GB2108153A - Method of chemically forming and coloring anodized coatings - Google Patents

Abstract

A method of chemically forming and coloring anodized coatings on cast or, in particular, die-cast Al alloys, simultaneously performs the chemical formation and coloration of the anodized coating by using a reversing electric current supplied to the alloy. The method also enables the chemically formed and colored anodized coating to be variously colored by further dipping it in a coloring liquid of an electrolyte containing either of inorganic and organic acids.

Description

SPECIFICATION Method of chemically forming and coloring anodized coatings.

This invention relates to improvements in methods of chemically forming an anodized coating on a cast Al alloy or, in particular, a die-cast Al alloy, as well as in methods of coloring such anodized coating. More specifically, the present invention relates to a novel technique of realizing in the same step the main portions of the chemical formation of the anodized coating on the cast or particularly die-cast Al alloy and of the coloration of the anodized coating.

For the methods of chemically forming and coloring anodized coatings on Al and ordinary Al alloys, there have been already suggested such various methods as, for example, referred to in the following, but they still involve certain defects as will be also detailed: (1) Method of alloying: An alloy element which easily develops a color upon the anodic oxidation is added in advance into an Al material so that the color will be naturally developed when the anodic oxidation coat is chemically produced. However, there are defects that, in this case, the tone of the color developed by the added alloy element is limited, that the color will not develop unless the thickness of the coat is made larger, and that, while the weatherproofness can be improved by such thicker coat, it has been required to employ a high voltage of more than 40V for chemically producing the coat.

(2) Method using an electrolyte: An alloy element easily developing a color upon the anodic oxidation is also added in advance into an Al material and a special electrolyte easily developing a color when the anodic oxidation coat is chemically produced is used to improve the color developing efficiency to be higher than in the foregoing method (1). There are defects that, in this case, though the color of this coat is high in weatherproofness, the electrolyte is more complicated to control and is more expensive than such sulfuric acid or electrolyte containing sulfuric acid as is used in the method (1), that a high electric voltage will be required when the anodic oxidation coat is chemically produced, and that the tone of the developed color is limited as in the case of (1).

(3) Method using chromic acid: This is a method of chemically producing an anodic oxidation coat on an Al alloy by adding chromic acid into the electrolyte and properly adjusting the chemically producing voltage. The appearance of the coat is opaque and presents an enamel-like color tone but there have been defects that the coat is so thin as to be 2 to 5ym and lacks mechanical durability.Further, since it is necessary to so adjust the chemically producing voltage as, for example, to be gradually elevated producing voltage as, for example, to be gradually elevated from 0 to 40V during the first 10 minutes, to be maintained at 40V during the next 20 minutes and to be held at 50V during the last 5 minutes, there have been defects that the adjusting operation is complicated, it is necessary to use a high voltage and, in addition, it is necessary to use chromic acid which is a detrimental substance.

(4) Ematal process: This is a method wherein such salt as of Ti, Zr or the like is added into the electrolyte (oxalic acid) and an oxide of such metal is adsorbed in an anodic oxidation coat while being chemically produced at a chemically-producing voltage of 1 20V. In this case, there are advantages that the anodic oxidation coat is opaque and presents an enamel-like milky white tone, whereas defects have been involved in such that a very high chemically-producing voltage and a costly metallic salt are required and the electrolyte in the electrolytic bath requires a complicated control.

(5) Secondary alternating current electrolysis method (Japanese Patent Application Publication No. 1715/1963): An anodic oxidation coat is chemically produced on an Al material in an electrolyte of sulfuric acid or the like and is then subjected to an alternating current electrolysis in a solution containing a heavy metallic salt so as to be colored. In this case, the tone of the developed color is comparatively rich and, therefore, the method is most extensively utilized as a coloring method of building materials.However, there are defects that the solution containing the heavy metallic salt, that is, the secondary electrolyte is so complicated in its composition and the range of controlling the electrolyzing conditions of the secondary electrolysis is so narrow that the operation is difficult to control and the developed color tone is likely to fluctuate and that, in order to obtain a product of many kinds of tones, electrolytic cells and current sources different with respective tones are required, whereby the equipment must be made large and the equipment cost is caused to be high.

(6) Electric current reversing electrolysis method (Japanese Patent Application Laid-Open Publication No. 145197/1980): While an applied electric current is being periodically reversed, an Al material or Al alloy material dipped in an electrolyte containing sulfuric acid is subjected to a chemical production of an anodic oxidation coat and a sulfur compound is caused to be contained and accumulated in the anodic oxdiation coat, after which the produced coat is dipped in a warmed metallic salt solution to be thereby colored.There are advantages in this method that, as compared with the foregoing methods (1) to (5), the anodic oxidation coat can be colored simply by being dipped in the warmed metallic salt solution after the chemical production of the coat, a color of various kinds of tones can be developed by varying the metallic salt and current reversing conditions, only a dipping vessel is additionally required for the coloring and, consequently, required costs can be well reduced.

Such conventional methods of chemically forming and coloring the anodized coatings as described above have been suggested only for use with Al and ordinary Al alloys and, relative to the cast Al alloys or specifically the die-cast Al alloys, the suggestions have been limited to the chemical formation of the anodized coatings by means of a direct current electrolysis. Yet, such formation has been defective in that the chemically forming voltage is high resulting in a large electric power consumption, the thickness of the anodized coating cannot be made larger, and the hardness of the anodized coating is comparatively low.

A primary object of the present invention is, therefore, to provide a novel method of chemically forming an anodized coating on a cast or particularly die-cast Al alloy.

Another object of the present invention is to provide a method of coloring a chemically formed anodized coating on a cast or particularly die-cast Al alloy by dipping and heating it in either of a warmed metallic salt solution and a warmed sealing liquid containing a metallic salt.

Other objects and advantages of the present invention will become apparent upon reading the following detailed explanation of the invention in respect of certain examples performing the method.

While the present invention shall be detailed in the following with reference to the examples of the method, it is not intended to limit the invention only to these examples but is to include all modifications, alterations and equivalent substitutions possible within the scope of appended claims.

Compositions of the cast Al alloys and die-cast Al alloys utilized in the examples are as in the following Table 1: Table 1 Cast Al Alloy Chemical Composition (Weight %) Die-Cast Al Alloy Cu Si Mg Zn Fe Mn Ni Sn Al 9.0 0.4 ADC 3* < 0.6 -10.0 -0.6 < 0.5 < 1.3 < 0.3 < 0.5 < 0.1 Rest 4.0 ADC 5 < 0.2 < 0.3 -11.0 < 0.1 < 1.8 < 0.3 < 0.1 < 0.1 Rest 1.5 10.0 ADC 12 -3.5 -12.0 < 0.3 < 1.0 < 1.3 < 0.5 < 0.5 < 0.3 Rest 6.5 0.2 Mn Ti ADC 4*C* < 0.2 -7.5 -0.4 < 0.3 < 0.5 < 0.3 < 0.8 < 0.2 Rest *ADC represents a die-cast Al alloy ** AC represents a cast Al alloy However, it will be clear that such other cast and die-cast Al alloys as listed in the following Table 2 can be used as objects of the present invention as will be evident from their composition:: Table 2 Cast Al Alloy Chemical composition (Weight %) Die-Cast Al Alloy Cu Si Mg Zn Fe Mn Ni Sn Al 11.0 ADC 1 < 0.6 -13.0 < 0.3 < 0.5 < 1.3 < 0.3 < 0.5 < 0.1 Rest 2.5 0.4 ADC 6 < 0.12 < 1.0 -4.0 < 0.4 < 0.8 -0.5 < 0.1 < 0.1 Rest 4.5 ADC 7 < 0.6 -6.0 < 0.3 < 0.5 < 1.3 < 0.3 < 0.5 < 0.1 Rest 2.0 7.5 ADC 10 -4.0 -9.5 < 0.3 < 1.0 < 1.3 < 0.5 < 0.5 < 0.3 Rest Referring now to the examples of the method performed According to the present invention: EXAMPLE I: Electrolyte: 20% by weight sulfuric acid Current conditions: 24 Hz;Reversing rate of 11 % Positive current density: 2 A/dm2 Positive-to-negative voltage ratio: 1 /2 Electrolytic bath temperature: 15"C Chemically forming time: 60 minutes Under the above listed conditions, anodized coatings were chemically formed using a carbon plate as an opposite pole.The final chemically-forming voltages in the chemical formation and the thicknesses and hardnesses of the chemically formed anodized coatings in this case were as shown in Table 3, in which results according to a conventional direct current process (abbreviated as DCP) are shown for comparison with those according to a reversing current process (abbreviated as RCP) employed in the present invention:: Table 3 Chemically Final Chemic- Coating Coating Cast Forming ally-Forming Thickness Hardness Al Alloy Process Voltage (V) (Um) (Hv) RCP 25.3 33.1 385 ADC 3 DCP 58.2 20.3 352 RCP 19.7 36.2 457 ADC 5 DCP 25.4 31.5 421 RCP 28.3 32.1 365 ADC 12 DCP 66.0 16.8 331 RCP 22.0 33.4 392 AC 4C DCP 45.1 21.3 365 Referring to Table 2 in view of Table 3, it is found that the final chemically-forming voltage rises with the increase of the content of Si but is reduced to be about one half of that of the conventional direct current process. The thickness of the anodized coating is twice as large as that of the conventional direct current process in some cases and is generally larger than in the case of the conventional direct current process.The hardness of the anodized coating is the highest in ADC 5 which is a die-cast Al alloy of Al-Mg series and is the lowest in the die-cast Al Alloy of the Al-Si-Cu Series. According to the reversing current process of the present invention, the hardness of the anodized coating can be generally made higher than that of the conventional direct current process. In short, it will be clear that the reversing current process of the present invention is superior to the conventional direct current process in all of the final chemically-forming voltage and the thickness and hardness of the anodized coating and can form an anodized coating of a large thickness and high hardness with a low chemically-forming voltage on a cast or die-cast Al alloy of a high Si content or a cast or die-cast Al alloy of Al-Si-Cu series.

