US3625841A

US3625841A - Color anodizing in an inorganic electrolyte

Info

Publication number: US3625841A
Application number: US805825A
Authority: US
Inventors: Bernard Ray Baker
Original assignee: Kaiser Aluminum and Chemical Corp
Current assignee: Kaiser Aluminum and Chemical Corp
Priority date: 1969-03-10
Filing date: 1969-03-10
Publication date: 1971-12-07
Anticipated expiration: 1988-12-07
Also published as: DE2010678A1; FR2034789A1; AU1231870A

Abstract

A process for the color anodizing of aluminum comprising subjecting the aluminum, as an anode to electrolysis in an aqueous electrolyte containing sulfuric acid and dichromate ions.

Description

United States Patent [72] Inventor Bernard Ray Baker Spokane, Wash. [21] Appl. No. 805,825 [22] Filed Mar. 10, 1969 [45] Patented Dec. 7, 1971 [73] Assignee Kaiser Aluminum & Chemical Corporation Oakland, Calif.

[54] COLOR ANODIZING IN AN INORGANIC ELECTROLYTE 16 Claims, No Drawings [52] US. Cl. 204/58 [5 1] Int. Cl C23b 9/02 [50] Field of Search 204/58, 35

[5 6] References Cited UNITED STATES PATENTS I 3,180,806 4/l965 Hollingsworth 204/29 3,023,l49 2/1962 Zeman 204/12 2,855,352 10/1958 Ernst 204/58 Surface Treatment of Aluminum, by Wernick & Pinner, 3rd edition, 1964, gs. 356- 357, 385, 387

Surface Treatment of Aluminum by Wernick et al., 3rd ed., 1964, pages 353- 355 Primary Examiner-John H. Mack Assistant Examiner-R. L. Andrews Attorneys-James E. Toomey, Paul E. Calrow, Harold L.

Jenkins and Frank M. Hansen ABSTRACT: A process for the color anodizing of aluminum comprising subjecting the aluminum, as an anode to electrolysis in an aqueous electrolyte containing sulfuric acid and dichromate ions.

COLOR ANODlZlNG IN AN INORGANIC ELECTROLYTE BACKGROUND OF THE INVENTION Heretofore many methods have been employed to place a protective oxide coating on aluminum surfaces. One of the more frequently used is the process of anodization wherein the aluminum surface as an anode is subjected to electrolysis in an electrolytic bath which is capable of yielding oxygen on electrolysis. The aluminum surface protected is commonly termed anodized. Most often the baths are acidic. As used herein, the term aluminum includes high purity aluminum, various commercial grades of aluminum and aluminum base alloys.

To produce an abrasion resistant coating it has been necessary in the past to anodize in sulfuric or chromic acid baths at low temperature, such as to 30 F., which entails considerable expense for refrigeration.

Under many circumstances, such as in architectural applications, where aesthetic considerations are quite important, it is desirable to have a colored or hued surface on the aluminum. Several processes have been developed to produce colored oxide coatings; however, only one has met with real commercial success, that being the integral color-anodizing process basically described in US. Pat. No. Re 25,566, assigned to the present assignee. Methods such as dyeing a previously anodized aluminum surface with organic dyes or introducing metallic salts or oxides into a previously prepared porous oxide coating suffer from the inherent disadvantage of requiring additional process coloring steps after the anodization, which increase the cost as well as inconvenience of the process. The dyeing has the additional disadvantage of producing a color which fades when exposed to ultraviolet light and also a color which is difficult to reproduce from batch to batch. The color anodizing process described in US. Pat. No. Re 25,566, involves the anodization of aluminum in an aqueous electrolyte containing sulfosalicylic acid and sulfuric acid. Other sulfonic acids such as sulfophthalic acid may be utilized in place of the sulfosalicylic acid. The color-anodizing process produces not only a wide range of colors which are light-fast but also an abrasion resistant oxide coating without the prior art requirement of'electrolysis at low temperatures. in general, the colors so produced are uniform, reproducible and have a high aesthetic appeal necessary for architectural applications.

