High corrosion-resistant Ni–P/Ni/Ni–P multilayer coatings on steel

Multilayer coatings having a thickness about 20 μm and consisting of different Ni–P and Ni layers were prepared by combining electrodeposition with electroless deposition. The microstructure of the coatings was analyzed by scanning electron microscope (SEM) and X-ray diffractometer (XRD). The corrosion resistance of the coatings was estimated by electrochemical polarization measurements and salt spray test. The salt spray test showed that the three-layer coating, whose composition is Ni–P (low phosphorus)/Ni/Ni–P (high phosphorus) from surface to substrate, exhibited the highest corrosion resistance. The time of the emergence of the first red rust spot on the coating surface can reach 936 h, which is 3.5 times higher than that of the common amorphous Ni–P alloy coatings. The electrochemical analysis revealed that the difference in the corrosion potential among layers plays a very important role in protecting the substrate from rusting.