US2995502A - Conditioning and anodizing system - Google Patents

Description

Aug. 8, 1961 E. R. RAMIREZ ETAL 2,995,502

CONDITIONING AND ANODIZING SYSTEM Filed 001,. 7, 1957 /yf 4 /jfa Ta ATTORNEYS aired States Patent O 2,995,502 CONDITIONING AND ANODIZING SYSTEM Ernestr Rt Ramirez, Richmond, and Erik F. Barkman, Henrico County, Va., assignors to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Filed Oct. 7, 1957, Ser. No. 688,669 12 Claims. (Cl. 204--15) This invention relates to a novel method for edge conditioning and anodizing of aluminum slit sheet orstrip. More particularly, the invention concerns a novel processfor the manufacture of anodically coated aluminum sheet or strip possessing superior dielectric surface characteristics.

The excellent electrical properties, light weight and resistance to corrosion of aluminum have led to the widespread adoption of aluminum in sheet, strip, and foil form. in the metal and electrical products industries.A

Properly anodized aluminum strip, for example, carries a surface. layer of aluminum oxide` which is capable of furnishing a dielectric coating of sufficiently high insulat# ing power to enable the strip to be utilized as such as an electrical conductor in laminated or coil form, such as, forexample, intransformers. However, anodized aluminum strip or foil has heretofore achieved only very limitedy application in electrical devices owing to the un-A dependable and nonuniform character of the anodc coatings obtainable by presently known methods.

From the standpoint of the aluminum manufacturer, prior art methods for the production of oxide coated aluminum strip or sheet have been slow, cumbersome and costly. From the standpoint of the electrical products manufacturer, the available strip has been unsatisfactory and its performance uncertain, owing to poor dielectric properties both at the edges and on the main surfaces of the strip, giving rise to pinholes, rough edges, and protuberan'ces causing short-circuiting when the material was put to use in applications such as coils, where contact was maintained directly between adjacent surfaces. Nevertheless, the dielectric advantages of an adequate and uniform coating of aluminum oxide are well known, and aluminum strip so coated would have been more extensively adopted had there been a suitable method of manufacturing sound material. Oxide insulating coatings make possible the elimination of other and separate insulation layers between adjacent aluminum surfaces when in coil form.

Aluminum sheet, strip and foil, particularly in the gages employed in electrical work, is manufactured in desired widths by conventional methods involving shearslitting'.l The nature of such operations is such as part to the sheared or slit edge a rough, jagged, or otherwise discontinuous aspect, such as depicted, for example, 4in FI'Grl of the drawing. 'I'he overhanging, sharp edges resulting from the slitting or shearing operation, will,l unless removed or' modified by special treatment,` adversely affect usefulness of the strip for elecf trical Npurposes.v The condition of the strip edge will particularly affect the result of any anodizing operation to whichthe strip may be subsequently subjected in order to provide the strip with an insulating oxide coating.

In accordance with the present invention, it has been found that insulating oxide coatings of high quality, depth, and uniformity may be obtained upon the sur' faee's of aluminum sheet or strip by a novel process in-l volvir'rg three treatment stages or steps. In the first step of our novel process, 'slit aluminum sheet or strip is subject t treatment confined to the rough edges and contiguous portions, which has as its purpose the elimination of lroughness, burrs and protuberances and the productio of smooth, preferably tapered, edge portions on the strip or sheet, In the second step of the process,

Patented Aug. 8, 1961 ice . edges and to protect them during the final step of the process. The third and final step of our novel process is to establish an oxide coating over the entire surface of the now preheated strip by means of an anodizing dperation.

The three process steps outlined above are interrelated and cooperate to bring about a remarkably improved final anodized sheet or strip product having an oxide coating of extraordinary uniformity. The preconditioning treatment eliminates the burrs and rough edges which interfere with subsequent anodizing steps and which might otherwise cause Contact and short circuiting when the finished sheet or strip is used for transformer coils or other electrical apparatus. The second or edge anodizing and precoating step serves to convert the soft and deformable edges resulting from the first or preconditioning step into diicultly bent and hard coated areas which will be protected during the final anodizing step and at the same time reduce the area over which final anodizing must be applied.

