Improvements in the electroplating of precious metals on to articles made of baser metals

摘要

A tarnish- and corrosion-resistant coating is provided by a layer of chromium with a coating of a precious metal supported on the chromium layer and adhered to a depassivated surface thereof. The chromium coating may be depassivated by subjecting it to a cathodizing current in an acid bath, e.g. it may be treated in 5%-10% hydrochloric acid or sulphuric acid at room temperature for 5-30 minutes with a cathode current of 30-40 amps/square foot. Phosphoric and acetic acids may also be used for depassivation. The surface is then rinsed in a weak acid bath, e.g. 0.01-1% hydrochloric or sulphuric acid and then electro-plated with the precious metal which may be gold, platinum, palladium or rhodium. Specific acid gold cyanide bath compositions are referred to for plating with gold. These baths may also contain salts of base metals such as iron, nickel, cobalt, tin, cadmium or zinc to produce a gold alloy coat. For plating the platinum group metal baths may be prepared from iodo complexes containing excess of iodides of alkali metals, ammonium or magnesium, repassivation of the chromium by liberated iodine being avoided by placing the anode in a porous compartment of unglazed porcelain or fritted glass, using acid or alkaline anolytes. The substrate may be coated first with copper, then with nickel, an adherent chromium coating being applied to the nickel surface.