US3136709A

US3136709A - Method of electroplating an aluminum containing coating

Info

Publication number: US3136709A
Application number: US827006A
Authority: US
Inventors: Edwin J Smith; Michael G Vucich
Original assignee: National Steel Corp
Current assignee: National Steel Corp
Priority date: 1959-07-14
Filing date: 1959-07-14
Publication date: 1964-06-09
Anticipated expiration: 1981-06-09

Description

Ju 1964 E. J. SMITH ETAL METHOD OF ELECTROPLATING AN ALUMINUM CONTAINING COATING Filed July 14, 1959 2 Sheets-Sheet 1 NGD INVENTORS EDWIN .1. SMITH MICHAEL 6. l/UO/GH ATTORNEY 5 June 9, 1964 E. J. SMITH ETAL METHOD OF ELECTROPLATING AN ALUMINUM CONTAINING COATING Filed July 14, 1959 2 Sheets-Sheet 2 INVENTORS EDWIN J. SMITH MICHAEL G. VUCICH ATTORNEYS -Patent No. 3,007,854, for

United States Patent 3,136,709 METHQD F ELECTROZ LATING AN ALUMINUM CONTAINING CDATING Edwin .l. Smith, Steubenville, Ohio, and Michael G. Vucich, Bridgeville, Pa., assignors to National Steel Corporation, a corporation of Delaware Fiied July 14, 1959, Ser. No. 827,006

Ciaims. (Cl. 204-28) 7 This invention relates to a novel method and improved apparatus for the electrodeposition of aluminum or aluminum alloys on a metal base. I

This application is a continuation-in-part of our application Serial No. 665,743, filed June 14, 1957, now US. Plating Apparatus and Method. Application Serial No. 665,743, in turn, is a continuation-in-part of application Serial No. 512,734, filed June 2, 1955, by Edwin J. Smith et al. for Plating Apparatus and Method, now abandoned.

The present invention will be described and illustrated hereinafter with reference to the electrodeposition of aluminum on ferrous metal strip using a fused electrolyte containing predominant proportions of aluminum chloride. However, it will be apparent to those skilled in the art that the principles of this invention are applicable to other base metals, forms of base metals, coating metals and alloys, or other fused salt electrolytes presenting the problems solved by the invention.

A fused electrolyte composition having an aluminum chloride content of approximately 7585% by weight of the composition, with the remainder being sodium and/ or potassium chloride, will produce good aluminum plate when plating ferrous metal strip. The above electrolyte composition may be either of the binary or ternary types, a satisfactory binary electrolyte composition being exemplified by a composition containing by weight 80% aluminum chloride and 20% sodium chloride; while a satisfactory ternary electrolyte may be exemplified by a composition containing by weight 80% aluminum chloride, 10% sodium chloride, and 10% potassium chloride. Although a fused electrolyte containing from 80-85% aluminum chloride produces objectionable visible fuming in the presence of water, an electrolyte containing more than 85% aluminum chloride fumes even more excessively in the presence of water and the plating results obtained are not improved by increasing the aluminum chloride above this value. However, if the aluminum chloride content of the fused electrolyte falls below about 75% by weight, poor plating results when using unmodified direct current and in the absence of special plating conditions. Thus, such electrolyte compositions are not generally preferred.

The above mentioned visible fuming is\ a result of a reaction between the aluminum chloride and atmospheric water vapor, or water from some other source. The fumes thus produced are highly corrosive and if vented to the atmosphere they offer serious health hazards. The fumes also tend to corrode metallic apparatus with which they come in contact that is constructed of the usual materials of construction, thereby reducing its useful life at an uneconomic rate. For example, if a blower system is used for removing fumes directly from the electroplating bath, the fumes are so corrosive that they attack the blower system and damage the same after a short period of time ice unless the materials of construction are non-corrosive, relatively expensive metals or alloys.

It has been proposed to eliminate the fuming problem by sealing off the electroplating bath by means of a hood having entrance and exit slots, these slots being closed by flaps of resilient material which permitted movement of the strip therethrough while largely preventing escape of fumes from the electroplating bath. In such case, whether or not atmospheric air leaked into the hood in small amounts via the entrance and exit slots was not considered to be of great importance. However, applicants have discovered that even though the fuming problem is eliminated by use of the slotted hood, after long periods of continuous plating a point is eventually reached where it is impossible to plate a satisfactory deposit of aluminum onto the strip. They discovered that these plating difficulties could be corrected only by replacing the electrolyte with fresh electrolyte. In applicants above-mentioned copending parent application, on the other hand, perfect hermetic sealing of the electrolyte bath surface and a conditioning of the strip are achieved by means of entrance and exit liquid seals through which the strip passes on entering and leaving the plating zone. The liquid seal type of sealing therefore eliminates the fuming problem and, even more important, eliminates the plating diificulties arising from contaminated electrolyte.

In both methods, namely, that using the liquid seals and that using the slotted hood, the strip to be plated was thoroughly dried prior to entry into the plating zone. Furthermore, the temperature of the electroplating bath in both cases was maintained well above the boiling point of Water. Despite the foregoing, applicants have discovered that water gets into the bath when using the slotted hood and acts to contaminate and interfere with plating. By detecting aluminum hydroxy chloride (AlCl OH), aluminum dihydroxy chloride AlCl(OH) or other products of hydrolysis, such as hydrochloric acid, which is'more acidic than the fused aluminum chloride electrolyte, applicants proved that despite the precautions associated with the slotted hood and despite the high temperature of the electroplating bath, water from some source gradually enters the electroplating bath and causes its contamination.

Applicants have found that the entrance into the plating.

zone containing fused aluminum chloride electrolyte of even minute quantities of atmospheric air, or the presence on the strip surface of minute quantities of free water, or combined water such as hydrated iron salts, or substances reacting under conditions present in the electroplating zone to form water, cause contamination of the electrolyte to a point where it is useless.

