Electrolytic alloys: Part -II Industrial alloy plating

Abstract

The cathode polarisation of a metal deposits, i.e. the difference between the potential of a growing cathode and 1 equilibrium potential in the same solution, is determined by the following factors: 1)The rate of supply of ions to the cathode, which depends on: (a) Concentration of the cations round the cathode during electrolysis. (b) The speed of dissociation of complex and/or hydrated ions. (c) Hindrance to diffusion or migration by : (i) porous semipermeable or impermeable film, (ii) congregations of undischargable cations, (iii) chemical conditions which remove a portion of the dischargeable cations before they can reach the cathode. 2)Time taken for the discharge of the ions, hydrated to different degrees, on the cathode and their incorporation in the lattice. Complex salts are often chosen for electrolysis for getting o fine-grained deposit. The idea is that the film round a cathode, especially of a colloidal nature, is well established for the deposition of metals, e.g., nickel , manganese, etc. This film is supposed to help in the formation of fine-grained nickel from the sulphate bath. Again cations which can increase bath conductivity without being discharged at the cathode are often added to electrolytic being for better deposition of fine grained metals or alloys. Thus in cathodes potential due to these effects dose not require any remedy. Removal of cations by reactions with the products of electrolysis is adjusted by controlling the bath, e.g. by adjusting the pH of a nickel-plating bath.