WO2001018281A1

WO2001018281A1 - Rapid colouring process for aluminum products

Info

Publication number: WO2001018281A1
Application number: PCT/CA2000/001034
Authority: WO
Inventors: Melville Douglas Ball; Robert Arthur Innes; Robert Jones; Mark Adrian Jozefowicz; Barry Best; Barry Roy Ellard; Robin Christopher Furneaux
Original assignee: Alcan International Ltd Canada
Current assignee: Rio Tinto Alcan International Ltd
Priority date: 1999-09-07
Filing date: 2000-09-06
Publication date: 2001-03-15
Anticipated expiration: 2002-03-07
Also published as: AU6975900A

Abstract

A process is described for the production of an anodized aluminum article coloured by virtue of the increasing amount of light scattering by deposited particles and consequent absorption of light within the anodic coating. The process comprises the steps of (a) establishing a porous anodic film on the aluminum surface, (b) treating the porous anodic film so that only a proportion of the pores remain active, and (c) electrolytically depositing an inorganic pigmentary material into the active pores at a current density of at least 10 A/m2. During treatment of the porous anodic film, a proportion of the pores may be encouraged to nucleate and develop pore colonies.

Description

RAPID COLOURING PROCESS FOR ALUMINUM PRODUCTS

Technical Field

This invention relates to a new process for rapidly generating coloured finishes on aluminum sheet or extrusions.

Background Art

The colouring of anodic oxide films by electrolytic deposition of inorganic particles is well known. In the electro-colouring process inorganic particles are deposited in the pores of the anodic oxide film by the passage of electric current, usually alternating current, between an anodized aluminum surface and a counter electrode, while immersed in an acidic bath of an appropriate metal salt. The most commonly employed electrolytes are salts of nickel, cobalt, tin and copper. The counter electrode is usually graphite or stainless steel, although other forms of electrodes may be employed.

It is believed that in the coloured anodic oxide coatings, the increasingly dark colours are the result of the increasing amount of light scattering by the deposited particles and consequent absorption of light within the coating. As the pores of the film become filled with deposited particles, the extent of the scattering by the particles and absorption of light within the film becomes almost total, so that the film acquires an almost completely black appearance. In commercial operations, direct-current anodizing in a sulphuric acid- based electrolyte is normally used for the production of a thick, clear, porous-type anodic oxide coating. In general, anodizing voltages employed for sulphuric acid- based electrolytes range from 12 to 22 volts depending upon the strength and temperature of the acid. Typically in sulphuric acid anodizing the electrolyte contains 15 to 20% sulphuric acid.

It has been shown (G.C. Wood and J.P. O' Sullivan: Electrochimica Acta 15 1865-76 (1970) that in a porous-type anodic aluminum oxide film the pores are at essentially uniform spacing so that each pore may be considered as a centre of an essentially hexagonal cell. There is a barrier layer of aluminum oxide between the bottom of the pore and the surface of the metal. The pore diameter, cell size and barrier layer thickness each have a virtually linear relationship with the applied anodizing voltage. Similar relationships hold true within quite small deviations for other electrolytes employed in anodizing aluminum, for example phosphoric acid, chromic acid and oxalic acid. U.S. Patents 3,382,160 issued May 1, 1968 and 3,616,309 issued

October 26, 1971 describe what is known as the Anolok® electrolytic colouring process where the colour derives from metal deposited in the pores of an anodic film layer. The colour effect is the result of light scattering and a black colour was achieved only after immersion times of 10 minutes or more. A later development was the standard Anolok II interference colouring process as shown in U.S. Patent 4,066,816 issued January 3, 1978 and U.S. Patent 4,251 ,330 issued February 17, 1981. In that process, the obtained colour is a result of light reflected from the top of the metal deposit interfering with light reflected from the aluminum substrate surface. The colour achieved is directly dependent on the distance between the upper surface of the deposit and the underlying aluminum surface, and in the case of Anolok II, this distance is determined solely by the height of the metal deposit.

A further prior procedure is described in U.S. Patent 4,310,586 issued January 12, 1982. That describes an interference colouring process (Anolok III) where the metal deposit itself is relatively shallow and the distance between the surface of the deposit and the substrate is adjusted by anodizing beneath the deposit.

