US3836439A

US3836439A - Method for forming a colored oxide coating on surfaces of aluminum or aluminum alloy

Info

Publication number: US3836439A
Application number: US00299329A
Authority: US
Inventors: M Ikegaya; F Shigeta
Original assignee: Riken Light Metal Industry Co Ltd
Current assignee: Riken Light Metal Industry Co Ltd
Priority date: 1971-10-22
Filing date: 1972-10-20
Publication date: 1974-09-17
Anticipated expiration: 1991-09-17
Also published as: IT969776B; CA1012088A; GB1391808A

Abstract

COLORED OXIDE COATING HAVING A BROAD RANGE OF BRONZE COLORS ON ALUMINUM OR ALUMINUM ALLOY MATERIAL ARE FORMED BY ANODIC OXIDATION WITH AN AQUEOUS SOLUTION CONTAINING 0.5-10% BY WEIGHT PER VOLUME OF OXALIC ACID AND 0.05-1.00% BY WEIGHT PER VOLUME OF SULFURIC ACID. WHEN THE ELECTROLYTIC BATH IS MAINTAINED AT A LOW TEMPERATURE OR THE ABOVE DESCRIBED AQUEOUS SOLUTION CONTAINS ADDITIONALLY ALUMINUM ION, NITRIC ACID OR A MIXTURE OF NITRIC ACID AND BORIC ACID, THE DESCRIBED COLORED OXIDE COATING CAN BE OBTAINED IN A SHORTENED TIME.

Description

United States Patent 3,836,439 METHOD FOR FORMING A COLORED OXIDE COATING ON SURFACES 0F ALUMINUM OR ALUMINUM ALLOY Masashi Ikegaya and Fumio Shigeta, Shizuoka, Japan, assignors to Riken Light Metal Industries Co., Ltd., Shizuoka, Japan No Drawing. Filed Oct. 20, 1972, Ser. No. 299,329 Claims priority, application Japan, Oct. 22, 1971, 46/813,336, 46/83,.337, 46/83,338; Nov. 30, 1971, 46/95,850; May 12, 1972, 47/46,487;

May 29, 1972, 47/52,528; July 5, 1972, 47/66,624

Int. Cl. C23b 9/02 US. Cl. 204-58 2 Claims ABSTRACT OF THE DISCLOSURE Colored oxide coatings having a broad range of bronze colors on aluminum or aluminum alloy material are formed by anodic oxidation with an aqueous solution containing 05-10% by weight per volume of oxalic acid and ODS-1.0% by weight per volume of sulfuric acid. When the electrolytic bath is maintained at a low temperature or the above described aqueous solution contains additionally aluminum ion, nitric acid or a mixture of nitric acid and boric acid, the described colored oxide coating can be obtained in a shortened time.

The present invention relates to a method for forming a colored oxide coating on surfaces of aluminum or aluminum alloy materials and particularly a method for forming colored oxide coatings having a broad color tone of bronze series colors by subjecting aluminum or aluminum alloy materials to an anodic oxidation in an aqueous solution containing oxalic acid and sulfuric acid by means of direct current.

In the specification, aluminum or aluminum alloys are referred to as merely aluminous materials and the direct current includes the ripple obtained by rectifying alternate current as well as current from an accumulator.

Recently, aluminous building materials have been utilized broadly and various coloring processes for these materials have been studied. However, the range of coloration of the aluminous materials is not said to be broader than that of the other building materials and for obtaining a certain color, for example, bronze, strict treating conditions are required and further the color tone range is narrow and the chemicals for such a treatment are usually expensive. Conventional processes for coloring aluminous materials to bronze are as follows:

(1) An aqueous solution of oxalic acid is used as the electrolytic bath.

(2) Anodic oxidation is effected in an electrolytic bath containing an aromatic sulfonic acid and sulfuric acid or a metal sulfate.

In the above described first process, oxalic acid to be used in the electrolytic bath is inexpensive and in this point this process is most valuable commercially. However, the color tone range of the obtained oxide coatings is very narrow and even if the color tone is bronze, it belongs to yellow series. Even if the quality of the aluminous material itself or the thickness of oxide coating is varied, the color tone range is limited and consequently this process has not been extensively used in practice.

The above described second process can provide a clear bronze color tone and a broad range and therefore this process has been used most broadly. However, the aromatic sulfonic acid which is the main component of the electrolytic bath is expensive and this is a great drawback in this process.

3,836,439 Patented Sept. 17, 1974 'ice Recently, such colored aluminous materials have been in great demand and in order to increase the yield, it is required to reduce the treating time but the above described processes do not fully satisfy this requirement.

