JP2953474B2 - Electrolytic treatment of aluminum and aluminum alloy - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for anodizing and electrolytically coloring aluminum and aluminum alloys and compositions useful therefor.

Various electrolytic coloring processes have been developed and include a basic "2" including an anodizing step followed by an electrolytic coloring step.
Certified as a "step" process.

In the anodization step, the aluminum workpiece is electrolyzed under conditions that form an aluminum oxide coating on the surface (commonly referred to as an "anodized film"). Electrolysis is generally performed by applying a direct current to a second metal source such as aluminum or an aluminum workpiece that acts as an anode in an electrolytic bath where graphite acts as a cathode. Aqueous strong acid electrolytes, such as sulfuric acid, are commonly used to provide anodic oxide films having sufficient hardness, corrosion resistance and coloring ability.

The resulting anodized film is an internal protective “barrier”
Including layers. It is dielectric, thin (ie, about 0.1 to
1μ), strong and non-porous. The anodized film also includes a non-dielectric outer layer, which is thicker (i.e., about 3-100), characterized by a pattern of holes extending into the layer, to varying degrees depending on the anodizing conditions.
μ or higher) (Hubner, WWE and A. Schiltkne
cht, The Practical Anodising of Aluminum, MacDonald
& Evans, London (1960), pp. 21-29). The porous outer layer of the anodized film provides a suitable substrate for the deposition of the colorant.

The second stage of the two-stage electrolytic coloring process involves the electrolytic deposition of a colorant, eg, a metal salt or a mixture thereof, into the pores of the anodized film, typically in the presence of an alternating current.

Various factors such as current density and treatment time, temperature and composition of the anodizing and coloring baths and the particular treatment will affect the morphology and properties of the resulting anodized film and its coloration.

For example, depending on the current density in the anodization step, the resulting anodized film varies from a "soft" or porous type film to a "hard" less porous and dense film. In general, porous anodized films can be used at ambient temperatures, ie, about 55-95 ° F. (about 13-35 ° F.), at current densities not exceeding about 25 amps per square foot (ASF) (2.7 amps / dm ² ). C)). About 24 or 25 ASF (2.6-2.7 amps / dm
² ) When anodized under higher conditions at higher current densities, a hard, dense type, low porosity film is obtained, with hardness varying with the anodizing temperature.

U.S. Pat. Nos. 4,180,443 and 4,179,342 disclose that a hard, dense type of anodic oxide coating is formed at about 24 to 36 ASF (2.6 to 39 amps / dm.) ² ) Generated at DC current density. While such a process offers certain advantages in hardcoat technology, it only provides a limited number of colors: dark red, bronze and black.

The present invention relates to improvements in porous anodized film technology, including, inter alia, processes for providing various light to medium color anodized aluminum or aluminum alloys.

[Summary of the Invention]

The method of the present invention comprises: (a) treating an aluminum or aluminum alloy workpiece;
55-90 ° F (13-32 ° C) in electrolyte solution containing strong acid
At a temperature of 5 to about 25 amps / square foot (0.
(B) anodizing by applying a direct current with a current density of 54-2.7 amps / dm ² ) to form a porous anodized film on the workpiece having a thickness of at least about 3μ; The anodized workpiece is treated in an aqueous electrolyte solution containing a strong acid and an organic carboxylic acid containing at least one reactive group of a hydroxy, amino, keto or carboxy group bonded to a carbon atom in the α-position with about 5 to 25%. (C) electrolytically coloring the workpiece by subjecting it to a substantial alternating current in an aqueous electrolyte solution containing at least one metal salt as a colorant. including.

In a preferred embodiment of the present invention, the "standby time"
Maintained at one or more stages of the above process,
During that time, substantially no current is passed through the workpiece in the electrolyte solution. It has been found that such "no current" standby time can provide a dark colored product that is particularly suitable for architectural applications and is advantageous.

In another embodiment of the present invention that provides improved uniform colorability, the workpiece is subjected to a pre-treatment prior to electrolytic coloring (step (c)), including applying a substantial direct current.

[Detailed description]

Prior to the anodizing step, for example, washing and degreasing with hot trichloroethylene or trisodium sulfate and
For example, a known pre-treatment of the aluminum workpiece, such as etching with caustic soda, may be performed.

