CA1039228A - Method of forming colored oxide film on aluminum or aluminum alloy material - Google Patents

Abstract

METHOD OF FORMING COLORED OXIDE FILM

ON ALUMINUM OR ALUMINUM ALLOY MATERIAL

Abstract of the Disclosure 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 elect-rodes in an electrolytic bath containing a metallic salt while applying a pulse voltage 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.

Description

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Bl\CICGROUND OF Tlll~ INVENTION
_______ ___ _ ~
Eield_of the Inv n tiO n This invention relates to a method of forminy a colored oxide film on tl~e surface o~ an ~luminum or aluminum alloy material (l~ereinafter 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 with a color tone characteristic of the metal in the metallic salt. ~ `
Descri~tlon of the Prior Art ~lerertofore, a variety of methods have been employed :i .
Eor ~orming a colored oxide Eilm on the surface of an alumi-num 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 color~
forming 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 after the , oxide film formation of the first step. This introduces dis--. advantages 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 i uniform colored oxide films at all times.
SUMMARY OF THE INVENTION ~-According to the invention there is provided a method of ,, .

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; 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 at least one electrode with a pulse voltage consisting of a plurality of unipotential pu:lses whose polarity is reversed every 0.2 to 240 seconds.
Various features and advantages of this invention will become apparent from the following description o~ preferred embodiments taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a waveform diagram of a pulse voltage which is obtained by half-wave rectification of a single-phase sinewave . voltage and whose polarity is periodically inverted;
F:iyure 2 is a waveform dlagram o~ a pulse voltage which is obtairled by phase-controllincl the waveform oE Fic~ure 1 with a silicon controlled rectifier;
Figure 3 is a waveform diagram of a rectangular-wave pulse voltage whose polarity is periodically inverted and whose positive and negative pulses are both of rectangular shape;
Figure 4 is a graph showing the interrelationships of a ~:
pulse period T/a pulse duration ra, a peak voltage and color tone of the resulting colored oxide film in an electrolysis which is effected in an electrolytic bath containing a metal-lic salt by applying a rectangular pulse voltage whose po- ~`
larity is periodically inverted;
Figure 5 is a graph showing the same relationships as in Figure 4 in the case of using a silver salt as a metallic :
salt; -Figure 6 shows an equivalent circuit of an aluminum material electrolytic cell;
Figures 7 and 8 show voltage variations between both ~ .;

electrodes cluc to a clischarcle frolll arl electrolytic cell during an elcctroly~sis elllployin<3 ~sucll a rectangular-wave pulse voltage as depicted in Figure 3;
Figures 9 and 10 are circuit diagrams :illustratinq an external impedance in conjunction with electrolytic cell;
Figure 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 voltaye as shown in Figure 3; and Figure 12 is a waveform diaqram of a rectangular-wave pulse voltage produced by half-wave rectification and phase control of a commercial AC.
DESCRIPTION OF T~IE PREFERR~D ~ IMENT~
The present invention will hereinafter be described in detail. ~ ~' At first, an aluminum material is subjected to pretreat-ment as is the case of ordinary electrolysis. This pre-treatment is not related directly -to the present invention and may be a mechanical or a chemical pretreatment. Further, an alu-minum material having an oxide film previously formed thereoncan also be used and, in this case, the mechanical or chemi-cal 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 color forming - ~
metallic salt, the aluminum material with no oxide film formed ~ -~3 thereon or the aluminum material with an oxide film previously formed thereon is electrolyzed by using the aluminum material as one or both electrodes and applying a voltage of the following characteristics thereto.
The voltage used in this case is a voltage of pulse waveform ~,li~ ' .:
~, X~
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~.~3~ 8 and the polarity of the pulses is periodically reversed with a reversal period longer than the period of a single pulse.
Further, this voltage of pulse waveform is desirably one in which the duration of each pulse is short, in which the initial and final values of the pulse are equal to each other ;~
and in which the pulse rises up to a predetermined level. This voltage may be such as shown in Figure 1 which is obtained by half-wave rectification of a single-phase sine-wave voltage, such as shown in ~igure 2 which is obtained by phase-controlling the voltage of Figure 1 with a silicon control-led rectifier or like rectifier, or such as shown in Figure

