Effect of grain size on oxidation behavior of oriented nickel coatings

Two kinds of Ni coatings with different grain size and preferred orientation were fabricated by controlling the cathode current density in the electro deposition procedure.Oxide film structure,morphology,phase composition and mass gain before and after oxidation of two Ni coatings with oxidation experiments carried out in air at 600 °C and 960 °C respectively were studied by transmission electron microscope(TEM),scanning electron microscopy/energy dispersive spectroscopy(SEM/EDS),X-ray diffraction meter(XRD) and thermo mass analyzer.The results show that,Ni coatings with the finer particle size of 120 nm and the stronger preferred orientation of 100 can be easier obtained at higher current density,while as,Ni coatings with the bigger particle size of 925 nm and the weaker preferred orientation of 111 can be easier obtained at lower current density.An important and interesting result is that after oxidation at 600 °C,the Ni coating obtained at lower current density has a better oxidation resistance,whereas after oxidation at 960 °C,the Ni coating obtained at higher current density has a more compact scale and a better oxidation resistance.For Ni coating obtained at low current density,the oxidation kinetic curves follows a parabolic law,while for Ni coating obtained at high current density,the oxidation kinetic curves follows a cubic law.The oxide film / metal interface is more curved for Ni coating obtained at low current density,compared with Ni coating obtained at high current density.