One result of this continued gradual contamination of the electroplating bath with water in one or more of its various forms is that the concentration of the resulting products of hydrolysis builds up at the expense of free aluminum chloride, and since the concentration of free aluminum chloride is critical, eventually the electrolyte is rendered useless and must be replaced. Another result of continued 'water contamination is an increase in the corrosivity of the electroplating bath due to the production of hydrochloric acid during hydrolysis of the aluminum chloride. Normally, the equipment in contact with the electrolyte may be constructed of aluminum. However, the hydrochloric acid which is more acidic than the electrolyte quickly corrodes the aluminum equipment and it must be replaced at frequent intervals. In addition, the hydrogen evolved by the reaction between hydrochloric acid and aluminum presents an appreciable explosive hazard. Still another diificulty resulting from Water contamination of the electroplating bath is a buildup in the concentration of iron salts. It has been found that the deposition of aluminum is adversely affected when the concentration of iron exceeds about 0.02%. Since the hydrochloric acidproduced during hydrolysis of aluminum chloride also attacks the ferrous metal strip passing through the electroplating bath, the electroplating bath is soon contaminated with a sufiicient amount of iron salts to cause difiiculty and the contaminated electrolyte must be replaced if a high-quality product is to'be produced.

The above described water contamination of the electrolyte is particularly unexpected in view of the fact that the difficulties usually arising from visible fuming are eliminated by the slotted hood type of sealing of the electroplating bath, taken with the high operating temperature of the electroplating bath. It would be expected that this attempted isolation of the bath and the hi h operating temperatures would not allow any entry of Water into the electroplating bath, particularly where the ferrous metal strip entering the electroplating bath is thoroughly dried. As mentioned above, where the liquid seal type of sealing is used, and the liquid of the liquid seal is maintained water-free, no contamination of the electroplating bath takes place due to the entrance of Water. One important feature of the present invention therefore resides in the discovery that the fused aluminum chloride plating bath must be completely isolated from water, particularly from water vapor in atmospheric air, free water or combined water such as hydrated metal salts on the strip, and substances capable of reacting under the conditions present in the electroplating bath to form water, and in the method and apparatus for accomplishing this purpose.

It has been further discovered that the temperature of a metal base to be electroplated with an aluminumcontaining coating is very critical, at least at the time of commencing to electroplate the coating, and that the metal base must be maintained within the temperature limits to be defined hereinafter if acceptable coatings are to be produced. Otherwise, non-adherent and/or porous coatings are produced which are entirely unsatisfactory.

FIGURE 1 is a diagrammatic side view, partially in cross section, of one embodiment of a continuous aluminum electroplating line constructed in accordance with the present invention and employing a liquid seal form of sealing;

FIGURE 2 is a cross-sectional view taken along the line 22 of FIGURE 1 but omitting a showing of the strip the rolls, and the electrodes for the purpose of clarity;

FIGURE 3 is a diagrammatic side View, partially in cross section, of still another embodiment of a continuous electroplating line constructed in accordance with the present invention and employing a slotted hood form of sealing; and

FIGURE 4 is a cross-sectional view taken along the line 4 4 of FIGURE 3 but emitting a showing of the strip, the sealing flaps, the rolls, and the electrodes for the purpose of clarity.

Referring now to the drawings and more particularly to FIGURES l and 2, a specific embodiment of the invention employing the liquid seal type of Scaling will be first described. For the purpose of wet pretreatment of the strip prior to electrodeposition of a coating including aluminum, the strip may be fed from coil 12 supported on coil support means through cathodic cleaning tank 16, brushing and spraying apparatus 18, anodic pickling tank 20, brushing and spraying apparatus 22, heater 45 and unit 24, by means of a path established by

rolls

26, 28, 30, 32, 34, 36, 33,

4t

42, 54, 46, 4S and 59. in cleaning tank 15, between rolls 26 and 28 the strip is made electronegative by connecting the negative side of generator 52 (or any other suitable source of current) to conductor roll 26 and the positive side of generator 52 to anodes 54. A suitable cleaning medium for tank 16 has been found to be a heated Orthosil" (Na SiO solution when employing a treating time of 10 to 15 seconds at a minimum current density of 35 amperes per square foot. Any other conventional cathodic cleaning solution employing an alkaline material of high conductivity, good wetting power and free-rinsing qualities normally used in the electrolytic cleaning of ferrous metal strip will, however, serve the purposes of this cleaning operation.

After leaving cleaning tank 1d, the strip 10 passes through brushing and spraying apparatus 13 where it is contacted with rotary brushes 56 both before and after water washing from spray devices 58. This operation removes dragout cleaning solution from tank 16 and mechanically scours the surface of the strip 10 to remove any loosened film from its surface. Squeegee rolls 3?. and 34 remove surface water from the strip.

From brushing and spraying apparatus 18, the strip it passes over roll 36 and into anodic pickler 20. Between rolls 36 and 38 the strip is made electropositive by connecting the positive side of generator 60 (or any other suitable source of current) to conductor roll 36 and the negative side of generator 6t, to cathodes 62. For the purpose of the anodic pickling operation occurring in tank 2%, a suitable electrolyte has been found to be an aqueous sulfuric acid solution of a maximum concentration of 35% sulfuric acid when employing anodic pickling for two to three seconds at to amperes per square foot. However, for the purposes of this pickling operation, other conventional pickling solutions and treating conditions may be employed.

Upon leaving anodic pickler 2d, the strip passes through brushing and spraying apparatus 22 where it is contacted by rotary brushes 64 both before and after cont ct by hot spray water from spray devices This apparatus serves to remove dragout pickle solution from the strip It? as well as residual loosened oxide film, hydrated metal salts, etc. Squeegee rolls 42 and 44 serve to remove surface water from the strip It, and the heat imparted to the strip by the hot spray water tends to cause it to flash dry.

