Process for the electrolytic coloring of aluminum or aluminum alloys

A process for the electrolytic coloring of aluminum or aluminum alloys consisting of subjecting the previously anodized piece to two electrolytic treatments in a coloring solution (solution of soluble metal salts). The first consists of an alternating current treatment with an elevated positive polarization, to cause a modification in the pores of the anodic layer. After this electrolytic treatment and in the same bath, the sample is colored by applying thereto an alternating current. The electrolytic treatment prior to coloring permits colors differing from those produced in conventional coloring to be obtained.

INTRODUCTION 
Currently the colouring of anodized aluminium is well known and it is 
possible to use different processes, although the conventional processes 
are colouring by dyeing, integral colouring and electrolytic colouring. 
Colouring by dyeing presents the advantage that the complete range of 
colours and shades can be obtained, however the resistance to light of 
this type of finish is poor, wherefore its application to articles for use 
outdoors is very limited. 
Integral colouring permits substantially lightresistant finishes to be 
produced, however the range of colours to be produced is limited to 
bronzes, blacks and greys, and also, due to the high energy cost of this 
technique, the application of these processes is becoming rather reduced. 
Electrolytic colouring permits different colours to be obtained, such as 
bronzes, blacks and reds with a good resistance to light and a lower 
energy cost; however it is necessary to have a colouring bath for each 
colour and therefore the majority of the installations only produce 
bronzes and blacks. 
This invention relates to an electrolytic colouring process which permits a 
wide range of colours and shades to be obtained using a single colouring 
bath. 
DESCRIPTION OF THE PRIOR ART 
The search for new colours using the electrolytic colouring technique has 
been a widely developed field due to the current limitations. In 1968 
French Pat. No. 1,605,100 described a method by means of which it was 
possible to obtain yellow and brick-red colours by the partial anodic 
dissolution of the particles deposited on the pores in a sodium 
thiosulphate solution. 
Thereafter German Pat. Nos. 2,106,388 and 2,106,389 recited a process for 
producing bluish colours which mainly consists in electrodepositing, on a 
chromic acid formed anodic layer, metals such as Cu, Co and Ni, and 
simultaneously sealing under special conditions; this patent has the 
disadvantage that colouring can only take place on samples previously 
anodized in chromic acid and not sulphuric acid which is currently the 
most widely used anodizing process and, furthermore, the use of a special 
sealing is another limitation of this process. 
From 1974 onwards various patents appeared, inter alia, Spanish Pat. No. 
437,604 (equivalent to U.S. Pat. No. 4,011,152) and French Pat. No. 
2,236,029 whereby the use of high sulphuric acid concentrations in the 
colouring bath produced colours differing from those normally obtained in 
electrolytic colouring baths. 
These processes present the typical disadvantages of working with high 
proton concentrations, that is, the possibility of producing spalling of 
the oxide layer at not very high voltages, difficulty of impossibility of 
producing very dark tones or blacks, and the production of very soft 
anodic layers due to the dissolution effect of the anodic layer by the 
acid during the colouring process, and finally the possibility of 
producing colour losses in the washing processes, subsequent to colouring, 
and in the sealing processes due to the acid hauled. 
Thereafter, U.S. Pat. Nos. 4,022,671; 4,066,816 and 4,152,222 use a double 
anodizing process, firstly in sulphuric and phosphoric acid solutions and 
a posterior colouring in a solution of metal salts. These patents claim to 
produce a wide variety of colours in a single bath but present the 
disadvantage of a reanodizing in a phosphoric acid bath, between the 
conventional anodizing in sulphuric acid and the colouring. 
In accordance with these patents, the colours are produced by optical 
interference and the fundamental requirement is to have a pore size equal 
to or more than 260 .ANG.. 
Other patents, such as U.S. Pat. No. 4,021,315, claim a process having two 
electrolytic treatments with direct current in the colouring bath to 
prevent spalling and to obtain finishes having better characteristics. 
However, neither the modification of the anodic layer nor the wide range 
of colours sought by this process can be obtained, due among other 
aspects, to the different values of the parameters involved in the 
process. 
Finally, U.S. Pat. Nos. 3,788,956, 3,704,209 and 3,915,813 refer to 
different wave forms to increase the quality of the product and the 
uniformity of the colour in the colouring process.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention refers to a process for producing anodic layers 
electrolytically coloured in soluble metal salt baths, mainly 
characterized by producing colours differing from those normally obtained 
in these baths. It is known that with copper salt electrolytes it is 
possible to obtain reds and blacks; however, blues, greens, greys, 
violets, etc. can be obtained with the process of the present invention. 
Likewise, with tin salts it is only possible to produce, with the 
conventional processes, bronzes and blacks; however, blues, greens, 
yellows, reds, etc. can be obtained with the process of the present 
invention. 
The process for electrolytically colouring anodized aluminium of the 
present invention mainly consists of the following. 
First, an anodic layer is produced through a conventional anodizing process 
in sulphuric acid having a concentration ranging from 150 to 200 g/l at a 
current density of from 1 to 2 A/dm.sup.2 corresponding to a potential of 
from 12 to 20 volts of direct current for treatment times of from 15 
minutes to 1 hour. 