Although the integral color-anodizing process described above is an effective method to produce a color-anodized coating on aluminum surfaces, the chemicals used, i.e.' the aromatic sulfonic acids, are comparatively expensive and amount to a large portion of the expenses involved in the color anodizing of aluminum articles.

The purpose of this invention is to provide an effective color-anodizing process for aluminum which utilizes low cost, readily available inorganic chemicals in the electrolytic bath and which produces a wide range of architecturally desirable colored oxide coatings which are highly abrasion and corrosion resistant.

Other purposes and advantages will become apparent from the ensuing discussion.

SUMMARY OF THE INSTANT INVENTION The present invention is directed to a novel electrolytic bath and to the process of color anodizing an aluminum surface in the bath. More particularly, it is directed to the color anodizing aluminum in an aqueous electrolyte containing sulfuric acid and dichromate ions, preferably from an alkali metal dichromate. The process is most advantageously operated by subjecting the aluminum surface as an anode to a substantially constant current density between l5 and 40 amps/ft until a peak voltage across the cell between 42 and 70 reached, then maintaining this peak voltage at a substantially constant level until the desired color density and oxide thickness is obtained.

The electrolytic bath embodied in the present invention is an aqueous solution preferably having a sulfuric acid concentration of from 0.06 to 0.1M and having a dichromate concentration of from 0.10M to 0.40M.

DETAILED DESCRIPTION In accordance with the present invention, it has been found that an aqueous solution of sulfuric acid and dichromate ions can be advantageously utilized as an electrolyte for the integral color anodizing of aluminum alloys. Furthermore, it has been discovered that a full range of light-fast, architecturally desirable colors can be obtained by anodizing aluminum and aluminum alloys in an aqueous electrolyte containing sulfuric acid and dichromate ions.

The dichromate concentration can range as low as 0.] gram-moles/liter up to the saturation point. Any soluble dichromate is operable in the present process if the compound yields dichromate ions in the desired concentration ranges. Potassium and sodium dichromate are preferred, although ammonium dichromate, lithium dichromate and ferric dichromate are fully operable. A dichromate concentration between 0.l0 to 0.40 gram-moles/liter is preferred. The sulfuric acid can vary between 0.05 and 0.30 gram-moles/liter, but it is preferred to maintain the sulfuric acid concentration between 0.06 and 0.10 gram-moles/liter. Furthermore, it has been found that small additions of up to 5 grams/liter of nitric acid to the electrolyte accelerates the color formation, considerably reducing the time necessary for anodizing.

In the process of this invention an aluminum article is immerged as the anode in an aqueous electrolyte containing the sulfuric acid and dichromate compound and subjected to anodization. The anodization can comprise a plurality of electrical programs; however, a two stage anodizing process is easiest to control and probably the least expensive. The two stage process comprises first subjecting the aluminum article to a substantially constant current density between 10 and 70 amp/ft until a peak voltage between 42 and 120 volts is reached, and subsequently maintaining the voltage at substantially this peak level until the desired color density and coating thickness is obtained. it is recognized that other electrical programs, such as maintaining the current density between l0 and 70 amp/ft at a substantially constant level until the desired color density and coating thickness is obtained, can also be employed to give substantially equivalent results.

in the two stage process it is preferred to maintain the current density in the first stage at a substantially constant level between l5 and 40 amp/ft and the voltage in the second stage at a substantially constant level between 42 and 70 volts. in the single stage process the current density should be maintained between the same limits, Le. 15 and 40 amp/ft. The bath temperature can range from 0 to 100 F. but it is preferred to maintain the temperature between 60 to F.

