Abstract
The imitation gold, which has been investigated by the authors, has the defect of tarnishing by the perspiration in practical use. The present investigation has been carried out on the tarnish-resistance properties of copper alloys by the measurement of the tarnishing rate and by the observation of the colours produced by tarnishing, using salt, lactic acid, urea and sodium sulphide as experimental reagents. In binary copper alloys, the tarnish-resistance to salt is improved by adding zinc, against lactic acid nickel and silver are effective, and α+β phase alloy of copper-zinc system is also good. No remarkable effects are obtained in the case of sodium sulphide particularly, neverthless it is full of interest that the tarnish-resistance property is improved by nickel, in spite of its weakness against sulphide. The tarnishing by urea is negligible. Oxide films, produced by selective oxidation of aluminium copper-aluminium alloys, are highly protective against tarnishing.
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