Once the anodic layer, whose thickness can range from 3 to 30 microns, has 
been produced by this process, the aluminium piece is introduced in the 
colouring bath. The colouring bath consists of a solution of metal salts 
having a pH above 0.8. Among the metal salts to be used in this process 
are metal sulphates, acetates and chlorides, such as copper, tin, cobalt 
and nickel. 
Once the aluminium piece has been submerged in the colouring bath, it is 
subjected to two electrolytic processes, both with alternating current. 
In the first electrolytic process, the aluminium sample is subjected to an 
alternating current in which the effective voltage applied is from 7 to 35 
volts and the peak voltage of the negative semi-wave is always less than 
that of the positive semi-wave and is from 2 to 15 peak volts. 
The term alternating current will be applied here to any type of variable 
current between the positive and negative polarity, having a positive and 
a negative cycle alternately, and can be purely wavy, modified or any 
other wave form. 
The objective of this treatment is to produce an anodic layer close to the 
aluminium/anodic layer interphase with pores having dimensions differing 
from those obtained in conventional anodizing with direct current. 
We have observed that this modification is particularly effective when a 
small cathodic peak potential ranging from 2 to 15 peak volts is used. 
Another characteristic of the process is that the ratio between the anodic 
and cathodic potential must be such that during the modification process 
the metal is not deposited, since even a minimum deposition (appearance of 
colour) inhibits the modification of the bottom of the pores. 
The time used in this reanodizing process can vary from 3 to 30 minutes if 
interference colours or colours differing from normal are to be obtained 
in the second electrolytic process, although preferably times of from 5 to 
10 minutes will be used. If bronze tones of higher uniformity than 
obtained by conventional methods are to be obtained in the second 
electrolytic process, this reanodizing process is conducted for from 15 
seconds to 2.75 minutes. 
Once the first described electrolytic process has terminated, the aluminium 
piece is subjected in the same bath to an alternating current, in which 
the cathodic peak voltage is equal or similar to the anodic peak voltage, 
to produce the electrodeposition of the metal pigment (colouring of the 
anodic layer). 
The term alternating current used in the preceding paragraph refers to any 
type of wave variable between the positive and negative polarities, having 
a positive and a negative cycle alternately. 
The voltages used in this treatment can vary from 7 to 25 volts and the 
treatment time from 2 to 30 minutes. 
It has been observed that both in the first as well as in the second steps, 
better results are obtained when programs are used instead of a constant 
voltage throughout the entire process. 
The pH of the solution plays an important role and in all cases it must be 
maintained above 0.8. The temperature of the colouring bath is not 
critical in the range of room temperature (15.degree. to 25.degree. C.). 
EXAMPLE 1 
After the workpiece has been treated with a neutral type degreasing agent, 
it is subjected to etching in a 6% caustic soda solution at 60.degree. C. 
for 5 minutes. It is then washed with water and neutralized in 1/1 v/v of 
nitric acid, whereafter it is introduced in a bath containing a 15% by 
weight solution of sulphuric acid. On this piece is applied a direct 
current having a voltage of 17 volts and a current density of 1.5 
A/dm.sup.2 for 30 minutes to form a transparent oxide layer of 12-14 
microns. 
The piece thus treated is submerged in a bath having the following 
composition 
______________________________________ 
SO.sub.4 Cu.5H.sub.2 O 
30 g/l 
Tartaric acid 
30 g/l 
pH - 1.5 
______________________________________ 
The pH is adjusted by adding sulphuric acid. The piece is firstly subjected 
to an alternating current at having an anodic peak voltage of 25 volts and 
a cathodic peak voltage of 5 volts for 7 minutes. After this period of 
time an alternating current at having a voltage value of 15 volts is 
applied. The colours obtained were the following: 
______________________________________ 
Time (minutes) Colour 
______________________________________ 
1 Violet-grey 
2 Neutral grey 
3 Blue-grey 
4 Violet 
5 Brown 
______________________________________ 
The thickness of the modified layer and the diameters of the pores in the 
normal and modified anodized zone were measured by electronic microscopy. 
______________________________________ 
Modified Layer Normal Layer 
Thickness .0. Pore .0. Cell 
.0. Pore 
.0. Cell 
Colour .ANG. .ANG. .ANG. .ANG. .ANG. 
______________________________________ 
Blue-grey 
3000 180 480 125 400 
______________________________________ 
EXAMPLE 2 
A sample treated in the same manner as in example 1, was subjected after 
anodizing and in the colouring bath to a continuous anodic voltage of 25 
volts for 7 minutes and then to a colouring step with an alternating 
current identical to that of the preceding example. The resulting colour 
was as follows: 
______________________________________ 
Time (minutes) Colour 
______________________________________ 
1 Pink 
2 Light Red 
3 Average Red 
4 Average Red 
5 Dark Red 
______________________________________ 
As can be observed, a range of colours as that obtained in example 1 is not 
obtained using the conditions of this example 2. 
This is due to the use of direct current in the first phase of this 
colouring step. 
Finally, it must be pointed out that the carrying out of the colouring 
method described can take place, under optimum conditions, using the 
colouring equipment described in U.S. Pat. No. 4011152 (granted on Mar. 8, 
1977).