Process for continuous electrolytic coloring of aluminum or aluminum base alloy strip and wire

An improved process for electrolytically coloring an anodically oxidized aluminum or aluminum base alloy strip or wire which comprises, while continuously travelling the strip or wire through an electrolytic coloring means, subjecting the anodically oxidized strip or wire as a cathode to direct current electrolysis using an electrolytic coloring means comprising at least one hollow body having an opening for continuously supplying a coloring bath onto the surface of the oxidized strip or wire and an electrode acting as an anode therein, and continuously supplying the coloring bath through the opening of the hollow body onto the surface of the oxidized strip or wire.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for the continuous electrolytic 
coloring of an article of aluminum or an aluminum base alloy (hereinafter 
referred to aluminum) in a windable shape, such as an aluminum strip or 
wire. 
2. Description of the Prior Art 
Various processes for the electrolytic coloring of aluminum comprising 
subjecting aluminum which has previously been anodically oxidized to 
electrolysis in an electrolytic coloring bath comprising an aqueous 
solution of at least one nickel salt, cobalt salt, tin salt, iron salt, 
copper salt or selenious acid, the aluminum serving as an electrode, are 
well known in the art. These electrolytic coloring processes are known to 
be economically advantageous as they enable one to produce 
electrolytically colored aluminum articles with excellent reproducibility 
which are uniform in color. In addition, because of their excellent 
weatherability or fade resistance, the resulting electrolytically colored 
aluminum generally can be used as structural materials such as extrusions, 
plates and the like. 
Known electrolytic coloring processes can be classified as alternating 
current electrolysis processes as disclosed in U.S. Pat. No. 3,382,160 or 
direct current electrolysis processes as disclosed in U.S. Pat. No. 
3,761,362, depending upon the kind of electric current which is passed 
through the electrolytic coloring bath. 
The process disclosed in U.S. Pat. No. 3,761,362 is characterized by the 
coloration of a previously anodized aluminum by subjecting the aluminum to 
a direct current electrolysis, with the aluminum as a cathode, in an 
electrolytic coloring bath comprising an aqueous solution containing at 
least one nickel salt, cobalt salt, tin salt, iron salt, copper salt or 
selenious acid. 
In this direct current electrolysis, the composition of the electrolytic 
coloring bath is suitably selected from the above-described components 
depending on the desired color. Generally, the color formed on the surface 
of the aluminum is bronze when a nickel salt is used, reddish-brown when a 
copper salt is used, bronze to black when a tin salt is used, bronze when 
a cobalt salt is used, yellow when an iron salt is used, and yellow to 
reddish-orange when selenious acid is used. 
The above direct current electrolytic coloring process can be carried out 
in a continuous manner which is advantageous in that electrolytically 
colored aluminum can be obtained economically by continuously anodically 
oxidizing a windable aluminum article such as strips, wires and the like, 
and then subjecting the resulting anodically oxidized aluminum article to 
a direct current electrolytic coloring process, followed by conventional 
final treatments such as a sealing treatment or various coating 
treatments. However, when the electrolytic coloring process is effected in 
a conventional electrolytic cell containing an electrolytic coloring bath 
by continuously supplying aluminum as a cathode from one end of the 
electrolytic coloring cell which is provided with an anode on an inside 
wall or on the bottom of the cell and with agitation means for the 
coloring bath, e.g., by injecting air or the coloring bath into the cell, 
it is very difficult to obtain a uniformly colored aluminum in a stable 
manner. That is, satisfactory coloring is difficult to conduct due to the 
problems hereinafter described, whereby the resulting color tone tends to 
be non-uniform, for example, a stripe-pattern comprising a deeply-colored 
portion and an unattractive lightly-colored portion results, and the 
electrolytically colored film on the aluminum tends to be partially 
spalled. These phenomena are frequently observed with an electrolytic 
coloring process conducted on a large industrial scale, and are liable to 
occur as the travelling speed of the aluminum passing through the coloring 
bath increases and the electric current applied to the coloring bath 
increases. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a direct current process for the 
continuous electrolytic coloring of a windable aluminum strip or wire. 
Another object of this invention is to provide a process for producing a 
uniformly colored aluminum strip or wire in a stable manner by a direct 
current electrolytic coloring process which eliminates the problems 
associated with conventional processes such as irregular stripe-pattern 
coloration and spalling of the electrolytically colored oxide film on the 
aluminum strip or wire. 
As a result of studies on processes for the continuous electrolytic 
coloring of aluminum, it was found that the above-described disadvantages 
associated with conventional processes are due to the fact that during 
electrolytic coloring the electric current is partially concentrated, in 
particular, at portions of the aluminum which come into contact with the 
coloring bath, and that hydrogen gas generated on the surface of the 
aluminum as a cathode is difficult to completely remove. 
