US 3616287 A
An apparatus for the hard-chromium plating elongated bodies of large surface area wherein the body is advanced in stages through the chromium plating bath relative to the anode, and chromium plating is carried out during the transverse in which the substrate is stationary. A pair of equipotential screens is provided in axially spaced relationship at each end of the plating zone, at least one of the screens being positioned upon the previously coated portion of the substrate at a location in which the prior plating has reached its maximum thickness.
Claims available in
Description (OCR text may contain errors)
Unite States atet  Inventors Antoaneta M. Draghicescu;
Aurel C. Radoi, both of Bucharest, Romania  Appl. No. 813,828  Filed Apr. 7, 1969  Patented Oct. 26, 1971  Assignee Institutul de Cercetari Technologice Pentru Constructii de Masini  METHOD FOR HARD-CHROME PLATING LARGE METALLIC SURFACES 6 Claims, 5 Drawing Figs.
 US. Cl 204/25, 204/28, 204/211  Int. Cl C23b 5/56, C23b 5/58, BOlk 3/00  Field of Search 204/DIG. 7, 25, 23,15, 28, 211
 References Cited UNITED STATES PATENTS 1,794,973 3/1931 McBride 204/28 2,232,019 2/1941 Beckwith 204/28 2,540,175 2/1951 Rosenquist.. 204/9 3,415,723 12/1968 Bediet al.. 204/15 3,477,920 11/1969 Bedi 204/15 FOREIGN PATENTS 734,414 5/1966 Canada Primary Examiner-T. Tung Assistant ExaminerT. Tufariello Attorney-Karl F. Ross Hard-Chrome Plating Befh PATENTEDnm 26 ml SHEET 1 [1F 4 mEQ W 5S 1 g Attorney PATENTEDum 26 Ian SHEET 2 [IF 4 Anfoanef'a M. Draghicescu Aurel C. Radoi Invenfors.
K rl 6R0 Attorney PAIENTEBncI 26 men 3.616287 sum 3 BF 4 x/CJ Anfoanefa M. Draghicescu Aurel C. Radol' lnvenfors.
' Attorney METHOD FOR HARD-CHROME PLATING LARGE METALLIC SURFACES Our present invention relates to a method of hard-chromium plating large-surface elongated bodies or substrates and especially elongated shafts, rods, bars and the like which may be used in mechanical-engineering systems, in petroleum engineering and in naval engineering, including (but not exclusively) pistons and other rams, elevator support tubes and shafts, turbine shafts, the power shafts for the screws of ships and like elongated metallic bodies which may have a length of the order of tens of meters.
Various methods have been provided heretofore for the hardchromium plating of large-surface bodies, such methods generally requiring plating tanks and installations of correspondingly large dimensions and high-intensity (high-amplitude) electroplating current sources.
The hard-chromium plating of a body requires a certain electrical current density to obtain the desired degree and quality of the coating and, as the surface area of the body to be placed increases, a corresponding increase in the current supply capacity of the power source is necessary.
Electroplating processes using high-intensity (high-amplitude) electroplating currents, large quantities of electrolyte and large tanks and other installations have heretofore created control and supervisory difficulties, thereby preventing the obtention of uniform hard-chromium plating over the entire surface of a large body, and rendering the plating of large bodies nonreproducible. In fact, the hard-chromium plating of elongated bodies of the character described has heretofore been troublesome and difficult.
It is the principal object of the present invention to provide an improved method of coating large-surface bodies by hardchromium electroplating.
Another object of our invention is the provision of an improved apparatus for the hard-chromium plating of large bodies for the purposes and of the character described.
Another object of this invention is to provide a method of and an apparatus for the hard-chromium plating of elongated bodies, e.g., of a length of the order of tens of meters, which makes use of current sources of unexpectedly low amplitude capacity, uses relatively small amounts of electrolyte, is readily controlled and supervised, and gives rise to uniform reproducible coatings over the entire length ofthe body.
These objects and others which will become apparent hereinafter are attained, in accordance with the present invention, by a method of hard-chromium plating elongated bodies whereby the body is advanced in stages through an electroplating bath relatively to the counterelectrode or anode which is juxtaposed with only a portion or increment of the length of the body to be plated flanking this anode and maintained at the potential of the cathode or substrate, there is provided a pair of equipotential screens which surround the body and define the plating zone for each increment or plating stage.
After an initial plating of the substrate or workpiece, at least one such screen is provided (at the junction between a prior plating increment and the zone to be subsequently plated) on the prior plating in a region in which the latter is of its maximum thickness. The equipotential screens, according to this invention, have a conductive peripheral outline at the same potential as the cathode or workpiece and geometrically similar to the periphery thereof, so that when the plated body is of circular section, the screens are likewise of circular outline.
