US3915835A - Method of improving plating distribution of elnisil coatings - Google Patents

Abstract

An electroplating system for cathodically plating an epitrochoidally shaped internal surface of a rotary engine housing. An anode assembly is provided which is comprised of a perforate walled container of titanium metal or other anodically inert metal to which a voltage potential can be applied; the basket contains anode pieces such as nickel which are shaped to be in intimate contact with each other during the plating operation. The perforate walls of the anode container is shaped from flexible expanded titanium sheet metal interfitted within semi-epitrochoidally aligned grooves respectively machined into titanium plates forming the ends of the anode assembly. The anode walls are thus shaped substantially complimentary to the epitrochoid configuration of the cathode but have a predetermined deviation adjacent the nodes of the trochoid for insuring a uniform but heavy coating thickness under high speed electroplating conditions.

Description

United States Patent 1 Cordone et al.

[ 1 Oct. 28, 1975 METHOD OF IMPROVING PLATING DISTRIBUTION OF ELNISIL COATINGS Mich.

[731 Assignee: Ford Motor Company, Dearborn, Mich.

[22] Filed: July 1, 1974 21 Appl. No.: 484,729

Related US. Application Data [62] Division of Ser. No. 413,154, Nov. 5, 1973, Pat. No.

FOREIGN PATENTS OR APPLICATIONS 4/1944 Germany 204/DIG. 7

OTHER PUBLICATIONS Leadbeater, Metal Industry, Apr. 28, 1944, p. 266.

Primary Examiner-F. C. Edmundson Attorney, Agent, or Firm-Joseph W. Malleck; Keith L. Zerschling [57] ABSTRACT An electroplating system for cathodically plating an epitrochoidally shaped internal surface of a rotary engine housing. An anode assembly is provided which is comprised of a perforate walled container of titanium metal or other anodically inert metal to which a voltage potential can be applied; the basket contains anode pieces such as nickel which are shaped to be in intimate contact with each other during the plating operation. The perforate walls of the anode container is shaped from flexible expanded titanium sheet metal interfitted within semi-epitrochoidally aligned grooves respectively maehined into titanium plates forming the ends of the anode assembly. The anode walls are thus shaped substantially complimentary to the epitrochoid configuration of the cathode but have a predetermined deviation adjacent the nodes of the trochoid for insuring a uniform but heavy coating thickness under high speed electroplating conditions.

3 Claims, 4 Drawing Figures US. Patent Oct.'28, 1975 SheetlofZ 3,915,835

METHOD OF IMPROVING PLATING DISTRIBUTION OF ELNISIL COATINGS This is a division of application Ser. No. 413,154, filed Nov. 5, 1973, now U.S. Pat. No. 3,891,534.

BACKGROUND OF THE INVENTION It is generally well known in electroplating that the density of current flow will be uneven at the sharp edges or contour changes on the object to be plated. This phenomenon involves a proposition that there is increased plating resulting from increased current density at any outstanding contour, while the opposite effect will take place at depressions. In the latter case, the

density of current flow becomes less than the average density of plating current over the full area being treated. This problem becomes exaggerated when an article to be plated has a compound curvature, such s in an epitrochoid, where the cathode is able to receive current throw from two different zones or anode locations due to the reverse or compound curvature. Accordingly, certain areas will be unduly thick because of the throwing power which is multiplied in some areas.

In applications such as a functional coating for a wear surface of an internal combustion engine, i.e. the internal rotor housing surface of a rotary internal combustion engine, the need for uniformity in the coating is extremely severe. The efficient electroplater not only seeks to obtain uniform thickness in such applications, but the plating must be of good sound density throughout; the latter will be degraded as a result ofinappropriate bath chemistry, electrode spacing, and change of the anode or cathode area during the plating process.

SUMMARY OF THE INVENTION A primary object of this invention is to provide an anode assembly useful in an electroplating system for cathodically plating an article having a compound or reverse curvature, the anode assembly being particularly adapted to maintain a proper current throw relationship so that a uniform thickness and density is maintained throughout the plated surface of said article.

Another object of this invention is to provide a semiconforming anode assembly or apparatus for use in an electroplating system of the type which is adapted to deposit a significantly heavy functional coating on a non-uniformly curved surface.

Features pursuant to the above objects comprise the use of an anode assembly having a foraminous or perforate sheet metal titanium wall shaped in a predetermined unique configuration and varied from the shape of the cathode at selected locations. The wall is retained by end plates formed of the same material but solid. The cross section of the foraminous wall is defined so that it is semi-conforming with respect to the shape of the cathode; the anode assembly progressively becomes more spaced from any portion of the cathode article which has a reversely curved portion, the progression of spacing increasing to a location intersected by radius of the reversely curved portion passing through the mid-point thereof.