The present invention provides an improved process for electrolytically 
coloring an anodically oxidized aluminum strip or wire which comprises, 
while continuously travelling the strip or wire through an electrolytic 
coloring means, subjecting the anodically oxidized aluminum strip or wire 
as a cathode to direct current electrolysis using an electrolytic coloring 
means comprising at least one hollow body having an opening for 
continuously supplying a coloring bath onto the surface of the aluminum 
strip or wire and an electrode acting as an anode in the hollow body, and 
continuously supplying the coloring bath through the opening of the hollow 
body onto the surface of the aluminum strip or wire.

DETAILED DESCRIPTION OF THE INVENTION 
The anodic oxidation carried out prior to the electrolytic coloring 
treatment of this invention is usually carried out in an anodic oxidation 
bath comprising an aqueous solution of about 10 to 55% sulfuric acid with 
a current density of from 3 to 50 A/dm.sup.2, but, if desired, the anodic 
oxidation bath may further contain a small amount of a salt such as 
magnesium chloride, sodium sulfate, magnesium sulfate, sodium chloride, 
etc.; a carboxylic acid; an organic sulfonic acid; and/or an amine. The 
electric current used for the anodic oxidation can be a direct current or 
direct and alternating superposed currents. 
The direct current electrolysis for coloring of an aluminum strip or wire 
according to the present invention can be carried out using conventional 
electrolytic coloring baths. 
The main component of the electrolytic coloring bath used herein is one or 
more water-soluble metal salts such as water-soluble nickel salts (such as 
nickel sulfate, nickel chloride, nickel acetate and the like), copper 
salts (such as cupric sulfate and the like), tin salts (such as stannous 
chloride, stannous sulfate and the like), cobalt salts (such as cobalt 
sulfate, cobalt acetate and the like), iron salts (such as ferrous sulfate 
and the like), like metal salts, and selenious acid. Further, if desired, 
the electrolytic bath may contain a suitable amount of boric acid or 
sulfuric acid to control the pH and the electric conductivity of the bath. 
For example, when the main component is nickel sulfate, boric acid is 
often used in combination with nickel sulfate, with both components being 
capable of use over a relatively wide range of concentrations, for 
example, about 15 to 100 g/liter of nickel sulfate and about 10 to 50 
g/liter of boric acid producing a superior colored oxide film. 
The term "direct current" as used herein means an electric current which 
always flows in a fixed direction, as is well known. Therefore, the direct 
current is not limited by the wave form thereof and it includes all 
electric currents which have a wave form periodically changing in current 
strength, so long as the direction of the current flow is not changed. 
The current density used in the electrolytic coloring process of this 
invention can range from about 0.05 to 3.0 A/dm.sup.2, but a range of 0.1 
to 2.0 A/dm.sup.2 is preferred from an operational point of view. Further, 
bath temperatures in the vicinity of room temperature are sufficient, but 
a temperature of from about 10.degree. to 40.degree. C can also be used 
for coloring. 
The period during which time the coloring phenomenon occurs, i.e., the 
actual coloring time, varies depending upon the distance between the 
surface of the aluminum strip or wire and the opening of the hollow body 
employed in the present invention, but is generally adjusted to be less 
than about 30 seconds, preferably from 2 to 15 seconds. The coloring time 
used herein means the actual time when the aluminum strip or wire contacts 
the coloring bath and the coloring phenomenon actually occurs. Therefore, 
the coloring time is not always identical with the contact time of the 
aluminum strip or wire with the coloring bath. That is, the region where 
the coloring phenomenon occurs is not determined with reference to the 
length of aluminum strip or wire contacting with the coloring bath, 
rather, it is determined by the region of current distribution having the 
specified current density through the opening of hollow body from the 
anode onto the surface(s) of the aluminum strip or wire treated. The time 
when the aluminum strip or wire passes through this region corresponds to 
the coloring time as defined above. 
The narrower the distance between the lower portion of the opening provided 
in the hollow body and the surface(s) of the aluminum strip or wire to be 
treated, the easier the coloring operation is. The minimum distance can be 
easily determined by a simple pre-experiment from the viewpoint of the 
size of the apparatus used, the bending degree of the aluminum strip or 
wire, the uniformity of coloring, etc. The maximum distance is not 
particularly limited, but is preferably controlled to be less than about 
100 mm from the viewpoint of the electric power consumed. 