The procedures described in the above patents are relatively slow. For example, in the Anolok® process, if a deep black is required, an immersion time in the colouring stage of at least 20 minutes is required. This becomes a considerable problem particularly for use in a continuous coil system where a long process time requirement means that the metal strip must progress slowly through relatively large tanks.

It is the object of the present invention to provide a very rapid colouring process for aluminum products which can be used in either a discontinuous anodizing process, or in a continuous strip anodizing process. Disclosure of the Invention

The present invention provides a process for the production of an anodized aluminum article by virtue of the increasing amount of light scattering by the deposited metal and consequent absorption of light within the anodic coating. In this process, firstly, a porous anodic film is established on the aluminum surface. Then the porous anodic film is treated so that only a proportion of its original pores remain active and an inorganic pigmentary material is electrolytically deposited into the active pores. The proportion of active pores is such as to maximize the amount of light scattering by the deposited material and consequent absorption of light within the coating.

One way of treating the porous anodic film according to the invention is to enlarge the pores in the film particularly toward their lower ends by anodizing for less than 4 minutes in the presence of an acid electrolyte having strong dissolving power for aluminum oxide whereby the enlarged pore region in the growth direction extends less than 100 nm. An inorganic pigmentary material is then electrolytically deposited into the enlarged pores at a current density of at least 10 A/m² , and preferably at least 15 A/m².

In one embodiment of this invention, a discontinuous anodizing process involves the sequential processing of an aluminum article or articles through a series of separate baths, in each of which, parts of the processing are carried out. In another embodiment, a continuous aluminum strip (coil) is passed through a series of tanks and is processed as it progresses.

It will be apparent that the discontinuous process is suitable for irregular shaped aluminum components, such as extruded sections, or for inflexible panels which cannot easily be coiled or uncoiled. In contrast, the continuous process is only suitable for thin sheet or foil which can be readily coiled and uncoiled. It is generally accepted that, where applicable, the continuous processing of sheet is economically advantageous.

The porous anodic film is formed by conventional anodizing using, for example, sulphuric acid to develop an anodic film of suitable thickness, e.g. greater than about 5μm, for the application of interest. In a typical procedure, the aluminum article is anodized in sulphuric acid at ambient temperature and a voltage of about 17 volts to produce an anodic film about 12 microns thick for architectural applications. At this stage the anodic film has a pore size of about 17 nm and pore spacing (in a hexagonal array) of about 34 nm (between centers). The pore modification stage is an important feature of the present invention. The objective of this stage is to reduce the number of pores which can actively participate in the subsequent deposition process. This can be achieved by means exemplified in the two methods described in the following sections:

Method A - Pore enlargement method. In this procedure, the diameter of a proportion of the pores is enlarged, particularly toward their lower ends, such that the enlarged region of the pores extends less than about 100 nm in the growth direction. This is typically achieved by anodising for a period of time of less than about 4 minutes. The pore enlargement is preferably carried out in a separate stage or tank in phosphoric acid. During the pore enlargement stage, some pores grow (selectively) until the new pore size and barrier thickness correspond to the new anodizing conditions. At the same time some other pores are "necked off' and no longer participate in the anodizing process or in the subsequent colouring process. Thus, there is a reduced number of active pores available for electro-deposition. It is the aim of the pore enlarging procedure to achieve this reduction in the number of active pores (by pore widening) without significantly inhibiting the subsequent electroplating process. The height of the modified pore region must therefore be minimized so that during electroplating, the modified region is quickly filled and the deposit grows into the normal pore structure. The reduced number of active pores means that they fill at a faster rate for a given plating current. Thus, the resulting deposits fill the active pore structure to variable heights up to several microns from the aluminum surface.

The invention contemplates the use of alternating current and/or direct current for pore enlarging. An alternating current is preferably applied at a voltage in the range of about 5 to 15 volts to obtain an enlarged pore region in the growth direction extending from about 50 to 100 nm. The preferred electrolyte is phosphoric acid in a concentration of about 50 to 150 grams/liter and at a temperature of about 15°C to about 20°C, and with these conditions a pore enlargement time of about 4 minutes or less is required. The time for pore enlargement can be reduced still further (to about 2.5 mins) if a slightly higher temperature e.g. 25°C is used. This is of particular advantage in the continuous coil processing embodiment.