EXAMPLE ll: Electrolyte: 20% by weight sulfuric acid Current conditions: 13.3 Hz; Reversing rate of 5% Positive current density: 2 A/dm2 Positive-to-negative voltage ratio: 1/1 Electrolytic bath temperature: 15"C Under these conditions, an anodized coating was chemically formed on die-cast Al alloy of ADC 12, using a carbon plate as an opposite pole.The chemically forming voltages (V) required for respective cases of different chemically forming times (in minutes) are shown in the following Table 4, in which results obtained with the conventional direct current process are also shown for comparison with those of the reversing current process used in the case of the present invention: Table 4 Chemically Chemically Forming Time (minutes) Forming Process 10 20 30 40 50 60 RCP 13.8(V) 20.2 23.5 25.0 27.2 27.8 DCP 28.0 38.0 51.0 58.0 62.5 66.0 As has been clarified in Example I, the chemically-forming voltage in the case of the reversing current process is less than about 1/2 of that in the case of the conventional direct current process. Therefore, the reversing current process of the present invention is lower in the electric power consumption so as to be economical as compared with the conventional direct current process.According to the reversing current process of the present invention, further, the absolute value of the chemically forming voltage is so low and the rise of the chemically-forming voltage with the lapse of time is so small that the Joule heat generated at the time of chemically forming the anodized coating can be well reduced, whereby any dissolution of the chemically formed anodized coating can be well prevented.This shall be referred to next: EXAMPLE 111: Electrolyte: 20% by weight sulfuric acid Current conditions: 24 Hz; Reversing rate of 11 % Positive current density: 2 A/dm2 Positive-to-negative voltage ratio: 1/2 Chemically forming time: 60 minutes Under these conditions, an anodized coating was chemically formed on the die-cast Al alloy of ADC 12, using a carbon plate as an opposite pole while varying the electrolytic bath temperature ("C). The thickness (um) of the anodized'coating thus obtained was as in Table 5.

As evident from Table 5 in which results according to the conventional direct current process are shown for comparison with those of the reversing current process of the present invention, the thickness of the anodized coating can be made larger at the respective temperatures than those of the conventional process. Consequently, the reversing current process of the present invention makes it possible to minimize required cooling facilities in contrast to the conventional reversing current process.

Table 5 Chemically Electrolytic Bath Temperature ("C) Forming Process 0 5 10 15 20 25 30 29.4 RCP (zm) 29.2 31.7 32.1 23.2 17.8 12.5 DCP - - 17.6 16.8 15.7 - - EXAMPLE IV: The hardness (Hv) of the anodized coating as chemically formed by means of the reversing current process under the same conditions as in Example Ill was as shown in Table 6. Results according to the conventional direct current process are also shown for comparison. It will be clear in view of Table 6 that the reversing current process achieves higher hardnesses of the anodized coating at the respective temperatures than those of the conventional process.