We have found that a pair of conductive shields, spaced outwardly from the cathodic workpiece or substrate by a holder or nonconductive material, cause the plating in the immediate regions of these screens and within the plating zone to taper off in a remarkably uniform manner. Thus, when an equipotential screen or shield is provided at the end of a preceding plating zone, the coating in this region tapers from its maximum thickness to zero at the equipotential screen, the coating terminating in a plane of the edge of the conductive screen perpendicular to the major dimension or direction of elongation of the workpiece body.
Thus, by using two such screens, one of which is positioned so that its plane of plating termination lies precisely at the end of the preceding taper at the thickest coating in the prior plating zone, it is possible to overlap the taper coating to obtain a hard-chromium plating of constant thickness over the length of the substrate, in spite of the fact that the latter is electroplated in increments or stages.
According to another aspect of this invention, the apparatus for hard-chromium plating elongated metallic bodies of tens of meters in length may include a tank for the chromium-plating bath which has a length corresponding to a small fraction of the length of the workpiece and is provided with a seal at its opposite ends so that the workpiece may be advanced in stages through this tank.
The anode, according to this invention, is a sleeve surrounding the workpiece over the major part of the length increment within the tank with all around uniform clearance and has a configuration geometrically similar to the cross section of the workpiece and the equipotential screens mentioned earlier. The latter are provided with means enabling them to be shifted along the workpiece to be detached or replaced thereon axially outwardly of the anode, the internal diameter of the latter preferably being greater than the diameter of the metallic screens or shields.
The apparatus may also be provided with polarity-reversing means whereby the workpiece may be connected to the positive terminal of the source while the counterelectrode is connected to the negative terminal for a brief anodization of the workpiece prior to hard-chromium plating, the anodization serving to increase the strength of the bond between the chromium coating and the substrate.
The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:
FIG. 1 is an elevational view, partly in longitudinal cross section in a vertical plane through a chromium-plating tank according to this invention;
FIG. 2 is a longitudinal partial section in a vertical plane through an equipotential screen according to this invention;
FIG. 2A is a section along line IIA-lIA of FIG. 2;
FIG. 3 is an elevational diagram showing the positions of the anode and screen during the initial plating operation; and
FIG. 4 is a diagram similar to FIG. 3 showing the positions of the screens during the subsequent plating operation.
The system as illustrated in FIG. 1 is an apparatus for the hard-chromium plating of an elongated body 1 of circular cross section, whose exterior is rough from a preliminary degreasing and/or descaling operation, which extends through the chromium plating tank 2 through openings 2a provided in the end walls 2b and 2c, thereof. These openings are provided with a gland-type seal as represented at 3 to prevent escape of the chromium-plating electrolyte M in which the region la of the body 1 within the bath 2d is immersed.
Coaxial with the body la and extending over the major part of the length thereof within the tank 2 is an anode 4 of geometrically similar configuration (cylindrical when the body I is a shaft of circular cross section) composed of an antimony-lead or a lead-tin-silver alloy and perforated with holes 4a. The anode 4 is spaced by to mm. from the workpiece. The surface of the anode relative to the cathodic workpiece is in a ratio therewith ranging between 2:1 and 1:1.
Outwardly of the anode 4 is provided a pair of shields 5a and 5b which are best shown in FIG. 2 and may be spaced from the anode 4 by a distance of 40 to 60 mm.
The holes or apertures 40 in the anode 4 are provided in longitudinally extending arrays with successive arrays about the periphery of the anode being staggered by approximately half the distance between the holes of each array.
As can be seen in FIGS. 2 and 2A, each shield 5a (or 5b as seen in FIG. 3) comprises a pair of semicircular ring members 6 held together by screws 6a through lugs provided on the ring halves (FIG. 2A) and composed of electrically insulating material. A metallic band 8 has a cylindrical sleeve 80 fixed to the outer periphery of the ring halves 6 by screws 7 and axially overhangs the ring at 822 while being provided with an inwardly turned flange 8c, the inner periphery 8d of which spacedly surrounds the workpiece 1 when the ring is placed thereon.
As shown in FIG. 1, a reversible power supply is provided for the hard-chromium plating apparatus. This power supply comprises the usual line source 10 of alternating current which energizes a rectifier arrangement 12 via a current-controlling component 11 such as an autotransformer. A reversing switch 13 of the double-poledIdouble-throw type is ganged with a single-pole/single-throw switch 14 so that, when the switch is in the position illustrated, the positive terminal 120 of the rectifier can be connected to the workpiece 1 while the negative terminal 12b is connected to the anode 4. In this position switch 14 is opened and the shields a and 5b are deenergized.
When the switch arrangement l3, 14 is reversed, the positive tenninal 12a is connected to the anode 4 and the negative terminal l2b to the workpiece 1 while switch 14 closes to apply the cathode potential to the shields 5a and 5b. The operation of the device will be described in greater detail in connection with a specific example.