Experience has shown that when a strip is continuously anodized without any edge pretreatment, a varying rate of anodizing takes place at the sht edges as compared with the flat surface. Two factors contribute to this phenomenon. During the slitting operation, the metal on the edges is cold worked and consequently left in a high energy state and with residual stresses. As a result, the edges require a lower anodizing potential than the flat surface, causing excessive current ux or current leakage through the edges. Extremely high current densities produced at the slit edges tend to dissolve oxide formed at those edges, which'are inherently more susceptible to anodizing action, almost as fast as formed,

resulting in so-called burning or bare points. Another factor causing inferior formation of oxide on untreated slit edges is the fact that a metal sliver projecting at a very small angle, as shown in FIG. l, will not anodize properly, since oxide formation takes place more for less perpendicularly to an aluminum surface, thus leaving the metal top bare. The pretreatment of step 1 of our novel process, by removing protruding metal burrs and slivers, promotes relatively `uniform oxidizing ofthe edges in step 2, The use of step 2, in turn, enables the building up of a substantial, protective, and uniform layer of oxide on the edges, imparting good dielectric properties thereto, and preventing nonuniform oxide formation at the edges in step 3, Precoating of the edges eliminates the tendency of untreated edges to draw olf anodizing current in step 3 or to concentrate it at the edges. Gradual building of an oxide coating on the metal edge prior to over=all anodizing permits increased thicknesses of oxidec'oatings in final anodizing, with greater uniformity and higher dielectric Value.

In terms of practical plant operation, the novel process of the present invention has demonstrated that important economies in time and electrical energy are possible in the final anodizing step. Thus, a net reduction in time consumed of from l0 to 15 percent is brought about, while simultaneously, current consumption for a given area of anodized product may diminish as much as 10 percent,

For a better understanding ofV the invention, reference is now made to the Vdrawing which depicts schematically the results obtainable in various stages of the process. ln the drawing: e

FIG. 1 depicts an untreated slit aluminum edge in cross-section; A

FIG. 2 shows the effect of edge pretreatment and edge anodizing, respectively;

FIG. 3 is a cross-sectional view of adjacent turns of untreated edges in a coil of strip;

FIG. 4 depicts the action of gas formation and the distribution of current density at the coil edges.

The various steps of our novel process will now be described in detail. While the process will be described with reference to the inclusion of all three steps mentioned above, it will be understood that, where strip or sheet is available with edges in suitable condition, the firs-t step may be omitted and that the process then cornprises the novel combination of edge anodizing or precoating, followed by over-all anodizing.

'I'he preconditioning or edge treatment of the first step may be performed in accordance with this invention either by electrolytic or chemical means, but preferably by electrolytic treatment. Whichever means is used, step l of our process involves the novel principle of limiting the electrolytic or chemical action substantially to the edges and contiguous portions and without subjecting the main flat areas of the strip or sheet to the etching, milling or polishing action. This is done preferably by subjecting the exposed rough edges to the action of the bath while the strip or sheet is in coil form. Prior to coiling, the sheet, strip or foil may be subjected to suitable alkaline or otherdegreasing treatments, and to water rinsing, in accordance with conventional procedures. Such degreasing and Washing may, however, also be confined to the edges of previously coiled material.

In carrying out the edge treatment by electrolytic l means, the principle of operation adopted is that of employing higher current densities and bath temperatures than are customary, for example, in anodizing baths. The attacking agent is preferably a mineral acid, such as, for example, sulfuric acid or uoboric acid. Where sulfuric acid is used, the concentration may range from about 5% to 70%, but it has been found preferable to use a strength of about The temperature range may-be from about 110-130 F. The current density, when using sulfuric acid, will preferably be of the order of 80 to 110 amperes per square foot of exposed edge area, while the bath temperature is maintained at a point suicient to result in solution of any formed oxide at a rate equivalent to its rate of formation.