After treatment of the ferrous metal strip in brushing and spraying apparatus 22, it passes through heater and unit 24 via rolls 46, 48 and 50. In unit 24 the strip i6 is completely dried and may be heated to an elevated temperature in a dry reducing atmosphere for the purpose of fur her conditioning the strip surface and removing all traces of metal oxides. Also, the strip may be additionally treated for the purpose of removing completely hydrated metal salts or other substances on the strip surface which are a source of water or which may react to produce water during subsequent passage of the strip through the electroplating line. Sealing flaps '79 are provided to reduce the loss of treating gas employed within unit 24. For reasons to be explained hereinafter, it is important that the strip 10 leave unit 24. at the proper temperature for introduction into unit 72. In the event the strip is heated to a temperature higher than this requisite temperature, it may be allowed to cool to a suitable temperature in section 74 of the unit 24- provided with baflie 76 and flaps 78. Once the strip has been thoroughly dried, the dry strip must not be subjected to any source of water while passing from the drying step to the electrolyte. It is preferred to surround the dry strip at all times with a dry medium such as a dry non-reactive gas, during passage from the drying step to the electroplating bath.

After wet pretreatment of the ferrous metal strip 19,

it would be conventionalin the art of electroplating ferrous metal with a coating containing aluminum to subject the strip as a cathode to a fused aluminum plating electrolyte employing aluminum anodes. However, when this was done it was found that portions of the strip would exhibit a dark undercoating. This dark undercoating is objectionable from a standpoint of appearance to such an extent that the resulting plate would be less acceptable as a commercial product. Even more important, the coating often exhibited poor adherence and high porosity even in instances where the appearance was good. Furthermore, this method of electroplating is subject to the many difficulties enumerated above in connection with water contamination and fuming of the electroplating bath.

In accordance with one important feature of the present invention, the problems resulting from Water contamination and fuming of a fused electrolyte containing from about 80 to 85% aluminum chloride may be eliminated. The clue to the liquid seal means of eliminating this problem came from the discovery that a fused composition containing critical proportions of aluminum chloride and one or more of the other constituents of the fuming electrolyte, although not a satisfactory electrolyte for plating, is substantially non-fuming in the plating temperature range. By use of such a composition, water contamination and fuming of the fused electroplating bath are entirely prevented by eliminating contact between the electroplating bath and the atmosphere, and by completely removing prior to passing the strip into the electrolyte free water, hydrated metal salts, or other substances on the strip surface which are a source of water or which will react to produce water under conditions present in the electroplating zone. Further, it has been discovered that the strip is preheated to an elevated temperature sufficiently high to eliminate the problems of poor adherence and high porosity in the aluminum-containing coating. Additionally, the electric current carrying characteristics of this non-fuming composition are such that it can provide a medium in which electrolytic surface treatment of the ferrous metal strip can take place whicheliminates objectionable dark undercoating in the plated product. All this is accomplished by using the non-fuming composition as a liquid seal through which the strip passes on its way to and from the electroplating bath and at the same time, if desired, using the liquid of the seal on the entrance side of the electroplating bath as an electrolyte for an anodic treatment of the strip which in accordance with the present discovery eliminates all objectionable dark undercoating in the final plated product.

In a preferred embodiment of the present invention in which ferrous metal strip is continuously plated with aluminum or aluminum alloys, the surface of the bath is enclosed and access to the bath is limited to strip inlet and outlet zones. Elimination of atmospheric contact with the electrolyte through the strip inlet and outlet zones in accordance with the liquid seal type of scaling is achieved by interposing a fused non-fuming composition of the type to be described hereinafter across the path of the inlet and outlet zones but out of contact with the fused aluminum plating electrolyte. The completely dried strip free of water or sources of Water as above described is then fed into the electrolyte by passing it through a liquid seal, i.e., the interposed fused non-fuming composition, electroplated and subsequently passed from the electrolyte via the exit zone through the fused non-fuming composition interposed across its path. As will be explained more fully hereinafter, immersion of the strip in the hot nonfuming fused composition preheats the strip to a temperature in the vicinity of the electrolyte temperature and anodic treatment of the strip during its passage through the fused composition interposed across the inlet zone and prior to its entry into the electrolyte Will further condition the strip so as to assure a highly acceptable aluminum or aluminum alloy plated product.

More particularly and with reference to FIGURES 1 and 2 of the drawing, a fused electrolyte containing, for example, 82% aluminum chloride, 9% sodium chloride and 9% potassium chloride is contained in electroplating unit 80. A hood 82 of rectangular cross section and open at its lower end is positioned above electroplating unit 89 with its lower edges 84 below the level of the upper edges 86 of unit 80. Unit '72 which is to contain the non-fuming fused composition for the above mentioned purposes is shown constructed so it completely surrounds unit 80 while permitting the lower edges 84 of hood 82 to lie at a level below the level of the upper and outer edges 88 of unit 72. Upon filling unit 72 with a fused non-fuming composition of the present invention, such as 70% aluminum chloride, 15% potassium chloride and 15 sodium chloride, to a level above that of the lower edges 84 of hood 82 but below that of the upper edge 86 of the unit 80, atmospheric contact with the fuming electrolyte 78 is prevented. Furthermore, by extending the exit end of unit 24 below the surface of the non-fuming composition in unit 72, oxidation of strip 10 by atmospheric contact is prevented.

Electroplating unit 80 is provided with anodes 90 of high purity aluminum. These anodes: are electrically connected to the positive side of a generator 92 (or some other source of electrical current) while the strip is made p electro-negative between rolls @4- and 96 by an electrical connection between roll 96 and the negative side of generator 92. Rolls 98, 1M and 103 serve to guide the strip through the fused non-fuming composition and into the electrolyte 78. Rolls 102 serve as squeegee rolls to remove dragout composition from unit 72. It will be appreciated that a plurality of passes may be made through the electrolyte 78 for the purpose of electroplating with suitably arranged anodes being provided for each pass. Also, the strip need not enter the electrolyte as an active cathode and may be subjected to one or more passes in the electrolyte for the purpose of further preheating the strip prior to commencing electroplating of an alumiumcontaining coating thereon. FIGURES 1 and 2 of the drawings illustrate a downward and upward pass only for the sake of simplicity.

