CA2029438C

CA2029438C - Process for electrolytically treating aluminium and aluminium alloys

Info

Publication number: CA2029438C
Application number: CA002029438A
Authority: CA
Inventors: Pinakin Patel
Original assignee: Sandoz AG; Clariant Finance BVI Ltd
Current assignee: Clariant Finance BVI Ltd
Priority date: 1989-11-08
Filing date: 1990-11-07
Publication date: 2001-10-02
Anticipated expiration: 2010-11-07
Also published as: GB2237817B; FR2654118A1; IT9048443A1; GB9023974D0; DE4034854C2; JPH03207895A; IT1242327B; FR2654118B1; HK100696A; DE4034854A1; IT9048443A0; CH682240A5; CA2029438A1; JP2953474B2; GB2237817A

Abstract

A process for providing a variety of light to medium colours of anodised aluminium or aluminium alloys which comprises the steps of anodising an aluminium or aluminium alloy workpiece in an aqueous strong acid electrolyte solution by application of direct current to form on the workpiece a porous anodic oxide film having a thickness of at least about 3 microns; subjecting the resulting anodised workpiece to alternating current in an aqueous strong acid electrolyte solution comprising an organic carboxylic acid containing at least one reactive group bound to the carbon atom in the alpha-position; and electrolyti-cally colouring the workpiece. In a preferred embodiment resulting in deepened colour tones, including those in the blue to blue-gray range, one or more currentless "waiting periods" are maintained at various stages of the process. In another embodiment, improved colour uniform-ity is obtained by subjecting the workpiece prior to electrolytic colouring to a direct current pre-treatment step.
The resulting electrolytically coloured aluminium or aluminium alloy is particularly useful for architectural product.

Description

20~~~~~~
Case 156-7114 PROCESS POR ELECTROLYTICALLy TREATING ALUMINIUM AND ALUMINIUM ALLOyS
Background of the Invention The present invention relates to a method for anodising and elec-trolytically colouring aluminium and aluminium alloys, and to compo-sitions useful therein.
Various electrolytic colouring processes have been developed, and can be viewed as fundamentally "two-stage" processes involving an anodising stage followed by an electrolytic colouring stage.
In the anodising step, the aluminium workpiece is electrolysed under conditions to result in the formation of a surface aluminium , oxide coating (commonly referred to as an "anodic oxide film"). The electrolysis is generally performed by applying direct current to the aluminium workpiece serving as the anode in an electrolytic bath wherein a second metal source, such as aluminium, or graphite, serves as the cathode. An aqueous strong acid electrolyte such as sulphuric acid is generally employed to provide anodic oxide film of satisfac-tory hardness, corrosion resistance and colouring ability.
The resulting anodic oxide film comprises an inner protective "barrier" layer which is dielectric, thin (i.e. about 0.1-1 micron), strong, and pore-free; and a nondielectric outer layer, of greater thickness (i.e. from about 3 to 100 or more microns) which to varying degrees depending on the conditions of anodisation is characterised by a pattern of pores extending within the layer, see Hubner, W.W.E. and A. Schiltknecht, The Practical Anodising of Aluminium, MacDonald &
Evans, London (1960), pp. 21-29. The porous outer layer of the anodic oxide film provides a suitable substrate for deposition of colouring agents.

- 2 - - ~~~~~~e 156-7114 The second stage of the two-stage electrolytic colouring processes comprises electrolytic deposition of colouring agents, e.g., metal salts or mixtures thereof, into the pores of the anodic oxide film, typically in the presence of alternating current.
Various factors such as current density and duration, temperature and composition of the anodisation and colouring baths, and special-ised treatments may affect the morphology and properties of the re-sulting anodic oxide film and its colouring.
For example, depending on the current density in the anodising step, the anodic oxide film which is produced varies from a "soft" or porous-type film to a "hard" dense film of lesser porosity. Generally, the porous anodic oxide film is obtained by anodising at current den-sities not exceeding about 25 amperes per square foot (ASF) (2.7 amps./dm2) at ambient temperature, i.e. about 55-95°F (ca. 13-35°C).
Anodising at current densities above about 24 or 25 ASF (2.6-2.7 amps./dm2) under certain conditions provides hard, dense-type film of lesser porosity, the hardness varying with the anodising temperature.
