Nanocrystalline copper oxide (CuO) powder of varying sizes (22, 25, 28 and 36 nm) have been successfully synthesized by hybrid electrochemical method using aqueous sodium nitrate electrolyte with Cu electrodes under galvanostatic mode at room temperature. The as-synthesized CuO sample was calcined for an hour at temperatures ranging from 60 to 900 °C. The crystallite size, morphology, and chemical state of the synthesized powders were characterized by powder XRD, XPS, SEM/EDAX, TEM, and UV–Vis spectral methods. The effect of calcination temperature on crystallite size and morphology was studied. The TEM result revealed that, the particles are hexagonal and the sizes are in 30–50 nm in diameter and 120–200 nm in length. The band gap values are 5.60 and 5.54 eV. The crystallite size increased with increase of calcination temperature. The CuO nanopowder is used to fabricate Zn–Ni–CuO composite thin films and its corrosion behaviour was analysed by Tafel extrapolation and electrochemical impedance spectroscopy. The results indicate that the Zn–Ni–CuO composite thin films provided good corrosion protection.