Electrodeposited nickel and nickel-yttrium oxide composite samples were studied in situ using scanning electrochemical microscopy (SECM). The monitored probe currents in phosphate-citrate buffer (pH 4.2) in the presence or absence of Ru(NH3)63+ as an oxidizing mediator near the Ni surface show that the SECM is a useful tool for study of the electrochemical activity of heterogeneous metal surface at micrometer scales. The SECM ultramicroelectrode probe tip provides information about the shape, activity and location of particles, such as Y2O3 introduced (co-deposited) in the Ni-matrix of the composite. Experiments show that the Ni-matrix in the composite coating is more active than the pure Ni-coating. This fact is expected, because of texture changes in the Ni structure upon introduction (by co-deposition) of Y2O3 particles. In the absence of mediator in the solution, the electrochemical activity of heterogeneous metal surface at a micro-level is investigated by using O2 concentration changes. The rate of reaction for O2 reduction was found to locally vary at electrodes floating at the open-circuit potential (o.c.p) when compared to an electrode potentiostatically polarized at a more positive potential than the o.c.p. This behavior suggests that local anode and cathode regions are being observed at the o.c.p. sample
