US3031387A - Anodic oxidation of aluminum - Google Patents

Description

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United States atent Ofihce 3 ,ll3 1,387 Patented Apr. 24, 1962 3,031,387 ANODIC OXIDATION OF ALUMINUM Bruce E. Deal, Palo Alto, Calif., and Larry Swanson,

Coeur dAlene, Idaho, assignors to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware N Drawing. Filed Dec. 7, 1959, Ser. No. 857,562

4 Claims. (Cl. 204-58) This invention relates to oxide coatings on aluminum metal. This application is a continuation-in-part of our co-pending application Serial Number 799,089, filed March 13, 1959, now abandoned.

For many purposes aluminum surfaces are protected by an oxide coating produced on the surface by exposure as an anode in an acid electrolyte capable of yielding oxygen on electrolysis, such as an aqueous solution of sulfuric. acid. The operation of forming such coatings is commonly termed anodizing and the aluminum surface thus protected is commonly termed anodized. Electrolysis in solutions of sulfuric acid with direct or alternating current generally provide colorless to grayish layers of oxide coating on aluminum. The abrasion resistance of these layers is in general not very great which can be demonstrated by the fact that the oxide layer can generally be scraped off with a knife. It has long been desired to produce abrasion resistant oxide layers and to produce colored oxide coated aluminum surfaces. In the past, however, abrasion resistant oxide layers were produced only by anodizing at low temperatures, i.e. from about 0 to 30 F. Such process possesses certain inherent disadvantages particularly with regard to the necessity for expensive refrigeration equipment to maintain the coating bath at temperatures below 30 F.

With regard to producing colors on oxide coated alu minum surfaces, in the past it has been necessary to first anodize the aluminum metal base and thereafter dye the anodic coating with organic dye. Another prior art process for producing colored aluminum surfaces is the ferric oxalate hydrolysis process which deposits ferric oxide or hydrate in the pores of the anodic coating. This process requires first anodizing the aluminum metal base followed by immersion of the anodized aluminum in a solution of ferric ammonium oxalate at controlled concentration, temperature and pH. A third means for producing colored aluminum surfaces involves the double precipitation of an insoluble colored inorganic compound. Such processes possess certain inherent disadvantages particularly with regard to the number of processing steps I required to produce a colored surface. For example such prior art processes require the anodized metal to be subjected to at least one additional processing step in order to color the anodic coating. In addition where the color is produced by impregnating the anodic coating with a dye, the colors tend to fade rapidly when exposed to ultraviolet light.

It has been discovered according to the present invention that an abrasion resistant oxide coating on aluminum metal can be produced at temperatures which do not necessitate subjecting the anodizing bath to refrigeration. For example, temperatures in the range of 55 to 85 F. have been found quite satisfactory. Further, it has been discovered that colored oxide coated aluminum metal surfaces characterized by pleasing appearance and superior light fastness can be produced without subjecting the coated surfaces to a separate coloring treatment. The colors are produced during production of the oxide coating in a bath with special reagents but wherein no dyes or pigments are used.

The aluminum metal is subjected to anodic oxidation in an aqueous solution consisting essentially of sulfosalicylic acid and at least one substance selected from the group consisting of metal sulfates and sulfuric acid, balance water. ,The aluminum metal may comprise a single layer or multiple layers of the same or differing aluminum metals bonded together.

in general the electrolytes of this invention consist essentially of an aqueous solution of from about 5 to 50% by weight of sulfosalicylic acid and at least one substance selected from the group consisting of metal sulfates'and sulfuric acid in an amount equivalent to not more than 15% by weight sulfuric acid. Of the recited group it is presently preferred to use sulfuricacid. Even a small amount of metal sulfate or sulfuric acid is beneficial to the electrolyte embodying the principles of this invention, e.g. an amount of metal sulfate or sulfuric acid equivalent to 0.1% by weight of sulfuric acid is beneficial. The preferred ranges are from about 7 to 15 by weight sulfosalicylic acid and an amount of metal sulfate or sulfuric acid equivalent to from about 0.3 to 4% by weight sulfuric acid. It has been found that anodic coatings on samples of various aluminum alloys anodized in electrolytes having compositions within the broad ranges recited above have superior abrasion resistance as demonstrated by the fact that the coatings cannot be easily scraped off with a knife.