The rate of attack of the electrolytic solution depends upon (1)*the physical condition of the metal surface, and (2) accessibility of the surface to the electrolyte.

'Ihe use, for example, of a sulfuric acid electrolyte under the conditions specified above, results in a small formation of oxide at first, but the step differs from conventional anodizing techniques, to which it may appear to bear some resemblance, in that this treatment preferentially attacks and dissolves away the sharp protruding areas, and leaves no oxide lm. Burrs and rough edges which might cause metal to metal contact when the final anodized product is used for electrical coiis, are eliminated, and in fact, the coiled aluminum strip or sheet is left with a unique tapered edge, as depicted in FIG. 2 of the drawing. The absence of an oxide coating at this stage'leaves the metal edge with a dielectric value of substantially zero.

The use of the coiled form of aluminum strip, sheet, or foil insures limited Iaccessibility of the attacking solution, and confines its action to the edges to be treated and the immediately contiguous areas. Electrolytic edge treatment of metals for the removal of burrs is known and is described, for example, in U.S. Patent 2,590,927. However, such treatment has heretofore been applied only to laminated products comprising layers of ferrous metal separated by intermediate layers of insulating material. Applicants were the first to discover that edge treatment for removal of burrs and rough areas could be successfully applied to aluminum in coil form with no intermediate layers of other material intervening between adjacent turns or surfaces of aluminum in the coil.

The principle of edge treatment in coil form is applicayble to both pretreatment of the edges, and to edge anodizing. In either case, the action of the bath is usually limited to a distance not exceeding about ls of an inch from the cxtreme edge, with the flat portions of the coiled strip remaining dry. An example of the action resulting in a rounded or tapering edge by this method of operation is depicted in FIGS. 3 and 4. FIG. 3 illustrates the observed position and shape of the adjacent edges of several turns of coiled metal which has not been pretreated. By subjecting the edges in coil form to the action of an attacking bath, the protruding edge portions dissolve more rapidly than the other exposed areas. 'This is believed due to (a) higher current density at the extreme edges, and (b) entrapped liberated gas which concentrates in the crevices and tends to oppose wicking or further penetration by the bath between the metal layers. The latter effect is illustrated in FIG. 4.

The following examples serve to illustrate the step of edge pretreatment, but this phase of the invention is not to be regarded as limited thereto.

Example 1 A coil wound aluminum strip is immersed in the following solution and made the anode (positive), with a lead or graphite element used as the cathode; and a current is passed through the bath:

Composition Sulfuric acid, 30% by weight. Temperature -130 F.

Current density 100 amps. per sq. ft.

Time of treatment 5 minutes.

At the end of the time of treatment. the coil is removed from the bath, and exhibits an edge structure similar to that shown in FIG. 2. Y

Example 2 A wound aluminum coil is made the anode, with stainless steel as a cathode, in a bath having the following composition:

Composition Fluoboric acid, 2.5% by weight. Temperature 65-75 F.

'I 'ank voltage 12-12.5 volts.

Time of treatment 5-10'minutes.

Example 3 A coiled aluminum strip is immersed in a solution having the composition:

Composition Orthophosphoric acid (85%), 94 parts by volume. Nitric acid j (60-63%), 6 parts by volume. Temperature -210" F. Time of treatment- 3-5 minutes.

The coil is removed and washed.

'Ihe coiled strip, after receiving pretreatment of the type described in the foregoing examples, is next prepared for continuous or other types of final anodizing by means of the edge anodizing second step of our novel process.

'I'he edge anodizing operation of step 2 of the process of this invention may be carried out either by immersing the aluminum in coil form in a suitable anodizing bath, or by unwinding the coil and passing the edge-treated strip or sheet through an edge anodizing apparatus of the type disclosed in our copending application, Serial No. 695,901, tiled November 12, 1957.

