US3423298A - Process for coloring aluminum - Google Patents

Description

United States Patent 3,423,298 PROCESS FOR COLORING ALUMINUM Christian E. Michelson, .New Haven, James F. Murphy, Hamden, and David C. Montgomery, Clinton, Conn., assignors to Olin Mathieson Chemical Corporation No Drawing. Filed July 5, 1966, Ser. No. 562,536 US. Cl. 204- 58 10 Claims Int. 'Cl. C2311 9/02 The present invention relates to anodizing aluminum. More particularly, the present invention resides in the production of integral, colored, anodic coatings on an aluminum article by making the aluminum article anode in an electrolytic cell utilizing a particular aqueous acid electrolyte.

Decorative and protective oxide coatings on aluminum have long been made by anodizing in electrolytes consisting of aqueous solutions of sulfuric acid. Such electrolytes are usually employed at or near room temperature and must be cooled to maintain such temperature. The coatings so produced are usually relatively clear or colorless, although on some alloys they may be tinted or colored by some constituents and they are characterized by generally poor abrasion resistance.

Processes have been developed for producing anodic oxide coatings which are more abrasion resistant by anodizing in aqueous sulfuric acid electrolytes at extremely low temperatures, i.e., from about 0 to 30 F. These processes are subject to the inherent disadvantage of requiring expensive refrigeration equipment to maintain the temperatures below 30 F. The coatings so produced, however, in addition to being abrasion resistant are attractively colored for numerous uses of aluminum, i.e., are colored in dark desirable colors, such as dark brown, etc.

It is, therefore, highly desirable to develop a practical process for anojclically obtaining relatively dark surface colors and good corrosion resistance.

Accordingly, it is an object of the present invention to provide a practical process as aforesaid for producing an integral, colored, anodic coating on an aluminum article and an electrolytic cell for performing said process.

It is a further object of the present invention to provide a process asaforesaid for anodizing aluminum which yields oxide coatings having relatively dark surface colors.

In addition, it is an object of the present invention to provide a process as aforesaid which yields an integral, colored aluminum article having good physical properties, such as excellent corrosion resistance, abrasion resistance and light stable colors.

It is a still further object of the present invention to provide a process as aforesaid which enables the attainment of a wide variety of desirable, integral, dark surface colors.

Further objects and advantages of the present invention will appear hereinafter.

In accordance with the process of the present invention it has now been found that the foregoing objects and advantages may be readily obtained. The present invention provides a process for coloring aluminum electrolytically, which comprises anodically oxidizing said aluminum at a current density of from 10 to 100 amps per square foot preferably between 15 and 35 amps per square foot, and a voltage of from 10 to 90 volts in an aqueous solution consisting essentially of from 0.2 to 2% sulfuric acid, from 3% to saturation of sulfamic acid, preferably from 3 to 15% sulfamic acid and from 0.5% to 5% of a lower aliphatic dibasic acid. The bath is maintained at a temperature of from 15 to 30 C. The preferred dibasic acids are maleic, tartaric or succinic acids.

It has been found in accordance with the present invention that the foregoing process surprisingly achieves all of the objects of the present invention and readily achieves a wide variety of light stable, abrasion and corrosion resistant surface colors having excellent physical characteristics and varying from light silver gray to jet black or a variety of browns and dark olive shades.

It is particularly surprising that the present process attains a wide range of highly desirable dark colors. One particularly unexpected feature of the present invention is the production of integral dark colors over a relatively wide range of sulfamic acid and dicarboxylic acid concentrations but over a very narrow range of sulfuric acid concentrations.

The use of the dicarboxylic acid additive surprisingly enables the attainment of darker colors than are obtained without the additive for the same treatment conditions and time.

An additional and significant feature of the present invention is that all of the foregoing advantages are obtained at a low cost which renders the process particularly attractive commercially.

It has been found that the process variables of the present invention are important in attaining the desired effect.

The sulfamic acid concentration may be varied over a wide range of from 3% to saturation, preferably should be maintained in the range of from 3% to 15% and optimally from 4 to 6%. The sulfuric acid concentration should be maintained in the range of from 0.2% to 2% by weight and preferably from 0.20 to 1.0%.

Any lower aliphatic dibasic acid may be conveniently employed. The preferred lower dibasic aliphatic acids are maleic, succinic and tartaric. Other representative lower dibasic aliphatic acids include the following: oxalic and malonic, for example. Naturally, the saturated or unsaturated dibasic aliphatic acids may be used.

The dibasic acid concentration can be varied from 0.5% to 5% and preferably is maintained from 1 to 2.5% by weight. The particular concentrations of all of the foregoing substituents may be varied within the foregoing ranges depending uppn the alloy in question and the particular colors desired.

The aqueous bath is maintained at a temperature of from 15 C. to 30 C. and preferably from 18 to 25 C.

The time of treatment is not especially critical, with the longer times providing the thicker coatings and darker colors. Generally, at least one minute is used and preferably from 1 to 150 minutes.

The aluminum article is anodically oxidized at a current density of from 10 to 100 amps per square foot and preferably at a current density of 15 to 35 amps per square foot and at a voltage of from 10 to volts, and preferably at a voltage of from 25 to 70 volts.

