WO1991002830A1

WO1991002830A1 - Improved electrolytic method for coloring anodized aluminum

Info

Publication number: WO1991002830A1
Application number: PCT/US1990/004618
Authority: WO
Inventors: Eliseo Benitez-Garriga
Original assignee: Wolf, David
Priority date: 1989-08-17
Filing date: 1990-08-16
Publication date: 1991-03-07
Also published as: BR9004077A; PT95029A; CA2023502A1; EP0413589A1; AU633132B2; IT9067011A0; NO903635L; KR910004848A; NO903635D0; JPH05125589A; AU6110390A; IT9067011A1; IT1240224B

Abstract

The invention provides an improved electrolytic process for producing anodized metal substrates such as aluminum or aluminum alloys colored using optical interference effects. In particular, the invention pertains to a novel electrolytic method of modifying the anodic barrier layer. The method results in all colors of the visible spectrum without the need for anodizing in a separate phosphoric acid-based electrolyte. The modification procedures consist of treating the metal substrate with a sequence of direct and alternating currents. The alternating current is symmetrical, the voltage of the positive half-wave equal to that of the negative half-wave. A preferred sequence is direct current, alternating current, direct current. As a result, metallic oxide is deposited within the enlarged pores to a defined height, thus achieving the decomposition of light and obtaining the different colors of the visible spectrum. The modification step can be performed in the anodizing bath, the electrocoloring bath, or in a specific bath with an acid electrolyte.

Description

IMPROVED ELECTROLYTIC METHOD FOR COLORING ANODIZED ALUMINUM

BACKGROUND OF THE INVENTION

The coloring of aluminum by formation of anodic oxide films and electrolytic deposition of inorganic particles therein has been known for many years and comprises several well-defined steps. First, anodization of aluminum or other light metal produces a porous oxide film (porous anodic layer) on the metal under alternating or direct current flow in an acid bath in which the metal is - suspended. The bath generally contains sulfuric, oxalic, phosphoric or chromic acid.

In the subsequent electrocoloring process, inorganic material, usually a metal, is deposited in the pores of the anodic oxide film by the passage of an electric current, usually alternating .current, between the anodized aluminum substrate and a counterelectrode, which counterelectrode usually consists of graphite or stainless steel, although nickel, copper, and tin electrodes can also.be used. The deposition of inorganic material functions to give the metal a colored appearance, the apparent color due to optical interference effects. In a porous anodic aluminum oxide film, the pores are evenly spaced apart and there is a barrier layer of aluminum oxide between the bottom

of the pore and the surface of the metal. Inorganic metallic pigments deposited in the pores of the anodic film result in light being scattered both from the lower ends of the individual deposits and from the aluminum/aluminum oxide interface. The color produced depends upon the difference in optical path length resulting from separation of the two light scattering surfaces, e.g. the ends of the deposits and the aluminum/aluminum oxide interface. The pore diameter and barrier layer thickness are directly related to the applied anodizing voltage.

Increase in the size of the deposits and changes in the colors produced can be achieved by modification of the pores adjacent to the barrier layer. In order to obtain coloring by optical interference effects, however, it is necessary to provide anodized metal in which the deposited particles are constrained to have an average size of at least 260 A at a separation distance from the aluminum/aluminum oxide interface of the order of 300-700 A.

Although electrolytic coloring permits colors to be obtained, the repertoire of colors produced is often limited to bronzes, blacks and reds. Furthermore, it is often necessary to have a coloring bath for each color. Therefore, the majority of procedures for anodizing aluminum pieces

only produce limited colors due to the higher costs involved in having multiple coloring baths. In addition, most conventional anodizing procedures use a double anodizing process, exemplified by methods utilizing both sulphuric acid and phosphoric acid anodizing solutions to modify the pores of the barrier layer. Use of a second acidic bath solution, such as phosphoric acid, is disadvantageous because it increases the likelihood of contamination by phosphate ions in the electrocoloring process. Contamination by phosphoric acid in this manner may prevent the effective sealing of the final product and lead to the gradual loss of color through weathering.

