U.S. patent number 3,929,593 [Application Number 05/507,216] was granted by the patent office on 1975-12-30 for method of forming colored oxide film on aluminum or aluminum alloy material.
This patent grant is currently assigned to Nippon Light Metal Co., Ltd., Riken Light Metal Industries Company, Ltd.. Invention is credited to Isao Hara, Yoshio Hirayama, Hirosuke Kanamori, Kazuyoshi Kaneda, Masahiro Mikamo, Toshihiro Nagano, Ken Sato, Noboru Sugiyama, Masahiro Takahashi, Toshiro Takahashi.
United States Patent |
3,929,593 |
Sugiyama , et al. |
December 30, 1975 |
Method of forming colored oxide film on aluminum or aluminum alloy
material
Abstract
A method of forming a colored oxide film on an aluminum or
aluminum alloy material by electrolyzing the aluminum or aluminum
alloy material used as one or each of the electrodes in an
electrolytic bath containing a metallic salt while applying a pulse
voltage consisting of a plurality of unipotential pulses whose
polarity is reversed at every predetermined conduction time. The
aluminum or aluminum alloy material may also be one that has an
oxide film previously formed thereon.
Inventors: |
Sugiyama; Noboru (Shizuoka,
JA), Takahashi; Masahiro (Shizuoka, JA),
Kanamori; Hirosuke (Shizuoka, JA), Sato; Ken
(Shizuoka, JA), Hirayama; Yoshio (Shizuoka,
JA), Mikamo; Masahiro (Tokyo, JA),
Takahashi; Toshiro (Shizuoka, JA), Nagano;
Toshihiro (Shizuoka, JA), Kaneda; Kazuyoshi
(Shizuoka, JA), Hara; Isao (Shizuoka, JA) |
Assignee: |
Riken Light Metal Industries
Company, Ltd. (BOTH OF, JA)
Nippon Light Metal Co., Ltd. (BOTH OF, JA)
|
Family
ID: |
14439131 |
Appl.
No.: |
05/507,216 |
Filed: |
September 18, 1974 |
Foreign Application Priority Data
|
|
|
|
|
Sep 21, 1973 [JA] |
|
|
48-106656 |
|
Current U.S.
Class: |
205/105;
204/DIG.8; 205/107; 204/DIG.9; 205/173 |
Current CPC
Class: |
C25D
11/14 (20130101); C25D 11/22 (20130101); Y10S
204/09 (20130101); Y10S 204/08 (20130101) |
Current International
Class: |
C25D
11/22 (20060101); C25D 11/04 (20060101); C25D
11/18 (20060101); C25D 11/14 (20060101); C25D
005/48 (); C25D 011/22 (); C25D 005/18 () |
Field of
Search: |
;204/35N,38A,58,DIG.8,DIG.9 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Mack; John H.
Assistant Examiner: Weinstuch; Aaron
Attorney, Agent or Firm: McNenny, Farrington, Pearne &
Gordon
Claims
We claim as our invention:
1. A method of forming a colored oxide film on an aluminum material
by electrolyzing said aluminum material in an aqueous electrolytic
bath containing a metallic salt by supplying said aluminum material
serving as each electrode with a pulse voltage consisting of a
plurality of unipotetial pulses whose polarity is inverted after
every predetermined conduction time.
2. The method according to claim 1, wherein said aluminum material
has an oxide film previously formed thereon.
3. The method according to claim 1, wherein said aluminum material
is electrolyzed by applying a pulse voltage whose positive and
negative pulse waveforms are rectangular ones.
4. The method according to claim 1, wherein said aluminum material
is electrolyzed by applying a pulse voltage which is obtained by
phase-controlling an AC wave.
5. The method according to claim 1, wherein said aluminum material
is electrolyzed in an aqueous sulfuric acid solution containing a
metallic salt.
6. The method according to claim 3, wherein said aluminum material
is electrolyzed in an aqueous sulfuric acid solution containing a
metallic salt.
7. The method according to claim 4, wherein said aluminum material
is electrolyzed in an aqueous sulfuric acid solution containing a
metallic salt.
8. The method according to claim 3, wherein the positive and
negative pulse wave forms are symmetrical.
9. The method according to claim 6, wherein the positive and
negative pulse waveforms are symmetrical.
10. The method according to claim 3, wherein said aluminum material
is electrolyzed by applying a rectangular pulse voltage which is
obtained by rectifying AC and whose pulse intervals of the positive
and negative pulse waveforms are determined based on the unit
period of said AC.
11. The method according to claim 6, wherein said aluminum material
is electrolyzed by applying a rectangular pulse voltage which is
obtained by rectifying AC and whose pulse intervals of the positive
and negative pulse waveforms are determined based on the unit
period of said AC.
12. The method according to claim 3, wherein the ratio of the unit
pulse period to the pulse duration and the peak value of said pulse
voltage are independently selectively controlled.
13. The method according to claim 3, wherein the pulse duration of
the positive rectangular pulse voltage and that of the negative
rectangular pulse voltage are controlled to be
10.times.10.sup..sup.-3 sec. or longer.
14. The method according to claim 3, wherein the time for the
positive rectangular pulse voltage and the negative rectangular
pulse voltage to fall to the value of 1/4 of its peak voltage value
is selected to be 1/3 of the pulse interval or shorter than it.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of forming a colored oxide film
or the surface of an aluminum or aluminum alloy material
(hereinafter referred to simply as an aluminum material), and more
particularly to a method of forming a colored oxide film on the
surface of an aluminum material by electrolyzing the aluminum
material in an electrolytic bath containing a metallic salt to
thereby color the oxide film in color tone characteristic of the
metal in the metallic salt.
2. Description of the Prior Art
Herertofore, a variety of methods have been employed for forming a
colored oxide film on the surface of an aluminum material by
electrolyzing the aluminum material by applying thereto a
predetermined voltage in an electrolytic bath containing a metallic
salt. In one such method, an oxide film is formed first by
electrolyzing the aluminum material used as an anode and then
colored by applying an AC voltage to the aluminum material in an
aqueous solution containing a metallic salt.
With this method, however, the colored oxide film forming process
is composed of two steps and, further, it is necessary that the
second step using the AC field be achieved in relation to the oxide
film formation of the first step. This introduces defects such as
difficulty in bath control, low productivity, a narrow range of
color tone of the colored oxide film and poor reproducibility of
color tone, making it difficult to obtain uniform colored oxide
films at all times.
SUMMARY OF THE INVENTION
It is one object of this invention to provide a colored oxide film
forming method that, in an electrolytic bath containing a metallic
salt, an aluminum material with no oxide film formed thereon or an
aluminum material having an oxide film previously formed thereon,
used as one or each of electrodes, is electrolyzed by supplying the
aluminum material with a voltage of pulse waveform which is
positive or negative for a period of time longer than one pulse
period at the shortest, whereby an oxide film colored over a wide
range of color tone is formed on the surface of the aluminum
material.
