US2825681A - Electroplating - Google Patents

Description

March 1953 s. s. JOHNSTON ELECTROPLATING Filed Feb. 10, 1955 INVENTOR SAMUEL S. JOHNSTON BY aw ATrRNEY United States Patent {3 F ELECTRDPLATING Samuel S. Johnston, Steubenvill'e, Ohio, assignor to National Steel Corporation, a' corporation of Delaware Application February 10, 1953, Serial No. 336,124 3 Claims. (CL 204-43) This invention relates to a method of electroplating strip material with a coating of metaland is primarily concerned with electroplating ferrous metal strip, such asblack plate, with a coating of protectivemetal, such as tin, while the strip is moving at a high speed.

When continuously and progressively electroplating strip steel with tin, the strip is continuously moved through a long electroplating line at speeds of about 1,000 feet per minute to 2000 feet per minute and higher. The electroplating line includes a large number of units disposed along the path of the strip for treating and electroplating the strip while traveling at such high speeds. These units include a unit for cleaning the strip and a unit for scrubbing and washing the cleaned strip to remove any cleaning solution dragged out of the cleaning unit by thestrip. In addition, the electroplating line includes a unit for pickling the cleaned and. washed strip and a unit for scrubbing and washing the pickled strip with cold water to remove any residual pickle liquor dragged out of the pickling apparatus. This wet, washed strip is then passed through the electroplating unit in which at least one surface of the strip is maintained in contact with a bath of electroplating solution. The clec: troplating unit may include one, and usually a plurality, of electroplating cells in each of which there is provided one or more tin anodes. As the strip moves across or through the electroplating cells, current is passed from the anodes through the electroplating solution to the strip to deposit thereon a layer of anode metal. The electroplating solution will include at least one electrolyte of the metal which is to be plated. Usually, only a thin film of tin is deposited by the electroplating process and for the tin plated product to have sufficient resistance to attack it is necessary that the coating be of the best quality that can be deposited. Accordingly, it is necessary that the units of the electroplating line including the electroplating unit all be operated at peak etficiency in order to produce satisfactory electro-tinplate. Further, it is diflicult to operate such a line at peak efliciency so that at times the quality of the coating will not be satisfactory for reasons which can not be dete'rmined. As a result, there is a need for an improved electroplating method and apparatus which will produce better coatings and which will not be as difficult to opcrate at peak efiiciency as has been the case heretofore.-

The value of a metal coating such as tin on a-base' metal depends in a general way on the uniformity of the thickness of coating obtained. Lack of uniformity in the thickness of a metal coating not only adversely aifects its protective value but also contributes to a product of poor appearance and renders it unsatisfactory for many uses. Although the exact cause of ununiform thickness of coatingv is not known in many instances, there are several knowntypes which may be classified by physical appearance. One such type is known as striation. Striation is a striped or streaked efiect running the length of the strip, having no particular fixed pattern, but being of difierent width along the 2,825,681 Fatented- Mar. 4, 1.958

length of the strip; Striations occur in the center twothirds portion of the strip, the outer limits of the width of the strip usually beingfree of striation. Another ef-" fect often occurring is one which is referred to as anode pattern. Anode pattern is a striped etfect, the stripes being parallel, fairly uniform in width and running along the length of the strip.

Both striation and anode pattern efiects may occur at the same time with the result that the product is highly unsatisfactory from the standpoint of both, protective value and appearance.

Accordingly, it isanobject of the present invention to provide anim'proved method of electroplating the con; ductivevsurfac of material with a metallic coating: which may include one or a plurality. of metals.

Another object of the present invention isto provide an improved electrodeposited coating of protective metal.

A further objec't of the present invention is to provide an improved method'of electroplating ferrous metal strip with tin.

It is also an. object of. the present invention to provide an improved method of electroplating strip material at a high rate of speed with a protectivecoating of metal of superior quality.

A further object of the present invention is to provide an improved method of electroplating strip materials at a high rate of speed with a protective coating of metal of uniform thickness.

Another object ofthe present invention is to provide an improved method of electroplating strip materials at a high rate of speed with a protective coating of metal relatively free of striation and anode pattern.

