US3669856A

US3669856A - Process for the production of colored protective coatings on articles of aluminum or aluminum alloys

Info

Publication number: US3669856A
Application number: US835668A
Authority: US
Inventors: Ove Christopher Gedde
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-06-21
Filing date: 1969-06-23
Publication date: 1972-06-13
Anticipated expiration: 1989-06-13
Also published as: DK126210B; GB1257047A; DE1930288B2; FI47385C; FR2011376A1; DE1902983A1; FI47385B; JPS4934287B1; DE1902983B2; NL152304B; DE1930288A1; SE351440B; NL6909024A; DE1902983C3; ES368153A1; AT285272B; CH520206A; BE734886A

Abstract

A process is applied for the production of colored, protective coatings on articles of aluminum or aluminum alloys by passing an alternating current between a previously anodically oxidized aluminum article and a counter-electrode immersed in an acid aqueous bath containing salts of metals capable of coloring the aluminum oxide layer, the alternating voltage supplied being modulated as to its amplitude and/or frequency so as to make it asymmetrical thereby to control the color tone on the aluminum article. The asymmetrical voltage may be produced, for example, by supplying two or more alternating voltages of different amplitude, frequency and/or phase, or by using a direct voltage superimposed on the alternating voltage.

Description

0 United States Patent 1 1 3,669,856 Gedde June 13, 1972 [54] PROCESS FOR THE PRODUCTION OF 2,901,412 8/1959 Mostovych et a1 ..204/58 COLORED PROTECTIVE COATINGS 2,443,599 6/1948 Chester ....204/58 0N 2,231,373 2/1941 Schenk ALUMINUM ALLOYS FOREIGN PATENTS OR APPLICATIONS [72] Inventor: Ove Christopher Gedde, Gaupefaret 24, 741,753 5/1943 Germany ..204/58 1713 Gralum, Norway 69,930 1/1946 Norway ....204/58 Filed: June 1969 662,063 4/1963 Canada ..204/58 211 App] 35 Primary ExaminerJohn H. Mack Assistant ExaminerR. L. Andrews Attorney-Waters, Roditi, Schwartz & Nissan [30] Foreign Application Priority Data June 21, 1968 Norway ..2459/68 [57] ABSTRACT A process is applied for the production of colored, protective [52] US. Cl ..204/58, 204/128 coatings on articles of aluminum or aluminum alloys by [51] ..C23b 9/02, BOlk 1/00 passing an alternating current between a previously anodically 5 8] Field of Search ..204/58, 128 oxidi ed alumin m a ticle d a cou e -elect de immersed in an acid aqueous bath containing salts of metals capable of 56] References Ci coloring the aluminum oxide layer, the alternating voltage supplied being modulated as to its amplitude and/or frequency UNITED STATES PATENTS so as to make it asymmetrical thereby to control the color tone on the aluminum article. The asymmetrical voltage may be 3,418,222 12/1968 Schaedel ..204/58 produced, for example by Supplying two or more alternating 1 6O 5/1968 A Sada "204/58 voltages of different amplitude, frequency and/or phase, or by L978 9/1960 D 'ckson et 204/58 using a direct voltage superimposed on the alternating voltage. 2,951,025 8/1960 Mostovych et al. ..204/58 2,935,454 5/1960 Tokumoto ..204/45 7 Claims, 4 Drawing Figures PROCESS FOR THE PRODUCTION OF COLORED PROTECTIVE COATINGS ON ARTICLES OF ALUMINUM OR ALUMINUM ALLOYS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for the production of colored protective coatings on articles of aluminum or aluminum alloys which have previously been provided with a protective oxide layer.

2. Description of the Prior Art With the increasing use of articles of aluminum or aluminum alloys both for indoor and outdoor use, a number of processes have been developed for surface treatment of such articles in order to make them decorative and/or resistant to abrasion and atmospheric influence.

Thus, there was early developed a process for chemical coloring of aluminum alloys with aniline dyes. The thus colored articles had poor resistance towards atmospheric influence. The coloring was carried out on aluminum articles which had previously been anodically oxidized.

