Study Of Corrosion Behavior And Mechanical Properties Of NiWP Duplex Deposits

Materials with high properties, such as corrosion resistance and mechanicalproperties, etc, are needed with the development of science and technology. Existingmaterials often cann’t meet the application requirements. The electroplate techniquefor film deposition can improve the corrosion resistance of materials without affectingtheir mechanical properties, thus expanding their applications. However, deposits mayfall off or bubble because of considerable differences in the structures andcharacteristics between deposits and substrates. New concept of multilayer deposits,which possess a more gentle transition of the structures and characteristics betweendeposits and substrates, is put forward in order to figure out this problem, with theimprovement of corrosion resistance.The NiWP alloy has been widely used in electronics, mechanical components,petroleum chemical, automobile, aerospace fields because of its good corrosionresistance, high hardness, fine wear resistance and high thermal stability.To enhance the corrosion resistance of NiWP alloys for their applications inextreme conditions, NiWP duplex deposits of different compositions with a thicknessof about22μm and inner and outer layer thicknesses of about11μm each were firstelectrodeposited. With the same thickness of82Ni13W4P monolayer film ascomparison, we studied the corrosion behaviour and mechanical properties of NiWPduplex deposits. Results showed that NiWP duplex deposits provided better corrosionresistance and mechanical properties than82Ni13W4P monolayer film. The moreresearch work was as follows:(1) a series of experiments were carried out to study the preparation, corr-sectionmorphology, corrosion behaviour and mechanism of NiWP duplex deposits. Scanningelectron micrograph showed that a dense and good adhesive strength between both thelayers was accomplished, the tungsten contents increased gradually in the direction ofthe thickness, while the nickel and phosphorus contents decreased accordingly.Results of3.5wt%NaCl solutions immersion test and neutral salt spray test showedthe corrosion rate of NiWP duplex deposits was less than that of82Ni13W4Pmonolayer film. That is to say NiWP duplex deposits provided better corrosionresistance than82Ni13W4P monolayer film, and82Ni13W4P/89Ni3W7P NiWPduplex deposits possess the best corrosion resistance.(2) The mechanism of corrosion resistance NiWP duplex deposits were studied according to the structures and electrochemical properties of NiWP alloys of differenttungsten content. XRD showed the structure terned turned into crystalline state fromamorphous state, with the increase of tungsten content. The stagger growth of twolayers can cover defects such as cracks and porosities each other in the NiWPduplex deposits. With the increase of W content, the corrosion potential of NiWPalloys is more and more negative in3.5wt%NaCl solution. When corrosion potentialdifference is140mV between two sublayers in the NiWP duplex deposits. It willprovide the substrate electrochemical protection.(3) It was studied that the influence of82Ni13W4P monolayer film andNi/82Ni13W4P duplex deposits on the mechanical properties of35CrMo sucker rods.The results showed that82Ni13W4P monolayer and Ni/82Ni13W4P duplex depositscan increase mechanical properties of35CrMo sucker rods in different degrees.Compared with the82Ni13W4P monolayer film, Ni/82Ni13W4P duplex deposits canbetter improve the tensile performance of35CrMo sucker rods. However, less thanthe bending performance than82Ni13W4P monolayer film.