A distinct advantage of the present invention is the production of abrasion resistant oxide coatings at bath temperatures such as 55 to F. rather than at freezing temperatures, i.e. 030 F. required by prior art methods wherein expensive refrigeration is needed. Further, depending on the aluminum base alloy and the composition and concentrations of the electrolyte employed, various shades of various colors may be produced. In general the coatings produced are uniform.

For the anodic treatment of the aluminum metal in the bath or electrolyte according to the present inven tion, there can be provided either direct current or, if desired, a combination of direct current and alternating current.

While abrasion resistant coatings equivalent to those produced by prior art processes are produced with the method and electrolyte of this invention at temperatures such as 55 to 85 F., both higher and lowertemperatures may be employed. Moreover, it has been found that with lower temperatures even harder coatings are produced.

Depending upon the lustre or brightness of the aluminum metal desired, that is the degree of specular reflectance, the aluminum metal may be subjected to various pretreatments prior to anodizing. For example, where high lustre or brightness is desired the-base metal may be subjected to conventional polishing or brightening treatments, e.g. mechanical, chemical or electrochemical. Where it is desired that the ultimate article have a matte or satin appearance, the base metal can be subjected to a suitable etching treatment. Examples of satisfactory pretreatment and anodizing procedure utilizing the novel electrolyte embodying the principles of this invention are set forth below, it being understood that the conventional water rinsing operations after various steps are not recited.

(1') Clean metal in an inhibited alkaline cleaner. An example of one suitable cleaning solution isone composed of 40 grams per liter sodium carbonate, 20 grams per liter trisodium phosphate, 5 grams per liter sodium metasilicate, balance water. The solution may be maintained at atemperature of about F.

(2) (a) Where high lustre or brightness of. the ultimate composite is desired, treat metal according to a suitable bright dip process such as that described in US. Patent No. 2,719,781.

. (b) Where a matte appearance is desired in the ultimate composite, subject material to a suitable etch treatment, e.g. in a solution of sodium hydroxide plus 2% sodium fluoride maintained at a temperature of 160 F. for a 5-minute immersion period.

(3) Rinse in 50% by volume nitric acid solution.

1 gram per liter cobalt acetate, 5 grams per liter boric acid, 5 grams per liter desugared calcium lignosulfonate, balance water, and (2) 5 grams per liter nickel acetate, 1 gram per liter cobalt acetate, 8 grams per liter boric with the solution being maintained at a temperature in the range of from about 70 to 170 F. Where color is emphasized but the aluminum does not contain a significant amount of copper, and sealing is desired, the presently preferred practice is. by immersion for a period of from about 10 to 25. minutes in a solution of water and a small amount of a sealing reagent and wherein the solution has a pH of from about 5.5 to 6.0 and is maintained at a temperature of from about 190 to 212 F. Examples of suitable solutions are: (l) 5 grams per liter nickel acetate,

(4) Anodize for 1 to 150 minutes in an electrolyte con- 5 acid, 1 gram per liter condensation product of naphsisting essentially of an aqueous solution of from 7 to thalene sulfonic acid and formaldehyde. by weight sulfosalicyclic acid and at least'one sub- In order to establish the superiority of the abrasion restance selected from the group consisting of metal sulsistance of anodic oxide coatings produced in accordance fates and sulfuric acid in an amount equivalent to from with this invention and the composite article coated thereabout 0.3 to 4% by weight of sulfuric acid at tempera- 10 with over those produced by conventional prior art anodiztures of from about 55 to 85 F. and at a current density ing methods and electrolytes, tests described hereinbelow of from about 10m 100 a.s.f. (amperes per square foot) have been conducted. f' and voltages from about to '120 volts. It is presently In these tests samples were fabricated from sheets of preferred practice to anodize with an initial current density various aluminum alloys (1100, 5052, 606 l'-T6 and 7075- of from about 20 to a.s.f. and use voltages from about 15 T6) cut into 4" x 4" squares providing a .total surface 25 to 70 volts. Anodizing times greater than 150 minutes area of 32 square inches per sample. The alloys cm.- can be used in certain instances. However, the length of ployed were commercial aluminum alloys with the ranges time isto some extent controlled by the thickness of the of constituents fixed by the Aluminum Association as indialuminum article being treated since aluminum dissolves cated in Table I below:

TABLE I Other Elements Alloy Al Cu Fe S1 Mn Mg Zn Cr Ti Each Total 99.99 min 00.00 min Remainder 5.0 -5.0 0.7 0.40 0.30 0. 05 0.15 .-...do 3.0 -5.0 1.0 0. 50-12 040-12 0.20-0.21 0.25 0.10 1.15 0. 05 0.15 as -1.9 0.50 0.50 0.3 .9 1.2-1.3 0.25 0.10 0.05 0.15 0.20 0.70 0.60 1.0-1.5 0.10 0.05 0.15 0.30 0.80 4.5 6.0 0. 05 0. 05 0.10 20 0. 05 0.15 0. 20 0.7 0. 0. 20 0. -11 0.25 0.10 0. 05 0.15 0.10 ('11 ta1).45 0.10 2.2 -2.8 0.10 0.15-0.35 0. 05 015 EX. 0.10 0.50 0. 40 0. 2150.7 3.5 -4.5 0. 25 0. 05-0. 25 0.15 0.05 0.15 0.07 0.17 0.12 0. 15-015 as -1.2 0.05 o. 15 0.15-0.40 0. 0.4 -0.8 0.15 0.8 -1.2 0.25 0.15-0.35 0.15 0. 05 0.15 0.10 0. 35 0. 20-05 0.10 0. 45-00 0.10 0.10 0.10 0. 05 0.15 1.2 -2.0 0.70 0.50 0.30 2.1 -2.9 5.1 -5.1 0.18-0.40 0.20 0. 05 0.15

during anodizing. Where abrasion resistance is the pri- 45 Prior to anodizing, the samples were subjected to the mary consideration, it is presently preferred practice to following pretreatment: .use an electrolyte temperature in the range of from about (1) Cleaned in an inhibited alkaline cleaner to water 60 to F. Where color is the primary consideration, break free surface. it is presently preferred practice to use an electrolyte (2) Rinsed in cold water. temperature in the range of from about 70 to F. 50 Bright dipped according to the teachings of US. and an anodizing time of from about 10 to 60 minutes. Patent N 2,719,781 ng a temperature of 200 F.

(5) If desired, the oxide coatings may be sealed by lns d 50% by volume nitric acid. various treatments. For example, the anodized metal can 111 C ld W t r. be immersed in hot water maintained at a temperature of The samples were anodiz d for a period of 60 min- .f t 212 R d having a H f fro 5 t 6 55 utes 1n the electrolytes as. set forth in Table 11 below, for a period of 10 to 30 minutes. Where abrasion re- $11611 electrolytes btfing f gf according to the P sistance is theprimary consideration and sealing desired, cfintage 0f sulfoallcyshc a and Sulfuric acid the aforementioned treatment is presently preferred. An- Tic Comallled imbalance. of ach 6160- other sealing procedure, and one presently preferred where troll/t6 13 Waief- All P e a are y Weight of color is the primary consideration and the aluminum con- 0 P 10ml Y The electrolyte? e Contained tains a significant amount of copper, involves immersion twenty bier rectangular y J flqP Pp with for from about 0.5 .to 10 minutes in an aqueous solution surfing d f lead cathOdBS- A e P of from about 0.1 to 5% of common soaps or their compp t y full Wave 86161110111 fiers 1n ponents, e.g. mixtures of the sodium salts of fatty acids Serlessuch as lauric, myristic, oleic, palmitic and stearic acids 65 All the Samples were anodllfid With dlrect current at 72 F. The current densities employed were 27 a.s.f. forthe samples anodized in the electro ytes of this invention and 12 a.s.f. for the samples anodized in the sulfuric acid electrolyte. Current densities appreciably higher than 12 a.s.f. generally cannot be employed commercially for anodizing in straight sulfuric acid electroly-tes since the resulting coatings are unsatisfactory. It is presently preferred practice to maintain the current density constant during the anodizing operation. In general the voltages necessary to maintain a given curof 27 a.s.f. ranged from about 25 to 60 volts while the 5 voltages required to maintain a constant current density of 12 a.s.'f. ranged'trorn about 12 to 16 volts.

