Oxidation behaviors of electrodeposited nickel-cobalt coatings in air at 960 °C
(State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China)
Abstract: Ni coating and Ni-Co alloy coatings were produced by adjusting the composition of the plating solution using a direct current electrodepositing process. The oxidation behaviors of nickel and nickel-cobalt alloys in air at 960 °C were studied by thermogravimetric (TG) analyzer and then the formed oxide scales were examined by scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), X-ray diffractometry (XRD), and Raman spectroscopy. The scale morphologies, composition, grain size and mechanism of oxidation were discussed in detail. The results show that oxidation rates of Ni, Ni-7%Co (mass fraction) and Ni-15%Co generally follow parabolic relationship, whereas that of Ni-30% Co alloy follows cubic relationship. The higher the Co content of the alloys is, the faster the oxidation rate is. Metal concentration profiles reveal cobalt depletion in the alloy surface beneath oxide scales, and a progressive enrichment in cobalt towards the outer surface of the scale.
Key words: nickel-cobalt alloys; electrodeposition; high temperature oxidation