3 which is a rectangular-wave voltage, or may be a triangu-lar-wave, exponential-wave or a partly sine-wave voltage.
Of these voltages, a pulse voltage of rectangular waveform is the easiest to obtain industrially and, by changing the -characteristic values of the rectangular-wave as required, ~;
i, 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 T and Ta, pulse periods T and Ta, peak voltages V and Vpa and pulse polarity reversal periods (hereinafter referred to as conduction times) t and ta, as shown in Figure 3. The particular metallic salt, which is contained in the electrolytic bath, is selected according to the desired color tone of the resulting colored oxide film and it may be a sulfate, nitrate or any other salt. Further, the electrolytic bath is required only to be conductive and an `~
aqueous sulfuric acid solution is the most inexpensive, and hence economical.
' 30 By electrolyzing the aluminum material under such conditions as described above, a colored aluminum oxide film is formed ~i - 5 -.. ~ . . . . . .. .

~L039ZZ8 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 electrolyzlncl an aluminum matc~rial by applying thereto ..
the rectangular-wave pulse voltage shown in Figure 3. In the rectangular-wave pulse shown in Figure 3, the positive and negative pulse waveforms are different from each other :~
but the characteristic values of the bath pulse waveorms, for example, the peak voltages V ancl Vpa, the pulse periods T and Ta~ the pulse durations T and I and the conduction .. .,~, times t and ta~ can be selected to be identical or symmetrical .
with each other. Accordingly, the rectangular-wave pulse voltage will hereinafter be described on the assumption that ...
the characteristic values of the both pulse waveforms are identical with each other. However, this invention is not .. ~.
limited specifically to the above. ~or example, by selecting the conduction times o~ the positive and negative pulses to ;~
be different from each other, the color tone of the colored .
. . .
oxide film can be changed as required. Furthermore, by selacting the peak voltage, the period and the duration of the positive pulse to be different from those of the negative -~
pulse, the color tone of the colored oxide film can be changed as desired. ` . :
Thus, in the present invention, if alternately sup- .
plied with the positive and negative pulses such as depicted -.
in Figure 3, the aluminum material serving as one or each of the electrodes ~ecomes positive and negative alternately ; ~`
with the predetermined conduction times t and ta. In this ~
case, while the polarity of the aluminum material remains .~ .

30 positive, the aluminum material is oxidized as the anode ~.
to form an oxide film. Then, when the polarity of the alu-Y

minum material becomes negative, metallic ions from the ', metallic salt in the electrolytic bath enter into the oxide film. (The above metall,ic ions 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 mentloned 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 be-comes negative again, thus forming a colored o~ide film.
With such a mechanism, the colored oxide film is form-1 ed on the aluminum material and the conditions for the above i~ coloring mechanism are satisfie~ by the electrolysls using the pulse voltage. The use of such a rectangular-wave volt-age as shown in Figure 3 Eacilitates fulfilment of-such con-ditions.
Namely, while the negative rectangular-wave pulse,volt-age is applied to the aluminum material to electrolyze it, the metallic salt invades the oxide film formed on the alu-'`~ 20 minum material during the application of the positive rectanyular-wave pulse voltage or the oxide film formed ~ ;
previously. Since the applied pulse voltage is of rectan-l gular waveform, a predetermined amount of energy to cause '! ~ , ,' invasion is obtained and the metallic salt invades the oxide ,~ film in the vicinity of the bottom of each pore therein.
~, In other words, the pulse voltaye, in particular, the rectangular-wave pulse voltac~e, has entirely no rise time as shown in Figure 3 and the negative peak voltage V
acts on the aluminum material with practically no rise time, 30 so that the energy to cause the invasion by the metallic salt ' is provided simultaneously with rising of the pulse voltage.

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~ Q392Z~ ; ~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 rectan-i gular-wave pulse voltage is electrolyzed and oxidized again by the application of the positive 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 m~tallic salt is oxidized ~
, but part of the resulting product is eluted from the pores of `
' the oxide film and, further, 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 oxlde 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-precipita- ~ ;
tion 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 fulfil the requirement.
Namely, the amount of the metallic salt oxidized -~
during electrolysis by the application of the positive ~ ~;
pulse voltage increases in proportion only to the magni-tude of such an applied voltage Vp as shown in Figure 3.

,~ .

