CS212644B1 - A method of treating a black chromated metal surface - Google Patents

Abstract

The invention relates to a method of treating a black chromated metal surface, especially a zinc or cadmium surface, characterized in that the metal surface is brought into contact with an aqueous solution containing acetate ions and a thio compound. After drying without rinsing, a uniform appearance of the chromate layer is achieved and its corrosion resistance is increased.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating a black chromated metal surface, particularly zinc or cadmium.

At present, chromating is used for the treatment of zinc and cadmium coatings, which serve as a protective decorative finish of steel.

The purpose of staggering is to create a chromate layer on the surface of the zinc and cadmium coatings, which improves the appearance of the coatings by producing the desired color shade on them and at the same time improves their corrosion resistance. Chromate layers on zinc and cadmium coatings can be formed very thin, clear with low corrosion resistance, over blue, yellow to yellow-brown with very high corrosion resistance.

A special type of chromating of zinc and cadmium coatings is the formation of black chromate layers. These layers look highly decorative and especially in recent times when black color is fashionable, their use is increasing. Also, this type of chromate layer improves the corrosion resistance of the metal coating itself, but not substantially.

Black chromate layers on zinc and cadmium coatings are formed from baths containing hexavalent chromium compounds as an essential component; baths contain so-called activators, which are most often sulphate, nitrate, phosphate, etc. The essential part of this type of chromating baths are silver compounds. The chromate layers formed from these baths are gelled in the wet state, easily wiped, only after drying they become hard and resistant to normal abrasion and touch.

The basic requirement for black chromated surfaces is uniform, deep black coloring of the entire surface. This is quite difficult to accomplish by conventional techniques, since after chromating, the subsequent cold rinse washes out the chromate layer, causing the layer not to be deep black but gray in color after drying. Thus, the rinsing operation after chromating becomes critical to the appearance of the chromate layer and it is primarily the rinsing length and rinse water temperature that adversely affect the appearance of the layer.

In places where rinse water is retained on the surface of the chromate panels, such as the edges of holes, cavities, lower edges of the panels, etc., gray defective stains are formed during drying, even when properly rinsed. Another negative phenomenon causing visual defects is the run-off of rinse water on the surface of the components placed on the hinge, gradually from top to bottom. Since the black type of chromate layer must be dried carefully, so that the temperature of the dried surface does not exceed 60 ° C to reduce corrosion resistance, relatively long exposure of the retained water to the chromate layer occurs, resulting in gray degrading spots.

These known drawbacks are addressed by a number of suppliers of black chromate chromating agents, for example by reducing the rinsing time of the chromate layer to a minimum, or by dispensing into the rinsing water a portion of the chromating bath or a chromium compound containing bath part.

All these drawbacks are overcome by the treatment method according to the invention. Its essence is that the black chromated metal surface, in particular zinc or cadmium, is contacted with an aqueous solution containing 0.01 to 1 mol. acetic acid and 0.01 to 1 mol. thioureas at a temperature of up to 35 ° C for 10 to 300 s.

By immersing the chromate layer in this stabilizing bath and then rinsing it dry, a uniform black surface is obtained without a gray tint and gray spots at the liquid retaining points. By immersing in a stabilizing bath, the advantage of which is that it operates at room temperature, the corrosion resistance of the stabilized chromate layer is further increased due to the presence of the thio compound.

242644

Example 1

The zinc-plated coating, just black-chromed, is rinsed with water and immersed in an aqueous solution of the composition:

acetic acid at a concentration of 0.4 mol.

thiourea at a concentration of 0.15 mol.

Solution temperature 20 ° C, immersion time 30 s. Drying is then carried out preferably with a warm air stream without rinsing.

Example 2

The zinc galvanized coating just black-chromated is rinsed with water and immersed in an aqueous solution of the composition:

0.2 M acetic acid.

Thiourea at a concentration of 0.07 Mol.

at 30 ° C and a dive time of 10 seconds.

Without rinsing, it is dried with warm air.

Claims (1)

SUBJECT OF THE INVENTION Process for treating a black chromated metal surface, in particular a zinc or cadmium metal, characterized in that the black chromated metal surface is contacted with an aqueous solution containing 0.01 to 1 mol. acetic acid and 0.01 to 1 mol. thioureas at a temperature of up to 35 ° C for 10 to 300 s.