Table 6 Chemically Electrolytic Bath Temperature ("C) Forming Process 0 5 10 15 20 25 30 421 RCP (Hv) 400 430 365 341 338 328 DCP - - 350 343 331 326 - In view of the foregoing Examples II to IV, it is found that, according to the reversing current process of the present invention, the anodized coating having a sufficient thickness and hardness can be chemically formed on the cast or die-cast Al alloy with a high temperature electrolytic bath within a short time.

EXAMPLE V: Electrolyte: 20% by weight sulfuric acid Positive current density: 4 A/dm2 Electrolytic bath temperature: 15"C Chemically forming time: 60 minutes Under these conditions, an anodized coating was chemically formed on die-cast Al alloy of ADC 1 2 with a carbon plate used as an opposite pole while varying the electrolyzing conditions, that is, the frequency (Hz), reversing rate (%) and positive-to-negative voltage ratio. The thickness (yam) of the anodized coating thus chemically formed on the die-cast Al alloy ADC 1 2 is shown in Table 7 for each of different lead wires of the ADC 1 2 sample, in comparison with certain results in the case of the conventional direct current process.

Table 7 Lead Wire Condition Current Condition Exposed Al Wire Coated Al Wire Coated ADC 12 13.3 Hz; 5%; 1/1 24.0 (um) 26.7 33.0 13.3 Hz; 14%; 1/1 15.6 26.1 13.3 Hz; 5%; 1/2 27.3 31.3 13.3 Hz; 5%; 1/3 18.3 24.7 24.0 Hz; 11%; 1/2 26.5 32.7 33.7 24.0 Hz; 11%; 1/3 24.1 28.5 DCP 13,8 18.0 In Table 7, the "exposed Al wire" is an Al wire which is used as a lead wire to the sample of the die-cast Al alloy of ADC 1 2 and is not insulatively coated.In this case, as the chemically forming voltage is lower in the Al wire than in the ADC 12, the current will flow more in the Al wire part but less in the ADC 1 2, so that the efficiency of chemically forming the anodized coating will be lower than in the case that the Al lead wire is insulatively coated and the coating obtained will be thin. The "coated ADC 1 2" is ADC 1 2 of the same material which is used as a lead wire to the sample of ADC 1 2 and is insulatively coated. The thickness of the anodized coating could be made larger with the coated ADC 12 than with the "coated Al wire" because, as the lead wire part is of the same material, the current will flow less in the exposed lead wire part but more in the sample.

The thickness of the anodized coating on the die-cast Al alloy of ADC 1 2 can be made large when the reversing rate of the current to be used, that is, the rate occupied by the negative current time width in a cycle is made small. If, in this case, the positive-to-negative voltage ratio, that is; the ratio of the peak value of the positive voltage to the peak value of the negative voltage is reduced to be about 1 /2, the thickness of the anodized coating will be able to be made large. However, if the positive-to-negative voltage ratio is further reduced to reach 1 /3, the thickness of the anodized coating will not be able to be made large.

It will be also clear that the frequency of the current to be used substantially has no influence on the thickness of the anodized coating to be obtained.

As will be evident from Table 7, the reversing current process of the present invention is less influenced than in the case of the conventional direct current process, by the condition of the lead wire to the cast Al alloy or die-cast Al alloy on which an anodized coating is to be chemically formed, and can chemically form an anodized coating of a large thickness.

EXAMPLE VI: Electrolyte: 20% by weight sulfuric acid Current conditions: 1 3.3 Hz; Reversing rate of 14% Positive current density: 4 A/dm2 Positive-to-negative voltage ratio: 1/1 Electrolytic bath temperature: 20"C Chemically forming time: 20 minutes Under these conditions, an anodized coating was chemically formed on each of the die-cast Al alloy of ADC 5 and cast Al alloy of Al-Mn(2%)-Fe(l %) by using a carbon plate as an opposite pole and was then dipped in various treating liquids of metallic salt solutions and sealing liquid for 20 minutes to obtain such colorations as shown in Table 8. The metallic salt solutions were boiled.