~39~Z8 The amount of the oxidized product re-elutec~ is in pro-portion 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 volt-`~ age of the waveform of Figure 3, it isin proportion to (Vpxl) and, at the same time, the oxide film is formed -., .
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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 is as large as possible and that the amount of positive charges or the amount of current is small.
~5 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 Figure 3 ~1 is easier to control than the other pulse voltages and 'I has an advantage that a colored oxide film of desired i color tone can be obtained.
Figure 4 generally shows the relationships of the pulse durations T and Ta, the pulse periods T and Ta ' and the peak voltages Vp and Vpa 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 Figure 3 in a sulfuric acid aqueous solution contain-ing a metallic salt. Figure 5 shows similar relation-, ships in the case of an electrolysis in a sulfuric acid aqueous solution containing Ag2SO4. As is apparent ' .

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from the both graphs, the relationships of Figure 4 and those of Figure 5 employing a special metallic salt , (Ag2SO4) 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 Figure 5, triangles, white circles, black circles and crosses indicate yellow, light reddish orange, reddish orange and unclear reddish orange colors, respectively.
, 10 In Figures 4 and 5, n=pulse period/pulse duration f (=T/~>l or Ta/Ta>1).
' In Figure 4, the ordinate represents the peak voltage f and the abscissa represents n. Re~erence characters A, B, C and D indicates zones o~ color tone of the oxide ~ilm.
I C'o~,~ sO~7 As is seen ~rom _ of Figures 4 and 5, ~or example, in the case of an electrolysis using the pulse ! voltage in a sulfuric acid aqueous solution containing f Ag2SO4, the zones A,B and C correspond to yellowish, light f reddish orange and partly deep reddish orange colors, res-1 20 pectively, and the zone E is one in which the oxide film f is destroyed even i~ any kind of metallic salt is employed.
As shown in Figure 4, in the zone E above the line I-I, the peak voltage is high and a current flows ex cessively, 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 f lower than the line I-I, by electrolyzing with the pulse voltage at different values of n and the peak voltage, _ ~ _ : ~L03~;221~
colors such as shown in Figure 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 `,, S 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 i~' and its eluted amount. For example, as the value of n increases, the color tone of the oxide film changes from ; 10 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 , is ~7~ flowed ~n 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 Figure 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 Figure 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 T iS small ¦ but the quiescent time t is short, so that the amount of . . .
positive current flowed decreases and the thickness of the ~i oxide film decreases but the peak value remains as it is.

Accordingly, in th~ zone C, since the eluted amount de-creases as compared with the oxidized and electro-~ z .. . .
:, ..... . . . . . . . . .

3~2Z8 precipitated amoun-t, the oxide film becomes deeper in color than in the other zohes. In the case of Figure 5, the oxide film becomes of a reddish orange color and is partly in a deep reddish orange color. In Figure 4, the lines III-III and IV-IV between ad~iacent 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 Vpa, the overall energy level is lowered.
I Further, in the zone D above the line II-II in Figure 1 4, the color of the oxide film becomes deep and its thick-ness 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 electro-precipitation and oxidation and the elution is remarkedly `j~ 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 Figure 5, a deep reddish orange color can be obtained.

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; In the foregoing, n and the peak voltaye which are color control factors have been described in connection with the pulse voltage.
Namely, the magnitude oE the positive peak voltage ~- 5 Vp is related mainly to oxidation of a metal and the magnitude of the negative peak voltage V a is related mainly to invasion of the metallic salt into the oxide film. Considering that coloring of the oxide fllm 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.
Z In the present inventlon, the depth of color tone ~;! 15 of the oxide film is dependent upon the energy balance Z 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 Figure 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 di-rection as to increase the current, the color of the Z oxide film becomes lighter.
Z, Z5 As described above, in the present invention, the aluminum material is electrolyzed by applying between ~e both electrodes, at least one of which is the aluminum material, positive and negative rectangular-wave pulse , :. , . . ,:: : :

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voltages such, for example, as depicted in Figure 3, for the predetermined conduction times t and ta ~refer to Figure 3), respectively, whereby oxide films of various colors are formed.
In the case of electrolyzing the aluminum material '~ `r~ as described above, unlike in the conventional ~e~
oxidation or AC electrolysis, the peak voltages V and Vp ' rise in a moment and they are impressed for the durations `~ T and la' respectively, and stopped for the predetermined quiescent times (T-la andTa-Ta)~ respectively, and then impressed again ~refer to Figure 3).
However, even if the rectanyular-wave pulse voltage of such a characteristic is applied to the aluminum material from a power souce such, for example, as a pulse generator, lS 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 Figure 6.
Therefore, even if the power souce is cut off at the time of decay of the rectangular-wave pulse voltage, since , 25 charges are stored in the electrolytic cell 1 at the time ~ of impression of the peak voltages Vp and Vpa, the charges ;1 are discharged even after decay of the pulse voltage and 3 the pulse voltage does not fall from the point 2 to 3 but ` A~ ~
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., ., ,. , . ,., . . .; . . . .. . ...
.. , ............... .. ` ............. ~ . ..

