Electrodeposition of protective Ni–Cu–Mo coatings from complex citrate solutions

Ni–Cu–Mo ternary alloy deposition was investigated in order to obtain protective corrosion-resistant coatings. The present work is aimed at the determination of the influence of co-discharge kinetics on certain physicochemical properties of Ni–Cu–Mo coatings electrodeposited from complex ammonia–citrate baths. A schematic diagram of partial polarisation curves as well as the coating composition as a function of the cathode polarisation is presented. Based on the dominant species in the baths examined and the polarisation behaviour, the main reduction reactions in four ranges of cathode potential have been defined. A strong correlation between alloy composition and cathode potential was observed, due to the Ni–Cu–Mo electrocrystallisation kinetics being dominated by the induced Ni–Mo co-deposition. Bright compact metallic layers with a regular nodular morphology were obtained at more negative potentials where the nickel discharge is the predominant reaction. The X-ray diffraction patterns of all electrodeposited Ni–Cu–Mo coatings show the fcc structure of a Ni–Cu solid solution. The coatings show corrosion resistance similar to bulk commercial Monel alloy.