capable of exerting a pressure of from /2 to 1 ton per inch width is located vertically above the electrophoresis bath and drying station, and the coated strip is passed from this rolling mill or from a roll located above this rolling mill to the rolling mill for effecting full compaction, which can be located at ground level.

There is a tendency for coating metal to adhere to the rolls which effect the first compaction of the coating (whether it be a preliminary or a full compaction) and in order to prevent coating metal being built up on the rolls in contact with the metal coating and the consequential damage to the coating such deposits would cause, it is preferred to provide the rolling mill with one or more pads of, or rolls covered with, a soft fibrous material, such as lambswool or felt, and with one or more doctor knives, a pad or roll and a knife bearing against each roll in contact with the coating so as to clean its surface. We have found that a more eflicient cleaning action is obtained with a soft fibrous material, such as lambswool or felt, than with a brush.

The drying effected by the drying station referred to above is substantially complete and since the amount of electrophoresis medium removed from the electrophoresis cell is quite significant and increases with the strip speed, it is preferred to associate the drying station with a solvent recovery plant wherein the organic solvent carried out of the electrophoresis cell by the coated strip and volatilised therefrom in the drying station, is condensed and recycled to the electrophoresis cell. Such a solvent recovery plant renders the electrophoretic coating process more economic, but its operating costs must, of course, form part of the overall operating costs of the process.

We have found that the operating costs of the process can be reduced by reducing the amount of drag-out, i.e. the amount of electrophoresis medium carried out of the cell by the coated strip, and that drag-out can be substantially and effectively reduced by the use of air knives, that is jets of air having a component of velocity directly opposite to the direction of travel of the strip and impinging at an acute angle on the strip across its width. The effect of the air knives is to blow the entrained electrophoresis medium back towards the electrophoresis cell and in order that the minimum amount of electrophoresis medium should be lost, it is preferred, again, that the coated strip should be passed vertically upwards from the electrophoresis cell so that entrained medium blown off the coated strip by the air knives can fall back into the cell. The reduction in the amount of drag-out by use of air knives, also reduced the heating required to dry the coated strip.

In order to indicate the effectiveness of air knives, comparative runs were made at different rates of withdrawal from the electrophoresis cell. In all cases the substrate was steel strip, the coating metal aluminium powder, the electrophoresis medium 20% aqueous methylated spirit containing 0.5 millimole/litre of aluminium nitrate, the current density 10 amps/sq. ft. and the coating thickness (after compaction) 0.001 inch. The air knives were airheaders tapering to a slit 3 mm. wide and extending across the width of the strip which produced jets impinging at an angle of 30 on to the strip.

Drag-out (Gallons/1,000 sqjt.) Velocity of With- Reduction, drawal, ftjmin. Percent Without With air knife air knife We have found that when the liquid content of the coating has been reduced, by air knives and/or by heating, to between 20% and 75% by weight, based on the weight of the coating, the coated strip can be contacted with a roll having a surface of a suitable non-adherent material 3 for the purpose of changing its direction of travel without damage to the coating. That is to say, provided that the coated strip is neither too dry nor too wet, it can be passed over a roll and directed to a floor mounted rolling mill for the purpose of effecting compaction of the coating and it is not necessary to use a mill mounted above the electrophoresis cell.

Suitable non-adherent materials for the roll surface are,

'for example, chromium or nickel plated steel, polyethgrammatically in the accompanying drawing. A run was carried out with steel strip 25 mm. wide and having a thickness of 0.6 mm. The strip was passed vertically up- Wards through between two vertical sheet anodes 9, spaced 2 inches apart, in an electrophoresis cell 10. The cell contained a continuously circulated suspension of aluminium' powder in 20% aqueous methylated spirit containing 0.5 millimole/ litre of aluminium nitrate. The current on each side of the strip was 2.25 amps and the voltage 75 volts; the aluminium coating thickness was 25 microns. The strip was passed vertically upwards between two air knives 11 and two radiant electric heaters 12 to a chromium plated steel roll 13 having a diameter of 0.2 metre. From the'roll, it was passed downwardly, at an acute angle to the horizontal through a drying chamber 15 continaing a high frequency electric drier 14 and through which a current of air indicated by arrows 16 was passed, to a floor mounted rolling mill 17 where compaction was eifected. The air knives were mounted 1 inch above the opening of the electrophoresis cell and directed jets of air at an angle of 30 on to the coated strip and the radiant heaters were mounted 2 feet above the air knives at a distance of 2 inches from the strip. The line speed was 3 metres/minute and the fall on the strip (that required to pull the strip over the roll) was 45 kilograms.