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falls from the point 2 to 4, as indicated by the solid and broken lines, respectively, in Figure 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 poten-tial to the peak voltage Vp but, in practice, the pulse only rises :Erom V
to the peak voltage Vp, so that if the value of Vl is large, the aforementioned effect resulting from sharp ~ rise of the pulse voltage is lost. Therefore, it is `~ necessary to select the value of Vl as small as possible.
To this end, it is preferred to control the conditions ~ ~ for ele~trolysis in accordance with the capacitance c and ! 15 the intcr~al resistance ~ 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.
i In this case, however, the capacitance c and the j internal resistance y 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 o~ide 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 Vp is ;~, ~'~

... .. . .
::: : . .. ~.
:. :: ., ,, : - , . .

shorter than 1/3 of the ~s9 ~lnterval h, a suffi.ciently 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 ~nder ~ch condition as menti.oned above, color tone of the colored oxide fi.lm can also be .: :.
controlled as desired~
Such a control of the applied voltage can be effected only by connecting an impedance between out put terminals of a pulse generator or like pulse source in the following manner.
Namely, as illustrated in Figure 9, an impedance 7 is connected in parallel between output terminals 5 and 6 of a .
pulse generator or like pulse source. With such an arrange- ~:
ment, the impedance 7 is connected in parallel with the capacitance c and the internal resistance r of the electro-lytic cell 1.
When the pulse voltage is applied to the electrolytic .; ~:.
cell 1 and the power source is cut off, charges in the electro-lytic cell 1 pass through 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 Vp may be shorter than 1/3 of the pulse interval h, the pulse voltage can be controlled as described above. The same effect cannot be obtained if, as shown for comparative purposes in Figure 10, the impedance 7 is connected in series.
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I~ the above, tile 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 i9 "
applied or detected between the 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, the control by the applied or detected voltage is the easiest and excellent from the industrial view point.
However, where the pulse voltage of such a wave form as shown in Figure 3 is applied to the aluminum material to electrolyze lt 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 Figure 6.
Accordlngly, 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 aadition to the electric capacitance c and the ineernal ::
. , .
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~ resistance ~ of the electrolytic cell is produced, ,;, .~. , --~ by which although a pulse voltage indicated by the broken line in Figure 11 is applied, the curren-t rises as indicated by the solid line and does not reach a peak value Ip in some cases.
Consequently, before the current I :reaches the peak - value Ip, 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 impress-ed 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 Vp and Vpa to the pulse durations r and T a. Especially, it is advisable to control the peak voltages Vp and V
in the range of 5 to 150V, preferably 10 to 80V and to control the pulse durations T and ra of the pulse voltages to be longer than lOxlO 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 lOxlO 3 sec.
In other words, where the peak voltages Vp and Vpa and the pulse widths T and Ta 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 _ ~ _ .-,.. - . . , - .. , . ............................................................ ~
f "" ` . ' '. ' ' ' ' . . ' : , . : . . ~ - . , ~ . . , then falls. Thus, the effec-t 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, lt 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, :1 a three-phase or other commercial AC voltage by means of, ;l for example, a silicon controlled rectifier or the like.
;1 In Figure 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 Figure 12 has six ripple components in the unit period T, Ta or the unit pulse duration T ~ ~a and the ripple components are saw-tooth ~ 25 in wave form. Consequently, when a positive pulse is il applied to the aluminum material, the applied voltage on the aluminum material rises from zero level to the peak voltage Vp in a moment and, by the impulsive energy : ~

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resulting from this abrupt rise o~ the voltage, the metallic salt is oxidized and electro-precipitated. Since the six r~pple components of the saw-tooth wave form are intermlttently 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 1`
of such a wave form as shown in Figure 12, the oxidation and electro-precipitation are promoted by the presence of the ripple ; components, so that the pulse width T 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 lnevitably held small, with the result that the balance between the electro-precipitation and the elution can be well ma intained.
Then, in the negative conduction time ta 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 æero level up to the peak value Vpa in a moment as is the case with the positive pulse voltage and the energy for the invasion by the metallic ' salt is applied and, Eurther, by the presence of the six saw-tooth ripple components, invasion of the metallic salt into the oxide film is promoted, thereby to further enhance the coloring effect.