These. and other objects and advantages willbecome more apparent whenconsidering the following detailed description, taken with the accompanying drawing, in which: 7

Figure l is a side view, partially in section, diagrammatically showing a portion of a strip plating line em bodying the principles of the present invention; and

Figure 2 is a plan view of the apparatus of Figure. 1.

In accordance with the present invention, the quality of an electroplated tin coating is improved if, before the wet-washed strip is electroplated, the strip is heated to a temperature of through F. by bringing a heated aqueous solution containing at least one of the electrolytes and preferably all of the electrolytes of tin present in the electroplating solution into intimate contact with the surface to be plated.

it is to be understood that the present invention is not limited to any particular theory but apparently the very thin aqueous film. which is present on the washed strip reduces the efiectiveness of the electroplating action .in' the electroplating unit and causes striation. It isfln'ot known whether the aqueous film insulates the strip to re; duce the current passing to the strip from the anodes, or whether the aqueous film dilutes the electroplatingsolution adjacent the strip, or' whether the aqueous film reduces the quality of the electrodeposit for some other reason. Whatever the exact cause is, it has been dis covered that the quality of the plated tin can be'improved and striation eliminated by replacing the aqueous film out the washed strip with a film solution that is more nearly like or is substantially the same as the electroplating solution. Film replacement is obtained by physically propel ling the tin electrolyte containing solution against the's un' faces to be plated. The aqueous rinsingsolution is pref-i erably bstantially the same as the electroplating 5 111: tion in so far asthe' electrolyte or electrolytes of in g concerned. A rinsing solution containing only a' very'. small amount of'tin electrolyte'canbe used but better re sultsare obtainedif the rinsing solution contains about the same amountof the same tin electrolyte present in the' 7 Referring to the drawings, the strip which usually will have been previously cleaned and washed by apparatus,not shown, progressively passes over the rotatably supported; rubber-covered roll. 10 to apickler 11 where the strip 'is treated in a suitable pickling solution containing one or more acids; The pickled strip then moves to a scrubbing and washing apparatus 12 where the residual pickling solution is washed off by water. The washed strip passes from the scrubber 12 through the rinsing unit 13 and then to the electroplating unit'14 which may be ofany suitable type. The electroplating unit 14 may be of the type in which the strip is passed through a bath so that both surfaces are simultaneously electroplated. Asis shown in the drawings, electroplating unit '14 is of the type in which the strip is passed across a plurality of electroplating cells in which the lower surface of the strip is in contact with the electroplating bath in each cell so that the lower face is plated with tin. If the opposite surface of the strip is to be plated, the direction of travel is reversed and the strip is brought back across. a second series of electroplating cellswith the opposite surface facing downwardly and in contact with the electroplating solution in the cells so that this opposite surface is' plated withtin. Such an electroplatingline is shown in the bookentitled Tinplate by Hoare and Hedges, l945, .page 263.i After the strip has been electroplated with tin, the plated strip may be treated in any desired manner.

' The pickling apparatus ll includes a tank 16 in which there are mounted a pair of rotatable, rubber-covered rolls 17 for depressing the strip '8 and causing it to pass through the body of pickling solution 18 in the tank 16. Apair of rotatable, rubber-covered wringer rolls 19 are provided to remove the excess pickle liquor and prevent the strip from dragging an excessive amount of pickling solution out of the tank 16. When the strip leaves the wringer rolls 19 at 20, it has thereon a film of pickling solutionand it then passes through the scrubber 12 which removes this residual pickling solution. The scrubber includes a first brush 21 and a cooperating lower back-up 'roll 22. A'plurality of spray pipes 23, each connected to a source of wash water, are arranged to direct water sprays 24 against the strip as it passes the brush21. The scrubber 12 includes a second brush 26 and a cooperating upper back-up roll 27. A plurality of pipes '28 are each connected to a source of wash water and are ar-i ranged to direct streams orsprays of water 29against the strip as it passes between the brush 26 and the backup roll 27. 'The brushes 21 and 26 maybe driven and rotated in'a direction opposite to the direction of strip travel. Any suitable means may be provided for driving the brushes; A tank 30 is provided for collecting the excess wash water. A pair of wringer rolls 32 are provided for removing 'excess liquid from the strip.