It is another object of this invention to provide a colored oxide
film forming method that, in an electrolytic bath containing a
metallic salt, an aluminum material with no oxide film formed
thereon or an aluminum material with an oxide film previously
formed thereon, used as one or each of electrodes, is electrolyzed
by supplying the aluminum material with a pulse voltage of
rectangular waveform which is positive or negative for a period of
time longer than one pulse period at the shortest, thereby to form
a colored oxide film on the surface of the aluminum material.
Other objects, features and advantages of this invention will
become apparent from the following description taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a waveform diagram of a pulse voltage which is obtained
by half-wave rectification of a single-phase sine-wave voltage and
whose polarity is inverted at every predetermined conduction;
FIG. 2 is a waveform diagram of a pulse voltage which is obtained
by phase-controlling the waveform of FIG. 1 with a silicon
controlled rectifier;
FIG. 3 is a waveform diagram of a rectangular-wave pulse voltage
whose polarity is inverted at every predetermined conduction and
whose positive and negative pulses are both of rectangular
shape;
FIG. 4 is a graph showing the interrelationships of a pulse period
T/a, pulse duration .tau.a, a peak voltage and color tone of the
resulting colored oxide film in an electrolysis which is effected
in an electrolytic bath containing a metallic salt by applying a
rectangular pulse voltage whose polarity is inverted at every
predetermined conduction;
FIG. 5 is a graph showing the same relationships as in FIG. 4 in
the case of using a silver salt as a metallic salt;
FIG. 6 shows an equivalent circuit of an aluminum material
electrolytic cell;
FIGS. 7 and 8 show voltage variations between both electrodes due
to a discharge from an electrolytic cell during an electrolysis
employing such a rectangular-wave pulse voltage as depicted in FIG.
3;
FIGS. 9 and 10 are circuit diagrams illustrating means for
preventing the influence of the discharge from the electrolytic
cell such as shown in FIGS. 7 and 8;
FIG. 11 shows the mode of a current flowing between both electrodes
in an electrolytic bath which is caused by the influence of
accessories to an electrolyzing equipment such as leads or the like
during an electrolysis using such a rectangular-wave pulse voltage
as shown in FIG. 3; and
FIG. 12 is a waveform diagram of a rectangular-wave pulse voltage
produced by half-wave rectification and phase control of a
commercial AC.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will hereinafter be described in detail.
At first, an aluminum material is subjected to pretreatment as is
the case with ordinary electrolysis. This pretreatment is not
related directly to the present invention and may be mechanical or
chemical pretreatment. Further, an aluminum material having an
oxide film previously formed thereon can also be used and, in this
case, the mechanical or chemical pretreatment is achieved prior to
the formation of the oxide film, so that such pretreatment need not
be effected again.
Then, in an electrolytic bath containing a metallic salt, the
aluminum material with no oxide film formed thereon or the aluminum
material with an oxide film previously formed thereon is
electrolyzed by applying a voltage of the following characteristics
to it, the aluminum material being used as one or both
electrodes.
The voltage used in this case is a voltage of pulse waveform, which
is applied to the aluminum material for a period of time longer
than one pulse period in its positive and negative cycles
alternately.
Further, this voltage of pulse waveform is one that the duration of
each pulse is short, the initial and final values of the pulse are
equal to each other and the pulse rises up to a predetermined
level. This voltage may be such as shown in FIG. 1, which is
obtained by halfwave rectification of a single-phase sine-wave
voltage, or as shown in FIG. 2, which is obtained by
phasecontrolling the voltage of FIG. 1 with a silicon controlled
rectifier or like rectifier, or such as shown in FIG. 3, which is a
rectangular-wave voltage, or may be a triangular-wave,
exponential-wave or a partly sine-wave voltage. Of these voltages,
the pulse voltage of rectangular waveform is the easiest to obtain
industrially and, by changing the characteristic values of the
rectangular-wave as required, oxide films colored over a wide range
of color tone can be obtained.
The characteristic values of the rectangular-wave pulse voltage are
pulse durations .tau. and .tau..sub.a, pulse periods T and T.sub.a,
peak voltages V.sub.p and V.sub.pa and conduction times t and
t.sub.a, as shown in FIG. 3. The metallic salt, which is contained
in the electrolytic bath, is selected in relation to the color tone
of the colored oxide film which is desired to obtain and it may be
a sulfate, nitrate or any other salt. Further, the electrolytic
bath is required only to be conductive and a sulfuric acid aqueous
solution is the most inexpensive, and hence economical.
By electrolyzing the aluminum material under such conditions as
described above, an oxide film which is composed of an aluminum
oxide and colored, that is, a colored oxide film is formed on the
surface of the aluminum material.
Now, a description will be given of the colored oxide film forming
mechanism mainly in connection with the case of electrolyzing an
aluminum material by applying thereto the rectangular-wave pulse
voltage shown in FIG. 3. In the rectangular-wave pulse shown in
FIG. 3, the positive and negative pulse waveforms are different
from each other but the characteristic values of the bath pulse
waveforms, for example, the peak voltages V.sub.p and V.sub.pa, the
pulse periods T and T.sub.a, the pulse durations .tau. and .tau.
and the conduction times t and t.sub.a, can be selected identical
or symmetrical with each other, respectively. Accordingly, the
rectangular-wave pulse voltage will hereinafter be described on the
assumption that the characteristic values of both pulse waveforms
are identical to each other. However, this invention is not limited
specifically to the above. For example, by selecting the conduction
times of the positive and negative pulses to be different from each
other, color tone of the colored oxide film can be changed as
required and, also by selecting the peak voltage, the period and
the duration of the positive pulse to be different from those of
the negative pulse respectively, color tone of the colored oxide
film can be changed as desired.
Namely, in the present invention, if alternately supplied with the
positive and negative pulses such as depicted in FIG. 3, the
aluminum material serving as one or each of the electrodes becomes
positive and negative alternately with the predetermined conduction
times t and ta. In this case, while the polarity of the aluminum
material remains positive, the aluminum material is oxidized as the
anode to form an oxide film. Then, when the polarity of the
aluminum material becomes negative, metallic ions dissociated and a
metal ionized in the form of a metallic salt in the electrolytic
bath enter into the oxide film. (The above metallic ions and metal
will hereinafter be referred to simply as the metallic salt.) Next,
when the polarity of the aluminum material becomes positive again,
an oxide film is formed as mentioned above and, in addition, the
metallic salt having entered into the oxide film is also oxidized
and the resulting products are electro-precipitated in the oxide
film when the polarity of the aluminum material becomes negative
again, thus forming a colored oxide film.
