CA1193572A - Method of forming coloured anodized coating on die-cast auminum alloy articles - Google Patents

Abstract

ABSTRACT

A method of chemically forming and colouring an anodized coating on die-cast Al alloys, the chemical formation and colouration of the anodized coating being performed simultaneously by applying to the alloy an alternating current the polarity of which is reversed to be negative for less than 14% of each cycle, in an electrolyte containing either inorganic or organic acid. The method also enables the chemically formed and coloured anodized coating to be variously coloured and further improved by dipping the coated object into a heated metallic salt solution or water.

Description

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The invention relates to improvements in methods of chemically Eorming anodized coatings on die-cast Al alloys, as well as in methods of colouring such anodized coatings. More specifically, the present invention relates to a novel technique of performing in the same step the main portions of the chemical formation of the anodized coating on the die-cast Al alloy and of the colouration of the anodized coating.
For the methods of chemically forming and colouring anodized coatings on Al and ordinary Al alloys, there have been already suggested such various methods as, for example, reEerred to in the following paragraphs, but they still involve certain disadvantages as will be also detailed:
(1) Alloying Method: An alloy element which easily develops a colour upon anodic oxidation is combined in advance with an Al material so that the colour will be naturally developed when the anodic oxidation coat is chemically produced. However, the method has proven unsatisfactory since the tone of the colour developed by the added alloy element is limited, the colour will not develop unless the thickness of the coat is increased, and while the weather proof condition can be improved by such thicker coat, it has been necessary to employ a high voltage of more than 40V for chemically producing the coating.

(2) Method of electrolysis: An alloy element which easily develops a colour upon anodic oxidation is also combined in advance with an Al material and a special electrolyte easily developing a colour when the anodic oxidation coat is ~3s~z chemically produced is used to improve the colour developing efficiency over that in the foregoing method (1). Although the colour of this coat is highly weather resistant, the electrolyte is more difficult to control and is more expensive than either the sulfuric acid or electrolyte containing sulfuric acid used in method (1). In addition a high electric voltage is required for the chemical production of the anodic oxidation coat and the tone of the developed colour i5 limited as in the case of method (1).

(3) Method using chromic acid: This is a method o~
chemically producing an anodic oxidation coat on an Al alloy by adding chromic acid to the electrolyte and carefully adjusting the voltage during the chemical produc-tion of the coating. The appearance of the coat is opaque and presents an enamel-like colour tone but there are disadvantages in that the coat is thin (in the order of 2 to 5 um) and lacks mechanical durability. Further, since it is necessary to adjust the voltage during the chemical production of the coating so as, for example, to be gradually elevated from 0 to 40V during the 20 first 10 minutes~ to be maintained at 40V during the next 20 minutes and to be held at 50V during the last 5 mi.nutes, there are disadvantages in that the adjusting operation is complicated, lt is necessary to use a high voltage and, in addition, it is necessary to use chromic acid which is an undesirable substance,

(4) Ematal process: This is a method wherein a salt of Ti, Zr or the like is added to the electrolyte (oxa].ic acid) 331;~72 and an oxide of such metal is absorbed in an anodic oxidation coat while being chemically produced at a voltage of 120V. In this case, there are advantages in that the anodic oxidation coat is opaque and presents an enamel-like milky white tone.
However, there are disadvantages in that a very high voltage is required during the chemical production of the coating, a costly metallic salt is required and the electrolyte in the electrolytic bath requires complicated control.

(5) Secondary alternating current electrolysis method (Japanese Patent Application No. 1715/1963) Asada, (published 5 March, 1963): An anodic oxidation coat is chemically produced on an Al ma-terial 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 coloured. In this case, the tone of the developed colour is comparatively rich and, therefore, the method is most extensively utilized as a colouring method of building materials. However, there are disadvantages in that the solution containing the heavy metallic salt, that is, the secondary electrolyte, is of such a complex composition and the range of electroly~ing condi-tions during the secondary electrolysis is so narrow that the operation is difficult to cont:rol. As a result the developed colour tone is likely to fluctuate. In addition, in order to obtain coatings of diEferent tones, diEferent electrolytic cells and current sources are required for each of the respective tones. As a result, the equipment cost is unacceptably high.