The edge anodizing procedure in general comprises immersing the material in coil form in a suitable acid anodizing electrolyte, such as sulfuric acid or oxalic acid, the coil being made the anode, with preferably a graphite cathode, and passing a current through the bath. In accordance with a preferred mode of operation of step 2, a slow build up of current density results in an oxide coating at the edges which is especially hard and abrasion resistant. When using sulfuric acid in the anodizing bath, concentration will usually be between and 40% by Weight, preferably about 30%, and temperature between about 30 and 80 F., depending upon desired results.

The following examples serve to illustrate edge anodizing in accordance with our invention:

Example 4 A wound aluminum coil, edge pretreated as described in Example 1, is immersed as anode in the following composition, using a lead cathode and 'a current is applied to the bath.

Composition Sulfuric acid, 30% by weight. Temperature 65-75 F. Current density 80-100 amps. per sq. ft. D.C. Time of treatment 5 minutes.

There is obtained as a result a relatively uniform coating of oxide of about 0.003" on the exposed edges, resembling in cross-section the structure shown in FIG. 2.

Example 5 A preconditioned edge treated coil is ,immersed in a 30% sulfuric acid solution and made the anode, with a graphite cathode. Current density is slowly built up from 40 to 80 amps. per sq. ft. in the course of l minute. Subsequently the coil is edge anodized at 80 amps. per sq. ft. current density for 5 minutes. The dielectric value of the edge of the aluminum strip is thereby increased from about zero to about 200 volts D.C. By increasing the anodizing time from five minutes to minutes, the dielectric value rises to 400-500 volts. These values are obtained in an insulation breakdown tester, using the edge of the strip (about a 2" sample) in a mercury pool, with the mercury the anode, in a D.C. arrangement.

Example An edge-conditioned aluminum coil is made the anode in an oxalc acid electrolyte containing from 5 to 7% by weight of oxalic acid in water. Current density is gradually built up from about 10 amps. per sq. ft. to about 50 amps. per sq. ft. at a a 'temperature of about 80 F., resulting in -a dielectric value of the coil edges of about 200 volts D.C. By increasing the anodizing time, this value can be built up to any desired value below 1000 voltsD.C.

Example 7 The edges of a wound coil are anodized in a 30% sulfurie acid bath employing a slow buildup of current density starting with about amps. per sq. ft. to a figure of about amps. per sq. ft. Anodization is carried on for about 5 minutes, giving a dielectric value at the edges of about 100 volts, D.C. In this mode of operation the oxide coating formed is quite hard and is more resistant to abrasion than the coatings obtained in Examples 4 to 7 inclusive.

The final over-all anodizing step 3 of our novel. process is applied to the material in uncoiled form in accordance with conventional methods. Thus, the coil may be unwound and passed through a regular continuous anodizing bath, employing e.g. 30% sulfuric acid at 7080 F. In this way an oxide coating is obtained over the entire surface of the strip or sheet which is suiciently thick and has requisite dielectric properties both at the edges and in the central portions to enable the strip to be utilized for electrical windings without the necessity of any additional insulation between adjacent turns.

While we. have illustrated and described present preferred embodiments of the invention, it will be recognized that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

We claim:

l. Process for the manufacture of anodized aluminum sheet, strip and foil, which comprises treating only the edges and contiguous portions of slit or sheared aluminum in helical coil form having rough edges in a strong acid bath to remove burrs and protuberances from said edges, then anodizing the edges and contiguous portions of said aluminum in coil form to build an oxide coating thereon, the adjacent layers of aluminum in said coil being in substantial contact with each other at said edges and contiguous portions, and thereafter anodizing said aluminum in uncoiled form to establish an oxide coating over the entire surface thereof.