Other electrolytic procedures use complex wave forms such as asymmetric sine waves to increase the quality of the final product by producing more consistent colors, but these wave forms require expensive equipment.

It is important, therefore, to develop an electrolytic process that can produce a wide variety of colors quickly and efficiently without the use of unnecessary baths and sophisticated electrical equipment which make the process more complicated and substantially increase the cost.

The main object of the invention is to obtain the range of colors of the visible spectrum by

electrolytic coloring of aluminum or other metals in a simple and uniform manner without contaminating the anodizing and electrocoloring baths with phosphoric acid. A further object of the invention is to provide an improved process for modifying the anodic barrier using as few separate baths as possible.

In particular, one object of this invention provides for barrier layer modification in the electrocoloring bath so that all the colors are obtained in the same tank, thus eliminating a second anodizing treatment.

SUMMARY OF THE INVENTION

This invention pertains to an improved process for the electrolytic coloring of a metallic substrate -such as aluminum or aluminum alloys. It has been discovered that the application of direct and alternating current in a defined sequence will modify the barrier film and the pores, to produce both a wider range and increased brightness of colors. The use of a defined sequence of direct and alternating current to modify the pores of the ■anodic aluminum oxide film and barrier film is designed to more precisely control the dimensions of said films without the need for a second anodizing

bath of phosphoric acid. The improved method uses inexpensive and easily obtained equipment.

In one aspect of the invention, a process for the electrolytic coloring of metallic substrate is provided by the following steps:

(a) developing a porous anodic film on the substrate in a sulphuric acid electrolytic bath;

(b) modifying the anodic barrier film by sequentially applying to the substrate a series of voltages; a first voltage of direct current, a second voltage of alternating current, and, optionally, a third voltage of direct current;

(c) electrolytically depositing an amount of inorganic material in the pores previously modified in step (b) .

In a preferred embodiment, the alternating current used in the modifying method is symmetrical so that the peak voltage of the positive half-wave is equal to the peak voltage of the negative half-wave. The final direct current application is designed to redissolve any electrolytically- deposited inorganic material and to insure uniformity of the barrier film.

In another aspect of this invention, the modification step as outlined above can be performed in either the sulphuric acid anodizing bath, in a separate modification bath, or preferably, in the electrocoloring bath itself.

In a further aspect of this invention, direct current is optionally applied to the metal substrate after the electrocoloring step to redissolve the inorganic deposits. This procedure allows for fine scale adjustment of the color tone, resulting in more precise control of the colors of the final aluminum or other metallic product.

An aluminum article having an anodic oxide coating on its surface is also described, said article produced according to the following process: a. developing a porous anodic film on the substrate in a sulphuric acid electrolytic bath; b. modifying the anodic barrier film by sequentially applying to the substrate a first voltage of direct current, a second voltage of alternating current and a third voltage of direct current; c. electrolytically depositing an amount of inorganic metallic material in an electrocoloring bath, a material deposited within the pores of the oxidizing layer.

Brief Description of the Drawings

FIG. 1 is a schematic illustration of the anodic layer formed on the substrate during the anodization step.

FIG. 2 illustrates the effect of the alternating current treatment of the modification step, serving to increase the diameter of the pore by forming a cavity.

FIG. 3 illustrates deposition of inorganic material in the pores of the anodic layer during the electrocoloring step.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1-3, the anodizing process and the improved method of the present invention are schematically illustrated . Before the metal substrate 10 is subjected to the anodizing process, it is prepared using conventional methods for achieving a uniform, smooth and attractive finish. Initial treatments can comprise degreasing, matting, polishing, rinsing and neutralizing.

The prepared piece is then deposited in the anodic oxidation tank 11 which tank generally contains an acid solution comprising sulphuric acid 12. In some cases, additives can be used in the sulphuric acid bath to diminish the dissolution strength of the electrolyte. Other acids or acid mixtures such as a mixture of sulphuric acid and chromic acid can also be used in the conventional anodizing bath.