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(6) Electrlc current reversing electrolysis method (Japanese Patent Application No. 145197/1980) Nagano Prefecture, (laid-open 12 November, 1980)o While an applied electric curren~ is being periodically reversed, an Al material or Al alloy material dipped in an electrolyte containing sulfuric acid is subjected to the chemical production of an anodic oxidation coat containing a sulEur compound, aEter which the produced coat is dipped in a warmed metallic salt solution to be thereby coloured. There are advantages in this method in that, as compared with the foregoing methods (1) to (5), the anodic oxidation coat can be coloured simply by being dipped in the warmed metallic salt solution after the chemlcal production of the coat, a colour 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 colouring and, consequently, required costs can be well reduced.
Such conventional methods of chemically forming and colouring the anodized coatings as described above have been suggested only for use with Al and ordinary Al alloys and, relative to the die-cast Al alloys, the suggestions have been limited to the chemical formation of the anodized coating by means o~ a direct current electrolysis. Yet, such formation has been unacceptable in that the voltage required for chemical formation of the coating is high resulting in a large electric power consumption, the thickness of the anodized coating cannot be increased, and the hardness of the anodized coating is comparatively low, '~, ?35~2 Summary of the Invention In order to overcome the disadvantages discussed above in connection with conventional methods of chemically forming and colouring anodized coatings, the present invention provides a method of chemically forming and colouring an anodized coating on a die-cast aluminum alloy, wherein the coating is simultaneously chemically formed and coloured under a supply of alternating current to the alloy in an electrolyte containing either one of inorganic or organic acids, the alternating current having a reversing period of less than 14 in each cycle and a positive-to-negative voltage ratio in the range of about 1/3 to about 2/3.
According to a further aspect of the method of the invention, the anodized coating chemically formed and coloured by the method is further subjected to dipping in a heated colour enhancing treating liquid, which may be either water or a metallic salt solution. Useful metallic salts for this purpose are nickel sulfate, cobalt sulfate, copper nitrate, silver nitrate, lead acetate and ferric ammonium oxalate.
Other advantages and objects oE the present invention will become apparent upon reading the following detailed explanation of the invention by way of particular illustrative examples.
While the present invention shall be detailed in the followings with reEerence to the examples of the method, it is not intended to limit the invention only to these examples but to include all modifications, alterations and equivalent substitutions possible within the scope of the appended claims.

i'7~2 Compositions of the die-cast Al alloys and an exemplary cast Al alloy utilized in the examples are as in the following Table 1:
Table 1 Chemical Composition (Weight %) _ _ _ . _ Die-Cast or Cast Cu Si Mg Zn Fe Mn Ni Sn Al Al Alloy _ _ 9.0 0.4 ADC 3* <0.6 -10.0 - 0.6 <0.5 <1.3 ~0. ¦~0.5 <0.1 Rest _ _ __ ~
4.0 ADC 5 <0.2~0.3 -11. ¦<0.1 ~1.8 ~0.3 <0.1 <0.1 Rest _ _ __ 1.5 10.0 ADC 12 - 3.5 -12.0 <0.3 Sl.o <1.3 <0.5 <0.5 <0.3 Res _ _ __ _ ** 6~5 0.2 Mn Ti AC 4C <0.2 _ 7 c - 0.4 <-~ <0. < ~ ~0.2 Rest * ADC represents a die-cast Al alloy ** AC represents a cast Al alloy Elowever, it will be clear that such other 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 compositions:

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Table 2 _ 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. 1~-3 ~- ¦<0.1 Rest 2.5 -0.4 .
ADC 6 <0.12 < l.C -4O0 <0.4 ~O.E -o c <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.~ _ ADC 10 - 4.0 _ 9.' < 0.^ <1.C ~1.^ ~0.~ CO.~ ~0.~ 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: 24Hz; Reversing period oE
11~ in each cycle of alter-nating current Positive current density: 2 A/dm2 Positive-to-negative voltage ratio: 1/2 Electrolytic bath temperature: 15C
Chemically forming time: 60 minutes 3~i7;~

Under the above listed conditions, anodized coatings were chemically formed! using a carbon plate as an opposite pole. The final voltages during 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 . Die-Cast Chemically Final Chemic- Coating Coating or Cast Formingally-Forming Thickness Hardness Al Alloy ProcessVoltage (V) _ (Hv) RCP25.3 33.1 385 ADC 3 DCP 58.2 20.3 352 __ __ RCP 19.7 36.2 457 ADC 5 DCP 2504 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 . 9 .