In U.S. Patents 4,180,443 and 4,179,342, hard, dense-type anodic oxide coatings are produced at direct current densities of about 24 to 36 ASF (2.6-3.9 amps./dm2) at ambient temperature in an aqueous acid electrolyte comprising sulphuric acid, a polyhydric alcohol and an organic carboxylic acid. Such processes offer certain advantages in hard coating technology but nevertheless apparently provide only limited colours, i.e., deep red, bronze and black.
The present invention relates to improvements in porous anodic oxide film technology including, in particular, processes which pro-vide a variety of light to medium colours of the anodised aluminium or aluminium alloy.

3 20204 ~~
Case 156-7114 Summary of the Invention The process of the present invention comprises the steps of: (a) anodising an aluminium or aluminium alloy workpiece in an aqueous electrolyte solution comprising a strong acid, preferably about 90-300 grams per liter (g/1) thereof, by application of direct current at a current density of about 5-25 ASF (0.54-2.7 amps./dmz) and a tempera-ture of from 55-90°F (13-32°C) to form on the workpiece a porous anodic oxide film having a thickness of at least about 3 microns;
(b) subjecting the resulting anodised workpiece to alternating current at a voltage of about 5-25 volts for about 1-25 minutes in an aqueous electrolyte solution comprising a strong acid and an organic carboxyl-is acid containing at least one reactive group bound to the carbon atom in the alpha-position, wherein said reactive group is- a hydroxy, amino, keto or carboxyl group, preferably 120-250g/1 of the strong acid and about 1-15% by volume of the organic carboxylic acid;
and (c) colouring the workpiece by subjecting it to substantially alternating current in an aqueous electrolyte solution comprising at least one metal salt as a colouring agent.
In certain preferred embodiments of the invention, a "waiting period" is maintained at one or more stages in the above process, during which essentially no current is passed to the workpiece in the electrolyte solution. It has been found that such "currentless"
waiting periods advantageously can provide deeply coloured product which is particularly suitable for architectural applications.
In a further embodiment of the invention resulting in product of improved colour uniformity, the workpiece prior to electrolytic colouring [step (c)] is subjected to a pre-treatment which comprises application of substantially direct current thereto. ' Detailed Description of the Invention The anodisation step may be preceded by known pre-treatments of the aluminium workpiece such as by rinsing and degreasing, e'g., with hot trichloroethylene or trisodium phosphate, and etching, e-g., with caustic soda.

- 4 - Case 156-7114 The anodisation is performed by conventional means generally known in the art. The aluminium workpiece, which is adapted to serve as the anode of a power source, is immersed in an electrolyte bath, together with another metal source, preferably also aluminium, or graphite, which serves as the cathode. Direct current is applied to the work-piece for a time and under conditions suitable for formation of the anodic oxide film.
The anodizing bath comprises an aqueous strong acid electrolyte, such as sulphuric or phosphoric acid, or a mixture thereof.
The acid concentration in the aqueous electrolyte bath is prefer-ably from about 90-300 g/1 of bath and more preferably 120-250 g/1.
Sulphuric acid is preferred, because it provides film of "architec-tural quality", i.e., having suitable hardness, thickness, and cor-rosion resistance for outdoor use.
It is advantageous that a certain amount of aluminium also be present in the anodising bath, which can be provided by the addition of suitable aluminium compounds, such as aluminium sulphate. The amount of aluminium which is present in the bath is about 1-10 g/1, preferably 1-5 g/1.
Direct anodic current is applied to the workpiece at a current density of about 5-25 ASF (0.54-2.7 amps./dmZ), more preferably 10-20 ASF (1.08-2.2 amps./dmZ), and even more preferably 15-20 ASF (1.6-2.2 amps./dm2).
The term "direct current" as used herein shall be understood to comprise not only direct current in the strict sense of the term but also other essentially identical currents such as, e-g., those pro-duced by fullwave rectification of single-phase alternating current or by rectification of three-phase alternating current.
The anodisation bath is desirably maintained at about room tempe-rature, i.e. 55-90°F (13-32°C), preferably about 65-75°F
(18-24°C), and more preferably about 68-72°F (20-22°C), and therefore it may be necessary to employ devices to regulate the temperature of the bath during anodisation.
In the process of the present invention, anodising conditions are preferably selected to provide a porous anodic oxide film of about 20-30 microns thickness, and it will be within the skill of the - 5 - Case 156-7114 practitioner in the art to obtain such film by practicing within the scope of the present invention.
According to the process of the invention, the resulting anodised aluminium or aluminium alloy workpiece is then subjected to an alter s nating current (AC) in an aqueous strong acid electrolyte solution which comprises about 1-15%, preferably about 1-10% by volume of an organic carboxylic acid containing at least one reactive group bound to the carbon atom in the alpha-position, wherein said reactive group is a hydroxy, amino, keto or carboxyl group.
It has been found that a treatment of the anodised workpiece with AC current prior to electrolytic colouring, employing an electrolyte solution comprising the said organic carboxylic acid compounds, per-mits obtainment of medium to light colours of aluminium, including colours in the blue and green range.
Examples of suitable organic carboxylic acid compounds include glycolic (hydroxyacetic), lactic (hydroxypropionic), malic (hydroxy-succinic), oxalic, pyruvic, and aminoacetic acids, and mixtures thereof. Glycolic acid is preferred in the present process.
It has been further found that the use of certain polyhydric alcohols together with the aforementioned organic carboxylic acid compounds in the AC-treatment step provides additional light and medium colour tones, particularly including colours in the the blue and blue-gray range.
Therefore, in an embodiment of the process of the present inven-tion, the AC-treatment electrolyte bath further comprises, in addition to the organic carboxylic acid compound or compounds, about 1-15%, and preferably 1-10% by volume of a polyhydric alcohol of 3 to 6 carbon atoms. Examples of suitable polyhydric alcohols are glycerol, butane-diol-1,4, pentanediol-1,5, mannitol and sorbitol, of which glycerol is preferred.
Most preferably, the AC-treatment electrolyte bath comprises equal parts by volume, e-g., 1-10 volume % each, of the organic carboxylic acid and the polyhydric alcohol.
It has also been found that the desired light and medium colours of aluminium can be achieved when the organic carboxylic acid and/or polyhydric alcohol compounds which are employed in the AC-treatment - 6 - ~ ~~ ~ ~ ~ ease 156-7114 step are also present in the anodisation bath, and accordingly in an embodiment of the invention, a common bath may be used both for anodisation and for the AC-treatment.
The preferred electrolyte for AC-treatment is sulphuric acid.
The voltage of the alternating current is about 5 to about 25 volts, preferably about 10-20 volts, more preferably about 12-18 volts, and most preferably about 12-15 volts to obtain colours in the blue range and 15-18 volts to obtain colours in the green range.
Current is applied to the workpiece for about 1 to 25 minutes.
The wave form may, for example, be symmetric and/or asymmetric, pulsed anodic and/or cathodic with a square or sinusoidal output. The current may be applied continuously or non-continuously.
The AC-treatment bath is maintained at about 55-90°F (13-32°C), preferably about 65-75°F (18-24°C).
The thus-treated anodised aluminium workpiece is then subjected to electrolysis under generally known conditions to deposit one or more colouring agents into the pores of the anodic oxide film.
The electrolytic colouring bath comprises an aqueous strong acid, preferably sulphuric acid, in a concentration of about 5-50 g/1 based on the total bath.
An alternating current is generally employed to deposit the colouring agent into the pores of the anodic oxide film. The applied voltage is generally in the range of from about 5 to about 25 volts, and preferably about 10-16 volts. The wave form is preferably sinusoidal.
Prior to electrolytic colouring, the workpiece is preferably sub-jected to an electrolytic "pre-treatment" which comprises application of a substantially direct anodic current thereto. This DC pre-treat-ment step has been found to provide product having improved colour uniformity.
To effect such improvements, a current density of preferably about 0.5 ASF to about 5 ASF (0.054-0.54 amps./dm2) is maintained for about 0.5 minute to 10 minutes.
This direct current pre-treatment step may most conveniently be carried out in the electrolytic colouring solution but can also be carried out in a separate electrolytic bath having an acid concentra-- Case 156-7114 tion substantially equivalent to the acid concentration of the colouring solution.
After the DC pre-treatment step, the workpiece is then subjected to electrolysis by conventional means as described above employing a colouring agent in an aqueous electrolyte solution. Suitable colouring agents are metals such as nickel, cobalt, silver, copper, selenium, iron, molybdenum and tin, and the salts thereof, such as sulphates, nitrates, phosphates, hydrochlorides, oxalates, acetates and tar-trates.
Additives such as aromatic sulphonic acids and organic thio-compounds may be used to aid in obtaining uniformity and depth of colour.
