The early stages of Re-Ni alloy deposition from citrate electrolytes were studied in terms of the apparent faradaic efficiency (FE) of the deposition process, the composition of the deposits and their morphology. The deposition time was varied in the range of 0.05 to 60 seconds. The apparent FE of the deposition process was calculated as the ratio between the charge consumed in electrochemical stripping of the deposit and the charge consumed during electrodeposition. The apparent FE and Re-content in the deposits (measured by energy dispersive X-ray spectroscopy) were shown to decrease with deposition time. Anomalous values of the FE, well above 100%, were observed at short deposition times. Under these conditions, the deposition of material consisting of nanoparticles with diameters of 50-80 nm was observed by scanning electron microscopy studies, supporting the anomalous high deposition rates. This behavior is indicative of chemical reactions taking place at short times in parallel with electrodeposition. The effects of deposition time on the apparent FE and Re-content in the deposits are explained by the changes in catalytic activity of the cathode surface during deposition. Electroplating and electroless plating of Re-based alloys has been investigated in our laboratory in recent years.1-7 Rhenium alone can be deposited, but the faradaic efficiency (FE) is very low and the layer formed is of poor quality and does not adhere well to the substrate. Addition of salts of one of the iron-group metals, (Me = Ni, Co or Fe) leads to high FE and high Re-content of the alloys formed. When alloy deposition is concerned, there are, by definition, at least two reactions occurring in parallel. Moreover, hydrogen evolution is commonly observed. In the particular case of electroplating of Re-Ni alloys, it is common to use citric acid as a complexing agent. This can give rise to several complexes of Ni with the citrate ions, depending on pH, as well as complexes of Re with citrate, and even a triple complex of Re-Ni-Cit. The overall reaction for deposition of Re is given by Eq. 1, in which 7 electrons are involved.Thus, a detailed evaluation of all the steps would be extremely difficult, if at all possible. In several papers in the literature it was shown that this reaction follows the stepwise reduction of the ReO − 4 ion. Deposition of Re from acidic electrolytes was shown to proceed through formation of ReO 2 as an intermediate, followed by the complete reduction to the metal. Possible disproportionation of one of the intermediates has also been suggested as a route for electrodeposition. 8-10The intriguing question is to understand the role of the divalent ion (Me) in inducing the deposition of metallic Re. In our recent papers we proposed the mechanism shown in Eqs. 2 and 3: According to this mechanism, freshly deposited Ni 0 catalytically reduces the perrhenate ion from its complex with citrate to the 5-valent oxidation state in the rhenate ion, ReO

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The electroplating of Re-Ni alloys in citrate electrolytes of different compositions was studied by cyclic voltammetry and by anodic stripping voltammetry, following galvanostatic deposition. The faradaic efficiency of the deposition process and the composition of the deposits were determined. Increasing the concentrations of either the nickel or the perrhenate ion in the electrolytes was shown to enhance the rate of deposition of the Re-Ni alloy. The process is influenced by mass transport and by concentration of citric acid in the electrolytes. Conductometry, UV-Vis and Raman spectroscopy were used for studying the ionic interactions in electrolytes. No evidence of the existence of perrhenate complexes with other components of the electrolyte was found. However, an increased deformation of the perrhenate ions with increasing molar ratio of [NiCit] − /ReO − 4 was clearly shown by Raman spectroscopy. Hence, a weak interaction is definitely observed, which is enough to distort the shapes of the ReO − 4 and [NiCit] − or [NiHCit] species, thus enhancing the deposition rate of the Re-Ni alloy. The induced co-deposition is believed to be a catalytic process, including the stage of simultaneous reduction of ReO − 4 and [NiCit] − or [NiHCit] species, which influences each other by weak interaction.Rhenium and its alloys are used for different purposes where stability at high temperatures is important, such as aerospace and nuclear industries, as well as in catalysis, mainly for the petroleum industry. It also has applications as coatings. We have published a detailed listing of its unique properties and applications. 1 Electrodeposition of Re itself is very difficult, and the resulting deposits are usually non-uniform and tend to peel off readily. However, addition of the salts of Ni, Co or Fe catalyzes the deposition of the respective alloys. A typical plating bath consists of ReO − 4 , Ni(SO 3 NH 2 ) 2 and citrate. 2-10 The mechanism of induced co-deposition of Re with one of the above iron-group metals has been studied recently in our research group, 5-8 with emphasis on the role of Me (Me = Ni, Co, or Fe). The composition of the alloy deposited could be controlled by the relative concentration of the three components of the bath, the pH of the solution, the temperature and the current density applied. The highest Re-content achieved was 93 at.% and the highest faradaic efficiency (FE) was 96% in different compositions of the baths in the above publications. A leading role of freshly deposited Ni in catalytic deposition of Re was observed. [5][6][7][8]11 . On the other hand, a simultaneous discharge of Ni and perrhenate ions was shown to be responsible for the increase in the partial current density of the Re deposition. 4 The concept of simultaneous discharge is supported by the active role of perrhenate in nickel co-deposition. In fact, no Ni is deposited from the Ni electrolytes with high citrate to Ni 2+ molar ratio (of about 4.2), 12 while about 20 at.% Ni is found in the deposits produced in electrolytes with t...