CA1155080A - Anodizing aluminium asymmetrically in sulfuric acid, polyhydric alcohol and polycarboxylic acid - Google Patents

Abstract

COATING SYSTEM
Abstract of the Disclosure A novel process is disclosed for the production of colored coatings on articles of aluminum or aluminum alloys. The process involves first forming a hard, dense anodic coating of between about .2 to about 1.1 mils on the aluminum or aluminum base alloys by anodizing the aluminum in an acidic aqueous electrolyte comprising sulfuric acid, a polyhydric alcohol of from 3 to 6 carbon atoms and an organic polycarboxylic acid containing at least one reactive group in the alpha-position coloring the coating by applying alternating current to an electrode system comprising the coating and a counterelectrode immersed in an acidic aqueous bath containing a metal salt while modulating the voltage externally of said electrode system so as to apply voltage with controlled asymmetry to said electrodes.

Description

~15S~80 Background of the Invention 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 been previously anodized in a special way in order to obtain products which are particularly suitable for use in architectural applicati~ns.

Description of the Prior Art - 10 Much time and attention devoted in the prior artto the production of aluminum articles which are deco-rative and resistant to abrasion under atmospheric influence. Early processes have included chemical coloring of aluminum articles which had previously been anodically anodized by the treatment of the same with dyes, such as aniline dyes. As the art is wcll awarej the thus resulting a~ticles have poor resistance towards atmospheric influence. Other developments have included anodic oxidation of aluminum articles, followed by submersion in chemicals which penetrate into the pores of the oxide layer, so that when the thus treated aluminum article is placed in.aqueous solutions of salts which also penetrate into the pores, combination with the first used chemical is possible. These processes have not proven practical for a wide variety of reasons.
It is also known in the prior art to simultaneously anodize and color aluminum articles. ~owever, the art is aware that processes of this type result in only a limited selection o colors and that the processes are expensive and difficult to carry out and very rigid requirements are made for the working and heat treatmen~t of the aluminum articles as the metallic structure therein is of the utmost importance forthe result obtained. These simultaneous processes also demand the 3 ~ ~ .

llSS080 use of large currents and high voltages and long times and are thus relatively-expensive.
United States Patents 3,669,856; 3,769,180 and 3,849,263 represent recent developments in the field of coloring aluminum or aluminum alloys. These patents are, in general, directed towards the coloring of anodized aluminum by immersing said article in a bath containing a salt of a particular metal and passing an alternating current between the previously anodized article and a counterelectrode.
Although the process of these patents represent a significant improvement in the field of coloring alumi-num,nevertheLess, no details are given as to how the previously formed anodic coating is formed on the aluminum and, in fact, at least the implication is present that conventional anodizing techniques are used.
It is also well known in the art of anodizing alumi-num that two separate and distinct types of an oxide layer can be obtained which are generally referred to in the art as a hard coat or a soft coat. The conventional anodizing techniques utilized in the art result in the production of a so-called soft coat. There are processes known in the art for the production of hard, dense anodic coatings, but the techniques employed in the art for the subsequent dyeing of these hard, dense coatings have involved the conventional immersion in a suitable dye, as opposed to an electrolytic coloring process.
The explanation for this might be the fact that the techniques for the production of hard anodized coatings result in the production of anodic layers which are significantly colored and thus darker and muddier colors are only obtainable by the use of organic or inorganic dyestuffs. The art is also aware that the l~S~:)8~ . -thicker the anodic l~yer is formed that the darker theanodic layer will b~ and, in general, those processes which produced hard anodic layers had as one of their criticalities the production of a thick layer. These S thick layers of anodic film are totally unsuitable for the novel process of this inve~tion.
As has heretofore been stat~d, there are processes I known in the art for the production of a hard anodized layer. Of such known processes, those which have the anodizing bath at low temperature (around 32F) or intermediate temperature (around 45F) are unsuited for the use of the novel process of this invent iOII for several reasons. In the first place, these processes are expensive and require substantial energy in order to operate. Further, these processes produce an anodized layer which is relatively thick (customarily 1.5 mil or ~ heavier) in order to obtain high uear resistance, and - which is a darkish, muddied color thereby rendering it unsuitable for use where light, unmuddied colors are desired. On the other hand, the known hard coat process which operates at room temperature (with the anodizing bath at about 70F) uses a higher current density than that of the present invention, resulting in the produc-tion of an anodized layer unsuitable for use in pro-ducing a wide range of colors.
U.S. Patent 3,524,799 is directed towards a room - temperature process for anodiæing aluminum in order to produce hard, dense anodic coatings and the novel process of the present invention utilizes as one step thereof a modification of the process disclosed by this patentee.

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The specification and claims of this patent are directed to the formation of hard, dense anodic coatings on aluminum or aluminum alloys by anodizing the aluminum in an aqueous electrolyte containing a mineral acid, such as sulfuric acid, a polyhydric alcohol of 3 to 6 carbon atoms, an organic carboxylic acid containing at least one reactive group in the alpha-position to the carboxylic acid group, such as lactic acid or glycine, and an alkali salt of a titanic acid complex of a hydroxyaliphatic carboxylic acid containing from 2 to 8 carbon atoms, such as, for example, titanium dilactate ammonium salt.
We have now discovered that the use of such anodizing techniques without the al};ali salt of a titanic acid complex provide extremely dense and hard anodic coatings 1~ optimally suited to architectural applications and that such anodized layers when color-ed using the tcchniques described in U.S. Patent No. 3,669,~56 provide aluminum and aluminum alloy surface of very pleasing, architec-turally pure colors of exceptional uniformity. Addition-ally, the use of the combination of these prior arttechniques apparently provides exceptional throwing power in the coloring operation. Throwing power is a term of art defining the ability of a coloring bath and process to provide color uniformly to all surfaces of the surface of a workpiece undergoing coloring. Thus, a process and bath which demonstrates high throwing power provides uniform .color to small creases, cracks, nooks, detents, etc., as well as the larger uniform surfaces of an aluminum or aluminum alloy workpiece being colored.
High throwing power also permits the introduction into the coloring bath of a mix of workpieces in terms of their alloy composition and overall physical configur-ation to obtain uniform color of all such workpieces. In 11~i5~8~:) prior art coloring techniques it was often difficult, if not impossible to obtain uniform coloring of work-pieces of different alloys or shapes in a single coloring bath at the same time. Furthermore, as is well recog-nized by the skilled artisan in the aluminum coloring field, spacing of the various workpieces in the coloring bath has been a critical factor in successfully uniformly coloring aluminum extrusions, particularly for archi-tectural purposes. Such spacing restraints often required leaving sufficient distances between the undivided pieces being colored that substantial portions of the working volume of a given coloring tank were left empty during a coloring operation resulting in very inefficient use of coloring tank capacity. The excep-tional throwing power of the technique of the instant invention permits minimal spacing of the workpieces in the coloring ba*h and thus optimum usage of the coloring capacity of a coloring tank. This results not only in a more optimum efficiency in terms of use of tank capacity, bu~ reduces substantially the chemical and power require-ments of the electrolytic coloring process.
According to the present invention, a novel process is disclosed for the production of colored coatings on articles of aluminum or aluminum alloys which are particularly adapted to be employed for architectural uses which involves first forming a hard, dense anodic coating on aluminum and aluminum base alloys by anodizing the aluminum in a specific electrolyte comprising sulfuric acid, a polyhydric alcohol of 3 to 6 carbon atoms and an organic carboxylic acid containing at least one reactive group in the alpha-position in order to ob-tain a material having a film thickness of between about 0.2 to about 1.1 mils and thereafter electrolytically .

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coloring said anodized aluminum by passing alternating current between said anodized aluminum and a counter-electrode immersed in an acidic aqueous bath and a metal salt, such as tin, while modulating as to amplitude or frequency the alternating voltage externally of said electrode system so as to apply voltage of controlled asymmetry of said electrodes.
In order to obtain the architecturally acceptable and desirable hard anodic coatings of pure clean color as described above, it is absolutely critical that the anodic layer be between about .2 and about 1.1 mils in thickness, as opposed to the 1-5 mils set forth at column 3, line 26 of said U.S. Patent 3,S24,799.
As already pointed out, the electrolyte used to anodize the aluminum must be of the type described in U.S. Patent No. 3,524,799 without any alkali salt of titanic acid complex.
Apparently, as described in U.S. Patent No. 3,524,799, the combination is an anodizing bath of a polyhydric alcohol containing from 3 to 6 carbon atoms, and an organic carboxylic acid containing a reactive group in alpha-position to the carboxylic acid group will react with the hot reaction products formed during anodizing with or adjacent to the surface of the pore base, and thereby suppress the attack or dissolution of the forming oxide film by these products.
The mineral acid component of the electrolyte is sul-furic acid. The anodizing bath concentration of sul-furic acid is generally maintained between about 12% and about 20% by weight, preferably about 15%.
Polyhydric alcohols containing from 3 to 6 carbon atoms which may be employed in the practice of the invention, singly or in admixture, include glycerol, r~
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ll~S~80 butane-diol-1,4, pentanediol-l,S, mannitol and sorbitol.
The total arnount of polyhydric alcohol employed ran~es from about 1~ to about 4% by volume of the anodizing electrolyte. The preferred polyhydric alcohol is S glycerol.
The organic carboxylic acids containing a reactive group in alpha-position to the carboxylic acid group include acids in which the reactice group is hydroxy, amino, keto, or carboxyl. Examples of such acids include glycolic (hydroxyacetic), lactic (hydroxypro-pionic), malic (hydroxysuccinic), oxalic, pyruvic, and aminoacetic acids. Acyclic carbo~ylic acids such as lactic, malic, and amino-acetic (glycine) acids are preferred. A mixture of two or more of these acids may be employed in combination with the mineral acid and the polyhydric alcohol. The amount of- carboxylic acid included in the electrolyte is preferably between about lg and about 4% by volume of the bath.
Glycolic acid is specifically preferred as the carboxylic acid.
- In order to achieve the results described above, the temperature at which anodizing is carried out must range from about 65 to about 85F with room temperature con-dition, i.e., 68-75F being preferred.
In order to achieve the exceptionally hard and readily colored anodic coatings, it is also necessary - that the current density which is used in the anodizin~
operation should be in the range of from about 24 to about 36 amperesfsq. ft.
The time required to achieve the desired film thic~ness of between about 0.2 and 1.1 mils will vary with the other parameters of temperature, current ! density, chemical composition of the bath, etc., but generally anodizing times on the order of from about 1 .
. '' I -A-, 1155~)80 8 to about 30 minutes produce acceptable results.
Following the special anodizing treatment, above-described, the aluminum article is thereafter colored electrolytically by passing alternating current between said article and a counterelectrode in an aqueous acidic solution containing a water soluble metal salt.
~ he particular manner of electrolytically coloring the anodized aluminum in accordance with the novel process of this invention is set forth in United States 3,669,856. It has been found that if the aluminum is anodized in the manner above-described and thereafter electrolytically colored in accordance with the teachings of U.S. 3,669,856 many significant advan-tages will be obtained as opposed to the use of conven-tional anodizing techniques. In the first place, the article which is obtained has a hard coating which makes it particularly adapted to be used in architectural applications. Additionally, the process of this in-vention permits the simultaneous electrolytic coloring of articles of varying sizes and shapes. This has been difficult if not impossible, to achieve in prior art processes due to the fact that uneven color was obtained when articles of different sizes and shapes were simul-taneously electrolytically colored. Another advantage of the novel process of this invention resides in the act that the aluminum article to be colored need only have electric contact at one edge thereof, as opposed to both edges. This results in a sïgnificant manpower 115S08~

savings. Another advantage of the novel process of this invention resides in the fact that it is possible to correct for too dark a color electrolytically which has heretofore been impossible with processes utilizing dyes or with processes involving simultaneous anodizing and coloring. According to this technique, after application of an excess of color the polarity of the coloring system is reversed and color can be subtracted from the anodized layer.
As has heretofore been stated, the electrolytic coloring process is carried out by passing an alternating current between the anodized article of aluminum or aluminum alloy and counterelectrode, which has been carried out in the manner above-described, and a counterelectrode immersed in an acid aqueous bath containing metal salts having coloring cations, wherein the colored 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 to achieve an advantageous transport of material and course of reaction with regard to said anodized aluminum article. The alternating voltage supplied is modulated with regard to its amplitude and/or -frequency so as to make them asymmetrical, thereby to control the color tone of the aluminum article. As is known in the art, the modulation of the alternating voltage can I be carried out in several ways, such as simultaneously ¦ supplying two or more different alternating voltages or ¦ 30 a superimposed direct voltage or by generating an ! alternating voltage having the desired frequency and curve shape.

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The material for the counterelectrodes can be stainless steel, titanium, copper, nickel, but prefer-ably tin because they lead to advantageously low - energy consumption.
The strength of the alternating voltage in S the modulation of the amplitude and/or fre~uency thereof according to the present process is from 5-50 volts, depending upon the composition of the electro-lyte and the properties of the oxide layer previously formed. Preferably there is used a current density of rom 0.1 to O.S A/dm2, dependent on the electrolyte employed and a low treatment period of from 1 to 10 minutes As is ~nown in the art, various soluble metallic salts can be employed. The preferred salts are those of tin, althouyh salts of nickel, cobalt, copper, silicomolybdic acid and silicotungstic acid can also be employed. The electrolytic coloring bath also contains a strong acid which is desirably either sulfuric or hydrochloric.
As is well known in the art, the metallic salts, e.g., sulfates, chlorides, acetates, etc. desired to provide the particular color can be~utilized at a concentration of from 0.5 to 20~ by weight, preferably about 2~ by weight based on the electrolyte. The pH
of the electrolyte may vary considerably within the acid range, but pHs of about 1.5.have been found to be useful.
A particularly preferred embodiment resides in having present in the electrolyte a certain amount of aluminum. In this connection, the aluminum can be provided by the addition of suitable aluminum compounds, such as aluminum sulfate or a certain part of a previously used electrolytic bath can also be used.
The amount of aluminum which is present in the electro-..~,,~,.
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- ~15508() lyte can range from 0-12 grams/liter, and more desir-ably, from 4-8 grams/liter.
As has heretofore been pointed out, the novel process of this invention is applicable to color articles made from aluminum, as well as from aluminum base alloys of all kinds.
It is particularly preferred initially to supply a symmetrical alternating current and then add asymmetrical alternating current.
In addition, the coloring takes place faster, more efficiently if the alternating current is regulated relatively slowly and in particular of the order of a few seconds, from zero voltage to the voltage which is desired for the coloring. This relates both to the alternating current at the start up of the coloring and to a later supply of alternating current at the voltage desired for coloring.
Further improvement in the throwing power of the coloring solution of the instant invention can be achieved by incorporation of material which serves as a complexing or sequestering agent for the coloring metal ion. Although the mechanism for this further improvement is not fully understood, it has been found that the addition of small amounts on the order of 5x10-5 to 5xlO 3 grams/liter of, for example a combination of ~-naphthol and gelatin in a ratio of -about 2:1 naphthol to gelatin or 4,4 di(dimethyl-amino) diphenyl methane to the coloring bath provides even more improved coloring baths.
The following examples will illustrate the novel - 30 process of this invention.

,--llS5080 An aluminum article was anodized in accordance with normal anodizing techniques utilizing a current density of 24 amperes/sq. ft. and an electrolytic bath comprising 20 weight percent sulfuric acid, and 8 grams/liter of oxalic acid. The temperature utilized ranged from 18-21C, and the resulting aluminum article had an anodized layer of 25 microns. The resulting product was not suitable for coloring due to the fact that it was darkish in color.
As is obvious from the above experiment, the anodizing solution used was other than that of the instant invention.

An aluminum article was anodized using a solution compxising 18 weight percent sulfuric acid, 1% glycolic acid and 1% glycerol. The anodizing was carried out at a current density of 36 amperes/sq. ft. at a tem-perature of about 19.5C. After 13 minutes an anodized layer of appro~imately 0.83 mils was obtained.
The anodized aluminum article was then electro-lytically colored by immersing the same into a bath comprising 25 grams/liter sulfuric acid, 22 grams/liter sulfonic acid, 25 grams/liter tin sulfate, 5 grams/liter aluminum sulfate, 0.2 grams/liter of ~-naphthol and 0.4 grams of gelatin per liter. The electrolytic coloring was carried out by applying alternating current through the electrolyte at a voltage of 8 volts for three minutes. Three minutes of alternating current of half-wave was then applied.

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An aluminum article having a blackish color was obtained.
EXAMPL~ 3 An aluminum article was anodized utilizing the electrolyte solution of E~ample 2 at a current density of 40 amperes/sq. ft. at a temperature of 20C.
The anodized article which was obtained was thereafter electrolytically colored in accordance with the techniques of United States 3,669,856.
This resulted in an article having poor color.
' ' EXAMPLE ~
An aluminum article was anodized utilizing the anodixing solution-set forth in ~xample 2 at a temperature of 20C and at a current density of 48 amperes/sq. ft. The anodizing was carried out until an anodized layer was obtained which had a thic~ness of about 1.65 mils. Subsequent color anodizing of this material in accordance with the techniques of this invention resulted in spalling on the anodic film.
It is apparent that the thickness of the anodized layer obtained during the anodizing step was simply too great to produce the satisfactory product desired by the techniques of this invention.

An aluminum article was anodized utilizing the electrolyte solution of Example 2 at a temperature of 21C until an anodized layer having a thickness of about .8 mils was obtained.
This material was then electrolytically colored utilizing the techniques of United States 3,669,856 ; ~ and the color solution of Example 2. Alternating ~ ; current was applied for 1 1/2 minutes and thereafter . ~ y~
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a half-wave alternating current was applied for a hal~-minute. The resulking material was colored satisfactorily and was capable for use as an archi-tectural material.
.

An aluminum article was anodized using the electrolytic solution of Example 2 at a temperature of 20C for six minutes in order to obtain an article which had a thickness of approximately 0.4 mils.
This material was then electrolytically colored utilizing the solution of Example 2 by passing normal AC current between the aluminum article and a cour-ter-electrode for two minutes, thereafter an alternating current having a minus half-wave which was asymmetrical was applied for one minute.
A very acceptable black color was obLained. It is to be noted that normal anodizing techniques could not have obtained color in an anodic film this thin.

An aluminum article was anodized utilizing the solution of Example 2 at a temperature of 20C, a current density of 36 amperes/sq. ft. in-ord~r to obtain a material which had a thickness of 1.1 mils.
The material was thereafter color anodized utilizing the tin solution set forth in Example 2 and the tech-nique of United States 3,669,856. Alternating current was applied for 1 1/2 minutes followed by half-wave at i, ~
one minute. A perfectly acceptable colored article was obta1ned.

i5080 EX~MPLE 8 The process of ~xa~ple 5 is repeated with the exception that ater the product was run to a bronze color, it was immersed in an oxidizing acid, preferably 20-30 volume % nitric acid at room temperature, which resulted in a uniform champagne color. This color is virtually impossible to produce in a uniform manner by any other known process.
The desirable hardness of coatings made in accordance with the invention is evidenced by high coating density. For example, four samples were prepared using 6063 alloy and the process of Example 5 except that the current times ir. the coloring bath were varied as follows, with the following results:
Alternating V2 Wave Coating Coating Sample Color Current Current Thickness ~Yeight -Minutes -Minutes -Mil ~in2 A Light 2 0 .84 ;68 Bronze 8 ~dium 3 O .75 70 Bronze C - ~ark 3 1 1/2 .83 90 Bronze D Black 3 4 .86 138 ,~It will be readily understood that the description '~ herein is ~or the purpose of illustration and that the ¦scope of the invention is limited only by the appended clai~s.

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Claims (18)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for the production of colored pro-tective coatings on previously anodized articles of alumi-num or aluminum alloys wherein an alternating current is passed between an electrode system comprising said pre-viously anodized aluminum article and a counterelectrode immersed in an acid aqueous bath containing a salt of a metal capable of coloring the anodized layer and wherein voltage with controlled asymmetry is applied to the electrodes so as to control the color tone of the aluminum article by modulating the voltage externally of said electrode system, the improvement which comprises anodizing said aluminum article in the absence of an alkali salt of titanic acid complex in aqueous acid electrolyte comprising from about 12 to about 24 weight percent sulfuric acid and from about 1 to about 4 volume percent of a polyhydric alcohol of from 3 to 6 carbon atoms and from about 1 to about 4 volume percent of an organic polycarboxylic acid containing at least one reactive group in the alpha-position, and carrying out the anodizing at a temperature of from 65-85°F at a current density of 24-36 amperes/sq. ft.
so as to obtain an anodized layer from 0.2 - 1.1 mils.

2. The process of claim 1 wherein said anodizing is performed at a temperature of between about 68 and 75°F.

3. The process of claim 1 wherein the organic carboxy-lic acid is either glycolic acid or lactic acid.

4. The process of claim 1, 2 or 3 wherein the poly-hydric alcohol is glycerol.

5. The process of claim 1,2 or 3 wherein the metal salt capable of coloring is tin sulfate.

6. The process of claim 1,2 or 3 wherein the metal salt capable of coloring is nickel sulfate.

7. The process of claim 1,2 or 3 wherein the metal salt capable of coloring is copper sulfate.

8. The process of claim 1,2 or 3 wherein the metal salt capable of coloring is cobalt sulfate.

9. The process of claim 1,2 or 3 wherein 4-8 grams/
liter of aluminum is present in the coloring bath.

10. The process of claim 1 wherein initially a symme-trically alternating voltage is passed followed by the asymmetrically alternating voltage.

11. The process of claim 1,2 or 3 wherein the metal salt capable of coloring is a tin salt.

12 The process of claim 10 wherein the metal salt capable of coloring is a nickel salt.

13. The process of claim 10 wherein the metal salt capable of coloring is a copper salt.

14. The process of claim 1,2 or 3 including the additional further step of reversing the polarity of the coloring bath and backing off the color in an already applied colored coating.

15. The process of claim 1 wherein said aqueous acid coloring bath also includes a sequestering agent for the metal salts.

16. The process of claim 15 wherein said sequestering agent comprises a mixture of .beta.-naphthol and gelatin or 4,4 di(dimethylamino) diphenyl methane.

17. The process of claim 16 wherein the sequestering agent is used at a concentration of between about 5X10 5 to about 5X10 3 grams/liter of bath.

18. The process of claims 1,2 or 3 including the ` 18 additional further step of reversing the polarity of the coloring bath and subtracting color from the anodized layer.