With such a mechanism, the colored oxide film is formed on the
aluminum material and the conditions for the above coloring
mechanism are satisfied by the electrolysis using the pulse
voltage. The use of such a rectangular-wave voltage as shown in
FIG. 3 facilitates fulfillment of such conditions.
Thus, while the negative rectangular-wave pulse voltage is applied
to the aluminum material to electrolyze it, the metallic salt
invades the oxide film formed on the aluminum material during the
application of the positive rectangular-wave pulse voltage or the
oxide film formed previously. Since the applied pulse voltage is of
rectangular waveform, predetermined invading energy is obtained and
the metallic salt invades the oxide film to the vicinity of the
bottom of each pore therein.
The positive pulse voltage, V.sub.p in particular, the
rectangular-wave pulse voltage, has practically no rise time as
shown in FIG. 3 and the negative peak voltage V.sub.pa also acts on
the aluminum material with practically no rise time, so that the
invading energy of the metallic salt is provided simultaneously
with rising of the pulse voltage. Hence, the metallic salt enters
deeply into the pores of the oxide film, that is, down to the
bottoms of the pores.
The metallic salt thus driven into the oxide film by the
electrolysis by the application of the negative rectangular-wave
pulse voltage is electrolyzed and oxidized again by the application
of the positve rectangular-wave pulse voltage. While the positive
rectangular-wave pulse voltage is applied, oxidation of the
metallic salt is promoted to provide an excellent colored oxide
film.
When the oxide film into which the metallic salt has entered is
electrolyzed by the application of the positive pulse voltage, the
metallic salt is oxidized but one part of the resulting product is
eluted and, further, one part of the metallic salt is eluted before
oxidized. During the next application of the negative pulse
voltage, the remaining oxidized metallic salt is
electro-precipitated in the oxide film and serves as a coloring
source. Accordingly, in order that the colored oxide film may be of
clear and deep color tone when it is gradually formed by repeatedly
effecting the above processes, it is necessary that
electro-precipitation and elution of the product are balanced with
each other. The rectangular-wave pulse voltage satisfies this
requirements most easily and, in the electrolysis using the
rectangular-wave pulse voltage, it is easy to control the voltage
to fulfill the requirement.
The amount of the metallic salt oxidized during electrolysis by the
application of the positive pulse voltage increases in proportion
only to the magnitude of the applied voltage V.sub.p as shown in
FIG. 3. The amount of the oxidized product re-eluted is in
proportion to the product of the value of the applied voltage and
the duration thereof, that is, the amount of positive charges. For
example, in the case of the voltage of the waveform of FIG. 3, it
is in proportion to (V.sub.p.times..tau.) and, at the same time,
the oxide film is formed in proportion to the amount of positive
charges or current flowed.
Consequently, for electrolyzing the aluminum material in such a
manner as to oxidize the metallic salt and to increase the
electro-precipitated product within a range in which the oxide film
can be formed on the aluminum material and to prevent re-elution of
the product, it is preferred that the value of the applied voltage
be as large as possible and that the amount of positive charges or
the amount of current is small.
In the case of the rectangular-wave pulse voltage, it is possible
to apply a high voltage instantaneously.
In the case of controlling the amount of positive charges and the
peak voltage at will as described above, the rectangular-wave pulse
voltage shown in FIG. 3 is easier to control than the other pulse
voltages and has an advantage that a colored oxide film of desired
color tone can be obtained.
FIG. 4 generally shows the relationships of the pulse durations
.tau. and .tau..sub.a, the pulse periods T and T.sub.a and the peak
voltages V.sub.p and V.sub.pa to color tone of the oxide film in
the case where an aluminum material A.A6063 was electrolyzed by the
rectangular-wave pulse voltage of FIG. 3 in a sulfuric acid aqueous
solution containing a metallic salt. FIG. 5 shows similar
relationships in the case of an electrolysis in a sulfuric acid
aqueous solution containing Ag.sub.2 SO.sub.4. As is apparent from
both graphs, the relationships of FIG. 4 and those of FIG. 5
employing a special metallic salt (Ag.sub.2 SO.sub.4) are
substantially the same but there are some occasions when chemical
and physical properties of the metallic salt used differ a little
in accordance with the kind of metallic salt added. In FIG. 5,
triangles, white circles, black circles and crosses indicate
yellow, light reddish orange, reddish orange and unclear reddish
orange colors, respectively.
In FIGS. 4 and 5, n=pulse period/pulse duration (=T/.tau.>1 or
T.sub.a /.tau..sub.a >1).
In FIG. 4, the ordinate represents the peak voltage and the
abscissa represents n. Reference characters A, B, C and D indicates
zones of color tone of the oxide film. As is seen from a comparison
of FIGS. 4 and 5, for example, in the case of an electrolysis using
the pulse voltage in a sulfuric acid aqueous solution containing
Ag.sub.2 SO.sub.4, the zones A,B and C correspond to yellowish,
light reddish orange and partly deep reddish orange colors,
respectively, and the zone E is one in which the oxide film is
destroyed even if any kind of metallic salt is employed.
As shown in FIG. 4, in the zone E above the line I--I, the peak
voltage is high and a current flows excessively, so that the oxide
film is broken and its color tone becomes unclear. Hence, it is not
preferred to raise the peak voltage above the line I--I. In the
zones lower than the line I--I, by electrolyzing with the pulse
voltage at different values of n and the peak voltage, colors such
as shown in FIG. 4 can be obtained with ease.
Further, the zone below the line II--II is divided into the zones
A,B and C in accordance with the values of n and the peak voltage.
In each zone, the oxide film is colored only by the balance between
the amount of the invaded metallic salt oxidized and
electro-precipitated and its eluted amount. For example, as the
value of n increases, the color tone of the oxide film changes from
A to B and C one after another. For example, in the zone A in which
the value of n is small, the amount of current flowed is large and
the amount of metallic salt eluted is larger than that oxidized and
electro-precipitated and, as a result of this, the color tone of
the oxide film becomes light and, in the case of FIG. 5, the oxide
film becomes yellowish. In the zone B in which the value of n is a
little larger than that in the zone A, the amount of metallic salt
eluted is a little smaller than that in the zone A and the color
tone of the oxide film becomes a little deeper. For example, in the
case of FIG. 5, the oxide film becomes of a light reddish orange
color. Further, in the zone C in which the value of n is larger
than that in the zone B, the pulse width .tau. is small but the
quiescent time h=T-.tau. is short, so that the amount of positive
current flowed decreases and the thickness of the oxide film
decreases but the peak value remains as it is. Accordingly, in the
zone C, since the eluted amount decreases as compared with the
oxidized and electroprecipitated amount, the oxide film becomes
deeper in color than in the other zones. In the case of FIG. 5, the
oxide film becomes of a reddish orange color and is partly in a
deep reddish orange color. In FIG. 4, the lines III--III and IV--IV
between adjacent zones below the line II--II are inclined upwardly.
this indicates that the amount of current flowed contributes to
coloring of the oxide film.