35'~2 Referring to Table 2 in view of Table 3, it is found that the final voltage during the chemical formation 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. In general the hardness of the anodized coating produced by the reversing current process of the present invention is higher than the hardness of the anodized coating produced by the conventional direct current process on the same alloy. In addition, it is clear that the final voltages during chemical formation are lower in the reversing current process of the present invention than in the conventional direct current process for the same alloy. It is also clear that the thickness of the anodized coating is increased using the reversing current process of the present invention relative to the thickness of the anodized coating produced by the conventional direct current process on the same alloy. It is also clear that the reversing current process of the present invention can form an anodized coating of a large thickness and high hardness with a low voltage during chemical formation on a die-cast Al alloy of a high Si content or a die-cast Al alloy oE Al-Si-Cu series.

~335~2 EXAMPLE II:
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Electrolyte: 20% by weight sulEuric acid Current conditions: 13.3 Hz; Reversing period of 5%
Positive current density: 2 A/dm2 Positive-to-negative voltage ratio: 1/1 Electrolytic bath temperature: 15 C

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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 voltages during chemical formation (V) required for respective cases of different chemical 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 1, the voltage during chemical formation 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 prèsent invention is lower in electric power consumption so as to be more economical compared with the conventional direct current process. ~ccording to the reversing current process of the present invention, .Eurther, the absolute value of the voltages, during chemical :Eormation is so low and the rise of the voltage with the lapse of time is ~,3~

so small that the Joule's heat generated at the time of chemically forming the anodized coating is substantially reduced, thereby preventing any dissolution of the chemically formed anodized coating. This shall be referred to next:
EXAMPLE III:
Electrolyte: 20~ by weight sulfuric acid Current conditions: 24 Hz; Reversing period 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 (DC). The thickness (Jum) 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 of the RCP method can be increased relative to the thickness of the coating of the DCP
method at the respective temperatures. 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.

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Tab]e 5 .. ._ . . . .
Chemicall ¦ Elec-trolytic Bath Temperature ( C) Forming Process 0 5 10 15 20 25 30 __ . _ _ RCP 29.4 29.231.7 32.1 23.2 17.8 12.5 (Jum ) __~ _ _.
DCP _ _ 17.6 16.8 15.7 _ EX.AMPLE 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 III was as shown in Table 6. Results according to the conventional direct current process are also shown for comparison. It will be clear in vi.ew 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 lS 20 25 30 (Hv) _ _ __ _ _ DCP _ _ 350 343 331 326 3S~'~

In view of the foregoing Examples II to IV, it is found that, according to the reversing current process of the present invention, an anodized coating having a sufficient thickness and hardness can be chemically formed on the 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: 15C
Chemically forming time: 60 minutes Under these conditions, an anodized coating was chemically formed on die-cast Al alloy of ADC 12 with a carbon plate used as an opposite pole while varying the electrolyzing conditions, that is, the fre~uency (Hz), reversing period (%) and positive-to-negative voltage ratio. The thickness (~um) of the anodized coating thus chemically formed on the die-cast Al alloy of ADC 12 is shown in Table 7 for different lead wires to the ADC 12 sample, in comparison with the case of the conventional direct current process.

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Table 7 Lead Wire Condition Current Condition __ . .

Exposed Al Wire Coated Al Wire Coated ADC 12 . .. _. _ 13.3 Hz;5~; 1/124.0(~um) 26.7 33.û
. . . ._ 13.3 Hz;14~; 1/115.6 26,1 13.3 Hz;5%; 1/227.3 31.3 13.3 Hz;5~; 1/318.3 24.7 , . _ __ 24.0 Hz;11%; 1/226.5 32.7 33.7 24.0 Hz;11%; 1/324.1 28.5 _ CCP 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 12 and is not insulatively coated. In this case, as the chemically -Eorming 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 12, so that the eEEiciency of chemically forming the anodized coating will be lower than in the case where the Al lead wire is insulatively coated and the coating obtained will be thin. The "coated ADC 12" is ADC 12 of the same material which is used as a lead wire to the sample of ADC 12 and is insulatively coated. The thickness of the anodized coating could be increased on the coated ADC 12 relative to 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 12 can be increased when the reversing period in each cycle of the applied current 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 can be increased. However, if the positive-to-negative voltage ratio is further reduced to reach 1/3, the thickness of the anodized coating can not be increased.
It will be also clear that the frequency of the current to be used has no substantial 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 die-cast Al alloy on which an anodized coating is to be chemically formed, and can chemical:Ly form an anodized coating of significant thickness.