In the present invention color generation within the oxide coating is a function of two separate phenomena 'which are presently not completely understood. A slight golden hue is developed at low voltages and probably exists throughout the entire anodic oxide coating. This color formation may be due to a slight extent to the occlusion of chromate within the anodic oxide structure. However, the level of chromate occlusion within the oxide coating, which normally runs between 0.6 and 0.9 percent CrO, does not warrant a conclusion that the occlusion of chromate is even a major cause of this color generation. The primary color generation in the present process results from the formation of a dark band of anodic oxidenext to the substrate metal. The color density of the final anodized product is dependent upon the thickness of this dark band. The color depends upon the color-imparting constituents within the dark band. The dark band, however, does not form unless a threshold voltage greater than 42 volts is reached during anodizing. Thus, in all anodizing programs the voltage across the electrolytic cell must at some time during the process exceed 42 volts, otherwise only the slight golden hue is developed.

Others such as Keeler in US. Pat. No. 1,574, 290 and Frasch in US. Pat. No. 2,338,924 have anodized metals in a mixture of sulfuric acid and dichromate ions, however, in these cases the color generation is primarily or entirely due to the chromium oxide deposition on the metal surface, which is in these cases magnesium. Speer in U.S. Pat. No. 2,437,620, employed an aqueous electrolyte containing sulfuric acid and dichromate ions in the anodic oxidation of aluminum, but he failed to recognize that within the composition ranges of the present invention integral colored oxide coatings canbe obtained.

During the startup of the anodizing program a short period, usually about 3 to 6 minutes, is required to attain the desired current density. This is commonly termed the run-in period. Care must be exercised during this period not to exceed the threshold voltage, because once this voltage is exceeded the dark band forms and this dark band is characterized by a relatively high resistance when compared with the oxide formed at the low voltages. if the dark band is formed during the run-in period a higher voltage is necessary during anodizing to overcome the higher resistance of this layer, thus increasing the cost of the process.

It has been found that with the two stage anodizing process the anodic oxide coating becomes momentarily passive when the constant voltage stage commences. This passivation results in a severe depression of the current density. However, after a short time the current density recovers a considerable portion of its level and thereafter slowly dies out to a much lower level. At a peak voltage of 65 volts, the recovery time is in the range of several minutes, but above 70 volts the recovery time is in the order of several hours.

It has been found that by adding small amounts of sodium sulfate to the electrolyte the current density level after passivation can be increased and the recovery time reduced. Up to 20 grams/liter of sodium sulfate have been found effective. Although sodium sulfate is preferred because of its cost and availability, any sulfate concentration above that produced from the sulfuric acid will produce equivalent results. Thus, up to about 0. l gram-moles/liter of additional sulfate will be effective.

As mentioned above and recognized in the art there are many interacting variables which effect the color of the anodic oxide coatings formed in the various electrolytes. in the present process lighter colors are obtained by increasing the dichromate ion concentration, increasing the sulfuric acid concentration, increasing the sodium sulfate concentration, decreasing the current density, decreasing the voltage and increasing the temperature. Darker colors are obtained by the converse. This is not to say, however, that all of the variables must be varied in the same direction to obtain either the dark or the light colors. For example, in commercial practice the bath composition will remain constant and the colors will be obtained by varying the current density, peak voltages and bath temperatures.

Although the dichromate concentration in the bath must be maintained at a level greater than 0.10 gram-moles/liter, preferably between 0.1 and 0.4 gram-moles/liter, to produce an integrally colored anodic oxide coating, the overall color appears to be most sensitive to changes in the total sulfate and sulfuric acid concentrations. Therefore, in commercial processes the total sulfate and sulfuric acid concentrations will have to be controlled within rather narrow limits to produce anodized products which match from batch to batch. During anodizing the aluminum concentration in the electrolyte increases due to the effect of the acidic electrolyte on the aluminum oxide coating. For efficient anodizing the aluminum concentration should not exceed 13 grams/liter. Moreover, it is recognized that a variable aluminum concentration below 13 grams/liter will produce a variance in the color response to a particularanodizing program, making it difficult to produce integrally colored coatings which match from batch to batch. Thus it is preferred to maintain the aluminum concentration at a constant level below 13 grams/liter within a maximum deviation of 10.5 grams/liter. This control is conveniently accomplished by passing a portion of the electrolyte through a cation exchange resin such as a sulfonated polystyrene. The reaction of the acid electrolyte and the oxide coating reduces the hydrogen ion concentration of the bath and when the acid is added to replenish the hydrogen ion concentration the sulfate concentration increases. This excess sulfate is herein considered as sulfate produced or provided by the sulfuric acid.

