Theoretical and Practical Aspects of Alloy Plating - The 16th William Blum Lecture - Part 2
This article is the second of three parts of a re-publication of the 16th William Blum Lecture, presented at the 62nd AES Annual Convention in Toronto, Ontario, Canada on June 23, 1975. Dr. Ernst Raub presents a comprehensive treatise on alloy plating.
Forschungsinstitut für Edelmetalle und Metallchemie
Recipient of the 1974 William Blum AES Scientific Achievement Award
Originally published as Plating & Surface Finishing, 63 (3), 30-43 (1976), this article is the second of three parts of a re-publication of the 16th William Blum Lecture, presented at the 62nd AES Annual Convention in Toronto, Ontario, Canada, on June 23, 1975. A printable PDF version of Part 2 is available by clicking HERE. The printable PDF version of the complete 44-page paper is available HERE.
Alloy deposits from complex compounds
As an example, cyanide complexes are widely used in commercial practice to approach deposition potentials of the metals to be simultaneously deposited. The classical example, copper-zinc, has already been mentioned.
Stability of cyanide complexes differs strongly and the complex from which discharge occurs changes with metal-to-cyanide ratio of the electrolyte.
The deposition potentials in cyanide solutions are not always close enough together to permit the codeposition desired. Simultaneous deposition of zinc and cadmium happens equally in acid sulfate and alkaline cyanide solutions.38 In Fig. 17 the potential curves in the acid and in the alkaline cyanide bath are shown. They are nearly 0.4 V apart, and codeposition of zinc with cadmium occurs only when the limiting current density of cadmium is exceeded. In the cyanide alloy bath at higher current densities a second limiting current density is observed which relates to the deposition of zinc. As in the acid bath, in the alkaline cyanide bath also, zinc deposition does not take place until the limiting current density for cadmium is exceeded. The often described simple method to transform a cadmium electrolyte into a zinc electrolyte just by substituting cadmium anodes for zinc anodes relies on the preferred cathodic discharge of cadmium and the anodic dissolution of zinc. Using this method one has to keep in mind that at current densities above the limiting current density for cadmium separate crystallization of both metals easily leads to the deposition of alloys with insufficient properties. It is better to work during the conversion of a cadmium into zinc bath at current densities below the limiting current density of cadmium.