Anodization is performed by conventional means generally known to those skilled in the art. The aluminum workpiece, acting as the anode of the power supply, is immersed in the electrolytic bath with another source of metal used as the cathode, preferably aluminum, or graphite. Direct current is applied to the workpiece for a time and under conditions suitable for forming an anodized film.

The oxidizing bath contains an aqueous strong acid electrolyte such as sulfuric or phosphoric acid or a mixture thereof.

The acid concentration in the aqueous electrolytic bath is preferably about 90-300 g /
, More preferably 120 to 250 g /. Sulfuric acid is preferred because it provides a film of "architectural quality", that is, having adequate hardness, thickness and corrosion resistance for outdoor use.

Advantageously, a certain amount of aluminum is present in the anodizing bath, which can be obtained by adding a suitable aluminum compound, for example, aluminum sulfate.
The amount of aluminum present in the bath is about 1-10 g /, preferably 1-5 g /.

DC at the anode is about 5~25ASF (0.54~2.7 amperes / dm ^2), more preferably 10~20ASF (1.08~2.2 amperes / dm ^2), a still preferred 15~20ASF (1.6~2.2
Applied to workpieces at current densities of amps / dm ² ).

As used herein, the term "direct current" refers not only to direct current in the strict sense of the term, but also to other substantially identical currents, such as those obtained by full-wave rectification of single-phase alternating current or rectification of three-phase alternating current. Including.

The anodizing bath is desirably at about room temperature, i.e. 55-90 ° C.
F (13-32 ° C), preferably about 65-75 ° F (18-25 ° C),
More preferably it is maintained at about 65-75 ° F (20-22 ° C), so it may be necessary to use equipment to regulate the temperature of the bath during anodization.

In the method of the present invention, the anodizing conditions are preferably selected to provide a porous anodized film having a thickness of about 20-30μ, and are suitable for obtaining such a film by practice within the scope of the present invention. Within the knowledge of the trader.

According to the method of the present invention, the obtained anodized aluminum or aluminum alloy workpiece is then subjected to alternating current (AC) in a strong acid electrolyte aqueous solution. The strong acid electrolyte aqueous solution contains about 1 to 15, preferably about 1 to 10% by volume of an organic carboxylic acid containing at least one reactive group bonded to a carbon atom at the α-position. The reactive group is a hydroxy, amino, keto or carboxyl group.

When the workpiece anodized before electrolytic coloring is subjected to an alternating current treatment using an electrolyte solution containing the organic carboxylic acid compound, it is possible to obtain neutral to monochromatic aluminum including colors in the blue and green ranges. It was found that.

Examples of suitable organic carboxylic acid compounds include glycol (hydroxyacetic acid), lactic acid (hydroxypropionic acid), malic acid (hydroxysuccinic acid), oxalic acid, pyruvic acid and aminoacetic acid and mixtures thereof. Glycolic acid is preferred for the method of the present invention.

It has further been found that the use of certain polyhydric alcohols with the organic carboxylic acid compounds in the AC treatment step results in additional light to medium colors, including colors ranging from blue to blue-grey.

Thus, in one embodiment of the method of the present invention, the AC-treated electrolytic bath further comprises about 1 to about 1 organic carboxylic acid compound.
-15, preferably 1-10% by volume of polyhydric alcohols having 3 to 6 carbon atoms. Examples of suitable polyhydric alcohols are glycerol, butanediol-1,4, butanediol-1,5, mannitol and sorbitol, of which glycerol is preferred.

Most preferably, the AC treated electrolyte bath contains equal parts by volume of, for example, 1 to 10% by volume each of an organic carboxylic acid and a polyhydric alcohol.

In addition, it has been found that the desired light to medium colored aluminum is obtained when the organic carboxylic acid and / or polyhydric alcohol used in the AC treatment step is also present in the oxidation treatment bath. Thus, in one embodiment of the present invention, a common bath is used for both anodization and AC treatment.

 The preferred electrolyte for AC treatment is sulfuric acid.

AC voltage is about 5 to about 25 volts, preferably about 10 to 20 volts
Volts, more preferably about 12-18 volts, most preferably about 12-15 volts for a color in the blue range and 15-18 volts for a color in the green range. The current is applied to the workpiece for about 1-25 minutes.

