July 29, 2009

The Difficulties of Aluminum Plating

Aluminum is an even more difficult metal to electroplate. Although aluminum is a very reactive metal, it is normally covered with a protective oxide film, which is rapidly reformed when a fresh metal surface is exposed. The oxide is also insoluble in most acids. It thus cannot be prepared in the normal way by acid dip. The most successful method of preparation is by immersion in a strongly alkaline solution of sodium zincate. The alkali dissolves off the oxide film, and then the naked metal reacts chemically with the zincate to form a double cyanide solution, and any other electroplate subsequently applied. Nevertheless aluminum is even more active than zinc, and thus is anodic to any metallic coating. The electroplating of aluminum and its alloys is therefore not advisable, especially as there are a number of protective and decorative processes, unique to aluminum which can be applied instead.

Magnesium and its alloys are too reactive to be electroplated. Control of electroplating process relies largely on measurements of currents, temperatures and pH values, also on periodic chemical analysis of the solution. However, even today control relies very largely on the skill, knowledge and experience of the operator.

Electroplating plants are large quantities of water, which is expensive to buy and equally expensive to purify before disposal. Considerable attention to the economical use of water and also to recovery of rinsed off chemicals is worth while. Water supplies suitable for drinking and domestic purpose may be unsuitable for electroplating, because of their contains of dissolve salts and are often worth while. Large ammount of heat are required to maintain solutions at the required temperature, and for heating air which is afterwards thrown out of the fume extraction ducts. Care in design can often materially reduce such cost.

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