The anomalous behavior of Al3+ in nickel electrowinning from sulfate electrolytes

The effect of varying concentrations of Al³⁺ on the morphology and current efficiency of electrodeposited nickel from an acid-sulfate electrolyte was studied. Concentrations of Al³⁺ of 20 to 100 ppm significantly degraded the nickel-cathode morphology and current efficiency. However, it was found that higher concentrations of Al³⁺, in the range of 1 to 5 g/L, depending on the electrolyte parameters and operating conditions, improved the deposit morphology compared to deposition from a pure electrolyte. However, the current efficiency remained low. Physical characterization of the nickel revealed that a smoother, more uniform surface resulted from deposition in the presence of 1 g/L or greater of Al³⁺. Impedance analysis indicated the presence of an inhibiting layer on the cathode when low concentrations of Al³⁺ were present. Concentrations of Al³⁺ greater than 1 g/L appeared to modify this layer and the nickel-deposition mechanism, allowing improved nickel morphology and structure.