Under these conditions, a uniform and well adherent aluminium coating was obtained and there was no damage to the coating as a result of its contact with the roll. The same result was obtained in another run under the same conditions but using strip 0.25 mm. thick.

As indicated above electrophoresis is preferably effected while moving the strip vertically upwards through the electrophoresis bath. The latter is advantageously contained in a vertically elongated tank, the bottom of which is provided with a stufling box through which the strip is passed into the tank. We have found that although there is a tendency for the electrophoresis medium to escape through the stufilng' box in the first few minutes of operation, particles of coating metal rapidly fill up any gaps and interstices so that an effective seal is formed.

Whilst various methods are-available formaintaining the coating metal particles in suspension, it is preferred to do this by circulating the suspension in a closed circuit which includes the electrophoresis tank, the suspension being introduced at the bottom of the tank andbeing taken ofr" at the top. Advantageously the cross-section of the tank is kept small, ie so that it is not much larger than is required to accommodate the strip and the anode or anodes, and the suspension is introduced through two inlets located at the base of opposite walls of the tank. Removal of the suspension from the top of the tank is preferably effected by using a tank which is open at the top and allowing the suspension to flow over the whole of the periphery of the top of the tank to be collected in a header located some way below the top of the tank, the suspension being discharged from the header to the return circuit.

With this vertical arrangement of strip and anode or anodes, it is preferred that the latter should not extend to the surface of the bath, but should be some 2 or 3 inches short of the surface, in order to reduce the occurrence of surface defects in the coating due to drag-out from the bath. We have found that the surface quality of the coating obtained is improved by circulating the suspension at a faster rate than the minimum required to maintain the coating metal particles in suspension.

After full compaction of the coated strip it'is subjected to heat treatment in order to develop maximum adhesion of the coating to the substrate and such heating is prefer ably effected with the coated strip in coil. Undue bending 'of the coated strip before the heat treatment tends to cause the coating to delaminate from the substrate and it is therefore desirable to coil the strip on a mandrel which is sufficiently large to avoid any undue bending taking place. The amount of bending which can be tolerated is indirectly related to the substrate thickness; thus, for example, it is preferred to use a mandrel having a diameter of not less than 12 inches when the substrate is 0.012 in. thick,

and having a diameter of not less than 16 inches when the substrate is 0.022 in. thick.

I claim:

1. In a process for the formation of metal coatings on metal strip by electrophoretically depositing the coating metal on to the strip from a suspension of finely divided coating metal, drying the coated strip to remove entrained electrophoresis medium therefrom, rolling the coated strip to compact the coating, and then heating the coated strip to obtain a tightly bonded coating, the improvement which comprises subjecting the coated strip onleaving the elec;- trophoresis cell to the action of air knives which direct jets of air having a component of velocity directly opposite to the direction of travel of the strip and impinging at an acute angle on the strip across its width.

2. A process according to claim 1, in 'which the jets of air impinge at an angle of 30 on both sides of the strip'.

References Cited UNITED STATES PATENTS 1,897,902 2/1933 Harsanyi 204 1s1 3,034,971 5/1962 Seaward 204 1s1 3,091,547 5/1963 Jones 117 -65.2

3,096,567 7/1963 Ross et al 117-652 FOREIGN PATENTS 884,797 12/1961 GreatBritain.

JOHN H. MACK, Primary Examiner. E. ZAGARELLA, Assistant Examiner,

Claims (1)

1. IN A PROCESS FOR THE FORMATION OF METAL COATINGS ON METAL STRIP BY ELECTROPHORECALLY DEPOSITING THE COATING METAL ON TO THE STRIP FROM A SUSPENSION OF FINELY DIVIDED COATING METAL, DRYING THE COATED STRIP TO REMOVE EXTRAINED ELECTROPHORESIS MEDIUM THEREFROM, ROLLING THE COATED STRIP TO COMPACT THE COATING, AND THEN HEATING THE COATED STRIP TO OBTAIN A TIGHTLY BONDED COATING THE IMPROVEMENT WHICH COMPRISES SUBJECTING THE COATED STRIP ON LEAVING THE ELECTROPHORESIS CELL TO THE ACTION OF AIR KNIVES WHICH DIRECT JETS OF AIR HAVING A COMPONENT OF VELOCITY DIRECTLY OPOSITE TO THE DIRECTION OF TRAVEL OF THE STRIP AND IMPINGING AT AN ACUTE ANGLE ON THE STRIP ACROSS ITS WIDTH.

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