Instability of Ferrous Sulfate Bath for Electrodeposition of Nanocrystalline Iron Coating

Abstract

Nanocrystalline iron coatings were electrodeposited at six different current densities, from 1 to 25 A dm^–2 from a bath containing iron(II) sulfate at 60 and 80°C. The impact of the current density on the bath deterioration was investigated. The bath color changed from emerald green (485 nm maximum absorption wavelength, λ_max) to turbid yellow (λ_max = 470 nm) with some orange precipitates after electrodeposition up to 10 A dm^–2. Further electrodeposition up to 25 A dm^–2 changed the bath colour to dark brown (λ_max = 435 nm). Ultraviolet-Visible spectra were recorded to verify the bath deterioration, which, in turn, prevented the coating growth. The color change and also the shift of the maximum absorption wavelength were discussed in terms of the crystal field theory and also of the anodic oxidation of the hexaaquairon(II) ion to the hexaaquairon(III) ion on the anode surface. The Pourbaix diagram for iron was used to interpret these results and the instability of the bath which was utilized here for electrodeposition of nanocrystalline iron coatings. Addition of saccharin also led to the bath instability. The influence of the bath instability on the average grain size, appearance, surface morphology and thickness of the coatings was discussed. The results proved that the iron(II) sulfate solution is an unstable bath for electrodeposition of nanocrystalline iron coatings.