EP0280886A1 - Process for the production of decorative coatings on metals - Google Patents

Abstract

The process for the production of decorative coatings on metals is mainly used in the jewelry industry, in the arts and crafts and in device construction. The inventive solution consists in matt, contour-true decorative layers with a homogeneous thickness of 3 µm to 30 µm by means of an electrochemical and plasma-chemical reaction in pulse operation in aqueous electrolytes with voltage peaks of 250 V to 750 V, pulse times of 20 µs to 2 ms, pulse frequencies of Cut off 35 Hz to 300 Hz, pulse currents from 20 A to 120 A, electrolyte temperatures between 318 K and 360 K and average current densities from 0.1 A cm to 1 A cm.

Description

The invention relates to a process for the production of decorative coatings on metals, in particular on aluminum, titanium, niobium, zirconium, tantalum and their alloys, preferably for the jewelry industry, the arts and crafts, equipment manufacturing, cladding and coins, metal plastics and badges.

In order to achieve non-glossy, matt surfaces on relatively cheap, easily machinable and processable metals, special multi-stage processes are currently used which roughen, structure, coat, color and seal the surface.
On the conventional metals of the jewelry and jewelry industry, for example silver, gold, platinum, such matt surfaces cannot be achieved directly and only indirectly through organic or inorganic cover layers. However, they visually reduce the value of the noble and expensive base material. Known processes for the production of decorative, also colored metal surfaces are the Kalcolor process or the color anodizing (US-3631384). The color is generated directly by the electrolyte, or organic dyes or inorganic color pigments are subsequently embedded in the transparent carrier layer for coloring. As with aluminum, this carrier layer is produced by anodizing using direct or alternating current in electrolytes containing sulfur, phosphorus, maleic or salicylic oxalate. Such surfaces always show a "cold" shine. Also is in this relatively expensive multi-stage process a homogeneous surface decoration on, for example, plastics, badges, hollow bodies, delicate structures, brooches etc. not, or only realizable with high technical effort. Furthermore, transparent layers of aluminum oxide produced by anodic oxidation of aluminum are known, which can then be colored with various dyes (US 3031387). Since these layers are transparent, the metallic gloss of the surface is always retained. The organic compounds used to color the layers are also influenced by environmental influences such as radiation, temperature and air humidity and change their color.
Various decorative metal surfaces are also known in the jewelry industry. For example, DD-WP 204845 describes the use of electronic parts as jewelry. DE-OS 1446289 presents a special enamelling process for decorative coatings on jewelry. Colored, anodized aluminum parts are also used as decorative elements.

The invention is based on the object of developing a process by means of which aluminum, titanium, niobium, zirconium, tantalum and their alloys on optically shaped surfaces of the metals are optically attractive, decorative-acting matt, white, black or colored layers on all sides with high dimensional accuracy, Adhesion strength and color homogeneity are generated.

According to the invention, the object is achieved in that aluminum, titanium, tantalum, zirconium, niobium or their alloys in an aqueous electrolyte on the barrier layer-forming metals by means of an electrochemical and plasma chemical reaction determined by pulse voltage in:
- Voltage peaks from 250 V to 750 V
- Pulse times from 20 µs to 2 ms
- Pulse frequencies from 35 Hz to 300 Hz
- Pulse currents from 10 A to 120 A.
- Electrolyte temperatures from 318 K to 360 K.
and
- Average current densities from 0.1 A cm⁻² to 1 A cm⁻²
matt, contour-true decorative layers with a homogeneous thickness of 3 µm to 30 µm are formed.

