Structure of nickel-iron and nickel-zinc electrodeposits
Electrodeposited nickel-iron and nickel-zinc alloys have been studied using `direct' methods e. g. XRD, microscopy, EDX and GDOES on coatings having different thicknesses and deposited on different substrates. The work has confirmed and extended the ideas of Finch et al. (2, 51), Piontelli et al. (3) and Pangarov (49.120, 121) applicable to alloy deposition. It has shown that a better understanding, particularly of the initial deposition stages, can be obtained by considering crystallographic, energy and electrochemical aspects in combination rather than individually. Initially alloy is deposited on a `foreign' substrate but subsequently the growing alloy itself serves as the `substrate'. Similarly `old' crystallites have to compete for growth with `newly' nucleated ones as the deposit develops. The three stages of growth observed in nickel-iron and nickel-zinc are discussed. Crystallographic strain, resulting from mismatch between substrate and deposit structures and the presence of impurities, along with the energies required to produce different structures are considered to play a major role in determining structure during alloy deposition. Competition for discharge between hydrogen and metallic ions at the changing deposit surface, including changes in its electrochemical nature are considered. In addition possible effects resulting from adsorbed species, including co-discharged hydrogen as well as other species such as hydroxyl ions or precipitated hydroxides are discussed. Whereas during initial nucleation the original substrate is the dominating influence, in the later stages the electrolytic parameters determine the structure of the deposit. If the structure of the outer deposit differs markedly from that of the deposit during the initial stages, then a transitional growth stage may be involved. The structures of deposits studied in the present work tended to be fine grained in the initial stages but developed coarser columnar structures due to selected grain growth with favoured grains becoming broader during the intermediate and final stages of growth.