Moreover, the line II--II separating the zones A, B and C from the
zone D is also inclined upwardly. This indicates that the line
II--II exists in the presence of a certain energy level,
considering that when the amount of current flowed is decreased by
an increase in n, even if the amount of current flowed is increased
by an increase in the peak voltage V.sub.pa, the overall energy
level is lowered.
Further, in the zone D above the line II--II in FIG. 4, the color
of the oxide film becomes deep and its thickness greatly increases
regardless of the value of n. In this zone D, the peak voltage is
high and the current density increases, so that the balance between
the electroprecipitation and oxidation and the elution is
remarkably different from those in the zones A,B and C.
Particularly, over a wide range of n, in other words, over a wide
range of current density zone, oxide films of generally deep colors
can be obtained and, for example, in the case of FIG. 5, a deep
reddish orange color can be obtained.
In the foregoing, n and the peak voltage which are color control
factors have been described in connection with the pulse
voltage.
In particular, the magnitude of the positive peak voltage V.sub.p
is related mainly to oxidation of a metal and the magnitude of the
negative peak voltage V.sub.pa is related mainly to invasion of the
metallic salt into the oxide film. Considering that coloring of the
oxide film in this invention is achieved by invasion, oxidation and
electro-precipitation of the metallic salt, any peak voltage,
whether it is positive or negative, has a close relation directly
to the depth of color tone of the oxide film.
In the present invention, the depth of color tone of the oxide film
is dependent upon the energy balance between the amount of the
invaded metallic salt oxidized and its eluted amount. Whether the
pulse voltage is positive or negative, in the zones below the line
II--II in FIG. 4, when the values of the peak voltage, n, etc. are
controlled in such a direction as to decrease the current, the
color of the oxide film becomes deeper and when the above values
are controlled in such a direction as to increase the current, the
color of the oxide film becomes lighter.
As described above, in the present invention, the aluminum material
is electrolyzed by applying between both electrodes, at least one
of which is the aluminum material, positive and negative
rectangular-wave pulse voltages such, for example, as depicted in
FIG. 3, for the predetermined conduction times t and ta (refer to
FIG. 3), respectively, whereby oxide films of various colors are
formed.
In the case of electrolyzing the aluminum material as described
above, unlike in the conventional anodic oxidation or AC
electrolysis, the peak voltages V.sub.p and V.sub.pa rise
practically instantaneously and they are impressed for the
durations .tau. and .tau..sub.a, respectively, and stopped for the
predetermined quiescent times [h=(T-.tau. and Ta-.tau.)],
respectively, and then impressed again (refer to FIG. 3).
However, even if the rectangular-wave pulse voltage of such a
characteristic is applied to the aluminum material from a power
source such, for example, as a pulse generator, there are some
occasions when exactly the same voltage as the rectangular-wave
pulse voltage is not applied between the both electrodes, at least
one of which is the aluminum material, under the influence of the
amount of charges stored in the electrolytic cell. An electrolytic
cell 1 for electrolyzing the aluminum material has a predetermined
electric capacitance c and an internal resistance r as shown in its
equivalent circuit diagram of FIG. 6. Therefore, even if the power
source is cut off at the time of decay of the rectangular-wave
pulse voltage, since charges are stored in the electrolytic cell 1
at the time of impression of the peak voltages V.sub.p and
V.sub.pa, the charges are discharged even after decay of the pulse
voltage and the pulse voltage does not fall from the point 2 to 3
but falls from the point 2 to 4, as indicated by the solid and
broken lines, respectively, in FIG. 7. Further, the decay time H of
the pulse voltage in this case is longer than the pulse interval or
quiescent time h, so that the next pulse starts to rise before the
preceding pulse reaches the zero level. Accordingly, in theory, the
pulse voltage should rise from the zero potential to the peak
voltage V.sub.p but, in practice, the pulse only rises from V.sub.1
to the peak voltage V.sub.p, so that if the value of V.sub.1 is
large, the aforementioned effect resulting from sharp rise of the
pulse voltage is lost. Therefore, it is necessary to select the
value of V.sub.1 as small as possible.
To this end, it is preferred to control the conditions for
electrolysis in accordance with the capacitance c and the internal
resistance r of the electrolytic cell 1 so as to ensure that each
pulse starts to rise after the preceding one falls down to
substantially zero potential.
In this case, however, the capacitance c and the internal
resistance r of the electrolytic cell 1 do not remain constant
during electrolyzing of the aluminum material. Especially, the
value of the capacitance c is dependent upon the surface area of
the aluminum material and the thickness of a barrier layer of the
oxide film and it is almost impossible, in practice, to detect the
instant when the pulse voltage lowers down to substantially zero
potential.
If the time necessary for lowering of the pulse voltage down to
about 1/4 of the peak voltage V.sub.p is shorter than 1/3 of the
quiescent time h, a sufficiently colored oxide film can be obtained
regardless of the value of the capacitance c of the electrolytic
cell. Further, by changing the voltage applied between the both
electrodes in the electrolytic cell under such condition as
mentioned above, color tone of the colored oxide film can also be
controlled as desired.
Such a control of the applied voltage can be effected only by
connecting an impedance between the output terminals of a pulse
generator or like pulse source in the following manner.
Namely, as illustrated in FIGS. 9 and 10, an impedance 7 is
connected in parallel or in series between output terminals 5 and 6
of a pulse generator or like pulse source. With such an
arrangement, the impedance 7 is connected in parallel or in series
with the capacitance c and the internal resistance r of the
electrolytic cell 1.
When the pulse voltage is applied to the electrolytic cell 1 and
the power source is cut off, charges in the electrolytic cell 1
by-pass the impedance 7, so that the mode of voltage drop is
changed with a change in the time constant of the impedance 7.
Therefore, only by setting the value of the impedance 7 such that
the time necessary for lowering of the pulse voltage down to 1/4 of
the peak voltage V.sub.p may be shorter than 1/3 of the quiescent
time h, the pulse voltage can be controlled as described above.
In the above, the influence of the electric capacitance c and the
internal resistance r of the electrolytic cell 1 has been described
mainly in connection with the voltage which is applied or detected
between both electrodes, at least one of which is the aluminum
material. The reason therefore is that even if the influence of the
current during electrolyzing is not considered, it is sufficient,
in practice, only to consider the applied or detected voltage as a
coloring control factor and that, in actual electrolysis, control
by the applied or detected voltage is the easiest and best from the
industrial viewpoint.
However, where the pulse voltage of such a wave form as shown in
FIG. 3 is applied to the aluminum material to electrolyze it in the
presence of a large electrolyzing current, a large difference
occurs between the applied pulse voltage and the current and it is
necessary to achieve the electrolysis taking this difference into
account.
