US2277668A - Zinc plating - Google Patents

Description

ating characteristics.

Patented Mar. 31, 1942 ZINC PLATING Clayton F. Ruebensaal, Cleveland, Ohio, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing.

Application October.19, 1938, Serial No. 235,782

3 Claims. (01. 204-55) This invention relates to the electrodeposition of zinc from sodium zincate solutions and is more particularly directed to the use of glycollic acid or its salts as an addition zincate plating baths.

. It is frequently the practice to produce zinc electrodeposits. from acid baths which are made up. for instance, with sulfuric acid. Though these acid baths are relatively inexpensive, they require special acid-resisting equipment and they have a relatively poor throwing power.

Zinc is plated with great success from alkaline baths containing cyanide. Deposits of excellent character are obtained from these cyanide-zinc plating baths and the baths are themselves relaagent for sodium therefore, that glycollic acid may be added initively much less corrosive than the acid baths.

Cyanide-zinc plating baths have excellent'opercyanide-zinc plating baths are relatively expensive. Alkaline zinc baths omitting cyanide are quite inexpensive but they have not heretofore proven satisfactory. These alkaline zinc baths are com- Unfortunately, however,

posed of sodium zincate and are customarily pre.-.

pared by dissolving zinc oxide in sodium hydroxide. The sodium zincate baths are less expensive than even the acid baths heretofore used, and they have a relatively great throwing power. Sodium zincate baths are relatively much less corrosive than acid baths, and they maybe handled in ordinary iron tanks.

Unfortunately, the deposits obtained from sodium zincate baths are spongy and are of .ex-

ceedingly poor character. Addition agents such as have been found valuable in improving the character of deposit in cyanide-zinc plating baths are without substantial effect or are actually deleterious in sodium zincate baths.

- I have found that by the use of glycollic acid (CH2OHCOOH) or of its salts as an addition agent, sodium zincate baths may be markedly improved and deposits of improved character may be obtained. Glycollic acid and glycolates increase the range of current densities over which a smooth and satisfactory deposit may be obtained, and the maximum permissible current density is considerably increased. Glycollic acid and its salts are apparently quite stable and no harmful decomposition. products are formed either upon standing of the bath or upon electrolysis.

The amount of glycollic acid or glycolate to employ will depend to some extent upon the exact character of the sodium zincate bath and upon the effect desired. In general, the optimum amount to use in a particular instance can best be determined by a few simple trials. In general I have found that from about seven to thirty grams per liter of glycollic acid, or of a glycolate, is satisfactory while more particularly I have found that around twenty-three grams per .liter is about the optimum amount of glycollic acid for a number of specific in stances. I

It will be understood that when glycollic acid is added to a sodium zincate bath it is likely that sodium glycolate forms and it'is this reaction product which is actually present as an active addi ion agent. It will be understood,

tially in the. form of sodium glycolate, and it will also be understood that other soluble glyco-'- lates such as the. potassium, ammonium and Zinc salts of glycollic acid may be used.

Glycollic acid or one of its salts may :conveniently be added to a sodium zincate bath in the amount required. The bath composition may be maintained by adding a'small additional amount of glycollic acid or glycolate from time to time. The addition agent may, if desired, be

incorporated in a prepared composition including anylor all of the bath ingredients so that upon dissolution-in water a sodium zincate bath 'will be formed.

It will be-understood that glycollic acid or its salts may be used according to my invention with any sodium zincate bath, and that such baths may contain additional addition agents, brighteners, and other bath-modifying constituents of a character known to the art.

While I have mentioned specific illustrative conditions in the foregoing, it will be understood that those skilled in the art may employ glycollic acid or its salts as addition agents for sodium zincate baths in. any desired manner without departing from the spirit of my invention. i I claim:

1. In a process for the electrodeposition of zinc from a sodium zincate bath, the step comprising depositing zinc from an electrolyte comprised essentially of sodium zincate and free of cyanide, deposition being effected in the presence of from about 7 to 30 gramsper liter of a substance 3.'A plating composition comprising sodium zincate and a minor amount of a substance selected from the group consisting of glycollic acid and its salts, the composition being free of cya nide. r

CLAYTONF. RUEBENSAAL.