8 - Deposition of Alloys

Abstract

This chapter summarizes several aspects of deposition of alloys. The first steps of alloy deposition differ from the nucleation of a pure metal. Special models have been developed for the nucleation process. If the deposition of the two components is independent of each other, the total current potential diagram is the sum of the current potential diagrams of the components. In the voltammogram, one observes at more positive potentials the deposition of pure copper, which is diffusion controlled and follows the rules of mass transport with the limiting current of Cu2+ diffusion. At the potential, where Ni deposition begins, the Ni deposition is first charge transfer controlled. If the charge transfer dominates at larger overvoltage, Tafel lines will be found. The increasing partial current of nickel deposition is added to the limiting partial current of Cu deposition. With increasing polarization in cathodic direction, the Ni ion discharge also becomes diffusion controlled and the total deposition rate is the sum of the limiting currents. A special case is observed for the electrodeposition of molybdenum and tungsten from molybdate or tungstate solutions. Deposition of the metallic form is not possible from a solution of the pure molybdate or tungstate ions. Only oxide layers of pure quality are formed. But a deposition is observed in combination with metals of the iron group. These alloys, especially NiW and NiMo, have become of interest because of their mechanical properties like hardness or wear resistance.