It was found that the voltage values, which are assigned to the arc discharge area in the current-voltage characteristic of the electrolyte-metal pairing and usually lead to the destruction of the layer, but result in a homogeneous layer formation on all sides in pulse operation up to 300 Hz.
It has been shown that the partial anode area is greatly minimized by the pulse operation, in particular with a needle pulse character, and the energy input into the base is localized in such a way that the individual discharge focal points overlap strongly and lead to a uniform layer thickness with only a small roughness. The pulse operation according to the invention also enables colored layers of high quality and depth of color to be formed even in electrolytes with a high concentration (up to 20%) of transition metal ions.
Furthermore, it was found that a temperature of 318 K to 360 K of the electrolytes with coloring additives promoted the color formation of the layer and average current densities of 0.1 A to 1.0 A cm⁻² led to particularly matt surfaces. Surprisingly, it has also been shown that, for example, in an electrolyte containing cobalt ions, the known black-gray deposits of cobalt oxide compounds on the metal surface do not occur when the pulse voltage is operated on aluminum, but rather optically and mechanically high quality cover made of matt blue cobalt aluminum spinel.
In this way, copper ions in the electrolyte at light concentrations of 1% to 20% produce light yellow to ocher, manganese ions pink to umber-colored, chrome ions green to black, iron ions light gray to deep black, molybdenum ions light gray to dark gray layers.

The layers produced by the process according to the invention have an average surface roughness of 4 μm to 20 μm and consist of two different layer areas. The layer area adjacent to the metal surface is transparent or white and has a thickness of 0.1 µm to 1.5 µm. The cover layer area following this transparent or white layer area is white, black, colored, matt and opaque and has a thickness of 3 µm to 25 µm. With a maximum achievable layer thickness of 30 μm according to the invention, a transition zone occurs between the transparent or white layer region and the top layer region.
The transparent or white layer area contains no coloring transition metal ions and acts as an adhesion promoter to the top layer area.
The lattice parameter deviations of the metal from the transparent layer area are small because of the field crystallization effect, so that no mechanical stresses occur at the metal-metal oxide interface areas of transparent or white layer areas. Since the transparent or white layer area and the top layer area also consist of the same base metal oxide, a high bond stability between the two oxide layers is ensured despite the lattice expansion of the oxides in the top layer area, also due to built-in transition metal ions. The color in the top layer area is due to the incorporation of transition metal ions. With these inorganic color bodies there is a risk of bleaching not given. Lightfast spinels or mixed oxides are formed. A multiple anodic treatment according to the invention in different electrolytes can form several differently colored cover layer regions, so that homogeneous and / or heterogeneous color distributions arise. The matt appearance of these decorative covers is characterized by their very low gloss. Gloss measurements according to RICHTER showed gloss numbers from 0.5 to 1.5 regardless of the angle of reflection. That means a completely matt surface. Different concentrations of the transition metal ions in the top layer area, the mass fraction of which ranges up to 20%, gave color shades combined with a homogeneous or marbled or speckled or flamed or mosaic-like appearance. All or part of the metal surfaces can be provided with the anodic oxide layer. By combining surface areas with different colored and transparent and / or interference colored and / or other conventional anodic coatings there are many design variants.

Jewelry articles that are treated with the method according to the invention are notable for good body tolerance because the chemically indifferent oxide layers prevent direct contact between metal and skin.