Namely, an equivalent circuit of the electrolytic cell containing
an electrolytic bath containing a metallic salt is regarded to have
the electric capacitance c and the internal resistance r connected
to each other as shown in FIG. 6. Accordingly, in the case of
applying the rectangular-wave pulse voltage to electrolyze the
aluminum material in the presence of a large electrolyzing current,
the influence of the load of a lead in addition to the electric
capacitance c and the internal resistance r of the electrolytic
cell is produced, by which although a pulse voltage indicated by
the broken line in FIG. 11 is applied, the current rises as
indicated by the solid line and does not reach a peak value I.sub.p
in some cases.
Consequently, before the current I reaches the peak value I.sub.p,
the power source is cut off and the pulse voltage rapidly falls.
This appreciably lessens the effect of the pulse voltage
impression.
In the present invention, in the case of the impressed voltage,
particularly in the case of the rectangular-wave pulse voltage, it
is sufficient, in practice, only to properly control the
relationships of the peak voltages V.sub.p and V.sub.pa to the
pulse durations .tau. and .tau..sub.a. Especially, it is advisable
to control the peak voltages V.sub.p and V.sub.pa in the range of 5
to 150V, preferably 10 to 80V and to control the pulse durations
.tau. and .tau..sub.a of the pulse voltages to be longer than
10.times.10.sup.116 3 sec. in the presence of a large electrolyzing
current. In the presence of an ordianry electrolyzing current, it
is sufficient that the pulse durations are shorter than
10.times.10.sup..sup.-3 sec.
In other words, where the peak voltages V.sub.p and V.sub.pa and
the pulse widths .tau. and .tau..sub.a of the pulse voltage are
controlled as described above and the aluminum material is
electrolyzed by such pulse voltage in the electrolytic bath
containing a metallic salt, the values of the loads of the
electrolytic cell 1 and the lead need not be considered and the
current rises up to its peak value and then falls. Thus, the effect
of application of the pulse voltage, that is, the effect of rapid
rise and fall of the voltage or current can be sufficiently
produced.
As described in detail in the foregoing, according to this
invention, the aluminum material is electrolyzed in an electrolytic
bath containing a metallic salt by applying to the aluminum
material a pulse voltage whose polarity changes from positive to
negative and vice versa alternately with a predetermined period,
thereby to form a colored oxide film on the surface of the aluminum
material. In this case, it is preferred from the industrial point
of view to obtain the rectangular-wave pulse voltage by half-wave
rectification and phase control of individual AC components of, for
example, a three-phase or other commercial AC voltage by means of,
for example, a silicon controlled rectifier or the like.
In FIG. 12, a six-phase commercial AC voltage is shown by broken
lines and voltages obtained by half-wave rectification and phase
control of its individual AC components are shown by solid lines.
The rectangular pulse voltage depicted in FIG. 12 has six ripple
components in the unit period T, T.sub.a or the unit pulse duration
.tau.,.tau..sub.a and the ripple components are saw-tooth in wave
form. Consequently, when a positive pulse is applied to the
aluminum material, the applied voltage on the aluminum material
rises from zero level to the peak voltage V.sub.p in a moment and,
by the impulsive energy resulting from this abrupt rise of the
voltage, the metallic salt is oxidized and electro-precipitated.
Since the six ripple components of the saw-tooth wave form are
intermittently applied to the aluminum material, the impulse energy
is intermittently provided, by which oxidation and
electro-precipitation of the metallic salt is further promoted.
However, while the electro-precipitation proceeds, the metal is
eluted but, in the case of such a wave form as shown in FIG. 12,
the oxidation and electro-precipitation are promoted by the
presence of the ripple components, so that the pulse width .tau.
need not be so large. Therefore, the amount of positive charges
applied to the aluminum material can be decreased and the amount of
the metal eluted can be inevitably held small, with the result that
the balance between the electro-precipitation and the elution can
be well maintained.
Then, in the negative conduction time t.sub.a after the positive
one t, a negative pulse voltage having the same characteristics as
the positive pulse voltage is applied. This negative pulse voltage
also rises from the zero level up to the peak value V.sub.pa in a
moment as is the case with the positive pulse voltage and the
invading energy is applied to the metallic salt and, further, by
the presence of the six saw-tooth ripple conponents, invasion of
the metallic salt into the oxide film is promoted, thereby to
further enhance the coloring effect.
Further, in the case of rectifying the commercial AC as shown in
FIG. 12, it is preferred that the pulse interval or the quiescent
time (T-.tau. or T.sub.a -.tau..sub.a) is determined based on the
unit period of the commercial AC. For example, in the waveform
shown in FIG. 12, its unit period is used as the pulse period.
In the case of applying the rectangular-wave pulse voltage, the
following values are appropriate.
______________________________________ V.sub.p /V.sub.pa / 5 to 150
(10 to 80V) f.f.sub.a (f=1/T, f.sub.a =1/T.sub.a) 5 to 500Hz (5 to
150Hz) t, t.sub.a 0.2 to 2.40 sec. (3 to 50 sec.)
______________________________________
In the above, the bracketed values indicate optimum ranges. The
time for sufficient electrolysis is usually about 60 minutes.
The reason why the above values are proper is as follows:
For example, where the peak voltages are lower than 5V, coloring is
deteriorated and where they are higher than 150V, it is difficult
to control the rate of forming the oxide film.
From the viewpoint of the coloring effect, it is preferred, in
general, to select the values of the peak voltages V.sub.p and
V.sub.pa as large as possible. However, the values of the peak
voltages V.sub.p and V.sub.pa are determined dependent upon the
kind of the metallic salt in the electrolytic bath selected in
accordance with color tone which is desired to be ultimately
obtained. For example, in the case of the silver salt, optimum
values of the peak voltages V.sub.p and V.sub.pa for forming an
oxide film of clear and deep color tone are about 20V or more and,
in this case, the electrolysis can be achieved at relatively low
peak voltages V.sub.p and V.sub.pa.
For convenience' sake, the foregoing description has been given
mainly in connection with the case of applying the rectangular-wave
pulse voltage that the characteristic values of its positive and
negative waveforms are partly or entirely equal to each other. With
the method of this invention however, even if the characteristic
values of the positive and negative waveforms are entirely
different from each other, a colored oxide film can be formed by
electrolyzing an aluminum material and, in addition, the color of
the oxide film can also be changed as desired. Especially, by
increasing the amount of charges of the negative waveform component
in the case of electrolyzing the aluminum material in the sulfuric
acid aqueous solution containing a metallic salt, degreasing or the
like of the aluminum material (except the aluminum material
previously anodized can also be achieved.
Further, the foregoing description has been made mainly with regard
to the case where the electrolytic bath is one containing only
sulfuric acid but, even if one or more of malonic acid, malic acid,
maleic acid, sulfosalicylic acid, sulfamic acid, tartaric acid and
oxalic acid are contained in the electrolytic bath, the effect does
not change. The electrolytic bath may be any aqueous solution
containing any of the above acids other than sulfuric acid, so long
as it is conductive.
