US2969314A

US2969314A - Electrolytic apparatus

Info

Publication number: US2969314A
Application number: US619184A
Authority: US
Inventors: George H Coxe
Original assignee: Sanford Process Co Inc
Current assignee: Sanford Process Co Inc
Priority date: 1956-10-30
Filing date: 1956-10-30
Publication date: 1961-01-24
Anticipated expiration: 1978-01-24

Description

Jan. 24, 1961 G. H. coxE ELECTROLYTICAPPARATUS 3 Sheets-Sheet 3 Filed 001;. 30. 1956 FIG. 12.

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ELECTROLYTIC arranarns George H. Coxe, Norwalk, Conn., assignor to Sanford Process Co. Inc, Los Angeles, Calif., a corporation of California Filed Oct. 39, 1956, Ser. No. 19,1s4

12 Claims. c1. zoo-242 T his invention relates to the production of hard, wear and corrosion resistant aluminum oxide film on aluminum and aluminum alloys by electrtolyte oxidation of the aluminum and the aluminum alloys. More particularly, the invention is concerned with novel apparatus for commercial production of such films on numerous parts simultaneously in a single tank, with substantially no burning or destruction of the oxide film during the eiectrtolytic coating process.

As is well known, protective oxide films are produced on aluminum or aluminum alloy parts by anodic oxidation of such parts in acid electrolytes such as chromic acid, sulfuric acid or oxalic acid. Generally, the tank is made the cathode by connection of the negative side of the DC. current generator to the tank, and the aluminum work pieces are made the anode by connecting them to the positive side of the DC. current source. An improved process of this general type for producing hard, tough coatings of oxide of aluminum is described in the Sanford Patent 2,743,221, and another process for advantageously obtaining particularly thick, dense and hard oxide coatings of this type, e.g., up to .010" thickness or greater, is described in the co-pending application of John E. Franklin, Serial No. 438,349, filed June 21, 1954, now Patent No. 2,897,125.

However, when a large number of parts are treated in a single tank containing an acid solution of the nature described above for commercial production of an oxide coating on all the parts simultaneously, it has been found that burning and destruction of the coating on many of the parts occur. This is particularly so in the case of high copper alloys of aluminum,'such as 148, 178 and 243, containing up to about 5% copper. Also, those parts positioned near the corners or bottom of the tank tend to burn more so than parts located in other portions, say, the middle of the tank. It is believed that this phenomenon is at least in part due to an unequal distribution of current and voltage through the aqueous electrolyte in the tank, and non-uniform growth of oxide coating especially on those parts placed near the bottom or sides of the tank.

One object of this invention is to provide apparatus for electrolytic anodic oxidation of a substantial number of parts simultaneously in an electrolyte solution contained in a single tank, substantially without burning of the parts, that is, without destruction of the oxide coating during the process of its formation on each of the parts.

Aother object is to design novel electrolytic tank apparatus for electrolytic oxidation of aluminum and its alloys, to facilitate commercial production of good, uniform oxide coatings on a large number of parts simultaneously placed in the tank solution, regardless of the location of the respective parts in the tank.

Yet another object is the provision of improved tank equipment for the electrolytic oxidation .of aluminum and its alloys, which is simple, relatively inexpensive and enables rapid production ofuniform oxidel'coatingsion Patented Jan. 24, 1961 "ice a plurality of parts simultaneously with a minimum of labor and supervision.

