Method of producing colored surfaces on parts of aluminum or aluminum alloy

A method is described for producing colored surfaces on parts of aluminum or aluminum alloy which is characterized by the following process steps: PA1 1. The parts are pre-treated by degreasing or cleaning, PA1 2. The parts are electrolytically anodically/alkalinely brightened, PA1 3. The parts are electrolytically anodized with the use of direct current, PA1 4. The parts are electrolytically and/or organically colored, and PA1 5. The oxide layer on the parts is compacted.

BACKGROUND OF THE INVENTION 
The present invention refers to a method of producing colored surfaces on 
parts of aluminum or aluminum alloy. 
Aluminum parts, such as aluminum stampings or rolled sections are used, 
inter alia, in the hardware and lighting industries or, in particular, 
also in automobile manufacture, for instance, for window mounting systems 
or else as ornamental frames, ornamental moldings and the like. In this 
connection, it is also known to use parts of aluminum or aluminum alloy 
which have a colored anodized surface. 
The object of the present invention is to provide a new method of producing 
colored surfaces on aluminum or aluminum- alloy parts in which the parts 
which are colored by this method have the following quality features: 
a) 1,000-hour light-fastness testing without fading and without change in 
the color pigmentation. Testing: Xenotest Hot Light Exposure 450 DIN 
75202. 
b) 6 cycles Kesternich DIN 50018 Testing for corrosion resistance 
c) 5,000 strokes--Testing of fastness to rubbing without change of surface 
according to Veslick DIN 53339. 
The new method is also characterized by the fact that multiplicity of 
colors with color gradations viz 
gold - bronze 
light - dark bronze 
gray - brown 
gray - blue 
anthracite 
light - dark blue 
blue - violet 
is made possible on coloring of the parts as well. The new method also 
allows for a simple verification of the results of the coloring and 
reproduceability at all times of the individual shades of color. 
SUMMARY OF THE INVENTION 
The objects of the invention are accomplished by subjecting aluminum or 
aluminum alloy parts to a sequence of steps comprising: 
(1) pretreating the parts by degreasing or cleaning. This may be suitably 
accomplished by first treating the parts in an aqueous alkaline phosphate- 
and borate-containing solution and then treating the parts in an acid 
solution containing phosphoric acid; 
(2) electrolytically anodically/alkalinely brightening the parts, as by 
subjecting them to direct current in an alkaline electrolyte containing 
NaPO.sub.4, Na.sub.2 Co.sub.3, AlPO.sub.4 and beechwood extracts; 
(3) electrolytically anodizing the parts to produce an oxide layer, with 
the use of direct current, suitably in an electrolyte containing H.sub.2 
SO.sub.4, Al and a surfactant; 
(4) electrolytically and/or organically coloring the parts. Such coloring 
may suitably be accomplished by first subjecting the parts to an 
alternating current in an electrolyte containing SnSO.sub.4, H.sub.2 
SO.sub.4 and an oxycarboxylic or sulfonic acid, preferably an aromatic 
sulfonic acid, for example, benzene sulfonic acid, and then treating the 
parts in an acid dyebath containing an azo dyestuff and a fungicide; 
(5) compacting the oxide layer on the parts. This may be accomplished by 
first treating the parts in a solution containing cobalt and nickel 
fluorides in desalinated water and then treating the parts in desalinated 
water containing a coating inhibitor such as a triazine derivative, 
preferably a 1,3,5-triazine such as isocyanuric acid. 
Between steps (2) and (3) the parts are preferably washed with water, 
exposed to a chromic acid solution and then washed in a sodium 
hydrosulfite solution.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred examples of a surface treatment of a part in order to obtain the 
desired quality features will be explained in detail below. 
An extruded section of aluminum alloy AlMg 1 or AlMgSi 0.5 is mechanically 
ground and polished. Degreasing or cleaning is effected in two steps, 
namely: 
1. Treatment by an aqueous solution of an alkaline phosphate and borate, pH 
9. 
1.1. Treatment by an acidic phosphoric acid containing aqueous solution, pH 
1.1. 
2. After the degreasing and cleaning steps, the section is subjected 
electrolytically to alkaline brightening in order to obtain a sufficiently 
bright surface reflection. 
The electrolyte employed in this step contains: 
______________________________________ 
Na.sub.3 PO.sub.4 (trisodium phosphate) 
120 g/l 
Na.sub.2 CO.sub.3 (disodium carbonate) 
330 g/l 
AlPO.sub.4 (aluminum phosphate) 
10 g/l 
Beechwood extracts 5 m/l 
______________________________________ 
The electrolysis conditions are as follows: 
______________________________________ 
Current density 3 amp/dm.sup.2 
Operating temperature 70-80.degree. C. 
