Structure and thermal stability of zinc-nickel electrodeposits

ZnNi alloys were electrodeposited from a chloride bath on steel substrates. The effect of nickel bath concentration on chemical composition, structure and microstructure of the deposits is demonstrated. From 0 to 13 nickel, the phases obtained do not correspond to that reported on the thermodynamic phase diagram. It is shown that the substitution of zinc by nickel is responsible for the formation of distorted eta(d) and gamma(d) phases corresponding to the supersaturated hexagonal eta phase of zinc and to the unsaturated cubic gamma phase of Zn-Ni alloy, respectively. Differential scanning calorimetry indicates that the thermal instability of the alloys containing up to 13 wt of nickel, results from the crystallization of the delta phase from the eta(d) and gamma(d) phases at around 200 degrees C and 250 degrees C, respectively.