EXAMPLE Using the apparatus of FIGS. 1-4 a shaft of circular cross section with a length of 30 m. was introduced into the chromium-plating tank 2. The anode 4 was provided with holes 4a with diameter of 5 to mm. and an interhole spacing of about 10 to 20 mm. The rows of holes were circumferentially spaced by about 10 to 30 mm. The surface area of the anode 4 confronting the workpiece had a ratio to the workpiece area of about l.5:l and the anode is spaced by 125 mm. from the workpiece. The shields 5a and 5b were mounted at a distance of about 40 to 60 mm. from the anode and positioned as described below.
For the initial chromium-plating operating only a single shield 5b, positioned at a distance d about 50 mm. from the anode 4 was used as shown in FIG. 3. A conventional hardchrornium plating electrolyte containing 200 to 250 g./l. chromium trioxide (Cro and 2 to 2.5 g./l. sulfate ion (S0 and constituted as described in the Encyclopedia of Electrochemistry, pages 201 ff., Reinhold Publishing Corp., N.Y., 1964, was used at a temperature of the plating bath of 55 C. An initial anodization (with the workpiece 1 connected to the positive terminal 12a) for a period of 90 seconds with increas ing current density 12 to 40 a./dm.") is carried out to prepare the workpiece for plating. The electroplating is carried out at 40 to 60 a./dm. (FIG. 3) to form a coating C of the desired thickness. This coating has a stretch C l of uniform thickness terminating in a tapering portion C of diminishing thickness terminating in a plane P which is the plane of the flange 8c of the shield 5b. The axial length of the tapering portion C is also represented by the distance d. The junction of the tapered portion and the uniform-thickness portion is represented at C For the next stage or increment, the workpiece l is advanced through the tank by a distance equal approximately to the axial length of the anode 4 and the distance d with the portion of the workpiece emerging from the tank being suggested to washing, e.g., via a spray nozzle 15 and drying as represented by the heating coil 16.
As shown in FIG. 4, shield 5b is mounted so that its flange 8a lies in the plane P of the junction between the tapered portion C and then conducts uniform-thickness portion C while a section shield or screen So is placed downstream at the other end of the plating zone at a distance d of 40 to 60 mm. from the anode 4 and in the case of the present example, at about 50 mm. from the end of this anode. Again plating is carried out under the indicated conditions, whereupon the shields are shifted to the next increment, etc.
As shown in broken line in FIG. 4, a tapered plating is formed over the previous tapered portion so that the junction between the two increments has the same thickness as the plating between these unctions. In each increment, a brief the steps of:
a. advancing said surface in increments through a plating bath to juxtapose successive portions of said surface with a counterelectrode;
b. positioning on said surface upstream of said counterelectrode in the direction of advance of said surface and at a predetermined spacing from said counterelectrode, a conductive equipotential shield substantially at the electrical potential of said surface;
c. cathodically connecting said surface and anodically connecting said counterelectrode to electrodeposit a metal from said bath onto said portion of said surface juxtaposed with said counterelectrode with the deposited coating tapering off between said counterelectrode and said shield;
d. positioning a successive portion of said surface in juxtaposition with said counterelectrode while locating a shield on the previously formed coating substantially at the junction of the tapered portion thereof with the remainder of the previously formed coating and at a distance from the counterelectrode corresponding to the distance between the counterelectrode and the shield during the electroplating of the preceding portion, positioning a further conductive equipotential shield upstream of said counterelectrode, and then electroplating said successive portion of said surface between the shields by cathodically connecting said surface and anodically connecting said counterelectrode; and
e. repeating step (d) until said surface is completely coated to a uniform thickness.
2. The method defined in claim 1 wherein said portions of said surface are plated with a hard-chromium electroplate at a current density of 40 to 60 a./drn. said counterelectrode has a surface area having a ratio to the confronting surface area of said portions ranging between 2:] and 1:1 and spaced from to mm. from said metallic surface, and said bath is a hard-chromium plating bath containing chromium trioxide and sulfate ion.
3. The method defined in claim 2 wherein said metallic sur face is the surface of an elongated body, said body being ad vanced axially through said bath in said increments, said shields being positioned around said body, said anode surrounding said body between the shields of step d).
4. The method defined in claim 3 wherein said counterelectrode is perforated and is provided with staggered rows of holes having a diameter of 5 to 10 mm., said counterelectrode being composed of a lead-antimony alloy or lesd-tin-silver alloy.
5. The method defined in claim 1, further comprising the step of briefly anodically connecting said surface and cathodically connecting said counterelectrode to anodize each portion of the surface juxtaposed with said counterelectrode prior to the electrode deposition of metal from said bath onto each portion of said surface.
6. The method defined in claim 5 wherein anodization is carried out for a period of about 90 seconds at a current density increasing gradually from 12 to 40 a./dm.*.