Still another object of this invention is to provide a novel and unique method for fabricating an anode assembly which will have a defined cross sectional configuration with a semi-conforming relationship to the cathode, a continuous wall of the assembly being fabricated of perforate sheet metal, such as titanium.

BRIEF SUMMARY OF THE DRAWINGS FIG. 1 is a schematic illustration of an electroplating apparatus having a stacked series of cathodically constituted articles for plating, and an anode assembly disposed within the interior of said seriesof cathode articles;

FIG. 2 is a plan. view of the apparatus of FIG. 1, shown similarly in a somewhat schematic manner;

FIG. 3 is a highly enlarged schematic layout of the cross sectional configuration of the anode assembly and the inner wall of the cathodic article to be plated;

FIG. 4 is an exploded view of the basic elements which interfit to form the cathode assembly according to the method of this invention.

DETAILED DESCRIPTION Turning now to the drawings and particularly FIGS. 1 and 2, there is schematically illustrated a preferred mode for an anode assembly and plating system according to this invention. An electroplating tank A is provided to contain an electrolyte B, such as an aqueous solution of nickel sulfamate containing inert particles of silicon carbide. Typically the bath may contain about 600 grams/liter of nickel sulfamate, about grams/liter of silicon carbide with a mesh size no greater than 400, about 2.5 grams/liter of a stress reliever such as saccharin, about 19 grams/liter of nickel chloride, and about 45 grams/liter of boric acid a a)- I A'cathode assembly C is disposed in the electrolyte consisting of several cathodic articles 10 each constituting a cast aluminum rotor housing useful as an element of a rotary internal combustion engine. The rotor housings are annular and must have a highly wearresistant epitrochoid surface 11 on the interior thereof and against which apex seals or other moving parts of a rotary engine must hear. The housings are separated, one from the other, by spacers 12 which may act as shields and prevent plating on the side faces 10a of the rotor housings. Such spacers can be formed as polypropylene or nylon sheets and have an inner edge 13 which is recessed from the interior surface 11 ofeach rotor housing. Alternatively, the spacers may be arranged as cathode elements which fit tightly between the housings and which draw current around the edges of the housings to overcome the problem of exaggerated thickness at such edges; again the spacers would be recessed as illustrated.

At the upper and lower ends of the stack of housings and spacers, there is employed a rigid annular shield 14 for the top and bottom faces 15 and 16 respectively. Each shield should be a plate comprised of aluminum coated with silicone rubber which stays clean and does not draw plating. Plates are supported by a harness (not shown) which facilitates the lowering and the raising of the entire cathode assembly from the electrolyte. The harness should similarly be coated so as to have an inert outer surface.

An anode assembly D is employed which is of a semiconforming type wherein only a portion of the anode is adapted to be proportioned identical to the cathode surface 11 to be plated; other portions are designed to progressively deviate from such configuration. The anode assembly, here, is a basket made from expanded titanium sheet metal (or may be woven from titanium wire). The walls 19 are foraminous and the bottom and top walls 20 and 21 are each a solid titanium plate. Resilient or elastic neoprene bands 22 may be mounted about the anode wall 19 to mask off or block the current throw in certain predetermined elevation zones along the anode assembly, particularly those areas where the edges of the cathode article would promote an uneven distribution. The masking also blocks off current throw to the spacing between the housings. Such masking is unnecessary if cathodic spacers are utilized as mentioned earlier. Active anode pieces 23, such as nickel, are collected and stacked in the basket for intimate interengagement with each other and with the basket.

An anodic film is formed on the titanium basket which affords corrosion resistance and electrical insulation, the basket thereby being rendered anodically inert. The titanium acquires a thin-dense inert oxide film which is chemically resistant to acidic electrolytes and has a high electrical resistance. The current density, of course, is controlled by the configuration of the titanium basket even though the nickel anode pieces therein are the active anode metal. Current will pass between the basket and pieces at a contact point between the anodic film and the nickel pieces; this is so even though the film on the titanium is an electrical insulation.

To realize the objects of this invention, the anode walls 19 are defined with a predetermined variation from the nodes 24 of the epitrochoid configuration of cathode wall 11. Such nodes or segments have a reverse curvature relative to the uniform curvature of the remaining portions 25 and 28 of the epitrochoid; in cross section, the portions 25 may substantially be arcs of circles. The cross sectional configuration of wall 19 has a pair of uniform arcuate segments 26 and 27 which are directly proportional and aligned with the segments 25 and 28 respectively of wall 11. At stations on wall 19, substantially adjacent the extremities of the reverse curvature segments 24, varying arcuate segments 30 and 31, each of which may have a different radius from that of the uniform segments, are employed. Each of these varying arcuate segments substantially continue or extend the curvature from each of the uniformly curved segments until they meet at ajuncture 32 which is on the minor axis of the epitrochoid, or in other terms, has a radius 33 of the reverse curvature segment 24 passing therethrough and through a midpoint 34 of the reverse curvature. In this manner the total segment 30 or 31 is each comprised of two arcs meeting at an abrupt juncture and thereby rendering the combination as varying in curvature.