In FIGS. 1, 2, 3, 7, 8, 11 and 12, the abscissa represents time and
the ordinate represents the peak voltage.
Now, this invention will be further described by the following
Examples.
EXAMPLE 1
Aluminum materials 1100, degreased and rinsed with water in usual
manner, were electrolyzed in an electrolytic bath composed of 150g
of H.sub.2 SO.sub.4 per liter of water and 60 mg of AgNO.sub.3 per
liter of water, with aluminum material used as both electrodes.
Pulse voltages of rectangular-waveform having a duration of 2 msec.
shown in the following Table 1 were applied, by which colored oxide
films of such color tone as shown in Table 1 were formed on the
aluminum materials. In the cases shown in Table 1, the time for
electrolysis was 60 minutes and the positive and negative waveforms
of the pulse voltages were the same.
Table 1.
__________________________________________________________________________
Conditions for electrolysis Formed oxide film
__________________________________________________________________________
Frequency Peak Mean Conduction Film Color tone Voltage current Time
of thickness f=1/T or 1/T.sub.a V.sub.p,V.sub.pa density Positive
(Color indication and of the Munsel (Hz) (V) (A/dm.sup.2) negative
(.mu.) solid) pulses t, t.sub.a (sec)
__________________________________________________________________________
30 1.2 11.3 1.4Y 5.6/7.2 50 20 0.6 5 3.9 5.6YR 4/8.8 10 0.2 1.3 2Y
4.1/5.1 30 1.1 10.3 10YR 5.2/4.8 40 20 0.5 5 3.0 6.8YR 4.1/6.9 30
10.6 7.1 2.5Y 5.7/7.9 30 20 0.5 5 3.0 3.8YR 3.4/0.1 10 6.16 1.0
1.5Y 4.2/4 30 1.0 4.2 0.9Y 5.3/8.7 20 20 0.5 5 2.5 5.4YR 3.8/8.7 30
0.8 3.3 7YR 3.9/8.7 10 20 0.4 5 2.0 2.4YR 3.1/5.8
__________________________________________________________________________
The relationships between the pulse durations .tau. and .tau..sub.a
of the positive and negative pulse voltages obtained (a) based on
the results given in Table 1 are shown in the following Table 2.
Further, (b) by our experiments in which the aluminum material was
electrolyzed under the same conditions as those in Table 1, with
the aluminum being used as one electrode and a carbon electrode as
the counter electrode, it was found that substantially the same
colored oxide films as shown in Table 1 could be obtained. In the
both cases (a) and (b), the time for electrolysis was also 60
minutes.
Table 2.
__________________________________________________________________________
Conditions for electrolysis Formed oxide film
__________________________________________________________________________
Duration Frequency Peak Mean Conduction Film Color Tone voltage
current time of thick- (Color indica- .tau., .tau..sub.a f=1/T or
V.sub.p, V.sub.pa density positive ness tion of the (m sec) and
Munsel Solid) 1/T.sub.a (V) (A/dm.sup.2) negative (Hz) pulses t,
t.sub.a (sec) (.mu.)
__________________________________________________________________________
40 1.9 22.6 7.8YR 4.6/5.4 2 30 1.6 5 17.2 9.9YR 20 0.6 3.8 100
6.9/1/5 40 3.6 28.3 3.4Y 5.8/2.9 3 30 2.1 5 12.8 8.9Y 5.2/5.8 20
0.7 5.9 5Y 7.4/6.1 40 1.4 13.5 7.4YR 4.5/8.5 2 30 1.2 5 11.3 1.4Y
5.6/7.2 20 0.6 3.9 5.6YR 4/8.8 50 40 2.0 20.0 2.7G 4.9/5.6 3 30 1.3
5 6.8 3.5Y 6.1/5.6 20 0.6 4.3 5Y 6.3/2.4
__________________________________________________________________________
It appears from Table 2 that color tone of the colored oxide film
can also be changed only by changing the pulse duration as
required.
EXAMPLE 2
Aluminum materials 6063, degreased and rinsed with water in usual
manner, were electrolyzed by applying the same rectangular pulse
voltage as that used in Example 1 under such conditions as shown in
the following Table 3, with the aluminums being used as both
electrodes. As a result of this, colored oxide films with color
tone as shown in Table 3 were formed on the aluminum materials. The
for each electrolysis was 60 minutes. In Table 3, additive
components in the electrolytic bath are shown in their weights per
liter of water.
Table 3.
__________________________________________________________________________
Bath Conditions for electrolysis Formed oxide film Composition
__________________________________________________________________________
Basic Added Duration Frequency Peak Mean Conduc- Film Color liquid
Metalic T, T.sub.a f=1/T or Volt- cur- tion thick- tone Salt age
rent time of ness (Color (msec) 1/T.sub.a dens- positive indica-
(Hz) V.sub.p, ity and tion of V.sub.pa negative the pulses Munsel
(A/ t, t.sub.a Solid) (V) dm.sup.2) (sec) (.mu.)
__________________________________________________________________________
H.sub.2 So.sub.4 HA.sub.u Cl.sub.4 3.5RP 2 50 25 0.98 5 6 150 g/l
100 mg/l 4.2/7.1 H.sub.2 SO.sub.4 Na.sub.3 SeO.sub.3 2 25 0.6 5 4.6
1.gamma. 40 6.7/2.4 150 g/l 5 g/l 6 25 11.0 5 10.4 8YR 7.2/2
__________________________________________________________________________
EXAMPLE 3
Aluminum materials of the same kind as employed in Example 1,
similarly degreased and rinsed with water, were electrolyzed in the
electrolytic bath of the same composition as that in Example 1,
with the aluminum materials being used as both electrodes. In this
case, a voltage obtained by half-wave rectification of a
single-phase sine-wave, shown in FIG. 1, and a voltage obtained by
controlling the above voltage with a silicon controlled rectifier
(refer to FIG. 2), were applied as positive and negative pulse
voltages to the aluminum materials. The electrolysis was achieved
under the conditions shown in the following Table 4.
Colored oxide films of such color tone as shown in Table 4 were
formed on the aluminum materials. The time for each electrolysis
was 60 minutes and the frequency used was 60 Hz.
Table 4.
__________________________________________________________________________
Formed oxide Conditions for electrolysis film
__________________________________________________________________________
Kind of Duration Peak Mean Conduction Film Color applied T, T.sub.a
Volt- current time of thick- tone voltage age density positive ness
(Color indica- V.sub.p,V.sub.pa and tion of the pulses Munsel
solid) t, t.sub.a (msec) (.gamma.) (A/dm.sup.2) (sec) (.mu.)