Copper has been found useful as a colouring agent in the process of the present invention. An example of a copper bath which may be employed comprises:
Sulphuric acid 10-25 Copper sulphate 5-15 Magnesium sulphate 0-25 Tin salts, optionally in combination with the sulphates or ace-tates of copper or nickel, are also desirably employed in the process.
A preferred electrolytic colouring bath which in the process of the present invention has been found to provide anodised aluminium product in light to medium colours comprises the following formula tion:

sulphuric acid 5 50 -copper sulphate 5 50 -stannous sulphate 1 10 -tartaric acid 1 10 -nickel acetate 1 10 -boric acid 1 10 -- 8 ~ ~ ~ ~ ~ '~ ~ Case 156-7114 A further preferred bath comprises:
g/1 sulphuric acid 20 - 40 copper sulphate 10 - 25 stannous sulphate 5 - 10 tartaric acid 5 - 10 nickel acetate 5 - 10 boric acid 5 - 10 Varying colours of aluminium may be obtained depending on the con-ditions of anodisation and electrolytic deposition.
For example, an aluminium workpiece having been anodised by direct current in an anodisation bath at 68°F (20°C) comprising:
sulphuric acid 170 g/1 aluminium 5 g/1 glycerine 1.0 Y by vol.
glycolic acid 1.0 Y by vol.
at a voltage of 18V for 40 minutes and at a current density of 15 ASF
(1.61 amps./dm~), which is then subjected to AC-treatment in the same bath at a voltage of 18V for 5 minutes, followed by electrolytic colouring in a bath comprising the following formulation:
sulphuric acid 10 copper sulphate 5 stannous sulphate 5 tartaric acid 5 nickel acetate 5 boric acid 20 at a voltage of 18V, for 0.5 min., 1 minute, 2 and 3 minutes, respectively, has the following colouration as a function of duration of applied current in the electrolytic deposition step:

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- 9 - Case 156-7114 duration of applied current colour (minutes) 0.5 light blue 1.0 blue Z0 light green 3.0 dark green It has been found that deeper colours including those in the blue, blue-gray, green and green-gray range, are obtainable by maintaining a "waiting period" at one or more stages of the process, during which essentially no current is passed to the workpiece in the electrolyte solution.
The cumulative duration of the currentless waiting periods is pre-ferably about 0.5 to 30 minutes.
Preferably such a waiting period is maintained following the AC
teatment step (b), and prior to the electrolytic colouring step (c), of the process of the invention.
For example, the workpiece, having been recovered from the AC-treatment solution of step (b), is then introduced into the electro-lytic colouring solution of step (c) (or another electrolytic solution having an acid strength substantially equivalent thereto), and main-tained therein for a period of time during which essentially no cur-rent is passed to the workpiece, after which the workpiece is sub-jected to further electrolytic treatments according to the invention.
In the case where a direct current pre-treatment of the workpiece is carried out prior to electrolytic colouring, as previously described herein, the currentless "waiting period" is generally effected prior to this DC pre-treatment step. (An additional such waiting period, generally about 0.5 minute in duration, is also preferably maintained between the DC pre-treatment step and the electrolytic colouring step.) More preferably, the workpiece, having been subjected to AC-treat-ment in the electrolytic solution of step (b) is then maintained in such solution (or in another electrolytic solution having substantial-ly equivalent acid strength thereto) for an initial currentless wait-ing period, and thereafter is transferred to the electrolytic colour-- 10 - Case 156-7114 ing solution of step (c) (or another electrolytic solution having substantially equivalent acid strength thereto), where one or more additional such currentless waiting periods are maintained, as de-scribed above, prior to electrolytic colouring according to step (c) of the invention. It is preferred in this case that the initial wait-ing period in the electrolytic solution of step (b) be about 1-20, and preferably 10-15, minutes in duration, and that the subsequent period or periods be about 4-10 minutes in cumulative duration. It has been observed that deepened colours of the resulting product, including deeper blue and blue-gray colour tones at lower AC-treatment voltages and deeper green colours at higher AC-treatment voltages, can be ob-tained by lengthening the duration of the waiting period in the AC-treatment solution (or equivalent acid strength solution) within the above-recited ranges.
The provision of blue, green and other colours of anodised alumi-nium and aluminium alloy by the process of the invention responds to a long-felt need in the art, particularly as concerns architectural alu-minium product.
Following electrolytic colouring, the pores in the anodic oxide film may be sealed by immersion in boiling water or by impregnation with wax-like substances, or by other means such as with chemical treatments, which are known in the art.
The process of the present invention can be applied to all aluminium and aluminium alloys which may be conventionally anodised and electrolytically coloured. Such alloys are well-known and contain at least about 80%, and preferably at least about 95%, aluminium.
In each of the following examples, the aluminium workpiece com-prises a panel of sheet stock type 1100 aluminium alloy about 10 x 15 cm, which has been pre-treated by degreasing with an alkaline cleaner comprising 60-70% by weight borax, ca. 10% sodium tripolyphosphate, ca. 5% trisodium phosphate, ca. 2% sodium gluconate with the balance of a carboxylated surfactant, followed by immersion in aqueous 6%
sodium hydroxide etching solution at 60°C for about 5 minutes.

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- 11 - Case 156-7114 A 45-liter tank equipped with a power source and temperature con-trol means which contains an electrolyte bath of the below-indicated composition, is used for anodisation of the panel, and also for the subsequent AC-treatment. An 18-liter tank also equipped with a power source, containing an electrolytic colouring bath of the below-de-scribed composition, is employed in the colouring step. In the anodisation tank, the panel is adapted to serve as the anode of the external power source, and six strips of aluminium extrusion alloy 6063T6, each approximately 2 x 25 cm, serve as counterelectrodes. The counterelectrodes are arrayed in two parallel rows equidistant from the panel on each side. The electrodes are completely immersed in the bath, current is then applied.
In each of the examples, anodisation is performed by applying direct current to one of the panels at the current density and for the length of time also below-indicated.
Except where otherwise indicated, the panel is thereafter sub-jected to the AC-treatment step of the process, wherein alternating current is applied at the voltage and for the length of time indicated in column (b) in the accompanying Table I.
The panel is then removed from the tank, rinsed with water, and transferred to the electrolytic colouring bath, which has the below-recited composition. Current is applied to the panel at the voltage and for a length of time recorded in column (c) of Table I.
The obtained colours of the panels are recorded in the last column of Table I.
Table II provides the results of standard testing of certain of the panels for weatherability and corrosion resistance.
Unless otherwise indicated, the temperature of the baths is about 20-22.2°C.

- 12 - Case 156-7114 Examples 1-10 (a) Anodisation [step (a)] is carried out employing a direct cur-rent voltage at a current density of 1.61 amps./dm2 for about 35 min-utes in a bath as follows:
165 g/1 sulphuric acid 6 g/1 aluminium 2 vol. Y glycolic acid (b) AC-treatment of the anodised workpiece [step (b)] is then carried out in the bath employed in step (a) under the current con-ditions given in Table I.
(c) Electrolytic colouring [step (c)] is conducted under the cur-rent conditions given in Table I in a bath comprising:
g/1 sulphuric acid 10 g/1 copper sulphate 15 20 g/1 magnesium sulphate Colours in the range of green-gray and blue-gray are obtained, with green generally predominating at the higher alternating current ranges, e-g., about 15 volts or above, and blue predominating at lower current ranges. Reddish colours are observed in Examples 9 and 10 following treatments in step (b) wherein current strength is about 6 volts; however, color tones in the blue and green range can be obtained at lower voltages by employing, e.g., increased acid concentration solutions, higher operating temperatures, etc.
Co~parative Bxamples 11 and 12 The general procedure of Examples 1-10 is employed using baths of the same composition, under the current conditions indicated in Table I, except that no glycolic acid is present in the electrolytic bath used in steps (a) and (b).
Reddish colours are obtained.

~~~~~r-~
- 13 - Case 156-7114 Example 13 The general procedure of Examples 1-10 is followed, an alternating current of 26 volts being employed in step (b). (In addition, 2 vol.9~
glycolic acid is added to the bath used in steps (a) and (b), pro-s viding a total of 4 vol.Y glycolic acid in the bath.) The product is poorly coloured and exhibits spalling. However, the desired colours of the invention may be obtainable under the given voltage conditions by, e-g., lowering acid concentration, reducing temperature, etc.
Examples 14-28 (a) Anodisation is carried out employing a direct current voltage of 18 V for 40 minutes at a current density of 1.61 amps./dmz in a bath comprising:
sulphuric acid 170 g/1 glycolic acid 2.0 vol.Y
glycerine 2.0 vol.Y
aluminium 5 g/1 (b) AC-treatment of the anodised workpiece [step (b)] is then carried out in the bath employed in step (a) under the current conditions given in Table I.
(c) Electrolytic colouring is then carried out under the current conditions indicated in Table I in a bath comprising:
copper sulphate 10 stannous sulphate 5 nickel acetate 5 tartaric acid 5 boric acid 5 sulphuric acid 20 - 14 - Case 156-7114 Comparative Examples 29-33 The general procedure of steps (a) and (c) of Examples 14-28 is repeated employing the same electrolytic baths and the same current conditions for anodisation. (The current conditions.for electrolytic colouring [step (c)] are provided in Table I.) However, step (b) is omitted.
The resulting panels exhibit colours in the red to black colour tones.
The resulting panels of Examples 14-28 and Comparative Examples 29-33 axe then subjected to tests of weatherability and corrosion resistance as recorded in the accompanying Table II.
In Table I, the different columns indicated below have the following meanings: -(b) AC-treatment step (c) electrolytic colouring step bl current (volts AC) cl current (volts) bz duration (min.) cZ duration (min.) - 15 - Case 156-7114 TABLE I
(b) (c) Examples bl b2 C1 CZ Colour 1 15 10 18V-AC 2 dark green-gray 2 16 20 " 1 medium-dark green-gray 3 15 5 " 1 dark green-gray 4 12 5 " 1 medium blue-gray 10 5 " 1 light blue-gray 6 20 5 " 0.5 medium-light green-gray 7 20 5 20V-AC 1.0 medium-dark green-gray 8 24 5 " 0.5 faint green/spalling 9 6 5 18V-AC 2 deep red 6 5 " 4 red-black ComparativeExamples 11 16 5 18V-AC 1 light red 12 16 5 " 2 rose Examples 13 26 5 18V-AC 4 no colour/spalling 14 18 5 " 2 light blue-gray 18 10 " 2 light green 16 18 15 " 5 light green, some spalling 17 23 5 " 0.5 light green 18 23 5 " 2 dark green 19 23 5 " 1 medium green-gray 23 10 " 0.5 light green 21 23 10 " 2 medium green 22 23 10 " 3 dark green gray 23 16 5 " 2 light gray 24 16 5 " 4 medium blue-gray 15 5 " 2 medium blue-gray 26 15 5 " 4 blue-green-gray - 16 - Case 156-7114 TABLE I (continued) (b) (c) Examples bl bz cl c2 Colour 27 20 10 18V-AC 1 green-gray 28 20 10 " 3 dark gray ComparativeExamples 29 -- -- 16V-DC 15 deep red 18V-AC 0.5 light rose 1.0 light red 2.0 medium red 3.0 deep red 5.0 black 30 -- -- 16V-DC 2 medium red 18V-AC 1 light red 31 -- -- 18V-AC 5 black 32 -- -- " 1 light red 33 -- -- " 3 deep red TABLE II
Note Weight Loss~>Admittance3~
corresponding to Grey Scalel~ mg/dm= (uS) 9~ Loss Observed Corrosion of ColourColour e Resistance4>
Chang Examples 23 <10 brighter 2.6 11.0 10 (no attack) 24 <10 " 5.0 7.5 10 ( " ) 25 <10 " 2.8 11.5 10 ( " ) 26 10 " 2.2 8.0 10 ( " ) 27 <10 " 3.2 12.5 10 ( "

28 <10 " 3.4 12.0 10 ( " ) ComparativeExamples 32 10 darker 2.6 8.5 10 ( " ) 33 10 darker 4.0 12.5 10 ( " ) - 17 -~~~~~~~~ase 156-7114 Inscription to Table II:
1> Panels tested on Atlas Weather-0-meter 65 WRC for 7'000 hours of total exposure.
The numeral "10" indicates a loss of colour of about 10%.
The observed change in colour of the panel after testing, whether brighter or darker, is also indicated.
z' Procedure of ISO 3210 -1983(E): - Assessment of quality of anodic oxide film by measurement of loss of mass after immersion in phosphoric-chromic acid solution.
3~ Admittance value (uS) obtained according to the procedure of ISO 2931 - 1983(E).
Results of Copper-accelerated acetic acid salt spray (CASS) test [ISO-3770-1976(E)].
Examples 34-37 - General Procedure (a) Employing the apparatus initially described above, anodisa-tion of the workpiece is carried out by applying direct current to the panel at a current density of 1.61 amps./dmZ for about 35 minutes in a bath comprising:
165 g/1 sulphuric acid 6 g/1 aluminium 2.0 vol.% glycolic acid 2.0 vol.% glycerine (b) AC-treatment of the anodised workpiece is then carried out in the same bath employed in step (a) by passing 14 volts for 10 minutes.
(c) The panel is then removed from the anodisation tank, rinsed with water, and transferred to an electrolytic colouring bath, which comprises:

- 18 - Case 156-7114 15 g/1 sulphuric acid g/1 copper sulphate g/1 magnesium sulphate.
Alternating current is passed at a voltage of 14 volts for 2 min-5 utes.
Example 34 (i) Prior to application of alternating current in step (c) above, the panel is maintained in the colouring bath for a currentless "waiting period" of 20 minutes.

10 The colour of the resulting panel is a deep blue.
Example 35 (i) Following the AC-treatment according to step (b) above, the workpiece is maintained in the electrolyte solution used in step (b) for a currentless "waiting period" of 5 minutes. The workpiece is then 15 removed from the anodisation bath and transferred to the colouring bath.
(ii) Prior to application of alternating current in step (c) above, the panel is maintained in the colouring bath for a currentless "waiting period" of 10 minutes.
20 The resulting panel is observed to have a somewhat deeper blue colouration than the panel of Example 34.
Example 36 The procedure of Example 35 is repeated, with the exception that the aluminium workpiece comprises a panel of 6063-T6 aluminium alloy about 5 x 50cm; the colouring tank comprises a 7-liter tank having dimensions 15 x 15 x 60 cm; and the counterelectrodes comprise 2 rods of stainless steel, 0.64 cm diameter, 15 cm in length, which are placed about 1.3 em from one end of the tank. Thus current density applied to the workpiece in the electrolytic colouring step (c) of the process varies depending on distance from the counterelectrodes.
The resulting workpiece exhibits an intense blue colour in the higher current density zone (i.e., nearest the counterelectrodes) and 2~~~,~s;t~
- 19 - Case 156-7114 a lighter blue colour in the low current density zone (furthest from the counterelectrodes).
Example 37 The procedure of Example 36 is followed, except that after sub-s jecting the anodic workpiece to a currentless waiting time of 10 min-utes in the colouring tank, and prior to application of AC current for electrolytic colouring under the conditions of Example 35, a direct current of 16 V is applied to the workpiece for 2 minutes, and the workpiece is then subjected to a currentless "waiting time" of 0.5 minute.
The resulting panel shows greater unifomity of blue colour, indi-cating that improved throwing power is obtained as a result of appli-cation of direct current in the electrolytic colouring bath prior to application of alternating current. A green colour is also observed in the high current density zone.
Example 38 Steps (a), (b) and (c) of the General Procedure described for Examples 34-37 are carried out, employing the apparatus initially described herein, with the exception that the workpiece and colouring tank apparatus are as described in Example 36. The following addition-al steps are carried out following step (b) (AC-treatment) and prior to step (c) (electrolytic colouring) of the General Procedure, in the order below-indicated:
(i) Following step (b), a panel is maintained in the AC-treatment bath of step (b) for a currentless waiting time period having a dura-tion of either of 0 min.; 2 min.; 10 min.; or 20 min.
(ii) The panel is then transferred to the colouring bath where it is maintained for a currentless "waiting period" of 5 minutes.
(iii) A direct current of 16 V is then applied to the workpiece for 2 minutes;
(iv) The workpiece is subjected to a currentless "waiting time" of 0.5 minute; and step (c) is then carried out.

- 20 - Case 156-7114 A primarily light blue colour of the resulting product is obtained with good colour uniformity in the absence of a waiting period in step (i). It was observed that deeper colours, including predominantly deep blue colours, can be obtained by lengthening the waiting period of step (i) from 0 to 20 minutes.
Example 39 The procedure of Example 38 is carried out, with the exception that in the AC-treatment step (b), an alternating current of 18 volts is employed.
A primarily light greenish-blue colour of the resulting product with good colour uniformity is obtained in the absence of a waiting period in step (i). It was observed that deeper greenish colours can be obtained by lengthening the waiting period of step (i) from 0 to 20 minutes.
The above examples demonstrate that desirable colours of anodised aluminium and aluminium alloy may be obtained by the process of the invention, and that the thus-prepared coloured anodic oxide film has satisfactory corrosion resistance and weatherability.
Of course, various changes and modifications may be made without departing from the invention and it is intended, therefore, that all matter contained in the foregoing description shall be interpreted as illustrative only and not limitative of the invention.

Claims

1. A process for electrolytically colouring aluminium or aluminium alloys comprising:
(a) anodising an aluminium or aluminium alloy workpiece in an aqueous electrolyte solution comprising a strong acid by application of direct current at a current density of 5 to 25 amperes per square foot (0.54-2.7 amps./dm2) and a temperature of from 55-90°F (13-32°C) to form on the workpiece a porous anodic oxide film having a thickness of at least 3 microns;
(b) subjecting the resulting anodised workpiece to alternating current at a voltage of 5-25 volts for 1-25 minutes in an aqueous electrolyte solution comprising a strong acid and an organic carboxylic acid containing at least one reactive group bound to the carbon atom in the alpha-position, wherein said reactive group is a hydroxy, amino, keto or carboxyl group; and (c) electrolytically colouring the workpiece by subjecting it to alternating current in an aqueous electrolyte solution comprising at least one metal salt as a colouring agent.

2. A process according to Claim 1 wherein the aqueous electrolyte solution used in step (a) comprises 90-300g/l of the strong acid.

3. A process according to Claim 1 wherein the aqueous electrolyte solution used in step (b) comprises 120-250g/l of the strong acid and 1-15% by volume of the organic carboxylic acid.

4. A process according to Claim 1 wherein the strong acid used in steps (a) and (b) is sulphuric acid.

5. A process according to Claim 1 wherein steps (a) and (b) are carried out in the same bath.

6. A process according to Claim 3 wherein in step (b) the aqueous electrolyte solution additionally comprises 1-15% by volume of a polyhydric alcohol having 3 to 6 carbon atoms.

7. A process according to Claim 3 wherein the organic carboxylic acid is glycolic acid.

8. A process according to Claim 6 wherein the polyhydric alcohol is glycerol.

9. A process for electrolytically colouring aluminium or aluminium alloys according to Claim 1 comprising:
(a) anodising an aluminium or aluminium alloy workpiece in an aqueous electrolyte solution comprising 120-250g/l of sulphuric acid by application of direct current at a current density of 10 to 20 amperes per square foot (1.08-2.15 amps./dm2) and a temperature of from 65-75°F (18.3-23.9°C) to form on the workpiece a porous anodic oxide film having a thickness of at least 3 microns;
(b) subjecting the resulting anodised workpiece to alternating current at a voltage of 10 to 25 volts for 1 to 25 minutes in an aqueous electrolyte solution comprising 120-250g/l of sulphuric acid and from 1 to 10 % by volume of glycolic acid and from 1 to 10 %
by volume of glycerol; and (c) electrolytically colouring the workpiece by subjecting it to alternating current in an aqueous electrolyte solution comprising a copper salt.

10. A process according to Claim 1 wherein following step (b), the workpiece is subjected to one or more currentless waiting periods.

11. A process according to Claim 10 wherein said one or more currentless waiting periods have a cumulative duration of 0.5 to 30 minutes.

12. A process according to Claim 10 or 11 wherein a currentless waiting period is maintained at least in part in the electrolytic colouring solution of step (c), or another solution having an acid strength equivalent thereto, prior to further electrolytic treatment.

13. A process according to Claim 12 wherein following said currentless waiting period and prior to electrolytic colouring with alternating current, direct current is applied to the workpiece in the electrolytic colouring solution, or an equivalent acid strength solution.

14. A process according to Claim 1 wherein prior to electrolytic colouring according to step (c), direct current is applied to the workpiece in the electrolytic colouring solution, or an equivalent acid strength solution.

15. A process according to Claim 14 wherein following application of direct current and prior to the electrolytic colouring step (c), the workpiece is subjected to a currentless waiting period in the electrolytic colouring solution, or an equivalent acid strength solution.

16. An electrolytic colouring solution comprising:
g/l of solution sulphuric acid 5 to 50 copper sulphate 5 to 50 stannous sulphate 1 to 10 tartaric acid 1 to 10 nickel acetate 1 to 10 boric acid 1 to 10.