US3475296A - Electrolyte production of a protective coating on articles - Google Patents

Description

Oct. 28, 1969 w. J. CAMPBELL. 3,475,2Q6

OLYTIC PRODUCTION OF A PROTECTIVE COATING ON ARTICLES Filed April 5, 1967 3 Sheets-Sheet l Inventor Mu. MM J. CAMPMLL By M A Horneyzi Um. 28, 1969 w. J. CAMPBELL ELECTROLYTIC PRODUCTION OF A PROTECTIVE COATING ON ARTICLES Filed April 5. 1967 3 Sheets-Sheet 2 Inventor Mu/lqM'dCnmacu A Horneys m. 28, 1969 w. J. CAMPBELL 3,475,296

ELECT OLYTIC PRODUCTION OF A PROTECTIVE COATING ON ARTICLES Filed April 5. 19s? a Sheetsheet a IIHIIIIHIIHIIIII]IIIIHIIIHHHHIIIIIHHIIIIIIHI I t nvenor I -l$ By Attorneys United States Patent ELECTROLYTE PRODUCTION OF A PROTECTIVE COATING ON ARTICLES William J. Campbell, 42 Hamilton Ave., Glasgow S.1, Scotland Filed Apr. 5, 1967, Ser. No. 628,664

Claims priority, application Great Britain, Apr. 5, 1966,

15,036/ 66 Int. Cl. C23b 9/02 US. Cl. 204-58 9 Claims ABSTRACT OF THE DISCLOSURE A method of electrolytically producing a protective coating on articles, particularly a method of anodising an aluminium cylinder by rotating only part of the periphery at any one time in an electrolyte which flows at right angles to the direction of rotation in an annular passage defined by a curved bafie which can be an electrode. The radius of curvature of the baffle is adjustable so that, with cylinders of different diameters, the bafiie can be equispaced at all points from each cylinder to form an even coating. Cooling electrolyte progressively increases in a direction away from the electrolyte entry to the passage.

This invention relates to the electrolytic production of a protective coating on articles by, for example, electroplating, electro-forming or anodising. For convenience of description, the invention will be described in connection with the anodising of articles of aluminium or aluminium alloy.

The invention is concerned particularly, but not exclusively, with the anodising of large aluminium articles of cylindrical cross-section, such as torpedoes.

It is well known that adequate agitation of the electrolyte, or of an aluminium article in the electrolyte, is essential to the production of a compact uniform anodic coating on the article and various methods have been proposed heretofore to provide this agitation condition. One such method is described in my prior British Patent 716,554. Another and more typical method is to inject air into the electrolyte from perforated tubes at the bottom of an anodising tank. None of these methods, however, are entirely adequate for the production of an anodic coating to a close tolerance of hardness over the periphery of a large cylindrical article.

If a large cylindrical article were to be completely immersed horizontally in an anodising tank of dimensions not much greater than those of the article, it will be understood that air agitation from the bottom of the tank would result in the formation of a softer coating in relatively still electrolyte on the top of the cylinder and said coating would become warmer, while the amount of current required would probably be in excess of the rated capacity of the tank and its electrical equipment.

While the method of agitation described in my prior British Patent 716,554 would theoretically be adequate for hard anodising a large cylindrical article located vertically in a circular cathode, and has proved excellent in production for anodising cylindrical articles about 8 inches in diameter and feet in length, such a method would require large resources of power, pumping and refrigeration for very large cylindrical articles.

The primary object of the present invention is to provide an economical means of anodising a varied range of large cylindrical articles having a diameter up to, for example, 5 feet and a length of 6 feet, to a close tolerance of maximum hardness over the entire periphery of the article.

The present invention is a method of electrolytically producing a protective coating on an article, including the steps of continuously rotating the periphery of the article, which serves as an electrode, in an electrolyte in such wise that only a portion of the periphery is immersed in the electrolyte at any one time, and causing the electrolyte to flow in contact with the immersed portion of the article substantially at right angles to the direction of rotattion of said periphery.

Preferably the electrolyte is caused to flow through an annular passage defined by the periphery of the rotating article and the inner wall of an arcuate bafile.

A first embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 is a front view of the apparatus used in the process:

FIG. 2 shows the apparatus for longitudinally and vertically adjusting the central pivot:

FIG. 3 is an end view of the rigid plate and the adjustable copper sheet:

FIG. 4 is part plan view showing the direction of electrolyte flow:

Referring to FIG. 1, a cylinder 1 is provided at each end with a central pivot 2 which rests on non-conductive hearings or rollers 3 longitudinally and vertically adjustably set at such a height above the base of a tank 4 of electrolyte 5 that the pivots 2 are located substantially above the surface level of the electrolyte 5 and part only of the periphery of the cylinder 1 is immersed in the electrolyte 5.

