| Nanocrystalline Ni-Co alloy coating had good corrosion resistance, wear resistance and beautiful appearance, it could be applied as a kind of decorative and protective coating. A variety of methods could be adopt to obtain nanocrystalline Ni-Co alloy coating, such as molecular beam epitaxy (MBE) and sputtering. It could be also prepared by the method of electrodeposition, which did not need the vaccum environment. Electrodeposition also had the advantages of fast, low cost, easy to control and so on. It could be divided into DC electrodeposition and pulsed electrodeposition according to the type of power supply, and pulsed electrodeposition could be divided into single pulsed electrodeposition and reverse pulsed electrodeposition. Comparing to DC and single pulsed electrodeposition, the most unique advantage of reverse pulsed electrodeposition was the reverse pulsed current (RPC), it could effectively dissolve the burrs on the coating surface and smooth it, it could also eliminate the hydrogen embrittlement, reduce the internal stress and enhance the adhesion.In this paper, the method of reverse pulsed electrodeposition was employed to prepare nanocrystalline Ni-Co alloy coating, and the influence of pulsed current parameters, electrolyte temperature, electrolyte composition on the structure of nanocrystalline Ni-Co alloy was investigated. X-ray Diffraction (XRD) was adopt to measure the structure of Ni-Co alloy, the grain sizes and intensity of different peaks were calculated. The results indicated the face-centered cubic(fcc) structure of nanocrystalline Ni-Co alloy electrodeposited by RPC,(111) and (200) were the main crystal orientations, and (220),(311) of fcc structure were rarely observed. The results of orthogonal test of RPC parameters suggested that the influence of five RPC parameters on grain size of nanocrystalline Ni-Co alloy had such a sequence:γ+>J+>y.> T+> T-. To prepare the nanocrystalline Ni-Co alloy with minimum grain size, the optimal RPC parameters were set as follows:γ+80%, J+2.0A/dm2, T+60～80ms, γ-20%～40%, T.0.1ms～0.2ms. The electrolytes containing different cobalt content were made up, and prepared nanocrystalline Ni-Co alloy respectively, the mole fractions of cobalt in the alloy range from6% to63%. The XRD patterns indicated that the intensity ratio I(111)/I(200) tended to increase with the increasing of cobalt content in the alloy, the preferred orientation gradually changed from (200) to (111). When the Co content in the coating was lower than15%, the grain sizes were17～19nm, as the mole percentage of Co was higher than47%, the grain size reduced to6～8nm.Adopting linear sweep voltammetry (LSV) to determine the polarization curves of electrolyte in temperatures range from20℃to80℃, the results show that the cathodic polarization weakened with the increase of electrolyte temperatures. XRD indicated the trend of grain size increasing with the raising of electrolyte temperatures, and the preferred orientation changed from (111) to (200). The enlargement of cathodic polarization could lead to the grain size refinement of crystal in electrodeposition, and the XRD measurement was corresponding to the LSV result. Ni-Co alloy coatings were prepared by electrolytes with saccharin concentration from0to5g/L respectively. XRD result indicated that saccharin significantly reduced the grain size of nanocrystalline Ni-Co alloy, while as the saccharin concentration ranged from1g/L to5g/L, the grain size didn’t change much. In addition, the results of LSV test and XRD measurement suggested that both2-Butyne-1,4-diol and sodium dodecyl sulfate (SDS) had very little effect on the cathodic polarization and grain size.The Tafel polarization measurement of Ni-Co alloy prepared in orthogonal test indicated that the smaller grain size led to more positive corrosion potential Ecorr, it proved that the nanocrystalline Ni-Co alloy with smaller grain size was more corrosion resistant. As the grain sizes of nanocrystalline Ni-Co alloys were smaller than20nm, the average value of polarization resistance Rp was about202000Ω·cm2, while the grain sizes were bigger than20nm, the average value of Rp was about73200Ω·cm2, this indicated that the smaller grain size of nanocrystalline Ni-Co alloy led to lower corrosion rate in3.5%NaCl. As the saccharin concentration and cobalt content in alloy increased, Ecorr shifted to more positive potential, and the corrosion resistance was enhanced. The increase of cobalt content in Ni-Co alloy could perform better corrosion resistance, it was due to the more easily formation of passive film composed of Co(OH)2colloid on the surface of Ni-Co alloy. The surface morphology of the coatings was observed by scanning electron microscope (SEM), the images showed the pitting corrosion of Ni-Co alloy with low cobalt content, while the high cobalt content led to stress corrosion. The pitting corrosion of nanocrystalline Ni-Co alloy could be divided into three stages:passive film damage, formation of holes and the expanding of pitting corrosion. The adsorption of Cl-on the film surface could cause the damage of passive film, and as the corrosion holes were formed, the corrosion battery was the motive of pitting corrosion to dig deeper. |
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