Investigation of electrochemical co-deposition of zinc and molybdenum from citrate solutions

Highlights: ► Zn–Mo alloy electrodeposition from citrate baths have been studied. ► It was found that it is possible to electrodeposit Zn–Mo alloy layers from aqueous citrate solutions. ► The optimal conditions for electrodeposition from studied citrate baths are in the narrow pH range of between 4 and 6. ► XPS measurements confirm the presence of metallic molybdenum in obtained deposits. ► The mechanism of Zn–Mo co-deposition has been proposed and discussed. -- Abstract: The co-deposition of molybdenum with zinc from citrate electrolytes was studied. Cyclic voltammetry with rotating disc electrode experiments and potentiostatic deposition in coulostatic conditions were carried out in order to analyse the mechanism of electrodeposition of molybdenum with zinc. The results of voltammetric studies indicate Zn and Mo interaction during the electrodeposition process. The electrodeposits were characterised by wavelength dispersive X-ray fluorescence (WDXRF), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Possibility of Zn–Mo alloy layers electrodeposition from citrate solutions was confirmed. The molybdenum content in metallic components (Zn–Mo alloy/compound or Mo) on the surface of deposit was close to 70% of the total molybdenum amount. The mechanism of the co-deposition of molybdenum with zinc has been proposed. It was found that the reaction of molybdenum deposition is limited by the presence of adsorbed Zn(I)-citrate ions, which are formed in the outer-sphere charge transfer mechanism. Further reduction processes are related to the formation of adsorbed mixed Zn(I)–Mo(VI)–citrate complexes, then reduced to metallic molybdenum and metallic zinc during a complicated, multi-stage process