US3385774A - Method and means of anodizing - Google Patents

Description

May 28, 1968 w. A. THOMPSON ETAL 3,385,774

METHOD AND MEANS OF ANODIZING 2 Sheets-Sheet 1 Filed March 16, 1964 INVENTORS. W/lZ/AM A. THOMPSON BY Jam l1! POWERS y 1968 w. A. THOMPSON ETAL 3,385,774

METHOD AND MEANS OF ANODIZING By JOHN H. P007525 United States Patent 3,385,774 METHOD AND MEANS OF ANODIZING William A. Thompson, Apollo, and John H. Powers, Arnold, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Filed Mar. 16, 1964, Ser. No. 352,230 4 Claims. (Cl. 204-58) ABSTRACT OF THE DISCLOSURE Anodizing apparatus having a submerged rotatable separate chamber with a movable pressure plate locatable to pressure pack a mass of articles therein in anodically energized condition and subject to a centrifugally induced flow of anodizing electrolyte therethrough.

This invention relates to the anodizing in bulk of relatively small aluminum, or aluminum base alloy, articles to produce on surfaces of such articles, oxide coatings, which may, if desired, be later modified to adapt the coated article to special purposes, or later colored if the coated article be not of the desired color.

The method of this invention is readily used in any anodic processing by which an oxide coating is electroformed on aluminum, or aluminum alloy, surfaces, and is not dependent upon the use of any particular electrolyte or electrochemical procedure in the anodizing process. The invention has, as will appear, special advantages when high current density anodizing coupled with the processing of small articles creates heat problems not readily solved by usual methods of cooling.

In general, it may be said that the uniformity of temperature throughout the electrolyte of an anodizing bath is desirable to the production of anodic coatings of specified characteristics on aluminum, or aluminum base alloy surfaces, all of which surfaces which are hereinafter, and in the appended claims, collectively defined as aluminum surfaces. When the article to be anodized is of such size and configuration as will allow it to be separately racked during the anodizing process, the very considerable heat normally released during the anodic oxidation may be readily dissipated and controlled by conventional agitation of the electrolyte bath, which agitation is usually sufficient to renew and maintain at the aluminum surface electrolyte interface a relatively constant temperature at a level which, if need be, may be set and maintained by refrigeration. Such is not the case, however, when size and configuration of the articles to be anodized require that they be anodized in bulk rather than separately. Examples of such aluminum articles include eyelets, rivets, ferrules, small screws and other articles presenting similar problems of individual size and configuration. In anodizing aluminum articles such as these, it is conventional to place them together in bulk in a perforated basket or holder which is made anode in an anodizing circuit, the cathode usually being a piece of lead submerged in an electrolyte contained in a conventional anodizing tank. The mass of such bulked articles prevents, by currently used methods, adequate circulation of the electrolyte during anodizing, the result being that the temperature at the aluminum surface, and consequently the current density, varies considerably over the surfaces of the thus held articles. If temperature rise within the bulk load becomes too great, and such does occur, the consequent localized overheating may produce rapid dissolution of the coating being formed, or of the article itself in the electrolyte. Because of such lack of temperature control, it is common in bulk anodizing of aluminum surfaces to limit the maximum current density applied, thus increasing the time required to produce on the surfaces of the bulked aluminum articles oxide coat- 3,335,774 Patented May 28, 1968 ings of the thicknesses desired. In some instances, the necessities in respect of limitation of applied current densities may limit coating formation to thicknesses less than those which would ordinarily, be desired. Indeed, in many instances, it has been found that the localized heat ditliculties attendant'upon anodizing of bulked aluminum articles may require lowering of applied current densities to onefourth or less of those usually employed in anodizing aluminum surfaces in a given electrolyte.

The general object of the present invention is to provide methods, and a preferred means, for uniformly circulating an electrolyte through a packed bulk load of small aluminum articles at a rate suflicient to prevent, in most instances, localized heating, thereby generally per mitting conventional application of oxide coatings at desirable current densities.

The basic object of this invention is achieved by forming a pressure packed mass of aluminum articles to be anodized between inner and outer perforated walls so that the mass has substantially circular continuity around a central orifice or passage, immersing said mass in an electrolyte of desired characteristics, and anodically energizing the mass while rotating it around the approximate longitudinal axis of the central passage at such speed that the electrolyte will, by centrifugal force, be impelled to flow from said central passage through the inner perforated wall, through said packed mass and thence through the'outer perforated wall. The convenience of the method of this invention is such that the cross-section and volume of the 'bore of the inner passage, the speed of rotation and the perforations in the inner and outer confining walls of the pressure packed mass of articles may be adjusted to give the flow required to reduce or eliminate localized heating in any electrolyte and at any practically applied current density. The adjustment of these factors will be in accord with known principles for centrifugal systems. Of course, the limiting case is where no perforations exist in the outside wall in which case circulation would not take place. Otherwise, if the perforations in the inner perforated wall are adequate to permit, the circulation rate will increase proportionally to the square of the diameter of the central passage and linearly as to the number and size of the perforations in the outer cylinder.

