ELECTRODEPOSITION AND HEAT TREATMENT STUDIES OF NICKEL-ZINC ALLOYS

Abstract

(1) Recently, the shortage of Nickel has stimulated the development of electrodeposits which could substitute it either partially or wholly. This prompted the study of Nickel-Zinc electrodeposition. The effect of temperature (35°C to 65°C), current density (1 to 50 MA/CM²) and Nitrogen agitation (0 to 450 CC/MIN) on the cathode current efficiency, appearance, composition, microhardness and microstructure of the Nickel-Zinc deposits, plated from a mixed sulphate bath, were investigated. Heat treatment of some selected deposits was carried out in an inert Nitrogen atmosphere up to 600°C at various annealing times. The anomalous sulphate bath was found to be under diffusional control at higher current densities (20-50 MA/CM²). The current efficiency of deposition of most deposits was about 80 to 90%. A decrease in temperature or an increase in agitation or current density increased the Zinc content of the deposit. The microstructures belonged to the common laminar, columnar, fibrous or dendritic types. Agitation had a considerable effect of the microstructure. The microhardness was not only a function on composition and structure but also of plating conditions. Deposits containing 17 to 86% Zinc (atomic) had microhardness values of approximately 350 to 630 kp/mm². (2) (i) Deposits which were yellowish white and bright can have a laminar or columnar microstructure. The microhardness ranged from about 430 kp/mm² to 620 kp/mm² for deposits containing 17 to 26% Zinc. The observed microcracks were correlated to the reported tensile stress. With heat treatment at 400°C to 500°C, the microhardness increased to about 630 to 670 kp/mm². Deposits containing a high Nickel content recrystallized at about 800°C. (ii) Deposits which were silverish grey had a laminar microstructure. The microhardness lies in the region of about 450 to 610 kp/mm². When heat treated at 400°C, the initial hardness decreased slightly. (iii) Deposits which were bluish-grey of an unknown nature, had a 'broken' microstructure. The current efficiencies were low, of about 70% ; the microhardness ranged from 320 to 450 kp/mm² which decreased on heat treatment. (iv) Deposits which were silverish white, had a columnar or laminated columnar microstructure. The microhardness ranged from about 520 to 610 kp/mm² which changed marginally with heat treatment. The deposits tend to soften with long periods of heat treatment or at high annealing temperature. (v) Deposits which were greyish had a dendritic microstructure with hardness of about 450 to 500 kp/mm². A deposit containing 84.4% Zinc recrystallized at 500°C.