2. Process for the manufacture of anodized aluminum sheet, strip and foil, which comprises .electrolytically treating only 'the edges and contiguous portions of slit or sheared aluminum in helical coil form having rough edges in an acid bath to remove burrs and protuberances from said edges, then anodizing the edges and contiguous portions of said aluminum in coil form to build an oxide coating thereon, the adjacent layers of aluminum in said coil being in substantial contact with each other at said edges and contiguous portions, and thereafter anodizing said aluminum in uncoiled form to establish an oxide coating over the entire surface thereof.

3. Process for the manufacture of anodized aluminum sheet, strip and foil, which comprises iirst anodizing the edge and contiguous portions of said aluminum in helical Y coil form to build an oxide coating thereon, the adjacent layers of aluminum in said coil being directly in contact substantially over the interface therebetween, and then separately anodizing the central areaof the aluminum to obtain a uniform oxide coating over the entire surface thereof.

4. Process for the manufacture of anodized aluminum sheet, strip and foil, which comprises treating slit or sheared aluminum in helical coil form having lrough edges in a strong acid bath to remove burrs and protuberances from said edges, and then anodizing the edges and contiguous portions of said aluminum in coil form to build an oxide coating thereon, the -adj-acent layers, of aluminum in said coil being directly in contact substantially over the interface therebetween.

5. Process for the removal of burrs and protuberances from the edges of slit or sheared aluminum sheet, strip and foil, which comprises treating the edges and contiguous portions of said aluminum in helical coil form in a strongacid bath to render said edges substantially free from bur-rs and protuberances, the adjacent layers of aluminum in said coil being `directly in contact substantially over the interface therebetween.

6. Process for the removal of burrs and protuberances from the edges of slit or sheared aluminum sheet, strip and foil, which comprises subjecting the edges and contiguous portions of said laluminum in hel-ical coil form to treatment in an electrolytic acid bath in which the aluminum isthe anode, by passing a current through said bath to render said edges substantially free from bu-rrs and protuberances, the adjacent layers of aluminum in said coil being directly in contact substantially over the interface therebetween.

7. Process for the removal of burrs andr protuberances from the edges of slit or sheared aluminum sheet, strip and foi-l, which comprises subjecting the edges and contiguous portions of said aluminum in helicaly coil form to electrolytic treatment in `a sulfuric acid bath in which the aluminum is the anode, at a temperature between y about -and 130 F., and ata current density of about in said coil being in substantial contact with each other at said edges and contiguous portions.

8. Process for the edge anodizing of aluminum sheet, strip and foil, which comprises electrolytically treating only the ediges and contiguous portions of slit or sheared aluminum in helical coil form having rough edges in an acid bath in which the aluminum is the anode by passing l a current through said ybatir to render said edges substantially free from roughness, and then anodizing the edges and contiguous portions of said aluminum in coil form in a sulfuric acid bath to build an oxide coating thereon, the adjacent layers of aluminum in the said coil being in substantial contact with each other at said edges and contiguous portions.

9. Process for the edge anodizing of aluminum sheet, strip and foil, which comprises immersing said aluminum in helical coil form as the `anode in a sulfuric acid bath, adjacent layers of aluminum in said coil being in substantial contact with each other Iat the coil edges and contiguous portions, access of said bath being limited substantially to the edges and contiguous exposed portions of the aluminum by said contact between layers, and passing a current through said bath at a temperature between about 30 and 80 F., maintaining a current density between abou-t 80 land 100 amperes per sq. ft., to deposit an oxide coating confined to said edges and exposed portions contiguous thereto.

10. Process for the formation of a hard, abrasion-resistant oxide coating on the edges of aluminum sheet, strip and foil,which comprises immersing said aluminum in helical coil formas the anode in a sulfuric acid bath, adjacent layers of aluminum in said coil being in substantial contact with each other at the coil edges and contiguous portions, access of said bath being limited substantially to the edges and contiguous exposed portions of the aluminum by said Contact between layers, and passing a current through said bath with an initial current density of about 2O amperes per sq. ft., gradualy increasing the current density to 'about 30 amperes per sq. ft. for a period of about 5 minutes.

