CZ20031237A3 - Passivation process - Google Patents

Abstract

The invention relates to a method for passivating zinc, cadmium, or the alloys thereof, especially zinc-nickel alloys, with a chrome (VI)-free solution containing a weak complexing agent, preferably dicarboxylic or tricarboxylic acids, preferably a chrome (III)-oxalate complex and Co<2+>, the Co<2+> concentration being higher than 30g/l.

Description

Method of passivation

Technical field

The present invention relates to a process for the passivation of zinc and zinc alloy layers as well as cadmium and cadmium alloy layers and claims priority to German Patent Application No. 100 55 215.3, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

It is known to electroplate metal surfaces with metals or metal alloys to improve corrosion resistance. It is further known that in order to further increase the anticorrosive effect of this layer, the layer is modified and covered with other systems. The system with good anticorrosive effect is a galvanically applied zinc-nickel alloy, which is subsequently chromium-plated and to which an organic or inorganic coating can additionally be applied. Good corrosion resistance is achieved by using chromium ions for the passivation of the zinc and nickel layers.

The toxicity of chromium compounds contrasts with their good anticorrosive properties. Therefore, efforts have been made for a long time to prepare systems which do not contain chromium ions but provide sufficient corrosion resistance.

For this purpose, it is known to use chromate ion-containing passivation solutions as described in US 4,171,231, which, however, do not lead to chromium-free passivation layers due to oxidizing agents also present in the solution, since chromium ions are oxidized during the operation .

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DE 41 35 524 C2, which is incorporated herein by reference and fully incorporated herein by reference, describes a passivation process which achieves improved anti-corrosion values due to a solution containing chromium ions with formation of an oxalate complex (Table II; Table III). Various examples of chromium compounds for passivation are given in Table IV of said reference. The good anticorrosive effect of the passivation process described herein is based on the use of oxalate as a complexing agent which, unlike other complexing agents, promotes the incorporation of chromium into the passivation layer.

Good anticorrosive results can already be achieved with this type of passivation process. Further, it is known to increase the anticorrosive protection provided by chromium ion passivation methods using cobalt.

Improvement of anticorrosive protection using cobalt is described in WO 97/40208, which is hereby incorporated herein.

As can also be seen from this publication, experts are trying to make the conversion layer as compact as possible in order to achieve the desired layer quality with respect to corrosion and chemical resistance.

The invention aims to provide both a passivation method with even better anti-corrosion protection properties and a corresponding coating system.

The object is achieved by the method and the passivating solution according to the independent claims. Preferred aspects are the subject of the dependent claims.

SUMMARY OF THE INVENTION

The invention is based on the discovery that, when the efforts to focus on the compact conversion layer and attempt to form a porous conversion layer are abandoned, this leads to improved end products when porosity is used to bind at least one additional layer.

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The invention may be practiced by using cobalt in a chromium-ion passivating solution and a weak complexing agent - preferably a di- or tri-carboxylic acid, for example oxalic acid.

By using a cobalt concentration higher than 30 g / l, preferably between 70 g / l and 100 g / l, in particular about 90 g / l (concentrated - 12%), surprising corrosion protection properties according to DIN 50961 are already obtained during manufacture. salt chamber test according to DIN 50021 SS. Preferably, the ratio of chromium to cobalt is 1.7: 2.0 = 0.85.

The process is preferably carried out at temperatures up to 55 ° C and pH values of 0.5 to 5.5. Preferably, the passivation is carried out at pH 4. Particularly good results can be obtained with a sulphate-free solution. This is based on the idea that the use of sulfate leads to disruption of the catalytic reaction and disturbance in the development of an anti-corrosive layer of chromium.

The organic or inorganic layer, which penetrates into the porous conversion layer, is again applied to the passivated layer. Preferably, a coating of Acares ™ (Enthone-OMI GmbH, Neuss) is applied to the passivation layer. Particularly good corrosion control can be obtained with a double layer of Aquares. Surprisingly, the Aquares layer is no longer evident in REM in some variants of the invention.

The corrosion control properties as well as the chemical resistance can be further improved with Topcoat containing a dry lubricant. This coating according to the invention also participates in special binding to the porous conversion layer. Thus, the desired frictional and flow properties can be achieved.

The combination of the dry lubricant Molykote® D-7100 (Dow Corning) and the zinc-nickel alloy passivated according to the invention exhibits surprising properties. Compared to known coating systems, this system is particularly resistant to chemicals and fitting cleaners. It is therefore particularly suitable for wheel bolts. In addition, a surprising insensitivity of the combination according to the invention to temperatures was found.

Preferably, this layer of dry lubricant is applied to the double layer of Aquares.

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Claims (15)

PATENT CLAIMS Method for the passivation of zinc, cadmium or alloys thereof, in particular zinc-nickel alloys, with a chromium-free solution comprising a weak complexing agent, preferably a di- or tricarboxylic acid, preferably a chromite-oxalate complex and Co, characterized in that the Co concentration ²⁺ is greater than 30 g / l. Method according to claim 1, characterized in that the Co ²⁺ concentration is 70 to 110 g / l. The method of claim 1, wherein the Co concentration is 90 g / L. Method according to any one of the preceding claims, characterized in that the passivation is carried out at 20 ° C to 55 ° C. A method according to any one of the preceding claims, characterized in that the passivation is carried out at a pH of 0.5 to 5.5. Method according to any one of the preceding claims, characterized in that the passivation is carried out at pH 4. The process according to any one of the preceding claims, wherein the ratio of chromium to cobalt is 1.7: 2.0. A process according to any one of the preceding claims, wherein the solution is sulphate-free. Method according to any one of the preceding claims, characterized in that the passivated layer is coated with another layer. A method according to any one of the preceding claims, characterized in that it is coated with an Aquares ™ layer. · · · * «* * * * * * * * * * * * * * * * Method according to any one of the preceding claims, characterized in that it is coated with a double layer of Aquares ™. Method according to any one of the preceding claims, characterized in that a dry lubricant is applied to the passivated layer. Method according to any one of the preceding claims, characterized in that the passivated layer is coated with an organic Topcoat layer. Method according to any one of the preceding claims, characterized in that the passivated layer is coated with a layer comprising polytetrafluoroethylene. Method according to any one of the preceding claims, characterized in that the passivated layer is coated with a Molykote® D 7100 layer.