Guide rolls 33 and 34 are provided adjacent the rinsing unit '13. The rinsing unit 13 includes a tank 35 and a source of supply 36 of rinsing solution. .The source of supply 36 of rinsing solution is arranged for directing jetsor streams pr rinsing solution'37against both sides of the 'central portion of the strip S as it entersthe tank 35. Rinsing solution 45 is preferably fed to supply means 36 from a recirculating. tank 38.; This is done by means 0t pipeline 3? :a'rid'pump 4(LfThe rinsing solution from V to one side of the generator 53.

pump 40 passes through a heat exchanger 41 to the sup ply means 36. Heat exchanger 41 will be described more fully hereinafter. An outlet pipe 42'is provided for tank 35 and preferably is valve controlled so as to'maintain the body of solution in tank 35 at the proper level. Excess rinsing solution is removed from strip S after passing through the rinsing unit 13 by contact roll 55 and back-up roll 57. V I

Electroplating unit 14 includes a plurality of similar electroplating cells 43, only one of whichwill be de scribed in detail. The electroplating cell 43 includes a tray 44 having side troughs 45 and end troughs 46 for collecting the overflowing electroplating solution. Electroplating solution is continuously supplied to the tray from a source 33 of electroplating solution by means described hereinafter. The sides of the tray 44 are of such a height as to maintain a body or bath of electroplating solutionin the tray. with the upper surface at or slightly above the level of the lower surface of strips. Electroplating solution is constantly fed to the tray to replenish the bath and the solution continuouslyflows over the sides of the tray 44 into the collecting troughs 45 and 46. In the tray there is a center anode support 48 and spaced side supports 49 on which are mounted a plurality of anodesSil, each formed of tin. The center support is provided with electrically conductive surfaces which engage the ends of the anodes 5i) and these surfaces are connected through a bus bar 51 and a wire-52 Such anarrangement of anodes is shownand described in'Patent No. 2,399,254, issued April 30, 1946 to Rieger 'et al. Below each end of the'tray 44 there is a collecting pan 54 and these pans 54 collect the electroplating solution-dragged out of the electroplating trays by the strip andalso collect the solution from the overflow troughs. At the entrance side of tray 44, there is a conductive contact roll 55 connected through wire 56 to'. the other sideof generator 53. A rotatable back-up roll'57 is provided forholding the strip in contact withthe contact roll 55. Theisides of the tray 44 are so arranged that the stripis in contact with the upper surfacefof the body'of electroplating'solution in the tray so that'electrical current flows from the anodes through the electroplating solution to .the stripto deposit a layer of tin .on the bottom surface of the strip. r i

The electroplating solution may be of any particular type, for example, the stannous sulphate acid type or any other suitable type which contains oneor more tin electrolytes (see United States Pa.tents.2,512,7l9 and 2,598,- 486). contains stannous sulphate which is anelectrolyte ofthe metal being plated. As the tin ions migrate to and are deposited on the strip which'is the cathode, the bath is replenished with tin dissolved from the anodes. N01? mally, the electroplating solution flows. from the collecting trays 54 through drain pipes 5 9 to a common collecta ing or recirculating tank 38 which isjreplenished when necessary with chemicalsto maintain the electroplating solution in the proper condition. Tank 38 preferably is located below and extends the combined length of Washingunit 13 and the electroplating units 14. Heat exchanger coils 69 are provided for controlling the temperature of the electroplating solution in tank 38. The temperature of this electroplating solution will vary in ac-' cordance with such conditions as length of time that the line has been in operation since the last shut down,'current density being employed for electroplating, ambient temperature,- temperatureof make-up solution added to the electroplating solution, 1 etc. Consequently, heat ex,-

changer coils 59 may be used either for heating or cooling the electroplating solution as required. Lines 61, pumps 62, manifold '63 and jet'or spray orifices- 64 are provided 7 for supplying electroplating solution'tothe individual cells 14 at the forward end thereof and physically pro-.3

The stannous sulphate acid bathgfor example, 7

pelllng the same against the central area of both surfaces of the strip.