The waveform may be, for example, symmetric and / or asymmetric, pulsed, anodic and / or cathodic, having a square or sinusoidal output. The current is applied continuously or discontinuously.

The AC treatment bath is maintained at about 55-90 ° F (13-32 ° C), preferably about 65-75 ° F (18-24 ° C).

The anodized aluminum workpiece thus treated is then subjected to electrolysis under generally known conditions to deposit one or more colorants in the pores of the anodized film.

The electrolytic coloring bath contains a strong aqueous acid, preferably sulfuric acid, at a concentration of about 5 to 50 g / total of the bath.

In general, an alternating current is used to cause the colorant to adhere to the pores of the anodized film. The applied voltage is generally
It is in the range of about 5 to about 25 volts, preferably about 10 to 16 volts. The waveform is preferably sinusoidal.

Prior to electrolytic coloring, the workpiece is preferably subjected to an electrolytic "pre-treatment" which involves applying a substantial direct current of the anode. This DC pretreatment step has been found to give a product with improved color uniformity.

To make such improvements, preferably from about 0.5 ASF to
The current density of about 5 ASF (0.054-0.54 amps / dm ² ) is about 0.5
Maintained for ~ 10 minutes.

Although this DC pretreatment step is usually performed in an electrolytic coloring solution, it may be performed in another electrolytic bath having an acid concentration substantially equivalent to the acid concentration of the coloring solution.

After the DC pre-treatment step, the workpiece is then subjected to electrolysis by conventional means as described above using the colorant in the aqueous electrolyte solution. Suitable colorants are metals such as nickel, cobalt, silver, copper, selenium, iron, molybdenum and tin and salts thereof, for example, sulfates, nitrates, phosphates, hydrochlorides, oxalates, acetates and tartaric acids Salt.

Additives such as aromatic sulfonic acids and organic thio compounds may be used to obtain color uniformity and depth.

Copper has been found to be useful as a colorant in the method of the present invention. Examples of useful copper baths include:

g / sulfuric acid 10-25 copper sulfate 5-15 magnesium sulfate 0-25 In the method of the present invention, it is also desirable to use a tin salt together with copper or nickel sulfate or acetate, if desired.

Preferred electrolytic coloring baths which have been found to provide light to medium color anodized aluminum products in the process of the present invention include:

 g / sulfuric acid 5-50 copper sulfate 5-50 stannous sulfate 1-50 tartaric acid 1-10 nickel acetate 1-10 boric acid 1-10 More preferred baths include:

g / sulfuric acid 20-40 copper sulfate 10-25 stannous sulfate 5-10 tartaric acid 5-10 nickel acetate 5-10 boric acid 5-10 Depending on the conditions of anodic oxidation and electrolytic deposition, aluminum of various colors can be obtained.

For example, in a 68 ° F (20 ° C) anodizing bath containing 170 g of sulfuric acid / 5 g of aluminum / 1.0 vol% of glycerin and 1.0 vol% of glycolic acid, 15 ASF (1.61 amp / dm ² ) with a DC voltage of 18 volts Anodize by direct current at a current density of 5% and then AC treated in the same bath at a voltage of 18 volts for 5 minutes, then: g / sulfuric acid 10 copper sulfate 5 stannous sulfate 5 tartaric acid 5 nickel acetate 5 boric acid 20 At a voltage of 18 volts in a bath containing
The aluminum workpiece electrolytically colored for 1 minute, 1 minute, 2 minutes and 3 minutes has the following coloring as a function of the processing time of the current applied in the electrolytic deposition process.

Processing time of applied current (minutes) Color 0.5 Light blue 1.0 Blue 2.0 Light green 3.0 Dark green At one or more stages of this process, there is a “standby time” that does not substantially conduct current to the workpiece in the electrolyte solution. It has been found that by maintaining, deeper colors can be obtained, including colors in the blue, blue-grey, green and green-grey ranges.

The total processing time of this non-current standby time is preferably about 0.5 to 30 minutes.

Preferably, such a waiting time is a factor of the method of the invention.
It is kept after the AC treatment step (b) and before the electrolytic coloring step (c).