In the table, several examples are given to illustrate particular embodiments of this invention. To prepare the samples for anodizing they were first cleaned in an inhibited alkaline solution, caustic etched in a solution containing 50 grams/liter sodium hydroxide, 1.5 grams/liter sodium gluconate and 5 to 25 grams/liter aluminum for 20 minutes at 130 F., rinsed, desmutted in a 35 percent nitric acid solution and finally rinsed. Alloys one through three were then immersed in an electrolytic bath containing grams/liter potassium dichromate, 8 grams/liter sulfuric acid and 7 grams/liter sodium sulfate and subjected to low voltages during a run-in period of about 3 minutes to bring up the current density to the desired level and then subjected to the two stage anodizing programs shown in the table. Alloy four was subjected to the same process except the bath contained 60 grams/liter sodium dichromate (Na,Cr,0-,-2H=O), 7 grams/liter sulfuric acid and l l grams/liter sodium sulfate.

As is shown in the table, a full range of architecturally desirable colors are obtained. These integrally colored anodic oxide coatings are fully equivalent and some instances superior in both abrasion resistance and corrosion resistance as those produced by the process employing an electrolyte containing sulfuric acid and an aromatic sulfonic acid, such as Deal et al. in U.S. Pat. No. Re 25,566. Several samples anodized in accordance with the present invention were subjected to jet abrasion tests to determine the abrasion resistance of the oxide coatings. The results of these tests shown in the table indicate that the oxide coatings have an average abrasion resistance of about 1.5 sec/spot/mil. These results are comparable with the abrasion resistance of coatings produced by the Deal et al. process which generally have an abrasion resistance between 1.0 and 1.7 sec/spot/mil.

The integrally colored oxide coatings produced by the present invention can be sealed according to conventional sealing practice.

As was discussed above, the addition of up to 5 grams/liter nitric acid to the electrolyte tends to accelerate the anodizing process. For example, Alloy four in the table was anodized at l5". C. at a constant current density of 30 amps/sq. ft. for 8.5 minutes during which time the voltage rose from 40 to 62 volts. The voltage was maintained at the 62-volt level for 38.4 minutes to obtain a statuary bronze anodic oxide coating. Another sample of this same alloy, prepared in the same manner, was anodized in an electrolytic bath of the same composition except that L8 grams/liter of nitric acid was added to the electrolyte. The sample was anodized at 30 amps/ft until a peak voltage of 55 volts was reached and this voltage was then maintained at a substantially constant level until the same statuary bronze anodic oxide color was obtained. With the nitricacid addition the statuary bronze was developed in 38 minutes. This is to be compared with the total period of 46.9 minutes without nitric acid a 20 percent reduction in anodizing time.