__________________________________________________________________________
Voltage 2.9Y obtained 30 2.4 5 15.0 6.2/8.2 by half- 3.0Y wave 5.7
20 1.14 5 4.8 5.9/7.6 rectification single-phase 4.1Y sine wave 10
0.64 5 1.5 5.7/3.5 (FIG. 1) Voltage 9.4YR obtained by 2.6 30 1.8 5
14.8 5.2/8.8 controlling 3.1YR the above 1.4 20 0.24 5 1.5 3.8/7.4
voltage 8.5YR with SCR 1.6 10 0.30 5 0.8 4.1/6.3 (FIG. 2)
__________________________________________________________________________
EXAMPLE 4.
Aluminum materials A.A6063, subjected to pretreatment in known
manner, were electrolyzed in an electrolytic bath containing 150g
of H.sub.2 SO.sub.4 per liter of water and 50mg of Ag.sub.2
SO.sub.4 (at a bath temperature 23.degree.C) under the conditions
shown in Table 5 for 60 minutes. The pulse width used was 16 msec.
By changing the values of the peak voltage and n(=T/.tau.) during
electrolyzing, colored oxide films shown in Table 5 were
formed.
By rearranging the results in relation to the peak voltage and
n(=T/.tau.), the relation-ships shown in FIG. 5 were obtained.
Further, when the aluminum materials were electrolyzed under the
conditions shown in Table 6 with different pulse durations, such
colored oxide films shown in Table 6 were obtained.
The distribution of the depth of color tone of the colored oxide
films obtained in this case was substantially the same as shown in
FIG. 5. Even when the pulse duration was changed based on the
above, the variation in the depth of color tone of the oxide films
was substantially the same as the basic tendency shown in FIG.
4.
The current density values given in Tables 5 and 6 are all those
obtained with a moving-coil ammeter.
Table 5.
__________________________________________________________________________
Conditions for electrolysis Formed oxide Film
__________________________________________________________________________
peak Voltage Period Mean current Film thick- Color tone V.sub.p n
density ness T (V) (A/dm.sup.2) (.mu.)
__________________________________________________________________________
10 100 0.32 1.7 Reddish orange 8 80 0.37 2.0 " 20 6 60 0.44 2.5 " 4
40 0.64 4.0 Right reddish orange 2 20 1.18 7.8 Yellow 10 100 0.54
4.0 Reddish orange 8 80 0.60 5.0 " 25 6 60 0.80 5.2 Light reddish
orange 4 40 1.12 6.7 " 2 20 2.04 15.0 Yellow 8 80 0.88 6 Light
reddish orange 27.5 4 40 1.60 11.5 Yellow 2 20 2.52 19 " 10 100
0.88 6.7 Light reddish orange 8 80 0.98 7.5 " 30 6 60 1.32 10.7 " 4
40 1.64 15.1 Reddish orange 2 20 2.46 21.7 " 6 60 1.16 11 Unclear
reddish orange 32.5 4 40 3.04 24 " 2 20 3.28 30 Reddish orange 10
10 1.50 17.8 " 8 80 2.28 24.4 " 35 6 60 2.86 28.2 Unclear reddish
orange 4 40 3.08 29 "
__________________________________________________________________________
Table 6.
__________________________________________________________________________
Conditions for electrolysis Formed oxide film
__________________________________________________________________________
Pulse Peak Frequency Mean Film Color tone width voltage n T current
thickness .tau..sub.p V.sub.p density (msec) (V) (msec)
(A/dm.sup.2) (.mu.)
__________________________________________________________________________
15V 2 10 0.7 3.9 Yellow 20 8 40 0.5 2.2 Reddish orange 5 25 4 20
1.4 10.5 Light reddish orange 30 2 10 3.0 25.7 Reddish orange 35 10
50 1.52 21.5 " 25 3 48 0.69 5.5 Yellow 25 7 118 0.4 2.5 Light
reddish orange 16 30 2 32 2.3 11.5 Reddish orange 33 2 32 2.2 22.0
" 33 4 64 1.8 10.0 "
__________________________________________________________________________
When metallic salts such as HAuC1.sub.4, Na.sub.2 SeO.sub.3,
CuSO.sub.4, SnSO.sub.4, NiSO.sub.4 and CoSO.sub.4 were added in a
H.sub.2 SO.sub.4 aqueous solution in place of the aforesaid
metallic salt and the peak voltage V.sub.p and n were changed,
colors shown in the following Table 7 were obtained.
Table 7. ______________________________________ No. Coloring metal
Metallic Salt Color of oxide film
______________________________________ 1 Au HAucl.sub.4 Purple 2 Se
Na.sub.2 SeO.sub.3 Cream 3 Cu CuSO.sub.4 Deep red to brown 4 Sn
SnSO.sub.4 White to dark brown 5 Ni NiSO.sub.4 Amber to black 6 Co
CoSO.sub.4 " ______________________________________
When aluminum materials A.A1099, 1100, 2011, 2014, 2024, 3003,
4043, 5005, 5086, 5357 6061 and 7075 other than 6063 were
electrolyzed during which the peak voltage V.sub.p and n were
controlled, substantially the same results as those in FIGS. 4 and
5 were obtained, although colors of the oxide films formed were a
little different from one another because these aluminum materials
were of different compositions and because their electrical
properties differed in accordance with the contents and kinds of
alloy elements contained in them.
The relationships of the peak voltage and the value n to the
distribution of color tone showed the same tendency as shown in
FIG. 4, though a little affected by such factors as the power
source, voltage adjusting means and the geometrical shape of the
electrolytic cell used (for example, distance between electrodes,
capacitance, etc.) used, a leakage current, etc. in addition to the
quality of each aluminum material and the kind of each metallic
salt used.
For example, when one or more of the above factors were changed,
there were some occasions when the sizes and shapes of the zones A,
B and C in FIG. 4 were changed, or the zone D became so narrow that
it was not necessary to distinguish the zone D between the zones A,
B and C and the zone E in actual electrolysis, or the width of the
zone D increased. Further, the aforesaid factors had relation to at
least some of the conditions for electrolysis, so that when one or
more of the factors were altered, the levels of the lines I--I and
II--II became higher or changed in inclination in some cases.
In any case, however, according to the method of this invention the
basic tendency shown in FIG. 2 can be maintained, in which one of
the features of the method of this invention resides.
EXAMPLE 5
Aluminum materials 1100, degreased and rinsed with water and then
neutralized in known manner, were electrolyzed by applying the
rectangular pulse voltage shown in FIG. 3 is an electrolytic bath
containing 150g of H.sub.2 SO.sub.4 per liter of water and 50mg of
Ag.sub.2 SO.sub.4 per liter of water (at a bath temperature of
25.degree.C), with the aluminum materials being used as both
electrodes.
In this case, an impedance was connected as shown in FIG. 9 and the
impedance used was a resistor. By changing its resistance value,
the by-pass current was changed. The results shown in the following
Table 8 were obtained.