Naturally, various additional ingredients conventionally used may be added to the anodic oxidation bath to achieve particular results or enhance particular characteristics, such as, for example, metal sulfates.

After the anodic oxidation treatment, the anodized article may if desired be subjected to a conventional hot water sealing step, e.g., immersion in water maintained at or near its boiling point.

The present invention is applicable to anodizing any aluminum article, i.e., high purity aluminum, aluminum in various commercial grades, and aluminum base alloys.

In the electrolytic cell, the cathode which may be used is not especially critical. Conventionally, lead or preferably stainless steel cathodes may be used.

The present invention and the improvements resulting therefrom will be more readiy apparent from a consideration of the following illustrative examples wherein the amounts of all ingredients are given in percents by weight.

EXAMPLE I In the following example the aluminum alloy utilized was aluminum alloy 6063 (aluminum association designation). The electrolyte used was an aqueous solution containing about 4% by weight sulfamic acid, about 0.3% by weight sulfuric acid and about 2% by weight of maleic acid. The bath was maintained at a temperature of 22 C. Maleic acid was added as the anhydride and allowed to hydrolize to the acid in the bath. The aluminum alloy 6063 was made anode in the bath with a cathode of lead. The voltage was raised to give a current density of 5 amps per square foot for an initial period of 5 minutes after which the voltage was slowly raised to maintain a current density of amps per square foot. The voltage was then held constant for an anodizing time of minutes. After 30 minutes the aluminum was removed, rinsed and sealed in the conventional fashion by immersing in boiling water. The resultant sample was found to have a dark bronze color. The resulting color may be shown by a photovolt reading wherein generally darker colors are indicated by lower photovolt readings, i.e., the lower photovolt reading indicates lower reflectivity, hence a darker color. The sample treated in accordance with this example gave a photovolt reading of 7 units. This should be compared with a comparative example which was run in the exact same manner except that the maleic acid was omitted. A lighter bronze color was obtained in the comparative example giving a photovolt reading of 9.5 units.

EXAMPLE II The procedure of Example I was repeated with the maleic acid added except that the constant current of 25 amps per square foot was allowed to flow until the maximum voltage (65 volts) of the rectifier had been reached. This sample was darker in color than the sample anodized the same way, but in a solution without maleic acid.

EXAMPLE III Example I was repeated with the exception that instead of maleic acid, succinic acid was used with the material being added as the anhydride as in Example I. The results were essentially the same as in Example I. The succinic acid addition increased the intensity of color produced in a given time.

EXAMPLE IV Example I was repeated with the exception that instead of maleic acid, tartaric acid was used with the material being added as the anhydride as in Example I. The color of the anodized sample was again darker than the sample anodized without the tartaric acid addition. Further, the

color intensity was greater than produced by Examples I,

II or III.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to 'be embraced therein.

What is claimed is:

1. A process for coloring aluminum electrolytically which comprises anodically oxidizing said aluminum at a current density of from 10 to amps per square foot and a voltage of from 10 to 90 volts in an aqueous solution consisting essentially of from 0.2 to 2% sulfuric acid, from 3% to saturation of sulfamic acid, and from 0.5% to 5% of a lower aliphatic dibasic acid, said bath being maintained at a temperature of from 15 to 30 C.

2. A process according to claim 1 wherein the sulfamic acid is present in a concentration of from 3 to 15%.

3. A process according to claim 1 wherein the sulfamic acid is present in a concentration of from 4 to 6%.

4. A process according to claim 2 wherein the sulfuric acid is present in a concentration of from 0.20 to 1.0%.

5. A process according to claim 2 wherein said dicarboxylic acid is present in a concentration of from 1 to 2.5%.

6. A process according to claim 2 wherein said dicarboxylic acid is selected from the group consisting of succinic acid, tartaric acid and maleic acid.

7. A process according to claim 2 wherein said bath is maintained at a temperature of from 18 to 25 C.

8. A process according to claim 2 wherein the current density is from 15 to 35 amps per square foot and the voltage is from 25 to 70 volts.

9. A process according to claim 2 wherein said aluminum is anodically oxidized for from 1 to minutes.

10. An electrolyte for coloring aluminum electrolytically, comprising an aqueous solution consisting essentially of from 0.2 to 2% sulfuric acid, from 3% to saturation of sulfamic acid and from 0.5 to 5% of a lower aliphatic dibasic acid.

References Cited UNITED STATES PATENTS 2,855,351 10/1958 Ernst 20458 3,020,219 2/1962 Franklin et al 204-58 JOHN H. MACK, Primary Examiner. R. L. ANDREWS, Assistant Examiner.

U.S. Cl. X.R. 204-35

Claims (1)

1. A PROCESS FOR COLORING ALUMINUM ELECTROLYTICALLY WHICH COMPRISES ANODICALLY OXIDIZING SAID ALUMINUM AT A CURRENT DENSITY OF FROM 10 TO 100 AMPS PER SQUARE FOOT AND A VOLTAGE OF FROM 10 TO 90 VOLTS IN AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF FROM 0.2 TO 2% SULFURIC ACID, FROM 3% TO SATURATION OF SULFAMIC ACID, AND FROM 0.5% TO 5% OF A LOWER ALIPHATIC DIBASIC ACID, SAID BATH BEING MAINTAINED AT A TEMPERATURE OF FROM 15 TO 30*C.