Patent GB-579109-A (GB)
Title: Process for metal plating and baths and compositions suitable for use therein
IPC: C23C18/16
Filed: 1943-07-07
Abstract: Articles are immersion or electroplated in a bath containing a salt of a hydroxythiol with the metal to be plated, the preferred composition being a polyhydroxythiol derived from a reducing sugar, for example, by catalytic hydrogenation of a hydroxy carbonyl compound, in both cases in the presence of hydrogen p  sulphide.  A metal salt of a polyhydroxythiol may be prepared either by dissolving a metal hydroxide or oxide in aqueous solution of the polyhydroxythiol or by dissolving a metal salt of a non-oxidizing acid in such solution, the resulting acid being separated, if desired, by evaporation or neutralization with alkali, and the salt being separated by evaporation. Alternatively, when a polyhydroxythiol is soluble in acetic acid or alcohol, its salt may be precipitated by adding the selected metal acetate in acetic acid, or the metal alcoholate.  The acid, if retained in solution, accelerates the rate of plating, but other non-oxidizing acids may be added, such as sulphuric, hydrochloric, or phosphoric acid of a concentration between 1 and 10 per cent.  Soluble salts of the metals to be plated, and, if desired, of non-plating metals, for example, sulphates, chlorides, double cyanides, phosphates and alkali metals, alkaline earth metals, may be added to increase the rate of plating. Metals to be plated generally comprise all those standing below calcium in the electromotive series, cadmium, chromium, nickel, cobalt, copper, gold, iron, lead, platinum and members of its group, silver, tin, manganese and molybdenum being specifically mentioned. The polyhydroxythiols preferred are those having one or more primary or secondary mercapto groups, and those having a hydroxyl group on a carbon atom in the a  or b  position with respect to a carbon atom bearing a mercapto group.  The reducing sugars from which the polyhydroxythiols are prepared are the aldo-and keto-hexoses and disaccharides, specific polyhydroxythiols being the penta- and tetra-, hydroxyhexane, tetrahydroxypentanethiols and trihydroxybutanethiols.  The hydroxythiols dissolve metal oxides and so obviate the need for pre-treatment.  They may be recovered by precipitation with hydrogen sulphide and filtering.  The process may be applied to plating alloy articles and graphite, and to non-metal articles which have first been coated with metal by sputtering, and in conjunction with other conventional plating processes such as cyanide, sulphate and chloride baths.  Nine detailed examples are described, in one of which a steel object is cleaned by brushing with a solution of phosphoric acid in methanol and then washing before immersion in a plating bath heated to 60-80 DEG  C.  In another example, 65 parts by weight of commercial dextrose and 30 of powdered sulphur were charged into a hydrogenation autoclave with 100 parts of water and 8 of CoS catalyst prepared by precipitating an aqueous solution of 13 parts of cobalt chloride hexahydrate and 16 of sodium polysulphide.  Hydrogen was admitted at 1500 lbs./in.2 and the autoclave agitated and heated to 125 DEG  C. with additional periodic admission of hydrogen, the heating being maintained for three to four hours.  After cooling, the product was rinsed and the catalyst filtered out.  The filtrate was heated at 100 DEG  C. at a pressure of 10-15 mm. of mercury and the residue of about 53 parts was redissolved in an equal volume of water, and the 1-thiosorbitol content estimated volumetrically with standard iodine solution.  The bath comprising 30 parts of this solution (corresponding to 13 of 1-thiosorbitol), 10 of copper sulphate pentahydrate and 150 of water, to which 5 of sulphuric acid were added, was heated to 80 DEG  C. and poured into a steel pipe previously cleaned by pickling, the solution being removed after 10 minutes.  With electroplating, no heating was necessary.