Oxidation and H2O/NaCl-induced corrosion behavior of sputtered Ni–Si coatings on Ti6Al4V at 600–650 °C

The oxidation and H2O/NaCl-induced corrosion behaviors of Ti6Al4V with and without Ni–Si coatings were investigated. The Ni–Si coatings were prepared by magnetron sputtering from a Ni3Si target. The isothermal oxidation tests were performed at 650 °C for 100 h in ambient air, while the corrosion experiments were carried out at 600 °C for 10 h in O2/H2O mixture with NaCl deposit on the specimens. The as-sputtered coating was characterized using SEM (scanning electron microscopy) and XRD (X-ray diffraction). The specimens after oxidation/corrosion tests were studied using SEM/EDS (energy-dispersive spectrometry), XRD, EPMA (electron probe microanalysis) and XPS (X-ray photoelectron spectroscopy). The as-sputtered coating was dual-phase mixture of Ni5Si2 and f.c.c. Ni, which transformed into ordered Ni3Si during oxidation or corrosion. The Ni–Si coatings possessed excellent oxidation and corrosion resistances and eliminated the corrosion-induced crack which was observed in the uncoated Ti6Al4V specimens after 10 h of corrosion. External NiO scales along with amorphous silica precipitates formed on the coating surfaces during oxidation. It is suggested that the formation of silica involves into the oxidation of Ni–Si, disturbs the growth of NiO scale and leads the kinetics deviate from the parabolic law. In the case of H2O/NaCl-induced corrosion, external oxide scale on the coating surface, as well as internal oxide at the columnar boundaries of the coatings and at the coating/substrate interface, was observed. The external scales showed alternative lamellar structures, which consisted of lamellae of NiO, amorphous silica and metallic Ni, might result from the combination effects of multi-step reactions between Ni and NaCl/H2O/O2, the low diffusivity of Ni3Si and a displacement reaction between Si and NiO.