Upon emerging from the electrolyte 78, the strip 10 passes through squeegee rolls 104 and is guided by rolls 196, 168 and 110 through the fused non-fuming composition of unit 72.

For the purpose of preventing heat leaks to the atmosphere, units 72 and are positioned within unit 112 and insulating material or a heated medium such as oil is employed in the space 114. Additionally, and in order to maintain the electrolyte '7 8 and the non-fuming fused com position in unit 72 at the proper temperature, immersion heating may be employed. It is even more convenient to preheat the strip to an elevated temperature higher than the sealing composition and the electrolyte and, upon passage of the strip therethrough, the proper operating temperature may be maintained without a further source of heat.

In accordance with still another feature of the present invention, it has been found that the passage of the strip 10 through the non-fuming fused electrolyte 72 prior to electrodeposition of aluminum surface conditions the strip in a manner serving to lessen the dark undercoating condition often found to exist in the absence of such treatment. The extent of the surface conditioning obtained in the fused non-fuming composition is dependent in some degree on the time of treatment therein. Thus, the speed of the strip, the length of the path through the fused composition and the volume of non-fuming fused composition determines the elfectiveness of this treatment. Still more satisfactory results may be obtained in the surface treatment by anodically treating the strip in the non-fuming composition. For the purpose of this anodic treatment, aluminum cathodes 116 are electrically connected to the negative side of generator 118 while the strip is made electro-positive between rolls 50 and 98. Current densities of to amperes per square foot for 5 seconds have been found satisfactory for this treatment. Minimum current densities and time conditions are dictated by the extent of the surface condition being rectified. Maximum current densities and time conditions are dictated by the extent this treatment can be carried out without pitting the base metal to an extent which will interfere with the quality of the finished product. Also, the strip residence time within the sealing composition should be such as to preheat the strip to a temperature eliminating poor adherence and high porosity in the coating.

Strip 10, after emergence from unit 72, has its dragout losses reduced by the action of squeegee rolls 120 and then passes via rolls 110 and 122 into unit 124 where hot water sprays 126 serve to remove any excess material carried from the electroplating unit. The strip subsequently passes via rolls 128, 130 and 132 to coiling unit 134. During this passage, the strip may be heated in unit 136 for the purpose of assuring complete drying prior to coiling.

In accordance with the present invention those compositions which are suitable for use as a liquid seal in eliminating water contamination and turning problems associated with the electroplating electrolyte and/or providing a medium for treating the strip prior to electroplating in order to obtain improved results are those aluminum chloride containing compositions in which the aluminum chloride content is less than by Weight. The balance of this fused non-fuming composition is made up of at least one salt selected from the group consisting of sodium chloride and potassium chloride. In the case of a binary plating electrolyte such as to aluminum chloride and 20 to 15% sodium chloride, the preferred non-fuming fused composition of the present invention would consist of aluminum chloride and sodium chloride, the aluminum chloride content being less than 70% by weight. In the case of a ternary electrolyte such as one containing from 80 to 85% aluminum chloride and the balance sodium chloride and potassium chloride, the preferred fused non-fuming composition would consist of aluminum chloride, sodium chloride and potassium chloride, the aluminum chloride content being less than 70% by weight.

More specifically, the non-fuming compositions of the present invention comprise those compositions containing aluminum chloride and at least one salt selected from the group consisting of sodium chloride and potassium chloride in which the proportion of aluminum chloride is less than about 52 mol percent. When the mol percent of aluminum chloride is less than about 52%, the vapor pressure of aluminum chloride at practical operating temperatures (below 400 F.) is not sufficient to permit fuming. At a preferred upper aluminum chloride proportion of not more than 50 mol percent, temperature appears to have no appreciable effect on fuming. This is thought to be, in the case of the binary non-fuming compositions, owing to the formation of the compound NaAlCL; or KAlCl In the case of the ternary electrolyte, a mixture of these compounds is apparently formed. In such cases, any excess sodium chloride or potassium chloride would be present as a free salt.

An important aspect of the present invention is the fact that the composition employed to eliminate the water contamination and fuming problems associated with the electrolyte contains or may contain substantially the same constituents as the electrolyte. Since this is true, dragout losses on the strip from the non-fuming composition will not introduce undesired components into the electrolyte. Furthermore dragout losses from the electrolyte introduced into the fused non-fuming composition as the strip passes through the latter will not contaminate the fused non-fuming composition with constituents other than those desired in a usable composition.

The fused non-fuming composition gradually becomes contaminated with undesirable products resulting from the hydrolysis of aluminum chloride, hydrated metal salts,

etc., as it performs its function of preventing water contamination and fuming of the electrolyte. As a result, some provision must be made for maintaining the concentration of contaminates at a low level in order to prevent gradual contamination of the electrolyte through dragout losses from the sealing bath. This may be accomplished by any one of a number of different methods such as by batch replacement of the contaminated sealing bath with a fresh fused non-fuming composition at intervals dictated by the rate of build-up in the concentration of contaminates, or by continuous or intermittent withdrawal of contaminated sealing composition and addition of fresh fused non-fuming composition to the sealing composition in quantities sufficient to maintain a low concentration of contaminates. An even more satisfactory metl od comprises combining either of the foregoing methods with continuously circulating the fused non-fuming composition to an enclosed storage tank filled with fused nonfuming composition and having a volume at least several times the volume of unit 72. Such continuous circulation is beneficial in that it assures a much more gradual buildup and better control over the concentration of contaminates.