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~, Further, in the case of rectifying the commercial - AC as shown in Figure 12, it is preferred that the pulse interval or the quiescent time (T-r or Ta-Ta) is deter-j mined based on the unit period of the commercial AC.
For example, in the waveform shown in Figure 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 /V / .......... 5 to 150 (10 to 80V) a T ~ fa T ) 5 to 500Hz (5 to 150HZ) a ~4O
t~ ta ........... 0.2 to ~T4~ sec. (3 to 50 sec.

In the above, the bracketed values indicate optimum ranges. The time for electrolysis is usually sufficient to be 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 Vp and Vpa as large as possible. However, the values of the peak voltages Vp and Vpa are determined dependent upon the kind of the metallic salt in the electrolytic bath selected in accordance with color tone .1 . .

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~L~3~2;~8 which is desired to be ultimately obtained. For example t in the case of the silver salt, optimum values of the peak voltages Vp and Vpa for Eorming an oxide film of clear and deep color tone are about 20V or more and, in this case, the electrolysis can be ac:hieved at rela-tively low peak voltages Vp and Vpa.
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 l 15 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, 1 by increasing the amount of charges of the negative .~ waveform component in the case of electrolyzing the . 20 aluminum material in the sulfuric acid aqueous solutioncontaining a metallic salt, degreasing or the like of :1 I the aluminum material (except the aluminum material ~ v ~,zec/
3~ previously a*~7~e~) can also be achieved.
Further, the foregoing description has been made ~ ' .
1, 25 mainly with regard to the case where the electrolytic j 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 :` ~ z3 :, .
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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 Figures 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 -1 the following Examples.
EXAMP~E 1 Aluminum materials 1100, degreased and rinsed with water in usual manner, were electrolyzed in an electro-, lytic bath composed of 150g of H2SO~ per liter of water and 60mg of AgNO3 per liter of water, with aluminum material used as both electrodes. Pulse volt-ages 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.

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Conditions for electrolysis Fo:rmed oxide film Frequency Peak Mean Conduction Film Color tone f-l or 1 Voltage current Time of thickne.ss T Ta Vp,V a density Positive (Color indication f , ~ 2 and of the Munsel ~HZJ (V) (A/dm ) negative(~) solid) pulses ,...
. 30 1.2 11.3 1-4Y 5.6/7.2 20 0.6 5 3.9 5.6YR
' 10 0.2 . . .1.3 2Y 4.1/5.1 30 1.1 10.3 5.2/4.8 _ 5 20 0.5 3.0 4.1/6.9 , 30 10.6 7.1 2-5Y 5.7/7.9 .j 30 20 O.S 5 3.0 3.4/0.1 ~'' 10 6.16 1.0 1.5Y 4.2/4 ! _ _ 1.0 4.2 0-9Y 5.3/8.7 _ _ 5 20 0.5 2.5 3.8/8.7 . 30 0.8 3.3 3.9/8.7 , 10 5 .3 20 0.4 2.0 2.4YR
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The relationships between the pulse durations T and T
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.

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1~392~3 Table 2.

Conditlons for electrolysis Formed oxide film Duration Frequency Peak Mean Conduction Film Color Tone . T ~ 1 voltage current time of thick- (Color indica-: ~ a f= T or V , V density positivenesstion of the (m sec) 1 P pa 2 and Munsel Solid) ~; _ (V)(A/dm ) negative :~ (Hz) pulses : (sec) (~) :' .. .
40 1.9 22.64.6/5.4 .

2 30 1.6 5 17~2 g.9YR

_ 100 20 0.6 3. a 6.9/1/5 :~ 40 3.6 28.3 3.4Y
, . 5.8/2.9 `~ 3 30 2.1 5 12.88-9Y 5.2/5.8 0.7 5.95Y 7.4/6.1 _ 1.4 13.54.5/8~.5 . 2 30 1.2 5 11.3_ 5 6/7.2 0.6 3.95 6YR4/8.8 . 50 40 2.0 20.0 2.7G
;l. 4.9/5.6 !, 3 30 1 3 5 6.86.1/5.6 . _ _ _ 20 0.6 4.3 5Y 6.3/2.4 ,", ~ 7 ' "

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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.