The samples anodized in'the sulfosalicylic acid-sulfuric acid anodizing baths were tested for abrasion resistance with abrasive jet apparatus of the type recommended by the ASTM for testing abrasion resistance. This is described by Roberts A.. G., Crouse W. A. and Pizer R. 'S., Abrasive Jet Method for Measuring Abrasion-Resistance of Organic Coatings. ASTM Bulletin No. 208, September 1955. With this apparatus abrasion resistance is measured by the time required to penetrate the coating and the results are given in Table II. In general for purposes of comparative data, several spots on each surface to be tested are abraded with the apparatus, the

It is readily seen from the results indicated above that the anodic oxide coatings produced on the aluminum base metal employing the electrolytes of this invention are superior in abrasion resistance to the oxide coatings produced with conventional sulfuric acid electrolytes. The above samples were characterized by attractive, light stable colors. However, for optimum results with regard to the production of colored surfaces on the aluminum alloys it is preferred to use an electrolyte with a sulfuric acid content of less than about 1%, e.g. .1

Further examples of the excellent abrasion resistance of the oxide coatings produced by practice of this invention involve tests conducted on alloys 1100, 5052, 6061 and 7075 having the following chemical compositions:

. v 1 same number of spots be1ng abradedon each sample. Alloy A1 Cu Fe Si Mn Mg Zn or Ti Ni The comparative abras1on resistance 1s then measured y {the m reqmred penetrate an F Spots of 1100-- Bal. 0.14 0.56 0.11 0.01 0.00 0.02 0.01 0.01 0.005 the sample. In Table II below, hardness is measured 282%.- gal. g. 1; 8.2% 1.1 1% 8.82 f-gi g. 6.3 1 8. 8 1 8.382 8.. 1 111 tfirms 0f total number of Seconds feqlllfed to 7075.- Bal. 1.70 0.33 0.15 0.10 2.46 5.54 0.25 0.05 0.006 penetrate ten spots on the sample.

TABLE 11 In these tests the sam les used were 4" x 6" x 0.040"

Typical Aluminum Alloys Anodzzed m Electrolytes of P This Invention and in Prior Art Sulfuric Acid Electrolyte in size. Prior to anodizing each sample was cleaned by immersion for about 5 minutes in an inhibited alkaline cleaner, rinsed in cold water, etched 5 minutes at 160 F. in a 5% sodium hydroxide solution, rinsed in cold water, de-smutted by immersion in a 50% by volume 10%sul1'o- 10%su1r6- 15%sulfonitric acid solution at room temperature for 2 minutes .Alloy salicylic salicylic salicylic 15'7 3 acid, 1% acid, 2% acid, 5% sulfugic and rinsed 1n cold water. The samples were anodlzed 1n 511111 1 10 suliuic 5 tfrgict 1 a cid t varlous electrolyte composltlons and under various time, gi fi ib 55,2 e 90 m y e current density, voltage and temperature conditions with resulting abrasion resistances as set forth in table III. 33.0 19.0 27.9 11.5 The electrolytes were contained in SO-gallon, rubber-lined gig 1 391% ii; I tanks and were circulated by means of mechanical stir- 31.0 21.5 7.0 ring or air agitation. Power was supplied by a motor generator and perforated lead cathodes were used.

TABLE III Tempera- Initial Final Time to Maxi- Total Alloy Electroture of current current maximum mum anodizing Abrasion lyte electrodensity, density, volts, volts time, resistance yte, F. amps/ft. ampsJft. minutes minutes as 24 24 31 65 65 22. 4 68 24 9 23. 5 65 120 32. 1 68 24 10. 5 25. 5 65 101 33. 7 e9 24 22. 5 65 69. 5 29. 6 cs 24 19. 5 65 00 24. 6 68 24 24 6O 25.2 68 24 12 j 44 05 120 37.4 Y 68 24 9 54 116.5 31.6 68 24 24 42. 92 37.8 2 6s 43 4s 52 31 27. 9 68 e0 60 60 20 22.8 6s 24 13. 5 e5 72' 24. 9 59 24 24 49 45 22.1 68 24 24 45 60 24.4 as 24 15 70 65 120 47.4 0s 24 19. 5 93 65 45. 4 77 24 24 50 60 21. s 68 60 60 25 65 30 34. 0 0s 24 24 31. 5 60 25. 5 0s 24 24 46 60 23.6 68 24 24 64.5 48.5 cs 24 24 47 I 120 53.5

1 10% by weight sulfosalicylic acid, 0 2 10% by weight sulfosalicylic acid, 1

by Weight sulfuric acid, balance water. by weight sulfuric acid, balance Water.

4 10% by weitgh sulfosalicylic acid,

by weight sulfuric acid, balance water.