When the direct current is used, it is preferably applied at a voltage in the range of about 15 to 20 volts, with the acid electrolyte preferably being at a temperature in the range of 20°C to 30°C. As with the alternating current procedure, the time for pore enlargement is reduced if higher temperatures are used.

Method B - Pore colony method.

In this method, a proportion of the pores is encouraged to nucleate and develop pore colonies in a similar way to that described in the paper by Takahashi et al., J. Electron Microscopy, 22, 149-157, 1973). A pore colony results from the bifurcation of a pore. Thus, a colony may consist of two pores or, after multiple bifurcations, many pores. However, all the pores of a colony originate from one active pore. The extent of pore growth between bifurcations may be very short, possibly less than 3nm. The development of pore colonies occurs in the following way. After growing an anodic film at a selected voltage e.g. 17v, in sulphuric acid, the voltage is reduced by a preselected amount. This voltage reduction can be carried out, using direct or alternating current, with the same acid, or optionally with a different suitable acid, e.g. phosphoric acid. The voltage reduction dramatically reduces the current which flows through the barrier layer, although a relatively high, but reduced, field is maintained across the barrier layer. As a result of normal dissolution and field assisted dissolution, the barrier layer gradually thins until the barrier oxide layer thickness corresponds to the adjusted voltage, when film formation resumes. This dissolution process is affected by randomly distributed flaws or slight variations in the barrier layer thickness process. Thus, for example, a slightly thinner barrier layer below a pore results in a locally higher field intensity and a concomitantly increased rate of field assisted dissolution, and this pore develops faster than adjacent neighbours. In contrast, a pore with a slightly thicker barrier layer results in lower field assisted dissolution and will, therefore, develop more slowly. The net result is that some pores grow faster than others.

The duration of the treatment to form pore colonies is sufficient to cause on average at least one bifurcation of each active pore. This treatment time is typically less than 2 minutes. The variations in rate of development of the pores as described here, means that only a proportion of the initial pore population will develop to the point where film formation can resume. In the absence of competing neighbouring pores, each pore which becomes active will initiate a radially growing pore colony and the anodising current begins to "recover". Eventually, more colonies are initiated and adjacent colonies begin to impinge. The anodising current peaks and then settles down to stable value as "normal" anodising resumes.

It will be noted that each of the above pore colonies is connected to the outer surface of the anodic film through just one active pore. In a subsequent electrodeposition (colouring) process using standard conditions in, for example, a Ni electroplating bath as for method A, the only available paths for the current to flow is through these active pores. Electrodeposition therefore occurs at the barrier layer regions in the pores of the colonies and as the porous colonies fill, the deposit extends from each colony into the associated active pore of the original anodic film. At this point, all the subsequent deposition occurs in the original "active" pores and therefore they fill very quickly for a given apparent current density.

The actual area of the barrier layer through which the plating current can flow is high (the area of the pore colony barrier layer region) and the tends to prevent localised high current densities from generating overheating or spalling effects. The density of active pores and associated pore colonies per unit surface area of the workpiece can be adjusted by means of the stepwise change in voltage which is used, and also by the time allowed for current recovery before the electrodeposition commences. In general, small voltage reductions produce closely spaced pore colonies and larger voltage reductions result in a smaller number of more widely spaced pore colonies. Increasing the time for colony development will initially increase the number of active colonies, but as the colonies impinge, the number of colonies reach a maximum.

For the purposes of this invention, preferably a relatively large proportion of pores, e.g. more than about 10%, needs to remain active to achieve dark or black colouring so that relatively small voltage reductions, e.g. less than 10 to about 20% are preferred. Larger voltage reductions give rise to fewer, though larger colonies and this can lead to difficulties in generating dark colouring effects, although other effects of interest may be achieved in this way. With the modified pores prepared by either of the above methods, the inorganic pigmentary material may be electrolytically deposited very quickly, e.g. less than 5 minutes, at relatively high current densities, e.g. >15 A/m². In the continuous process, using temperature controlled, flowing electrolytes, higher current densities can be used and in this way, a black finish has been achieved in electrodeposition times of less than one minute. The deposited pigmentary material fills the modified portions of the active pores and partially fills the upper non-modified portions thereof, e.g. 15% to 75% of the upper non-modified portion. The pigmentary material may be selected from the group consisting of tin, nickel, cobalt, copper, silver, cadmium, iron or lead. It has been found to be particularly advantageous to electrolytically deposit the inorganic pigmentary material using a low frequency, low duty, AC voltage controlled, square wave with a superimposed positive bias known as the UNICOL® power system. It is described in UK Patent No. 2 063 300A incorporated herein by reference. This waveform results in a higher metal deposition rate because unlike the sinusoidal AC wave, the voltage goes immediately to its maximum value and stays there over the entire positive part of the cycle and metal deposition takes place over the full negative part of the cycle. With the AC wave on the other hand, deposition occurs over a smaller part of the wave form, i.e. only over that portion where the voltage is high enough to overcome the barrier layer threshold voltage. It has also been found to be advantageous to deposit the pigmentary material from an electrolyte having a low level of monovalent cation, e.g. a low level of ions of metals from Group 1 A of the Periodic Table, such as sodium.. Preferably the monovalent cation is present in an amount of less than 20 ppm.

The process of the invention has the important advantage over the prior art of the speed at which the colouring effect is achieved. Using the process of the invention, the colouring effect is achieved in less than 5 minutes while the prior art procedures require at least 20 minutes. This is particularly significant when producing dark blue or black, which normally require the longest processing times. While the process is particularly advantageous in the production of dark blue and black colours, shorter electroplating times have resulted in, for example, a champagne bronze colour and a pewter colour.

Because of the short processing times possible with the process of this invention, it is particularly valuable when used in a continuous coil system. Thus, it is possible to operate a continuous coil system at a greater continuous strip speed and it is also possible to design new installation with smaller processing tasks.

Brief Description of the Drawings

References made to the accompanying drawings which are diagrammatic sections, not drawn to scale, through anodic oxide coatings of an aluminum article, where:

FIG. 1 is a sectional view of one embodiment of an anodic oxide coating of this invention with pore enlargement;

FIG. 2 is a sectional view of an anodic oxide coating of the prior art; FIG. 3 A is a sectional view of an initial anodic oxide coating; FIG. 3B is the view as in FIG. 3 A with some pore development; FIG. 3C is the view as in FIG. 3B with pore colonies initiated;

FIG. 3D is the view as in FIG. 3C with further pore colony growth;

FIG. 3E is the view as in FIG. 3D with well developed pore colonies;

FIG. 3F is the view as in FIG. 3E showing impingement of pore colonies; FIG. 4 A is a sectional view as in FIG. 3C with beginning electrodeposition;

FIG. 4B is a sectional view as in FIG. 3D with beginning electrodeposition;

FIG. 4C is a sectional view as in FIG. 3E with beginning electrodeposition;

FIG. 4D is a sectional view as in FIG. 4A at a later stage of electrodeposition;

FIG. 4E is a sectional view as in FIG. 4B at a later stage of electrodeposition; and FIG. 4F is a sectional view as in FIG. 4C at a later stage of electrodeposition.

Fig. 1 shows an aluminum article 10 carrying an anodic oxide film 12 on its surface. The film contains pores 14 which extend from the outer surface thereof down to a distance 18 from the aluminum/aluminum oxide interface 20. The region between the bottom of the pores and the interface 20 is usually known as the barrier layer.

As shown, the bottom ends of some of the pores 14 are enlarged as enlarged areas 16. The upper ends of the enlarged area 16 extend a distance 21 above the interface 20 and in a typical product this distance is about 80 nm. It will be seen from Fig. 1 that the pigmentary material not only fills the enlarged area 16 of the pores but also extends partway up the upper-non-enlarged portions of the pores 14.

Fig. 2 shows a typical diagrammatic section of an anodic oxide coating obtained by a standard Anolok II interference colouring process. The pores 22 are also provided with enlarged lower areas 24. The lower ends of the enlarged portions are spaced a distance 26 from the aluminum/aluminum oxide interface 20 but in this prior art arrangement, the distance 28 from the top ends of the enlarged portions to interface 20 is typically greater than about 300 nm. In this prior art arrangement, the pigmentary material typically fills only part of the enlarged portions 24 of the pores 22. Thus, Figs. 1 and 2 very clearly demonstrate a fundamental difference between the anodic oxide film obtained according to this invention and that obtained by the standard Anolok II process.

The prior Anolok I® process can be compared to the first stage of the process of this invention in that the anodic oxide film contains the pores 14 as shown in Fig. 1, and the colour is the result of the light scattering. However, in the Anolok I process, the enlarged areas 16 are not formed and in this case, all the pores extend to the barrier layer. When metal is deposited in the pores, a colour effect is achieved as a result of light scattering, but an immersion time of more than 20 minutes is usually required to achieve a black colour. Figs. 3 and 4 illustrate the development of pore colonies according to the invention. Fig. 3 A shows an initial anodic film structure with pores 14 in the film 12, while Fig. 3B shows some pores 30 beginning to develop preferentially. In Fig. 3C colonies 31 are beginning to develop and colony growth 32 can be seen in Fig. 3D. Well developed pore colonies 33 are shown in Fig. 3E, while Fig. 3F shows pore colonies 34 that are beginning to impinge.

In each of Figs. 3B-3F, a plot of anodizing current as a function of anodizing time is also shown. It will be seen that the current progressively increases and peaks in Fig. 3E when the colonies are well developed. From Fig. 3F it can be seen that there is a slight drop in the current as the colonies begin to merge.

Figs. 4A-4F are schematic representations of electrodeposition of pigment material in the pore colonies shown in Figs. 3C, 3D and 3E. Thus, Figs. 4A, 4B and 4C show early stages of deposition 31a, 32a and 33a respectively, while Figs. 4D, 4E and 4F show later stages of deposition 31b, 32b and 33b respectively with the colony structures filled with pigment material. Best Modes for Carrying Out the Invention

Example 1

Tests were conducted using a 1 mm thick sheet of AA1100 anodizing quality (AQ) aluminum with a surface area of approximately 0.2 m². This was initially anodized in 165 gpl sulphuric acid at 20°C and the pores were modified in 120 gpl phosphoric acid at 20°C. Nickel was electrodeposited from a standard, commercially available nickel sulphate based colouring solution (Anolok 5 IX) using the Unicol square wave power supply as described hereinbefore. It was operated at a 20% duty cycle, i.e. the electrodeposition was carried out with the sample positive for 20% of the time and negative for 80% of the time. The conditions used and the results obtained are summarised in Table 1 below.

TABLE 1

Example 2 Another series of colouring experiments were undertaken as shown in Table

2 below. All experiments are based on pore widening techniques to produce coloured finishes. The samples were given an etch pretreatment prior to anodizing. For most of the tests, an initial anodic film thickness of 12 microns was used. The colouring electrolytes employed are detailed in Table 3. In all cases where the colouring was carried with Unicol square wave power supply, a duty ratio of 0.2 and a frequency of 10 Hz were used.

Tests 20 to 21 use an electrolyte with low sodium level and these show that a black finish can be achieved in five minutes after pore widening. This also demonstrates that monovalent cations affect the square wave part of the Unicol colouring process. The electrolyte B contained low magnesium level, using magnesium oxide for pH adjustment only, as it had previously been shown that magnesium sulphate provides no assistance in aiding uniform colour when using the Unicol power source.

Tests 22 to 24 show that the use of an additional DC pre-electrolysis voltage to slightly thicken and equalize the barrier layer oxide, gives a black finish with improved uniformity on 5005 aluminum sheet and 6063 aluminum extrusion. Test 25 demonstrates that the same method can be applied to a cobalt electrolyte; cobalt electrolytes are usually used to produce black finishes in extrusion finishing operations.

Tests 29 and 31 show that a black finish can be produced on a 7.5 micron anodic film in five minutes using the method either with or without the DC pre- electrolysis.

Ten of the black finishes produced were submitted for S.E.M. examination of the cross sections to evaluate the pigment distribution. In all cases with widened pores the pigment was neatly positioned at the base of the pore.

TABLE 2

TABLE 3

ELECTROLYTE USED

Black samples from the above experiments were hot water sealed, and then subjected to seal quality (by admittance only), abrasion resistance and colour measurement tests. The admittance and abrasion resistance results are shown in Table 4 below.

TABLE 4

ADMITTANCE AND ABRASION TEST RESULTS

Samples 25 and 31 failed the (y x t) seal quality requirement (< 500) for black finishes, while sample 21 failed the normal requirement for abrasion resistance. Otherwise, the samples all passed the tests. Thus, the surface finishes met the durability requirements expected of most architectural coatings.