Exposure time 18 minutes 
______________________________________ 
2.2. After the brightening step, the aluminum alloy which has been treated 
in this manner is washed in water. The oxide film formed in the 
brightening is then removed in a chromic acid solution of 50 g/l CrO.sub.3 
at 98.degree. C. with an exposure time of 3 minutes. 
2.3. In a further process step, the aluminum alloy is washed in a sodium 
hydrosulfite solution in order to reduce the hexavalent chromium to 
trivalent chromium. 
3. The aluminum alloy section is then electrolytically anodized with the 
use of direct current. 
The electrolyte employed in this step contains: 
______________________________________ 
H.sub.2 SO.sub.4 (sulfuric acid) 
180 g/l 
Al 8 g/l 
Surfactant (wetting agent) 
30 ml 
______________________________________ 
The anodizing conditions are as follows: 
______________________________________ 
Current density 1.5 amp/dm.sup.2 
Operating temperature 18-20.degree. C. 
Exposure time 35 minutes 
Anodic layer thickness 
12 .mu.m 
______________________________________ 
4. In the next process step of the coloring process, the section is exposed 
in an electrolyte containing a metal salt and subjected to an alternating 
current. Gold-bronze and light-dark bronze shades are obtained under the 
same operating conditions when using different exposure times. 
The electrolyte employed in this step contains: 
______________________________________ 
SnSO.sub.4 (tin sulfate) 
15 g/l Sn (tin as tin sulfate) 
H.sub.2 SO.sub.4 (sulfuric acid) 
15 g/l 
Benzene sulfonic acid 
30 ml/l 
______________________________________ 
The operating conditions are as follows: 
______________________________________ 
Operating temperature 20-22.degree. C. 
Current density 1.5 amp/dm.sup.2 
pH 1.0 
______________________________________ 
In the cathodic alternating current phase, tin is incorporated into the 
pores of the oxide layer by electrolytic metal deposition. 
The gold-bronze and light-dark bronze shades are obtained with variable 
current densities and different exposure times of 10 to 15 minutes. 
If the section is to be imparted a shade other than the one described 
previously, then the exposure time upon the deposition of the metal is 
limited to 10-30 seconds, variable current densities giving different 
color gradations. By this parameter-controlled metal deposition a basic 
coloring is obtained, the adsorbability of the oxide layer being further 
retained. 
4.1. In another coloring treatment, the adsorbability of the oxide layer is 
utilized in order variably to change the basic color by chemical 
incorporation of an organic azo dyestuff. 
The chemical dyebath employed in this procedure contains: 
______________________________________ 
Azo dye 1 g/l 
Fungicide 0.4 ml/l 
______________________________________ 
The operating conditions are as follows: 
______________________________________ 
pH 3.5-3.9 
Operating temperature 
50.degree. C. 
Exposure times 90-180 seconds 
______________________________________ 
With different times of exposure in the chemical dyebath, the shades 
gray-brown, gray-blue, anthracite, light-dark blue, and blue-violet are 
obtained. 
In the adsorption process, pigment parts of the azo dyestuff deposit, in 
addition to the previously deposited metal salt, into the pores of the 
oxide layer and thus change the basic color. 
5. Finally, the oxide layer is compacted in two process steps and is thus 
protected against external influences. 
In the first of these steps the section is pre-treated for about 10 minutes 
at about 30.degree. C. in a solution of 6% cobalt fluoride and 30% nickel 
fluoride in completely desalinated water. 
In this connection, the following basic reaction between the oxide layer 
and the nickel fluoride takes place: 
##STR1## 
In this first step, a consistent precompacting of the oxide layer is 
obtained. 
In the second step, the section is treated for 50 minutes at 70.degree. C. 
in completely desalinated water with the addition of a coating inhibitor 
consisting of 2 ml/l of isocyanuric and in this connection, a chemical 
reaction takes place first of all with the binding of the water 
ti (Al.sub.2 O.sub.3 +H.sub.2 O.fwdarw.2 AlO+OH). 
The increase in volume of the layer produces a closing of the pores. The 
layer is now protected from external influences. 
Although the present invention has been described in relation to particular 
embodiments thereof, many other variations and modifications and other 
uses will become apparent to those skilled in the art. It is preferred, 
therefore, that the present invention be limited not by the specific 
disclosure herein, but only by the appended claims.