GB474449A - Improvements in the production of zinc coatings by electrodeposition - Google Patents

Abstract

474,449. Electrodeposition of zinc IMPERIAL CHEMICAL INDUSTRIES, Ltd. (Du Pont de Nemours & Co., E. I.) March 27, 1936, Nos. 9190, 9526, and 12738. [Class 41] The electrolyte, an aqueous solution containing zinc cyanide, an alkali metal cyanide and an alkali metal hydroxide, is purified by one or a combination of the following methods and then electrolyzed, at certain cathode current densities. The methods are : (1) Addition to the bath of a reducing agent, e.g. sodium bisulphite or thiosulphate, phenyl hydrazine hydrosulphate or formaldehyde ; this precipitates the impurities which fall to the bottom of the bath ; (2) preliminary electrolysis of the solution, using low anode current densities, e.g. up to 15 amps. per sq. ft. ; the impurities are precipitated on the (zinc) anode ; (3) subjecting the bath to a galvanic action, as by hanging zinc anodes in intimate contact with the sides of the iron or steel tank containing the solution, or covering the bottom of the tank with granulated zinc ; the impurities precipitate on the anode ; (4) forming in the solution flocculent, colloidal precipitate, which apparently adsorbs impurities ; e.g. adding an aqueous solution of ferric sulphate, ferric hydroxide being produced ; (5) adding to the bath sulphur compounds which are soluble therein and which precipitate heavy metals, e.g. cadmium, lead and tin, as insoluble salts ; such compounds are sodium, potassium, barium, hydrogen and zinc sulphides ; sodium or potassium thiosulphate ; sodium hydrosulphite, thiocyanate or thiocarbonate ; potassium ethyl xanthate ; thiourea ; and tetramethylthiuram monosulphide. Using methods (1) to (4), the cathode current efficiency is at least 90 per cent. Using method (5) the efficiency may be 90 per cent or more or less. In the higher part of the range 90-100 per cent, bright coatings are obtained; in the lower part, dull coatings. Below 90 per cent, i.e. using the sulphur compound method, the deposits have a yellow to brownish coating. The dull or coated deposits may be given a bright, mirror-like appearance either by dipping in (a) sulphuric acid and either hydrogen peroxide or chromic acid, (b) nitric acid, or (c) sodium cyanide solution containing hydrogen peroxide, or by electrolytic treatment, making the coated article the anode in an alkaline electrolyte containing sodium cyanide. The brown film may also be removed by allowing the wet article to stand in the air for a short time. Before dipping, the deposits may be washed and treated with sulphuric or hydrochloric acid. The anodes used may be of pure zinc or a zinc-mercury alloy containing 1 to 2 per cent of mercury. A spherical cathode concentric with a hollow spherical anode may be used. According to the third Provisional Specification, organic colloids may be added to the bath.