Other objects and advantages will be apparent from the following description of my'invention:

I have discovered that by placing one or more buttons or lugs on the inside surface of the tank employed in electrolytically anodizing aluminum, said buttons being in contact with the electrolyte solution, and electricaliy connecting such lug or lugs to the negative terminals of the DC. current source, a large number of aluminum or aluminum alloy parts which are connected to the positive terminals of said D.C. source and which serve as anodes can be anodically oxidized simultaneously in the solution contained in the tank substantially irrespective of the location of the parts in the tank, to produce uniform oxide coatings on substantially all of the parts, essentially without any'noticeable burning or pitting of the coatings. Preferably a plurality of such lugs or buttons are employed, such lugs being positioned in preferred practice along the side walls of the tank, 211- I though in certain instances one or more lugs may be located on the bottom inside surface of the tank, or at the corners of the tank either at the bottom or above the botto-rn'of the tank. When such buttons are properly distributed along the inner surfaces of the tank, as will be described hereinafter, the parts can be placed in any position of the tank including the corner areas and the bottom portion without resulting in burning of the parts, particularly those located in such corner areas or bottom portion. Further, the provision of such buttens or lugs permits the voltage to be raised faster and higher without burning of the parts, than is normally the case in the absence of such buttons. This enables production of thick oxide coatings on the parts in a minimum time period, thus improving the economics of the system. Of particular significance, these advantages apply also to high copper alloys such as 148, 178 and 24S, heretofore difficult'to anodize uniformly when treating a large number of parts'in a single tank.

It has been found in practice that each of the buttons should have at least one angularly positioned surface which intersects the adjacent surfaces of the button along sharp edges. By the term angularly positioned surface is meant a surface having an element which forms a sharp angle With an adjacent surface 'at the intersection of these surfaces. The angularly positioned surface may be a conically'shaped surface or a truncated porton thereof, or it may be a planar surface which intersects another surface, e.g., a planar or curved surface, at an angle ,to form a sharp intersecting edge. Each of the buttons may have more than one such angularly positioned surface, e.g., two or three such surfaces. However, :it has been found that proper voltage and current distribution is provided by said buttons only if each of these angularly arranged surfaces intersects the adjacent s'urfaces along a sharp edge. The shape of the lugs or buttons may be round, square, triangular, rectangular, or may have any other suitable geometric configuration.

are positioned and spaced along theinside walls of the tank so as to result in a substantially uniform voltage and current distribution pattern throughout the entire tank, as will be described more fullyhereinafter. in

this manner, locationsof low or non-uniformjvoltageand current distribution zones through the solution, generally near the corners and sides of the tank, are eliminated, so that parts electrolytically treated in the bath contained in these zones of the tank are as uniformly and rapidly coated as those parts treated in other portions of the tank, e.g., near the center thereof.

Usually, the buttons are positioned on the tank walls intermediate the upper and lower edges thereof, preferably about half the height of the tank from the bottom thereof. Said buttons are generally all located substantially in a horizontal plane above the bottom of the tank, although the buttons need not actually be in the same horizontal plane. As will be seen more clearly hereinafter, the buttons are preferably positioned at points of equal potential drop from the corners of a portion of a side wall, or from the corners of the entire area of a side wall of the tank. However, if desired, the optimum positioning of the buttons may be ascertained by trial and error.

In some instances, particularly when eight or more buttons are employed, it has been found advantageous to mount bus bars along the inside walls of the tank, or preferably on the tank walls out of contact with the electrolyte, and to connect the buttons to such bus bars. The provision of such bus bars improves the action of the buttons, and has the added advantage of permitting the use of a single electrical cable from the negative terminals of the DC. source to the bus bar, instead of employing a plurality of wires for connecting each of the buttons with such negative terminals.

In practice it has been observed that the action of the buttons is particularly effective when a peat extract additive is incorporated in the acidic electrolyte solution according to the process of the above-noted Sanford patent and especially when used in conjunction with the operating voltage technique described in the above-noted Franklin application. However, my novel tank equipment provided with the buttons described above is also effective in affording uniform oxide coating of aluminum parts in conventional electrolytes and processes for anodic oxidation of aluminum.

The invention will be more clearly understood from the description below of certain preferred embodiments taken in connection with the accompanying drawings wherein:

Fig. l is a generally schematic plan view of a tank according to the invention, equipped with the buttons noted above;

Fig. 2 is a generally schematic vertical sectional view taken on line 22 of Fig. 1;

Fig. 3 is a plan view of a tank similar to that of Fig. 1, showing the electrical connections to the buttons;

Fig. 4 is a vertical section taken on line 44 of Fig. 3, showing in schematic form a number of work pieces being anodized in the tank;

Fig. 5 is a detail showing a button connected to the side wall of a tank, and the electrical connector attached to the button;

Fig. 5a is a plan view of the button shown in Fig. 5;

Fig. 6 is a schematic representation illustrating the preferred procedure for positioning the buttons on the side walls of the tank;

Fig. 6a is a plan view of a tank showing another button arrangement;

Fig. 7 is a plan view of a tank showing another embodiment of the invention with respect to the number and location of the buttons along the side and end walls of the tank;

Fig. 8 is a plan view of a tank showing still another arrangement of the buttons on the tank side walls;

Fig. 9 is a perspective view of a tank showing an arrangement of buttons similar to Fig. 8, and incorporating bus bars along the side walls of the tank, in conjunction with the buttons;

Fig. 10 is a partial sectional view taken on line 10-10 of Fig. 9;

Fig. 10:: is a fragmentary section showing a modification of the bus bar and button arrangement of Fig. 10;

Fig. 11 is an elevation of another form of button which may be employed according to the invention;

Fig. 12 is a plan view of the button of Fig. 11;

Fig. 13 is a plan view of still another form of button which can be employed;

Fig. 14 is an end view of the button of Fig. 13;

Fig. 15 is a plan view of yet another form of button which I can use; and

Fig. 16 is an end view of the button of Fig. 15.

Referring particularly to Figs. 1 and 2 of the drawing, numeral 20 represents a conventional tank employed in the electrolytic oxidation of aluminum or aluminum alloy parts. The sides 22 and ends 24 of the tank are connected in conventional manner by an electrical lead (not shown) to the negative side of a DC. current source such as a DC. generator (not shown). It has been observed that when aluminum parts to be anodized are placed in an electrolyte solution in the tank, and such parts are electrically connected to the positive side of the DC. generator, those parts placed within the central volume 26 of the tank bounded by the dotted lines 28 and 30 are more evenly coated with an oxide coating than those parts placed in the border portions 32 of the tank between lines 28 and 30 and the side and end

walls

22 and 24, and in lower portion 34 of the tank, between line 30 and the bottom 36 of the tank. Dotted lines 28 and 30 represent schematically the boundary defining the central volume 26 of the tank, wherein there is apparent even current and voltage distribution or some other phenomenon, unknown to applicant, occurs permitting relatively uniform and satisfactory coating of parts within volume 26, while parts placed in the side and

bottom border spaces

32 and 34 do not coat properly. Thus, those parts particularly positioned in the side portions 32 of the tank adjacent the side and end

walls

22 and 24, and in the bottom portion 34 of the tank tend to burn easily, that is, the oxide coating thereon readily breaks down resulting in defective coatings on these parts. This burning" of such parts also tends to adversely affect the coating of the parts in the central portion 26 of the tank.

According to the instant embodiment of the invention, one or preferably more, e.g., a pair of buttons 38, are spaced along one side wall 22 of the tank, and one or preferably more, e.g., a pair of buttons 40, are spaced along the opposite side wall 22 of the tank. It will be seen that buttons 38 are spaced from each other and are approximately equidistant from the end walls 24, and each are positioned substantially the same distance above the bottom 36 of the tank. Buttons 40 are disposed on the opposite tank wall 22 directly opposite buttons 38. When current is passed through the solution to anodically oxidize parts placed in the solution, the buttons function in some unknown manner so that all of the parts within the major portion of the tank bounded by the

solid lines

41 and 42 in Figs. 1 and 2 are uniformly coated with oxide coating, and substantially no burning of the coatings takes place.

