Characterisation of electrodeposited Co–W–P amorphous coatings on carbon steel

Electrochemistry and nucleation mechanism of induced co-deposition of cobalt, tungsten and phosphorus from a citrate bath was investigated using voltammetry and chronoamperometry techniques. It was found that the induced co-deposition of the species occurred under diffusion control and followed instantaneous nucleation mechanism. Co–W–P coatings were electrodeposited potentiostatically on plain carbon steels from a citrate bath containing CoSO4, Na2WO4 and NaH2PO2. X-ray diffraction studies of the coatings revealed that the as-deposited Co81W10P9 coatings had amorphous structure. The formation of some stoichiometric compounds like Co3W, Co2P and WP2, however, was observed upon annealing at 600 °C. The hardness of the Co–W–P coatings increased with annealing temperature which was possibly due to the formation of these inter-elemental compounds. The corrosion resistance of the Co–W–P coatings increased with increase in annealing temperature which might also be due to the formation of the stoichiometric compounds at elevated temperatures. A comparison between coating characteristics of Co–W–P and chromium showed that the Co–W–P coatings exhibited nobler corrosion potential than chromium coatings.