Study on corrosion behaviour of nanocrystalline and amorphous Co–P electrodeposits

In this study, corrosion properties of nanocrystalline and amorphous Co–P coatings prepared using dc electrodeposition were investigated. The morphology of amorphous coatings was smoother and brighter than nanocrystalline coatings with 'cauliflower' morphology. Tafel polarisation tests of the coatings in 0·1M H2SO4 solution revealed that the amorphous coating had better corrosion resistance than the nanocrystalline one. Corrosion resistance of both coatings decreased after annealing. On the nanocrystalline coating, corrosion attacks were localised around the nodules on the surface while a more uniform type of attack was observed on amorphous coatings. Neither nanocrystalline nor amorphous coatings showed passivation in 0·1M H2SO4 solution, but both of them showed an active–passive behaviour in 10%NaOH solution due to the formation of Co(OH)2 as fine hexagonal shape products which acted as a passive layer. The passive current density of the amorphous coating was higher than that of the nanocrystalline one, but it was decreased markedly by annealing. However, annealing had no significant effect on the passivation behaviour of the nanocrystalline coating.