It is believed that the provision of buttons such as 38 and 40 along the side walls of the tank provides uniform voltage and current distribution throughout practically the entire volume of the tank as defined substantially within the

solid lines

41 and 42, whereas in the absence of said buttons only those central portions of the tank within the dotted lines 28 and 3t afford current and voltage distribution suflicient to produce parts which have any useful degree of coating uniformity. Current and voltage distribution are noted to be directly proportional to rate of formation of satisfactory oxide coating during the anodizing process. It will be observed that the capacity of the tank is materially increased by provision of the buttons, enabling parts placed close to the sides and bottom of the tank, as illustrated in Fig. 4, to be properly coated. Thus, for example, employing a tank 32 inches long, 12 inches wide and 16 inches deep, utilizing the above described button arrangement, the parts can be placed less than an inch from the side and end walls, and the same distance from the bottom of the tank, with good uniform coatings obtained on all parts, whereas in the absence of my buttons employing the same tank, the parts must be placed not closer than about 3 inches from the side walls, end walls, and the bottom of the tank, in order to obtain some degree of uniformity of coating on all parts. In addition, however, 1 have found that by provision of my buttons, more uniform coatings are produced on the parts even though highly irregular in shape, and high copper aluminum alloys are particularly advantageously coated with uniform oxide coatings throughout the enlarged working volume of the tank. The theory as to how the buttons function to produce my improved results is not presently known to me.

As to the structure of

buttons

38 and 40, all of which are the same in the embodiment of Fig. 1, Figs. 5 and 5a show one of these buttons in detail. The

button

38 or 40 is round and comprises a head having a fiat central portion 44 and an outer conically shaped or frusto conical surface 46 generated by an element forming an obtuse angle with the central surface 44. The outer periphery of the conical surface portion 46 intersects a short cylindrical surface portion 48, the axis of which is perpendicular to the central surface 44. Hence, the head of button 33 or 40 is composed of a frustum formed by

surface portions

44 and 46 and an integral cylindrical portion 48. The salient structural feature of this button is the provision of the surface 46 disposed at an angle to the

surfaces

44 and 48, and forming sharp intersecting edges at 50 and 52 with said surfaces. The angularly disposed or conical surface 46 and the sharp intersecting edges between said surface and the adjacent surfaces of the button have been found necessary in order to produce the results of the invention. If the angularly disposed surface 46 does not intersect the adjacent surfacesalong a sharp edge, I have observed that the effectiveness of the buttons is materially reduced.

The button has a centrally disposed aperture 54, through which a shank or stud 56 is threaded, the shank being welded at 58 about its periphery to the adjacent central surface 44 of the button. The inside surface 60 of the button is placed in close proximity to or against the inner wall 22 of the tank, and the button is welded to the wall about its outer periphery as indicated at 62. The tank has an outer wall or jacket 64 which is separated from the

inner walls

22 and 24 by spacers 66. The space between the

inner walls

22 and 24 and the outer wall 64 of the tank is filled with heat insulation 68, to prevent substantial temperature variation of-the solution in the tank. The shank 56 and button are further supported in position by a nut 70 which clamps a washer 72 against the outer wall 64 of the tank. A cable connector 74 threadably engages'the outer end of the shank 56, and a wire or electrical cable 76 is held in a central aperture 78 of the connector by means of a threaded keeper 80 which is adjustable along the longitudinal axis of the connector, and has an inner plate 82 which, when the keeper 80 is turned down, clamps the cable in firm position in the connector. The cable or wires 76 are each connected to the negative terminals of a DC. generator (not shown).

