Abstract: In acidic Sn(II)-containing electrolytes for coloring oxide layers on anodized aluminum or aluminum alloys, or for electroplating, the Sn(II)-ions are oxidized. By the addition of substances, so called stabilizers, to the electrolytes this oxidation can be more or less prevented i.e. Sn(II) can be more or less stabilized.Furthermore, the effectiveness of the stabilizer can be decisive for the quality of the deposit or the coloring of the oxide layer. It is known that the presence of substantial amounts of Sn(IV) along with Sn(II), as a result of inadequate stabilization of the Sn(II), impairs the quality of the surface treatment.An acidic, Sn(II)-containing electrolyte containing an addition of at least one soluble diphenylamine or substituted diphenylamine derivative stabilizes the Sn(II) and produces flawless coloring.

Abstract: The object of this invention was to find a Sn.sup.II -containing electrolyte for galvanizing or coloring anodized aluminum and its alloys which, in the first place, contains substances which stabilize Sn.sup.II better than substances used up to now for this purpose, secondly, exhibits better throwing power and therefore better current distribution and thirdly is not harmful to the environment, or at least not to such a degree as the electrolytes used up to now. The electrolyte of the invention is such that it contains one or more additives having the general structure R.sub.1 --S--R.sub.2, where R.sub.1 can be a hydrogen atom or a hydrocarbon group with or without a hydroxy or carboxy group, R.sub.2 a hydrocarbon with hydroxy or carboxy group. A particularly preferred additive is 2.2'-di-hydroxy-di-ethyl-thio-ether (or 2.2'-thio-di-ethanol) HOCH.sub.2 CH.sub.2 --S--CH.sub.2 CH.sub.2 OH.