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 such 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 wate~.

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~ Table 3.
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BathConditions for electrolysis Formed oxide film Compos Ltion Basic Added Duration Frequency Peak Mean Conduc- Film Color liquid Metalic T, T 1 1 Volt- cur- tion thick tone . Salt af=T or T- age rent time of ness (Color : (msec) dens- positive indica-., (Hz) Vp~ ity and tion of V r.egative the pa pulses Munsel , (A/2 t, ta Solid) 1 (V) dm ) (sec) (~) .
H2So4 HAuCl4 3.5RP
~ 2 50 25 0.98 5 6 J150 100 4.2/
lg/Q mg/~ 7.1 :~ _ ~;H2SO4Na3SeO3 2 25 0.6 5 4.6 ly 6.7/
2.4 5 g/Q 6;2~ 11.0 51( 4 ~ 8YR

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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 S 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 Eigure 1, and a voltage obtained by controlling the above voltage with a silicon controlled rectifier (refer to Figure 2), were applied .1 as positive and negative pulse voltages to the aluminum , materials. The electrolysis was achieved under the ', conditions shown in the following Tahle 4.
Colored oxide films of such color tone as shown in L5 Table 4 were formed on the aluminum materials. The , time for each electrolysis was 60 minutes and the ` frequency used was 60 Hz.

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Table 4.

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Formed oxide ` Conditions for electrolysis film :`, _ Kind of Duration Peak Mean Conduction Film Color . applied T, T Volt- current time of thick- tone .~ voltage a age density positive ness (Color indica- :~
V ,V and tion of the p pa pulses Munsel solid) t, ta (msec) (Y) (A/dm2) (sec) (~ _ _ _ i Voltage 2.9Y .
obtained 30 2.4 5 15.0 6.2/8.2 by half- 3.0Y
rectification 5.7 20 1.14 5 _4 8 5.9/7.6 . single-phase 4.lY
Fi . 1) 10 0.64 5 1.5 5.7/3.5 lq Voltage 9.4YR
, obtained by 2.6 30 1.8 5 14.8 5.2/8.8: controlling 3.lYR
. the above 1.4 20 0.24 5 1.5 3.8/7.4 voltage ::
with SCR 8.5YR :
(Fig. 2) 1.6 10 0.30 5 0.8 4.1/6.3 ,~ .

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-EXAMPLE 4.
Aluminum materials A.A6063, subjected to pretreat-ment in known manner, were electrolyzed in an electrolytic ~` bath containing 150g of H2SO4 per liter of water and 50mg of Ag2SO4 (at a bath temperature 23C~ under -the conditions shown in Table 5 for 60 minutes. The puls~ width used was ' ~ ~ 16 msec. By changing the ~t~ of the peak voltage and . ~.
- n(=T/T) during electrolyzing, colored oxide films shown in Table 5 were formed.
By rearranging the results in relation to the peak ` voltage and n(=T/T), the relation-ships shown in Figure 5 were obtained.
Further, when the aluminum materials were electrolyzed under the conditions shown in Table 6 with different pulse ! 15 durations, such colored oxide films shown in Table 6 were ~ obtained.
. ~ .
The distribution of the depth of color tone of the I colored oxide films obtained in this case was substantially j! the same as shown in Figure 5. Even when the pulse dura-i 20 tion 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 Figure 4.
The current density values given in Tables 5 and 6 ~, ~ are all those obtained with a moving-coil ammeter.

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- ~392~8 . Table S.