Referring to Fig. 4, a number of workpieces 84 to be anodized are hung from metal wires or fixtures 85 which are in turn suspended from one or more bus bars 86 positioned across the top of the tank. The bus bars 86 are connected by means of wire or cable 88 to the positive terminals'of thellC.v generator. During the anodizing process current flows from the negative terminals of the generator to the buttons 38 and .40, via cables '76 and the shanks 56, placing a negative potential on the buttons and the

inner walls

22 and 24 of the tank, since such walls are composed of a conductive metal such as stainless steel. Current is conducted through the solution 90, the pieces 84 to be coated serving as anodes, and the circuit is completed through the positively polarized bus bars 86, to cause electrochemical formation of an oxide coating on said work pieces, the thickness of such coating being proportional to the amount of current and time of treatment, and also being dependent on other factors such as temperature of the bath, the chemical nature of the bath and the composition of the aluminum alloy work pieces.

The placement of the buttons on the inner walls of the tank in order to obtain-optimum results for a particular tank can be accomplished by trial and error, but preferably is achieved in the following manner in the embodiment of Fig. 1. Each wall 22 of the tank on which the buttons are to be mounted is geometrically divided in half along a line indicated at 92 in Fig. 6 to form two equal

half wall portions

94 and 95. A pair of electric leads 96 and 98 are connected from the opposite terminals of a battery 100 to the diagonally opposite corners 102 and 104 of half portion 94. One probe 106 of a pair of probes connected to a galvanometer 110, is connected to corner 102, while the other probe 108 is manipulated until the point of equipotential drop across the plate from corners 102 and 104 is obtained. This point, indicated at 112 is generally on a line 113 close to the geometric diagonal between corners 102 and 104, although it need not necessarily be directly on such diagonal. The same procedure is followed to determine the point of equipotential 115 across the

opposite corners

114 and 116 of wall portion 94, using

probes

106 and 108 and galvanometer 110. A line 118 is drawn from corner 114 through point 115 and where this line intersects line 113, drawn from corner 106 through point 112, is the place, indicated at 120, where a button 38 is to be located. A similar procedure is followed with respect to the other half portion 95 of tank wall 22, for location of the point 122 where the second button 38 is to be positioned.

It will be seen that the buttons 38 need not necessarily be exactly the same distance away from the adjacent end walls 24, nor need they both be exactly the same height above the bottom of the tank. The same applies to buttons 40 on the opposite side wall 22.

Referring to Fig. 60, if the tank is of a different shape or size, a different button arrangement may give best results. Here, the tank 124 is more square shaped than tank 20, and has two buttons 126 of a structure like 38, on one pair of opposite walls 128 within the tank, and another pair of buttons 130 directly opposite each other on the opposite pair of walls 132.

In Fig. 7 is shown still another modified button arrangement on a large rectangular tank 134. Here, three buttons 136 of a structure similar to 38, are disposed along one side wall 138, and three buttons 140 are positioned on the opposite side wall 142, directly opposite the respective buttons 136. Two buttons 144 are each disposed centrally along opposite end walls 146. Buttons 136 are substantially equally spaced from each other, as are buttons 140. All of the buttons are about the same distance above the bottom of the tank.

Fig. 8 illustrates still another button arrangement on a rectangular form of tank. Here, four buttons 148 of a structure similar to 38 are disposed along one side wall 150 of tank 152, with four buttons 154 positioned on the opposite side wall 156 of the tank directly opposite the respective buttons 148. The buttons on each of the side walls are approximately equally spaced from each other.

In Fig. 9 is shown schematically a button arrangement similar to that of Fig. 8. However, instead of connecting the buttons 14% and 154 directly to the inside surface of the

tank walls

150 and 156, an electrically conductive bus bar 158 (see also Fig. 10) is mounted, such as by welding, longitudinally on the inner surface of one wall 150, and a. second similar bus bar 160 is mounted in a similar fashion on the opposite side wall 156 of the tank directly opposite bus bar 158. The buttons are mounted on the bus bars, with the shanks 161 of the buttons passing through said bus bars. One lead 162 connects bus bar 158 to the negative side of the D.C. generator (not shown), and one lead 164 also connects the other bus bar 160 to the negative terminals of the generator. By using bus bars such as 158 and 160, the number of leads required from the buttons to the generator is reduced, inasmuch as each button does not require a separate connection to the generator, but only the bus bars on which the buttons are mounted need be electrically connected to the negative side of the generator.