It is desirable, as explained above, to employ a nonfuming sealing composition substantially free of water, products resulting from the hydrolysis of aluminum chloride, hydrated metal salts, iron salts, etc., and containing the same constituents as those present in the fuming electrolyte. This eliminates dragout contamination of the electrolyte with constituents other than those already present. When employing a ternary electrolyte of aluminum chloride, potassium chloride and sodium chloride and a ternary non-fuming sealing composition of the same constituents, it is preferred to establish and maintain a weight ratio of potassium chloride to sodium chloride in the sealing composition identical with the weight ratio of potassium chloride to sodium chloride in the electrolyte. By doing this even though dragout losses change the composition of the electrolyte slightly by reducing the aluminum chloride content and change the non-turning composition by increasing its aluminum chloride content, the weight ratio of potassium chloride to sodium chloride remains the same in both the electrolyte and sealing compositions. As a result, when it becomes desirable to adjust the composition of either or both of these compositions so as to maintain proper operating conditions, there is no practical problem. In the case of the electrolyte, simple addition (either continuous or batch) of the proper amount of aluminum chloride will serve the purpose. In the case of the non-fuming composition simple addition (eithcr continuous or batch) or potassium chloride and sodium chloride in the same weight ratio as already present will serve the purpose. I this weight ratio were not initially the same in both th electrolyte and the sealing compositions, readjustment or the compositions during or between plating operations would be exceedingly complicated instead of a routine operational problem within the skill of operating personnel.

The following exemplifies the meaning of identical weight ratios of potassium chloride to sodium chloride in the ternary electrolyte and non-fuming compositions. If the electrolyte initially consists of 84% aluminum chloride, 8% potassium chloride, and 8% sodium chloride (a 111 Weight ratio of potassium chloride to sodium chlo ride), then a satisfactory non-fuming composition would be one containing 67% aluminum chloride, 16.5% pota sium chloride, and 16.5% sodium chloride (also a 1:1 weight ratio of potassium chloride to sodium chloride). It is not necessary for this feature of the invention that this weight ratio be a 1:1 ratio. Instead, another ratio such as a 4:1 ratio for the electrolyte would be suitable if the same weight ratio existed in the non-fuming sealing composition.

Since a change in the composition of the fused electrolyte will alter its melting or solidificationpoint and since preferred conditions for electroplating are those at which the electrolyte composition is maintained at a relatively low temperature while still remaining in the fused state, it is impossible to state a range of preferred temperature for the electrolyte itself. It may be broadly stated that satisfactory plating will occur if the temperature of the electrolyte is maintained at some value lying between approximately 250 and 40K)"v F. providing the electrolyte remains in the fused state. Thus, for example, a ternary composition containing 80% aluminum chloride, and 10% sodium chloride and 10% potassium chloride by Weight was found to give good deposits at plating temperatures as low as 250 F. However, better results generally are obtained at an electrolyte temperature of about SOD-350 F. and preferred results at about 325 F. Assuming sufiicient agitation, good deposits can be obtained at cathode efiiciencies as high as 80% when employing current densities up to 150 amperes per square foot, and current densities up to 300 amperes per square-foot are possible.

Referring now to FIGURES 3 and 4 of the drawing, which illustrate a specific embodiment of the invention employing the slotted hood type of sealing, the ferrous metal strip 210 after receiving a wet pretreatment in cathodic cleaning tank 16, brushing and spraying apparatus 18,-anodic pickling tank 20 and brushing and spraying apparatus 22 identical with that previously described in connection with FIGURE 1, is shown passing under roll.227 and upward through strip conditioning unit 239, where the ferrous metal strip is thoroughly dried to remove all traces of free Water and then passes over rolls 237 and 238. on passing through heater 236, the strip is heated to an elevated temperature to evaporate any free water remaining on the surface and to preheat the strip prior to its entry into the electrolyte. The ferrous metal strip 210 should be treated within strip conditioning unit 239 in an inert to reducing atmosphere, i.e., a non-oxidizing atmosphere or otherwise treated for the purpose of further conditioning the strip surface and preventing oxidation or removing oxides from the surface. Two pairs of suitable sealing flaps 240 and 241 are provided near the entrance and exit ends, respectively, ofstrip conditioning unit 239 to prevent undue loss of any dry gaseous treating agent which may be employed in strip conditioning unit .239, as well as aiding in preventing entry of atmospheric water vapor. The dry gaseous treating agent may be supplied to strip conditioning unit 239 by means of conduit 242 and Withdrawn by means of conduit 24-3. The flow rate of dry gaseous treating agent to strip conditioning unit 239 via conduit 242 is controlled by valve 244, while the withdrawal rate via conduit 243 is controlled by means of valve 245. Any suit- I able dry, i.e., water free gaseous treating agent may be employed in strip conditioning unit 239, depending upon the nature of the treatment desired. However, it is essential that the treatment be such so as to insure the surface of ferrous metal strip 210 being free of free water, combined water such as hydrated ferrous metal salts or substances reacting under conditions present in the electroplating zone to form water upon exit from strip conditioning unit 239. In addition, it is essential once the strip has been thoroughly dried and freed of all sources of water that the dry strip not be subjected to any source of water while passing from the drying step to the electrolyte. It is preferred to surround the strip at all times with a dry medium, such as a dry, non-oxidizing or non-reactive gas, during passage from the drying step to the electroplating bath. In instances where a special conditioning treatment is not necessary, the gaseous treating agent may be dry air, dry nitrogen, dry carbon dioxide, dry argon, and the like. Usually, it is preferredthat the gaseous treating agent be supplied to strip conditioning unit 239 at substantially atmospheric pressure although higher pressures may be used where desirable.

' The ferrous metal strip 210 passes downwardly from strip conditioning unit 239 into electroplating tank 246 containing fused electrolyte 247. The fused electrolyte 247 has a high aluminum chloride content such as for example 82% aluminum chloride, 9% sodium chloride and 9% potassium chloride, by weight, and fumes in the presence of water or Water vapor under operating conditions. A hood 2 58 is positioned above electroplating tank 246, as best seen in FIG. 4. The exit end 297 of strip conditioning unit 239 extends downwardly a short distance through opening 286 in hood 248 and, similarly, the entrance end 2533 of unit 268 extends downwardly a short distance through opening 287 in hood 24%, thereby providing an entrance and an exit, respectively, to electroplating tank 246 for ferrous metal strip 21% The hood 248 is joined to both strip conditioning unit 239 and unit 268 in air tight reiationship, and since sealing flaps 241 and 281 are provided near ends 267 and 2%, respectively, this combination of elements substantially seal off electrolyte 247 from the surrounding atmosphere. A dry gas such as dry nitrogen, dry carbon dioxide, dry argon, or dry air, is fed by means of conduit 24% into the space 285 within electroplating tank 246 above the level 284- of electrolyte 247 and the undersurface of hood 248. A valve 25!) is provided in conduit 249 for the purpose of controlling the feedrate of dry gas fed to space 285.

