SK5442003A3 - Passivation method - 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

Technical field

The 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 hereby incorporated by reference.

BACKGROUND OF THE INVENTION

It is known to electroplate metal surfaces with metals or metal alloys in order 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 a chromium ion for the passivation of the zinc-nickel layer.

The toxicity of chromium compounds contrasts with their good anticorrosive properties. Efforts have therefore 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 passivation solutions containing chromium ions, as described in U.S. Pat. No. 4,171,231, which, however, do not lead to chromium-ion-free passivation layers due to oxidizing agents also present in the solution, since chromium ions are oxidized during the operation.

DE 41 35 524 C2, which is incorporated herein by reference and fully incorporated herein by reference, describes a passivation process which achieves improved anticorrosive value 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 the aforementioned file. The good anti-corrosive 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.

With this type of passivation method, good anticorrosive results can already be achieved.

Furthermore, it is known to increase the corrosion protection provided by chromium ion passivation methods using cobalt.

An improvement in anticorrosive protection using cobalt is disclosed in WO 97/40208, which is hereby incorporated herein.

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

I

It is an object of the present invention 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 of 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 if we refrain from focusing on a compact conversion layer and try to create a porous conversion layer, this leads to improved end products when porosity is used to bind at least one additional layer.

The invention can be carried out by using cobalt in a passivating solution containing chromium ions and a weak complexing agent, preferably a di- or tricarboxylic 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 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.

An organic or inorganic layer is again applied to the passivated layer and penetrates into the porous conversion layer. Preferably, a coating of Aquares ™ (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 variations of the invention.

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

The combination of Molykote® D-7100 dry lubricant (Dow Corning) and zinc-nickel alloys 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 metal screws. In addition, it has been found surprisingly that the combination of the invention is insensitive to temperatures.

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

Claims (15)

PATENT CLAIMS Method for the passivation of zinc, cadmium or alloys thereof, in particular zinc-nickel alloys by means of a solution of chromium-free ions containing a weak complexing agent, preferably a di- or tricarboxylic acid, preferably a chromite-oxalate complex and CO ² ', CO ²⁺ is higher than 30 g / l. Method according to claim 1, characterized in that the concentration of CO ²⁺ is 70 to 110 g / l. The method of claim 1, wherein the CO ²⁺ 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. 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. Method according to any one of the preceding claims, characterized in that the ratio of chromium to cobalt is 1.7: 2.0. Process according to any one of the preceding claims, characterized in that the solution is free of sulphates. 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. A 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 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. I ¹ 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.