I have found that where 8 or more buttons are used, it is economical to employ bus bars such as 158 and 160. Also the bus bars tend to improve the action of the buttons to some extent. It is noted that when the bus bars are mounted on the inside surface of the tank as indicated in Fig. 10, it is necessary that the surfaces of the bus bars in contact with the solution be covered with an inert masking material 166 or that such bars be formed of a material which is inert with respect to the electrolyte, in order to prevent corrosion of the bus bars. Hence, it is preferred that the bus bars not be mounted on the inside surface of the tank, but rather on the outside surfaces of the tank walls, e.g., as illustrated in Fig. 10a, with the buttons of course always located inside the tank along the inner surfaces of the tank walls. In this modification the bus bars, e.g., 158 are located on the outer surface of the inner wall 150 of the tank within the insulated portion 163 between the inner wall 150 and the outer tank wall 165, the shanks 161 of the buttons passing through the bus bar and the outside surfaces 165 of the tank walls. If desired, said bus bars may be mounted on the outside walls 165 with the shanks of the buttons connected to said bus bars.

While the buttons described above are shown with but a single angularly disposed or frusto conical surface, as 46 in Fig. 5, the button may have a plurality, say two, three or four of such surfaces. Thus, as seen in Figs. 11 and 12, button 170 has adjacent, angularly disposed or

conical surfaces

172, 174 and 176, forming three frustrums, with a cylindrical surface portion 178 and a central portion 180 which is apertured by the central bore 182 passing axially through the button. It will be observed that surface portion 172 intersects

adjacent surfaces

174 and 178 along the sharp

peripheral edges

184 and 186; surface 174 intersects

adjacent surfaces

176 and 172 along the sharp

peripheral edges

188 and 184; and surface 176 has a sharp outer periphery 190 defined by the outer end of bore 182 and a sharp inner peripheral edge 188 at its intersection with surface 174.

In Figs. 13 and 14 are shown another form of lug or button 191, which is rectangularly shaped, and has a pair of bevelled planar, or angular longitudinal side surfaces 192 intersecting a top planar surface 194 and extending to a lower base portion 196. Button 191 also has a pair of bevelled planar end surfaces 198 intersecting the bevelled side surfaces 192 along edges 200.

Figs. 15 and 16 illustrate yet another form of button 202 which can be employed. Button 282 is of triangular shape having three bevelled surfaces 204 adjacent the outer edges of the button, said surfaces intersecting the top planar surface 206 of the button and intersecting each other along edges 208. Angular surfaces 204 extend downwardly to a base portion 210.

It has been found that the buttons or lugs are most effective when mounted on the side or end walls of the anodizing tank, and ordinarily have less effect when placed on the bottom surface of the tank, although this may be done if desired, e.g., in the case of a hemiy spherically shaped tank. From experience it has been observed that the buttons should be positioned on the tank walls a substantial distance above the bottom of the tank, preferably at least one-third the distance from the bottom to the top of the tank. It is noted that the buttons described above have a short base portion adjacent the bottom of the buttons, e.g., cylindrical portion 48 in Fig. 5 and 178 in Fig. 11. This base portion can be omitted if desired. However, as a practical matter in constructing the button and facilitating fastening thereof, e.g., by welding, to the tank surface, a short base portion is provided, such as 48 or 178. The buttons may be of varying sizes, e.g., from A" to 5" in diameter in the case of essentially round buttons. The size of the buttons employed in a particular tank depends on the amount of electric current to be carried by the tank and the total number of buttons to be used in such tank. In preferred practice, the buttons should be large enough so that their temperature and the temperature of the bath are not raised to any material extent by passage of the current through the buttons. The buttons and shanks thereof are constructed of a conductive material, preferably stainless steel for electrolytic anodizing, to prevent corrosion of the buttons by the acid electrolyte solution employed. However, it will be understood that my apparatus may be employed for electrolytic operations generally, utilizing either alkaline or acid electrolytes.