During the process of the present invention a hexavalent dichromate ion is reduced to the trivalent state, and this requires a periodic replenishment of the dichromate ion to maintain the dichromate ion concentration within the desired limits. Obviously, if the rate of dichromate reduction is known, the dichromate compound may be added continuously to the bath. The inventor has discovered that the dichromate reduction can be substantially eliminated by utilizing a metallic cathode which has been coated with a plastic material which is resistant to the corrosive environments of the electrolytic bath, said coating having a plurality of small (1 millimeter) holes to expose the bare metal substrate and allow current to flow. The perforation diameter can vary from 0.5 to 1.5 millimeters although 1 millimeter is preferred. The perforations may be of any shape, but preferably round and the diameter disclosed above is the maximum diameter. The perforations should be spaced between 4 and 10 millimeters throughout substantially the entire cathode surface. Suitable plastics include, but are not limited to epoxy resins, polytetrafluoroethylene and polyvinyl chloride. Obviously any material which is resistant to the particular electrolyte em- 5. A process for forming integrally colored anodic oxide coatings on aluminum comprising subjecting said aluminum as the anode in an aqueous electrolyte at a temperature between 60 and 90 F. containing in solution at least 0.10 gramployed for anodizing and which has a high resistivity, for ex- 5 moles/liter of a compound selected from the group consisting ample above 10 ohm-centimeter, can be used as a coating Of sodium dichromate not more than 13 gram per liter alumaterial. Apparently, by drilling a plurality of these small mlnum and potassium dichromate and from 0.05 to 0.30 holes the current density through these holes is quite high, and grammoles/lllef Sulfuric acid to a Substantially constant at the high current densities, the diffusion of the hydrogen ion rem density between 10 and 70 P until a P l to the metal substrate of the cathode occurs preferentially to 10 age between 42 and 100 311515 reached and maintaining Said the dichromate difi i and th f hydrogen gas f peak voltage at a substantially constant level until the desired tion occurs preferentially to the dichromate reduction. in one color and Coamlg thlckness are obtainedtest, an epoxy resin, Epibond No. 122 resin produced by the T Precess of 5 wherel" aqueous electrolyte p plastics Corporation was p|aced on an aluminum contains between 0.10 and 0.40 gram-moles/liter of a comcathode base and l millimeter holes were drilled through this 5 Pound Selected f h group conslstmg of Sodium dichroplastic coating to the substrate metal at intervals of 5 mm. mate and pofasslum dlchrqmate and between 006 and 010 After 15 runs in a preferred electrolyte composition containgram'lmles/llter Sulfur": aclding 8 grams/liter sulfuric acid and 90 grams/liter potassium T P s of Claim 6 wherein the current density is di h the epoxy coated h was m] ki maintained atasubstantially constant level between 15 and 40 satisfactorily and the coating was practically intact. As a comamPts/fiz and P321k Voltage mamtained at a Substantially parison, the anodized aluminum cathode, which had similar Constant level hem/661142 and 70 Voltsholes drilled through the anodic oxide coating, ceased produc- T process f l im 7 wherein the aqueous electrolyte ing hydrogen gas after four runs and began reducing the contains asulfate concentration in excess of that produced by dichromate ions to the trivalent state. Moreover, the cathode the Sulfuric acid in amounts p l0 15 g m-m /literstructure discussed above may be utilized in the anodization of 9. The process of claim 7 wherein the aqueous electrolyte magnesium as well as aluminum. contains nitric acid in amounts up to5 grams/liter. It is obvious that various modifications to the present 10. The process of forming an integrally colored anodic process can be made without departing from the spirit of the oxide coating on aluminum comprising subjecting said aluinvention or the scope of the appended claims. minum as the anode in the aqueous electrolyte at a tempera- EXAMPLES Bath Cur- Time temrent Peak to Total Total Oxide Abrasion peradenvoltpeak, time, eurthickresisttnre, sity, age, minminrent, ness, ance, sec./ a.s.i. volts utes utes a.h.s.f. Color mils spot/mil Alloy:

