Effects of electrolyte composition and additives on the formation of invar Fe–Ni alloys with low thermal expansion electrodeposited from sulfate bath

The effects of solution composition and additives on the formation of electrodeposited invar Fe–Ni alloys with low thermal expansion were investigated. In all solutions, increasing the current density from 10A·m ⁻² significantly decreased the Ni content in the deposits and resulted in anomalous codeposition, in which the electrochemically less noble Fe was preferentially deposited. Further increasing the current density increased the Ni content in the deposits as Fe deposition reached the diffusion limitation of the Fe ²⁺ ions. Increasing the concentration of Fe ²⁺ ions increased the current density required to initiate an increase in the Ni content in the deposits because of increase in the diffusion-limited current density of Fe. With the increasing concentration of malonic acid, the current density region in which Ni deposition was suppressed was extended, and the potential at which Fe deposition reached the diffusion limitation of the Fe ²⁺ ions was shifted to a less noble direction. As a result, the relationship between the Ni content in the deposits and the current density shifted to a higher-current-density region with increasing malonic acid concentration. At malonic acid concentrations above 0.05mol·dm ⁻³ , the current efficiency for alloy deposition was greatly reduced owing to the promotion of hydrogen evolution. The Ni content in the deposits was significantly increased with the addition of thiourea. Boric acid somewhat increased the Ni content in the deposits in the lower-current-density region. In solutions containing both boric acid and saccharin, an invar alloy of 36 mass% Ni was obtained in the wider-current-density region.