EP0127774B1 - Process for protecting anodised aluminium - Google Patents

Description

The invention described here is a method for protecting anodically oxidized aluminum by a metal-containing salt solution at low temperature.

• a) Verdichtung durch heisses Wasser bei einer Temperatur von vorzugsweise über 85°C,
• b) Passivierung durch chromsaure Salze,
• c) Ueberzug durch organische Stoffe, vorzugsweise Acrylharze.

In the industry that electrolytically oxidizes aluminum, a finishing treatment is carried out to improve the anodized layer, which can look as follows:

• a) compression by hot water at a temperature of preferably above 85 ° C,
• b) passivation by chromic acid salts,
• c) coating with organic substances, preferably acrylic resins.

The compression by hot water or steam, which is often used, requires high energy costs and long process times until the pores in the anodized layer are closed, which are said to improve the corrosion protection.

The same high costs arise when using acrylic resins as protective coating, since they have to be dried at a temperature of approx. 180 ° C.

Passivation in solutions with chromates has the advantage that it is possible to work at lower temperatures. However, it adversely changes the decorative aspect of the aluminum profiles.

According to USA-3,071,494, the anodized aluminum is treated in a solution containing copper as well as nickel and / or cobalt ion at a temperature close to the boiling point of the solution. The pores are closed by hydrolysis.

According to JP-A-54116350, the anodized aluminum is treated in a solution which contains at least one compound such as nickel, cobalt, copper, chromium or lead acetate or zirconium fluoride at 50 to 100 ° C.

Nickel fluoride solutions are also used in the prior art to produce high-quality protection of the anodized layer. Although these solutions are used at temperatures between 20 ° C and 42 ° C and therefore represent a significant saving in energy costs, they have the disadvantage that the anodized aluminum is given a "green tinge" by this treatment and thereby loses the decorative natural appearance.

The purpose of the present invention is to prevent the so-called "green tinge" caused by the use of nickel fluoride as a passivating agent and to maintain the natural decorative effect of the anodized aluminum, while at the same time providing a high level of corrosion protection against environmental influences, which can be termed "high quality". is achieved. “High quality” is understood to mean a dirt-free surface that does not show any unnatural colors such as the “green cast”, and which is protected against corrosion according to ISO standard 3210, the maximum weight loss of which has been set at 30 mg / dm2.

It has been found that the green cast caused by the use of nickel salts in the presence of anions such as fluoride, fluoborate and fluosilicate can be prevented if a mixture of nickel and cobalt salts is used in aqueous solutions at temperatures below 50 ° C.

The present invention therefore relates to a method for protecting anodized aluminum by passivating the anodized layer in a solution containing nickel and fluoride, fluoroborate or fluosilicate ions at a temperature of 20 to 50 ° C., the solution additionally containing cobalt ions in a ratio Contains nickel: cobalt from 1.0: 0.1 to 1.0: 1.0 and has a metal salt concentration of at least 0.02 mol / I to at most 0.2 mol / 1.

The basis of the invention is that the concentrations of the cations nickel and cobalt must be kept in relation to one another in order to maintain the excellent quality of the protection of the anodized layer and at the same time to prevent the occurrence of undesirable color tones such as "greenish tinge".

The effectiveness of the products was determined as follows.

A degreased and heated aluminum sheet was anodized in an electrolyte of 190 g / l sulfuric acid at a temperature of 19 ° C. with direct current for 40 minutes at a current density of 1.5 A / dm ² . In order to protect the anodized layer obtained, the aluminum sheets were then immersed in a solution of various chemical products for 12 minutes at a temperature of 30 ° C.

After the passivation layer obtained had dried, the weight loss was determined in accordance with the ISO 3210 standard.

At the same time, color comparisons were carried out with a sample part anodized under the same conditions but compacted in hot water at 95 ° C. for 60 minutes.

Drying of the anodized and treated aluminum in the passivation solution can be achieved by treating for 5 minutes in boiling water or during 48 hours storage at an air temperature of 20 ° C to 30 ° C. Warm air drying devices can also be used.

