US3382160A - Process for inorganically coloring aluminum - Google Patents

Description

Ulllf ABSTRACT F THE DISCLOSURE Inorganically colored coatings are produced by first anodizing an aluminum article, as in sulfuric acid solution, to form an anodic coating, and then subjecting the anodized article to electrolytic treatment with alternating current in an acidic bath containing metal ions selected from the group consisting of the following cations and anions: Ni++, Co++, Fe++, Cu++, Ag Cd++, Zn++, Pb++, and anions consisting of oxygen combined with one of the metals Se, Te and Mn. By the described process there is deposited in the anodic coating the oxide or hydroxide of metal of the selected ions, resulting in a colored coating which can be sealed and which has good permanence.

This application is a continuation-in-part of my copending applications, Ser. No. 468,141, now abandoned, filed June 29, 1965, and Ser. No. 536,627 filed Mar. 23, 1966, the first of which was copending with, and a continuation-in-part of, my application Ser. No. 63,733, filed Oct. 20, 1960, now abandoned; the benefit of all of their filing and foreign priority dates are claimed under U.S.C. 119 and 120.

This invention relates to a method for producing an inorganic colored coating on aluminum. In particular, this invention relates to a method for producing characteristic black or deeply colored coatings on aluminum without using special aluminum alloys.

In prior processes for producing an inorganic colored coating on aluminum, aluminum alloys have been used containing special metals in excess of its solid solubility range in aluminum at the working temperature of the alloy. The alloy is anodized in sulfuric acid or sulfosalicylic acid containing small amounts of metal sulfates or sulfuric acid.

But this process is not effective to provide a homogeneous deep color since the alloy constituents existing as a solid solution or dispersion in the aluminum greatly affect its color.

In the process of this invention for producing inorganically colored aluminum, the characteristic properties of aluminum, itself, are utilized. Pure aluminum (Al: approximately 99.099.9%) is most effective to produce a product having a characteristic and deep colored coating. This colored sheet of aluminum does not discolor permanently even after exposure to sunlight and weather.

In this invention, the metallic ions added in an acid solution are attracted to the aluminum matrix in the micropores of oxide cell and is therein deposited as a hydroxide or oxide during the treatment with alternating current through the acid solution between an anodized aluminum and a carbon.

It is thought that the strong irreversible absorption of the added metallic cation in the micropores of the anodic coating on an aluminum is due to the fact that the aluminum matrix functions as a semi-conductor and a condenser.

States Patent 0 Therefore, this invention etfects an anodic oxidation on the surface of an aluminum in an acid liquor, such as, for example, sulfuric acid, chromic acid and oxalic acid or a suitable sulfonic acid such as sulfosalicylic acid, for 30 to 60 minutes. Then an alternating current is supplied through the acid solution which contains metal ions, such as Ni Co++, Pe Cu++, Ag+, Cd++, Zn++, Pb++, and anions consisting of oxygen combined with one of the metals Se, Te and Mn, between an aluminum electrode which is previously oxidized and a carbon electrode. The metallic ions in the acid solution are attracted to the aluminum matrix in the micropores of oxide cell, and is therein deposited as hydroxide or oxide for several minutes while hydrogen gas is generated at the aluminum electrode. The metallic ions, being the selected member or members of the above group of cations and anions that are all here defined as metal ions, can be added to the aqueous bath as any compound which will provide the requisite ions in the solution. When the metallic ion is a cation, it is not necessary that the anion of the metallic ion added to the bath be the same as the anion of the acid component of the bath. Indeed, it will be readily appreciated from the foregoing and the examples that the defined bath, consisting essentially of an aqueous acidic solution containing metal ions of the stated group, may comprise various acids or salts as may be appropriate or desired for such bath, permitting a coloring function due to the stated ions, so long as the bath is acidic in nature. Examples of a Variety of baths are given below, made up to have an acid pH, whether by direct inclusion of an acid or acids or by suitable salts that provide a solution of acidic character.

The deposition of the metal hydroxide or oxide in the micropores of the anodic coating on the aluminum provides a variety of coating colors, depending on the particular metal salt employed. After the metal ion has been deposited in the oxide coating, the aluminum is preferably sealed in boiling water or steam to provide a more adherent coating.

In this process, instead of carbon (which would normally be used in the form of graphite), other substances, exemplified by nickel or silicon carbide, can be employed as the other electrode in the step of treating the anodized aluminum article by passing alternating current between it and such other or counter-electrode in the aqueous bath containing metal ions from the group defined above. In view of the described nature of the treatment, selection of a counter-electrode of any suitable electrically conductive material can be readily made in the light of ordinary understanding of the art or with simple testing as in respect to corrosion of the electrode or undesirable introduction of other ions. Electrodes have been employed to function opposite the electrode constituted by the anodized aluminum article under treatment, which have a solution potential equal to or more noble than aluminum, being materials such as aluminum silicon carbide, carbon, and nickel and other metals more noble than aluminum in the conventional series of electrode potentials. Metals of the above stated group of metal ions useful in the bath are included in such category, it being thus readily apparent that besides carbon or silicon carbide, or aluminum where found to be useful, it is in general quite appropriate to employ a counter-electrode of a metal of which ions, from the prescribed group, are contained in the bath that is used.

The following specific but non-limiting examples are provided to illustrate the characteristic aspects of the present invention:

Example 1 A sheet of aluminum (99.099.9% Al) was oxidized with a direct current in a bath of aqueous solution con- 3 taining 15 by weight of sulfuric acid at ambient temperatures for 30 to 50 minutes.

The sheet of uniformly oxide-coated aluminum and a carbon electrode were connected to a source of alternating current in a bath containing g./l. of sulfuric acid, and 0.15 g./l. of silver sulfate at room temperature for 3 minutes at a potential of 8 volts.

A light gold color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 2 A sheet of aluminum (99.099.9% Al) was oxidized with direct current in a bath of aqueous solution containing by weight of sulfuric acid at ambient temperature for 30 to 50 minutes.

The sheet of uniformly oxide-coated aluminum and a carbon electrode were connected to a source of alternating current in a bath containing 7 g./l. of sulfuric acid and g./l. of cupric sulfate at room temperature for 3 minutes at a potential of 13 volts.

A dark red color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 3 A sheet of aluminum (99.099.9% Al) was oxidized with direct current in a bath of aqueous solution containing 15% by weight of sulfuric acid at ambient temperature for 30 to 50 minutes.

The sheet of uniformly oxide coated aluminum and a carbon electrode were connected to a source of alternating current in a bath containing g./l. of boric acid, 20 g./l. of cobalt sulfate, and 15 g./l. of ammonium sulfate at room temperature for 10 minutes at a potential of 15 volts.

A black color was created. The coated aluminum was then sealed in boiling water for minutes.

Example 4 A sheet of aluminum (99.0-99.9% Al) was oxidized with a direct current in a bath of aqueous solution containing 15% by weight of sulfuric acid at ambient temperature for 30 to minutes.

The sheet of uniformly oxide-coated aluminum and a carbon electrode were connected to a source of alternating current in a bath of 7 g./1. of sulfuric acid and 1.5 g./l. of sodium selenite at room temperature for 10 minutes at a potential of 13 volts.

A light gold color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 5 A sheet of aluminum (99.099.9% Al) was oxidized with direct current in a bath of aqueous solution containing 15 by weight of sulfuric acid at ambient temperature for 30 to 50 minutes. The sheet of uniformly oxide-coated aluminum and a carbon electrode were connected to a source of alternating current in a bath containing 7 g./l. of sulfuric acid and 1.5 g./l. of sodium tellurite at room temperature for 10 minutes at a potential of 13 volts.

A dark gold color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 6 A sheet of aluminum (99.099.9% Al) was oxidized with direct current in a bath of aqueous solution containing 15 by weight of sulfuric acid at ambient temperature for 30 to 50 minutes. The sheet of uniformly oxide-coated aluminum and a carbon electrode were connected to a source of alternating current in a bath containing 7 g./l. sulfuric acid and 1.5 g./l. of potassium permanganate at a potential of 13 volts. A light gold color was created. The coated aluminum Was then sealed in boiling water for 30 minutes.

4 Example 7 A sheet of aluminum (99.099.9% Al) was oxidized with direct current in a bath of aqueous solution containing 8% by weight of chromic acid heated to 50 C. at a potential of 40 volts, for 30 minutes.

The sheet of uniformly oxide-coated aluminum and a carbon electrode were connected to a source of alternating current in a bath containing 25 g./ l. of boric acid and 10 g./l. of zinc sulfate at room temperature for 10 minutes at a potential of 25 volts.

A dark brown color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 8 A sheet of aluminum (99.099.9% Al) was oxidized with direct current in a bath of aqueous solution containing 8% by weight of chromic acid heated to 50 C. at a potential of 40 volts for 30 minutes. The sheet of uniformly oxide-coated aluminum and a carbon electrode were connected to a source of alternating current in a bath containing 25 g./l. boric acid and 10 g./l. of cadmium sulfate at room temperature for 10 minutes at a potential of 25 volts.

A dark brown color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 9 A sheet of aluminum (99.099.9% Al) was oxidized with direct current in a bath of aqueous solution containing 8% by weight of chromic acid heated to 50 C. at a potential of 40 volts for 30 minutes. The sheet of uniformly oxide-coated aluminum and a carbon electrode were connected to a source of alternating current in a bath containing 20 g./.l. of sulfamic acid and 10 g./l. of lead acetate at room temperature for 10 minutes at a potential of 25 volts.

A black color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 10 A sheet of aluminum (99.0-99.9% Al) was oxidized with direct current in a bath of aqueous solution containing 8% by weight of chromic acid heated to 50 C. at a potential of 40 volts for 30 minutes. The sheet of uniformly oxide-coated aluminum and a suitable electrode were connected to a source of alternating current in a bath containing 25 g./l. of boric acid, 20 g./l. of nickel sulfate and 15 g./l. of ammonium sulfate at room tem perature for 2 minutes at a potential of 20 volts.

A bronze color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 11 A sheet of aluminum (99.0-99.9% Al) was oxidized with direct current in a bath of aqueous solution containing 8% by weight of sulfuric acid and 1.5% oxalic acid at ambient temperature at a potential of 20 volts for 40 minutes.

The sheet of uniformly oxide-coated aluminum and a carbon-electrode were connected to a source of alternating current in a bath containing 7 g./l. of sulfuric acid and 20 g./l. of cupric sulfate at room temperature for 13 minutes at a potential of 20 volts.

A black color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 12 A sheet of aluminum (99.099.9% Al) was oxidized with direct current in a bath of aqueous solution containing 8% by weight of sulfuric acid and 1.5% oxalic acid at ambient temperature at a potential of 20 volts for 40 minutes.

The sheet of uniformly oxide-coated aluminum and a carbon electrode were connected to a source of alternating current in a bath containing 25 g./l. of nickel sulfate and g./l. of ammonium sulfate at room temperature for 10 minutes at a potential of 13 volts.

A light bronze color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 13 A sheet of aluminum (99.099.9% Al) was oxidized with direct current in a bath of aqueous solution containing 8% by weight of chromic acid heated to 50 C. at a potential of 40 volts for 30 minutes.

The sheet of uniformly oxide-coated aluminum and a carbon electrode were connected to a source of alternating current in a bath containing 7 g./l. of sulfuric acid, 1.5 g./l. of sodium selenite and 1.5 g./l. of cupric sulfate at room temperature for 6 minutes at a potential of volts.

A brown color was created. The coated aluminum was then sealed in boiling water for minutes.

Example 14 A sheet of aluminum (99.099.9% Al) was oxidized with direct current in a bath of aqueous solution containing 8% by weight of chromic acid heated to 50 C. at a potential of volts for 30 minutes.

The sheet of uniformly oxide-coated aluminum and a carbon electrode were connected to a source of alternating current in a bath containing 25 g./l. of tartaric acid and 1.5 g./l. of selenic acid at room temperature for 10 minutes at a potential of 20 volts.

A light brown color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 15 A sheet of aluminum (99.099.9% Al) was oxidized with direct current in a bath of aqueous solution containing 15% by weight of sulfuric acid at ambient temperature for 3050 minutes.

The sheet of uniformly oxide coated aluminum and a carbon electrode were connected to a source of alternating current in a bath containing 25 g./l. of phosphoric acid and 1.5 g./l. of sodium selenite at room temperature for 10 minutes at a potential of 15 volts.

A reddish gold color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 16 A sheet of aluminum (99.099.9% Al) was oxidized with a direct current in a bath of aqueous solution containing 15% by weight of sulfuric acid at ambient tem perature for 30 to 50 minutes.

The sheet of uniformly oxide-coated aluminum and a carbon electrode were connected to a source of aternating current in a bath containing 5 g./l. of sulfuric acid, and 0.15 g./l. of silver nitrate at room temperature for 3 minutes at a potential of 8 volts.

A light gold color was created. The coated aluminum was then sealed in boiling water for 30 minutes.

Example 17 A sheet of aluminum was oxidized with direct current in a bath of aqueous, 15 sulfuric acid a ordinary temperature. After minutes of such treatment, the sheet of uniformly oxide-coated aluminum and a nickel electrode were connected to a source of alternating current in a bath of 2.5% boric acid which contained 4.0% nickel ammonium sulfate (pH 4 to 4.5) at room temperature for 5 minutees at a potential of about 15 volts. The coated article was then sealed in boiling water for about 30 minutes. By this process a coating having a bronze color was created.

Example 18 A sheet of aluminum was oxidized with direct current in a bath of aqueous, 15 sulfuric acid at ordinary temperature. After 45 minutes of such treatment, the sheet of uniformly oxide-coated aluminum and a lead electrode were connected to a source of alternating current in a bath of 2.5% boric acid which contained 2.5% cobalt sulfate and 1.5% ammonium sulfate (pH 4 to 4.5) at room temperature for 13 minutes at a potential of about 17 volts. The coated article was then sealed in boiling water for color 30 minutes. By this process a coating having a black color was created.

Obviously, many modifications and variations of the invention as hereinbefore set forth can be made without departing from the essence and scope thereof, and only such limitations should be applied as are indicated in the claims.

What is claimed is:

1. A method for the production of uniformly colored protective coatings on metal articles of aluminum which comprises the steps of first anodizing said article to produce an anodic oxide coating thereon and secondly treating said anodized article by passing an alternating current between said article and an electrode selected from the group consisting of carbon and silicon carbide, said article and said electrode being immersed in a bath consisting essentially of an aqueous acidic solution containing metal ions selected from the group consisting of the following cations and anions Ni Co++, Fe++, Cu++, Ag Cd++, Zn++, Pb anions consistinx of oxygen combined with one of the metals, Se, Te and Mn, said bath having a composition effective, during said second, treatment, for producing a deposit of metal, of the selected ions, which is in chemical combination with oxygen, said deposit being effected in the coating produced by the aforesaid anodizing treatment.

2. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with a direct current in a bath of aqueous solution containing 15 by weight of sulfuric acid for 30 to minutes, said second step occurring with a carbon electrode in a bath including 5 g./l. of sulfuric acid, and 0.15 g./l. of silver sulfate at room temperature for 3 minutes at a potential of 8 volts, whereby a light gold color is created.

3. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 15 by weight of sulfuric acid for 30 to 50 minutes, said second step occurring with a carbon electrode in a bath including 7 g./l. of sulfuric acid and 20 g./l. of cupric sulfate at room temperature for 3 minutes at a potential of 13 volts, whereby a dark red color is created.

4. A method as claimed in claim ll, said first step including oxidizing said aluminum articles wtih direct current in a bath of aqueous solution containing 15% by weight of sulfuric acid for 30 to 50 minutes, said second step occurring with a carbon electrode in a bath including 25 g,/l. of boric acid, 20 g./l. of cobalt sulfate, and 15 g./l. of ammonium sulfate at room temperature for 10 minutes at a potential of 15 volts, whereby a black color is created.

5. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 15% by Weight of sulfuric acid for 30 to 50 minutes, said second step occurring with a carbon electrode in a bath including 7 g./l. of sulfuric acid and 1.5 g./l. of sodium selenite at room temperature for 10 minutes at a potential of 13 volts, whereby a light gold color is created.

6. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 15% by weight of sulfuric acid for 30 to 50 minutes, said second step occurring with a carbon electrode in a bath including 7 g./l. of sulfuric acid and 1.5 g./l. of sodium tellurite at room temperature for 10 minutes at a potential of 13 volts, whereby a dark gold color is created.

7. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 15% by weight of sulfuric acid for 30 to 50 minutes, said second step occurring with a carbon electrode in a bath including 7 g./l. of sulfuric acid and 1.5 g./l. potassium permanganate at a potential of 13 volts, whereby a light gold color is created.

8. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 8% by weight of chromic acid heated to 50 C. at a potential of 40 volts for 30 minutes, said second step occurring with a carbon electrode in a bath including 25 g./l. boric acid and g./l. of zinc sulfate at room temperature for 10 minutes at a potential of 25 volts, whereby a dark brown color is created.

9. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 8% by weight of chromic acid heated to 50 C. at a potential of 40 volts for 30 minutes, said second step occurring with a carbon electrode in a bath including 25 g./l. of boric acid and 10 g./ l. of cadmium sulfate at room temperature for 10 minutes at a potential of 25 volts, whereby a dark brown color is created.

10. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 8% by weight of chromic acid heated to 50 C. at a potential of 40 volts for 30 minutes, said second step occurring with a carbon electrode in a bath including 20 g./l. of sulfamic acid and 10 g./l. of lead acetate at room temperature for 10 minutes at a potential of 25 volts, whereby a black color is created.

11. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 8% by weight of chromic acid heated to 50 C. at a potential of 40 volts for 30 minutes, said second step occurring in a bath including 25 g./l. of boric acid, 20 g./l. of nickel sulfate and g./l. of ammonium sulfate at room temperature for 2 minutes at a potential of volts, whereby a bronze color is created.

12. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 8% by weight of sulfuric acid and 1.5% oxalic acid at a potential of 20 volts for 40 minutes, said second step occurring with a carbon electrode in a bath including 7 g./l. of sulfuric acid and 20 g./l. of cupric sulfate at room temperature for l3 minutes at a potential of 20 volts, whereby a black color is created.

13. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 8% by weight of sulfuric acid and 1.5% oxalic acid at a potential of 20 volts for 40 minutes, said second step occurring with a carbon electrode in a bath including g./l. of nickel sulfate and 15 g./l. of ammonium sulfate at room temperature for 10 minutes at a potential of 13 volts, whereby a light bronze color is created.

14. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 8% byweight of chromic acid heated to 50 C. at a potential of 40 volts for minutes, said second step occurring with a carbon electrode in a bath including 7 g./l. of sulfuric acid, 1.5 g./l. of sodium selenite and 1.5 g./l. of cupric sulfate at room temperature for 6 minutes at a potential of 25 volts, whereby a brown color is created.

15. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 8% by weight of chromic acid heated to 50 C. at a potential of volts for 30 minutes, said second tep occurring with a carbon electrode in a bath including 25 g./l. of tartaric acid and 1.5 g./l. of selenic acid at room temperature for 10 minutes at a potential of 20 volts, whereby a light brown color is created.

l6. A method as claimed in claim 1, said first step in cluding oxidizing said aluminum articles with direct current in a bath of aqueous solution containing 15% by weight of sulfuric acid for 30-50 minutes, said second step occurring with a carbon electrode in a bath including 25 g./l. of phosphoric acid and 1.5 g./l. of sodium selenite at room temperature for 10 minutes at a potential of 15 volts, whereby a reddish gold color is created.

17. A method as claimed in claim 1, said first step including oxidizing said aluminum articles with a direct current in a bath of aqueous solution containing 15% by weight of sulfuric acid for 30 to 50 minutes, said second step occurring with a carbon electrode in a bath including 5 g./l. of sulfuric acid and 0.15 g./l. of silver nitrate at room temperature for 3 minutes at a potential of 8 volts, whereby a light gold color is created.

18. A method as defined in claim 1 in which. the electrode is carbon.

19. A method as defined in claim 18 in which said acidic solution has an acid concentration of not substantially more than about 25 grams per liter.

20. A method for the production of uniformly colored protective coatings on metal articles of aluminum which comprises the steps of first anodizing said article with direct current to produce an anodic oxide coating thereon and secondly treating said anodized article by passing an alternating current between aid article and an electrode, said article and said electrode being immersed in a bath consisting essentially of an aqueous acidic solution having a pH value of at least about 4 and containing metal ions selected from the group consisting of Ni++, Co and Zn++, said bath having composition effective, during said second treatment, for producing a deposit of metal, of the selected ions, which is in chemical combination with oxygen, said deposit being effected in the coating produced by the aforesaid anodizing treatment.

21. A method as defined in claim 20 in which the metal constituted in the metal ions selected from said group and contained in the bath consists of nickel.

22. A method as defined in claim 21 in which the electrode is selected from the group consisting of carbon and nickel.

23. A method for the production of uniformly colored protective coatings as defined in claim 20, said step of first oxidizing said article with direct current occurring in an aqueous bath of about 15% sulfuric acid for about minutes, said step of secondly treating said article comprising connecting said uniformly oxide-coated aluminum article and a nickel electrode to a source of alternating current in a bath of about 2.5% boric acid containing about 4.0% nickel ammonium sulfate (pH 4 to 4.5) at room temperature for about 5 minutes at a potential of 15 volts and sealing the coated article in boiling water for about 30 minutes, whereby a bronze color is created.

24. A method as defined in claim 20, in which the electrode is of an electrically conductive material which is not less noble than aluminum in respect to solution potential.

25. A method for the production of uniformly colored protective coatings on metal articles of aluminum which comprises the steps of first anodizing said article to produce an anodic oxide coating thereon and secondly treating said anodized article by passing alternating current between said article and an electrode of an electrically conductive material which is selected from the group consisting of carbon and metals more noble than alun1inum in respect to a solution potential, said article and said electrode being immersed in a bath consisting essentially of an aqueous acidic solution having a pH of a value of at least about 4 and containing nickel ions, said first anodizing step being effected in an anodizing bath which is free of nickel ions, said bath during said second treatment having a composition effective for pro ducing a deposit of which is in chemical combination with oxygen, said deposite being efl'ected in the coating produced by aforesaid anodizing treatment.

26. A method as defined in claim 25 in which the electrode is of nickel.

27. A method for the production of uniformly colored protective coatings on metal articles of aluminum which comprises the steps of first anodizing said article with direct current to produce an anodic oxide coating thereon and secondly treating said anodized article by passing an alternating current between said article and an electrode, said article and said electrode being immersed in a bath consisting essentially of an aqueous acidic solution having an acid concentration of not substantially more than about 25 grams per liter and containing metal ions selected from the group consisting of the following cations and anions: Cd+ Zn+ and anions consisting of oxygen combined with one of the metals Se, Te and Mn, said bath having a composition effective, during said second treatment, for producing a deposit of metal, of the selected ions, which is in chemical combination with oxygen, said deposit being effected in the coating produced by the aforesaid anodizing treatment.

28. A method for the production of uniformly colored protective coatings on metal articles of aluminum which 25 comprises the steps of first anodizing said article with direct current to produce an anodic oxide coating thereon and secondly treating said anodized article by passing an alternating current between said article and an electrode,

said article and said electrode being immersed in a bath consisting essentially of an aqueous acidic solution containing lead ions and having an acid content which consists essentially of sulfamic acid, said bath having a composition elfective, during said second treatment, for producing a deposit of lead which is in chemical combination with oxygen, said deposit being elfected in the coating produced by the aforesaid anodizing treatment.

References Cited UNITED STATES PATENTS 1,923,539 8/1933 Jenny et al. 204-58 2,008,733 7/1935 Tosterud 20435.1 2,022,798 12/1945 Bengston 204-58 X 2,231,086 2/ 1941 Muller et al. 204-58 2,231,373 2/1941 Schenk 204-58 X 2,260,278 10/1941 Schenk 20458 2,262,967 11/1941 Schenk 20458 2,290,364 7/1942 Tosterud 204-35.1 X 2,785,098 2/ 1957 Cunningham et al.

FOREIGN PATENTS 741,753 11/1943 Germany. 69,930 1/ 1946 Norway.

HOWARD S. WILLIAMS, Primary Examiner.

JOHN H. MACK, Examiner.

G. KAPLAN, Assistant Examiner.