Color anodizing of aluminum

A process for producing a colored coating on aluminum comprises anodizing the aluminum at a voltage in the range of 6 to 24 volts in a sulfuric acid electrolyte and thereafter reducing the voltage to not greater than 3 volts while further anodizing the aluminum in the aforementioned electrolyte thereby producing the colored coating.

INTRODUCTION 
This invention relates to anodizing aluminum and more particularly it 
relates to producing colored anodic coatings on aluminum. 
In the prior art, various methods have been used to obtain colored anodic 
coatings on aluminum. These methods include dyeing an otherwise colorless 
anodic coating or anodizing in a special electrolyte, e.g. combination 
sulfuric/sulfophthalic acid, to produce an integral colored coating. It is 
also known in the prior art that colored anodic coatings can be produced 
on aluminum using sulfuric acid. For example, Terai et al. U.S. Pat. No. 
3,935,084 disclose that colored oxide films can be formed on aluminum by 
anodizing in sulfuric acid using D.C. voltage followed by an A.C. voltage 
treatment, the voltage of which is lower than the D.C. voltage. However, 
in this process to increase the degree of coloring in the oxide film, the 
A.C. voltage has to be raised and lowered in a voltage range lower than 
the D.C. voltage. 
Quite surprisingly, I have discovered a process for producing colored 
anodic coatings on aluminum which employs controlled D.C. voltages and a 
sulfuric acid type electrolyte. 
SUMMARY OF THE INVENTION 
An object of this invention is to produce a colored anodic coating on 
aluminum. 
Another object of this invention is to produce a colored anodic coating on 
aluminum by using controlled D.C. anodizing voltages. 
In accordance with these objectives, a process for producing colored anodic 
coatings on aluminum comprises anodizing the aluminum at a voltage in the 
range of 6 to 24 volts in a sulfuric acid electrolyte and thereafter 
reducing the voltage to not greater than 3 volts while further anodizing 
the aluminum to produce the colored coating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
In FIG. 1, there is provided a schematic for producing a colored anodic 
coating on aluminum. In the first step, the aluminum material to be 
treated in accordance with the invention is subjected to a cleaning 
pretreatment well known to those skilled in the art which includes 
deoxidizing, caustic etching and desmutting. Thereafter, the aluminum 
material, which can be sheet or extrusion or other product, is subjected 
to a conventional anodizing treatment using D.C. voltage and a sulfuric 
acid electrolyte, for example, as taught by Gower U.S. Pat. No. 1,869,058. 
Having thus provided an anodic coating as described, in accordance with 
the present invention, a controlled color is obtained therein by reducing 
the voltage to a fraction of the original anodizing voltage and by 
continuing the anodization process for a short period of time thereafter. 
It should be noted that while reducing the voltage, the anodization 
treatment should be continued. That is, the passage of current should be 
continued while the reduction of voltage is taking place. If the 
anodization process is allowed to stop while reducing the voltage, color 
can fail to develop or if it does develop, often it is non-uniform. 
The reduced voltage can be in a range of 0.5 to 3 volts, with a preferred 
voltage being in the range of about 1 to 2.5 volts. Treatments at higher 
than 3 volts normally results in lack of color development in the anodic 
coating and at lower than 0.5 volt the color development can require an 
unduly long anodizing period. It should be noted that the reduced voltage 
referred to should normally be measured at or relatively close to the 
anodizing surface to minimize the effect of the electrolyte and the like. 
With respect to the original anodizing voltage referred to, by anodizing in 
a sulfuric acid electrolyte, this voltage can range from 6 to 24 volts. 
For purposes of the present invention, preferably this voltage should be 
regulated between 12 and 20 volts. For example, with reference to FIG. 2, 
it will be seen that a starting voltage of 18 volts is used to produce a 
conventional coating having a thickness of about 1 mil. Also, by reference 
to FIG. 2 it will be observed that a reduced voltage of about 2 volts can 
be used to impart a controlled color to what would otherwise be regarded 
as a substantially colorless coating. The time period employed to reduce 
the original anodizing voltage to the color developing voltage can be 
relatively short with a preferred time period being in the range of 0.5 to 
5 minutes. FIG. 2 indicates that a time period of 3 minutes to reduce the 
voltage is quite suitable. It is preferred that the voltage be reduced 
from the original voltage to the color developing voltage without 
interruption of the anodizing process, substantially in a manner similar 
to that shown in FIG. 2. The voltage can be reduced in steps as shown in 
FIG. 2 or it can be reduced gradually to the lower voltage. The preferred 
rate of reduction is a rate in the range of 1 to 16 volts/minute with a 
typical voltage reduction rate being about 4 volts/minute. 
The length of time from the start of the reduction of the voltage to the 
end of the color producing step can be as much as half the higher voltage 
anodizing period. For example, if the higher anodizing period is 30 
minutes, the color anodizing or lower voltage period can be as much as 15 
minutes. In certain instances, though, the color anodizing step may 
require only a few minutes. Typically, the color anodizing step requires a 
time period in the range of 4 to 12 minutes. 