The amount of the coloring bath which is supplied to the surface(s) of 
aluminum strip or wire via the opening of the hollow body is not 
particularly limited, and can generally be varied depending upon the 
degree of desorption of hydrogen gas generated on the surface(s) of the 
aluminum strip or wire during the electrolysis step. 
The process of this invention will be further illustrated in greater detail 
by reference to the accompanying drawings. 
FIG. 1 is a schematic sectional side view of an embodiment of the apparatus 
used in the process of this invention. In FIG. 1, 1 is an aluminum strip 
or wire which has previously been anodically oxidized by a conventional 
process. This anodically oxidized aluminum strip or wire 1 is used as a 
cathode by applying electric current using the liquid electric supply 
method or the roll electric supply method. 2 is an electrolytic coloring 
cell containing a coloring bath 3 for obtaining the desired color as 
earlier described. 4 represents a pair of rollers for travelling the 
aluminum strip or wire to be electrolytically colored, which rollers can 
be provided at any convenient positions. 5 is a heat exchanger for 
controlling the temperature of the coloring bath. 6 is a pump for 
supplying the coloring bath to an electrolytic coloring means comprising a 
hollow body 7 while circulating the coloring bath. The hollow body 7 which 
characterizes this invention is provided with an anode 8 inside the cavity 
of the hollow body, which anode 8 is electrically insulated from the 
hollow body, and has an opening 9 for continuously supplying the coloring 
bath onto the surface of the travelling aluminum strip or wire and has at 
least one inlet 10 for introducing the circulating coloring bath into the 
hollow body 7. 
In FIG. 1, two hollow bodies are positioned facing the upper surface and 
the lower surface of the aluminum strip or wire 1, respectively, but this 
pair of hollow bodies can be modified so as to integrate the two hollow 
bodies into a single hollow body or, alternatively, two or more hollow 
bodies can be provided at each of the upper and lower surfaces of the 
aluminum strip or wire to be electrolytically colored. 
In carrying out the process of this invention, the anodically oxidized 
aluminum strip or wire 1 is first thoroughly rinsed with water and then 
introduced by means of driving rollers, guide rollers or the like into the 
electrolytic coloring cell 2 where the aluminum strip or wire is brought 
into contact with the coloring bath 3. The aluminum strip or wire 1 
further travels in the coloring bath and approaches the hollow bodies 7 
where the aluminum strip or wire is to be electrolytically colored. The 
coloring rate of the aluminum strip or wire increases as the aluminum 
strip or wire approaches the hollow bodies and decreases as the strip or 
wire travels away from the hollow bodies. A strip or wire which is 
completely colored finally emerges from the coloring bath. 
Of course, during the above coloring procedure, direct current is passed 
through the coloring bath between the aluminum strip or wire as a cathode 
and the anode provided in the hollow body or bodies 7. 
As described above, the characteristic feature of the present invention 
resides in that coloring occurs at areas where the aluminum strip or wire 
to be electrolytically colored approaches close to the hollow bodies 7. 
The hollow body 7 used in the present invention as the electrolytic 
coloring means includes any type of hollow member which can be provided 
with an anode therein electrically insulated from other portions of the 
hollow body, and which has at least one inlet for introducing a coloring 
bath and an opening for supplying a coloring bath by flowing the same 
toward a travelling aluminum strip or wire disposed adjacent the bath 
supply opening. The hollow body can be of any shape, for example, 
cylindrical or box-like. The opening in the hollow body is preferably a 
slit shape, the longitudinal direction of the slit being perpendicular to 
the travelling direction of the aluminum strip or wire, but it may be of 
any shape so long as the opening is capable of flowing the coloring bath 
uniformly toward the travelling aluminum strip or wire. 
The anode is also preferably arranged perpendicular to the travelling 
direction of the aluminum strip or wire, and is advantageously made from a 
material which is convenient for the maintenance of the coloring bath 
used, for example, a nickel plate when the coloring bath contains nickel 
ions. The amount of the coloring bath flowing or ejected through the 
opening can be controlled by liquid flow control means such as a valve, 
after pump 6. 
The flow rate of the coloring bath and the slit width of the opening are 
not critical, and vary depending upon various parameters used in the 
process, but in large scale industrial apparatus it is preferred to use 
higher flow rates and larger slit widths as the travelling rate of the 
aluminum strip or wire increases. 
In another embodiment of the process of this invention, it is possible to 
electrolytically color only one surface of an aluminum strip by, for 
example, using an apparatus as illustrated in FIG. 2. 
Referring now to FIG. 2, members 1 to 10 have the same meanings as 
explained with reference to FIG. 1, and the shape and the function of 
these members is the same as in FIG. 1; there are, however, two 
significant differences between the systems shown in FIGS. 1 and 2. 
