US3546088A - Anodizing apparatus - Google Patents

Description

I Dec.; 8; I B R ETAL 3,546,088

ANODIZ'I'NGAPPARATUS 2 lsheets-vsheet 1 Filed March 14, 1967 im I m) DIRECTION OF MOVEMENT INVENTORS HAROLD J. COATES G. SANFORD JONES ERIK F. BARKMAN TAMATTORNEYS Dec. 8 1970 Filed March 14, 1967 F. BARKMAN ETAL' AINODIZIVNG APPARATUS 2 Sheets-Sheet 2 INVENTORS HAROLD J. COATES G. SANFORD JONES ERIK F. BARKMAN y QM wi TM ATTORNEYS United States Patent Office 3,546,088 ANODIZING APPARATUS Erik F. Barkman, Harold J. Coats, and Garth Sanford Jones, Henrico County, Va., assignors to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Filed Mar. 14, 1967, Ser. No. 623,134 Int. Cl. B23p 1/02; C23b 5/76 US. Cl. 204-224 6 Claims ABSTRACT OF 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 85568 (72 Stat. 435; 42 U.S.C. 2457). Ownership of the invention and this application is retained by Reynolds Metals Company in accordance with the Administrators determination and waiver of title, subject to applicable regulations.

This invention concerns the electrolytic treatment of metal surfaces. More particularly, the invention relates to a novel method for continuously anodizing successive portions of an anodizable metal workpiece and to apparatus for maintaining an enclosed electrolyte-containing compartment adjacent the surface to be anodized and for rapidly passing anodizing electrolyte over a predetermined portion of such surface.

The protection against corrosion of spacecraft components such as propellant tanks for periods during which they are stored or are in process of being installed prior to use is a matter of considerable importance. These components are made of light metals, such as aluminum and aluminum base alloys, and frequently attain substantial size. Welding is commonly employed in their construction. Prior to the present invention, chemical conversion coatings were applied to the workpieces, and particularly to the welded portions, as a means of corrosion protection.

Since the light metals employed are anodizable, it was recognized that anodizing as a means of corrosion protection would be superior to chemical conversion coatings, but the components in question were to large to be treated in an anodizing tank of practical size. Thus there was presented the problem of devising a successful portable anodizing system and of developing a novel anodizing method whereby the system could be brought to the part or portion to be anodized to produce at high speed and in a short time an anodic coating of suflicient quality to meet stringent specifications. Moreover, the system had to be versatile enough to anodize either flat or curved parts in horizontal or vertical position. The electrolyte had to be contained so as not to contaminate surrounding equipment or injure the operating personnel.

3,546,088 Patented Dec. 8, 1970 In the ensuing discussion, aluminum alloy 2219 will be employed for purposes of illustration of the invention, but it will be understood by those skilled in the art that the novel method and apparatus of the invention are applicable to the treatment of anodizable metals generally, as well as to the treatment of other aluminum base alloys, such as, for example, Nos. 1100, 3003, 2014, 5052, and 6061.

It was found, in accordance with the invention, that in order to anodize successive predetermined portions of a workpiece in a dynamic system within as short a time as possible, certain fundamental conditions had to be met, if a hard, corrosion resistant anodic coating were to be obtained. These conditions include the application of high electric power input, while at the same time passing the anodizing electrolyte in contact with the metal surface at a rate sufficient to keep the temperature rise attendant upon the use of such heavy power input below that at which the anodic coating formed would tend to dissolve.

Anodizing is conventionally performed in a static system in a suitable electrolyte, such as sulfuric acid solution, by setting a potential and passing current through the solution at a given ratio of amperes per unit area (current density) until the desired coating thickness is attained. When current is passed through the surface of the aluminum anode in a sulfuric acid solution, the reaction which takes place in the first few seconds is the formation of a barrier film. At a potential difference sufficient to cause current to flow through the barrier film, the oxide layer starts to grow thicker. The current flowing through dis crete points in the barrier film causes point heat build-up. The sulfuric acid, being a dissolving type electrolyte, attacks the oxide coating in these regions of greatest heat build-up. Once anodizing has begun, a rather complicated equilibrium is achieved, involving current density, voltage, barrier film thickness, electrolyte temperature, concentration, and flow rate. If any of these anodizing conditions or parameters are changed, the others may be affected. Thus, raising the electrolyte temperature increases the solubility of the oxide in the electrolyte, lowers the barrier film thickness, lowers the voltage (if the power source is constant current), or raises the current (if the power source is constant voltage). The equilibrium is also affected by differences in the electrolyte flow rate.

causing localized temperature differences which result in variations in the thickness of the barrier film. Localized differences in barrier film thickness may result in a number of coating defects, which include burning and associated nonuniformities, burning being defined as the total disintegration of the oxide and dissolution of the metal at a particular point on the surface.