..... _ _ : Conditions for electrolysis Formed oxide Film,, ..... _ _ ., peak Voltage Period Mean curren-t Film thick- Color tone Vp ' n T 2 ness (V) (A/dm ) (~) .. , . . ._ _ , 10 lO0 0.32 1.7Reddish orange . 8 80 0.37 2.0 : 20 6 60 0.44 2.5 ll

4 40 0.64 4.0Right reddish orange ~, 2 20 1.18 7.8 Yellow _ lO lO0 0.54 4.0Reddish orange '~ 8 80 0.60 5.0 . "
, 25 6 60 0.80 5.2Ligh-t reddish li . orange 4 40 1 1.12 6.7 ,.
2 20 2.04 15.0 Yellow -j , l 8 80 0.88 6 Light reddish :~ orange :.
27.5 4 40 1.60 11.5 Yellow :
~' 2 20 2.52 19 I. :
:7 I lO lO0 0.88 6.7LicJht reddish ¦ orange 8 80 0.98 7.5 .
6 60 1.32 10.7 ..
~ 4 40 1.64 15.1Reddish orange .~ 2 20 2.46 21.7 ..
l .. . .__ .
:1 6 60 1.16 ll Unclear reddish orange ~, 32.5 4 40 3.04 24 ..
.j 2 20 3.28 30 Reddish orange .1 .
.l lO lO 1.50 17.8 ..

- 8 80 2.28 24.4 ,l 6 60 2.86 28.2 Unclear reddish orange i 4 40 3.08 29 ., .~
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Table 6.

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Conditions for electrolysis :Formed oxide film Pulse Peack Frequency Mean Film Color tone :l width voltage n T current thickness Tp Vp density (msec~ (V) (msec) (A/dm2) (~) 15V 2 lO 0.7 3.9 Yellow 8 40 0.5 2.2 Reddish orange 4 20 1.4 10.5 Light reddish 2 lO 3.0 25.7 Reddish orange 1.52 21.5 ..
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:, 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 lO.0 ll ',1, .. .

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,' ~. : ' ' . ' ' ' ' ' ' ' ~' ~0392Z8 When metallic salts such as HAuCl4, Na2SeO3, CuSO4, SnSO4, NiSo4 and CoSO4 were added in a H2SO~ aqueous solution in place of the aforesaid metallic salt and the peak voltage Vp 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~l Purple l 2 Se ~a2SeO3 Cream i lO 3 Cu CuSO4 Deep red to brown ' 4 Sn SnSO4 White to dark .~ brown ~ 5 Ni NiSO4 Amber to black 6 Co CoSO4 ,~ .
When aluminum materials A.A1099, 1100, 2011, 2014, 1 2024, 3003, 4043, 5005, 5086, 5357 6061 and 7075 other -~ than 6063 were electrolyzed during which the peak voltage ~' Vp and n were controlled, substantially the same results ,, as those in Figures 4 and 5 were obtained, although ,;j 20 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.
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The relationships of the peak voltage and the value n to the distribution of color tone showed the same tendency as shown in Figure 4, though a little affected by such factors as the power source, voltaye ad~usting 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 j .
were changed, there were some occasions when the sizes ; and shapes of the zones A, B and C in Figure 4 were ~; changed, or the zone D became so narrow that it was not necessary kodistinguish the zone D between the zones ;
- lS A, B and C and the zone E in actual electrolysis, or the width of the zone D increased. Further, the afore-! said factors had relation to at least some of the condi-tions 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 Figure~ can ; ~ 7 be maintained, in which one of the features of the .~
method of this invention resides.
!

Aluminum materials 1100, degreased and rinsed with water and then neutralized in known manner, were electro-, ., ~ j .
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g~2 lyæed by applying the rectangular pulse voltage shown in Figure 3 is an electrolytic ba-th containing 150g of H2SO4 per liter of water and 50mg of Ag2SO4 per liter of water (at a bath temperature of 25C), with the -~ 5 aluminum materials being used as both electrodes.
,~ In this case, an impedance was connected as shown . `~j!
'( in Figure 9 and the impedance used was a resister.
~ By changing its resistance value, the by-pass current ., .
was changed. The results shown in the following Table ; 10 8 were obtained.
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`,`.,.'` ' ~ .' ' :': :' ' ' " ,'`,' ' '; ., Table 8 : Sample Pulse Pulse Peak Surface Resistance No. duration interval voltage area of Value T = Ta h=ha Vp=¦Vpa¦ sample (sec) (sec) (V) (cm ) (Q) ., -3 -3 . 1 2xlO 18xlO 20V 50 10 2 ll ll ll ll 40 3 ll ll ll ll 54 4 ll ll ll ll 100 ll ll ll ll 150 6 ll ll ll ll 220 .
.~ 7 ll ll ll ll 500 8 ll ll ll 100 10 ., 9 ll ll ll ll 150 ll ll ll ll 500 ~ Sample Mean current Time for lowering Color of .;, No. Electrolytic By-pass to - V or V film :
cell circuit 4 p pa (A) (A) (sec) ~ 1 0.21 0.22 0.3xlO 3 Dark brown i 2 0.23 0.07 0.85x10-3 Jf 3 0.20 0.06 l.lxlO- ll 4 0.22 0.04 2.0xlO A little dark 0.25 0.03 2.5x10-3 ll i 6 0.23 0.03 4.3xlO Brown , 7 0.23 0.02 9xlO 3 Light dark 3 yellow 8 0.44 0.27 0.5x13- Dark brown f 9 0.39 0.04 5xlO Light brown 1 10 0.41 0.015 14.5xlO Light drak . yellow ... :f.' _ ~ _ ~ - ~