Specific examples illustrating colors obtained by anodizing various alloys, compositions of which are given in Table I, for thirty minutes in an electrolyte embodying the principles of this invention consisting of sulfosalicylic acid, 0.5% sulfuric acid, balance water at 72 F. are given in Table IV. In anodizing the samples direct current was employed and the current densities were maintained at 27 a.s.f. The voltages required to maintain the current densities at 27 a.s.f. varied as anodizing progressed ranging from about 25 to 60 volts.

TABLE IV Anodizing Characteristics and Colors Produced on Samples Anodized in a 10% Sulfosalicylic Acid 0.5 Sulfuric Acid Electrolyte As can be seen from the above table, alloys such as 2011, 2014, 2024 and 7075 containing material amounts of copper; e.g. 1 to 6% produce a blue color when anodized under the above conditions.

The anodized samples of Table IV were sealed for two minutes in .a 1 2% aqueous soap solution maintained at a temperature of 120 F.

The colors'produced on the samples of Table IV were attractive and light stable. Tests made on samples of 5052, 3003 and 2024-T3 aluminum alloys anodized as above described have withstood 24,000 hours of ultraviolet exposure with no appreciable fading as determined by visual inspection and colormetric measurements. Similar exposure of 606l-T6- alloy for 8,000 hours has given the same results.

Further examples illustrating colors obtained by pracnice of this invention are set forth in Table V below which includes alloy designation, color, and operational conditions, i.e. initial current density, average time to maximum volts, maximum volts and total anodizing time. The samples used were in the form of a sheet or extrusions, the sheets ranging in size from about one foot square to four feet by eight feet. The extrusions used involved various typical cross-sectional shapes in lengths up to about 10 feet.

TABLE V Initial Time Total current to max- Maxianodiz- Alloy Color density, irnurn mum ing amp/ volts, volts time,

ft. minutes minutes 50((l511fll) with 5005 Amber gray 24 20 50 30 s as Do Charcoal brown. 24 60 45 1100 clad with 1100 Tan 24 20 50 30 (sheet) Do Olive 24 30 60 45 2024T3 (sheet Light blue l2 15 65 45 and extrusion) 3003 clad with 3003 Dove gray 24 10 50 710 (sheet) Do Charcoal gray.-- 24 20 65 Black" 24 25 65 40 D0 Gray 24 15 50 20 6052 (shee Light bron 24 15 4O 30 D0.-." Golden brown 24 35 60 5357 (sheet)- Light brown 24 20 30 Do Brown 24 30 45 6061-16 (sheet Antique bronze. 24 10 50 20 and extrusion) Do Jet black 24 30 40 6063-15 (extrusion) Amber 24 20 50 30 Do Light brown. 24 35 60 45 The examples shown in Table V were produced by using the following procedure:

(1) Cleaned by immersion for five minutes with agitation in a solution composed 0f'40 grams per liter sodium carbonate, 20 grams per liter t'risodium'phosphate, 5 grams per liter sodium metasilicate,balance water and maintained at approximately 160 F. 1

(2) Rinsed in cold Water.

(3) Etched by immersion in a 5% sodium hydroxide (caustic soda) solution for five minutes at 160 F.

(4) Rinsed in cold water.

(5) De-smutted in a 50% by volume nitric acid solution at room temperature for two minutes.

(6) Rinsed in cold water.

(7) Anodized in electrolye or bath consisting of 107 by weight sulfosalicylic acid, 0.5% by weight sulfuric acid, balance Water. The anodizing bath was maintained at a temperature of 77 F. In each example (with the exception of alloy 2024-T3), anodizing began'with a current density of 24 amperes (D.C.) per square foot of work surface and the anodizing was continued at. this current density until the cell voltage reached the value indicated in Table V under Maximum volts for the particular alloy and color involved. The Work was held at the maximum voltage for the remainder of the an odizing time set forth in the subject table. Where color is the primary consideration, it is presently preferred practice to follow the aforesaid procedure, i.e. start with and maintain a selected current density value until such time as the voltage reaches a selected maximum value andthen conduct theremaining anodizing period at such voltage. 1

(8) Rinsed in cold water. I p

(9) Sealed in the following solution at the indicated conditions:

grams/liter Nickel acetate 5 Cobalt acetate 1 Boric acid- 5 Desugared calcium lignosulfonate 5 Balance Water.

11 Dry.

The colored aluminum product of the present invention is characterized by a dense anodic coating. For example, the products set forth in Table V generally are found to have an oxide coating Weighing not less than about 20 milligrams per square inch per side of surface and the oxide coating generally is at least .0005 inch thick.

