July 19, 2008

Etching for Metallic Coating

The degreased articles are not yet ready to receive the electroplated coating, because the metal surface may not be sound and it may have superficial oxide films or stains on it. If a clean, sound metal surface can be exposed the subsequent electroplate will bond atomically to it, but any impediment to this intimate contact will lead to poor adhesion, and the plating may peel off later in service. The extreme outer surface of the metal is indeed not representative of the sound metal below, because it may have been disturbed and fragmented by previous manufacturing operations and by the smoothing processes. For this reason, after degreasing, the articles are lightly etched in a dilute acid solution, e.g. by dipping into 10% sulfuric acid.

A cyanide solution is often used for lightly etching copper alloys and brass. Where thick and particularly adherent deposit must be formed on steel and the adhesion is very important, an electrolytic anodic etching process may be used. After this etching treatments the articles are gain thoroughly rinsed and placed without delay into the electroplating tank. It is advisable for their entry to complete the electrical circuit so that plating starts immediately.

The technique of electroplating is fairly similar for different metals, but naturally the composition of the solution is different, and temperature, current density and other conditions may also vary. The process will therefore be illustrated by describing nickel and chromium plating in some detail, since these are by far the most extensively used electroplating used electroplating process, and then outlining process for other metals.

No comments: