DE3741456A1 - Method for the electrolytic colouring of anodically generated oxide layers on aluminium and aluminium alloys - Google Patents

Abstract

Method for the electrolytic colouring of anodically generated oxide layers on aluminium and aluminium alloys, which provides for automatic control of the process from the anodic oxidation via the colouring up to densification. Based on the finding that the layer structure of the anodised surface and the number of the pores occupied are responsible for the result of colouring, it was found that the variation of current with time during a direct-current treatment carried out prior to the colouring process provides information on the layer structure. Based on the difference between the maximum and final values of the direct-current treatment, and employing known rules, the effective capacitor current can be calculated. This, together with a constant, forms the basis for the calculation of the initial current and, together with a term taking into account the desired brightness, the calculation of the switching-off current.

Description

The application relates to a process for electrolytic coloring anodic oxide layers on aluminum and aluminum alloys.

Numerous processes for electrolytic coloring are already anodic generated oxide layers on aluminum and aluminum alloys known become.

In the extensive literature, the composition is preferred of the coloring electrolytes and the electrical conditions of the coloring treatment described. The aim of the various measures is almost exclusively the achievement of colorations in bronze tones and derivatives thereof.

All of the previously described methods for electrolytic coloring are common that in practical application in the company on a matching the goods to be dyed against a given standard pattern and if necessary, it is not possible to do without recoloring. The matching interrupts the dyeing process, with the result that the throughput overall and thus the economy is significantly reduced.  

Added to this is the fact that the visual sampling takes subjective influences by the operating personnel, which causes considerable errors and deficiencies in the uniformity of batch colorations Enter batch. Differences in the structure of the anodized Oxide layers can also adversely affect the coloring result even within the batch.

Suggestions have already been made to achieve uniform coloring made. In DE-PS 22 23 254 a method for electrolytic Manufacture of colored oxide layers of aluminum and aluminum alloys described, which consists of the instruction during the first Treatment stage, the anodic oxidation, the voltage at an initial value and then gradually increase this to two thirds to several Reduce tenths of the initial voltage and then constant this value or to keep pulsating for a period of time. The applicant this prior publication assumes that depending on the last current density obtained any desired thickness of the barrier layer and that an even barrier layer is good dyeing the oxide layer and the risk of uneven color density is reduced. A method of the type described, with the irregularities the color density to be reduced is without the Dyeing process interrupting sampling does not operate what the use e.g. B. excludes systems to be operated automatically.

Incidentally, the Fig. 10 of this document shows that the uniformity in color density is not given (see. Also notes in column 5 + 6).

DE-PS 26 09 146 recommends to achieve a permanent and uniform Color the anodized base metal first in the dye bath undergo electrolysis with a direct current and then the perform electrolytic coloring with alternating current. The applicant wants to achieve that the current continuity in the electrolytic coloring takes place evenly over the length and height of the batch, causing differences in brightness be avoided when coloring. The applicant leaves but from coloring electrolytes containing at least two metal salts from and also from adding strongly reducing compounds to the electrolyte. The examples show that barrier layer images such as boric acid, Glycolic acid, maleic acid and others are required. For a sample cannot be dispensed with here either.

The interruption of the dyeing process to test the objects but becomes impossible when handling large series of objects Need to become. This can be the case if, for example, parts for the Motor vehicle construction are to be manufactured, which require the greatest possible uniformity the coloring exists because individual parts are put together will. Even small color differences would catch the eye.

The object underlying the invention was accordingly the method for electrolytic coloring of anodized aluminum parts and to develop aluminum alloys so that the entire treatment process from anodic oxidation to coloring and compression can be carried out fully automatically. Such a practice but excludes a sample with subsequent re-coloring. This problem is solved with the in the labeling part of the Features listed claim 1.  

The invention is based on the finding that the structure of the anodic produced oxide layer a significant influence on the uniformity the coloration. The layer structure is characterized by the Formation of the barrier layer, the pore size and the number of pores per unit area.

It was shown that regardless of the color tone, the brightness of the coloring is an important parameter for assessing the uniformity of a coloring. According to the CIELAB system, the brightness is defined by the numbers L * = 0 for black to L * = 100 for white. The number of pores occupied per unit area is decisive for the intensity or brightness of a coloring. There is much to suggest that the color scheme is a resolution problem in a grid. The layer which has the most pores covered with tin per unit area appears darker. However, if the occupied pores are further apart, the appearance of the colored surface appears lighter. In practice, the values L * = 20 for black and L * = 90 for uncolored oxide layers are at best achieved.

Further investigations showed that the brightness of a coloring responsible degree of occupancy of the pores is determined by the quotient from the Difference between the brightness of the uncolored layer and the desired one Brightness on the one hand and the difference between that achievable in practice maximum and minimum brightness on the other hand can be described:

It was found that after applying a DC voltage to the anode (= Goods) and cathode (= counter electrode) existing system make a statement  can be obtained via the individual layer structure of the anodized goods can. DC treatment can be less in any medium Aggressiveness than that of the anodizing bath occur, otherwise the barrier layer is attacked would. For practical reasons, however DC treatment carried out in the coloring electrolyte.

After applying the DC voltage, the level of the order of magnitude of the anodizing voltage corresponds and is kept constant, the current initially increases and then drops towards the end of the DC treatment. From the difference between maximum and minimum current and the duration of treatment, which, for. B. Is 60 seconds, can be applied using those known in electrical engineering Formulas for calculating the RC element the effective capacitor voltage and the capacitor current can be determined. The quotient of capacitor current and degree of pore occupancy results in the cut-off current during dyeing. The initial dye flow can then be derived from the formula:

calculate.

The dyeing voltage corresponding to the initial dyeing current is during the electrolytic coloring constant until the cut-off current is reached held. In the described procedure, which is done by installing a Computer can be fully automated, can be on the matching and on the dispense with this necessary re-coloring and reproducible, uniform colorations are obtained.  

example

A batch of aluminum profiles of the alloy AlMgSi 0.5, which was anodically oxidized in an electrolyte containing sulfuric acid with the application of direct current, is run after intermediate rinsing in the coloring electrolyte containing metal salt and is exposed here as an anode to a treatment with direct current. The capacitor current is determined from the current profile. This value is divided by the degree of pore occupancy. For example, if the capacitor current is 400 A and the desired brightness L * = 65 (according to the CIELAB system), the switch-off current after electrolytic coloring is 1120 A. After multiplying the capacitor current by

becomes the initial dyeing stream from 1981 A.

After setting this current, the corresponding voltage is kept constant until the switch-off current of 1120 A is reached

As a result, a uniformly colored product with a brightness of L * = 65 is obtained without intermediate sampling and re-coloring being necessary.

Different alloys are to be anodized and colored To determine correction factors.

Claims (3)

1. A process for the electrolytic coloring of anodically produced layers on aluminum and aluminum alloys with alternating current in electrolytes containing metal salt, the coloring process with alternating current being preceded by a treatment with direct current, characterized in that the direct voltage over 10 to 300 seconds, preferably 50 to 70 seconds, at a value of 5 to 30 volts, preferably 12 to 20 volts, kept constant, the difference between the maximum and final value of the direct current is tapped and after the known calculation of the RC element to determine the effective capacitor current I _Kon the initial current I _Beginning the AC staining treatment according to the equation is determined. 2. The method according to claim 1, characterized in that the switch-off current as a function of the desired brightness L * is the coloring of the quotient of I _Kon and the term 1.29-0.014 L * . 3. The method according to claim 1 and 2, characterized in that the DC voltage before the coloring treatment corresponds to the voltage value in anodic oxidation in sulfuric acid electrolytes corresponds.