Abstract
Aluminum was anodized for different durations in a 13M H2SO4 solution (20°C) at constant voltages of 30, 35, 40 and 45V.
It was possible to deposit nickel uniformly into the pores of the oxide films formed without any pretreatment. Electrodeposition did not take place with films formed in dilute H2SO4 solutions. The behavior and mechanism of the electrodeposition in relation to film structure were examined and the results are summarized as follows:
Film structure: Reanodizing behavior of the specimens in a neutral borate solution suggests that the 13M H2SO4 films have numerous micro-voids in the barrier layer. The formation of the microvoids, which probably penetrate through the oxide to the substrate metal, is caused by incorporation of SO42- ions in the oxide and their partial removal when the film is washed with water.
Deposition behavior: Electrodeposition was carried out in two steps in a solution of 70g·dm-3 NiSO4⋅7H2O-30g·dm-3 H3BO3 (20°C) by applying a small cathodic current of 0.05A·dm-2 for 15min, followed by a current of 1.45A·dm-2 for 45min.
The first step with the small current is essential to form nickel nuclei in the micro-voids, and a uniform deposit was obtained only by the two step procedure. The maximum thickness of the uniform deposit increased with the voltage at which films formed. The distribution of nickel in the pores and the surface of films was observed by XMA.