The substrate 10 is then subjected to an anodizing flow of direct 13 current wherein the substrate is the positive electrode (anode) and electrodes 14 made of aluminum, carbon, lead, stainless steel and the like, are the negative electrode (cathode).

In this anodizing step, if the substrate 10 is aluminum, an anodic layer 16 is formed on the substrate (FIG. 1). The layer 16 is porous, containing a plurality of evenly spaced pores 18, the distance between the bottom of a pore 18 and the substrate 10, being defined as the barrier film 20. The thickness of the layer 16 and the length and depth of the pores 18 will vary depending on many variables such as time, which will determine the pore thickness; voltage, which will determine the barrier film 20 size; temperature, which will determine the diameter of the pore in addition to the dissolution rate of the anodic layer, and current density.

The types of current used to develop the anodic layer 16 are not critical to the functioning of this invention. Direct current, alternating current; or alternating current with direct current components, either in sine, square, or pulsed waves, in any of their frequencies, can be employed in this conventional anodizing step. In general, direct

current voltages in the range of 16-22 volts are used in sulfuric acid-based electrolytes depending upon the strength and temperature of the acid. Generally, the thickness of the resulting barrier film is on the order of 10 A per volt applied. Typically, in sulfuric acid anodizing bath 12 the electrolyte contains 15-20% (by weight) sulfuric acid at a temperature of 20°C and a voltage of 17-18 volts. In normal sulfuric acid anodizing, the pore diameters 19 are in the range of 150-180 A (15-18nm). The barrier film thickness 20 is typically about equal to the pore diameter 19 in the anodization step. These same conditions hold true with mixed sulfuric acid-oxalic acid electrolytes 12.

The operating ranges which may be used most effectively in this anodizing step are those in which the sulphuric acid electrolyte has a concentration of 50 to 250 g/1, temperatures range from -5 to 40°C, and D.C. voltages range from 5 to 50 volts with preferred voltages of 15-20 volts D.C. The time during which the current is applied may vary from 1 to 100 minutes.

It is essential that the anodic layer 16 has a consistent thickness, height of barrier film 20, and diameter of pore 19. Therefore, established conditions must be maintained within a narrow tolerance range. .Any variation may induce a

different color from that desired. This will result because the color depends upon the thickness of the anodic layer 16 and, especially, on the thickness of the barrier film 20, i.e., the distance between the substrate 10 and the bottom of the pore 18. When inorganic material is deposited within the pores 18 under alternating current conditions by electrocoloring of the porous anodic layer 16 (FIG. 5), the barrier film 20 distance will directly influence the wavelength of visible light, thus producing a wavelength corresponding to a given color of the visible spectrum by optical interference.

Modification of the anodic layer 16 (FIG. 2) is performed in an acid electrolyte 12 which permits the flow of the current through the barrier film 20 and subsequent formation of hydrogen within the pores 18. ^• Generally, this is done by modifying the pore walls surrounding the barrier film 20 to form a cavity 22 with a volume and dimensions proportional to the temperature, concentration, voltage, treatment time, and the like. When the cavity 22 is formed, enlarging of the. volume of the bottom of the pore 18 determines the level of color which may be obtained when inorganic materials are deposited on the pores 18 in the subsequent electrocoloring step. For example, in a small cavity, the barrier

film 20 between the inorganic materials deposited and the metallic substrate 10 is small, resulting in a short wavelength with the appearance of a violaceous color. If the above distance is increased, other colors will begin to emerge.

The barrier film 20 is a semi-conductor which resists the passage of the current. This resistance will be directly proportional to the thickness of - the layer.

