US3795590A

US3795590A - Process for coloring aluminum and alloys of aluminum having an anodized surface

Info

Publication number: US3795590A
Application number: US00886330A
Authority: US
Inventors: J Patrie
Original assignee: Cegedur GP
Current assignee: Cegedur GP
Priority date: 1968-12-23
Filing date: 1969-12-18
Publication date: 1974-03-05
Anticipated expiration: 1991-03-05
Also published as: BE743617A; DE1963587B2; RO54852A; ES374797A1; CH508734A; NL6919219A; GB1291607A; FR1605100A; DE1963587A1; CA926808A; JPS4927492B1; SE360393B; LU60070A1

Abstract

THE PRODUCTION OF CORROSION RESISTANT REPRODUCIBLE COLORS ON THE SURFACE OF ALUMINUM AND ALLOYS OF ALUMINUM WHEREIN AN ALUMINUM OXIDE LAYER IS FORMED ON THE SURFACE BY ANODIZATION AND THE COLORED PARTICLES OF A METAL OR COMPOUND OF A METAL ARE DEPOSITED IN THE POROUS SURFACE BY ALTERNATING CURRENT ELECTROLYSIS WITH THE OBJECT MOUNTED AS AN ELECTRODE IN AN ELECTROLYTE CONTAINING THE CONSTITUENT METAL AND WHEREIN THE OBJECT WITH THE COLORED PARTICLES DEPOSITED THEREIN IS SUBJECTED TO ANODIC TREATMENT IN AN ELECTROLYTE CONTAINING AN ALKALI METAL OR AMMONIUM THIOSULPHATE, WITHOUT AND PREFERABLY WITH SUBSEQUENT SEALING.

Description

United States Patent 3,795,590 PROCESS FOR COLORING ALUMINUM AND ALLOYS 0F ALUMINUM HAVING AN ANODIZED SURFACE Jos Patric, Grenoble, France, assignor to Cegedur GP, Paris, France -No Drawing. Filed Dec. 18, 1969, Ser. No. 886,330 Claims priority, applicatigog9France, Dec. 23, 1968, 17 0 The portion of the term of the patent subsequent to May 23, 1989, has been disclaimed Int. Cl. C23!) 9/02; C23f /00, 17/00 U.S. Cl. 204-35 N 24 Claims ABSTRACT OF THE DISCLOSURE This invention relates to aluminum and alloys of aluminum having a protective colored coating and to a method for producing same.

In the copending application filed on or about Dec. 3, 1969, entitled Objects of Aluminum and Alloys of Aluminum Having Colored Coatings and Process, description is made of a process for obtaining clear, uniform and reproducible colored coatings formed on anodized aluminum or alloys of aluminum by anodic dissolution of a more darkly colored coating formed of colored particles deposited by an alternating current electrolysis. In the described process, it is possible to effect the desired dissolution in the same bath which was used for alternating current coloring.

In contrast, the process of this invention makes use successively of two different electrolysis baths whereby new colors are capable of being obtained while retaining all of the advantages of the process previously described. For example, with the previously described process, the use of copper salts in the coloring bath operates to produce colors ranging from pink to copper red to bright red whereas the process of this invention makes it possible to obtain additional colors such as bright yellow and brick color with the same copper salts.

Thus it is an object of this invention to produce and to provide a method for producing colored coatings on aluminum and alloys of aluminum in which colors of a greater variety can be obtained, in which colored coatings of greater stability and color intensity can be obtained, in which the colors can be reproduced in a manner to provide colored aluminum products having uniform color of high weather resistance and in which such colors can be produced in a simple, economical and eflicient manner.