Table 8 ______________________________________ Sample Pulse Pulse
Peak Surface Resis- No. duration interval voltage area of tance
.tau. = .tau..sub.a h=ha V.sub.p =.vertline.V.sub.pa .vertline.
sample Value (sec) (sec) (V) (cm.sup.2) (.OMEGA.)
______________________________________ 1 2.times.10.sup.-.sup.3
18.times.10.sup.-.sup.3 20V 50 10 2 " " " " 40 3 " " " " 54 4 " " "
" 100 5 " " " " 150 6 " " " " 220 7 " " " " 500 8 " " " 100 10 9 "
" " " 150 10 " " " " 500 ______________________________________
Sample Mean current Time for lowering Color of No. Electrolytic
By-pass to 1/4 V.sub.p or V.sub.pa film cell circuit (A) (A) (sec)
______________________________________ 1 0.21 0.22
0.3.times.10.sup.-.sup.3 Dark brown 2 0.23 0.07
0.85.times.10.sup.-.sup.3 " 3 0.20 0.06 1.1.times.10.sup.-.sup.3 "
4 0.22 0.04 2.0.times.10.sup.-.sup.3 A little dark brown 5 0.25
0.03 2.5.times.10.sup.-.sup.3 " 6 0.23 0.03
4.3.times.10.sup.-.sup.3 Brown 7 0.23 0.02 9.times.10.sup.-.sup.3
Light dark yellow 8 0.44 0.27 0.5.times.10.sup.-.sup.3 Dark brown 9
0.39 0.04 5.times.10.sup.-.sup.3 Light brown 10 0.41 0.015
14.5.times.10.sup.-.sup.3 Light dark yellow
______________________________________
The frequency of the applied voltage was 50Hz.
As seen from the above Table, when the resistance value was
changed, especially in the case of the surface area of the specimen
being 50cm.sup.2, when the resistance value was 500.OMEGA., the
time for lowering of the applied voltage to 1/4 of the peak voltage
was longer than 1/3 of the pulse duration and the coloring mode of
the oxide film was not good. The same was true of the case of the
specimen surface area being 100cm.sup.2.
EXAMPLE 6
Aluminum materials A.A6036, chemically pretreated, in known manner
were electrolyzed in an aqueous solution containing 150g of
sulfuric acid per liter of water and 50mg of Ag.sub.2 SO.sub.4 per
liter of water, with the aluminum materials being used as both
electrodes. In this case, a pulse voltage, obtained by half-wave
rectification of a six-phase AC of the waveform shown in FIG. 12
was applied.
The results shown in the following Table 9 were obtained.
Table 9.
__________________________________________________________________________
Characteristics of pulse voltage Peak Pulse Pulse Conduction Mean
Color of No. Voltage width period time Current oxide V.sub.p
=V.sub.pa .tau. = .tau..sub.a T = T.sub.a t = t.sub.a density film
(V) (sec) (sec) (sec) (A/dm.sup.2)
__________________________________________________________________________
1 25 16.times.10.sup.-.sup.3 48.times.10.sup.-.sup.3 5 0.63 A
little deep yellow 2 25 33.times.10.sup.-.sup.3
132.times.10.sup.-.sup.3 " 0.62 " 3 20 16.times.10.sup.-.sup.3
48.times.10.sup.-.sup.3 " 0.44 A little light yellow 4 20
33.times.10.sup.-.sup.3 132.times.10.sup.-.sup.3 " 0.43 " 5 33
16.times.10.sup.-.sup.3 16.times.10.sup.-.sup.3 " 1.50 Deep orange
6 " 33.times.10.sup.-.sup.3 33.times.10.sup.-.sup.3 " 1.40 " 7 "
50.times.10.sup.-.sup.3 50.times.10.sup.-.sup.3 " 1.50 " 8 20
16.times.10.sup.-.sup.3 64.times.10.sup.-.sup.3 " 1.20 Light orange
9 " 33.times.10.sup.-.sup.3 132.times. 10.sup.-.sup.3 " 1.00 " 10 "
50.times.10.sup.-.sup.3 200.times.10.sup.-.sup.3 " 1.30 " 11 25
16.times.10.sup.-.sup.3 64.times.10.sup.-.sup.3 " 1.00 Orange
__________________________________________________________________________
It appears from Table 9 that an increase in the values of the peak
voltages V.sub.p and V.sub.pa causes the color of the oxide film to
become deeper and that, in the case of the same frequency, that is,
when the pulse periods T and T.sub.a are equal to each other, an
increase in the pulse widths .tau. and .tau..sub.a causes the color
of the oxide film to become lighter.
EXAMPLE 7
Aluminum materials 1100 were degreased and rinsed with water and
then neutralized to clean the surfaces of the aluminum materials.
These aluminum materials were electrolyzed in an aqueous solution
containing 150g of H.sub.2 SO.sub.4 per liter of water and 50mg of
Ag.sub.2 SO.sub.4 per liter of water, with the aluminum materials
being used as both electrodes. The pulse voltage shown in FIG. 3
was applied and the electrolysis was effected with a current of
1000A for 60 minutes.
The results shown in the following Table 10 were obtained. The
positive and negative waveforms of the applied voltages were the
same and the peak voltages and the pulse widths were all the same
in their absolute values, respectively.
Table 10. ______________________________________ Conditions for
electrolysis Frequency Peak voltage Pulse width Color of f=1/T or
V.sub.p = V.sub.pa .tau. = .tau..sub.a oxide 1/T.sub.a film (Hz)
(V) (sec) ______________________________________ 10 20
20.times.10.sup.-.sup.3 Yellow 5 25 40.times.10.sup.-.sup.3 Light
Orange 3 30 60.times.10.sup.-.sup.3 A little light orange 1 30
200.times.10.sup.-.sup.3 Deep orange
______________________________________
In all cases of the above Table, the currents all reached the peak
values and sufficiently colored oxide films were formed.
In the method of this invention, the frequency used is determined
in relation to the pulse width but it was sufficient to be lower
than 100Hz.
Although the foregoing description has been given mainly, in
connection with the cases in which the aluminum materials A.A1100
and A.A6063 are employed, the present invention can also be easily
applied to other aluminum materials.
However, a change in the composition and electrical properties of
the aluminum materials used causes a change in the color of the
oxide film. For example, under the conditions for electrolysis that
when the aluminum material A.A1100 is electrolyzed in the sulfuric
acid aqueous solution, an oxide film of an orange color is formed,
when the aluminum materials A.A3003, A.A4043, A.A5052, A.A6061 and
A.A6063 are electrolyzed in the above electrolytic bath, oxide
films of grayish orange, dark orange, light orange, dark reddish
orange and orange colors are formed, respectively.
It will be apparent that many modifications and variations may be
effected without departing from the scope of the novel concepts of
this invention.
* * * * *