As used herein, the term aluminum is meant to cover high purity aluminum, commercial purity aluminum and aluminum alloys.

It will .be understood that various changes, omissions and additions may be made to this invention without departing from the spirit and scope thereof as set forth in the appended claims. All percentages in the electrolyte.

What is claimed-is:

1. The method offorming colored coatings on aluminum electrolytically comprising the'steps of subjecting said aluminum to anodic oxidation for a period of time of from 1 to 150 minutes at an initial current density of from about 10 to a.s. f. and a voltage of from about 20 to volts in an aqueous solution consisting essentially of from 5 to 50% sulfosalicylic acid, at least one substance selected from the group consisting of metal claims are by weight of the total sulfates and sulfuric acid in an amount equivalent to from about 0.1 to 4% sulfuric acid and the balance water, said solution being maintained at a temperature of from about 55 to 85 F., maintaining said initial current density approximately constant until a selected maximum voltage is reached at which voltage a coating of a desired color is obtained, and thereafter maintaining said selected maximum voltage approximately constant until said anodic oxidation forms a coating of a desired thickness.

2. An anodized aluminum article prepared by the method of claim 1.

3. The method of forming colored coatings on aluminum electrolytically comprising the steps of subjecting said aluminum to anodic oxidation for a period of time of from to 60 minutes at an initial current density of from about to a.s.f. and a voltage from about 25 to about volts in an aqueous solution consisting essentially of from 7 to 15% sulfosalicyclic acid, from 0.1

to about 1% sulfuric acid and the balance water, said solution being maintained at a temperature of from about 70 to F., maintaining said initial current density approximately constant until a selected maximum voltage is reached at which voltage a coating of a desired color is obtained and thereafter maintaining said selected maximum voltage approximately constant until said anodic oxidation forms a coating of a desired thickness.

4. The method of claim 3 wherein said aqueous solution consists essentially of 10% sulfosalicyclic acid,"0.5% sulfuric acid and the balance water.

References Cited in the file of this patent UNITED STATES PATENTS 2,233,785 Korpiun Mar. 4, 1941 2,260,278 Schenk Oct. 21, 1941 FOREIGN PATENTS 657,902 Germany Mar. 16, 1938 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,031,387 April 24 1962 Bruce E. Deal et alo 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.

Columns 3 and 4 TABLE I tenth column line 4 thereof for "1.15" read 0.,15 columns 5 and 6 TABLE III, third columm for "69" read 68 same TABLE III, eighth line 4 thereof, column line 1 thereof for "65" read 6O same TABLE III footnote 4 thereof, for "weitgh" read weight Signed and sealed this 18th day of December 1962.

SEAL) Attest:

DAVID L. LADD Commissioner of Patents IRNEST W. SWIDEH Kttesting Officer

Claims (2)

1. THE METHOD OF FORMING COLORED COATINGS ON ALUMINUM ELECTROLYTICALLY COMPRISING THE STEPS OF SUBJECTING SAID ALUMINUM TO ANODIC OXIDATION FOR A PERIOD OF TIME OF FROM 1 TO 150 MINUTES AT AN INITIAL CURRENT DENSITY OF FROM ABOUT 10 TO 100 A.S.F. AND A VOLTAGE OF FROM ABOUT 20 TO 120 VOLTS IN AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF FROM 5 TO 50% SULFOSALICYLIC ACID, AT LEAST ONE SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF METAL SULFATES AND SULFURIC ACID IN AN AMOUNT EQUIVALENT TO FROM ABOUT 0.1 TO 4% SULFURIC ACID AND THE BALANCE WATER, SAID SOLUTION BEING MAINTAINED AT A TEMPERATURE OF FROM ABOUT 55* TO 85*F., MAINTAINING SAID INITIAL CURRENT DENSITY APPROXIMATELY CONSTANT UNTIL A SELECTED MAXMUM VOLTAGE IS REACHED AT WHICH VOLTAGE A COATING OF A DESIRED COLOR IS OBTAINED, AND THEREAFTER MAINTAINING SAID SELECTED MAXIMUM VOLTAGE APPROXIMATELY CONSTANT UNTIL SAID ANODIC OXIDATION FORMS A COATING OF A DESIRED THICKNESS.

2. AN ANODIZED ALUMINUM ARTICLE PREPARED BY THE METHOD OF CLAIM 1.