The rinsing solution 37 contains an electrolyte of tin which is the same as at least one of the tin electrolytes in the electroplating solution. The character of the final coating is improved when the rinsing solution contains tin electrolyte and apparently the reason is that the aqueous film on the strip is replaced with a him of rinsing solution which is more nearly like the electroplating solution. The greatest degree of improvement is obtained when the rinsing solution is the same as the electroplating solution in the electroplating unit 14-. Consequently, the preferred embodiment of the present invention utilizes electroplating solution from tank 38 and receives it through line 36 via line 59, pump it! and heat exchanger 41. If it were desirable to provide rinsing solution containing at least one tin electrolyte from another source, this could be accomplished by directing the line 39 to that source and providing for the return of overflow from tank 35 through line 32 to that same source. Heat exchanger 41 provides the heat necessary for the rinsing solution to obtain a strip temperature of 120 through 160 F. Steam or hot water may be used for this purpose.

If the strip is electroplated when it is at a temperature below 120 F., anode pattern effects result. If, on the other hand, the strip is heated to a temperature above 160 F., decomposition of certain components in the electroplating solution, such as ferro-cyanide compounds will occur. These factors therefore determine the upper and lower temperature ranges of the strip at the time of commencing the electroplating operation. Although the pickling solution 18 is maintained at a temperature of approximately 180 F the rinse water in tank 12 is at the prevailing water supply temperature because of the fact that cold rinse water removes the pickling acid better and prevents the strip from drying. Therefore, upon emergence from roll 32, the strip is at a temperature below 120 F. Thus, in order to obtain the benefits of the present invention, it is necessary to heat the strip prior to electroplating.

Although the preferred temperature of the strip upon commencing electroplating is somewhat dependent upon the coating weight of the tin to be applied, it has been found that a more important factor is the current density being used. For example, when using a current density of 200 amperes-per square foot, it has been found that the temperature of the strip should be at least 125 F. for preferred results. Similarly, at current densities of 300 and 600 amperes per square foot, the minimum temperature should be 135 and 145 F., respectively. In no case should the strip temperature exceed 160 F.

The present invention has been described in connection with the electroplating of tin on ferrous metal strip, but may be used for plating other metals onto other strip materials. Although the rinsing solution preferably is substantially the same as the electroplating solution, it need not be the same and it does constitute an inprovement when the rinsing solution contains only a small amount of the same electrolyte as the electroplating solution.

I claim:

1. The method of eliminating striation and anode pattern in the progressive electrotinplating of a surface of ferrous metal strip at strip speeds of about 1000 feet per minute and higher, comprising the steps of pro gressively passing the strip at a speed of at least about 1000 feet per minute through a washing zone, a rinsing zone and an electrotinplating zone, washing the surface of the strip to be plated with relatively cold water in the washing zone, passing a stream of hot aqueous rinsing solution into the rinsing zone, heating the strip in the rinsing zone in the absence of electrolytic treatment of the strip to a temperature within the range Fahrenheit to Fahrenheit by continuously bringing the hot aqueous rinsing solution into intimate contact with the surface to be plated, and electrotinplating the surface of the heated strip in the electroplating zone while the surface to be plated is in contact with a bath of aqueous electrotinplating solution, the electrotinplating solucontaining at least one tin electrolyte and the rinsing solution containing said one tin electrolyte.

2. The method of eliminating striation and anode pattern in the progressive electrotinplating of a surface of ferrous metal strip at strip speeds of about 1000 feet per minute and higher, comprising the steps of progressively passing the strip at a speed of at least about 1000 feet per minute through a washing zone, a rinsing zone and an electrotinplating zone, washing the surface of the strip to be plated with relatively cold water in the washing zone, passing a stream of hot aqueous rinsing solution into the rinsing zone, heating the strip in the rinsing zone in the absence of electrolytic treatment of the strip to a temperature within the rang 120 Fahrenheit to 160 Fahrenheit by continuously bringing the hot aqueous rinsing solution into intimate contact with the surface to be plated, and eleetrotinplating the surface of the heated strip in the electroplating zone while the surface to be plated is in contact with a bath of aqueous electrotinplating solution, the electrotinplating solution containing at least one tin electrolyte and the rinsing solution containing substantially the same percentage of said tin electrolyte as the electrotinplating solution.