Firstly, aluminum strip 1 is not contacted with coloring bath 3 contained 
in coloring cell 2, but is contacted only with a coloring bath layer 12 
formed on the surface of the travelling aluminum strip. When direct 
current is applied in this state between anode 8 and aluminum strip 1 
while travelling the aluminum strip, an aluminum strip electrolytically 
colored on only one surface (the surface which is contacted with the 
coloring bath layer 12) is obtained. Secondly, in this embodiment the 
coloring time can be adjusted by changing the distance between 
coloring-adjustment rollers 11 and 11', i.e., the longer the distance 
between the rollers, the longer the coloring time at constant rates of 
travel. Rollers 11 and 11' function only to adjust the length of the 
coloring bath layer 12, and therefore can be in any shape, such as round 
or square, and can be made from a wide variety of materials, such as 
rubber, sponge, plastic and the like. Rollers 11 and 11' may be eliminated 
if the rollers 4 are shifted to a position closer to each other so as to 
function as rollers 11 and 11'. 
The characteristic feature of the embodiment of FIG. 2 is that electrolytic 
coloring occurs only at the area near the hollow body 7 and the coloring 
bath layer 12. Thus, a hollow body can be provided facing the lower 
surface of the aluminum strip to thereby electrolytically color the lower 
surface of the aluminum strip, or the hollow body can be provided facing 
both the upper and lower surfaces of the aluminum strip, thereby 
electrolytically coloring both surfaces of the aluminum strip. Further, it 
is possible to electrolytically color the aluminum strip in special 
patterns, for example, in a stripe-pattern, by providing a particular type 
of hollow body. For example, a stripe-pattern coloring in which 
deeply-colored portions and lightly-colored portions are parallel to the 
travelling direction of the aluminum strip can be attained when the 
opening of one or more hollow bodies is partially covered with an 
appropriate member, such as a rubber plate or sponge, and electrolytic 
coloring is conducted while setting the partially covered hollow body or 
bodies close to the surface of the travelling aluminum strip. 
In FIG. 2, the hollow body 7 is provided at the upper surface of the 
aluminum strip, but as previously described, two or more hollow bodies can 
be provided at the upper and/or lower surface of the aluminum strip. 
In the embodiment illustrated in FIG. 2, the coloring bath sometimes tends 
to flow down to the edge portions of the lower surface of the aluminum 
strip, thereby partially coloring the edge portions. Such undesirable 
coloring can be prevented by increasing the flow rate of the coloring bath 
or by blowing pressurized air toward the lower surface of the aluminum 
strip. 
The aluminum thus colored is then washed with water and can then be 
subjected to a conventional sealing treatment or various lacquer coating 
methods including electrodeposition, dipping and spraying, as are known in 
the art. 
As will be apparent to one skilled in the art, when the electric polarity 
is reversed and an aqueous solution of sulfuric acid is used in place of 
the coloring bath in the embodiment shown in FIG. 2, only one surface of 
the aluminum strip can be anodically oxidized; such an embodiment is not 
contemplated in the present invention. 
As previously described, in accordance with the process of this invention, 
a stable, uniformly colored aluminum strip or wire can be obtained without 
partial spalling of the colored film. Also, since the process of this 
invention makes it possible to electrolytically color only one surface of 
an aluminum strip and, if desired, to color an aluminum strip in a 
stripe-pattern, various colored aluminum strips suitable for use in wide 
variety of utilities can be obtained. 
In addition, as a most significant feature of the process of this 
invention, anodically oxidized aluminum having a very thin anodic oxide 
film thereon, for example, an anodic oxide film having a thickness of 1 to 
2 .mu., which could not be effectively electrolytically colored by 
conventional electrolytic coloring processes, can be colored stably and 
uniformly by the process of this invention. The above fact indicates that 
electrolytically colored strips and wires having an excellent resistance 
to weathering can be produced at low cost and, therefore, the effects 
brought about by the present invention are very important economically. 
The present invention will be illustrated in further detail by reference to 
the following Examples, but is not restricted thereto as the examples were 
conducted on a small scale for purposes of illustration. 
EXAMPLE 1 
A coiled aluminum strip 65 mm (width) .times. 0.3 mm (thickness) having an 
aluminum content of 99.2% was mounted on the uncoiling means of an 
apparatus comprising an uncoiler, etching cell, washing cell, current 
supplying cell, anodic oxidation cell, washing cell, electrolytic coloring 
cell, washing cell, sealing cell and a recoiler, and passed through the 
recited units in the order given. The apparatus used was substantially 
identical to that disclosed in U.S. Application Ser. No. 450,259, filed 
Mar. 11, 1974, now U.S. Pat. No. 4,002,549 except, of course, for the 
novel coloring means of the present invention. 
