DD235081A1 - METHOD FOR THE ELECTROLYTIC PRODUCTION OF ALUMINUM OXIDE LAYERS ON ALUMINUM OR ALUMINUM ALLOYS - Google Patents

Abstract

The invention relates to a process for the electrolytic production of aluminum oxide layers on aluminum or aluminum alloys. The aim of the invention is to reduce the expenditure on equipment in the formation of 0.04 to 0.15 mm thick oxide layers. A substance is to be found which prevents the addition of electrolytes to the local redissolution of the aluminum oxide and allows anodic oxidation at bath temperatures of 15C and final cell voltages of 50V. According to the invention, the electrolyte solution is added with 0.5 to 10 g / l of potassium hexacyanoferrate (II) and the anodization is carried out at a bath temperature of 15 to 35 ° C, an anodic current density of 1 to 10 A / dm 2 and a cell voltage of 16 to 50 volts. The oxide layers produced according to the invention are used for wear protection, the absorption of electromagnetic radiation and decorative purposes.

Description

Field of application of the invention

The invention relates to an improved process for producing 0.04 to 0.15 mm thick aluminum oxide layers by anodic oxidation in sulfuric acid electrolyte solutions at a constant current density.

Such layers are deposited on the surfaces of workpieces, substrates and consumer goods made of aluminum and aluminum alloys for the purpose of wear protection, the decorative effect and the absorption of electromagnetic radiation.

Characteristic of the known technical solution

It has long been known to provide articles of aluminum or aluminum alloys by anodic oxidation with a protective layer.

For the production of deep colored, hard and abrasion resistant oxide layers up to about 150 microns thick and more, a number of Hartanodisierverfahren have been known using 1-20% sulfuric acid electrolyte solutions, either with constant current density and increasing voltage, with constant voltage or with gradual increase in Current density work

 (Galvanotechnik 1972, p. 122-131).

A common disadvantage of all these methods is the low working temperature to be used of -26 ° C to +7 ⁰ C and the high end voltage in the range of 60 to 550V when stopping the electrolysis.

These working conditions must be observed in order to prevent the redissolution of the oxide layer formed in the sulfuric acid or to allow the layer thickness to increase to the desired level. But they require the construction of special Hartanodisieranlagen with elaborate cooling units and occupational safety measures because of the high DC voltage. It is known that the sulfuric acid electrolyte glycerol, methanol, sugar, dextrin u.a. Add chemical substances to reduce the re-dissolution of the layer formed during the electrolysis. In this regard, however, the mentioned electrolyte additives are of little effectiveness. Their main effect manifests itself in the possibility of depositing higher layer thicknesses at elevated electrolyte temperature and lower final tension.

So it is z. B. possible to reduce by adding about 50g of glycerol per liter of 10% sulfuric acid necessary to achieve a layer thickness of 140μττι final voltage of 80 to 40 V (Plating 1972, pp 643-644). However, the method only allows to increase the temperature to 18 ° C. in the case of particularly intensive electrolytic movement. However, a lowering of the electrolyte temperature leads to smaller pores in the layer and thus to a deficient dyeability of the layers in solutions of organic dyes.

Object of the invention

The aim of the invention is to reduce the expenditure on equipment in the production of 0.04 to 0.15 mm thick, wear-resistant, colored and dyeable in aqueous solutions of organic dyes aluminum oxide layers on aluminum and aluminum alloys.

Explanation of the essence of the invention

The invention has for its object to find a substance that allows it to be added to the sulfuric acid electrolyte by largely suppressing the local redissolution of the aluminum oxide formed in the anodic deposition of aluminum oxide layers of the quality described above with a bath temperature of the electrolyte solution of ^ 15 ° Cund a final cell voltage S 50 V to work.

This object is achieved by a process for the electrolytic production of aluminum oxide layers on aluminum or aluminum alloys, wherein the articles of aluminum or aluminum alloys connected as an anode in an aqueous sulfuric acid and chemically added electrolyte solution at constant DC density and with increase of the cell voltage to form thick oxide layers are anodically oxidized on their surface, according to the invention dissolved in that the sulfuric acid electrolyte solution with 0.5 to 10g potassium hexacyanoferrate (II) per liter of solution is added and the anodic oxidation at a T3i..peraturder the electrolyte solution from 15 to 35 ° C, an anodic current density of 1 to 10 A / dm ² and a cell voltage of 16 to 50 V is performed.