It has been discovered that the application of direct and alternating current 17 in a defined sequence will modify the barrier film 20 and the pores 18, to produce both a wider range and increased brightness of colors. The use of a defined sequence of direct and alternating current to modify the pores 18 of the anodic layer 16 and barrier film 20 is designed to more precisely control the dimensions of said components without the need for a second anodizing bath of phosphoric acid. Use of phosphoric acid is problematic since it can be carried over as a contaminant into subsequent treatments and makes the metallic substrate more difficult to seal. Thus, weather-resistance can be impaired. The improved method uses inexpensive and easily obtained equipment.

The term "alternating current" denotes a type of current varying between positive and negative

polarity, having a positive and negative cycle alternately. Alternating current can be a pure sine wave or it can be modified in any other wave form. In a particularly preferred embodiment, the A.C. voltage is symmetrical. The term "symmetrical" refers to the mode of application of alternating current as to well as the values thereof. The term is meant to denote an alternating current in which the peak voltage of the negative half-wave is equal to the peak voltage of the positive half-wave.

In a particularly preferred embodiment of this method, the anodic layer 16 and barrier film 20 are modified by sequentially applying to the substrate 10 a tripartite voltage sequence comprising a first voltage of direct current, a second voltage of symmetrical alternating current, followed by a third voltage of direct current.

The effect of the different voltage treatments on modification of the anodic film is not yet .fully understood. It is probable that the first direct current application provides for a uniform barrier film thickness, while the alternating current serves to increase the diameter of the pore 19 at the bottom of the pore 18 by formation of a cavity 22. Formation of the cavity tends to reduce the size of the barrier film 20. Typically, alternating current treatments in conventional procedures lead to

elevated temperatures in the bath. This, in turn, increases the rates of reaction. Higher temperature will result in variable conditions within the anodizing layer so that the pore diameters/ the cavity dimensions, and the barrier film thickness may not be entirely uniform. The final DC current treatment is applied for a time sufficient to adjust the thickness of the barrier film 20 to the extent necessary to form a film appropriate for the chosen color and to ensure uniformity of barrier film thickness. The uniformity of the barrier film is directly related to the uniformity of the color once inorganic materials are deposited in the pores (Figure 3). By not providing a final D.C. treatment to ensure uniformity of barrier film thickness, the resulting colored metallic substrate will often have a speckled appearance with rainbow-like patches interspersed throughout a colored background. The duration of D.C. treatment is preferably less than about 20 minutes. The duration is strongly dependent on the voltage; low voltages will require a long time to adjust the barrier film 20. Shorter times and higher voltages result in a thinner barrier film 20 and a correspondingly shorter wavelength of light produced by optical interference. Use of a sequential treatment of direct and alternating current serves precisely to

control the barrier film distance 20, thus enabling more control of the final colors when inorganic materials are deposited in the electrocoloring step (FIG. 3). The best results can be obtained by following a tripartite sequence as described above. Other combinations of direct and alternating currents can be used, provided that the alternating current is symmetrical. The final D.C. treatment can be eliminated, but the resulting color may .not be uniform. In all embodiments of the improved modification steps described herein, the D.C. voltage is less than 20 volts. The alternating current is also less than 25 volts.

It is important to maintain a constant temperature with a variation of less than about 2-3°C during the modification process. A temperature of about 20°C is preferred. At temperatures much higher than about 30^βC, the anodic layer 16 is rapidly dissolved due to the higher chemical activity at the higher temperatures. The pores are then enlarged and more metal will be deposited in the subsequent electrocoloring step. This results in darker colors which may be less desirable under certain circumstances.

The time of each voltage treatment will depend on the temperature and other parameters but should be preferably less than 20 minutes, since at times

beyond this point, the process becomes less efficient and consequentlymore expensive.

In a further embodiment of this modification step, special voltage characteristics are chosen to overcome the electrical resistance of the barrier film. As mentioned previously, the barrier film 20 is a semi-conductor, and as the barrier film increases in size, the electrical resistance of the anodic layer 16 also increases concommitantly. Therefore, a preferred method of applying the direct current and symmetric alternating current is to apply the voltage in a linearly increasing manner, in other words, in a "ramped" configuration. This ramping may be particularly important during the A.C. treatment sequence, since the increasing resistance of the barrier film as the film enlarges tends to distort the symmetric sine wave input.