The depth of colour of the coatings was assessed using the L,a,b, colour scale and the results are shown in Table 5 below.

TABLE 5

Colour MEASUREMENTS

From these results, it can be seen that black finishes were obtained that compare to conventionally produced black anodized finishes. Using this method, a nickel black was as black as a cobalt black

Example 3

Experiments have been carried out using a pilot scale continuous anodizing and colouring line where a continuous strip of aluminum sheet is passed through a series of tanks. After passing through a cleaning and etching stage, the strip passes through a rapid sulphuric acid anodizing stage to grow a porous anodic film of about 10 microns in thickness. After a rinse section, the strip then passes through a pore modification anodisation tank where the electrolyte is warm ( ~ 25°C) phosphoric acid. After a further rinse section, the strip passes through the colouring section in which nickel is electrodeposited. After another rinsing stage, the strip passes through a sealing section where the porous film is sealed prior to a final drying and recoiling.

The time spent in each section depends on the speed of the strip through the line and on the length of the strip in each of the sections. By circulating and controlling the temperatures of the various electrolytes in the system, it has proved possible to achieve relatively short processing times and correspondingly faster line speeds. For example, a black finish was obtained with the following conditions in the principal stages :

Stage 1 : Primary anodizing to produce a 12 micron porous film. Electrolyte - 250 g /l H2SO4

Temperature - 50°C

Voltage- 16 V DC

Contact time (in the tank) ~ 100 sees.

Stage 2 : Pore Modification Electrolyte - 120 g/l H3PO4

Temperature - 25°C.

Voltage- 18 V DC

Contact time - 150 sees.

Stage 3 : Electroplating (colouring) Electrolyte - 140 g/l NiSO / 35 g/1 H3BO4

Temperature - 20°C.

Voltage - Biassed square wave 13V AC (sample dc biassed - 6V)

10 Hz, 20% + ve duty cycle. Contact time - 50 sees. Example 4

Tests were conducted using two 1 mm thick sheets of anodizing quality aluminum having an anodizing area of about 10 cm x 10 cm (2 sides). These were initially anodized in sulphuric acid at 20°C and 16 volts DC. Using the same acid bath, a Sample A sheet was further anodized at a reduced voltage of 15 volts DC and a Sample B sheet was further anodized at a reduced voltage of 14 volts DC. The current was monitored and it was found that full current recovery was achieved within about 5 seconds.

Nickel was then electrodeposited from a standard, commercially available nickel sulphate base colouring solution (Anolok 5 IX) as described hereinbefore, using a current density of about 15 AJvcfi. Colours changed similar to the regular Anolok I process with Sample A changing from light bronze through progressively darker bronzes to near black in a period of about 5 minutes. In Sample B the final colour was a mid-bronze colour.

Claims

Claims:

1. A process for the production of an anodized aluminum article coloured by virtue of the increasing amount of light scattering by deposited particles and consequent absorption of light within the anodized coating, comprising the steps of (a) establishing a porous anodic film on the aluminum surface, (b) treating the porous anodic film so that only a proportion of the pores remain active, and (c) electrolytically depositing an inorganic pigmentary material into the active pores at a current density of at least 10 A/m².

2. The process of claim 1 wherein the porous anodic film is treated to enlarge a proportion of the pores in the film particularly toward their lower ends by anodizing for less than 4 minutes in the presence of an acid electrolyte having strong dissolving power for aluminum oxide whereby the enlarge pore region in the growth direction extends less than 100 nm.

3. The process of claim 2 wherein the pigmentary material is deposited for a time period of less than 5 minutes.

4. The process of claim 3 wherein the enlarged pore region in the growth direction extends at least 50 nm.

5. The process of claim 2 wherein the acid electrolyte is a phosphoric acid based electrolyte.

6. The process of claim 2 wherein an alternating current is used for anodizing the film to enlarge the pores.

7. The process of claim 6 wherein the alternating current during pore enlarging is applied at voltages in the range of about 5 to 15 volts, with acid electrolyte temperatures in the range of about 15°C to 25°C.

