Effect of additives on the deposition behavior and micro structure of invar Fe–Ni alloys with low thermal expansion electrodeposited from Watt’s solution

The electrodeposition of Invar Fe–Ni alloy with low thermal expansion was performed at 100–5 000 A·m⁻² and 10⁵ C·m⁻² in agitated Watt’s solutions containing NiSO₄, NiCl₂, FeSO₄, malonic acid (C₃H₄O₄), saccharin sodium (C₇H₄NNaO₃S), and H₃BO₃ at 50°C. With increasing concentration of malonic acid, the Ni content in deposits formed at current densities greater than 2 000 A·m⁻² decreased, whereas the Ni content increased in deposits formed at current densities less than 1 000 A·m⁻². The current efficiency for alloy deposition decreased with increasing concentration of malonic acid. The deposits were composed of granular crystals whose size decreased with increasing concentration of malonic acid. With the addition of saccharin, the Ni content in the deposits decreased substantially, and the current efficiency for alloy deposition increased. With the addition of boric acid, the Ni content in the deposits somewhat decreased, and the current efficiency for alloy deposition increased. The surface morphology of deposits changed as the current density was varied and with the addition of saccharin, whereas it rarely changed with the addition of boric acid. The morphology was found to depend on the Ni content in the deposits. The deposits with a Ni content of 29–38 mass% were composed of granular crystals approximately 300 nm in size, whereas the deposits with a Ni content of 41–52 mass% exhibited a smooth surface that consisted of fine crystals. The effects of additives on the Ni content in deposits and current efficiency can be explained by the changes in the partial polarization curves for Fe and Ni depositions and H₂ evolution during Fe–Ni alloy deposition.