26 20 43. 2 27. 2 27. 2 0. 87 25 24 22.3 23 0.89 25 30 17.5 18 10 do 0.90 20 24 60 18.1 23. 2 10 Light amber. 0.87 1- 20 30 50 11.4 20.4 10 .do 0.88 20 30 60 11. 7 24. 4 10 Amber" 0.89 15 30 57 8.5 32. 4 10 d0 0. s0 15 30 57 7.9 47 12 Stat. bronze 1.08 16 36 56 5. 0 58 14 d0 l. 18 20 24 50 17. 1 23. 0 10 Light ambeL 0.97 20 24 17.9 24.1 10 Amber 0. 90 2 20 30 56 11.0 22.6 10 do 1.03 20 30 59 11.3 82.8 12 Stat. bronze 1.12 15 30 57 7. 2 49. 4 12 do 1.15 3 16 30 65 4.0 48 10 Brown. 0.93 4 15 30 62 8.6 46.9 10 Stet.bronze 1.00

I Alloy composition in weight percent: 0.10 Si, 0.54 Fe, 0.09 Cu, 0.75 Mg, 0.06 Cr, 0.01 Ti, balance Al. 11 Alloy composition in weight percent: 0.34 Si, 0.17 Fe, 0.25 Cu, 0.13 Mn, 0.51 Mg, 0.01 Ti, balance Al.

9 Alloy composition in weight percent: 0.10 Si, 0. balance Al.

What is claimed is: H V

1. A process for forming an integrally colored anodic oxide coating on aluminum comprising anodizing said aluminum as the anode in an aqueous electrolyte at a temperature between and 90 F., said electrolyte consisting essentially of at least 0.1 gram-mole/liter of dichromate ion, from 0.05 to 0.30 gram-mole/liter sulfuric acid, not more than 13 grams/liter aluminum, and the balance water, the voltage during a period of said anodizing exceeding at least 42 volts. 7 V

2. The process of claim 1 wherein the aqueous electrolyte contains in solution between 0.10 and 0.40 gram-moles/liter of a compound selected from the group consisting of sodium dichromate and potassium dichromate and between 0.06 and 0.10 gram-moles/liter sulfuricacid. V g V 3. The process of claim 2 wherein the aqueous electrolyte contains sulfate ions in excess of that produced by the sulfuric acid in amounts up to 0.15 gram-moles/liter. V v

4. The process of claim 2 wherein the electrolyte contains nitric acid in amounts up tofi grams/liter.

7 Fe, 0.06 Cu, 0.48 Mn, 4.00 Mg, 0.18 Cr, 0.05 Zn, 0.02 T1, 9 Alloy composition in weight percent: 0.34 Si, 0.17 Fe 0.25 11, 0.13 Mn, 0.51 Mg, 0.01 Ti, balance Al.

ture between 60 and F., said electrolyte containing in solution at least 0.1 gram-mole/liter of a compound selected from the group consisting of sodium dichromate and potassium dichromate, not more than 13 grams per liter aluminum, and from 0.05 to 0.30 gram-mole/liter sulfuric acid to a substantially constant density between 10 and 70 amperes/ft, the voltage during a period of said anodizing exceeding at least 42 volts, until the desired color and coating thickness are obtained.

l l. The process of claim 10 wherein the aqueous electrolyte contains in solution between 0.10 and 0.40 gram-moles/liter of a compound selected from the group consisting of sodium dichromate and potassium dichromate, and sulfuric acid between 0.06 and O. 10 gram-moles/liter.

12. The process of claim 10 wherein the current density is maintained at a substantially constant level between 15 and 40 amps/ft? 13. The process of claim 12 wherein the aqueous electrolyte contains a sulfate concentration in excess of that produced by the sulfuric acid in amounts up to 0. l5 gram-moles/liter.

balance water not more than 13 grams per liter aluminum.

16. An aqueous electrolyte suitable for the integral color anodizing of aluminum surfaces consisting essentially of from 0.10 and 0.40 gram-moles/liter of a compound selected from the group consisting of sodium dichromate and potassium dichromate, sulfuric acid from 0.06 and 0.10 grammoles/liter, nitric acid in amounts up to 5 grams/liter and the balance water not more than 13 grams per liter aluminum.