The inventors have made efforts to solve the above described drawbacks and accomplished the present invention.

The inventors have found that it is possible to obtain colored oxide coatings having a broad range of bronze series color by a simple electrolytic bath, that is an aqueous solution of oxalic acid to which sulfuric acid is only added.

An object of the present invention is to provide a method for forming easily a colored oxide coating having a broad range of color tone of bronze by means of an electrolytic bath of an aqueous solution containing sulfuric acid in addition to oxalic acid.

A further object of the present invention is to provide a commercially advantageous method for forming colored oxide coatings from bronze to amber in a short time by subjecting the aluminous materials to an anodic oxidation in an aqueous solution of oxalic acid added with sulfuric acid kept at a low temperature.

Another object of the present invention is to provide a method for forming colored oxide coatings from amber to bronze rapidly by subjecting the aluminous materials to an anodic oxidation in an aqueous solution of oxalic acid and sulfuric acid containing aluminum ion.

A further object of the present invention is to provide a method for forming stable colored oxide coatings from amber to bronze by subjecting the aluminous materials to an anodic oxidation in an aqueous solution of oxalic acid and sulfuric acid added with nitric acid or a mixture of nitric acid and boric acid.

The invention will be explained hereinafter.

FIRST ASPECT OF THE PRESENT INVENTION For the practice of the method of the present invention, firstly, an aluminous material to be treated is degreased with an aqueous solution of sodium hydroxide and then washed with water and successively neutralized with an aqueous solution of nitric acid and then washed with water. Then, the thus treated aluminous material is subjected to anodic oxidation with an aqueous solution containing 0.510.0% by weight per volume of oxalic acid and 0.05-1.0% by weight per volume of sulfuric acid.

When the amount of oxalic acid is less than 0.5% by weight per volume, the colored oxide coating is not fully formed and the mechanical strength is not sufficient and the color is not fully developed, while when the amount of oxalic acid is more than 10% by weight per volume, oxalic acid is not dissolved in water at room temperature. Accordingly, the above limitation in the amount of oxalic acid is based on the above described reason.

When the amount of sulfuric acid is less than 0.05% by weight per volume, the coloration of bronze is not sufiicient, while the upper limit of sulfuric acid of 1.0% is based on the reason that an amount of more than 1.0% is not necessary for obtaining bronze color from the relation to the amount of oxalic acid.

The conditions of the anodic oxidation by using such an electrolytic bath are as follows:

(1) Current: direct current (2) Bath temperature: 0-40 C., preferably 10-40 C. (3) Treating time: 5-120 minutes (4) Current density: 0.5-5 a./dm.

Voltage: 5-150 v.

It seems that in the above described bath composition, oxalic acid and sulfuric acid contribute to the hue of the oxide coating and sulfuric acid mainly contributes to the value (luminosity).

However, under the above described conditions, when the content of sulfuric acid in the electrolytic bath is less than 0.75% by weight per volume, the content of oxalic acid is preferred to be more than 5.0% by weight per volume.

However, even when the content of sulfuric acid is less than 0.75% by weight per volume, if the content of oxalic acid is less than 5.0%, the object of the present invention can be attained, but in this case unless the bath temperature and the current density are properly limited, the treating time must be greatly prolonged in order to obtain a satisfactory colored oxide coating having a sufficient strength. Namely, if the bath temperature and the current density are 15:1" C. and 4-5 a./dm. respectively, a bronze colored oxide coating having a satisfactory weather resistance can be obtained in about 20 minutes. In this case, even if the current density is less than 4 a./dm. when the treating time is prolonged (for example, 1 hour), a bronze colored oxide coating may be obtained but in order to shorten the treating time to 20 minutes (at most 30 minutes), 4 a./dm. of current density is necessary and even when the current density is increased to more than a./dm. the effect of the increase of current density does not appear, so that the upper limit of the current density is 5 a./dm.

Too high a bath temperature is not preferred and it has been found that the bath temperature at which the desired colored oxide coating is obtained by the above described bath composition and current density in less than 20 minutes, is :1" C.

Furthermore, even under the above described conditions (1) to (4), when the content of sulfuric acid is more than 0.75% by weight per volume, the colored oxide coatings having the desired bronze color can be obtained within the range of oxalic acid of 05-10% by weight per volume.

In the above described electrolytic bath, the bronze coloration can be obtained in a very short time by increasing the current density, for example, when the current density is 4 a./dm. the treating time is about minutes and if the current density is 5 a./dm. the treating time is about 5 minutes.