The rollers 3 are suspended from cross channels 6 (see FIG. 2) which are adjustably mounted on upper side members 7 of the tank 21. The pivot 2 at one end is operatively connected either directly or through a universal transmission link 8 to a driving motor (not shown), preferably a variable speed motor. One of the pivots 2 has provision for a brush type of electrical connection (not shown) to supply electrical current to the cylinder 1 while it is rotating.

A hard rolled copper sheet 9 serving as a bafile is located in the tank 4 to form an arcuate portion around but spaced from the immersed portion of the cylinder 1, each of the two longitudinal edges of the sheet 9 being bounded by angle sections 10 of copper with adjustable cross links 11 (see FIG. 3) at either end so that the radius of the arc of the copper sheet 9 may be adjusted by shortening or increasing the effective lengths of the end links 11. The copper sheet 9 is sufficiently flexible for the purpose and, as it is always cathodic when in use, it does not deteriorate in an electrolyte of, for example, sulphuric acid and it can be independently maintained in cathodic protection when the process is not in operation.

One end of each of the angle sections 10 is extended to rest on a vertical rigid plate 12 of polyvinyl chloride which is secured to the lowest point of the curved copper sheet 9 by a single semi-rotatable mounting 13 and which has a top edge curved so that in any radius of the copper sheet 9 the sections 10 always rest on the plate 12 which is held close to the sheet 9 by projections 14 (FIG. 1)

on said sections close to the plate 12 on the face remote from the sheet.9. By this means one end of the segment formed by the curved sheet 9 in any position is masked by the plate 12 which has an opening connected to a pump (not shown) which is arranged to pump electrolyte 5 to flow through said opening 15 and along the annular passage presented between the copper sheet 9 and the cylinder 1.

This lowest point of the curved sheet 9 rests on the bottom of the comparatively shallow acid resistant tank 4 which has an adjustable weir 16 in the end of the tank 4 adjacent to the plate 12. The height of this weir 16 controls the level of electrolyte 5 in the tank 4 before the electrolyte 5 reaches a connection back to the suction 17 of the pump. The end of the curved sheet 9 remote from the plate 12 is open and sufiiciently spaced from the adjacent end wall of the tank 4 so as not to impede the flow of electrolyte 5 within the arc of the copper sheet 9 and return flow of the electrolyte 5 along the outside of the copper sheet 9 back to the weir 16 and the suction 17 of the pump.

It will be understood that the flexible copper sheet 9 can be arranged to maintain a substantially uniform annular passage between the sheet 9 and the submerged portion of a large truncated cone by appropriate adjustment of the links 11 at both ends to different effective lengths. In this case, the lowest point of the sheet 9 at the end remote from the plate 12 would be raised from the bottom of the tank 4. As the level of the electrolyte 5 inside the curved sheet 9 is the same as the level outside the curved sheet 9 there will be no load on the sheet 9 to cause distortion from a substantially true radius in any position.

Although the invention described so far with the electrolyte 5 pumped at right angles to the direction of movemnet of the periphery of the cylinder 1 provides very vigorous and uniform agitation, the temperature at the end of the annular passage remote from the plate 12 can, on a large cylinder of considerable length increase considerably from the temperature at the input end. Even some vaporization of the electrolyte 5 from the unimmersed end of the cylinder remote from the plate 12 may occur and the hard anodic coating would then become considerably paler in colour and of progressively less hardness toward this remote end. The rotating cylinder 1 can in these circumstances become warmer to touch at this remote end in the latter stages of the process when the power input is at its maximum.

This inequality in the quality of the anodic coating is obviated or mitigated on a long cylinder 1 by arranging a longitudinal sparge tube 18 above and parallel to the top of the unimmersed portion of the cylinder 1. The tube 18 has holes 19 of diameters decreasing in size from the end of the cylinder 1 remote from the plate 12 to the other end and fresh cold electrolyte is introduced into the apparatus through this tube 18 to cool the cylinder 1. The larger holes at the end of the tube 18 remote from the plate 12 cause a proportionately larger amount of fresh electrolyte to be directly projected onto what would have become the warmer end of the cylinder 1 and a progressively smaller amount of electrolyte to be directed toward the end of the cylinder 1 adjacent to the input end of the main electrolyte flow 5. The sparge tube 18 mounts slidably adjustable rubber sleeves 20 which can be used to control the flow from any of the holes 19 so as to provide for maintaining a uniform temperature over the whole length of the cylinder 1.