, 11. Process for the manufacture of -anodized aluminum sheet, strip and foil, which comprises electrolytically treating the edges and contiguous portions of slit or-sheared aluminum in helicalvcoil form having rough edges n an acid bath to remove bur-rs and protuberances from said 8 edges, the aluminum being the anode in said bath, then edge anodizing said aluminum 4in coil form by immersing the coil as the anode in a sulfuric acid bath, adjacent layers of aluminum in said coil being in substantial contact with each other at the coil edges and contiguous portions, access of said bath being limited substantially to the edges and contiguous exposed portions of the aluminum by said contact between layers, passing a current through said bath at a temperature between about and F., and at a current density of about 80 to 110 amperes per sq. ft., to deposit an oxide coating confined to said edges and exposed portions contiguous thereto, and thereafter anodizing said aluminum in uncoiled form in a sulfuric acid bath at a temperature between about 70 and 80 F.

12. Process for the manufacture of anodized aluminum sheet, strip and foil, which comprises electrolytically treating the edges and contiguous portions of slit or sheared aluminum in helical coil form having rough edges in an acid bath to remove burrs and protuberances from said edges, the aluminum being the anode in said bath, then edge anodizing said aluminum in coil -form by immersing the coil as the anode in a sulfuric acid bath, adjacent layers of aluminum in said coil being in substantial contact with each other at said edges and contiguous portions, access of said bath being confined substantially to the edges and contiguous exposed portions by contact of adjacent coil surfaces, passing a current through said bath with an initial current density of about 2O amperes per sq. ft., gradually increasing the current density to about 30 Iarnperes per sq. ft. for a period of about 5 minutes to deposit a hard, abrasion resistant oxide coating on said' edges, and thereafter said aluminum in uncoiled form in a sulfuric acid bath at a temperature between about 70 and 80 F.

References cited in the me of this patent UNTTED STATES PATENTS 2,057,315 Robinson Oct. 13, 1936 2,094,048 Siegel Sept. 28, 1937 2,374,449 Mulcahy Apr. 24, 1945 2,436,227 Phillips Feb. 17, 1948 2,668,936 Robinson y Feb. 9, 1954 2,863.796 'Theodosopoulos et al. Dec. 9, 1958

Claims (2)

1. PROCESS FOR THE MANUFACTURE OF ANODIZED ALUMINUM SHEET, STRIP AND FOIL, WHICH COMPRISES TREATING ONLY THE EDGES AND CONTIGUOUS PORTIONS OF SLIT OR SHEARED ALUMINUM IN HELICAL COIL FORM HAVING ROUGH EDGES IN A STRONG ACID BATH TO REMOVE BURRS AND PROTUBERANCES FROM SAID EDGES, THEN ANODIZING THE EDGES AND CONTIGUOUS PORTIONS OF SAID ALUMINUM IN COIL FORM TO BUILD AN OXID COATING THEREON, THE ADJACENT LAYERS OF ALUMINUM IN SAID COIL BEING IN SUBSTANTIAL CONTACT WITH EACH OTHER AT SAID EDGES AND CONTIGUOUS PORTIONS, AND THEREAFTER ANODIZING SAID ALUMINUM IN UNCOILED FORM TO ESTABLISH AN OXIDE COATING OVER THE ENTIRE SUFACE THEREOF.

5. PROCESS FOR THE REMOVAL OF BURRS AND PROTUBERANCES FROM THE EDGES OF SLIT OR SHEARED ALUMINUM SHEET, STRIP AND FOIL, WHICH COMPRISES TREATING THE EDGES AND CONTIGUOUS PORTIONS OF SAID ALUMINUM IN HELICAL COIL FORM IN A STRONG ACID BATH TO RENDER SAID EDGES SUBSTANTIALLY FREE FROM BURRS AND PROTUBERANCES, THE ADJACENT LAYERS OF ALUMINUM IN SAID COIL BEING DIRECTLY IN CONTACT SUBSTANTIALLY OVER THE INTERFACE THEREBETWEEN.

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