Saturday, October 18, 2008

Electroless Plating of Nickel

Metals can be recovered in metallic form from aqueous solution of their salts by means of reducing agents. Development of photographic film and the silvering of mirrors are examples of this. The more noble metals such as gold and silver are easily turned out of the combined form: the more electronegative metals require much more powerful reducing agents. It is also essential that the metal deposits on the solid surface as a coherent and continuous film, and highly desirable that it only coats the surface desired, and not the walls of the containing vessel as well. In the silvering of glass mirrors, for example, the silver deposits on the glass but also on the walls of the containing vessel.

A process has been devised using the very powerful reducing agent, sodium hypophosphite, no produce nickel coatings on metals, and has the peculiar and advantageous property of proceeding only on nickel itself or on metals of the platinum group. Once started, it is therefore generally known by the ugly term ‘electroless’ nickel plating. The solution contains nickel sulfate, chloride or acetate, sodium hypophosphite and an organic acid, such as nitric acid or lactic acid to control the change of pH value, which has maintained at about pH 4.0. The solution is operated at about 90 oC and deposit nickel at the rate of about 0.0008 in/h. In due course the solution become spent and must be renewed. The formation of the nickel coating on a different metal such as steel can be ensured by a prior treatment of the surface with a dilute palladium chloride solution; a thin film of palladium is formed by chemical replacement which catalyses the subsequent reduction of the nickel salt by the hypophosphate.

The metallic deposit has an appreciable content of phosporus. It is smooth and often lustrous, very fine grained and extremely hard (500 Vickers pyramid number and indentation measure of hardness), it is very brittle. Since the process is chemical and autocatalytic, the thickness of the deposit can be uniform even on the most irregular and re-entrant areas. The process is naturally rather expensive, but it is uniquely suited to coating the interior of complicated chemical plant, tubes, condensers, etc., even after assembly or installation.

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