It was found, in accordance with the invention, that successive portions of an anodizable metal workpiece could be anodized with the production of a hard coating by passing the anodizing electrolyte in contact with a limited portion of the metal surface, at a flow rate of at least 50 feet per minute, preferably between about and 500 feet per minute. Adjustment of the electrolyte flow rate in this manner controls the temperature at the workpiece-electrolyte interface by removing heat from the interface at a rate sufiicient to keep the temperature below that at which excessive dissolution of the anodic coating occurs. This novel technique achieves two unexpected results: (a) it permits the use of a high electric power input, and (b) surprisingly it results in the deposition of a hard anodic coating employing an electrolyte and conditions which in a conventional static anodizing system would provide only a soft anodic coating.

The electric power input is selected to provide the desired film thickness in a given period of time during which the electrolyte is in contact with the surface area being treated. The flow rate of the electrolyte in contact with the area being treated is controlled to prevent dissolution of the formed anodic film and to effect heat removal. The flow rate is higher at higher electric power input and higher electrolyte temperatures.

In accordance with the method of the invention, hard uniform anodic coatings are formed on predetermined areas of anodizable metals by flushing the electrolyte across the surface being anodized. An anodizing head suitable for this purpose is described below.

Under controlled conditions in accordance with the invention, uniform anodic coatings can be successfully produced by employing a sulfuric acid electrolyte at a high flow rate, and high power input. The concentration range of the sulfuric acid is advantageously at least about 17%, preferably 30-40% H 80 The temperature range of the electrolyte is between about and 40 C. The density of the anodic coating increases with increasing current density. The current density may range from about 15 to about 2000 amperes per square foot, and higher. This anodizing rate is about 100 times that which is used in ordinary immersion anodizing operations. It permits, for example, the production of 0.5 mil thickness anodic coatings at a rate of 15 inches per minute. The electrolyte flow rate across the treated surface is at least about 50 feet per minute, preferably between 150 and about 500 feet per minute. The applied voltage range is from about 14 to 50 volts D.C., depending upon the temperature and concentration of the electrolyte.

It has been found further that the workpiece being anodized exhibits superior corrosion resistance and a more uniform response to anodizing when the workpiece is pretreated prior to anodizing. Suitable pretreat-ments for this purpose include electropolishing, or etching and desmutting. The electropolishing step may be performed by employing any of the usual reagents for this purpose, but it is preferred to use a 50%60% sulfuric acid solution. This not only permits controlled metal removal, but eliminates a rinse step between the pretreatment and anodizing steps. For this purpose, the flow of treating solution need not be rapid, and can be under about feet per minute. The time of exposure should be brief, e.g. from 10 to 45 seconds, and the temperature of the pretreatment is about to 50 C. The pretreatment is performed electrolytically, utilizing the apparatus of the invention, at current densities of 2 to 600 amperes per square foot.

If desired, the electropolishing step may be supplemented by a conventional caustic etch for about 5 minutes at 30 F., employing any conventional NaOH or Na CO etchant at a concentration of about 30 to grns. per liter. Any smutting which results from the etching step is then removed electrolytically using 26% sulfuric acid solution at 15 volts. The electropolishing with %60% sulfuric acid may be used as an alternative to the caustic etching step. In such case the workpiece may be anodized immediately after the electropolish. A similar desmutting step may be employed after anodizing to minimize staining.

Following the anodizing step, the article is rinsed with tap water nad the anodic coating is sealed. Conventional sealing techniques employing hot water or steam are suitable, but in certain instances cannot be employed as a practical matter due to the character of the workpiece. However, in accordance with another aspect of the in vention, sealing is successfully accomplished utilizing the portable spray head of the invention, to effect a chemical conversion treatment at room temperature.

For chemical conversion sealing, there may be advantageously employed a solution having the following composition:

Gpl. Phosphoric acid H PO 3.18 Aluminum fluoride 5.0 Potassium dichromate 10.0 Hydrochloric acid 4.6

The foregoing solution is applied for approximately 4 minutes at a temperature of about F.

There may also be employed for sealing purposes various phosphate-chromate solutions available commercially under the designations Alodine 1200, Alodine 12005 and Alodine 600, at concentrations of 15 grams per liter for 2 minutes at about 74 F. The spray head of the invention may be used for sealing purposes.

In accordance with another aspect of the invention, there is provided a portable anodizing device for performing the anodization of localized areas as described above. The main elements of this device are means for maintaining an enclosed electrolyte-containing compartment adjacent the workpiece to be anodized, electrode means for applying the anodizing current, and means for rapidly moving the electrolyte through said compartment relative to the workpiece. The apparatus system also includes a reservoir for the electrolyte, and means providing a vacuum shroud to avoid leaking of electrolyte.

The practice of the method of the invention, and the operation of the anodizing apparatus wil be more readily understood by reference to the accompanying drawings, showing a presently preferred embodiment.

In the drawings:

FIG. 1 is a perspective view of the anodizing head, shown semi-schematically associated with certain auxiliary equipment;

FIG. 2 is a cross-sectional elevation of the anodizing head in FIG. 1;

FIG. 3 is a bottom plan view of the anodizing head; and

FIG. 4 is a transverse section along the line 4-4 in FIG. 3, perpendicular to the section of FIG. 2.

Referring to FIG. 1 of the drawings, the system includes a portabel anodizing head 10 adapted to be moved arcoss the surface of a workpiece. The anodizing head is connected with an electrolyte reservoir 12, and the electrolyte is circulated through the anodizing head 10 by means of the pump 13 in inlet conduit 15 and the return pump 14 in conduit 16. In addition, a vacuum line 18 is connected between the anodizing head and the interior of reservoir 12. A vacuum pump 20 is provided to maintain a reduced pressure on line 18 and in the space overlying the electrolyte in the reservoir. A suitable mist filter 19 is disposed between the vacuum pump 20 and vacuum line 18, as shown, in order to remove any electrolyte carried back through line 18.

Electrolyte enters the anodizing head 10 through its inlet 22 and, as may be seen in FIG. 2, it then passes through an internal passageway system including manifold 23 and from there into the compartment 24. A partition 26 is provided in compartment 24 to direct the electrolyte around cathode support structure 28, as indicated by arrows showing the path of flow, thence outwardly through manifold 30 to an outlet 32 for return to the reservoir through conduit 16.

The cathode system includes spaced contact plates 34 and 36 which are made of lead backed with copper. Electrical connections to these plates are made through terminal posts 38 extending outwardly of the anodizing head (see FIG. 4). By dividing the cathode in this manner, provision is made for independently controlling the current density on each of plates 34 and 36. Ordinarily the current density can be the same on both plates. When an increased anodizing rate is desired, however, it is advantageous to be able to apply an increased current density on plate 34 after initiating formation of an oxide layer on the workpiece at a lower density.

An anodizing chamber 39 is formed along the bottom of compartment 24, between the cathode plates and the workpiece 40 as shown in FIG. 2, and it can be seen that an inlet throat for the electrolyte is provided (to the left of plate 34) which is narrower than the outlet throat to the right of plate 36. Also, the end walls 42 and corresponding cathode support sections 44 have respective curved portions which cooperate to define such inlet and outlet throats. This arrangement causes rapid and uniform flow of electrolyte across the adjacent surface of the workpiece.

A gasket arrangement is provided along the periphery of chamber 39 to retain the electrolyte and minimize leakage. The contact members 46 which are disposed outwardly of the end walls 42 (and the associated side walls 43 shown in FIG. 4) are made of Armalon felt, and a moderate contact pressure is maintained by resilient foamed PVC elements 48.

The vacuum recovery system includes passageways 50 communicating with vacuum outlets 52 atop the anodizinz head, which in turn are connected with the reservoir 12 through vacuum line 18 previously mentioned. In this manner any leakage of electrolyte past the gasket members 46 is withdrawn through the vacuum system, and control is maintained over the area of contact between the electrolyte and the adjacent surface of the workpiece.

As shown best in FIGS. 3 and 4, the anodizing head is mounted on wheels 54 which are made of acid-resistant material such as a phenolic-fabric composite. The axles 56 on which the wheels are mounted was made of Delrin AF, a Teflon filled material which is suitable acidresistant. The wheel treads 58 are Hypalon rings. The anodizing head itself was machined from Plexiglas acrylic plastic in separate sections convenient for assembly.

Operation of the anodizing head has been found to correspond to the empirical formula:

Thickness of coating wherein speed is in inches per minute and width is in inches. Thus, in the operation of the apparatus, the anodizing head is put in place, the vacuum is applied to keep the electrolyte from splashing out, and then the electrolyte is pumped against the workpiece surface, with the current applied. The electrolyte is contained within the anodizing head area by the sliding gasket, which has a low coefficient of friction and is resilient, permitting the head to conform to an irregular shape of the workpiece. Any electrolyte which escapes is picked up by the suction passageways which surround the outer perimeter of the gasket.

The spray anodizing system may be operated on a vertical, horizontal or overhead flat or curvilinear surface. The sulfuric acid electrolyte needed to anodize is pumped continuously through a closed loop system. It is taken from the storage reservoir through hoses, passes through the moving anodizing head, and returned to the reservoir. The sliding gasket which stops most of the acid flow prevents contamination of the surroundings. The vacuum system around the perimeter of the anodizing head picks up any seepage outwardly of the sliding gasket.

The operating conditions, e.g. current density and the speed of the head, may be selected to correspond to a predetermined coating thickness by the use of the above formula, as shown by the following examples:

EXAMPLE 1 The welded joint of a simulated section of a tank, measuring. 6' x 6 x 0.5" and having a curvature radius of about 16.5 feet, was anodized using the apparatus of FIGS. 14, with 30% sulfuric acid at 30 C. The area of contact between the metal surface and electrolyte (defined by the gasket contact members of the anodizing head) was 4" x 4", and the effective anodizing area was about 13 square inches. Electrolyte was pumped to and from the anodizing head at about 7.5 gal./min., corresponding to a flow rate of approximately 300 feet per minute through the anodizing compartment (for a cathode-workpiece spacing of A3"). A hard anodic coating of 0.5 mil thickness was produced. It was also observed that a coating thickness of about 0.65 mil was formed on the weld bead as a result of being closer to the cathode. This was an additional benefit because it provided increased corrosion protection to the area of the weld where it was particularly needed.

EXAMPLE 2 To show the effect of electrolyte fiow rate on the abrasion resistance of 2219 alloy, a series of small flat specimens were anodized to several coating thicknesses. A current density of a.s.f. was used, such as would produce at least a 0.3 mil coating at a linear rate of about 6 inches per minute.

The results are summarized in the following table:

ABRASION RESISTANCE AS AFFECTED BY FLOW RATE AND COATING THICKNESS [Taber Abrasion Tester sed with (33-17 wheels and 1,000 gram load. xloy]2219'[87, 0.050 gauge, anodized in 28% H2804 at 100 a.s.f. and

Weight Weight loss loss after after Flow first second rate, 1, 000 1, 000 f.p.m rev., mg. rev., mg.

While the abrasion resistance characteristics are approximately the same for the 0.3 and 0.5 mil coatings, it is apparent that the 1.0 mil coatings showed a marked increase in abrasion resistance with higher electrolyte flow rates. Thus, if heat is removed rapidly enough from the reaction site, a more abrasion resistant oxide coating is produced. This effect is more pronounced for thicker coatings.

It has also been found that the density of the coating varies with the anodizing current density employed, probably as a result of the greater cell wall thickness brought about by the higher forming voltage needed at increased current density. The effect of current density is apparent from the following:

EXAMPLE 3 Specimens of 2219-T87 alloy, 0.050" thick, measuring 4 x 6", were anodized at various current densities including 15, 50, 100 and 200 amps/sq. ft., in 26% by weight sulfuric acid at 30 C. An electrolyte flow rate of about feet per minute was employed. An additional specimen was anodized at 800 amps./ sq. ft. under otherwise the same conditions except that the flow rate had to be increased to about 330 f.p.m. After anodizing and weighing (coatings were not sealed), the coating thicknesses were measured on Ultrasonoscope and Dermitron instruments. Ten measurements were made on each specimen, including both sides, using each instrument. Two samples measuring about 2 x 4" (5 x 10 cm.)

were cut from each 4" x 6" specimen, and these were stripped and weighed in accordance with MILA8625A 4.4.3.2.

The results are tabulated below:

8 means contacting the surface of said workpiece, said gasket means limiting the area of contact between the surface and the electrolyte; and means for recovering electrolyte which may escape out- Oxide Electrolyte Current Sample Size Thiek- Oxide Apparent flow rate, density, Voltage length, width, noss, weight, density, ttjmin. amps/it. range em cm. mils gm. gmJcc.