As shown in FIG. 3, the uniform segments 27 and 26 are formed from cricles which overlap. It is possible that for some types of epitrochoids or compound cathode surfaces, the circles should be made tangent. In any event, the deviation (distance 36 distance should be progressively varied according to the relationship whereby the deviation is inversely proportional to VC.D., where CD. is current density, provided such factors as the conductivity of the solution and temperature are constant.

Method of Making Anode Assembly A preferred method mode according to this invention with respect to FIG. 4, comprises:

1. Prepare solid titanium end plates 44 and 45 with identical continuous groove 46 and 47 respectively, each groove defining a semi-conforming configuration to that of the cathode surface. In this case, the semiconforming configuration comprises two uniform arcs 40 and 41 connected by varying segments 42 and 43. The varying segments are adapted to render a predetermined deviation away from the cathode surface at these areas to promote uniform plating thickness.

2. Assemble a flexible web of titanium expanded sheet metal (having a mesh size no greater than with the longitudinal edges 50 and 51 of the web in the grooves 46 and 47 respectively. The web is overlapped upon itself at a seam 52 to define a sleeve-like wall with a uniform cross section reflecting the uniformity and deviations of said grooves.

3. Locate the ends of posts 53 and 54 in mating seats 55 in the end plates to effect a strong stable joint between said plates.

4. Stitch the seam 52 with titanium wire, and fill the assembly with a collection of nickel anode pieces.

5. Provide suitable electrical means for applying a potential to the web.

We claim as our invention:

1. An anode basket for an electroplating process wherein the cathode has a compound curvature with a transition point separating the different curvature thereof, said anode basket having a generally complimentary compound curvature except opposite said compound curvature to render a first anode effect on said cathode, said anode curvature being progressively deviated up to a maximum opposite said transition point, said deviation occurring where the anode effect on any one point on the compound curvature of said cathode is increased beyond said first anode effect, said deviated curvature at said maximum terminating in a sharp apex.

2. An anode basket adapted to fit within and promote electroplating of an inwardly directed epitrochoidally shaped cathode, said cathode having lobes which present a compound curvature with reversely curved segments, said anode basket having first portions adapted to be complimentary in shape to corresponding segments of said cathode and to reside at a uniform distance therefrom, said anode basket further having at least two second portions residing opposite each lobe and differing from the curvature of said lobes, said second portions having a radius of curvature smaller than any radius defining the curvature of said lobes, said second portions meeting at a sharp juncture lying on the minor axis of said epitrochoidally shaped cathode.

3. An anode basket as in claim 2, in which said second portions are arranged to be substantially tangent to each other, said juncture lying on a line of tangency.

Claims (3)

1. AN ANODE BASKET FOR AN ELECTROPLATING PROCESS WHEREIN THE CATHODE HAS A COMPOUND CURVATURE WITH A TRANSITION POINT SEPERATING THE DIFFERENT CURVATURE THEREOF, SAID ANODE BASKET HAVING A GENERALLY COMPLIMENTARY COMPOUND CURVATURE EXCEPT OPPOSITE SAID COMPOUND CURVATURE TO RENDER A FIRST ANODE EFFECT ON SAID CATHODE, SAID ANODE CURVATURE BEING PROGRESSIVELY DEVIATED UP TO A MAXIMUM OPPOSITE SAID TRASITION POINT, SAID DEVIATION OCCURRING WHERE THE ANODE EFFECT ON ANY ONE POINT ON THE COMPOUND CURVENTURE OF SAID CATHODE IS INCREASED BEYOND SAID FIRST ANODE EFFECT, SAID DEVIATED CURVATURE AT SAID MAXIMUM TERMINATING IN A SHARP APEX.

2. An anode basket adapted to fit within and promote electroplating of an inwardly directed epitrochoidally shaped cathode, said cathode having lobes which present a compound curvature with reversely curved segments, said anode basket having first portions adapted to be complimentary in shape to corresponding segments of said cathode and to reside at a uniform distance therefrom, said anode basket further having at least two second portions residing opposite each lobe and differing from the curvature of said lobes, said second portions having a radius of curvature smaller than any radius defining the curvature of said lobes, said second portions meeting at a sharp juncture lying on the minor axis of said epitrochoidally shaped cathode.

3. An anode basket as in claim 2, in which said second portions are arranged to be substantially tangent to each other, said juncture lying on a line of tangency.

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