8. The process of claim 2 wherein a direct current is used for anodizing the film to enlarge the pores.

9. The process of claim 8 wherein the direct current during pore enlarging is applied at voltages in the range of about 15 to 20 volts, with acid electrolyte temperatures in the range of about 20°C to 30°C.

10. The process of claim 5 wherein the phosphoric acid concentration is from about 50 to about 150 grams/liter.

11. The process of claim 10 wherein the inorganic pigmentary material is a metal-containing material in which the metal is selected from the group consisting of tin, nickel, cobalt, copper, silver, cadmium, iron and lead.

12. The process of claim 11 wherein the metal is nickel or cobalt and the aluminum article is coloured black.

13. The process of claim 12 wherein the metal is deposited using an alternating current with a square voltage waveform.

14. The process of claim 13 wherein the metal is deposited using an electrolyte containing a low level of ions of metals from group 1 A of the periodic table.

15. The process of claim 12 wherein the deposited metal fills the enlarged portions of the pores and partially fills the upper non-enlarged portions thereof.

16. The process of claim 15 wherein the deposited metal fills from about 15% to about 75% of the upper non-enlarged portions.

17. The process of claim 15 wherein the porous anodic film is formed to a thickness greater than about 5 microns.

18. A process for the production of an anodized aluminum article coloured by virtue of the increasing amount of light scattering by the deposited particles and consequent absorption of light within the coating, comprising the steps of (a) establishing a porous anodic film on the aluminum surface, (b) enlarging a proportion of the pores in the film particularly toward their lower ends by anodizing for less than 4 minutes in the presence of an acid electrolyte having strong dissolving power for aluminum oxide and (c) electrolytically depositing an inorganic pigmentary material into the enlarged pores at a current density of at least 10 A/m² and a time of less than 5 minutes.

19. A process for the production of an anodized aluminum article coloured by virtue of the increasing amount of light scattering by the deposited particles and consequent absorption of light within the coating, comprising the steps of (a) establishing a porous anodic film on the aluminum surface, (b) enlarging a proportion of the pores in the film particularly toward their lower ends by anodizing in the presence of an acid electrolyte having strong dissolving power for aluminum oxide such that enlarged pore region in the growth direction extends less than 100 nm, and (c) electrolytically depositing an inorganic pigmentary material into the enlarged pores at a current density of at least 10 A/m² and a time of less than 5 minutes.

20. A process for the production of an anodized aluminum article coloured by virtue of the increasing amount of light scattering by the deposited particles and consequent absorption of light within the coating, comprising the steps of (a) establishing a porous anodic film on the aluminum surface, (b) enlarging a proportion of the pores in the film particularly toward their lower ends by anodizing for less than 4 minutes in the presence of an acid electrolyte having strong dissolving power for aluminum oxide whereby the enlarged pore region in the growth direction extends from about 50 nm to about 100 nm, and (c) electrolytically depositing an inorganic pigmentary material into the enlarged pores to fill the enlarged portions of the pores and partially fill the upper non-enlarged portions thereof.

21. The process of claim 20 wherein the pigmentary material fills from about 15% to 75% of the upper non-enlarged portions.

22. The process of claim 1 wherein during treatment of the porous anodic film, pore colonies are formed in association with active pores.

23. The process of claim 22 wherein the treatment of the anodic film is carried out in the same electrolyte as that used to establish the porous anodic film on the aluminum surface.

24. The process of claim 22 wherein the treatment of the anodic film is carried out using direct current.

25. The process of claim 24 wherein the duration of the treatment to form pore colonies is less than 2 minutes.

26. The process of claim 22 wherein the duration of the treatment to form pore colonies is sufficient to cause on average at least one bifurcation of each active pore.

27. The process of claim 22 wherein the inorganic pigmentary material is a metal- containing material in which the metal is selected from the group consisting of tin, nickel, cobalt, copper, silver, cadmium, iron and lead.

28. The process of claim 27 wherein the metal is nickel or cobalt and the aluminum article is coloured black.

29. The process of claim 28 wherein the metal is deposited using an alternating current with a square voltage waveform.

30. The process of claim 29 wherein the metal is deposited using an electrolyte containing a low level of ions of metals from group 1 A of the periodic table.

31. The process of claim 28 wherein the metal is deposited in the pore colonies and partially fills the active pores above the pore colonies.