Direct and Pulse Plating of Metastable Zn-Ni Alloys

A widely used method to protect steel surfaces from corrosion is the application of a sacrificial layer of a less noble metal, which undergoes preferential corrosion due to galvanic coupling. The most used material is zinc, a metal readily available and easy to apply on components presenting large areas. However, zinc is far more reactive than iron, and the thickness of material needed to effectively prevent corrosion represents an issue for some applications, such as coatings for the automotive sector or the aerospace field. A possible solution can be the alloying of zinc with a metal of the iron group, thus increasing the nobility of the layer. By doing this the corrosion potential is shifted to values closer to iron, inducing a slower consumption of the sacrificial layer. Nickel, having a suitable equilibrium potential, is the most used metal, providing an excellent corrosion protection with respect to pure zinc. The coatings obtained are times more resistant to corrosion than pure zinc and are currently studied as alternatives to cadmium. Zn–Ni alloys are reasonably easy to electrodeposit, and the plating of corrosion efficient coatings was performed in the past from acidic and alkaline electrolytes. It is well-known that the best protective properties are achieved when the alloy presents a single γ phase, a condition that can be attained by controlling properly the plating parameters and electrolyte formulation. It is interesting to point out that the plated γ phase can be a metastable structure, as the Zn–Ni phase diagram predicts lattice distributions of the Ni atoms in contrast with the experimental results. The reasons for the observed metastability and the thermodynamic properties of Zn–Ni have been elucidated previously starting from the analysis of the equilibrium conditions of the different phases. It was demonstrated that Zn–Ni electroplated alloys from an alkaline bath follow the theoretical simulations, thus validating the interpretation provided. Zn–Ni can be plated also by means of pulsed electrodeposition, a method suitable to achieve more refined microstructures and compact layers. This change in microstructure also improves the corrosion behavior and the distribution of the residual stresses. Zn–Ni alloys having compositions in the range 14–18 wt-% are electrodeposited using direct current DC and pulse current PC in a cyanide-free alkaline electrolyte. Homogeneous and compact layers, suitable for corrosion protection, are obtained. Contrary to the common acceptance, X-ray diffraction spectra and differential scanning calorimetry thermal analysis show that the electrodeposited γ alloy by DC plating in electrolytes with complexant-additive agents has to be regarded as a metastable phase, whose atomic arrangement is different from that of the equilibrium γ intermetallic compound. A model for atomic distribution and the Gibbs free-energy function for the DC electrodeposited phase are discussed. It is found that PC deposited alloys from additive-free electrolytes present the same metastable behavior attributed to the DC plated homologues in electrolytes with complexant-additive agents. The corrosion behavior is found to be dependent on the duty cycle applied during PC deposition, as evidence of the different microstructures obtained in varying the parameters.