• 1. Manschettenknoepfe aus Titan sind in einem Elektrolyten welcher 0,1 mol/l Na ₂ B₄ O₇ und 0,5 mol/l KH₂ PO₄ enthaelt als Anode geschaltet und bei einer Spannung von 60 V mit einer intensiv roten arteigenen Oxidschicht versehen. Die Knoepfe werden je zur Haelfte mit einer Schutzvorrichtung abgedeckt und in einem waessrigen Elektrolyten der Zusammensetzung 0,5 mol/l NaF, 0,3 mol/l NaH₂ PO₄, 0,1 mol/l Na₂ B₄O₇ und 0,5 mol/l K₄[Fe(CN)₆] bei einem pH-Wert von 8 bei einer Impulsstromdichte von 0,1 A/cm² und einer Zeit von 60 Sekunden anodisch oxidiert. Es entsteht eine Kombination von einer tiefroten interferenzfarbenen arteigenen Schicht mit einer tiefschwarzen matten oxidkeramischen Schicht.
• 2. Eine Nachbildung einer antiken Gemme (Portraet) aus einer Aluminiumlegierung ist in einem waessrigen Elektrolyten der Konzentration 0,5 mol/l NaF, 0,5 mol/l NaH₂ PO₄ und 0,1 mol/l Na ₂ B₄ O₇ bei einer Impulsstromdichte von 0,2 A/cm² und einer Impulsspannung von 110 V anodisch oxidiert worden. Auf dem Schmuckgegenstand entsteht eine weisse, matte, porzellanartige Schicht mit einem elfenbeinartigen Aussehen. Die Oberflaechenrauheit betraegt 7,6 µm und die mittlere Schichtdicke 10,1 µm. Das Portraet ist konturengetreu wiedergegeben.
• 3. Eine Aluminiumbrosche mit 12 cm² Oberflaeche und den Initialen C.D. wird in einem waessrigen Elektrolyten von 338 K, der 2 %ig an NaF, 7 %ig an NaH ₂ PO₄ , 4 %ig an Na₂ B₄ O₇ , 0,5 %ig an NH ₄ F und 1 %ig an ammoniakali­schem Co(OH)₂ ist, als Anode geschaltet und mittels Impulsspannung bei Spannungsspitzen von 500 V, einer Impulszeit von 1 ms und einer Impulsfrequenz von 100 Hz beschichtet. Der maximale Impulsstrom wurde mit 50 A gemessen. Es entsteht eine mattblaue, nichtglaenzende Oberflaeche von hohem dekrativem Wert. Die konturentreue Wiedergabe der Initialen ist mit einer Abweichung von nur 8 µm nach der Beschichtung gewaehrleistet.
• 4. Eine Frontplatte eines elektronischen Verstaerkers von 400 cm² Oberflaeche aus Aluminium Al 99,5 wird in einem waessrigen Elektrolyten, der 4%ig an NaF, 6 %ig an NaCO₃ und 4 %ig an Na₂ B₄ O₇ ist, anodisch mit Impulsspannung behandelt. Dabei betrugen die Impulsspannungsspitzen 410 V, bei Impulszeiten von 0,5 ms, einem Impulsstrom von 35 A und einer Impulsfrequenz von 100 Hz. Nach 10 min Behandlungszeit ist die Frontplatte mit einer weissen, matten, porzellanartig aussehenden, dekorativ wirkenden Schicht von 9 µm Staerke allseitig homogen ueberzogen.
• 5. Ein Bilderrahmen mit Mustergravur und 840 cm² Oberflaeche wird vorderseitig nach Maskierung der Rueckseite dekorativ in einem Elektrolyten, der 2 % KMnO₄ , 6 % NaF, 7 % NaH₂ PO₄ , 3 % NH ₄ F und 4 % Na ₂ B₄ O₇ enthaelt, bei Impulsspannungsspitzen von 550 V, Impulsstromspitzen von 58 A, Impulszeiten von 1,2 ms und Impulsfrequenzen von 100 Hz beschichtet. Die gebildete hell- bis rosabraune Schicht von 7 µm Staerke ist glanzlos, hat keramikartiges Aussehen und ist von besonderer dekorativer Ausstrahlung. Die Mustergravur wird konturengetreu mit einer gleichmaessigen Veraenderung von nur 12 µm nach der Beschichtung wiedergegeben. Die mittlere Rauheit betraegt 8 µm.

The invention will be explained in more detail below using 5 exemplary embodiments.

