Effects of electrolysis conditions on the formation of electrodeposited invar Fe-Ni alloys with low thermal expansion

To elucidate the effects of the electrolysis conditions on the formation of electrodeposited invar Fe-Ni alloys with low thermal expansion, Fe-Ni electrodeposition was performed at 10-5 000 A·m⁻² and 5 × 10⁵ C·m⁻² in an agitated solution containing NiSO₄, NiCl₂, FeSO₄, H₃BO₃, C₇H₄NNaO₃S, and C₃H₄O₄ at 40°C-60°C. In the low-current-density region, the Ni content in the deposits significantly decreased with increasing current density, reached a minimum, and then increased after reaching the diffusion-limiting current density for Fe deposition. With the increasing concentration of FeSO₄ in the solution, the Ni content in the deposits decreased in the lower-current-density region, reached a constant at a moderate current density, and then began to increase at higher current density. As a result, the current density range in which the Ni content in the deposits reached a minimum and remained constant became wider with increasing FeSO₄ concentration. With the decreasing pH of the solution, since the partial polarization curve for H₂ evolution and the total polarization curve shifted to a higher-current-density region, the curve of the Ni content in the deposits as a function of the current density shifted toward a higher current density. The change in the composition of the Fe-Ni alloy deposits because of the electrolysis conditions can be explained by the changes in the total polarization curve and the partial polarization curves for Fe and Ni deposition and H₂ evolution.