Electroplating tank 246 is also provided with spaced pairs of anodes 251 and 253259 of high purity aluminum. The ferrous metal strip 210 passes downwardly from roll 238, into fused electrolyte 247, and is subsequently passed between the spaced pairs of anodes 251 and 253-252 by means of a path established by rolls 252 and 26ti-265. The spaced pairs of anodes 251 and 253259 are electrically connected to the positive side of generator 266 (or other suitable source of current), while ferrous metal strip 21%) is made electro-negative between contact rolls 238, 252, 261, 263 and 265 and the corresponding pairs of electrodes operably arranged with respect to each of the rolls by means comprising an electrical connection between each of the rolls and the negative side of generator 266. The contact roll 238 may be electricaly disconnected by opening switch 2%, if desired, and thereby allow the strip to enter the electrolyte as an inactive cathode and be heated to a suitable temperature by the electrolyte prior to commencing the electrodeposition of a coating thereon. The ferrous metal strip 210 is electroplated with a coating ofaluminum of desired thickness while within electroplating tank 246, with the electroplating conditions being substantialy the same as described herein for FIGURES 1 and 2.

After passing under roll 265, the ferrous metal strip 21% passes upwardly through electrolyte 247 and after emerg ing therefrom passes between squeegee rolls 267, which tend to reduce the dragout of electrolyte. The ferrous metal strip 21% continues to pass upward through unit 268, and then horizontally over roll 269. A pair of sealing flaps 272 are provided at the exit end of unit 268 for the purpose of sealing off the exit and thereby allow a slight suction to be applied to unit 268 through exhaust conduit 273, if desired, as well as preventing the entry of atmospheric Water vapor. The exhaust conduit 273 is provided with valve 274 for the purpose of controlling exhaust of unit 268 ininstances where unit 268 is exhausted. The plated ferrous metal strip 21%, after passing over roll 269, passes from the exit end of unit 268 and is then washed with water and dried in the manner described for FIGURES 1 and 2.

For the purpose of minimizing loss of heat, electroplating tank 246 may be surrounded by an insulating material 283. Additionally, and in order to maintain the fused electrolyte 247 at the proper operating temperature, heating means (not shown) may be employed as conventional in the art. For example, immersion heating may be em- 1L ployed in the conventional manner or the insulating material 283 may be spaced from the electroplating tank 246 and the electroplating tank 246 surrounded within the space thus formed between insulating material 283 and electroplating tank by suitable heating means such as fin type strip heaters. Preferably, the strip is preheated to a temperature above the temperature of the electrolyte to thereby supply sufiicient heat to maintain the proper operating temperature Without an additional source of heat.

In accordance with the present invention, it is essential that the gas or gaseous mixture fed through conduit 249 be dry. The temperature of the entering gas may be at any suitable convenient temperature, such as room temperature. The pressure employed Within space 285 may be any suitable convenient pressure, a pressure of substantially atmospheric pressure or slightly above having been found to be sufficient. Inasmuch as it is only necessary to replace the atmosphere within space 285 with a dry gas, and since a slight suction may be maintained, if desired, on strip conditioning unit 239 and unit 268 by means of

exhaust conduits

243 and 273, respectively, thereby allowing the gas fed to space 285 together with any moisture or water content contained therein to more easily escape between the flexible sealing flaps 241 and 271 and ferrous metal strip 210, it is not usually necessary that the gas be supplied or the atmosphere within space 285 be maintained at a substantially elevated pressure. However, a pressure within space 285 above atmosptheric pressure may be preferred in some instances for best results. The composition of the specific dry gas or dry gaseous mixture supplied to space 285 may vary widely. It is only necessary that the dry gas or gaseous mixture be substantially non-reactive under the conditions of operation of the electroplating line with the electrolyte 247, the ferrous metal strip 210, and the apparatus contacted by the gas or gaseous mixture. Suitable dry gases have been found to be nitrogen, argon, carbon dioxide, or mixtures thereof. However, best results are obtained when using a relatively heavy dry gas or gaseous mixture, with reference to air, such as carbon dioxide, argon, etc., rather than gases or gaseous mixtures lighter than air. The amount of dry gaseous treating agent supplied to space 285 will vary depending upon a number of factors such as the amount of contaminating moisture entering space 2-85, the volume of space 285, the rate of loss of dry gaseous treating agent, etc. When the volume of space 285 is 90 cubic feet, a. dry gaseous treating agent feed rate of about -100 cubic feet per hour is generally sufficient under normal operating conditions. However, in every instance the feed rate of dry gaseous treating agent to space 285 is at least sufficient to provide a dry atmosphere therein, but larger quantities may be used. The gaseous treating agent may be supplied to space 285 intermittently but, preferably, it is supplied continuously for best results.

The provision of a dry gaseous atmosphere within space 285 has been found to be highly effective in controlling the gradual entry of water into the electroplating bath and the deleterious effects thereof. As a result, it is possible to continuously operate the slotted hood type of electroplating apparatus when using the method of the present invention over extended periods of time with substantially no Water contamination or fuming of the electrolyte.

It is essential that the strip be preheated to a ternperature above the melting point of the fused electrolyte at least at the time of commencing to electroplate the aluminum-containing coating. Otherwise, poor adherence and/or porosity invariably result. Preferably, the strip should be at a minimum temperature of about F. below the temperature of the fused electrolyte. At strip temperatures more than 25 F. below the electrolyte temperature, the best results are not obtained. Higher temperatures may be employed and are usually preferred within reasonable limits. For example, it is very advantageous to preheat the strip to a temperature above the operating temperature of the electrolyte and thereby use the strip as a source of heat. Obviously, the strip should not be at a sufficiently high temperature to overheat the electrolyte at the rate of strip feed. Usually, when continuously plating metal strip, an entry temperature into the electrolyte of about 25 F. above the electrolyte operating temperature is preferred. Thus, in view of the foregoing, the preferred strip temperature at least at the time of commencing to electroplate the coating is within about 25 F. of the operating temperature of the electrolyte. When an electrolyte is used containing by weight about parts aluminum chloride, 10 parts sodium chloride and 10 parts potassium chloride, the strip entry temperature may be 250-400 F. and preferably 300-350 F.

