Note: Descriptions are shown in the official language in which they were submitted.
~35~
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)
-- 3 -
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.
~ ~3S7~
(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:
;`'~`-;
~33~7~
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
-- 8 --
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:
.
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
- lOa -
3~7;~
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.
- 12 -
~7~
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
RCP 421 400430 365 341 338 328
(Hv)
_ _ __ _ _
DCP _ _ 350 343 331 326
- 13 -
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.
- 14 -
.
3~7~
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
-- 15 --
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.
- 16 -
3~7~:
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
7Z
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.
- 18 -
~33~
(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.
- 19 -
(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.
~0
- 20 -