To illustrate the effectiveness of the invention structure, a stainless steel tank about 4 ft. long, 2 /2 ft. wide and about 2% ft. deep was filled with an electrolyte comprising aqueous sulfuric acid. The tank was of a type similar to that of Figs. 3 and 4, but without the buttons. In this tank not more than four bell-type piston forgings of 14S aluminum alloy (4.4% copper) could be successfully anodized to a coating thickness of .003", and such process required approximately 2 hours.

Using the same tank, electrolyte and procedure as noted above, except that a button arrangement similar to that shown in Figs. 3 and 4 was incorporated in the tank, as many as 35 of the same bell-type piston forgings were anodized at the same time in the tank to produce uniform oxide coatings of .003" thickness in about 35 minutes on all of the parts without burning. Using the button principle of the invention, as many as 2,000 parts have been simultaneously anodized to uniform coating thicknesses in a single tank, a substantial number of the parts being positioned closely adjacent the walls and bottom of the tank. This clearly illustrates the great increase in capacity of a given tank and the economic advantages of the use of my buttons, according to the invention, particularly for the electrolytic oxidation of aluminum and its alloys, especially high copper aluminum alloys, heretofore especially difficult to properly anodize in production quantities.

In add.tion to the various types of buttons or lugs described above, I can also employ buttons in the form of a diamond, having numerous planar surfaces or facets disposed at various angles, each intersecting adjacent surfaces along sharp edges.

While I have described a particular embodiment of my invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

I claim:

1. Electrolytic apparatus which comprises an electrically conductive tank, a plurality of electrically conductive buttons mounted on and electrically connected directly to the inside walls of said tank, said buttons being spaced from each other, each of said buttons having a plurality of surfaces, at least one of said surfaces being angularly disposed to and intersecting an adjacent surface of said button along a sharp intersecting edge, and an electrical connection to said buttons.

2; Electrolytic apparatus, which comprises an electrically conductive tank having a plurality of side walls, a plurality of electrically conductive buttons electrically connected directly to the inner surfaces of at least two of said side walls opposing each other, said buttons being spaced from each other, each of said buttons having a plurality of surfaces, at least one of said surfaces being conically shaped, said last named surface intersecting an adjacent surface of said button along a sharp intersecting edge, and an electrical connection to said buttons.

3. Electrolytic apparatus which comprises an electrically conductive tank, a plurality of electrically conductive buttons mounted on and electrically connected directly to the inside walls of said tank, said buttons being spaced from each other, each of said buttons having a plurality of surfaces, at least one of said surfaces being angularly disposed to and intersecting an adjacent surface of said button along a sharp intersecting edge, a source of direct current, an electrical connection between said buttons and the negative terminals of said direct current source, aluminum or aluminum alloy anodes disposed in said tank, a positive bus bar, said anodes being connected to said bus bar, and an electrical connection between said bus bar and the positive terminals of said direct current source.

4. Electrolytic apparatus as defined in claim 3, wherein said at least one of said surfaces intersects at least two adjacent surfaces along at least two sharp intersecting edges.

5. Electrolytic apparatus as defined in claim 3, wherein said tank has a plurality of side walls, and wherein a plurality of electrically conductive buttons are connected to the inner surface of at least two of said walls opposing each other, said buttons being spaced from the bottom and top of said tank.

6. Electrolytic apparatus as defined in claim 3, wherein said at least one of said surfaces is a frusto conical surface.

7. Electrolytic apparatus as defined in claim 3, wherein each of said buttons has at last three adjacent intersecting surfaces, the intermediate one of said adjacent surfaces being conically shaped, another of the adjacent surfaces being cylindrical and the third one of said adjacent surfaces being a substantially fiat surface positioned normal to the axis of said cylindrical surface, said conically shaped surface intersecting the adjacent cylindrical and flat surfaces along sharp circular edges.