In the electrolytic coloring cell, the aluminum strip was immersed in a 
coloring bath and two hollow bodies were positioned facing both surfaces 
of the aluminum strip, as shown in FIG. 1. Each of the hollow bodies was 
in the form of a circular cylinder having a diameter of 30 mm and a length 
of 100 mm. The hollow body was provided with a slit extending axially 
throughout the length thereof, the width of the slit being 1 mm. An anode 
plate of nickel was housed along the axis thereof, the dimensions of the 
anode plate being 20 mm .times. 80 mm .times. 0.5 mm. The hollow bodies 
were disposed perpendicularly to the travelling direction of the strip, 
the distance between the strip and each hollow body being 1 mm. The 
aluminum strip was treated as described hereinafter. 
The aluminum strip was introduced into the etching cell through the 
uncoiler at a rate of 20 cm/min, and contacted therein with a 10% aqueous 
sodium hydroxide solution at a bath temperature of 50.degree. C for 1 
minute, and passed therethrough to the washing cell where the aluminum 
strip was passed through a water bath. 
The resulting strip was then introduced into the current supplying cell 
which contained a 30% aqueous sulfuric acid solution in which was disposed 
a carbon anode plate, wherein the aluminum strip was subjected to 
electrolysis at a current density of 2.0 A/dm.sup.2 at a bath temperature 
of 25.degree. C for 4.5 minutes (electrolysis time), the aluminum strip 
acting as a cathode. 
The aluminum strip was then introduced into the anodic oxidation cell which 
contained a 30% aqueous sulfuric acid solution in which was disposed a 
carbon cathode plate, wherein the aluminum strip was subjected to anodic 
oxidation at a current density of 2.0 A/dm.sup.2 and a bath temperature of 
25.degree. C for 5 minutes (electrolysis time) to thereby form an oxide 
film of a thickness of about 3 microns on the aluminum strip. 
After subsequently passing through a washing cell as above, the aluminum 
strip was introduced into the electrolytic coloring cell in which the 
hollow bodies were disposed, as earlier described, and subjected to 
cathodic electrolysis. The electrolytic bath in the electrolytic coloring 
cell comprised an aqueous solution of 50 g/liter of nickel sulfate and 30 
g/liter of boric acid. While injecting this electrolytic bath at a rate of 
8 liter/min per 1 cm.sup.2 of the opening area of the hollow body, the 
aluminum strip was subjected to cathodic electrolysis at a current density 
of 1.0 A/dm.sup.2 and a bath temperature of 25.degree. C for a coloring 
time of 20 seconds (electrolysis time). As a result, both surfaces of the 
aluminum strip were uniformly colored bronze without undesirable defects 
such as stripe-pattern or film-breakage. 
The aluminum strip thus colored was then passed through a washing cell as 
above described, then introduced into the sealing cell in which the 
aluminum strip was sealed with pure water at 93.degree. C for 8 minutes 
(the color tone of the aluminum strip thus treated did not change) and 
coiled. 
The colored aluminum strip thus obtained had excellent processability, and 
cracks in the oxide film on the aluminum strip were hardly encountered 
even when the aluminum strip was subjected to press processing or roll 
processing. 
EXAMPLE 2 
An aluminum strip was colored on one surface using the same apparatus as 
employed in Example 1 except that a hollow body was provided at only one 
surface of the aluminum strip as shown in FIG. 2. 
The aluminum strip was treated under the same conditions as employed in 
Example 1 to the anodic oxidation cell. After passing through the anodic 
oxidation cell, the aluminum strip was further passed through the washing 
cell and then introduced into the electolytic coloring cell containing 
therein the same electrolytic bath as was employed in Example 1. In this 
step, while injecting the electrolytic bath at a rate of 10 liter/min. per 
1 cm.sup.2 of the opening area of the hollow body, the aluminum strip was 
subjected to cathodic electrolysis at a current density of 1.0 A/dm.sup.2 
and at a bath temperature of 25.degree. C for a coloring period of 10 
seconds (electrolysis time). After completion of this treatment, the 
aluminum strip was successively passed through the washing cell and the 
sealing cell under the same conditions as employed in Example 1 and then 
coiled. 
As a result only the surface of the aluminum strip facing the hollow body 
was uniformly colored bronze. The aluminum strip thus obtained was found 
to have excellent processability the same as that obtained in Example 1. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.