• a) Nur in Gegenwart von Kobaltsalzen gewährt Nickelfluorid, ohne das natürliche Aussehen des anodisierten und passivierten Aluminium zu verändern, einen hervorragenden Schutz, der als "Hochqualität" bezeichnet werden kann;
• b) das Verhältnis zwischen dem Kation Nickel und Kobalt muß 1:0,1 bis 1:1 vorzugsweise 1:0,35 sein;
• c) als Anion sind Fluorderivate wie Fluoborat, Fluorsilikat, Fluorid, u.s.w. unerläßlich.

The results obtained allow the following statement:

• a) Only in the presence of cobalt salts does nickel fluoride, without changing the natural appearance of the anodized and passivated aluminum, provide excellent protection, which can be described as "high quality";
• b) the ratio between the cation nickel and cobalt must be 1: 0.1 to 1: 1, preferably 1: 0.35;
• c) as an anion, fluorine derivatives such as fluoborate, fluorosilicate, fluoride, etc. are indispensable.

Protection of the previously anodized aluminum by the aqueous solution is achieved at temperatures between 20 ° C and 50 ° C and immersion times between 5 and 20 minutes, the pH should be between 5.0 and 7.0 and the metal salt concentration of at least 0.02 mol / l to at most 0.2 mol / I must be maintained.

Aluminum, which was anodized in oxalic acid, sulfuric acid or glycerol-sulfuric acid, also provided high-quality protection with the passivation solution described, even at anodizing bath temperatures above 21 ° C.

In order to clarify the previous results, some 5x10 cm aluminum sheet samples were treated according to the following procedure.

The aluminum sheets were cleaned by an alkaline degreasing agent before anodizing and then pickled with a mixture of 4% caustic soda and 0.2% sodium chromate at a temperature of 55 ° C. Then they were 10 minutes in nitric acid and 1 min. aftertreated in hydrofluoric acid before being subjected to the anodizing and passivation process as follows:

example 1

The aluminum sheet sample was anodized after the pretreatment described above under the following conditions.

After producing a 20 μm thick anodizing layer and rinsing for 5 minutes under running water, the anodized sheet was added to a solution of distilled water with 6.1 g / l nickel fluoride and 2.2 g / l cobalt acetate with a pH of 6.3 immersed at a temperature of 30 ° C for 12 minutes. After drying for 48 hours at 25 ° C, the weight loss according to ISO standard 3210 was 11.8 mg / dm ² and no change in color of the pattern could be determined.

Example 2

An aluminum sheet sample was anodized under the following conditions:

After producing a 20 pm thick anodizing layer and subsequent rinsing with running water, the sample was immersed in the following solution:

The sample was immersed in this solution at 30 ° C and pH 6.4 for 12 minutes.

After a 5 minute rinse in boiling water, the weight loss of the dry aluminum sheet was determined according to ISO standard 3210 to be 10.5 mg / dm ² .

No changes in the decorative appearance of the treated sheet were found.

Example 3

An aluminum sheet sample was anodized under the following conditions:

After a 20 µm thick anodizing layer and 5 minutes of rinsing under running water, the anodized sheet was immersed in a solution containing the following products:

The pH of the solution was adjusted to 6.1 and after an immersion time of 15 minutes at 32 ° C. with subsequent rinsing for 5 minutes in hot water, the weight loss according to ISO 3210 was determined to be 13.4 mg / dm ² .

No change in the natural color tone was found in comparison with an anodized aluminum sheet compressed in hot water.

Claims (2)

1. A process for the protection of anodised aluminium by passivation of the anodised layer in a solution containing nickel ions and fluoride, fluoborate or fluosilicate ions at a temperature of 20 to 50°C, wherein the solution also contains cobalt ions in an amount such that the nickel:cobalt ratio is from 1.0:0.1 to 1.0:1.0, and has a metal salt concentration of at least 0.02 mol/I but not exceeding 0,2 mol/I.

2. A process according to claim 1, wherein the nickel:cobalt ratio in the solution is 1:0.35.