It has been found where the inside and outside of a hollow cylinder are being simultaneously hard anodised that this external cooling of the cylinder had an almost identical effect on the inner surface of the cylinder. Although no fresh electrolyte was directed at the inner surface, an increased flow of fresh electrolyte applied externally from the sparge tube adequately prevented overheating of the hard anodic coating forming on the in- Cit terior wall of the cylinder. Hollow cylinders are usually sealed at their ends if it is desirable to prevent any electrochemical action on the interior surface.

On a cylinder of short length, or where the maximum power input is well within the cooling capacity achieved by the cross flow method of agitation, the sparge tube may be unnecessary and the fresh cold replacement electrolyte which is always necessary to maintain a uniform temperature in the tank can be introduced into the apparatus at any more convenient plate than through the sparge tube which usually necessitates the use of splash guards.

A bypass 21 from the pump can be used to pass electrolyte into an elevated heat exchanger (not shown) with a gravity feed of cooled electrolyte hack to the anodising tank 4 in order to maintain a uniform temperature of electrolyte 5 or a separate pump can be used for this purpose to return cooled electrolyte from a heat exchanger at the level of the apparatus.

In the electrolyte flow along the outside of the copper sheet back to the pump, cooling coils may be immersed to control the temperature of the electrolyte as an alternative to the elevated heat exchanger referred to in the preceding paragraph.

In place of the copper cathode sheet, a sheet of stainless steel or other metal, or a sheet of plastics material with straps of metal attached, may be used as the cathode.

In a second embodiment where the axis of rotation of the member is below the level of the electrolyte, the driving pivot is electrically connected by a very flexible woven wire mesh or braided cable alongside the submerged universal link to a point beyond the non-submerged universal link when it is electrically connected to the motor driven spindle which is insulated from the motor and receives its electrical power from a suitable brush connection. The whole arrangement is protected as required from the electrolyte by a flexible rubber sleeve.

Both embodiments accommodate rotating articles of different diameters but where it is required only to process one given diameter the curved sheet which constitutes both the cathode and the annular passage restricting wall would be rigid and constitutes also the outer wall of the process apparatus. It would be entirely of metal or of metal partly covered to reduce the effective electrode surface, or it could be of plastics or rubber with metal strips constituting the effective electrode.

Although these embodiments have been described in connection with the hard anodising of large aluminium cylinders of different diameters it will be appreciated that long large articles of hexagonal or similar section could be treated in the same manner but some of the adjustable elements to accommodate the difierent diameters may be excluded when the cylinders being processed are uniform in diameter.

The above described method of anodising is applicable also to other methods of electrolytic production of protective coatings, such as electro-plating or elcctro-forming.

I claim:

1. A method of electrolytically producing a protective coating on an article, including the steps of continuously rotating the periphery of the article, which serves as an electrode, in an electrolyte in such wise that only a portion of the periphery is immersed in the electrolyte at any one time, and causing the electrolyte to flow in contact with the immersed portion of the article substantially a right angles to the direcion of rotation of said periphery through an annular passage defined by the periphcry of the rotating article and the inner wall of a flexible arcuate baffle, means being provided for adjusting the radius of curvature of the baflie.

2. A method as claimed in claim 1 in which the baffle constitutes an electrode.

3. A method as claimed in claim 1, in which the axis 5 of rotation of the article is located above the level of the electrolyte.

4. A method as claimed in claim 1, in which the axis of rotation of the article is located below the level of the electrolyte.

5. A method as claimed in claim 1 in which the baffle constitutes the wall of a processing tank.

6. A method as claimed in claim 1, in which the electrolyte is recirculated outside the baffle and the level of the electrolyte within the baflle is substantially the same as the level of the electrolyte outside the baflle.

7. A Inehod as claimed in claim 1, in which cooling electrolyte is projected upon the unimmersed portion of the rotating electrode.

8. A method as claimed in claim 7, in which the volume of the cooling electrolyte applied to said unimmersed portion of the article increases progressively in 6 the direction away from the end of the annular passage to which electrolyte is admitted.

9. A method as claimed in claim 1, in which the method is anodising.

References Cited UNITED STATES PATENTS 1,862,745 6/ 1932 Fuller et al 2049 2,076,909 4/1937 Miller et a1 20425 2,155,392 4/1939 Ballard 204-112 2,905,604 9/ 1959 Kennedy et a1. 204-5 6 JOHN H. MACK, Primary Examiner 15 R. L. ANDREWS, Assistant Examiner US. Cl. X.R.

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