The strip may be preheated by any suitable prior art means such as by resistance heating in a non-oxidizing atmosphere prior to entry into the bath, or it may be heated by immersion in the electrolyte as an inactive cathode for a period of time suilicient to reach the necessary temperature level and then the electrodeposition of the coating on the heated strip is commenced. For -100 pound strip, about a 10 second immersion period usually is required for both good adhesion and low porosity, about 7 seconds immersion will give deposits of good appearance and low porosity but of questionable adhesion, and less than 7 seconds immersion time results in poor adhesion and porosity. Longer or shorter immersion periods may be selected for heavier or lighter weights of strip, respectively, than 90-100 pounds as will be recognized by those skilled in the art.

While the electrodeposition of aluminum coatings has been specifically described herein, it is understood that aluminum-containing coatings in general may be electroplated in accordance with the teaching of the invention. Also, other suitable electrolytes and/ or plating conditions may be used than the ones specifically described herein, such as those disclosed for electroplating aluminum and/or aluminum-manganese coatings in copending application Serial No. 819,298, filed June 10, 1959, for Method of Coating and Product, by Edwin J. Smith, Michael G. Vucich and Lowell W. Austin, now abandoned. The teachings of this copending application are incorporated herein by reference.

The foregoing detailed description and the following specific examples are for purposes of illustration only, and are not intended as limiting to the spirit or scope of the appended claims. All percentages appearing in the following examples are percent by weight.

Example I Pretreated ferrous metal strip at a temperature of 375 F. and at a speed of 10 feet per minute is fed through plating apparatus similar to that illustrated in FIGURE 1 employing a fused non-fuming sealing composition for the purpose of eliminating water contamination and fuming of the electrolyte. The ferous metal strip was completely free of free water, combined Water such as hydrated metal salts, and substances reacting under conditions present in the electroplating zone to form water at the time of entering the electroplating zone.

The fused non-fuming composition is maintained at a temperature of 350 F. and contains 68.2% aluminum chloride, 15.9% potassium chloride and 15.9% sodium chloride. After passage through the inlet non-fuming composition, the strip is introduced into an electrolyte which is maintained at a temperature of 325 F. and comprising 80% aluminum chloride, 10% potassium chloride and 10% sodium chloride. Employing high purity aluminum anodes 10 feet in length, a 0.18 lb. per base box coating weight is obtained at current conditions of 42.5 amperes per square foot. Under these conditions.

the cathode efi'iciency is 80%. After plating, the strip is fed from the electrolyte through the outlet zone having the non-fuming fused composition interposed across its path. During this operation, no fumes escape to the atmosphere and there is no indication of water contamination of the electrolyte even after an extended period of operation. The resulting plated product has a dense matte deposit of fine grain which is pleasing in appearance except for an occasional grayish appearance over small percentage areas of the strip. In addition, the coating exhibits a low porosity and is adherent, soft and ductile and can be mechanically brightened by light cold rolling. Both the sealing composition and the electrolyte were readily maintained at their respective operating temperatures since the entering strip was preheated in a protective atmosphere to a higher temperature.

Example II The conditions of Example I are repeated except for the use of an anodic treatment in the non-fuming fused composition occupying the inlet zone. The anodic treatment employs high purity aluminum cathodes of about 2 foot length with current conditions of 40 amperes per square foot for 5 seconds.

The product of this example is identical with the prod- The conditions of Example I are repeated except for the use of electroplating apparatus similar to that illustrated in FIGURE 3 employing the slotted hood type of sealing. The heater 236 is employed to heat the strip to a temperature of 375 F. in a nitrogen atmosphere maintained in unit 239. Dry nitrogen is fed to space 285 at the rate of 72 cubic feet per hour, for the purpose of eliminating water contamination and fuming of the electrolyte. The volume within space 285 was about 90 cubic feet.

During this operation, no fumes escape to the atmosphere and there is no indication of water contamination of the electrolyte even after extended periods of operation. The product of this example is similar in appearance to that of Example 11. However, the coating weight is considerably heavier due to the greater number of passes between the various pairs of electrodes. The coating is very adherent and exhibits a low porosity when the entering strip has a temperature of 375 F.

When the entering strip temperature is below 250 F. and the strip enters as an active cathode, a very porous and non-adherent coating is obtained. The adherency and porosity improve with increasing strip temperature and the coating is very satisfactory at entering strip temperatures of 300 F. or higher. The deposit is adherent and non-porous regardless of the entering strip temperature when 90 lb. strip is allowed to enter the bath as an inactive cathode and then immersed for at least seconds before becoming an active cathode such as when opening switch 206. Shorter periods of immersion result in porous and/or non-adherent coatings even though appearance is satisfactory in some instances. For heavier strip than 90 lb., longer periods of immersion prior to commencing the electrodeposition are equally efiective.

What is claimed is:

1. In a method of electroplating an aluminum-containing coating on a metal sheet wherein the metal sheet is electroplated with the aluminum-containing coating while being passed through a body of fused electrolyte for electroplating an aluminum-containing coating comprising aluminum halide and at least one alkali metal halide selected from the group consisting of sodium halide and potassium halide in a proportion containing free aluminum halide, the improvement which comprises maintaining the temperature of the fused electrolyte at about 250- 400 F., improving the adherence of the aluminum-containing coating by preheating the metal sheet and then electroplating the aluminum-containing coating on the heated metal sheet while immersed in the electrolyte the sheet being preheated to a temperature substantially higher than the minimum temperature necessary to prevent deposition of frozen electrolyte thereon and to a temperature which will substantially improve adherence of the aluminum-containing coating, the heated metal sheet being at a temperature of about 300-400 F. at the time of commencing to electroplate the aluminum-containing coating.