For example, the workpiece recovered from the AC treatment solution of step (b) is then placed in the electrolytic coloring solution of step (c) (or another electrolyte solution having substantially equivalent acid strength). The current is held there for a predetermined time, substantially without passing current through the workpiece, and then the workpiece is subjected to another electrolytic treatment according to the present invention. When the DC pre-treatment of the workpiece is performed before the electrolytic coloring as described above, the "stand-by time"
Is generally performed prior to this DC pretreatment step. Between the DC pretreatment step and the electrolytic coloring step, typically for about 0.5 minutes,
It is preferable to maintain an additional such waiting time.

More preferably, the workpiece that has been subjected to AC treatment in the electrolyte solution of step (b) is then placed in such a solution (or in another electrolyte solution having substantially equivalent acid strength). The initial electroless waiting time is maintained and then transferred to the electrolytic coloring solution of step (c) (or another electrolyte solution having substantially the same acid strength), where the step (c) of the present invention is carried out. One or more additional such electroless waiting times are maintained, as described above, before electrolytic coloring according to. In this case, the initial standby time in the electrolyte solution of step (b) is about 1 to 20, preferably 10 to 15 minutes, and the subsequent standby time is preferably about 4 to 10 minutes in total. The resulting product has deeper coloration, including deeper blue to blue-grey at lower AC processing voltages and deeper green at higher AC processing voltages, within the above range, the AC processing solution (or equivalent). In acid strength solutions).

The blue, green and other colors of the anodized aluminum and aluminum alloy obtained by the method of the present invention are:
Particularly for architectural aluminum products, those skilled in the art have long sought after.

After electrolytic coloring, the pores in the anodized film are immersed in boiling water or impregnated with a substance such as wax,
Alternatively, it can be sealed by other means, such as chemical treatment as known to those skilled in the art.

The method of the present invention can be applied to all aluminum and aluminum alloys that have been conventionally anodized and electrolytically colored. Such alloys are well known and contain at least about 80%, preferably at least about 95% aluminum.

In each of the examples below, the aluminum workpiece comprises a panel of 1100 aluminum alloy of sheet stock type of about 10 x 15 cm, which comprises 60-70 wt% borax,
Degrease with an alkaline cleaner containing about 10% sodium tripolyphosphate, about 5% trisodium phosphate, about 2% sodium glyconate and the balance of carboxylated surfactant, and then in a 6% aqueous sodium hydroxide etch solution. It was pretreated by immersion at 60 ° C. for about 5 minutes.

Forty-five tanks equipped with power and temperature control means and containing an electrolyte bath of the following composition were used for the anodization of the panels and subsequent AC treatment. Eighteen tanks equipped with a power supply and containing an electrolytic coloring bath having the following composition were used in the coloring step. In the anodization bath, the panel served as the anode of an external power supply, and six aluminum extruded alloys 6063T6, each approximately 2 × 25 cm, were used as counter electrodes. The counter electrodes were arranged in two parallel rows on each side of the panel at the same distance. The electrode was completely immersed in the bath and then a current was applied.

In each example, anodization was performed by applying a direct current to one of the panels for a predetermined time at the current densities indicated below.

Except where noted, the panels were then subjected to an AC treatment step, in which alternating current was applied for a predetermined time at the voltages shown in Table I, column (b).

The panel was then removed from the bath, washed with water and transferred to an electrolytic coloring bath having the following composition. A current was applied to the panel at a voltage shown in column (c) of Table I for a predetermined time.

 The coloring of the resulting panel is shown in the last column of Table I.

Table II shows the results of standard tests for weather and corrosion resistance of certain panels.

 Unless otherwise stated, the bath temperature was about 20-22.2 ° C.

Examples 9, 10 and 13 are provided as reference examples.

Examples 1-10 (a) Anodization (step (a)) was performed in a bath below for about 35 minutes using a DC voltage with a current density of 1.61 amperes / dm ² .

165 g / sulfuric acid 6 g / aluminum 2% by volume glycolic acid (b) Next, AC treatment of the anodized workpiece in the bath used in step (a) under the current conditions shown in Table I (step ( b))
Was done.

(C) In a bath containing the following, electrolytic coloring (step c)) was performed under the current conditions shown in Table I.

15 g / sulfuric acid 10 g / copper sulphate 20 g / magnesium sulphate Coloring in the green-grey and blue-grey range is obtained, and in the high alternating voltage range, for example, about 15 volts or more, the green color is generally more In the lower current range, the blue color became stronger. In Examples 9 and 10, the current intensity was about 6 volts. Reddish coloring was observed after the treatment of step (b). However, for example, by using a solution with a higher acid concentration, a higher processing temperature, and the like, coloring in the range of blue to green was obtained even at a lower voltage.