SECOND ASPECT OF THE PRESENT INVENTION The inventors have further found that the desired color tone can be obtained independently of the thickness of the formed oxide coating layer by eifecting the anodic oxidation at a given lower bath temperature. That is, colored oxide coatings from amber to bronze having various excellent properties can be obtained in a short time of 3-15 minutes by subjecting the aluminous materials to an anodic oxidation by means of an aqueous solution containing (LS-5.0% by weight per volume of oxalic acid and ODS-0.50% by weight per volume of sulfuric acid at a low temperature of 0-12" C. under the same current density and voltage as described above.

In the above described treatment, the desired color tone can be obtained by keeping the bath temperature low. In general, the color tone of the oxide coating is influenced by the thickness of said coating layer but as described above it has been found that the above described color tone can be attained at the low bath temperature in the above described short time. The upper limit of the bath temperature is important and if the bath temperature is higher than 12 C., it is necessary to increase the electric power per unit area, that is, to increase the thickness of the oxide coating in order to obtain a given coloration. For the purpose, the treating time is to be pronlonged and the object of the present invention for shortening the treating time can not be attained and therefore the desired economic effect can not be attained. While, in thi treatment, the desired color tone can be attained without increasing the thickness of the oxide coating and particularly since the bath temperature is low as described above, the amount of anode alumniurn dissolved off is small and therefore it is possible to prevent the aging of the electrolytic bath and such a treatment is very advantageous also in this respect.

THIRD ASPECT OF THE PRESENT INVENTION The inventors have found that the anodic oxidation can be effected in a short time by using the above described electrolytic bath to which aluminum ion is intentionally added.

It has been previously proposed to eifect the anodic oxidation in the presence of the dissolved aluminum ion. The purpose of the presence of the aluminum ion in this case is to prevent local corrosion caused by excess current which flows on the aluminous materials upon the anodic oxidation treatment. The amount of aluminum dissolved 01f in this case is, for example about 0.1-0.4 g./l. in an aqueous solution of oxalic acid. The upper limit is defined for the reason that the electric corrosion on the whole surface can be prevented by such an amount and the lower limit is defined for the reason that the elfect for preventing the excess current can be attained from such an amount.

The inventors have found that aluminum ion in an aqueous solution containing oxalic acid and sulfuric acid can reduce the treating time and give uniform color tone in addition to the prevention of the above described excess current.

Furthermore, it is known that anodic oxidation for forming the colored oxide coating is carried out in the presence of aluminum ion or the salts thereof. However, the functions of the dissolved off aluminum ion in this known art are as follows:

(1) Aluminum ion itself gives the inherent function for generating color in the oxide coating.

(2) Aluminum ion coprecipitates with colored oxide or hydroxide in the oxide coating to develop color.

Accordingly, in the above described prior art methods, it is very difficult to obtain the colored oxide coating on the aluminous materials in a short time.

The electrolytic bath to be used in this aspect of the present invention is an aqueous solution containing 05-10% by weight per volume of oxalic acid, 0.05-l.0% by weight per volume of sulfuric acid and ODS-6.0 g./l. of aluminum ion.

Aluminum ion promotes the formation of colored oxide coating, shortens the treating time and stabilizes the color tone of the oxide coatings. As mentioned above, aluminum ion generally has the effect of preventing excess current but this is a negative effect. On the contrary, the addition of a relatively large amount of aluminum ion in the present invention does not directly seek to prevent the excess current but to accomplish the anodic oxidation even at a lower current density, whether the excess current is formed or not. Aluminum ion itself promotes the coloration reaction and from this point of view the treating time also can be reduced and such a function can be attained by the addition of 0.05-6.0 g./l. of aluminum ion in the electrolytic bath containing sulfuric acid in addition to oxalic acid.

The conditions for the anodic oxidation are as follows:

(a) Bath temperature: 0-40 C.

(b) Current density: 0.5-5 a./dm. (c) Voltrge: 30-80 v.

(d) Treating time: about 10 minutes.

The above described aluminum ion is obtained by the following means.

(1) Dissolving off the anode aluminum during the anodic oxidation step.

(2) Addition of aluminum oxalate or aluminum sulfate.

(3) immersing the aluminum anode in the electrolytic bath containing oxalic acid and sulfuric acid according to the present invention in which the temperature has been raised by continuing the anodic oxidation.