`:
~L039~:2 `: 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 , when the resis-- tance value was 500Q, the time for lowering of the applied voltage to l/4 of the peak voltage was longer `5 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 lOOcm2.
, EXAMPLE 6 ~ . Aluminum materials A.A6036, chemically pretreated, ,l in known manner were electrolyzed in an aqueous solution '~ containing 150g of sulfuric acid per liter of water and ..
50mg of Ag2SO4 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 Figure 12 was applied.
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i The results shown in the following Table 9 were ~i obtained.
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; Table 9.

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Characteristics of pulse voltag~ MeanColor of No Peak Pulse PulseConduction Current ox'de . Voltage width period time Vp=Vpa I = I T = Tat = ta . (V) (sec) (sec) (sec) (A/dm2) .. 1 25 16x10 48x10 5 0.63 A little .~.,,i . ydeleleoPw ~. 2 25 33x10-3 132x10-3 ll 0.62 ll ::
'":

,, 3 20 16Xlo 3 48x10 3 .l 0.44 ~ little 4 20 33x10 3 132x10 3 .. 0.43 ye11owt i~ :'' 33 16x10 16x10 ll 1.50Deep . orange ~, 6 ll 33x10-3 33x10-3 . 1.40 7 ll 50x10 50x10--3~l 1.50 :i ~i~ 8 20 16x10 3 64x10 3 n 1.20Light i 9 ll 33x10-3 132x10-3 ll 1.00 ..
~! 10 ll 50x10 3 200x10 3ll 1.30 .
.~
s 11 25 16 lD-3 64x10 3 D0 Orange .',, ' ~ . .

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It appears from Table 9 that an increase in the values of the peak voltages Vp and Vpa causes the color ` of the oxide film to become deeper and that, in the case `d of the same frequency, that is, when the pulse periods ; 5 T and Ta are equal to each other, an increase in the ~' pulse widths land Ta causes the color of the oxide film to become lighter.

'' Aluminum materials 1100 were degreased and rinsed with water and then neutralized to clean the surfaces i of the aluminum materials. These aluminum materials were electrolyzed in an aqueous solution containing 150g of H2SO4per liter of water and 50mg of Ag2SO4 per liter of water, with the aluminum materials being used as both electrodes. The pulse voltage shown in Figure 3 was applied and the electrolysis was effected with a i ~ current of ~0e~ for 60 minutes.
J. The results shown in the following Table 10 were y 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, respecti.ely.

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Conditions for electrolysis Color of ~ f-1 cr ~ Peak ~oItage Pu1se w1dth fi1m --(Hz) (V) (sec) 20xl0 Yellow 40xl0 Light Orange 3 30 60xl0 3 A little ~` light orange l 30 200x10 3 Deep orange l0In 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 l00Hz.
, Although the foregoing description has been given mainly, in connection with the cases in which the aluminum materials A.All00 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 ~,~

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conditions for electrolysis that when the aluminum ` material A.AllO0 is electrolyzed in the sulfuric acid .
. aqueous solution, an oxide film of an o:range color is j formed, when the aluminum materials A.A3003, A.A4043, -. 5 A.A5052, A.A6061 and A.A6063 are electrolyzed in the -:
'~.
; above electrolytic ba-th, 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.
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Claims (15)

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

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 at least one electrode with a pulse voltage consisting of a plurality of unipotential pulses whose polarity is reversed every 0.2 to 240 seconds.

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 waveforms 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 base 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 10x10-3 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.

15. The method according to claim 1 wherein the polarity of the pulses is reversed every 3 to 50 seconds.