Another important feature of the invention is that these controlled direct. and alternating current treatments can be performed in the anodizing bath (FIG. 1) as well as in the electrocoloring bath (FIG. 3).

Where the anodizing electrolyte is substantially free of metallic salts typically used in electrocoloring, metallic deposits cannot form during the modification step. Where the anodic layer and barrier film are modified by the method of

this invention in the electrocoloring bath itself, the electrolyte is not substantially free of metal salts and pigmentary deposits can form under alternating current conditions. Thus, when the improved modification step of this invention is performed in the electrocoloring bath, pore modification can commence simultaneously with formation of inorganic deposits. The specific voltage sequences described herein can, however, be employed to more precisely control the barrier film thickness and eliminate metal deposition prior to actual electrocoloring.

The alternating current voltage treatment of the modification step, which treatment would normally deposit unwanted metallic pigments in -an electrocoloring bath, is chosen so that the extent of metallic deposition is kept to an absolute minimum. One way to accomplish this is to apply alternating current so as to provide for an extremely thin barrier film.

The final DC treatment of the modification step will cause slight redissolution of any metallic deposits that have been inadvertently formed during the alternating current treatment of the modification step in the electrocoloring bath. This step is advantageous because it provides a more precise control over the barrier film depth prior to

actual electrocoloring, thus leading to more definition in the final color production when electrocoloring does take place under alternating current. When the modification step is performed in the electrocoloring bath, the procedure eliminates the need for separate modification and electrocoloring baths and more efficiently uses available chemicals and electrical equipment.

In yet another embodiment, the improved modification step can also be performed in a completely separate bath. This separate bath is typically an acidic electrolyte containing a carboxylic acid, sulphonated organic, or inorganic mineral acid such as sulphuric acid, oxalic acid, - tartaric acid and the like.

Once the pores 18 and the height of the barrier film 20 are modified according to the improved process of this invention, inorganic materials are deposited in the thus-enlarged end region of the pores in the electrocoloring step (FIG. 3).

The general procedures used in the electrocoloring step are conventional. Inorganic material 24 is contained within a pigmented acidic salt 26 wherein the material is a metal selected from one or more of tin, nickel, cobalt, copper, silver, cadmium, iron, lead, manganese, and molybdenum. Preferably, electrolytic coloring is

performed in a solution of metal salt 26 having a concentration ratio to sulfuric acid of less than 10:1. A counter-electrode 27 is immersed in the metal salt bath 28 and connected to the alternating current source 30. The counter-electrodes can vary with the type of metal salt used. In typical applications, graphite, carbon, nickel, or stainless steel can be used. Since the color produced depends on the^' difference in optical path resulting from separation of the two light scattering surfaces, the separation will depend upon the barrier film distance 20. To obtain colors in the visible range, separation between the surfaces of the deposits 24 and the substrate 10 should be in the range of about 300-700 A. Resulting colors range from blue-violet due to interference effects at the shorter wavelengths and dark green due to interference effects at the longer wavelengths. In the preferred electrocoloring steps, alternating current most efficiently deposits inorganic pigment 24 from metallic salt solutions to the bottom of the pores 18. Typically, alternating current of less than 20 volts is preferred.

The alternating current will deposit the metal oxide to obtain a desired color, the color depending on the height of the barrier film 20. The time during which the alternating current treatment is

applied will determine the tone of the color, and the barrier film distance 20 will determine the actual color itself. For example, a one minute treatment with an alternating current will give a lighter tone of color than a five minute treatment, the difference in tone being primarily a function of the amount of deposited inorganic pigmented material.

It has been shown that, after formation of initial pigment deposits, there is some increase in resistance of the barrier film leading to a change in chemical conditions within the pores that may favor the growth of an additional anodic layer.

