Effect of sweep rate on the active-passive transitions of the Ni/H 2SO 4 systems

Abstract

The passivation of Ni in 0·1 IN H 2SO 4 has been studied potentiokinetically at sweep rates ranging from 6·5 to 1000 mV/min, and it has been shown that the passivation potential becomes more positive with increase in sweep rate. It is suggested that above a certain critical sweep rate, the rate of diffusion of H 2O dipoles and OH − ions is insufficiently fast for the nucleation of nickel hydroxide, which is considered to be the precursor of true passivation, so that the nickel remains active at a potential at which passivation would normally occur at sweep rates less than the critical value. The passivation potentials at slow sweep rates correspond with the reversible potentials for NiO ⇌ Ni and NiO 2 ⇌ Ni at low and high potential, respectively. However, no evidence was obtained of passivation potentials that corresponded with the Ni 3O 4 ⇌ Ni and Ni 2O 3 ⇌ Ni reversible systems, and the values obtained experimentally fell between these two potentials.