Crystallization and electrochemical corrosion behaviors of nanometer amorphous Ni-P alloys

Ni-P alloy coatings with a phosphorus content of 11.65 wt% were deposited by electroless plating. The as-plated nano-amorphous Ni-P alloy coatings were heated in air for 1 h at 200–500 °C. By using an atomic force microscope and the x-ray diffraction method, the nanocrystalline morphology and the crystallization phase identification of the coatings were studied. The corrosion behavior of the coatings in the 3.5 wt% NaCl solution was analyzed using polarization curves and impedance spectroscopy. It was observed that the corrosion behavior of the coatings was affected by the change in the nanocrystalline structure; during the heat treatment, the nanocrystalline structure of the coating alloy undergoes sequential changes from amorphous → the adjustment of amorphous → precipitated phase → crystalline. Meanwhile, the corrosion behavior of the coating alloy has also changed from uniform corrosion controlled by charge transfer → uniform corrosion controlled by diffusion → grain boundary corrosion. The Ni phase and the Ni3P phase developed a compact and dense nanocrystal layer on the surface of the coating alloy, in which the best performance by the corrosion resistant coating can be found.