The formation of a thick aluminum oxide layer in the case of anodic oxidation is limited by the redissolution of the aluminum oxide in the sulfuric acid starting from the pores of this oxide layer. Particularly detrimental is the increased redissolution, which sometimes begins with very limited areas of the surface, which usually forces the demolition of the electrolysis and in many cases results in the destruction of the parts to be coated.

The addition of potassium hexacyanoferrate (II) to the sulfuric acid electrolyte has been shown to block such pores, probably due to partial oxidation of the potassium hexacyanoferrate (II) to potassium hexacyanoferrate (III) with subsequent release of trivalent iron ions, which in turn is associated with excess potassium hexacyanoferrate (II) is a precipitate of poorly soluble Berlin blue form in the pore, is due. This assumption is supported by the blue color of the layers, which occurs especially at elevated electrolyte temperatures. Due to this fact, it is possible to deposit absorbent, further wear-resistant, wear-resistant, thick aluminum oxide layers on aluminum and aluminum alloys without the risk of spontaneous local redissolution of the aluminum oxide layer.

-2- 736 76

embodiments

The invention will be explained in more detail below with reference to four examples. Example 1:

A workpiece made of aluminum, which is to be provided with a radiation-absorbing, wear-resistant oxide protective layer, is first subjected to a pretreatment in a known manner for the purpose of cleaning and degreasing the surface

-'Beizen in 20% sodium hydroxide solution,

- Do the washing up,

Pickling in 5% hydrofluoric acid,

- Do the washing up,

Pickling in 30% nitric acid,

- Do the washing up

includes. Subsequently, the workpiece is immersed to carry out the anodic oxidation in an electrolytic solution containing per liter of solution 100 g of sulfuric acid, 50 g of glycerol and 5 g of potassium hexacyanoferrate (II).

The electrolyte solution is, for example, in an aluminum vessel, which simultaneously serves as a cathode and is cooled from outside by running water. The aluminum workpiece itself is switched as an anode.

The anodization is carried out at a constant DC density of 6 A / dm ² and increasing cell voltage from 20 volts initial voltage to 34 volts final voltage. To ensure a constant bath temperature of 25 ° C, the electrolyte solution is also moved during the anodization by introducing compressed air or by stirring. This also causes a uniform temperature distribution in the electrolyte. After a treatment time of one hour, the workpiece surface is covered with a 40μΐη thick, medium blue oxide layer. The oxide layer has good radiation absorptivity and high wear resistance. It can be dyed in a conventional manner in an aqueous solution of an organic aluminum dye.

Example 2:

An object which is likewise made of aluminum is pretreated in accordance with Example 1 and oxidized anodically on its surface in a glycerol-free electrolyte bath consisting of 100 g / l of H ₂ SO ₄ and 5 g / l of potassium hexacyanoferrate (II). However, it is worked with a bath temperature of 20 ° C and a current density of 7 A / dm ² .

After 1 hour treatment time, the workpiece has a medium blue, abrasion-resistant, radiation-absorbing oxide layer also 40 ^ m thickness.

Example 3:

Analogously to Example 1, a workpiece made of an aluminum alloy, which is provided as a moving component in a device, after appropriate pretreatment in an electrolytic solution consisting of 100g / l H ₂ SO ₄ and 2.5g / l Kaliumhexacyanoferrat (II) existing electrolyte solution on its surface , At a bath temperature of 35 ⁰ C and a current density of 7 A / dm ² , a 70μΓη thick, medium blue colored oxide layer is formed at a treatment time of one hour, which provides good wear protection.

Example 4:

An Al workpiece is pretreated according to Example 1 and in an electrolyte solution prepared from 100 g / l sulfuric acid and 0.5 g / l Kaliumhexacyanoferrat (II) with a temperature of 20 to 23 ⁰ C at a current density of 6 A / dm ² Anodized for 5 hours. It is a gray-blue colored, wear-resistant oxide layer in a thickness of 120μ, ιη deposited.

Claims (1)

-1- 736 Claim: A process for the electrolytic production of aluminum oxide layers on aluminum or aluminum alloys, wherein an aluminum oxide or aluminum alloy bonded aluminum anode or aluminum alloy articles are anodized at a constant anodic DC density and an increase in cell voltage to form thick oxide layers on its surface in an aqueous sulfuric acid and chemical supplemented electrolyte solution characterized in that the sulfuric acid electrolyte solution is added with 0.5 to 10 g of potassium hexacyanoferrate (II) per liter of solution and the anodic oxidation at a temperature of the electrolytic solution of 15 to 35 ° C, an anodic current density of 1 to 10A / dm ² and a cell voltage of 16 to 50 V is performed.