Chromium Plating

Abstract

A method of chromium coating a substrate is described, which method comprises: a) depositing, preferably electrodepositing, a metallic layer onto the substrate comprising chromium from a plating bath that includes trivalent chromium and no, or substantially no, hexavalent chromium; b) physical vapor depositing a layer comprising chromium cn the layer deposited from step a). In one embodiment, in step a), the layer is chromium and is electrodeposited from a plating bath that includes trivalent chromium and no, or substantially no, hexavalent chromium. The metallic layer deposited in step a) is preferably a metal or an alloy of two or more metals and may be selected from the group consisting of chromium, copper, nickel, cobalt, zinc, lead iron, palladium and tin and alloys thereof, e.g. copper tin, copper zinc, tin lead, tin nickel, tin cobalt, tin zinc, iron nickel, cobalt nickel, zinc nickel, zinc iron, zinc iron nickel, zinc iron cobalt or palladium nickel. Copper and nickel are especially preferred. The coating formed by the method has a blue sheen that is usually associated with chromium electrodeposited from hexavalent chromium baths without having to use a hexavalent chromium bath, which is highly toxic.

Description

TECHNICAL FIELD[0001]The present invention relates to chromium plating of substrates, including metal and plastic substrates.BACKGROUND ART[0002]Chromium plating is a well known technique for protecting substrates and giving them an attractive appearance. It is known to electroplate both metallic and plastic substrates using an electroplating bath containing hexavalent chromium, typically chromic acid or chromates (H2CrO4 and CrO42−). Electroplated deposits from hexavalent plating baths provide a strong, abrasion-resistant layer that will protect the underlying substrate from corrosion and provide an attractive decorative finish. One feature that is seen to be particularly attractive is the “blue sheen” of the chromium layer that is electrodeposited from hexavalent chromium baths.[0003]Hexavalent chromium, however, is believed to be toxic and carcinogenic and its use is being phased out. Accordingly, alternatives are being sought. One alternative that has been proposed is a bath bas...

AI Technical Summary

Benefits of technology

[0013]It has now been found that coatings having a blue sheen can be obtained without using hexavalent chromium baths.

Problems solved by technology

Hexavalent chromium, however, is believed to be toxic and carcinogenic and its use is being phased out.

However, trivalent chromium electroplating baths are not stable and are difficult to control.

Thus it is not easy to obtain a consistently smooth finish using a trivalent chromium bath; furthermore electrodeposits from trivalent chromium baths do not have the characteristic blue sheen of electrodeposits from hexavalent chromium baths, which is seen to be highly desirable.