Influence of Plating Conditions on Nickel-Chromium Alloy Electrodeposition

Abstract

Nickel-chromium alloys present high corrosion and high creep resistance. Due to the large difference between the standard potentials of the two metals, E0(Ni2+/Ni)=-230 mV and E0(Cr3+/Cr)=-760 mV, the alloy composition usually strongly depends on the current density chosen for deposition, on the nickel concentration in the electrolyte and the electrolyte temperature.The electrodeposition of chromium from the trivalent oxidation state is more challenging compared to its hexavalent state. In aqueous solutions Cr(III) ions form complexes some of which have a very low ligand exchange rate. Thus, electrochemical deposition is difficult or only possible at a very slow deposition rate. In addition to the aquo complexes, complexes can also be formed with the anion of the corresponding metal salt. Normally, complexing agents, such as carboxylic acids, are added to the electrolyte in order to increase the deposition rate.This paper will present recent results from authors’ lab on the electrodeposition of Ni-Cr alloys from aqueous electrolytes containing Cr(III) salts. The effect of formic and oxalic acids on the qualities of the deposits was investigated, as well as the influence of the deposition technique (direct or pulse plating), current density, nickel concentration in the electrolyte, temperature of the electrolyte, and the hydrodynamic conditions.We could show that the chromium content in the deposits can be easily tuned via the composition and temperature of the electrolyte, and the current density. At high current densities and low nickel concentrations, chromium-rich layers with up to 64 wt-% chromium could be deposited. An increase in the temperature led to a significant decrease in the chromium content of the deposits. The layers grow compactly and show a globular structure. The deposited films changed from a crystalline to an amorphous structure with increasing chromium content.