US3522166A - Electrical system for anodizing - Google Patents

Description

July 28, 1970 G, s, JQNES 3,522,166

ELECTRICAL SYSTEM FOR ANODIZING Filed April 21, 1967 2 sheds-sheet 1 \\\la l g`\ u lo lo Io KN w l 1 l lo 'o h) u h y I e lo '0 l NVENTOR Qfe/w ju//ae Jn/f5 A ORNEYS l I 3y/Mm@ @gw July 28, 1970 G. s. JONES 3,522,166

ELECTRICAL SYSTEM FOR ANODXZING Filed April 21. 1967 L 2 Sheets-5heet 2 N w w INVENT OR Ww 52W/mei Z2/vas N. wk QNQSQ Nk IJ v \NN l: @SW6 www@ MJQK TN. .ma .\l wh@ W m@ United States Patent O 3,522,166 ELECTRICAL SYSTEM FOR ANODIZHNG Garth Sanford `lones, Henrico County, Va., assgnor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Filed Apr. 21, 1967, Ser. No. 632,632 Int. Cl. B23p 1/02; B01k 3/00 U.S. Cl. 204-206 2 Claims ABSTRACT F THE DISCLOSURE The invention disclosed herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457); Ownership of the invention and this application is retained by Reynolds Metal Cornpany in accordance with the Administrators determination and waiver of title, subject to applicable regulations.

Conventional anodizing techniques of the immersion type are characterized |by the utilization of relatively low current densities in the order 12 to l5 amperes per square foot. Under such conditions the time required for anodization is quite long. For example, the anodization of a layer one mil in thickness can require an hour to be performed.

In order to accelerate the anodization process, continuous anodizing techniques have been developed in which the current densities have been increased tremendously. For example, current densities l0() times those conventionally employed in immersion techniques have been utilized. However, such yhigh current densities introduce critical problems in controlling the thickness of the anodized layer and preventing burning of the product.

It is well-known that anodization is proportional to the current density utilized. When the resistivity of a portion of the surface being anodized varies, as for example due to thermal, metallurgical or chemical anomalies at the metal surface, the resultant change in current density causes localized thickness variations in the anodized layer. More particularly, when the resistivity of a particular area is less than its surroundings, the current flow concentrates in that area increasing the current density, with a corresponding decrease in current density occurring in the surrounding areas. Such an effect results in localized build up of the anodized layer to exceed that of the surrounding metal from which anodizing current has been diverted.

The increase in current density just described generates heat in the affected localized area. This heat tends to further increase the electrical conductivity of the area, as well as to increase the solubility of the anodized layer in the anodization electrolyte. When the increased anodizer layer thickness caused by higher current density insuiciently increases the resistivity of the localized area so as to compensate for the eiects brought on by the aforementioned heat generation, burning occurs on the anodized surface. This burning is in the form of thermochemical attack on the anodized layer.

The principal object of the present invention is to provide an anodizing system which is operable at high cur- Lce rent densities and which reduces the problems of localized thickness variations and burning of the anodized surface. The attainment of this object permits the system to be used in a continuous, high speed anodizing operation with resultant improvements in quality, productivity and economy.

Briefly, the invention comprises an anodizing system in which a plurality of spaced, electrically isolated cathode elements are positioned in close proximity with respect to the metal being anodized. Each cathode element is dimensioned so as to afrect the anodization of only a portion of the metal. The spacing between cathode elements is greater than 4between the elements and the surface being arnodized thereby eiiectively preventing the concentration of current paths from several cathode elements to a localized anode area. By providing each cathode element with a constant current density, uniform anodization is achieved.

The invention will become more fully apparent when considered in light of the following detailed description of an illustrative embodiment of the invention and from the appended claims.

The illustrative embodiment may best be understood by reference to the accompanying drawings, wherein;

FIG. l is a diagrammatic representation of an anodizing system according to the invention illustrating a typical cathode arrangement;

FIG. 2 is a cross-sectional representation of an electrolytic cell for anodizing illustrating, according to the invention, the spacing relationship between adjacent cathode elements and between the cathode elements and the surface being anodized;

FIG. 3 is a schematic diagram illustrating a circuit arrangement for providing each of the plurality of cathode elements with a constant current density; and

FIG. 4 is a schematic diagram of a circuit capable of functioning as the series isolation element illustrated in FIG. 3.

Referring now to the drawings, the invention will be described in detail. In FIG. l there is diagrammatically illustrated an anodizing system comprising a metal strip 10, such as aluminum, which is connected to a suitable voltage source (-l) so as to serve as an anode. Of course, in the anodizing process, the strip 10 is the material to be anodized. A plurality of spaced cathode elements 12 are positioned in close proximity to the surface of strip 10. yElements 12 are individually connected to a source of constant current as is schematically depicted by each element being connected to a separate voltage source An electrolyte is passed between strip lil and the cathode elements 12 as indicated by the arrowheads. In the present invention, the electrolyte must flow at high velocity in order to remove the heat generated by the large current densities employed.

As can be appreciated from consideration of FIG. l, the cathode elements 12 are individually dimensioned so as to cover only a portion of the strip 10. Since these elements are closely spaced with respect to the strip, each element participates in the anodizing of only a particular localized area of the strip. Thus, as the strip and the cathode elements are moved relative to one another, the strip is completely anodized by the cumulative effect of the separate anodizations involving each of the elements 12. In order to insure an even thickness of the anodized layer, the cathode elements are arranged in staggered groups, these groups being indicated by the dash lines extending transversely of the length of strip l. In the illustrative embodiment, ve such groups are shown.

The relative spacing relationship which exists between adjacent cathode elements and between the anode and cathode elements so as to achieve electrical isolation of each element can be appreciated by reference to FIG. 2. For illustrative purposes, this gure depicts the anodizing system as part of a complete electrolytic cell. The cell includes a tank structure 14 having side walls and a bottom formed of suitable material to resist attack by the electrolyte which is passed at high velocity through the tank. Tank 14 includes a cover member 16 within which the cathode elements 12 are embedded such that a surface of each cathode is exposed to the electrolyte within the tank. Only one group of cathode elements is viewable in FIG. 2. The individual elements 12 are spaced from one another by a distance x which is .greater than the distance y between the cathode elements and the anode 'within the cell. Similarly the spacing between cathode elements of adjacent groups is greater than the distance y.

By placing the cathode elements closer to the anode than to each other, each element is substantially independent of the anodizing activity of adjacent cathodes. Consequently, by proper selective positioning of cathode elements relative to one another, an anodized layer of constant thickness can be developed as the anode and cathode elements are moved relative to one another.

Typically, a cathode element may have a linear size in the range of 1/2l in the major dimensions forming the cathodic surface. The spacing of such an element from the anode should be less than 1A". In practice, a distance of approximately s" has proved satisfactory. A spacing of approximately 1/2 between adjacent cathode elements is also appropriate in systems utilizing the dimensions just cited.

The invention requires that each of the cathode elements be electrically isolated from one another. This prevents rundesirable results experienced by prior art arrangements wherein the current path from a large cathode area is directed to a localized anode area having a lower resistivity than the surrounding anode portions with the resultant adverse effect that a disproportionate current density condition exists causing uneven anodization and burning problems. The present invention provides constant current density for each cathode element. Since these elements are selectively positioned with respect to the anode and one another, anode current flow via a cathode element other than that immediately adjacent the area being anodized is prevented, thereby contributing to constant anodization thickness and reduction of burning.

One type of circuit capable of providing constant current density for each cathode element 12 is illustrated in FIG. 3. This circuit includes a constant voltage supply 18 across which a potentiometer 20 is connected. As is obvious from the drawing, potentiometer is at the apex of a pyramid of voltage dividers. The voltage taken from potentiometer 20 serves as the total voltage across potentiometers 22, 24 and 26 at a level in the pyramid below potentiometer 20. Similarly, the voltages picked off each of the potentiometers 22, 24 and 26 serve as the respective total voltages across separate groups of three additional potentiometers at the next lower level of the pyramid. For example, the voltage taken from potentiometer 22 is connected across potentiometers 28, 30 and 32. The voltages taken from potentiometers 28, 30 and 32 are applied to the three cathode elements 12 of group 1 illustrated in FIG. l. In a like manner, the remaining cathode elements are connected to the voltage divider pyramid in the same fashion. Although the circuit of FIG. 3 illustrates output arrangements for only three groups, it is apparent that the circuit can be expanded as desired.