The reduced voltage anodizing treatment increases the thickness of an oxide 
layer, sometimes referred to as a barrier zone, located intermediate the 
oxide coating and the aluminum substrate metal. The barrier zone which is 
typical of that obtained by conventional sulfuric acid anodizing of 
aluminum is shown in FIG. 3. Referring now to FIG. 4, it will be seen that 
the barrier zone or oxide layer has been increased rather significantly by 
anodization treatments in accordance with the present invention. It is the 
increase in thickness together with composition of the second oxide layer 
provided at low voltage which is believed to provide color in an oxide 
coating which otherwise, for all practical purposes, would be a clear or 
colorless coating. These oxide coatings were produced in 15 wt.% sulfuric 
acid as described in Example 1. 
With respect to the sulfuric acid anodizing electrolyte, its concentration 
should be in the range of 7 to 35 wt.%. Higher concentrations are 
undesirable since they can affect the integrity of the coating. A 
preferred concentration of sulfuric acid is in the range of 12 to 18 wt.%, 
the remainder water. 
The temperature of the electrolytic bath throughout the anodizing process 
can be kept at or near room temperature. However, in certain instances the 
temperature may be raised as high as 95.degree. F depending to a certain 
extent on the color desired. A suitable electrolyte temperature is in the 
range of 60.degree. to 95.degree. F. It should be noted that temperature 
can influence the color development. For example, in certain instances as 
the temperature is increased there is a tendency for the coating to 
darken. That is, aluminum alloys which would produce a light bronze color 
in an electrolyte at or near room temperature can produce darker colors as 
the temperature is raised. 
While the inventor does not necessarily wish to be held to any theory of 
invention, it is believed that the color development obtained by the low 
voltage anodizing step results from a thin oxide layer formation 
intermediate the conventional oxide layer and the aluminum substrate. It 
is believed that this thin oxide layer operates to provide color by 
entrapment of alloying constituents or precipitates, e.g. Mg.sub.2 Si in 
6000 series aluminum alloy (Aluminum Association designation), in the thin 
oxide layer. By entrapment of alloy constituents or precipitates it is 
meant that such material is not dissolved in the anodizing electrolyte as 
appears to be the case at normal or higher voltage anodizing conditions. 
The present invention contemplates within its purview any alloy which 
responds by developing color under this reduced voltage technique. 
However, aluminum alloys preferred include those referred to as the 
aluminum-magnesium-silicon alloys. Examples of such alloys under the 
Aluminum Association Alloys designation are 6061, 6063, 6463, 6351 and 
6262. The temper of these alloys is important and the preferred temper is 
that known as T52. The T52 temper condition can be obtained by 
artificially aging an aluminum alloy which has been provided in the T4 
temper, the T4 temper indicating that the alloy has been solution heat 
treated, quenched and permitted to reach a stable condition. A typical 
artificial aging treatment which may be employed to obtain the T52 
condition is a treatment at 460.degree. F for about 2 hours. The T52 
temper might also be referred to as an over aged condition. 
The aluminum-magnesium-silicon alloys anodized in accordance with the 
present invention can produce colors ranging from light tan and bronze to 
light and medium grays. 
Coatings produced in accordance with this invention may be sealed by 
conventional procedures. For example, the coating may be sealed in boiling 
water containing sealing salts such as nickel acetate and sealing smudge 
formed may be removed with an acid treatment without any adverse effects 
to the coating. 
The following examples are further illustrative of the invention. 
EXAMPLE 1 
Specimens of Aluminum Association Alloy 6063 in the T52 temper condition 
were anodized in sulfuric acid for 30 minutes at a current density of 24 
amperes/ft.sup.2 which resulted in a starting voltage of 16 volts and an 
ending voltage of 17 volts. Thereafter, the voltage was reduced to 2 volts 
and the anodization process continued for 8 minutes. The sulfuric acid 
concentrations and temperatures are tabulated below. Also, the colors 
obtained are tabulated below. For comparison purposes three specimens of 
6063 alloy were treated as above except for the following differences. A 
first specimen was anodized without using the 2 volt treatment. In the 
anodization treatment of a second specimen, the voltage was reduced from 
17 volts to zero volts and then anodized at 2 volts for a period of 8 
minutes. In the treatment of a third specimen, the voltage was reduced 
from 17 volts to 4 volts and the anodization process continued for 8 
minutes. These three anodizing treatments produced a substantially clear 
or colorless anodic coating. 
Table I 
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H.sub.2 SO.sub.4 Electrolyte 
Concentration 
(wt. %) Temperature Color of Coating 
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7 90.degree. F light gray 
15 75.degree. F light bronze 
15 85.degree. F light gray 
25 75.degree. F light bronze 
25 90.degree. F medium gray 
35 75.degree. F light bronze 
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EXAMPLE 2 
Specimens of Aluminum Association Alloy 6463 in the T52 temper were 
anodized as in Example 1 in an electrolyte containing 15 wt.% H.sub.2 
SO.sub.4. The temperatures of the electrolyte used in this instance was 
75.degree. F and 85.degree. F. The 75.degree. F treatment resulted in a 
light metallic bronze color and the 85.degree. F treatment resulted in a 
medium metallic bronze color. 
Thus, it can be seen from these examples that colored anodic coatings are 
obtained on aluminum using controlled D.C. voltages and an electrolyte 
which would normally produce a clear or substantially colorless coating. 
It will also be observed that the colors can be varied by varying the 
temperature and concentration of the electrolyte. 
While the invention has been described in terms of preferred embodiments, 
the claims appended hereto are intended to encompass other embodiments 
which fall within the spirit of the invention.