The growth of a further anodic layer in the electrocoloring bath is a function of the pH value of the electrolyte which must be set at a level which results in an appropriate rate of layer without excessive redissolution of the deposited pigment material. Although the pH of the electrocoloring solution plays an important role, and should under all circumstances be maintained above 0.8, the exact pH is not critical to this modification step of the invention. Preferably, the electrocoloring solution electrolyte has a pH from 0.5 to 2.0.

In a further embodiment of this invention, a short anodic direct current treatment can be employed after the alternating current treatment in

the electrocoloring bath. This short DC treatment functions to yield the same effect as would a short DC treatment in the modification step, when said step is performed in the electrocoloring bath. The purpose of this final DC treatment after electrolytic deposition of pigment under AC ^•conditions is to reduce the intensity of the color by redissolving the metal oxide deposits. The DC treatment is continued for a short time (about.1/2 to 3 minutes). The current is at a voltage less than about 25 volts. This procedure allows for a fine scale adjustment of the final barrier film thickness and amount of deposits, resulting in muted colors and a more precise control of the final aluminum or other metallic colored product.

The invention is illustrated further by the following examples:

EXAMPLE 1

This Example illustrates modification of the anodic layer in a separate bath using a tripartite sequence of direct current, alternating current, and direct current.

Two pieces of 6063 aluminum alloy were cleaned in a soap solution, etched in.5 percent caustic soda at 60^βC, desmutted in nitric acid solution (1:1) and

anodized in a sulphuric acid bath at a temperature of 20°C with a direct current charge density of 2.5A/dm for 30 minutes. This resulted in an anodic porous layer of at least 18.microns.

One of the pieces was rinsed and transferred to a modification bath having sulphuric acid (50 g/1) at 20°C. The aluminum piece was used as a positive cathode and lead electrodes were the negative anode. A D.C. current of 16 volts was applied for 3 minutes. Following this the piece was subjected to a symmetrical alternating current of 4 volts for 3 minutes, followed by a D.C. current of 3 volts for 4 minutes.

The piece was then rinsed and transferred to an electrocoloring bath containing 16 g/1 tin sulphate, 17 g/1 sulphuric acid, 2 g/1 phenosulphuric acid. The electrodes were stainless steel and the aluminum was subjected to an alternating current of 18 volts for 4 minutes. A bright green color was obtained (Table 1) .

Table 1 further sets out the different colors that can be obtained when the D.C. current of the_, modification step is temporally varied in accordance with the improved modification procedure of the invention.

TABLE I

Film Barrier film modification Resultin

Heiσhts (A) under DC or AC current (minutes) Colors

400 to 415 2 grey

415 to 490 3 blue

490 to 560 4 green

560 to 580 5 yellow

580 to 660 6 red

660 to 700 8 purple

The second of the two anodized pieces was treated identically except that the three part modification step was eliminated. A bronze color was obtained.

EXAMPLE 2

This Example illustrates modification of the anodic layer in the electrocoloring bath using a dual sequence of direct current and sequence wave alternating current.

An aluminum piece was degreased in an alkaline cleaner and desmutted for.10 minutes in a 10% sodium

hydroxide solution at 60^βC. It was then rinsed, neutralized, and then anodized in a bath comprising 180 g/1 sulphuric acid and 15 g/1 aluminum sulphate at a temperature of 19 + 0.5C and a direct current of 3 A/dm for 20 minutes with a positive charge. The electrodes were carbon.

The piece was rinsed and transferred to an electrocoloring bath containing 16 g/1 stannous sulphate, 20 g/l nickel sulphate, 25 g/1 sulphuric acid, 2 g/1 phenosulphuric acid and 2 g/1 citric acid. The aluminum piece as the positive pole was treated to a.direct current of 0.4 A/dm for 3 minutes. Stainless steel electrodes were the negative pole. Next, the piece was subjected to a symmetrical square wave alternating current having a current density of 0.5 A/dm for 4 minutes and then to a symmetrical sinusoidal alternating current of 18 volts for 3 minutes. A green color was produced.