The output circuits of potentiometers 20, 22, 24 and 26 shown in FIG. 3 includes buffer arrangements. These are conventional unit-gain, non-inverting isolators, the purpose of which is to prevent the operation of portions of the overall arrangement of FIG. 3 from overloading other portions of the circuit. ln each of the output circuits from potentiometers 28, 30, 32, etc., a series isolation element (SIE) has been provided. This circuit insures the operation of its associated cathode element at a prescribed current regardless of variations in anodizing voltage at the cathode, or of voltage variations at the power supply. The details of a typical series isolation element will hereinafter be described with reference to FIG. 4.

From the above description of FIG. 3 it can be appreciated that by the pyramided voltage divider arrangement, current flow between the anode and each of the cathode elements 12 can be separately adjusted. This permits current flow to all cathode elements and groups of cathodes to be proportionally controlled, thereby resulting in close supervision of the anodizing operation.

FIG. 4 illustrates a circuitry arrangement which may be utilized as the series isolation element of FIG. 3. The circuit includes a driver transistor TR-1 of the NPN type to the base of which the top of its associated voltage divider pyramid potentiometer is connected. A PNP type transistor TR-Z is interconnected with transistor TR-l. More particularly7 the emitter and collector of TR-l are respectively joined to the collector and base of TR-Z. The collector of transistor TR-2 is connected through a resistor R-1 to the negative terminal of the power supply illustrated in FIG. 3. This terminal is also joined through a resistor R-2 to the base of transistor T R-l. Suitable resistors R-3 and R-4 are interposed between the emitter and base, respectively, of transistor TR-2 and a cathode element 12.

The circuit of FIG. 4 operates so as to maintain the voltage across resistor R-l constant. This is accomplished 'by the compensating operation of transistor TR-2 to changes in the conduction of transistor TR-l. The operating details of this circuit are well known and therefore need not be described further. It is sufficient to say that the output current to the cathode element 12 is maintained substantially constant by the series isolation element independently of changes in the supply voltage of the cathode-to-anode resistance.

The circuitry described with respect to FIGS. 3 and 4 has been presented for illustrative purposes only. A number of other conventional circuit arrangements may be employed so as to regulate and distribute the anodizing current such that the current is unaffected by voltage variations at the cathode elements or at the power supply.

The above-described embodiment is illustrative of a preferred embodiment of the invention but is not intended to limit the possibilities of reducing in a high current density anodizing system the problems of uneven thickness of the anodized layer and burning of the anodized surfaces. For example, although one surface of the metal strip 10 is disclosed as being anodized, it is apparent that by appropriately positioned additional cathodes, opposite surfaces of the strip can be anodized, or only portions of the surfaces may be so treated. The arrangement presented herein is an example of a system in which the inventive features of this disclosure may be utilized, and it will become apparent to one skilled in the art that certain modifications may be made within the spirit of the invention as defined by the appended claims.

What is claimed is:

1. Apparatus for uniformly anodizing a surface of a moving metal strip immersed in an electrolyte by passing current between a cathode and said strip, wherein:

said cathode comprises a plurality of elements spaced from one another and from said surface, said elements being positioned so that moving strip Will be closer to said elements than the spacing between said elements, the elements being dimensioned to substantially restrict each elment to participation in the anodization of only a portion of said surface; and means individual to each of said elements for supplying current at adjustable constant current density between an element and the portion of said surface proximate said element whereby the cumulative 6 effect of anodizing the portions of said surface is 3,240,685 3/ 1966 Maissel 204-224 XR the uniform anodization of said surface. 3,361,662 1/ 1968 Sutch 2.04-224 2. Apparatus as set forth in claim 1, wherein: 3,391,065 7/ 1968 Gerhard 204--224 XR said elements are arranged in groups, the elements of one group being positioned in staggered relationship JOHN H. M ACK, primary Examiner with respect to elements of an adjacent group. o

References Cited UNITED STATES PATENTS U.S. Cl. XR.

3,008,892 11/1961 owen 204-224XR 10 264-228 3,132,080 5/1964 Gann 20422SXR D. R. VALENTINE, Assistant Examiner

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