Table 8 Die-Cast Cast Al Alloy Al Alloy Al Alloy Al Alloy Coloring Liquid ADC 5 Al-Mn(2%)-Fe(l%) Nickel Sulfate (20g/l) Thick Gray Light Black Cobalt Sulfate (20g/l) Grayish Red Grayish Red Metallic Copper Nitrate (5g/l) Greenish Brown Greenish Brown Salt Silver Nitrate (2g/l) Ochre Ochre Solutions Lead Acetate (2g/l) Lusset Lusset Ferric Ammonium Oxalate (50g/l) Cocoa Cocoa Sealing Liquid Silvery Gray Light Black Among the die-cast and cast Al alloys, such alloys low in Si content as Al-Mg series alloys and Al-Mn-Fe series alloys are low in the natural color development of the anodized coatings and can be effectively colored by selecting the coloring liquid as in Table 8. It will be apparent that any other coloring liquid may be selected as desired.

On the other hand, the die-cast and cast Al alloys show a series of natural color developments in grayish color with the anodized coating and will not be suitable for the coloration into any other colors than black.

On the basis of the foregoing Examples I to VI, the method of chemically forming the anodized coating on the cast Al alloy or particularly die-cast Al alloy and the method of coloring the anodized coating according to the present invention may be viewed as follows: (1) According to the reversing current process of the present invention which periodically including a negative current zone, the required voltage for chemically forming the anodized coating on the cast Al alloy or particularly die-cast Al alloy can be reduced to be about one half of that in the conventional direct current process.

(2) According to the reversing current process of the present invention, further, the thickness of the anodized coating on the cast or particularly die-cast Al alloy can be made twice as large as that according to the conventional direct current process.

(3) According to the inventive reversing current process, the hardness of the anodized coating on the cast or particularly die-cast Al alloy can be made higher than that according to the conventional direct current process.

(4) According to the inventive reversing current process, the voltage for chemically forming the anodized coating on the cast or particularly die-cast Al alloy can be reduced so that the heat generation during the chemical formation can be prevented and the anodized coating can be chemically formed even at a higher temperature than in the conventional direct current process, whereby the required cooling facilities can be effectively minimized.

(5) According to the inventive reversing current process, the hardness of the anodized coating can be made high even when the anodized coating on the cast or particularly die-cast Al alloy is chemically formed at a higher temperature than in the conventional direct current process.

(6) According to the inventive reversing current process, the anodized coating on the cast or particularly die-cast Al alloy can be chemically formed to be of a sufficient hardness and thickness within a short time through the electrolytic bath at a higher temperature than in the conventional direct current process.

(7) According to the inventive reversing current process, the thickness of the anodized coating on the cast or particularly die-cast Al alloy can be made large by making the current reversing rate small.

(8) According to the inventive reversing current process, the thickness of the anodized coating on the cast or particularly die-cast Al alloy can be made large by reducing the positive-tonegative voltage ratio to be about 1/2.

(9) Yet according to the inventive reversing current process, the thickness of the anodized coating on the cast or particularly die-cast Al alloy can be made large by making the reversing rate small and reducing the positive-to-negative voltage ratio to be about 1/2.

(10) Further according to the inventive reversing current process; the anodized coating on the cast or particularly die-cast Al alloy can be made to naturally develop a color when it is chemically formed and, even when the natural development of the color is insufficient, a sufficient coloration can be easily achieved by means of a proper treating liquid therefor.

Claims (9)

1. A method of chemically forming and coloring an anodized coating on a cast Al alloy or particularly die-cast Al alloy, wherein said coating is chemically formed and colored under a supply of an alternately reversing electric current to said alloy within an electrolyte containing either one of inorganic and organic acids.

2. A method according to claim 1, wherein said reversing current is of a comparatively small reversing rate.

3. A method according to claim 1, wherein said reversing current is of a positive-to-negative voltage ratio of about 1/3 to about 2/3.

4. A method according to claim 3, wherein said positive-to-negative voltage ratio is about 1/2.

5. A method according to any preceding claim, wherein said chemically formed and colored coating is further dipped in a coloring liquid.

6. A method according to claim 5, wherein said coloring liquid is a heated metallic salt solution.

7. A method according to claim 5 or claim 6, wherein said coloring liquid contains a sealing agent.

8. A method according to claim 1 and substantially as hereinbefore described.

9. A method of chemically forming and coloring an anodized coating substantially as described herein with reference to the Examples.