Although the foregoing discussion has dealt with operwardly of said gasket means, lIlCiUdll'lg vacuum pasatiOns which involve moving the anodizing head across a stationary workpiece, the workpiece itself may also be moved or the anodizing head may be kept stationary while the workpiece is moved relative thereto. Thus, for example, metal sheets or coilable strips may be passed across the open side of the anodizing head, with the workpiece below the anodizing head or with the anodizing head inverted (open side facing upwardly) and the workpiece above; and, similarly, the anodizing head may be turned on its side so as to accommodate the treatment of a metal surface moving in a vertical plane.

In anodizing light-gage aluminum sheet, such as .030" thickness or less, the use of a very high current density in accordance with the invention, may require supplemental cooling applied to the reverse side as, for example, by flowing water or other suitable coolant across the metal sheet.

For purposes of this application, electrolyte flow rate is used in the sense of the volume of electrolyte 4 per unit time through the cross-sectional area of the electrolyte chamber adjacent the workpiece. A typical spacing between the workpiece and the cathode surface is one-quarter inch (which has been found to provide adequate clearance for a V3" weld bead on the surface being anodized, as well as for moderate curvature of such surface).

While the presently preferred embodiments of the invention and the practice thereof have been illustrated and described, it will be apparent that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. An apparatus including an anodizing head having a chamber for anodizing a workpiece comprising, in combination:

a source of electrolyte;

cathode means in said chamber to apply electric current to an adjacent workpiece;

an anodizing head adapted for relative movement over the workpiece, said head having a chamber adapted to receive electrolyte from the source, said chamber having inlet and outlet means arranged in a manner to permit the continuous and substantially parallel flow of electrolyte through the chamber between the cathode and the adjacent workpiece;

means connecting said anodizing head and said source,

including an inlet line to pass electrolyte into the chamber from said source and an outlet line to return electrolyte from said chamber to the source; pumping means for circulating electrolyte through said inlet line, the chamber, and said outlet line; gasket means for restricting the seepage of electrolyte from said chamber when the anodizing head is placed adjacent a workpiece to be anodized, with said gasket sageways in said anidizing head along the periphery of said chamber to collect any escaped electrolyte and return it to the source.

2. An anodizing apparatus according to claim 1, wherein the source is a reservoir, and further including a vacuum conduit connecting the reservoir to said passageways in the anodizing head, the outlet end of said conduit communicating with a space above the electrolyte in said reservoir;

a vacuum pump to maintain reduced pressure in said space, and in said vacuum conduit and passageways; and mist filter interposed between said pump and the conduit outlet.

3. Apparatus for electrolytically treating successive portions oil a metal workpiece, comprising:

means defining a chamber for receiving electrolyte having inlet and outlet means arranged to direct electrolyte through said chamber in contact with and substantially parallel to an adjacent surface of the workpiece;

a cathode comprising a first and a second contact plate in said chamber to apply electric current to said workpiece through the electrolyte, wherein said first contact plate has a larger surface area than said second contact plate; and

means for directing electerolyte through said chamber past said first contact plate prior to said second contact plate.

4. Apparatus for the electrolytic treatment of an anodizable metal surface, comprising:

an anodizing head providing a chamber for electrolyte, said chamber being adapted to maintain electrolyte therein in contact with the adjacent surface of a metal workpiece, wherein the anodizing head includes an inlet throat for entry of electrolyte into the chamber and an outlet throat which is larger than said inlet throat;

gasket'means peripherally enclosing said chamber and adapted in cooperation with the workpiece to restrict the area of contact between said surface and the electrolyte;

a cathode mounted in said chamber, and positioned therein to define a space for the passage of electrolyte between said cathode and said surface of} the workpiece; and

means to provide a flow of electrolytes through said chamber in a direction substantially parallel to said cathode.

5. An anodizing system according to claim 1, in which said cathode means comprises spaced contact plates within said chamber and separate electrical connections to said plates.

9 6. Apparatus according to claim 5 in which said cathode means comprises a first and a second contact plate wherein said first contact plate is larger in surface area than said second contact plate.

References Cited UNITED STATES PATENTS 10 FOREIGN PATENTS 4/1958 Switzerland 20456 3/1939 Germany 204224 US. Cl. X.R.

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