For obtaining aluminum ion, the combination of the above described means (1) and (3) is commercially advantageous, because the means (2) requires particular addition of the above described chemicals to the electrolytic bath and therefore in the following Examples 6 to 9, the aluminum ion was formed by the combination of the above described means (1) and (3).

The conditions for the anodic oxidation in the above described electrolytic bath are as follows:

(a) Bath temperature: -40 C.

(b) Current density: 0.5-5 a./dm. (c) Voltage: 30-80 v.

(d) Treating time: about minutes.

The above described explanation is made with respect to the case where the aluminous materials, after being degreased and washed with water, are directly subjected to the anodic oxidation, but if a porous oxide coating has been previously formed on the aluminous material by conventional anodic oxidation, the treating time can be reduced and further the color tone of the oxide coating can be stabilized.

FOURTH ASPECT OF THE PRESENT INVENTION The inventors have further found that when the electrolytic bath of an aqueous solution containing oxalic acid and sulfuric acid is supplied with nitric acid, the resulting bath can provide colored oxide coatings having stable bronze series colors in a shorter time than the electrolytic bath containing oxalic acid and sulfuric acid.

The electrolytic bath in this treatment is an aqueous solution containing 0.5-10.0% by weight per volume of oxalic acid, 0.05-1.0% by weight per volume of sulfuric acid and 0.05-0.5% by weight per volume of nitric acid.

The addition of nitric acid lowers the electrical conductivity of the electrolytic bath containing sulfuric acid and oxalic acid. Accordingly, when a given current density is applied to the electrolytic bath containing nitric acid in addition to oxalic acid and sulfuric acid, the increased rate of voltage in said bath is earlier than that of the electrolytic bath containing oxalic acid and sulfuric acid and consequently colored oxide coating having a desired color tone can be obtained in a shorter time.

However, nitric acid is apt to cause electric corrosion on the aluminous material during the anodic oxidation and nitric acid should not be added until the electric corrosion occurs and therefore the upper limit of nitric acid is defined in view of this point.

The lower limit of nitric acid is based on the reason that the effect of adding nitric acid does not clearly appear in an amount less than 0.05%.

Furthermore, the inventors have found that the electrolytic bath of an aqueous solution containing 0.1-10.0 by weight per volume of boric acid in addition to oxalic acid, sulfuric acid and nitric acid can provide colored oxide coating having stable greyish green to brown series colors by an anodic oxidation treatment in a further shorter treating time.

Boric acid also has the function of decreasing the electrical conductivity of the electrolytic bath, as in nitric acid, but it does not have the danger of the electrical corrosion. Accordingly, it has been found that when boric acid is added to further decrease the electrical conductivity, the time for the anodic oxidation can be further reduced as compared with the case when only nitric acid is added.

Furthermore, it has been found that the addition of boric acid can give a bluish bronze color to the oxide coating.

In this case the conditions for the anodic oxidation are as follows:

(a) Bath temperature: 10-40 C. (b) Current density: 0.5-5 a./dm. (c) Voltage: 15-120 v.

(d) Treating time: 3-120 minutes.

The treating time is sufiicient in about 3 minutes but in general 20-30 minutes are preferable and in this case the preferred bath conditions are as follows:

Oxalic acid: 1.5-3.5% by weight per volume Sulfuric acid: 0.05-0.3% by weight per volume Nitric acid: -0.05-0.2% by weight per volume Boric acid: 1.0-3.0% by weight per volume Bath temperature: 20- -2 C.

The following examples are given for the purpose of illustration of this invention and are not intended as limitations thereof. In the examples, percent by weight means percent by weight per volume.

EXAMPLE 1 Aluminum 1100 was immersed in 8.0% by weight of an aqueous solution of sodium hydroxide at 75 C. for 20 seconds and washed with water and then the thus degreased aluminous material was immersed in 16% by weight of an aqueous solution of nitric acid at room temperature for 10 seconds to effect neutralization and it then was washed with water.

The thus pretreated aluminous material was anodized as an anode in an electrolytic bath of an aqueous solution containing oxalic acid and sulfuric acid as shown in the following Table 1 at a temperature of 20il C., under conditions of a direct current of a current density of 2 a./dm. and a voltage of 30-85 v. for minutes to obtain colored oxide coatings as shown in Table I.