2. The method of claim 1 wherein the heated metal sheet is at a temperature of about BOO-400 F. and the temperature of the electrolyte is within about 25 F. of the temperature of the heated metal sheet at least at the time of commencing to electroplate the aluminum-containing coating.

3. The method of claim 1 wherein the aluminum halide is aluminum chloride and the alkali metal halide is alkali metal chloride.

4. The method of claim 3 wherein the electrolyte is maintained at a temperature of about 300-350 F. and the heated metal sheet is at a temperature of 325350 F. at least at the time of commencing to electroplate the aluminum-containing coating.

5 In a method of electroplating an aluminum-containing coating on ferrous metal strip wherein the strip is electroplated with the aluminum-containing coating while immersed in a body of fused electrolyte for electroplating an aluminum-containing coating comprising aluminum halide and at least one alkali metal halide selected from the group consisting of sodium halide and potassium halide in proportions containing free aluminum halide, the improvement comprising maintaining the temperature of the fused electrolyte at about 250-400" F., improving the adherence of the aluminum-containing coating by heating the metal strip in a non-oxidizing atmosphere, continuously passing the heated strip through the body of fused electrolyte and continuously electroplating the heated metal strip with the aluminum-containing coating, the strip being heated to a temperature substantially higher than the minimum temperature necessary to prevent deposition of frozen electrolyte thereon and to a temperature which will substantially improve adherence of the aluminum-containing coating, the heated strip being at a temperature of about 300-400 F. at the time of commencing to electroplate the aluminum-containing coating to improve the adherence thereof.

6. The method of claim 5 wherein the heated strip is at a temperature of about 300-400 F. and the temperature of the electrolyte is within about 25 F. of the temperature of the heated strip at least at the time of commencing to electroplate the aluminum-containing coating.

7. The method of claim 5 wherein the aluminum halide is aluminum chloride and the alkali metal halide is alkali metal chloride.

8. The method of claim 7 wherein the electrolyte is maintained at a temperature of about 300-350 F. and the heated strip is at a temperature of 325-350 F. at least at the time of commencing to electroplate the aluminum-containing coating.

9. In a method of electroplating an aluminum-containing coating on ferrous metal strip wherein the strip is electroplated with the aluminum-containing coating while immersed in a body of fused electrolyte for electroplating an aluminum-containing coating comprising aluminum halide and at least one alkali metal halide selected from the group consisting of sodium halide and potassium halide in proportions containing free aluminum halide, the improvement comprising maintaining the temperature of the fused electrolyte at about 25 0-400 F., heating the metal strip in a non-oxidizing atmosphere, continuously passing the heated strip through the body of fused electrolyte and continuously electroplating the heated metal strip with the aluminum-containing coating, the heated strip being passed into the body of fused electrolyte while initially at a temperature above that of the electrolyte to provide at least a portion of the heat required for maintaining the temperature of the electrolyte and the heated stripbeing at a temperature of about BOO-400 F. at the time of commencing to electroplate the aluminum-containing coating to improve the adherence thereof.

10. The method of claim 9 wherein the aluminum halide is aluminum chloride and the alkali metal halide is alkali metal chloride.

References Cited in the file of this patent UNITED STATES PATENTS Goodson May 9, 1905 Aylsworth Apr. 10, 1906 Monnot Oct. 3, 1911 Wright May 12, 1931 Pink Jan. 2, 1934 Bornhauser Jan. 21, 1936 Sendzimir June 4, 1946 Alferieif Oct. 9, 1951 OTHER REFERENCES Murphy: Metal Finishing, April 1952, pages 76-79.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,136 709 June 9 1964 Edwin J. Smith et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3-, line 44, strike out "the"; line 56, for "strip" read strip, column 6, lines 21 and 22, for "atmospheric" read atmosphere column 8, line 51, for "or",i..second occurrence, read of column 10, line 20,

for "281" read 271 Signed and sealed this 20th day of October 1964.

SEAL fittest:

ERNEST W; SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents

Claims

1. IN A METHOD OF ELECTROPLATING AN ALUMINUM-CONTAINING COATING ON A METAL SHEET WHEREIN THE METAL SHEET IS ELECTROPLATED WITH THE ALUMINUM-CONTAINING COATING WHILE BEING PASSED THROUGH A BODY FUSED ELECTROLYTE FOR ELECTROPLATING AN ALUMINUM-CONTAINING COATING COMPRISING ALUMINUM HALIDE AND AT LEAST ONE ALKALI METAL HALIDE SELECTED FROM THE GROUP CONSISTING O SODIUM HALIDE AND POTASSIUMHALIDE IN A PROPORTION CONTAINING FREE ALUMINUM HALIDE, THE IMPROVEMENT WHICH COMRPISES MAINTAINING THE TEMPERATURE OF THE FUSED ELECTROLYTE AT ABOUT 250400*F., IMPORVING THE ADHERENCE OF THE ALUMINUM-CONTAINING COATING BY PREHATING THE METAL SHEET AND THEN ELECTROPLATING THE ALUMINUM-CONTAINING COATING ON THE HEATED METAL SHEET WHILE IMMERSED IN THE ELECTROLYTE THE SHEET BEING PREHEATED TO A TEMPERATURE SUBSTANTIALLY HIGER THAN THE MINIMUM TEMPERATURE NECESSARY TO PREVENT DEPOSITION OF FROZEN ELECTROLYTE THEREON AND TO A TEMPERATURE WHICH WILL SUBSTANTIALLY IMPROVE ADHERENCE OF THE ALUMINUM-CONTAINING COATING, THE HEATED METAL SHEET BEING AT A TEMPERATURE OF ABOUT 300-400*F. AT THE TIME OF COMMENCING TO ELECTROPALTE THE ALUMINUM-CONTAINING COATING.