The present invention concerns alkaline-cyanidic baths for the galvanic deposition of bright to brilliant, leveled copper-tin alloy coatings. The baths contain 1 to 60 g/l copper in the form of copper(I) cyanide, 1 to 50 g/l tin in the form of alkali metal stannate, 0 to 10 g/l zinc in the form of zinc cyanide, 1 to 200 g/l of one or more complex binders, 1 to 100 g/l free alkali metal cyanide, 1 to 50 g/l free alkali metal hydroxide, 0 to 50 g/l alkali metal carbonate, 0.01 to 5 g/l brightener and 0 to 100 mg/l lead as lead(II) acetate or lead(II) sulfonate. The present invention also concerns a method of electrolytically depositing a bright to brilliant, leveled copper-tin alloy coating on a metal substrate which involves employing the alkaline-cyanidic bath described herein at a temperature of 40.degree. to 62.degree. C. Furthermore, the present invention concerns a metallic base coated with a copper-tin alloy coating prepared by the method described herein.
Baths are required in decorative surface technology which coat the surface of the base support in a uniform manner which is true to the contour of the base support and which compensate for any unevenness of the substrate (leveling). In addition, they must selectively produce a matte finish, satiny-matte or brilliant luster. These requirements are met in particular by galvanic nickel baths known in the art both for the deposition of nickel layers as final support and also as foundation prior to a subsequent coating with noble metals. However, there is the disadvantage that nickel is allergenic for a considerable part of the population.
The deposition of copper-tin coatings from galvanic baths has been known for many years. In particular, coatings are used which contain 45 to 60% copper since they have a bright silver luster and do not tend to tarnish, or coatings are used which comprise 75 to 85 % copper since they have a yellow to golden yellow color. The former are therefore used in decorative galvanotechnology as a replacement for e.g. silver, nickel, chromium or aluminum. However, copper-tin coatings are also increasingly being used industrially in other areas on account of their very good soldering properties, their wear resistance, corrosion resistance and their low electric transition resistance. The latter are used primarily in decorative galvanotechnology as a replacement for brass and as the undercoating prior to a galvanic gilding. Layers of copper-tin alloys do not cause any known allergies on the human skin.
Copper-tin alloys are deposited primarily from alkaline, cyanide-containing electrolytes containing copper as copper(I)-cyanide and tin as sodium stannate. Other electrolytes contain phosphate and/or polyphosphate as complex binder and, moreover, colloids such as e.g. polypeptides as brighteners (DE 860,300). These known baths must be operated at high constant temperatures (65.degree. C. and higher) in order to obtain uniform layers with a constant composition. Working with these baths is therefore difficult and complicated.
The copper-tin baths can also contain zinc salts, as a result of which a few percent zinc is also deposited.
Recently, copper-tin alloying baths have become known (U.S. Pat. Nos. 4,565,608 and 4,605,474 which are incorporated by reference in their entirety; DE 3,339,541) which, in addition to copper cyanide, alkali stannate, phosphates, free alkali cyanide and free alkali hydroxide, also contain organic substances in the form of fatty acid-imido-alkyl-dialkyl-amino oxides, fatty acid-amido-alkyl-dialkyl amine betaines and/or ethoxylated naphthols as complex binders and contain polyethylene diamines, benzaldehydes, ethinols and/or benzylpyridine carboxylates as brightening agents. These baths also require a monitoring of the content of free cyanide and hydroxide. Moreover, they exhibit only a weak leveling action. The same applies to copper-tin alloying baths containing 3 to 12 g/l monosaccharides as complex binder (Pat. Abstr. of Japan, C-122 Sep. 2, 1982, vol. 6/No. 169, Jp 57-82492).