Briefly described, in accordance with the process of this invention, the surface of the aluminum or alloy of aluminum to be colored is first provided in the conventional manner with a porous layer of aluminum oxide, as by anodization in an acid bath, preferably a sulphuric acid. Colored particles of the metal or metal compound are deposited into the portion of the porous layer by alternating current electrolysis in which the aluminum or alloy of aluminum is suspended as an electrode in the elec- 3,795,590 1C Patented Mar. 5, 1974 trolysis bath containing a salt of the metal of the compound dissolved therein in amounts of at least .15 g./ liter to an amount to form a saturated solution and preferably in an amount of .3 to 30 g./liter to provide a provisional or temporary color effect. The other electrode, referred to as the counter-electrode, is composed either of the constituent metal of the salt dissolved in the electrolysis bath or of a material which is resistant to attack by the bath and does not give rise to secondary reactions during alternating current electrolysis, such as represented by stainless steel.

The aluminum object containing the temporary colored coating, which has been deposited by alternating current electrolysis, is then subjected to anodic treatment by mounting the object as an anode in an electrolysis bath through which direct current or rectified unfiltered current is passed. In the practice of the invention, the electrolysis bath which is employed in the anodic treatment with direct current electrolysis comprises an aqueous solution containing sodium thiosulphate in an amount within the range of 1 to 100 g./liter and preferably 5 to 20 g./liter. Instead of sodium thiosulphate, ammonium thiosulphate or other alkali metal thiosulphate such as potassium thiosulphate may be used.

As the cathode, use can be made of platinum, stainless steel, or a metal such as copper or nickel. The eflfect of anodic treatment is to modify the temporary color finish. The anodic treatment is carried out at substantially constant current density within the range of 0.1 to 5 a./dm. and preferably within the range of 0.1 to 1 a./dm. or at a substantially constant voltage within the range of 10 to volts or preferably within the range of 15 to 30 volts. The electrolyte is employed generally within the range of 15 to 50 C. and preferably within the range of 18 to 25 C. The desired results can be obtained by anodic treatment under the conditions described in a time span of /6 to 30 minutes and generally within the range of /2 to 15 minutes. During anodic treatment, the temporary color effect lightens and may even disappear completely if continued for a sufficiently long period of time. This is indicative of some type of dissolution of the colored particles under the anodic treatment described.

The colored finish obtained after anodic treatment can be fixed by sealing in the conventional manner such as by boiling water, preferably containing dissolved nickel salts, or by other conventional means such as varnishing or the like.

Even when the anodic treatment appears completely to eliminate the temporary color eifect of the previously deposited colored particles of metal or metal compound, the sealing nevertheless reveals a new color effect, even when the boiling water sealing bath does not contain nickel salts.

The final color finish obtained is governed both by the temporary color effect obtained during the coloring treatment by alternating current electrolysis and by duration of the modifying anodic treatment. The appearance of the final color effect is somewhat related to the surface quality of the object before the layer of alumina is formed by anodization. An originally brilliant or shiny surface yields color finishes which will be brilliant. 0n the other hand, on a dull or mat surface, the final color will appear dull or mat, but it is therefore of advantage to modify the surface characteristics of the object before coating, such modification being effected by such well known means as polishing, butfing, satinizing, and the like, depending upon the final color effect that is desired. For example, a mat surface is obtained by cleaning the object for 10 minutes at 50 C. in a soda solution having a concentration of 50 g./liter, after which the object is Washed with running water, followed by immersion in nitric acid (36 B.) and then by further washing in running water. A brilliant appearance is obtained by mechanical polishing or by chemical or electrolytic glossing.

The following examples are given by way of illustration, but not by way of limitation, of the practice of this invention. In the following examples, the objects having a surface of aluminum or alloy of aluminum are generally subjected to the following:

(1) a surface preparation by one of the processes described above;

(2) anodization in a sulphuric acid bath;

(3) coloring with alternating current in an acid solution containing salts of a metal such as copper, nickel, cobalt and the like with counter-electrodes made of the constituent metal of the salts or stainless steel, at current densities within the range of 0.5 to a./dm. for periods ranging from 1 to 15 minutes; the result will be a black, dark red or deep bronze color finish;

: (4) anoclic treatment to modify the temporary color effect wherein the object is mounted as an anode of an electrolysis cell through which direct current or unfiltered rectified current is passed and in which the electrolyte contains sodium thiosulphate and/or other alkali metal or ammonium thiosulphate and then preferably,

though not essentially, by sealing by immersion in boiling water for 20 to 45 minutes with the boiling water optionally containing from 0.5 to 2 g./liter of nickel salt such as nickel acetate.

EXAMPLE 1 v A 99.5% pure and previously cleaned aluminum sheet is anodized by mounting as an electrode in a bath containing 200 g./liter of sulphuric acid for a period of 30 minutes at 20 C. at a current density of 1.5 a./dm. A

copper sulphate in solution for a period of minutes at p 12 volts with the result that a black color efiect is obtained. The sheet is then mounted as the anode in an electrolysis cell in which the electrolyte is an aqueous solution of sodium thiosulphate, Na S- O -5H O', having a concentration of 10 g./liter. Direct current is passed for a perod of 10 minutes at a density of 0.4 a./dm. with the temperature of the bath being about 20 C.

The sheet changes color to copper red which becomes a brick-like shade after 30 minutes treatment in boiling distilled water. The color effect that is obtained is uniform and resistant to corrosion and to ultra-violet rays.

EXAMPLE 2 An aluminum alloy object containing 0.5% of Si and 0.5 of Mg is mechanically surface polished and then anodized in a sulphuric acid bath to produce a porous surface layer of alumina having a thickness of 16 -microns. The object is then colored in an electrolysis cell fed with alternating current in which the electrolyte is formulated of an aqueous solution of 10 g./liter copper sulphate'in sulphuric acid solution. After 3 minutes treatment at 12 volts, an object which is dark red in color is produced; The object is then mounted as an anode in another electrolysis cell containing 15 g./liter of sodium thiosulphatein aqueous solution. After passing a direct current of 0.2 a./dm. for 15 minutes at C., the surface becomes colorless. After 30 minutes in boiling distilled water containing 2 g./liter of nickel acetate, the section assumes a deep yellow color which is highly uniform and resistant to corrosion and to ultra-violet rays.

EXAMPLE 3 A sheet of aluminum alloy containing 0.6% of Mg is provided chemically with a gloss surface finish and then anodized in a conventional manner in a sulphuric acid bath to provide a porous surface layer of alumina having a thickness of 20 microns. It is then mounted as an elec trode in an electrolysis cell fed with alternating current in which the electrolyte is an aqueous solution of 3-10% copper sulphate in sulphuric acid, until a dark red color effect is obtained. The colored sheet is then mounted as an anode in another electrolysis cell fed with continuous direct current in which the electrolyte is an aqueous solution of sodium thiosulphate in a concentration of 20 g./ liter. After 1 minute treatment at 0.6 a./dm. and' at 20 C., a colorless sheet is produced. After 40 minutes in boiling distilled water, a bright yellow color develops on the surface which is highly uniform and which is resistant to corrosion and to ultra-violet rays.

EXAMPLE 4 An object of an aluminum alloy containing 1% Si, 1% Mg and 1% Mn is electrolytically polished to provide a gloss surface finish and then anodized in a sulphuric acid bath to provide a porous surface layer of alumina having a thickness of 8 microns. The anodized layer is then colored in a sulphuric acid solution of 15% by weight copper sulphate, using alternating current, until a black color finish is obtained. The section is then mounted as the anode of another electrolysis cell containing an aqueous solution of ammonium thiosulphate in a concentration of 12 g./liter. The cell is fed with rectified unfiltered current of 0.2 a./dm. for 10 minutes at 20 C. The color intensity is reduced as by partial dissolution. It takes on a brick-like color after treatment in boiling distilled water containing 1 g./liter of nickel acetate. The brick-like color is highly uniform and resistant to corrosion and to ultra-violet rays.

It will be apparent from the foregoing that there is provided a new and economical process for permanently coloring the surfaces of aluminum and alloys of aluminum with a wide variety of colors, many of which have not heretofore been capable of being obtained, and in which the color is uniform and reproducible while being highly resistant to corrosion and to ultra-violet rays.

It will be understood that changes may be made in the details of formulation and operation without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. In a process for color coating objects of aluminum and alloys of aluminum in which the surface to be colored is provided with a porous anodized aluminum oxide layer, the steps of depositing colored particles of a metal or a compound of a metal by alternating current electrolysis of the object mounted as an electrode in an electrolysis bath containing a salt of the metal in solution, mounting the object with the colored particles as an anode in an electrolysis bath containing a thiosulphate in solution while passing direct or unfiltered current therethrough to modify the color effect of the colored particles previously deposited.

2. The process as claimed in claim 1 in which the porous aluminum oxide layer is provided on the surface by anodizing in a sulphuric acid bath. 7

3. The process as claimed in claim 1 which includes the step of sealing the colored coating that is formed.

4-. The process as claimed in claim 3 in which the colored coating is sealed with boiling water.

5. The process as claimed in claim 3 in which the colored coating is sealed with boiling water containing a nickel salt in solution.

6. The process as claimed in claim 5 in which the nickel salt is in the form of nickel acetate and present in an amount within the range of 0.5 to 2 g./liter.

7. The process as claimed in claim 1 in which the thiosulphate is an alkali metal or ammonium thiosulphate.

8. The process as claimed in claim 6 in which the thiosulphate is present in the electrolysis bath in an amount within the range of 1 to g./liter.

9. The process as claimed in claim 6 in which the thiosulphate is present in the electrolysis bath in an amount within the range of 5 to 20 g./liter.

10. The process as claimed in claim 1 in which the cathode in the electrolysis cell wherein the object is mounted as the anode is selected from the group consisting of platinum, stainless steel, copper and nickel.

11. The process as claimed in claim 1 in which the current is passed through the electrolysis cell for anodic treatment of the object at a current density of 0.1 to 5 a./dm.

12. The process as claimed in claim 1 in which the current is passed through the electrolysis cell for anodic treatment of the object at a current density of 0.1 to 1 a./dm.

13. The process as claimed in claim 1 in which the current that is passed through the electrolysis cell during anodic treatment has a voltage within the range of 10 to 80 volts.

14. The process as claimed in claim 1 in which the current that is passed through the electrolysis cell during anodic treatment has a voltage within the range of to 30 volts.

'15. The process as claimed in claim 1 in which the electrolysis bath during anodic treatment is maintained at a temperature within the range of 15 to 50 C.

16. The process as claimed in claim 1 in which the electrolysis bath during anodic treatment is maintained at a temperature within the range of 18 to 25 C.

17. The process as claimed in claim 1 in which the anodic treatment of the object in the electrolysis cell is continued for a period within the range of /6 to 30 minutes.

18. The process as claimed in claim 1 in which the anodic treatment of the object in the electrolysis cell is continued for a period within the range of /2 to 15 minutes.

19. The process as claimed in claim 1 in which the metal salt which is dissolved in the electrolysis bath for the deposition of colored particles into the porous surface of alumina is a salt of a metal selected from the group consisting of copper, nickel and cobalt.

20. The process as claimed in claim 19 in which the metal salt is dissolved in the bath in an amount greater than 0.15 g./liter to an amount to form a saturated solution.

21. The process as claimed in claim 19 in which the alternating current has a density within the range of 0.5 to 5 a./dm.

22. The process as claimed in claim 1 which includes the step of subjecting the object to be colored to a surface treatment prior to electrolysis treatment to modify the appearance of the colored coating that is formed.

23. The process as claimed in claim 1 in which the metal compound deposited in the porous layer of alumina contains a compound of copper.

24. An object of aluminum or alloy of aluminum having a uniform colored coating produced by the method of claim 1.

References Cited UNITED STATES PATENTS 3,382,160 5/1968 Asada 204- N FREDERICK C. EDMUNDSON, Primary Examiner