Comparative Examples 11 and 12 The general procedure of Examples 1 to 10 was carried out by using glycolic acid in the electrolyte baths used in steps (a) and (b) under the current conditions shown in Table I, using the same bath composition. Went, except that it did not exist.

 A reddish coloration was obtained.

Example 13 Example 1 was performed using 26 volt alternating current in step (b).
Ten general operations were repeated. Further, 2% by volume of glycolic acid was added to the bath used in steps (a) and (b), so that the concentration of glycolic acid in the bath was 4% by volume in total.

The product coloring was poor, showing spalling. However, the desired coloring of the present invention could be obtained under this voltage condition, for example, by lowering the acid concentration, lowering the temperature, and the like.

Examples 14-28 (a) Using a DC voltage of 18 volts, in a bath containing:
Anodization was performed at a current density of 1.61 amps / dm ² for 40 minutes.

170 g of sulfuric acid / 2.0 vol% of glycolic acid 2.0 vol% of glycerin Aluminum 5 g / (b) Then, under the current conditions shown in Table I, the AC of the anodized workpiece in the bath used in step (a) The treatment (step (b)) was performed.

(C) Next, electrolytic coloring was performed under the current conditions shown in Table I in a bath containing the following.

g / copper sulfate 10 stannous sulfate 5 nickel acetate 5 tartaric acid 5 boric acid 5 sulfuric acid 20 Comparative Examples 29-33 Using the same electrolyte bath and the same current conditions for anodic oxidation, the steps of Examples 14-28 (a ) And the general procedure of (c) was repeated. The current conditions for the electrolytic coloring (step (c)) are shown in Table I. However, step (b) was omitted.

 The resulting panel exhibited a red-black coloration.

The panels obtained in Examples 14 to 28 and Comparative Examples 29 to 33 were then subjected to weathering and corrosion resistance tests. Table II shows the results
Shown in

 In Table I, the different columns have the following meanings.

(B) AC treatment step (c) Electrolytic coloring step b ₁ current (volt AC) c ₁ current (volt) b ₂ treatment time (min) c ₂ treatment time (min) Examples 34-37 General Procedure (a) Using the equipment described above, anodize the workpiece, applying a direct current to the panel at a current density of 1.61 amps / dm ² for about 35 minutes in a bath containing: Made by.

165 g / sulfuric acid 6 g / aluminum 2.0 vol% glycolic acid 2.0 vol% glycerin (b) Then anodized workpiece by passing 14 volts for 10 minutes in the same bath used in step (a) Was subjected to AC treatment.

(C) Next, the panel was taken out of the anodized layer, washed with water, and transferred to an electrolytic coloring bath containing the following.

 15 g / sulfuric acid 10 g / copper sulfate 20 g / magnesium sulfate An alternating current of 14 volts was passed for 2 minutes.

Example 34 (i) Prior to the application of the alternating current in step (c) above, the panel was held in the color bath for a 20 minute current free "standby time".

 The color of the obtained panel was dark blue.

Example 35 (i) After AC treatment according to step (b) above, the workpiece was kept in the electrolyte solution used in step (b) for 5 minutes during a no-current "standby time". Next, the workpiece was removed from the anodizing bath and transferred to a coloring bath.

(Ii) Prior to the application of the alternating current in step (c) above, the panel was maintained in the color bath for a 10 minute current free "standby time".

The resulting panel was found to have a somewhat darker blue color than the panel of Example 34.

Example 36 The procedure of Example 35 was repeated, except that the aluminum workpiece included a panel of approximately 5 x 50 cm of a 6036-T6 aluminum alloy, and the colored layer included a 7 liter layer having dimensions of 15 x 15 x 60 cm. , The counter electrode includes two stainless steel rods 0.64 cm in diameter and 15 cm in length, which are approximately one end of the layer.
It was placed at 1.3cm. The current density applied to the workpiece in the electrolytic coloring step (c) was varied according to the distance from the counter electrode.

The resulting workpiece exhibited a strong blue color in the high current density region (ie, closer to the counter electrode) and a lighter blue color in the lower current density region (far from the counter electrode).