8. Apparatus as defined in claim 3, and including electrically conductive'studs passing through the wall of said tank, each stud being centrally connected to a separate one of said buttons, said electrical connections being connected to said studs.

9. Apparatus as defined in claim 3, wherein two of said buttons are positioned on the inner surface of one side wall of said tank, one on each half portion of said side wall, and two of said buttons are positioned on the inner surface of the opposite side wall substantially in alignment with said first two buttons, each of said buttons being located substantially at the point of equipotential diagonally across that half portion of the side wall on which each such button is positioned.

10. Electrolytic apparatus, which comprises in combination, an electrically conductive tank, a plurality of electrically conductive buttons mounted on and in direct electrical contact with the inside walls of said tank, said 10 buttons being spaced from each other, each of said buttons having a plurality of surfaces, at least one of said surfaces being angularly disposed to and intersecting an adjacent surface of said button along a sharp intersecting edge, a source of direct current, an electrical connection between said buttons and the negative terminals of said direct current source, metal anodes disposed in said tank, a positive bus bar, said anodes being connected to said bus bar, and an electrical connection between said bus bar and the positive terminals of said direct current source.

11. Apparatus for the electrolytic oxidation of aluminum and its alloys, which comprises an electrically conductive four-sided tank, a plurality of electrically conductive buttons connected to the inner surface of at least one pair of opposite walls of said tank, said buttons being spaced from each other and from the bottom of said tank, each of said buttons having at least three adjacent intersecting surfaces, the intermediate one of said adjacent surfaces being conically shaped, another of the adjacent surfaces being cylindrical and the third one of said adjacent surfaces being a substantially flat surface positioned normal to the axis of said cylindrical surface, said conically shaped surface intersecting the adjacent cylindrical and flat surfaces along sharp circular edges, a source of direct current, an electrical connection between said buttons and the negative terminals of said direct current source, aluminum or aluminum alloy anodes disposed in said tank, a positive bus bar, said anodes being connected to said bus bar, and an electrical connection between said bus bar and the positive terminals of said direct current source.

12. Electrolytic apparatus, which comprises a foursided tank, bus bars, one connected to each of at least two opposite walls of said tank, said bus bars being spaced from the bottom and top of the tank and" disposed substantially parallel thereto, a plurality of electrically conductive buttons mounted on said bus bars and positioned adjacent the inner surfaces of said at least two opposite walls of said tank, said buttons being spaced from each other, said buttons each having a plurality of angularly disposed surfaces, one of said surfaces intersecting an adjacent surface of said button along a sharp edge, a source of direct current, an electrical connection between said buttons and the negative terminals of said direct current source, aluminum or aluminum alloy anodes disposed in said tank, a positive bus bar, said anodes being connected to said bus bar, and an electrical connection between said bus bar and the positive terminals of said direct current source.

References Cited in the file of this patent UNITED STATES PATENTS 994,405 James June 6, 1911 1,093,384 Charlton Apr. 14, 1914 1,366,257 Gush Jan. 18, 1921 1,556,752 Blake Oct. 13, 1925 2,370,463 Herrick Feb. 27, 1945 2,462,968 Hogaboom et al Mar. 1, 1949 2,743,227 Waite et al. Apr. 24, 1956 FOREIGN PATENTS 546,334 France Aug. 18, 1922 UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No 2369,3141 January 24 1961 George' fl. Coxe It is hereby certified'that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the grant, line 1 address of inventor for :"Norwalk Connecticut" read Norwalk California in the heading to the printed specification line 3,, for "Norwalk'g Conn."- read Norwalk Calif Signed and sealed this 25th day of July 1961.,

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attestlng Offlcer Commissioner of Patents