• 1. Titanium cufflinks are in an electrolyte which contains 0.1 mol / l Na ₂ B₄ O₇ and 0.5 mol / l KH₂ PO₄ as an anode and at a voltage of 60 V with an intensely red species-specific oxide layer. Half of the buttons are covered with a protective device and in an aqueous electrolyte with the composition 0.5 mol / l NaF, 0.3 mol / l NaH₂ PO₄, 0.1 mol / l Na₂ B₄O₇ and 0.5 mol / l K₄ [Fe (CN) ₆] at a pH of 8 with an impulse current density of 0.1 A / cm² and a time of 60 seconds oxidized. The result is a combination of a deep red, interference-colored, native layer with a deep black matt oxide ceramic layer.
• 2. A replica of an antique gem (portrait) made of an aluminum alloy is in an aqueous electrolyte with a concentration of 0.5 mol / l NaF, 0.5 mol / l NaH₂ PO₄ and 0.1 mol / l Na ₂ B₄ O₇ at a pulse current density of 0.2 A / cm² and a pulse voltage of 110 V anodized. A white, matt, porcelain-like layer with an ivory-like appearance is created on the jewelry item. The surface roughness is 7.6 µm and the average layer thickness is 10.1 µm. The portrait is reproduced true to the contours.
• 3. An aluminum brooch with 12 cm² surface and the initials CD is in an aqueous electrolyte of 338 K, the 2% NaF, 7% NaH ₂ PO₄, 4% Na₂ B₄ O₇, 0.5% NH ₄ F and 1% ammoniacal Co (OH) ₂ is connected as an anode and coated by means of pulse voltage at voltage peaks of 500 V, a pulse time of 1 ms and a pulse frequency of 100 Hz. The maximum pulse current was measured at 50 A. The result is a matt blue, non-glossy surface with a high decrative value. The true-to-contour reproduction of the initials is guaranteed with a deviation of only 8 µm after coating.
• 4. A front plate of an electronic amplifier of 400 cm² surface made of aluminum Al 99.5 is treated anodically with pulse voltage in an aqueous electrolyte which is 4% NaF, 6% NaCO₃ and 4% Na₂ B₄ O₇. The pulse voltage peaks were 410 V, with pulse times of 0.5 ms, a pulse current of 35 A and a pulse frequency of 100 Hz. After 10 minutes of treatment, the front panel is homogeneously coated on all sides with a white, matt, porcelain-looking, decorative-looking layer of 9 µm thickness.
• 5. A picture frame with pattern engraving and 840 cm² surface is decorative on the front after masking the back in an electrolyte containing 2% KMnO₄, 6% NaF, 7% NaH₂ PO%, 3% NH ₄ F and 4% Na ₂ B bei O₇ Pulse voltage peaks of 550 V, pulse current peaks of 58 A, pulse times of 1.2 ms and pulse frequencies of 100 Hz coated. The light to pinkish brown layer of 7 µm thickness is lackluster, has a ceramic-like appearance and has a special decorative charisma. The pattern engraving is reproduced true to the contours with an even change of only 12 µm after coating. The average roughness is 8 µm.

Claims (5)

1. A process for producing decorative coatings on metals in an aqueous electrolyte using the barrier layer-forming metals aluminum, titanium, tantalum, zirconium, niobium or their alloys, characterized in that by means of an electrochemical and plasma chemical reaction determined by pulse voltage in:
- Voltage peaks from 250 V to 750 V
- Pulse times from 20 µs to 2 ms
- Pulse frequencies from 35 Hz to 300 Hz
- Pulse currents from 10 A to 120 A.
- Electrolyte temperatures from 318 K to 360 K.
and
- Average current densities of 0.1 A cm ² to 1 A cm ²², true-to-contour, decorative white layers with a homogeneous thickness of 3 µm to 30 µm are formed. 2. The method according to claim 1, characterized in that colored or black matt layers are produced by additives containing transition metals with a concentration of 1% to 20% in aqueous electrolytes. 3. The method according to claim 1 and 2, characterized in that a multiple process application with different electrolytes produces different colored homogeneous and heterogeneous color distributions. 4. The method according to claim 1 to 3, characterized in that decorative objects or objects with engravings are used as the base. 5. The method according to claim 1 to 3, characterized in that flat large-area elements are used as a base.