Study on properties of double-layered Ni–P–Cr composite coating prepared by the combination of electroless plating and pack cementation

The method of pack cementation was applied to prepare a chromized layer on the surface of an electroless Ni-P coating to further improve its mechanical property and corrosion resistance, and then the microstructure, chemical composition, and performance of the achieved coatings were investigated. The results indicated that the double-layered Ni-P-Cr composite coating was composed of an inner Ni-P layer and an outer Cr-Ni-P layer. Meanwhile, the thickness of the composite coating become larger and more Cr was found as chromizing time increased. By comparison, the double-layered coating with the chromizing time of 9 h showed a higher hardness than electroless Ni-P coatings and double-layered coatings with a shorter chromizing time. In addition, all the double-layered coatings generally displayed higher corrosion resistance than electroless coatings. Also, increasing chromizing time was beneficial to reduce the defects of the double-layered coatings ascribed to the dense oxide film, and thus improved corrosion resistance. •Novel double-layered Ni-P-Cr composite coatings were prepared and studied.•An inner Ni-P layer and an outer Cr-Ni-P layer was found in double-layered coatings.•Double-layered coatings displayed an increasing hardness as chromizing time lasts.•Double-layered coatings showed higher corrosion resistance than electroless coatings.•Fewer defects were found in double-layered coating with a longer chromizing time.