Microstructure and composition of pulse plated Re–Ni alloys on a rotating cylinder electrode

The effect of pulse electrodeposition parameters on the composition and microstructure of Ni–Re alloys were studied on a rotating cylinder electrode. Ni–Re alloys were deposited from a citrate bath where the nickel-to-rhenium ratio was 45:1. The composition of the deposited alloy was monitored by energy dispersive spectroscopy (EDS) and compared to a single component transport-based model describing the pulse limiting current of perrhenate. At long pulse times and high duty cycles, the partial current density of rhenium is predicted by the model, and rhenium inclusion into the alloy can therefore be described as being completely transport controlled. At shorter pulse times and low duty cycles, rhenium deposits under mixed or activation control. The morphology of the surface and cross-section were studied by SEM. The oxidation states and phases were studied by XPS and XRD, respectively, and discussed in view of the pulse parameters. By examining the effect of pulse parameters on composition and microstructure, we can deduce the practical range of pulse parameters for use in Ni–Re alloy film synthesis from dilute perrhenate electrolytes. Nickel-Rhenium alloys are unique materials due to their favorable mechanical and chemical properties [1]. They are exploited in various industries such as aerospace, aircraft, nuclear, electrical and catalysis. It was found that co-deposition of rhenium with iron-group metals (Fe, Ni, Co) can dramatically improve the efficiency and quality of the deposit compared to direct rhenium deposition from aqueous solutions [2–4]. Rhenium exhibits induced co-deposition with iron-group metals. The mechanism, composition, and microstructure of Ni–Re alloys have been studied by our group under a variety of d.c. conditions [5–7]. In brief, iron-group metals act as mediators in the reduction of the perrhenate ion. After the iron-group metal ion is electrochemically reduced on the surface, it can act as the reducing agent for the perrhenate ion to a lower valence oxide or to the metal atom. Application of a pulsed current, however, can allow for current densities, Faradaic efficiencies, compositions, and microstructures that are unattainable with d.c. plating [8,9]. Pulse plating has widely been used to increase the uniformity, adhesion, and speed of electrodeposition as well as to generate unique morphologies. There are only a few papers in the literature that discuss the pulse plating of rhenium [10]. We therefore wish to contribute to the fundamental knowledge of the formation of Ni–Re alloys in view of the unique transport parameters attainable under rotation, and the influence of a …