TABLE 1 Composition of electrolytic bath (percent by weight) Thickness of l t Sulfuric Voltage coating 00 or one Oxalic acid acid (v.) Color Munsell notation 5.0 0. 05 45-65 29 5. 7 2. 1 0. 10 40-60 31 5. 4. 7/2. 1 0. 30 35-50 30 Bronze 6. 3. 8/1. 7 0. 50 35-50 31 Slightly light bronze. 4. 9/1. 3

7.5 5 55-70 29 Greyish ye1low...- 5.8 GY 5. 2 1. 8 0. 10 50-65 29 Bronze 6.7 GY 4. 1/1. 8 0. 30 50-60 28 Slightly light bronze. 7.9 GY 4. 3/1. 4 0. 50 45-50 27 Slightly light greyish yellow 9.5 GY 5. 8/1. 3 0. 45-55 30 o 9.8 GY 5. 9/1. 5

10.0 0. 5 50-65 31 Light bronze 8.3 G 5. 6 3. 1 0. 10 50-60 27 Greyish yellow 3.0 GY 5. 5/2. 0 0. 30 45-55 28 Light grayish yellow 6.5 GY 4. 8/1. 5 0. 5 40-55 30 Slightly light bronze- 7.9 GY 5. 1/ 1. 3 0. 75 3 30 Bronze 9.0 GY 6. 0/1. 1

The conditions of the anodic oxidation in this case are as follows:

(a) Bath temperature: 10-30 C.

(b) Current density: 0.5-5 a./dm. (c) Voltage: 15-80 v.

Furthermore, the most favorable bronze is obtained at 0.10% of sulfuric acid in the case of a level of 7.5% of oxalic acid and at 0.75% of sulfuric acid in case of the level of 10.0% of oxalic acid, respectively. The colors obtained by varying the amount of sulfuric acid at each level of oxalic acid belong to the bronze series, which is distinguished from yellow, as shown in the Munsell notation. In this respect, it is understood that a broad range of bronze series colors is obtained according to the greased aluminousmaterial was immersed in 15% by weight of an aqueous solution of nitric acid at room temperature for 30 seconds, to effect neutralization and it was then washed with water. v

The thus pretreated aluminous material was anodized as an anode in an electrolytic bath of an aqueous solution containing oxalic acid and sulfuric acid as shown in the following Table 4 at a temperature of 201-1 0, with a direct current of a current density of 2 a./dm. and a voltage of 3085 v. for 60 minutes to obtain present invention. 10

EXAMPLE 2 colored oxide coatings as shown in Table 4.

Aluminum 1100 was immersed in 8% by weight of TABLE4 c an aqueous solution of sodium hydroxide at 80 C. for Compositionelectrolytic 20 seconds and washed with water and then the thus de- 15 both (percent by weight) Thickgreased aluminous material was immersed in 16% by Sulfuric Voltage 232;; weight of an aqueous solution of nitric acid at room tem- Oxalic acid acid (v.) (a) Color perature for seconds to efi'ect neutralization and it then 1 075 6&8; 29 Bronze was washed with water. 3

The thus pretreated aluminous material was anodized 20 2&2; 33 Rg' as an anode in an electrolytic bath of an aqueous solu- 5 B tion containing oxalic acid and sulfuric acid as shown in $5. 2,2128 3? fi the following Table 2 at a temperature of i1 C., with 7 5 0 a direct current of a current density of 4 a./dm. for 12$ 3 light bmme' minutes to obtain colored oxide coatings as shown in 10.0 0.75 -45 30 Do. Table 2.

TABLE 2 Composition of electrolytic bath (percent by weight) Thickness of Color tone Suliuric Voltage coating Oxalie acid acid (v.) (a) Color Munseil n tati n 1.0 0.05 65-85 27 Light bronze 4.9 G 4.3/1.9 0.10 60-80 28 Deep bronze 10 GY 3. 5/1.8

3.0 0.05 50-70 30 Light grayish yellow 2 0 GY 5. s/i. 5 0.10 45-60 28 Greyish yellow 5.5/2.0 0.30 -55 31 Bronze 6.9 GY 3.7/1.7

5.0 0.05 -65 29 Greylsh yellow 2.5 GY 5.7/2. 1 0. 10 40-60 31 Light bronze 5.0 G 4. 7/2. 1 0.30 35-50 30 Bronze- 6.5 GY 3.5/1.7 0. 35-50 31 Deep bronz 7.5 GY 4. 9/1. 3

EXAMPLE 3 As mentioned above, the present invention is directed Two tests were effected in the same manner as described in Example 2, under the following conditions as shown in Table 3, to obtain the results as shown in Table 3. In this case the voltage in the electrolytic baths was not measured.

As seen from the results of Table 3, bronze is obtained in a treating time of about 20 minutes only when the bath temperature is maintained at 151-1 0., while if the bath temperature exceeds 15 :1" C., the treating time should be prolonged.

As seen from the above results, in this example the aluminous material can be colored in bronze for a comparatively short time under the defined bath temperature and current density and therefore the cost of chemicals used is remarkably decreased. Since the bath temperature is 15 (2., the electrolysis is very easy. Furthermore, the oxide coatings thus obtained are very broad in color tone and have substantially the same weather resistance as that obtained with the expensive aromatic sulfonic acid bath.

EXAMPLE 4 Aluminum 1100 was immersed in 8.0% by weight of an aqueous solution of sodium hydroxide at 75 C. for 30 seconds and washed with water and then the thus deto a method of obtaining bronze series colors by using inexpensive oxalic acid and sulfuric acid and in this respect, it has high industrial and economic merit. Furthermore, the treating time can be shortened up to the order of 5 minutes when the current density is raised, and consequently the amount of chemicals consumed largely decreases and further the obtained coating has a suflicient weather resistance.

EXAMPLE 5 Aluminum 1100 was immersed in 8.0% by weight of an aqueous solution of sodium hydroxide at 60 C. for 1 minute and washed with water and then the thus degreased aluminous material was immersed in 16% by weight of nitric acid at room temperature for 20 seconds to effect neutralization and it was then washed with water. Then, the thus pretreated aluminous material was anodized as an anode in an electrolytic bath of an aqueous solution containing oxalic acid and sulfuric acid as shown in the following Table 5 at a temperature of 1li1 C., with a direct current of a current density of l a./d1n. and a highest voltage of 70 v. for 3 minutes or 15 minutes to obtain colored oxide coatings varying from bronze The sample obtained in this example and a comparative sample obtained by a conventional sulfuric acid electrolytic bath were determined with respect to the following properties to obtain results as shown in the following Table 6.

TABLE 6 Comparative sample obtained by Sample plate of Test conventional sulfuric acid bath (6 Example (3.5

1. 350 sec- 310 sec. 2- 60 sec 70 sec. 3- More than BN9 More than RNlO.

From the results of Table 6, it was confirmed that the properties of the oxide coatings according to the present invention are superior to those of the comparative sample even when the thickness of the coating is thinner.

As mentioned above, the present invention is directed to a method of obtaining a color from bronze to amber in a very short time by using inexpensive oxalic acid and sulfuric acid and in this respect, has a high industrial and economic merit.

Moreover, the obtained oxide coatings have a broad range of color tone and are the same as those obtained with the expensive aromatic sulfonic acid bath in weather resistance, abrasion resistance and the like.

EXAMPLE 6 Aluminum 1100 was degreased, washed with water and neutralized in the same manner as described in Example 5 and then the thus pretreated aluminous material was anodized as an anode in an electrolytic bath of an aqueous solution containing oxalic acid, sulfuric acid and aluminum ion as shown in the following Table 7 at a temperature of 20:2 C., with a direct current of a current density of 2 a./dm. and a voltage of 48 v. for 5 minutes, 7 minutes or 10 minutes to obtain colored oxide coatings showing uniformly from amber to bronze on the aluminous material as shown in Table 7.

The same aluminous material as used in Example 6 was degreased and washed with water in the same manner as described in Example 5 and then the thus pretreated aluminous material was anodized as an anode in an electrolytic bath of an aqueous solution containing by weight of sulfuric acid in a conventional manner for 3 minutes to form a porous oxide coating on the surface thereof and thereafter it was washed with water.

Then, the thus treated aluminous material was further anodized in the same electrolytic bath containing alumi num ion under the same conditions as used in Example 6 until the same colored oxide coating as obtained in Example 6 was formed, whereby colored oxide coatings showing uniformly from amber to bronze similar to those of Example 6 were obtained on the aluminous materials for 4 minutes, 6 minutes and 8.5 minutes, respectively. Accordingly, it was confirmed that when the conventional sulfuric acid electrolysis is carried out as an intermediate treatment for a very short time, the treating time for developing the color can be shortened.

EXAMPLE 8 The same aluminous material as used in Example 6 was degreased, washed with water and neutralized in the same manner as described in Example 5 and then the thus pretreated aluminous material was anodized as an anode in an electrolytic bath of an aqueous solution containing 2.2% by weight of oxalic acid, 0.22% by weight of sulfuric acid and aluminum ion as shown in the following Table 8, under the same conditions for the anodic oxidation as described in Example 6, for 7 minutes to obtain colored oxide coatings showing uniformly from amber to bronze on the aluminous material as shown in Table 8. It was confirmed that the concentration of aluminum ion dissolved has a relation to the increase of electrical resistance and has a favorable influence on the coloring of the aluminous material.

