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 100-5000 A.m(-2) and 10(5) C.m(-2) in agitated Watt's solution containing NiSO4, NiCl2, FeSO4, C3H4O4, C7H4NNaO3S and H3BO3 at 50 degrees C. With increasing the concentration of malonic acid, the Ni content in deposits decreased at higher current density region than 2000 A.m(-2), while it increased at lower current density region than 1000 A.m(-2). The current efficiency for alloy deposition decreased with increasing the concentration of malonic acid. The deposits were composed of granular crystals whose size decreased with increasing the concentration of malonic acid. With an addition of saccharin, the Ni content in deposits significantly decreased, and the current efficiency for alloy deposition increased. With an addition of boric acid, the Ni content in deposits somewhat decreased, and the current efficiency for alloy deposition increased. The surface morphology of deposits changed with the current density and an addition of saccharin, and rarely changed with an addition of boric acid. It was found that the morphology depended on the Ni content in deposits. The deposits with Ni content of 29-38 mass% were composed of granular crystals approximately 300 nm in size, while the deposits with Ni content of 41-52 mass% showed the smooth surface consisted of fine crystals. The effects of additives on the Ni content in deposits and current efficiency can be explained by the change of partial polarization curve for Fe and Ni deposition and hydrogen evolution in Fe-Ni alloy deposition.