It will be at once apparent that the method of this invention has great adaptability to control the tendency to localized heating caused by bulk loading of the articles to be anodized since, by using known centrifugal principles, flows can be readily created to meet even unusual heating conditions. We have further found that, in the practice of our method, the average thickness of the pressure packed mass of aluminum articles may conveniently range from about one to six inches, measured radially, depending on the density of the pressure packed mass, the denser the packing the better the result obtained by limiting the thickness to the lesser part of this range. In any event, we have found that by the use of this invention the temperature of the pressure packed mass may be held relatively uniform throughout so that a relatively uniform anodizing is achieved whether measured by the average thickness of coating for article or the number of the bulk packed articles adequately coated.

In the practice of the methods of our invention, the bulked mass of aluminum articles is pressure packed during anodizing; that is to say, a positive mechanical pressure is imposed on the mass at the outset and is thereafter maintained so that there is positive contact between adjacent articles within the mass thereby to preserve to a practical extent a constant contact between adjacent articles regardless of the speed of rotation of the mass.

To further illustrate and explain the principles of the methods of this invention, and to illustrate means of this invention, it is convenient to refer to the accompanying drawings, in which:

FIG. 1 shows in elevation, and partially in cross-section, a presently preferred device used by us in the performance of this invention;

FIG. 2 illustrates the parts of the devices of FIG. 1 in exploded form;

FIG. 3 illustrates in partial cross-sectional elevation a combination of electrical and mechanical means arranged and related for the practice of this invention.

The assembly of means shown in FIGS. 1 and 2 illustrate the preferred manner in which We form a pressure packed bulk mass of aluminum articles for the practice of the invention.

As will be apparent, other means can be used to practice the method of this invention, but we have found the assembly herein illustrated and hereinafter described as one which is simple, efficient, and flexible and is presently preferred by us in the practice of the method of the invention.

Essentially, the illustrated assembly comprises a central rotatably mounted rod '14 on which are mounted an inner perforated wall or cylinder, and an outer perforated wall, or cylinder 11 which, taken with the bottom member 12, define a chamber in which may be received the mass of small aluminum articles, the surfaces of which are to be anodized. After such a mass of articles is placed in the chamber, the mass is pressure packed therein, and maintained under said pressure, by a top plate 13 which is suitably sized to fit within the mass receiving space and is pressed into, and locked in, pressure contact with said mass of articles by a pressure device consisting of a pressure plate 22 on which are mounted pressure transmitting legs 23 and a tubular member 24 which fits over the central rod 14. Locking nuts 25 operating on the threaded portion 26 of said rod 14 bear on tubular member 24 and may be manipulated first to exert the desired pressure and then to lock and hold member 24, plate 22 and, consequently, top plate 13 under the desired pressure. The central rod 14 is provided with a reduced end 28 (see FIG- URE 2) which is either threaded or pressure fitted into a cross-bar 16 to which bar the inner cylinder 10 and the bottom plate =12 are separately fastened as by welding or bolting. Outer cylinder 11 is welded or otherwise joined to the mripheral portion of the bottom member 12. This assembly is rotatably mounted at the upper extremity of rod 14, in any suitable manner, on a gear located in gear box 19 (see FIGURE 3), thus allowing the imparting of rotatable movement to rod 14 through a prime mover such as motor 17. The entire said assembly is so positioned and assembled that the mass of aluminum articles contained therein are immersed in the electrolyte 21 which is maintained in a tank 30 of a conventional anodizing system. The anodizing circuit, including the cathode and the electrolyte 21 is completed by making the rod 14, and consequently the mass of aluminum articles contained in the assembly, the anode of the circuit by any convenient electrical connection to the rotating post. In the device as shown it is, of course, practical to make any of the parts of the assembly of non-conducting materials of sufficient strength so long as provision is made to electrically connect the pressure packed mass of aluminum articles contained therein as anode in the anodizing circuit. For practical purposes, we have found it suitable that all the parts be made of aluminum, and that all surfaces in contact with the mass of aluminum articles pressure packed between the two perforate walls except the outer surface of the inner perforate wall be rendered substantially non-conductive by treatment with suitable coatings, so that electrical contact is through the contact of said surface with the mass of articles. However, this is not necessary, and is at the choice of the operator.

It will be readily apparent, as best shown in FIGURE 2, that the pressure plate 22 be possessed of a central aperture 31 so sized as to pass the rod 14 and that the bottom plate 12 he possessed of a central aperture 32 so sized as to readily pass the inner perforate wall 10, so that wall 10 may be fastened in electrical conducting relationship to the bar 16 and if desired, insulated from the bottom plate 12. Also, the top plate 13 is possessed of a central aperture 33 sized to readily pass the inner perforate wall 10, and the area of plate 13 is such as to effectively contact the upper surface of the mass of articles contained between the inner and outer perforate walls of the articles receiving chamber.

In the practice of the method of the invention, the rotation of the assembly above described centrifugally induces flow through the central passage 34, which is defined by the inner perforate wall 10 and through which central rod 14 passes. To the end that such induced flow may be at a rate desired, these parts may be conveniently sized as desired, the drawings herein being not to scale for any particular purpose but merely to a convenient scale for illustrative purposes. The flow induced in central passage 34 passes through the perforate inner wall 10, through the mass of aluminum articles pressure packed between the inner wall 10 and the outer perforate wall 11, and thence through the outer perforate wall 11. In usual practice, we have found that the best anodizing results are obtained when the thickness of the mass contained in the chamber, i.e., as to the assembly above described, the distance between the inner surface of the inner wall 10 and the inner surface of the outer wall 11, is maintained, as aforesaid, between one and six inches. Such thickness dimensioning of the pressure packed mass tends to insure not only a practical absence of localized heating but also an acceptable overall uniformity of coating on the surface of the aluminum articles which, in the pressure packed mass, are subjected to the anodizing process.

The method and means of this invention may be used in any anodizing process which utilizes a liquid electrolyte, and said method and means are useful wherever the problem of anodizing small articles in bulk is complicated by a tendency to localized heating during the anodizing process. For example, the claimed method generally and by the preferred assembly means above described, has been utilized successfully in the anodizing of aluminum rivets, of aluminum shoe eyelets, of aluminum pencil ferrules, of small bolts made of aluminum and of like articles, whether the electrolyte be of the sulphuric acid type, the chromic acid type, the oxalic acid type, or of other inorganic acid, organic acid or mixtures of acids useful in anodizing aluminum.

We claim:

'1. In a process of anodizing in bulk the surfaces of small aluminum articles, the steps of forming said articles into a pressure packed mass of substantially circular continuity defined by inner and outer perforate walls, said mass being shaped to define inwardly thereof a central passage, anodically energizing said mass while submerged in an electrolyte and during said anodic energization rotating said mass within said electrolyte to cause a continuous centrifugally induced flow through said central passage, through said inner wall, through said mass and, thence, through said outer wall.

2. The process of claim 1 characterized by the fact that said mass has a thickness of about one to six inches.

3. In an assembly adapted to maintain a mass of aluminum articles as anode in an anodizing electrolyte in pressure packed relation one to another, means defining an electrolyte containing vessel, means defining a mass receiving and formnig chamber disposed within said electrolyte containing vessel, said mass receiving and forming chamber including inner and outer perforate walls defining the sides of said chamber, said inner perforate wall being shaped to define by its outer surface a passage axially central of said chamber and said outer wall being spaced from said inner wall to define said chamber circularly around said passage, a pressure device associated with said chamber and adapted to contact, and pressure pack,

b in said chamber a mass of aluminum articles located therein, means for rotating said mass receiving and forming chamber about its central axis and within said electrolyte containing vessel and means for maintaining the mass in a pressure packed condition during said rotation whereby a continuous centrifugally induced flow of anodizing electrolyte moves through said inner perforate wall, through said pressure packed mass of articles and through said outer perforate wall.

4. Apparatus for immersing a mass of aluminum articles as anode in an anodizing electrolyte comprising means defining an upwardly open annular mass receiving and forming chamber including inner and outer cylindrically shaped perforate walls disposed in concentric spaced relation dependently joined by a base member, said inner perforate wall defining an axial bore, a rotatable shaft disposed within said bore drivingly supporting said chamber, an annular pressure plate sized to be received intermediate said inner and outer Walls and vertically displaceable relative to said axial bore for application of pressure to a mass of articles contained in said chamber, means engageable with said rotatable shaft for maintaining said pressure plate in preselected pressure contact with a mass of articles contained within said chamber, means drivingly engaged with said shaft for rotating said chamber while submerged in an anodizing electrolyte and with said pressure packed mass in anodically energized condition to cause a centrifugally induced flow of anodizing electrolyte through said axial bore, through said inner wall, through said mass and through said outer wall.

References Cited UNITED STATES PATENTS 3/1909 Reid 204-212 X 12/1909 Bates et a1. 204-212 3/1912 Lacroix 204-212 10/ 1935 Merkenschlager 204-11 10/ 1943 Halvorsen 204-58 2/1'945 Herrick 204-58 10/1955 Backer 204-58 3/ 1958 Gempe 204-199 10/ 1965 Huffman et a1 204-23 FOREIGN PATENTS 10/ 1942 Norway.

2/ 1922 Great Britain.

3/ 1926 Germany. 12/ 1932 Germany.

8/ 1931 France.

2/ 1957 Germany.

HOWARD S. WILLIAMS, Primary Examiner.

JOHN H. MACK, Examiner.

W. VAN SISE, Assistant Examiner.

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