3. The method of eliminating striation and anode pattern in the progressive electrotinplating of a surface of ferrous metal strip at strip speeds of about 1000 feet per minute and higher, comprising the steps of progressively passing the strip at a speed of at least about 1000 feet per minute through a washing zone, a rinsing zone and an electroplating zone, washing the surface of the strip to be plated with relatively cold water in the washing zone, passing a stream of hot aqueousrinsing solution into the rinsing zone, heating the strip in the rinsing zone in the absence of electrolytic treatment of the strip to a temperature within the range 120 Fahrenheit to 160 Fahrenheit by continuously bringing the hot aqueous rinsing solution into intimate contact with the surface to be plated, and electrotinplating the surface of the heated strip in the electroplating zone while the surface to be plated is in contact with a bath of aqueous electrotinplating solution, the electrotinplating solution containing at least one tin electrolyte and the rinsing solution and the electrotinplating solution being substantially the same.

References Cited in the file of this patent UNITED STATES PATENTS 2,497,894 Luke Feb. 21, 1950 2,513,859 Glock July 4, 1950 2,557,823 Holbrook June 19, 1951 2,563,229 Faust et al Aug. 7, 1951 2,569,577 Reading Oct. 2, 1951 2,689,399 Gray Sept. 21, 1954 OTHER REFERENCES Metal Finishing, February 1944, pages 77-79. The Iron Age, April 30, 1942, pages 35-44.

U. S DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE OF CORRECTION Patent N00 2,825,681 March 4, 1958,

Samuel S, Johnston It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 43, for "Wet==Washed" read Wet, washed --5 column 6, llne 24, for "rang" read range Signed and sealed this 29th day of April 1958a (SEAL) Attest:

KARL Ha MINE ROBERT C. WATSON Attesting Officer Commissioner of Patents

Claims (1)

1. THE METHOD OF ELIMINATING STRIATION AND ANODE PATTERN IN THE PROGRESSIVE ELECTROTINPLATING OF A SURFACE OF FERROUS METAL STRIP AT STRIP SPEEDS OF ABOUT 1000 FEET PER MINUTE AND HIGHER, COMPRISING THE STEPS OF PROGRESSIVELY PASSING THE STRIP AT A SPEED OF AT LEAST ABOUT 1000 FEET PER MINUTE THROUGH A WASHING ZONE, A RINSING ZONE AND AN ELECTROTINPLATING ZONE, WASHING THE SURFACE OF THE STRIP TO BE PLATED WITH RELATIVELY COLD WATER IN THE WASHING ZONE, PASSING A STREAM OF HOT AQUEOUS RINSING SOLUTION INTO THE RINSING ZONE, HEATING THE STRIP IN THE RINSING ZONE IN THE ABSENCE OF ELECTROLYTIC TREAT MENT OF THE STRIP TO A TEMPERATURE WITHIN THE RANGE 120* FAHRENHEIT TO 160* FAHRENHEIT BY CONTINUOUSLY BRINGING THE HOT AQUEOUS RINSING SOLUTION INTO INTIMATE CONTACT WITH THE SURFACE TO BE PLATED, AND ELECTROTINPLATING THE SURFACE OF THE HEATED STRIP IN THE ELECTROPLATING ZONE WHILE THE SURFACE TO BE PLATED IS IN CONTACT WITH A BATH OF AQUEOUS ELECTROTINPLATING SOLUTION, THE ELECTROTINPLATING SOLUTION CONTAINING AT LEAST ONE TIN ELECTROLYTE AND THE RINSING SOLUTION CONTAINING SAID ONE TIN ELECTROLYTE.

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