Claims

2. The process of claim 1 wherein the aqueous electrolyte contains in solution between 0.10 and 0.40 gram-moles/liter of a compound selected from the group consisting of sodium dichromate and potassium dichromate and between 0.06 and 0.10 gram-moles/liter sulfuric acid.
3. The process of claim 2 wherein the aqueous electrolyte contains sulfate ions in excess of that produced by the sulfuric acid in amounts up to 0.15 gram-moles/liter.
4. The process of claim 2 wherein the electrolyte contains nitric acid in amounts up to 5 grams/liter.
5. A process for forming integrally colored anodic oxide coatings on aluminum comprising subjecting said aluminum as the anode in an aqueous electrolyte at a temperature between 60* and 90* F. containing in solution at least 0.10 gram-moles/liter of a compound selected from the group consisting of sodium dichromate not more than 13 gram per liter aluminum and potassium dichromate and from 0.05 to 0.30 gram-moles/liter sulfuric acid to a substantially constant current density between 10 and 70 amperes/ft2 until a peak voltage between 42 and 100 volts is reached and maintaining said peak voltage at a substantially constant level until the desired color and coating thickness are obtained.
6. The process of claim 5 wherein the aqueous electrolyte contains between 0.10 and 0.40 gram-moles/liter of a compound selected from the group consisting of sodium dichromate and potassium dichromate and between 0.06 and 0.10 gram-moles/liter sulfuric acid.
7. The process of claim 6 wherein the current density is maintained at a substantially constant level between 15 and 40 ampts/ft2 and the peak voltage is maintained at a substantially constant level between 42 and 70 volts.
8. The process of claim 7 wherein the aqueous electrolyte contains a sulfate concentration in excess of that produced by the sulfuric acid in amounts up to 0.15 gram-moles/liter.
9. The process of claim 7 wherein the aqueous electrolyte contains nitric acid in amounts up to 5 grams/liter.
10. The process of forming an integrally colored anodic oxide coating on aluminum comprising subjecting said aluminum as the anode in the aqueous electrolyte at a temperature between 60* and 90* F., said electrolyte containing in solution at least 0.1 gram-mole/liter of a compound selected from the group consisting of sodium dichromate and potassium dichromate, not more than 13 grams per liter aluminum, and from 0.05 to 0.30 gram-mole/liter sulfuric acid to a substantially constant density between 10 and 70 amperes/ft2, the voltage during a period of said anodizing exceeding at least 42 volts, until the desired color and coating thickness are obtained.
11. The process of claim 10 wherein the aqueous electrolyte contains in solution between 0.10 and 0.40 gram-moles/liter of a compound selected from the group consisting of sodium dichromate and potassium dichromate, and sulfuric acid between 0.06 and 0.10 gram-moles/liter.
12. The process of claim 10 wherein the current density is maintained at a substantially constant level between 15 and 40 amps/ft2.
13. The process of claim 12 wherein the aqueous electrolyte contains a sulfate concentration in excess of that produced by the sulfuric acid in amounts up to 0.15 gram-moles/liter.
14. The process of claim 12 wherein the aqueous electrolyte contains nitric acid in amounts up to 5 grams/liter.
15. An aqueous electrolyte suitable for the integral color anodizing of aluminum surfaces consisting essentially of from 0.10 and 0.40 gram-moles/liter of a compound selected from the group consisting of sodium dichromate and potassium dichromate, between 0.05 and 0.30 gram-moles/liter sulfuric acid, a sulfate concentration in excess of that provided by sulfuric acid in amounts up to 0.15 grams-moles/liter and the balance water not more than 13 grams per liter aluminum.
16. An aqueous electrolyte suitable for the integral color anodizing of aluminum surfaces consisting essentially of from 0.10 and 0.40 gram-moles/liter of a compound selected from the group consisting of sodium dichromate and potassium dichromate, sulfuric acid from 0.06 and 0.10 gram-moles/liter, nitric acid in amounts up to 5 grams/liter and the balance water not more than 13 grams per liter aluminum.