TABLE 8 Concentration of aluminum ion, g./l.: Color 0.6 Light amber. 2.0 Amber. 3.5 Bronze.

EXAMPLE 9 TABLE 9 Thickness of coating Color Aluminous material t) (Munsell notation) Pure aluminum (99.99%) 7. 5 1.5 Y 4. 97/12. 0 6063 7.5 7.8 GY 4. 21/6 5 7. 8 7.5 GY 4.13/11 8 7.5 IOGY 4.90 .0

From the above description, it can be seen that according to the present invention, the presence of aluminum ion in the electrolytic bath of an aqueous solution of oxalic acid and sulfuric acid controls the dissolution of aluminum ion from the aluminous material during the anodic oxidation and prevents the local electrical corrosion of the aluminous material to increase the electric resistance and therefore the voltage is effectively raised while maintaining a low current density, whereby the formation of the colored oxide coating can be considerably promoted. Accordingly, uniform coloration which has been obtained with difi'iculty in a conventional process, can be easily obtained and it is possible to obtain the colored oxide coating commercially and practically.

EXAMPLE 10 Aluminum 1100 was immersed in 8% by weight of an aqueous solution of sodium hydroxide at 70 C. for 30 seconds and washed with water and then the thus degreased aluminous material was immersed in 16% by weight of an aqueous solution of nitric acid at room temperature for 10 seconds to effect neutralization and it was then Washed with water. Then, the thus pretreated aluminous material was anodized as an anode in an electrolytic bath of an aqueous solution containing oxalic acid, sulfuric acid and-nitric acid as shown in the following Table 10 at a temperature of 2012 C., with a direct current of of 20:2 C., with a direct current under the same electrolytic conditions as shown in Table 11 to obtain colored oxide coatings of greyish green to greyish brown as shown a current density of 2 a./dm. and a voltage of 50-60 v. in Table 11.

TABLE 11 Composition electrolytic bath (percent by Electrolytic condition weight) Thick- Current uses 01 Color tone Sulfuric Nitric Borio Time Voltage density coating Oxalic acid ac acid acid (min.) (v.) (aldmfl) (11) Color Munsell notation 2.0 0.1 0. 0.5 30 4770 2.0 14 131111511 brown.-. 5.7 G 4. 4/1. 0. 1 0. 05 1.0 30 47-72 2. 0 d0 7. 1 G 4. 2/1. 4 0.1 0.05 3.0 30 45-72 2.0 Deep bluish brown- 7.1 G 4.2/1. 4

3.0 0.15 0.1 0.5 30 42-53 2.0 17 Slightlyllght bronze-..-.- 1.1 G 4.8/1.2 0.15 0.1 1.0 30 4H7 2.0 10 ronze 1.1 G 4.5/1.2 0.15 0.1 3.0 30 40-60 2.0 16 Bluish brown" 2.2 G 4.2/1.2 0.20 0.1 0.5 30 40-52 2.0 16 Bronze.- 1.8 G 4.4/1. 1 0. 0. 1 1. 0 -54 2. 0 16 -d0- 0. 7 G 4. 2/1. 4 0.20 0.1 3.0 30 38-55 2.0 15 Blulsh brown 0.5 G 4.0/1.2

for 30 minutes to obtain colored oxide coatings as shown in Table 10.

TABLE 10 Sulfuric acid] Thick- Oxaiic acid nitric acid ness of Color tone (percent by (percent by Voltage coating weight) weight) (v.) (0) Color Munsell notation 2 0. 05/0. 05 -60 14 Light grayish brown 7.3 GY 5. 4/1.0 0 05/0.10 -60 16 Greyish brown..-. 8.1 GY 5.5/1.2 0.10/0.05 -60 10 Brown 3.3 G 4.3/1.2 010/010 -60 15 Dark grayish brown" 9.7 CY 3.3/1.4 0.15/0.05 -60 17 Dark brown 9.5 GY 4.0/1.3

3 0. /0. -50 15 Greyish yellow 6.2 GY 5.6/1.2 0.10/0.10 -50 10 Light brown--- 6.3 GY 5.3/1.0 0. 10/0. 15 -50 15 Greylsh brown. 6. 5 GY 5. 1/1. 2 0.15/0.05 -50 do 3.8 GY 4. 3/0. 9 0.15/0.10 -50 8.2 GY 4.0/1.2 0.15/0.15 -50 8.7 GY 4.9/1.4 0. 20/005 -50 9.1 GY 3.2/1.0 0. 20/0. 10 -50 9.2 GY 3.4/0.9 0. 25/0. 05 -50 6.4 GY 3.9/1.3