There has further been developed a process whereby aluminum articles are subjected to anodic oxidation and immersed in chemicals which penetrate into the pores of the oxide layer, whereupon the thus treated aluminum article is placed in aqueous solutions of salts which also penetrate into the pores and there combine with the first used chemical. The practical realization of this process has proven difficult, and the process is essentially of theoretical interest only.

Processes have also been developed for simultaneous anodizing and coloring of aluminum articles. With these processes only a limited selection of colors is obtainable. The processes are expensive and difficult to carry out, and very rigid requirements are made for the working and heat-treatment of the aluminum articles, as the metallic structure therein is of the utmost importance for the result obtained. The processes also demand the use of large currents and high voltages and a long time, and they are relatively expensive.

From German Pat. application No. 562,615 it is known to impart a colored coating on aluminum or aluminum alloys by means of an acid bath containing chromic acid, wherein an article of aluminum or aluminum alloy having a previously applied oxide layer is used as a counter-electrode. The counter electrode and the article to be coated are immersed in the bath containing chromic acid, and an alternating current is passed between the counter-electrode and the article to which the protective coating is to be applied. This method is uneconomical and offers few possibilities for selection of colors.

According to German Pat. specification No. 655,700 aluminum articles which have already been provided with an oxide layer may be provided with a further, opaque, white, protective layer by electrolytic treatment of the article with alternating current or direct current in an aqueous bath containing dissolved titanium salts. The deposited, opaque film may then be colored with organic dyes in aqueous or organic solvents. The process is expensive and cumbersome.

British Pat. specification No. 1,022,927 relates to a process for the production of colored, protective coatings on articles of aluminum or aluminum alloys by passing an alternating current through an aqueous bath containing a salt yielding a colored metal oxide or hydroxide between a previously oxidized article of aluminum or aluminum alloy and a counterelectrode of carbon, carborundum or aluminum. This process is limited to the use of particular counter-electrodes in order to obtain the colored coatings, and the process has little flexibility as regards the obtainable color tones.

BRIEF SUMMARY OF THE INVENTION It has now suprisingly been found that by passing an alternating current between a previously anodically oxidized article of aluminum or aluminum alloy and a counter-electrode immersed in an acid aqueous bath containing metal salts having coloring cations the color tones of the coatings can be controlled in a simple manner by modulating the shape of the curve of the applied alternating voltage in such a manner that during the coloring process the alternating voltage will provide a suitable ratio between the two current directions for an advantageous transport of material and course of reaction with regard to the previously anodically oxidized aluminum article.

The present invention thus provides a process for the production of colored protective coatings on articles of aluminum or aluminum alloys by passing an alternating current between a previously anodically oxidized aluminum article and a counter-electrode immersed in an acid aqueous bath containing salts of metals capable of coloring the alumina layer, and the process is characterized in that the alternating voltage supplied is modulated as regards its amplitude and/or frequency so as to make it asymmetrical thereby to control the color tone on the aluminum article.

The modulation of the alternating voltage can be carried out in several ways, e.g., by simultaneously supplying two or more different alternating voltages or a superimposed direct voltage or by generating an alternating voltage having the desired frequency and curve shape.

As the material for the counter-electrode, stainless steel or titanium are preferably used because they are practically insoluble in the acid bath used for the coloring and because they lead to an advantageously low energy consumption.

It is assumed that the incorporation of metal salts in the oxide layer of the aluminum article is due to deposition and dissolution of metal at the surface of the aluminum article caused by the alternating current as the current changes its direction, however, in such a manner that some of the metal forms chemical compounds with other substances, e.g., oxygen, sulphur, hydroxyl groups or other groups, and remains in the pores of the aluminum oxide layer.

The strength of the alternating voltage and the modulation of the amplitude and/or frequency thereof according to the present process is from 5 to 50 volts dependent upon the composition of the electrolyte and the properties of the oxide layer previously formed. Preferably there is used a current density of 0.1 to 0.5 A/dm, dependent upon the electrolyte employed, and a low treatment period of 1 to 10 minutes.

BRIEF DESCRIPTION OF THE DRAWING FIG. la schematically shows a circuit diagram which can be used to modulate the applied alternating voltage according to the present process;

FIG. lb shows the curve of a such modulated alternating voltage;

FlG.2a shows an alternative embodiment of a circuit diagram which can be used for supplying a modulated alternating voltage in the present process; and

FIG. 2b shows one form of the curve of an alternating voltage modulated by using the circuit of FIG. 2a.

DETAILED DESCRIPTION In FIG. 1 S and S are two oppositely directed rectifiers coupled in parallel relationship with regard to one another, each rectifier being connected in series with a rheostatR and R Substantially the same circuit diagram as in FIG. 1 is shown in FIG. 2, however, here two controlled rectifiers T and T are used. These are arranged such that in a controllable manner they can be respectively used to disrupt the current in a part of each of the positive and negative half periods of the alternating current. It is then possible to omit the rheostats shown in FIG. 1, and there is obtained a control of the two currents in respectively positive and negative direction through the electrolysis bath essentially without any energy losses.

In order to be able to regulate these two currents in the positive and negative directions and the pertaining potentials between the electrodes of the electrolysis vessel, the circuits are provided with an ammeter A in series with each rectifier and with two voltmeters V connected in series respectively with oppositely directed rectifiers corresponding in direction with the rectifiers S and S or T, and T The circuits shown can be used for voltages of from to 200 volts, preferably 0 to 50 volts, for current strengths of from 0 to 10,000 A, preferably 0 to 1,000 A, and for a frequency of 5 to 500 cycles per second, preferably 50 cycles per second.

Since in the rheostats used in the circuit according to FIG. 1 an appreciable loss of electric energy will occur, it is preferred to use a circuit as shown in H6. 2 and comprising controlled rectifiers. By the term controlled rectifiers" as used herein is meant such rectifiers which can be made current carrying by the use of an auxiliary potential which is synchronous with the main voltage, i.e., thyristors. By the use of two such thyristors in parallel it is possible in a controllable manner to cut off a part of the positive and negative half waves of the alternating current. Thus, it is possible to regulate the effective value of the two half waves.

In the present process it has been found by the use of a number of electrolytes that it is possible in a simple manner to control the amount of the deposited, coloring compounds in the oxide layer and the color tone caused thereby. The colored coating is very resistant towards abrasion and has a very high resistance towards atmospheric influence, and the aluminum articles produced by the present process have proven to be particularly well suited for outdoor use.

EXAMPLE 1 An aluminum article with a previously oxidized surface was immersed in an aqueous bath containing 2.0 percent sulphuric acid, 7 percent aluminum sulphate and 1.5 percent copper sulphate. An electrode of stainless steel was used as a counterelectrode.

Alternating current was passed through the electrolyte at a voltage of 20 volts between the aluminum article to be colored and the counter-electrode. The current density was 0.4 Aldm The applied alternating voltage was made asymmetrical from two different A.C. sources applied simultaneously and having different phases in such a manner that the total negative half wave of the alternating voltage with regard to the aluminum was greater than the positive one. A deep red color was obtained in 10 minutes.

EXAMPLE 2 A previously oxidized aluminum article, the same counterelectrode and the same electrolyte as in Example 1 were used.

The alternating current was now supplied in such a manner that the positive half wave of the alternating voltage was greater than the negative one.

The coloring was carried out in the same time and with the same voltage and current density as in Example 1. There was obtained a red color which was less intense than the color obtained in Example 1.

EXAMPLE 3 An aluminum article which had previously been anodically oxidized in aqueous sulphuric acid was immersed in an electrolyte containing 1.5 percent boric acid and 1 percent cadmium sulphate. A lead electrode was used as a counter-electrode.

During the first 10 seconds of the coloring, a symmetrical alternating voltage was supplied to the aluminum article to be coated and to the counter-electrode. The alternating voltage was then modulated so as to make its negative half wave greater with regard to the lead electrode than its positive half wave. The alternating voltage was made asymmetrical by superimposing thereon a rectified alternating voltage.

The current strength remained substantially constant, and a deep, black color was obtained on the aluminum.

The current density used was adjusted with regard to the alumina layer and was 0.4 A/dm. if too high current densities are used the previously formed oxide layer on the aluminum will be damaged. The treatment period was 10 minutes.

EXAMPLE 4 An aluminum article which had previously been anodically oxidized in aqueous sulphuric acid was treated in the same electrolyte as in Example 3, and the same counter-electrode, alternating voltage and current density were used.

During the first 2 minutes a symmetrical alternating voltage was used, and the alternating voltage was then made asymmetrical and the coloring continued for 8 minutes. Thus, the total treatment took 10 minutes. A brown color was obtained on the aluminum article.

EXAMPLE 5 An aluminum article which had previously been anodically oxidized was used in an electrolyte together with a counterelectrode as described in Example 3. The same alternating voltage and current density were also used. The alternating voltage supplied was modulated from the beginning so as to be asymmetrical, and after a treatment period of 10 minutes there was obtained a beige color on the aluminum article.

The aluminum articles colored by using the present process have proven to possess a coating which is very resistant to abrasion and atmospheric influence. By means of the modulation of the amplitude and/or frequency of the applied alternating voltage according to the present process, the color tone of the aluminum article can also be varied within a very broad range of color tones, and the necessary time for obtaining a colored coating which is very resistant towards abrasion and atmospheric influence can be kept very short. There is thereby obtained a considerable saving both as regards labor and ener gy consumption.

It is particularly preferred initially and for a brief period to supply a symmetrical alternating voltage and then an asymmetrical alternating voltage.

In addition, the coloring takes place faster and more efficiently if the alternating voltage is regulated relatively slowly, of the order of a few seconds, from zero to the voltage which is decided for the coloring. This relates both to the starting up of the coloring and to a later supply of another alternating voltage than the one initially used.

What is claimed is:

l. in a process for the production of colored, protective coatings on articles of aluminum or aluminum alloys, said process comprising passing an alternating current between an electrode system comprising a previously anodically oxidized aluminum article and a counter-electrode immersed in an acid aqueous bath containing salts of metals capable of coloring the aluminum oxide layer, the improvement wherein an alternating voltage with controlled asymmetry is applied to the electrodes whereby to control the color tone of the aluminum article, the alternating voltage having been made asymmetrical by modulating the alternating voltage externally of said electrode system.

2. A process claimed in claim 1, wherein initially a symmetrical alternating voltage is passed followed by the asymmetrical alternating voltage.

3. A process as claimed in claim 1, wherein the asymmetrical voltage is produced by supplying at least two alternating voltages of different amplitude, frequency and/or phase.

4. A process as claimed in claim 1, wherein the asymmetrical alternating voltage is produced by superimposing a direct voltage on the alternating voltage.

5. A process as claimed in claim 1, wherein the alternating voltage is regulated relatively slowly from zero to the voltage used for the coloring.

6. A process as claimed in claim 1, wherein the alternating current is passed through two oppositely directed rectifiers connected in parallel each connected in series to a rheostat, and regilating the current by means of the two rheostats.

7. A process as claimed in claim 1, wherein the alternating current is passed through two oppositely directed controlled rectifiers connected in parallel, and controlling the rectifiers so as to disrupt the current during a pan of each of the positive and negative half waves of the alternating current.

Claims

2. A process claimed in claim 1, wherein initially a symmetrical alternating voltage is passed followed by the asymmetrical alternating voltage.
3. A process as claimed in claim 1, wherein the asymmetrical voltage is produced by supplying at least two alternating voltages of different amplitude, frequency and/or phase.
4. A process as claimed in claim 1, wherein the asymmetrical alternating voltage is produced by superimposing a direct voltage on the alternating voltage.
5. A process as claimed in claim 1, wherein the alternating voltage is regulated relatively slowly from zero to the voltage used for the coloring.
6. A process as claimed in claim 1, wherein the alternating current is passed through two oppositely directed rectifiers connected in parallel each connected in series to a rheostat, and regulating the current by means of the two rheostats.
7. A process as claimed in claim 1, wherein the alternating current is passed through two oppositely directed controlled rectifiers connected in parallel, and controlling the rectifiers so as to disrupt the current during a part of each of the positive and negative half waves of the alternating current.