EXAMPLE 3

This Example illustrates modification of the anodic layer in the electrocoloring bath using a dual sequence of direct current and symmetrical alternating current.

A 6063 aluminum alloy piece was introduced into an anodizing bath having an electrolyte containing

155 g/l sulphuric acid, 3 g/1 boric acid, 2 g/1 glycerin, at a temperature of 24 + 0.5C. Lead electrodes were used under pulsating direct current of 4 A/dm for 40 minutes. The piece was then transferred to a modifying bath and a direct current of 0.5 A/dm was applied for 5 minutes, the piece having a positive charge, in an electrolyte containing 200 g/1 sulphuric acid. The piece was next treated by applying a symmetrical alternating current under current density of 0.8 A/dm for 2 minutes. Finally, the piece was rinsed and colored in an electrolyte containing 18 g/1 stannous sulphate, 1 g/1 ascorbic acid, 2 g/1 citric acid with tin electrodes and subjected to alternating current at a voltage of 18 volts for 5 minutes until the desired color gray was obtained.

Even though the invention has been described and shown in connection with specific embodiments thereof, it is understood by those skilled in the art that modifications may be made to the invention itself or to any of its applications mentioned herein and that the same are encompassed within the spirit and scope of the invention, as defined in the following claims.

Claims

1. A process for the electrolytic coloring of a metallic substrate by optical interference effects, comprising:

(a) developing a porous anodic barrier layer on the substrate in an electrolytic bath;

(b) modifying the anodic barrier layer by sequentially applying to the substrate a defined voltage sequence of direct current and alternating current;

(c) electrolytically depositing an amount of inorganic material in an electrocoloring bath, the material deposited in the pores of the anodic barrier layer by alternating current.

2. The process of claim 1, wherein the defined voltage sequence comprises a first direct, current, followed by a first alternating current.

3. The process of claim 2, wherein the first alternating current is symmetrical.

4. The process of claim 3, further comprising modifying the anodic barrier layer by applying a second voltage of direct current.

5. The process of claim 4, wherein the modification step is performed in the bath selected from the acid electrolytic bath and the electrocoloring bath.

6. The process of claim 5, wherein the third voltage of direct current is applied for less than about 20 minutes.

7. The process of claim 6, wherein the direct current voltage is less than about 25 volts.

8. The process of claim 4, wherein the modification step is performed in a separate bath, comprising an electrolyte containing an acid selected from sulphonated acid; carboxylic acid and mineral acid.

9. The process of claim 1 further comprising adjusting the amount of inorganic material deposited in step (c) by further subjecting the substrate in the electrocoloring bath to direct current.

10. The process of claim 8, wherein the applied voltages are ramped.

11. The process of claim 10, wherein the metallic substrate is selected from aluminum and aluminum alloy.

12. In a process for electrolytically coloring a metallic substrate by optical interference having the steps of establishing on the substrate a porous anodic film in an anodizing bath, modifying the pores in the film; and electrolytically depositing an inorganic material in the pores in an electrocoloring bath, wherein the improvement comprises, modifying the porous anodic film by subjecting the substrate to a first voltage of direct current and to a first voltage of alternating current.

13. The process of claim 12, wherein the alternating current is symmetrical.

14. The process of claim 13, wherein the improvement further comprises subjecting the substrate to a second voltage of direct current.

15. The process of claim 14, wherein the modification is performed in the anodizing bath.

16. The process of claim 14, wherein the modification is performed in a separate bath comprising an electrolyte containing an acid selected from sulphonated acid, carboxylic acid, and mineral acid.

17. The process of claim 14, wherein the second direct current voltage is applied for less than about 20 minutes.

18. The process of claim 17, wherein the modification is performed in the electrocoloring bath.

19. The process of claim 18, wherein the direct current voltage is from less than about 25 volts.

20. A colored aluminum article having a porous -anodic oxide coating on its surface, produced by the method of claims 1 or 12.