6 0.05/02) -50 2.9 GY 5. 7/1.4 0.10/0.20 -50 2.7 GY 5.6/1.4 0.15/0.15 -50 3.1 GY 4.9/1.5 0. 15/0. 20 -50 3. 1 GY 5. 2/ 1. 4 0.20/0. 15 -50 3.0 GY 5.3/1.3 0. 20/020 -50 2.7 GY 5.5/1.7 0. 25/0. 10 -50 3.5 GY 5.2/1.2 0.25/0.15 -50 4.3 GY 5.1/0.9 0.25/0.20 -50 0 4.0 GY 5.0/0.9 030/005 -50 14 Light grayish yellow.. 4.2 CY 5.2/1.1 0.30/0.10 -50 10 Greyish yellow.- 3.7 GY 5.4/1.3 0.30/0.l5 -50 14 d0 3.6 GY 5.6/1.4 0.30/0.20 -50 17 --d0 2.9 GY 5.5/1.7 0.35/0. 05 -50 16 Light greyish yellow 3.3 GY 4. 9/1. 3 0.35/0.10 -50 16 Greyish yellow.-. 3.8 GY 4.8/1.2 0.35/0.15 -50 16 do 4.1 GY 5.0/0.0 0. 40/0. 05 -50 17 4. 2 GY 5. 1/1. 1 0. 40/0. 10 -50 15 Light yellow 3. 7 GY 5.8/1. 5 0. /005 15 Light grayish yellow 3.8 GY 4.9/1. 4

EXAMPLE 11 EXAMPLE 12 Aluminum 1100 was immersed in 8% by weight of an aqueous solution of sodium hydroxide at 70 C. for 30 seconds and washed with water and then the thus degreased aluminous material was immersed in 16% by weight of an aqueous solution of nitric acid at room temperature for 10 seconds to eifect neutralization and it was then washed with water.

The thus pretreated aluminous material was anodized as an anode in an electrolytic bath of an aqueous solution containing oxalic acid, sulfuric acid, nitric acid and boric 7 acid as shown in the following Table 11 at a temperature The same aluminous material as used in Example 11 '13 colored oxide coatings of greyish green to greyish brown in a short time as shown in Table 12.

'From this fact it is considered that the voltage mainly contributes to color development.

of 12 C. for 3-15 minutes with a current density of 0.5- a./dm.

2. A method as claimed in claim 1, wherein said aqueous solution contains either 0.050.5% by weight TABLE 12 Composition of electrolytic bath (percent by weight) Electrolytic condition Color tone Thick- Sul- Current ness of Oxalic (uric Nitric Boric Time density coating acid acid acid acid (min.) Voltage (v.) (a./dm. 4) Color Munsell notation 2.0 0. 1 0. 05 1. 0 15 -1 min6 50 v.; -15 1. 8-4. 1 8. 0 Slightly light bronze 10 G 5. 0/1. 3

n., v. 0.1 0.05 3.0 .'..do 1 65-3.0 8.0 do 6.8 G 4.8/1.7 0. 1 0.05 3. 0 10 -ni min.5 55 v.; 8-10 1 2-3.8 5.2 Greyish brown 7.5 G 4. 6/1. 5

n. v. i 0. 1 0.05 3.0 10 60-. 5.8 Bluish brown 4.7/1.0 0.1 0.05 3.0 15 58-63 12.0 Greyish yellow 1.8 G 4.5/1.0 0. 1 0. 05 3.0 48-65 13.5 Dark grayish purple 7.0 GY 4. 5/0. 2 0.1 0.05 3.0 20 50-65 8.5 do 2.4 G 4.5/1.1

dizing the aluminum or aluminum alloy material in an 3 aqueous solution of 0.55% by weight per volume of oxalic acid, 0.05-0.5% by weight per volume of sulfuric acid and 0.05-6.0 g./l. of aluminum ion at a temperature per volume of nitric acid or a mixture of 0.05-0.5% by 20 weight per volume of nitric acid and 0.1-10.0% by weight per volume of boric acid.

References Cited UNITED STATES PATENTS 3,252,875 5/ 1966 Economy 20458 3,616,308 10/1971 Cooke et al 204-58 3,616,297 10/1971 Cooke et a1. 204-58 3,597,338 8/ 1971 Matsuyama et a1 2045 8 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner