CA1045576A - Coloring methods for aluminum and aluminum alloys - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An aluminium or aluminium alloy article is coloured in a method in which the article having an oxide film thereon is placed in a solution of at least one salt of at least one metal of Group VIII of the Periodic Table an alternating current is passed through the article, and then a direct current is passed through a solution of electrolyte, in which the article is used as an anode.

Description

The present invention relates to the colouring of alumi-nium or aluminium alloys. More especially, it pertains to a meth- :
od of colouring in which aluminium or aluminium alloys haviny an oxide film thereon formed by anodic oxidation are subjected to a :.
t.reatment with an alternating current in an aqueous electrolytic solution of at least ,one metal salt of a metal of Group VIII of the Periodic Table, subsequently a direct curren-t is passed in a solution of electrolyte in which the aluminium material forms .:
the anode. :
The following methods have been used to colour alumin- `::
ium and aluminium alloys: .
1) Colouring aluminium or aluminium alloys with a paint which contains a pigment or dye,

2) Colouring by means of a dye on an oxide film :Eormed . . .
anodically: .

3) Colouring by means of a special chemical treatment,

4) Colouring which takes p~ace simultaneously with anodic . .
oxidation in a solution containing organic acids such as sulfonic acid:

5) Colouring aluminium alloys having special alloys compo-nent by means of anodic oxidation,

6) Colouring by a process in which alwninium or aluminium alloys treated by anodic oxidation are treated with an alternating current in an acidic solution of metallic salts containing inorganic o.r organic acids, and /or ~ :
with their ammonium, amino and imino salts.
The methods described above, however have the follow-ing defects: ..
1) The method of colouring with pigments or dyes does not always produce uniformity of colour shades, because of ~
the non-uniformity in the thickness of a coating of .~.
paint containing pigments or dyes. In additio~ ~on .~,~ . . .

_ colouring with piyments, the lus trous metallic appearan-ce of aluminium is impaired because the surface of the aluminlum is covered with an opaque coloured film.
Furthermore, when a painted aluminium surface is scratch-ed, the metallic surface is exposed easily, ~ -2) The method of colouring anodically formed oxide film with a dye produces a beautiful and clear colour. How-ever, because the dyes used are organic materials the coloured film on the surface of the aluminium is unsta-ble and shows a weak resistance against weather. There-fore, coloured aluminium articles produced by this meth-od are unsuitable for use where they will be exposed to the weather, 3) The colouring method by means of a special chemical treatment is not satisfactory due to the weakness of adhesion between the coloured layer and the aluminium surface. Thus, this method has not been accepted widely in industry:
4) The method of colouring by anodic oxidation in a solu-tion containing organic acids produces a durable colour-ed film. Therefore, this method can be applied to articles for use out of doors. However, this process ha~ been found to be difficult to control for the manu-facturlng of articles of uniform tone of colour when the pxoduction is continuously operated; ;
5) The method of colouring aluminium alloy having a special component by means of anodic oxidation is suitable for architectural applications because of its good weather resistance. However, a specially prepared aLuminium alloy is usually used for ~he coLouring and it is difficult to prQduce a variety of colour tQnes using the same aluminium allo~? : ~ ~:
6~ The method of colouring performed in the acidic solu-~SS76 tion of metallic salts containing inorganic and organ-ic acids or their salts is useful and aluminium arti-cles produced by this method do not lose colour easily.
According to this method, it is found that although metallic oxide may be inserted into the surface of the aluminium treated by anodic oxidation, the tone of colour on aluminium articles prepared by this method is not constant, and varies with different batches. Fur-thermore, the tone of colour disperses as the solution in the electrolytic bath changes with time and sub-standard products are found. Therefore, this method is not really suitable for industrial use.
As described above, colouring methods for alumini~l and aluminium alloys are known. However, each has some disad-vantageous features, and therefore an improvement in colouring methods has long been desired in industrial circles.
Accordingly the invention is directed to a method of colouring an aluminium or aluminium alloy article comprising - placing said article having an oxide film thereon in a solution of at least one salt of a~ least one metal of Group VIII of the Periodic Table, the solution being free from added acid and/or ammonium, amino or imino salts thereo~, passing an alternating cunrent through said articles, said alternating current being -applied at 5 to 40 C below ~ volts, and then passing a direct current through a solution of electrolyte, in which the article is an anode, said direct current being applied at a temperature of O to 30C at a current density of from 0.5 to 3.0 A/dm .
The present invention overcomes the wea~ points of the colouring methods mentioned above. Hence anodic oxidation of aluminium or aluminium alloys is carried out, the material having an oxide film is put in an aqueous solution of a metal salt containing metal of Group VIII of the Periodic Table and an ' ~ -3-.~ ~ .

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1~45S76 alternating current is passed throu~h the solution. After the passage of the alternating current, a direc-t current is passed through the solution using the aluminium material as an anode, thus forming a coloured aluminium article. The special features of this invention are that the treatment of alum-'nium or alumi-nium alloys is accomplished by the process described above and that the colour appears in a final step. By using this method, superior uniformly coloured articles are produced in contrast to previously known methods.

The colouring mechanism of the present invention is not yet fully understood but one theory is as follows:
On passage of the alternating current through the solu-tion of metal salt, the metal ions enter pores on the surface of the aluminium havin~ an oxide film. Followin~ this, a direct current is passed with the aluminium material as an anode, the metal ions present in the pores are changed to a colored, stable state.
When the metal and/or metallic o~ide are present in the pores of the aluminium, the electroconductlvity is centered large-ly in this area; the amount o-f electric current increases in this area to a greater degree than in other areas and the metal ions in aqueous solution also concentrate in this area. Thus, the surface of the aluminium is coloured irregularly. However, according to the present method, only metal ions are introduced . .
into the pores and the colour forms uniformly. Furthermore, this :- :.
method is relatively easlly operated and gives a substantially constant tone of colour in aluminium articles regardless of dif-.
ferent batches.

According to the method, only metal salts are involved in the solution of electrolyte, the acids and/or their ammonlum salts which have been used in other methods are not involved.

~ Consequentlyl the~composition and the pH of the solution which cr~l _4_ ~: .," ' ".

~45576 are necessary to produce the aluminium articles having uniform tone of colour~are easily adjusted.In addition a constant colour toné isobtained because the solution isrelatively easy to maintain.
Formerly, for example, when an inorganic or an organic acid was included in a solution of electrolyte, a~trace amount of the acid often remained on the surface of the aluminium even t~lough the surface of the aluminium was washed with water. Thus when the aluminium material was immersed in a solution of resin, the acid contaminated the solution. Also a non-uniform resin film or bubbles were often formed on the surface of the aluminium. How-ever, by not using the acid in solution the defective features described above are eliminated.
The present invention is explained in more detail as follows:
In the process of the invention, anodic oxidation of the aluminium or aluminium alloys is preferably used to form an oxide film. A suitable temperature is from about 0 to 30 C, but a temperature of from 15 to 25C is preferred and limitations of changes of temperature within + 2 C are desirable in order to maintain uniformity in the aluminium or aluminium alloy articles being treated. Further,direct current density is suitable from 0~5 to 3.0 A/dm2, but a direct current density of 0.8 to 2.0 A/dm is preferred. The method is applicable not only to aluminium or aluminium alloys treated by anodic oxidation but also to those ~ -treated chemically~
After anodic oxidation, aluminium or aluminium alloys having an oxide film thereon are subjected to an alternating cur-rent in an aqueous electrolyte solution of metal salt containing metal of Group VIII of the Periodic Table.

Especially desirable metal salts are those of iron, cobalt and nickel.
Suitable metal salts include ferrous sulphate, Eerrous '.

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phosphate, ferrous nitrate, ferrous acetate, ferrous fumarate, nickel sulphate, nickel phosphate, nickel nitrate, nickel acetate, nickel propionate, nickel formate, colbalt sulphate, cobalt nitrate, cobalt formate, cobalt acetate, jcobalt phosphate~ :
and cobalt propionate. Usually these metal. salts are employed as a mixture, but a single salt in aqueous solutlon is sufficient for colouring.
In this method, the counterelectrode in the alternating curren~ step may be a material, for example graphite, which is ' -5a-~ ....

55i~
inert to the electrolyte, or may be formed of other conductive materials, for example metals, which may advantageously be select-ed from metals having a solution potential more noble than alumi-nium in the conventional series of electrode potentials~
~ le temperature of the solution is suitably from 5 to 40C, but a temperature of from 10 to 35C is preferred, in 'the passage of alternating current, changes of the temperature are desirably within ~ 3C in order to maintain a uniformity in the " B " colour.
The voltage of the alternating current is suitable be- ' low about 30 volts, and 5 to 25 volts is preferred, accor~ing to ;~-the desired colouring, the alternating current is passed through -for from about 2 to 30'minutes. The aqueous electrolytic solu-tion of metal salts suitably has a concentration of the salts of greater than 0.2 weight percent, 'but 0.5 to 15 weight percent is preferred.
The pH o~ the solution is decided by the variety and concentration of the metal salts. The p~I o the solution goes '~
down when the metal ions enter the pores on the surface of alumi-nium having the oxide film. When the pH goes down, it is desira-ble that excess anions be removed because they exert a bad in-fluence upon the colouring of the oxide film. A conventional anion-exc'hanger or other suitable procedure may be used for remo- ' ving the excess anions. ~ ' In connection with these process steps, three related ~ - "
methods will be described.
In the first method of the electrochemical treatment, ;' aluminium or aluminium alloys oxidized by anodic oxidation are subjected to an alternating current in an aqueous electrolytic '' solution of metal salts containing metals of Group VIXI of the Periodic Table, a direct current is then passed through the alu- '-minium màterial in an aqueous solution having a concluctivity -1 -1 '' ' ' greater than 10~ ~ cm ~L0~5~76 Subsequently, the metal ions present in the pores act electrochemically to colour the oxide film on the aluminium.
The metal ions introduced into the pores change immediately to metal and/or metallic oxide and as a result the oxidized film is coloured. The electrical conductivity of the aqueous solution for the direct current electrolysis should suitably be above 1~

--1 .
cm When the conductivity of the solution is less than 10~ lcm 1 such as with deionized water, the current is too small and the colouring tends to be insufficient.
The colouring is not directly affected by large conduc-tivity but the operation requires special control, namely, the process capacity and/or the heat of reaction. Thus, the control of the voltage through the electrolyte must take into considera-tion, for example, the process capacity and/or heat of reaction.
In this case, it would be preferable to use an aqueous solution for colouring having the least possible amount of solute and/or being free of chlorine ion.
A strongly alkaline or acid solution especially should be avoided because a strong alkaline solution corrodes the oxide film of aluminium while a strong acid solution dissolves the heavy metals. Consequently, the pH of the aqueous solution is suitably from about 2 to 11.
These aqueous solutions are chosen so that the concentra-tion in an aqueous solution has the necessary electrical conducti-vity, according to their respective degree of dissociation. When a strong electrolyte for example sodium nitrate is used, the concentration in aqueous solution should suitably be above 0.0001 . ~ , ; : .

~4557~
weight pe:rcent.
In the second method aluminium having an oxidized film is subjected to an alternating current in an aqueous solution of metal salt containing metal of Group VIII o~ the Periodic Table, with this solution subsequen-tly being used continuously for the treatment carried ou-t with the direct current.
In this case, it is considered that -the oxide film on the surface of the aluminium may be coloured by electrochemical reac-tion occurring around the elec-trode.
In this method, a suitable counter-elec-trode is one made o~, for example, graphite or metal.
~urther, the desirable conditions for the direct current treatment are at room temperature, a di~ect curren-t voltage of 10 to 200 vol-ts and the ch~rging time is 0.5 to 10 rnin~ltes.
In the third me1hod, after the anodic oxidation o the aluminium or aluminium alloy, an alterna-ting current is passed as ..
described in the first and the second methodsfollowed by a direct current through a solution of water-soluble resin using aluminium or alurninium alloy treated as in the above processes, as an anode.
According to this method, both the process of colouring : ~
and the process of electrodeposi-tion progress simultaneously. . .
The special features of this method are that the oxidized film of aluminium is coloured electrochemically -through the pro- ..
cess of electrodeposition, while its material lS coa-ted spontane- :
ously with resin.
In the case of elec-trodeposition coating, the.following .. .. ..
resin solution can be employed: an aqueous solution, emulsion or .
dispersion of resin selected ~rom acrylic, al]~yd, acryl.ic-al~yd, ..
epoxy and pheno~lic resins,reaction products of such resins, with an or~anic amine or arnmoniurn hydroxide, and a mixture o~ said resins, and a mixture thereo~ with melamine resins.

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~045576 . As the cathode there is generally used an aluminium plate or a stainless-steel plate.
The desirablè conditions for electrodeposition coating ~ '~
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are that the resinous liquid have a concentration of 3 to 40 weight percent' that the temperature be from 10 to 30C, and that the direct current has a voltage of 50 to 300 volts; that the pH of resinous liquid is 7.5 to 10.5, and that the charging time is 0,5 to 5 minutes.
In each step of the process, the aluminium material is washed before the next step.
The temperature of the water in which the aluminium material is washed after both the anodic oxidation and the passa-ge of alternating current is suitably any temperature from room temperature to 100C.
The coloured aluminium articles produced by such a oethod can b~ used without a resin coating.
~hen both the treatment with an alternating current and the treatment with a direct current are carried out in the same solution of metal salt, or when the treatment with a direct current is carried out in an aqueous solution of more that lO~ n cm 1 conductivity, the aluminium articles produced can be coated with resins, if desired.
The well-known methods of applying powder, solvent and water-501uble paints such as electrostatic deposition, spray-ing, electrodeposition and dip-coating can be practiced with this invention.
The various paints can be applied according to known coating methods.
This invention is further explained in connection with the following examples, but it is to be understood that the scope of this invention is not to be limited to the examples described herein.

An aluminium plate was degreased with trichloroethyle-ne and anodic oxidation was carried out for 30 minutes in a 5 wt.%
solution of sulphuric acid, at 20 C, with a direct current having " .' _ 9 _ ~L~45S~6 a current density of 1.5A/dm2.
The oxidized aluminium plate was washed with water at room temperature and the surface was coloured using the follow-ing treatment. -The aluminium plate was immersed in a 5 wt.% aqueous solution of ferrous sulphate and subjected for 10 minutes to an alternating current of 10 volts at 25C using the plate and a carbon bar as the electrodes.
Subsequently, the plate was treated for 2 minutes in the same solution with a direct current of 50 volts at 20C7, using the aluminium plate as the anode.
As a result of this treatment, a uniform, lustrous, beautiful gold-coloured aluminium plate was obtained.

An aluminium plate was degreased with trichloroethyle-ne and the plate was treated f~r 30 minutes in a 10 wt.% solu-tion of trisodium phosphate at 30C., with a direct current hav-ing a current density of lA/dm2 with the aluminium plate as the anode.
The plate was washed with water and immersed in a 10wt.
% solution~of nickel sulphate. Upon passage through the solu-tion of an alternating current of 10 volts at 20C., for 10 minu-tes, the aluminium plate and a carbon bar being employed as the electrodes, a pale brown, coloured film was obtained on the sur-face of the aluminium plate.
Subsequentlyl the plate, as the anode was treated fur-ther in the same solution with a direct current of 50 volts at 20C~, for 2 minutes.
Upon immediate passage of the direct current the sur~
3~ face of the plate was coloured a uniform lustrous beautiful goldO ~ -A degreased aluminium plate was anodized in a 5 wt.%
~ ,,~,...

solution o-f sulphuric acid at 20 C. for 30 minutes with a direct current having a current density of l.OA/dm2. ;:~

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~4~iS76 After the aluminium plate was washed, it was immersed in an aqueous solution of 50 parts by weight of ferrous sulphate and 950 ~arts by weight of water and an alternating current of 10 volts was passed through the solution at 20C. for 10 minutes, using the aluminium plate and a carbon bar as electrodes.
This treatment resulted in the formation of a pale yellow coloured film on the aluminium plate.
Thereafter, the aluminium plate was washed with hot water at 40C., and immersed in 1~% aqueous solution O~an acryl-ic resin' a direct current of 150 volts was passed through thesolution at 20C. for 2 minutes, using the treated aluminium plate as an anode and another aluminium plate as a cathode.
The solution of acrylic resin was prepared as follows:
Into a reactor equipped with an agitator, a thermometer and a reflux condenser, ~2.5 parts of ethyleneglycol monobutyl ether and 44.5 parts of methanol were fed and heated at 70C.
A reflux temperature of 70C was maintained and 5.5 parts of methacrylic acid, 25 parts of ethylacrylate, 44.5 parts of methylmethacrylate, five parts of styrene, 1.5 parts of benzoyl-pero~ide and one part of dodecylmercaptan all parts being byweight were mixed together and fed into the reactor together with 20 parts by weight of methylolacrylamide, in ~ive portions at half-hourly intervals. The reactants were heated and agitated for four hours under reflux at the same temperature. `
After the conclusion of the reaction, the contents were cooled to 60C, and the acid component was neutralized with 28 percent ammonia solution, and the reaction product was diluted with water until the solid matter content was 15 percent by weight.
Since both the electrodeposition and the colouring re-action progress simultaneously in this .solution, not only the coating of transparent resin but also the gold colouring is formed -~
on the anodized aluminlum plate.
Subsequently, when the plate was washed with water and ~ 11 - . : ~
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~L5576 baked at 180Co for 30 minutes. a uniform, lustrous, gold-colored aluminium plate was obtained.

A degreased aluminium plate was oxidi2ed in a 1.5 wt.%
solution of potassium hydroxide at 30C. with a direct current having a current density of 2A/dm . The plate was washed with water and immersed in an aqueous solution of 70 parts by weight ~ -of nickel nitrate and 930 parts by weight of water.
An alternating current of 10 volts was passed through the solution at 20C for 10 minutes, using the aluminium plate and a carbon bar as the electrodes. This resulted in a pale yellow-coloured aluminium plate being obtained.
Subsequently, the plate was treated in th~ same solution with a direct current of 100 volts at 20C. for 2 minutes, using the aluminium plate as an anode, a deep brown coloured aluminium plate was obtained.
EX~MPLE 5 A degreased aluminium plate was oxidized for 30 minutes in a 10 wt.% solution of sulphuric acid at 20C. with a direct current having a current density of lA/dm2.
The plate was washed with hot water at 60C. and immers-ed in an aqueous solution of 8 wt.% of nickel nitrate and 2 wt.%
of cobalt nitrate.

, .:
Upon passage of an alternating current of 15 volts through the solution for 4 minutes at 20C, using the aluminium plate and a carbon bar as electrodes, a pale brown coloured film was formed on the aluminium plate.
The aluminium plate was washed and used as an anode, after which it was introduced into a 15% clear aqueous solution of acrylic melamine resin and treated with a direct current of 150 volts at 20C for 2 minutes while using another aluminium plate as ~ -a cathode.
The aqueous solution of resin was prepared as follows~

, .

1C~45S7~i Into a reactor equipped with an agitator, a thermometer and a re-flux condenser were fed 50 parts of ethylenegl~ycol monobutyl ether and 37 parts of methanol which were heated to initiate refluxing and maintain the reflux condition.
Thereafter, to the reactor was added a mixture in parts by weight of five parts of acrylic acid, five parts of hydro~yethylacrylate, 20 parts of ethylacrylate, 45 parts of ~ -methylmethacrylate, 5 parts of butylacrylate, 1.5 parts of benzoylperoxide and 0.5 parts of dodecyl mercaptan, in five portions at half-hourly intervals.
After addition of the five portions the mixture was heated and stirred for 5 hours under r~flux, and after the con-clusion of the reaction, it was cooled.
Acid components were rendered neutral with triethyl-amine, and thereafter the mixture was diluted with water until the solid matter content was 30 percent by weight. . : .
In this manner, a resin solution freely miscible with water was produced. :
This solution and a water-soluble melamine resin were .
mixed in a ratio of solid matter content of 1:1 and were reacted . .
together at 80C for 1 hour. The product was diluted with water. : :
Subsequently, the aluminium plate was washed with hot . :
water at 50C. and baked at 130C for 20 minutes to produce an .
aluminium plate having a uniform, lustrous, and beautiful brown coating. : .
E~AMPLE 6 ::
A degreased aluminium plate was oxidized for 30 minutes in a 15 wt.% aqueous sulphuric acid solution with a direct current having a current density of lAjdm2. .
The plate was washed with water at ~0C, placed in an aqueous solution of 10 wt.% ferrous sulphate and 3 wt.o/~ cobalt nitrate and subjected to an alternating current of 15 vo:Lts at ' - 13 - .

~ 5S~ ~
20C. for 4 minutes, using the aluminium plate and a carbon bar ..
as the electrodes.
The oxide film on the all~iinium p:Late became pa~e brown in colour. .
The aluminium plate was washed, used as an anode in a 10% a~ueous solution of alkyd modified acrylic resin and treated with a direct current of 180 volts at 20C. for 3 minutes while using another aluminium plate as a cathode.
The aqueous solution of resin was prepared as follows:
Into a reactor equipped with an agitator, a thermometer and a reflux condenser attached with a blowing inlet for nitrogen .
gas and a separating tube, were fed 330 parts of safflower oil, : ;
100 parts of glycerol and 0.2 parts of sodium methylate, all parts being parts by weight. : .
Nitrogen gas was blown into the mi~ture and an ester- .
exchanging reaction was carried out at a temperature between 230C .:.
and 240C for 2 hours.
After cooling 220 parts by weight of phthalic anhydride .
and 50 parts by weight of cyclohexanone were added and esterified ..
at a temperature of between 180C and 210C to produce an alkyd .
resin with an acid value of 30. ...
An alkyd resin having an acid value of 30 was produced by adding 220 parts by weight of phthalic anhydr.ide. ;.
. ' ' .:
Into a reactor similar to that used in example 3, were ~ -fed in parts by weight 20 parts of alkyd resin produced above, 55 ; parts of ethyleneglycol mono-ethyl ether and 32 parts of methanol. .. ~
Further, a mixture of in parts by weight, five parts of :: :
methacrylic acid, five parts of hydroxyethyl methacrylate, 40 parts of ethyl acrylate/ five parts of styrene, 1.5 parts of benzoyl :
peroxide and 20 parts of methylol acrylamide were divicled into five:
portions, the portions were added at intervals of 30 minutes to the contents of the reactor, while maintaining the contents of the ;.

reactor at the boiling point. Thereafter, the mixture was heated , . ~

and stirred for 4 hours under reflux.
At the completion of the reaction, the con-tents of the reactor were cooled and the acid values of methacrylic acid and alkyd resin were rendered neutral with diethylamine~ The mixture was diluted with water until the content of solid matter was 10 percent.
The oxide film of the aluminium plate was thus provided with a transparent resin coating by means of electrodeposition and has a blackish gold-coloured film.
Subsequently, the aluminium plate was washed and baked at 160C for 20 minutes to obtain an aluminium plate having a uniform, lustrous, beautiful coloured surface. :. . .
EXAMPLE 7 :
A degreased aluminium plate was oxidlzed for 30 minutes . .
in a 10 wt.% aqueous solution of sulphuric acid with direct current having a current density of lA/dm2. ~.
The aluminium plate was washed and used as an anode in ~::
an aqueous solution of 3 wt.% of nickel acetate and subjected to an alternating current of 15 volts at 20C for ~ minutes, usin~
a carbon bar as a cathode. ~..
Subsequently, the aluminium plate was placed as an anode ~ ~
in an a~ueous solution of 10% by weight concentration of acrylic . . .
resin and treated for 1.5 minutes with a direct current of 150 .
volts, using another aluminium plate as a cathode. The solution ~
of resin was prepared from the solution of acrylic resin in ..
example 3. ' The aluminium plate was washed and baked at 150~C or 20 minutes to produce a brown coloured aluminium plate coated with a lustrous, transparent resin.

An aluminium plate was oxidized as described in example -:

7, and washed with hot water at 60~C. ~ :~

The aluminium plate was inserted in an a~ueous solution ' ' ~6)4SS76 of 10 wt.% of ferrous citrate and 3 wt.% of cobalt acetate and subjected to an alternating current of 1.5 volts at 20C for 4 minutes using the aluminium plate and a carbon bar as the electrodes.
The aluminium plate was washed wlth water at room temperature and inserted in a 15% aqueous solution of alkyd modified acrylic melamine resin.
The solution of alkyd modified acrylic-melamine resin was prepared as follows:
Into a reactor similar to that of Example 3, was fed, in parts by weight, 192 parts of trimethylolpropane, 132 parts of sebacic acid, 67 parts of safflower aliphatic acid and eight parts of cyclohexanone, the mixture obtained was heated to 200C. to produce, by esterification, an alkyd resin with an acid value of 50.
Into a reactor similar to that of example 3, was fed in parts by weight, 20 parts of the alkyd resin obtained above 35 parts of cyclohexanone, 13 parts of isopropyl alcohol and 10 parts of methanol.
The content of the reactor was maintained at boiling and a mixture of, in parts by weight, three parts of methacrylic acid, 40 parts of methyl methacrylate, 42 parts of ethyl acrylate and 1.5 parts of benzoyl peroxide was added thereto in f:ive por-tions at half-hourly intervals. At the conclusion of the addi-tion, polymerization was continued for 6 hours under reflux.
The content of the reactor was cooled and triethylamine was added ko render the acid value of -the alkyd resin and metha-crylic acid neutral.
The thus obtained resin solution was diluted with water until the content of solid material was 30 percent by weight, and a resin solution freely soluble in water was obtained.
This resin solution and a water-soluble melarnine resin were mixed at the ratio of solid material content of 1:1 and then diluted with water until the content of solid material was 15 percent by weight.

. - - .. . . : .. . . ~ ~ .

The electrodeposition and the baking were carried out by the same method as described in example 7 and an amber gold coloured aluminium plate was obtained.
EXAMPLE 9 ;
A degreased 99.8% pure aluminium plate was oxidi~ed for 30 minutes in 15% aqueous sulphuric acid with a direct cur- -rent~having a current density of lA/dm2.
The aluminium plate was used as an anode in an aqueous solution of 15 wt.% of nickel sulphate and 2 wt.% of cobalt sul-phate and subjected for 10 minutes to an alternating current of 10 volts, using the aluminium plate and a carbon bar as the elec-trodes.
Subse~uently, The aluminium plate was immersed in a 0.001% by weight aqueous solution of ammonium sulphate having an electrical conductivity of 200~ ~Llcm 1 after which a direct cur-rent of 50 volts was passed through the solution for 3 minutes, using the aluminium plate as the anode and stainless steel as the cathode.
As a result of this treatment, a deep brown-coloured film was obtained.

A 6063 aluminium alloy was oxidized and subjected to an alternating current in a 1% by weight aqueous solution of nickel sulphate under the same conditions as de~cribed in example 9.
Thereafter the al~minium alloy plate was treated for 5 minutes with a direct current of 20 volts in a 0.1% by weight aqueous solution of sodium carbonate having an electrical conduc-tivity of 2000~ ~l~lcm 1.
As a result of this treatment, the oxide film of the aluminium alloy has a deep brown colour.

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As described in example 9, a 99.8% aluminium plate was oxidized. After the anodic oxidation, the aluminium plate was subjected to an alternating current of 13 volts for 5 minutes in a 10% by weight aqueous solution of nickel acetate.
Subse~uently, the aluminium plate was treated for 5 minutes with a direct current of 150 volts in a 0.01% by weight aqueous solution of potassium nitrate, using the aluminium plate as an anode. A reddish brown coloured aluminium plate was obtain-ed.
Thereafter the coloured aluminium plate was immersed in a 23% aqueous solution of a water-soluble acrylic resin and was then baked at 180C for 30 minutes.
The 23% solution of water-soluble acrylic resin was prepared as described in example 5.
rrhe aluminium plate thus obtained was reddish brown in colour, and fade-resistant. The colour film persisted under sunshine for 2000 hours.

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Claims (23)

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

1. A method of colouring an aluminium or aluminium alloy article comprising placing said article having an oxide film thereon in a solution of at least one salt of at least one metal of Group VIII of the Periodic Table, said solution being free from added acid and/or ammonium, amino or imino salts thereof passing an alternating current through said article, said alter-nating current being applied at 5-40°C and below 30 volts, and then passing a direct current through a solution of electrolyte, in which the article is an anode, said direct current being applied at a temperature of 0-30°C at a current density of from 0.5 to 30.0 A/dm2.

2. The method of claim 1 in which the solution of elec-trolyte is an aqueous solution having a conductivity greater than 10??-1cm-1.

3. The method of claim 1 in which the solution of electro-lyte is the same solution as that through which the alternating current is passed.

4. The method of claim 1 in which the solution of electro-lyte contains a water-soluble resin.

5. The method of claim 2 or 3 in which the coloured article is coated with resin after the passage of direct current.

6. The method of claim 4 in which said water soluble resin is at least one selected from the group consisting of acrylic, acrylic-alkyd, epoxy, phenol, alkyd with organic amine, alkyd with ammonium hydroxide, alkyd with organic amine and ammonium hydroxide.

7. The method according to claim 6 in which said resin is in admixture with a water-soluble melamine.

8. The method according to claim 1 in which said oxide film is formed by anodic oxidation.

9. The method according to claim 8 in which the current density is 0.8 to 2.0 A/dm2 and the temperature is in the range of 15° to 25°C.

10. The method according to claim 8 or 9 in which tempera-ture variations are maintained within ? 2°C.

11. The method according to claim 1 in which said metal of Group VIII of the Periodic Table is iron, cobalt or nickel.

12. The method according to claim 1, 2 or 9 in which said salt is selected from the group consisting of ferrous sulphate, ferrous phosphate, ferrous nitrate, ferrous acetate, ferrous fumarate, nickel sulphate, nickel phosphate, nickel nitrate, nickel acetate, nickel propionate, nickel formate, cobalt sul-phate, cobalt nitrate, cobalt formate, cobalt acetate, cobalt phosphate, and cobalt propionate and mixtures thereof.

13. The method according to claim 1 in which the alternating current is passed through the solution for from about 2 to 30 minutes.

14. The method according to claim 13 wherein the voltage for the alternating current stage is in the range of 5 to 25 volts and the temperature is within the range from 10° to 35°C.

15. The method according to claim 13 or 14 in which temper-ature variations are maintained within ? 3°C.

16. The method according to claim 1 in which the metal salt in solution has a concentration greater than 0.2 weight percent.

17. The method according to claim 16 in which the concentra-tion is from 0.5 to 15 weight percent.

18. The method according to claim 1 in which the solution of electrolyte has a pH from about 2 to 11.

19. The method according to claim 1 in which the direct current has a voltage of 10 to 200 volts and is maintained for 0.5 to 10 minutes.

20. The method according to claim 4 or 6 wherein the resin is present in a concentration of 3.0 to 40 weight percent.

21. The method according to claim 4 or 6 wherein the solu-tion of electrolyte during passage of direct current has a pH of 7.5 to 10.5 and is maintained at a temperature of 10° to 30°C.

22. The method according to claim 4 or 6 wherein the direct current has a voltage of 50 to 300 volts.

23. A coloured aluminium of aluminium alloy article pre-pared by the process of claim 1.