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EXAMPLE VI-Electr~lyte: 20% by weight sulfuric acid Current conditions: 13.3 Hz; Reversing period of 1~%
Positive current density: 4 A/dm2 Positive-to-negative voltage ratio: 1/1 Electrolytic bath temperature: 20C
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(1%) by using a carbon plate as an opposite pole and was then dipped in various treating liquids of metallic salt solutions and water Eor 20 minutes to obtain such colourations as shown in Table 8. The treating liquid was being boiled.
Table 8 Die-Cast Cast Al ALloy AL Alloy Colouring Liquid _ _ _ _ . ADC 5 Al-Mn(2%) - Fe(l~) _ Nickel Sulfate (209~e) Dark Gray Light Black Cobalt Sulfate (20g/Q) Grayish Red Grayish Red Copper Nitrate ( 59/,~) Greenish Brown Greenish Brown Metallic Salt Silver Nitrate ( 2g/~) Ochre Ochre So:Lutions .ead Acetate ( 2g/~) LussetLusset Ferric Al~onium Cocoa Cocoa Oxalate (509/~) _ _ hater Silve~yGrayLight Black . - 17 -,~ i Among the die-cast and cast Al alloys, such alloys low in the Si content as Al-Mg series alloys and Al-Mn-Fe series alloys are low in the natural colour development of the anodized coatings and can be effectively coloured by selecting the colouring liquid as in Table 8. It will be apparent that any other colouring liquid may be selec-ted as desired.
On the other hand, the die-cast and cast Al alloys show a series of natural colour developments in grayish colour with the anodiæed coating and will not be suitable for the colouration into any colours other than black.
On the basis of the foregoing Examples I to VI, the method of chemically forming the anodized coating on the die-cast Al alloy and the method of colouring 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 includes a negative current zone, the re~uired voltage for chemically forming the anodized coating on the die-cast Al alloy can be reduced to 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 die-cast Al alloy can be made twice as thick as that oE anodized coatings produced according to the conventional direct current process.

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(3) According to the inventive reversing current process, the hardness of the anodized coating on the die-cast Al alloy can be made higher than that of anodized coatings produced 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 die-cast Al alloy can be reduced so that the heat generation during the chemical formation can be minimized 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 oE the anodized coating can be made high even when the anodized coating on the die-cast Al alloy is chemically formed at a higher temperature than used in the conventional direct current process.
(6) According to the inventive reversing current process, the anodized coating on the 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 die-cast Al alloy can be increased by making the current reversing rate small.

(8) According to the inventive reversing current process, the thiclcness oE the anodized coating on the die-cast Al alloy can be increased by reducing the positive-to-negative voltage ratio to be about 1/2.

(9) Yet according to the inventi~e reversing current process, the thickness oE the anodized coating on the die-cast Al alloy can be increased by making the reversing rate small and reducing the positive~to-negative voltage ratio to about 1/2.

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

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:

1. A method of chemically forming and colouring an anodized coating on a die-cast Al alloy, wherein said coating is chemically formed and coloured by supplying an alternating current to said alloy in an electrolyte containing either an inorganic or an organic acid, said current having a negative polarity for less than 14% of the alternating cycle and a positive-to-negative voltage ratio of about 1/3 to about 2/3.

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

3. A method according to claim 1 wherein said anodized coating chemically formed and coloured is further dipped in a heated colour enhancing treating liquid selected from a group consisting of a metallic salt solution and water.

4. A method according to claim 3 wherein said colour enhancing treating liquid is a metallic salt solution.

5. A method according to claim 4 wherein said metallic salt solution is a nickel sulfate solution.

Page 1 of Claims

6. A method according to claim 4 wherein said metallic salt solution is a cobalt sulfate solution.

7. A method according to claim 4 wherein said metallic salt solution is a copper nitrate solution.

8. A method according to claim 4 wherein said metallic salt solution is a silver nitrate solution.

9. A method according to claim 4 wherein said metallic salt solution is a lead acetate solution.

10. A method according to claim 4 wherein said metallic salt solution is a ferric ammonium oxalate solution.

Page 2 of Claims