Example 37 The procedure of Example 36 was repeated. However, here, after subjecting the anodized product to the non-current standby time of 10 minutes in the colored layer,
Prior to the application of AC current for electrolytic coloring under the conditions of Example 35, 16
Apply bolt direct current to the workpiece for 2 minutes and then remove the workpiece
A 0.5 minute no-current "standby time" was applied.

The resulting panel exhibited a more uniform blue color, indicating that an improved throwing power was obtained as a result of applying a direct current in the electrolytic coloring bath prior to the application of the alternating current. Green coloring was observed in a high current density region.

Example 38 Using the apparatus described above, steps (a), (b) and (c) of the general procedure described in Examples 34 to 37 were performed. However, here the work and the color layer apparatus were the same as described in Example 36. The following additional steps were performed in the following order after step (b) (AC treatment) and before step (c) (electrolytic coloring).

(I) After step (b), the panel was maintained in the AC treatment bath of step (b) for a current-free standby time of 0, 2, 10, or 20 minutes.

(Ii) The panel was then transferred to a color bath where it was maintained for a 5 minute current-free "standby time".

(Iii) A 16 volt direct current was then applied to the workpiece for 2 minutes.

(Iv) The workpiece was subjected to a 0.5 minute non-current "standby time" followed by step (c).

A light blue product with good color uniformity was obtained without the waiting time in step (i). It has been found that by extending the waiting time of step (i) from 0 minutes to 20 minutes, it is possible to obtain a deeper color, mainly including a deep blue color.

Example 39 The procedure of Example 38 was repeated, except that in the AC treatment step (b), an 18 volt alternating current was used.

A light greenish blue product with good color uniformity was obtained without the waiting time in step (i). It has been found that extending the waiting time in step (i) from 0 minutes to 20 minutes results in a darker green tint.

The above examples show that the desired coloring of anodized aluminum and aluminum alloys can be obtained by the method of the present invention, and that the colored anodized films so produced have sufficient corrosion and weather resistance. .

Of course, various changes and modifications can be made without departing from the scope of the invention, and therefore all matters included in the description of the above examples are for illustrative purposes only and are not intended to limit the invention. Please understand that it is not something to do.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-54-19437 (JP, A) JP-A-51-26651 (JP, A) JP-A-62-107095 (JP, A) 500501 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C25D 11/22

Claims (7)

(57) [Claims] (1) When electrolytically coloring aluminum or an aluminum alloy into a blue and / or green and / or gray tone, (a) processing the aluminum or aluminum alloy workpiece by:
In an aqueous electrolyte solution containing 90 to 300 g / 1 sulfuric acid, 55 to 90 ° F
At a temperature of (13 to 32 ° C.), 5 to 25 by anodizing by applying a DC current density of amps / square foot (0.54 to 2.7 Amps / dm ^2), porous having a thickness of at least 3μ Forming an anodized film on the workpiece; (b) obtaining the anodized workpiece from 120 to 250 g / 1 sulfuric acid and 1 to 15 vol.% Of hydroxy, amino bonded to the carbon atom at the α-position. Applying an alternating current of 10 to 25 volts for 1 to 25 minutes in an aqueous electrolyte solution containing an organic carboxylic acid containing at least one reactive group of keto or carboxy groups; / 1 sulfuric acid and at least one metal salt as a colorant in an aqueous electrolyte solution,
Electrolytically coloring by applying an alternating voltage of volts. 2. The method according to claim 1, wherein the electrolyte solution used in step (b) further comprises 1 to 15% by volume of a polyhydric alcohol containing 3 to 6 carbon atoms. 3. The method according to claim 1, wherein the carboxylic acid is glycolic acid. 4. The method according to claim 2, wherein the polyhydric alcohol is glycerin. 5. The method of claim 1, wherein steps (a) and (b) are performed in the same bath. 6. After step (b), a total of 0.5 to 3 workpieces are obtained.
The method of claim 1, wherein the method is subjected to one or more currentless wait times of 0 minutes. 7. A direct current is applied to the workpiece prior to the electrolytic coloring of step (c), in an electrolytic coloring solution or in another electrolytic bath having an acid concentration equivalent to the acid concentration of the coloring solution. Claim 1
The described method.