Research On Anticorrosive Property Of Electroless NiCuP And Coating Passivation

Due to excellent comprehensive properties of uniform deposition, electrical conductivity, wear-resistance and anticorrosive performance, electroless NiP has been widely used in every walk of life, such as petrochemical industry, industrial machinery, aerospace, electric and electronic. However, in the field of corrosion protection, electroless NiP still cannot replace electrodeposited chromium; in the field of decorate, corrosion and oxidation lead to the discoloration of electroless NiP, which affects the appearance; in the field of electronic and microelectronics, the oxide film reduces the electrical conductivity and solderability of electroless NiP. Therefore, it is necessary for improving the anticorrosive property of electroless NiP. In this research, two methods were employed for improving the anticorrosive property of electroless NiP. The first method was adding copper ions into neutral electroless plating solution and obtained electroless NiCuP coating. The relationship between Cu content and corrosion resistance, and the effect of Cu on anticorrosive performance has been investigated systematically. The second method was respectively fabricating chromate passive film, manganate passive film and manganate/silicate composite passive film on the coatings, the formation mechanisms and anticorrosive properties were investigated. The conclusion as follows:(1) As the increase of copper ions concentration in electroless plating solution, the deposition rate increased at first and then decreased; porosity gradually increased; hemispheric structures became bigger and smoother; phosphorus content gradually decreased; the coating exhibited the trend of change from amorphous to crystal. The result of potentiodynamic polarization curve and EIS pattern showed that NiCuP-1, which Cu content was 6.3 wt.%, exhibited higher anticorrosive property than NiP, however, the corrosion resistance of NiCuP-2 and NiCuP-3, which Cu content were 11.2 wt.% and 26.7 wt.% respectively, were not as good as NiP. The analysis suggested that the corrosion resistance of coating was affected not only by Cu content, but also by P content, morphology, et al., NiCuP-1 was the better one under the comprehensive function of many factors. The Mott-Schottky curves showed that the native oxide film on NiP and NiCuP-1 exhibited semiconductor character of n type. The value of ND for NiCuP-1 was smaller than NiP, suggesting that the native oxide film on NiCuP-1 exhibited better anticorrosive performance, which leaded to higher corrosion resistance of Ni CuP-1.(2) After chromate passivation treatment, the chromate passive film(CPF) were fabricated on the surface of NiP and NiCuP, which were named as CPF-NiP and CPF-NiCuP. The thickness of CPF-Ni P and CPF-NiCuP were ultrathin, about 30 ~ 40 nm, therefore there was no effect on the metallic luster and surface morphology of the coating. The main compositions of CPF-NiP and CPF-NiCuP were Cr2O3 and Cr(OH)3, for CPF-NiP, the molar ratio of Cr2O3 to Cr(OH)3 was 52:48, however, for CPF-NiCuP, the molar ratio of Cr2O3 to Cr(OH)3 was 72:28. Obviously, the Cr2O3 content of CPF-NiCuP was higher, indicating that CPF-NiCuP was more compact than CPF-NiCuP. The analysis suggested that the composition difference of CPF was attributed to Cu in NiCuP coating, which played a role on accelerating the passivation reaction. The result of contact angle test showed that the hydrophobic property of CPF-NiCu P was higher than CPF- NiP, which was one of the reasons for CPF-NiCuP exhibiting better anticorrosive property than CPF- NiP. Potentiodynamic polarization curve and EIS pattern showed that the polarization resistance and impedance of CPF-NiCuP were two times higher than that of CPF-NiP, suggesting CPF-NiCuP can better prevent the coating from ion erosion. The result of neutral salt spray test indicated that CPF can promote the corrosion resistance, oxidation resistance and anti-tarnish resistance of NiP/NiCuP in serious corrosion environment.(3) The manganate passive film(MPF) and manganate/silicate composite passive film(MSPF) were respectively fabricated on the surface of NiP. MPF and MSPF were ultrathin, about 36 ~ 48 nm, therefore there was no effect on the metallic luster and surface morphology of the coating. The main compositions of MPF and MSPF were Ni(OH)2, NiO and MnO2. Ni(OH)2 and NiO were the oxidation products of Ni on the surface of coating, MnO2 was reduction products of KMnO4. However, on the outermost layer of MSPF, the detection of Si-O bonds component indicated an outer layer of polymeric silicate formed on the surface of MSPF. The analysis suggested that the outer layer of polymeric silicate acted as a physical barrier to inhibit the transfer of ions and block the cracks of passive film, therefore potentiodynamic polarization curve and EIS pattern showed that the corrosion resistance of MSPF was higher than MPF. The result of neutral salt spray test indicated that MSPF can promote the corrosion resistance, oxidation resistance and anti-tarnish resistance of NiP in serious corrosion environment.