AN ELECTROCHEMICAL IMPEDANCE STUDY OF THE PASSIVE FILM ON SINGLE-CRYSTAL NI(111) IN PHOSPHATE SOLUTIONS

An electrochemical impedance study of the passive film on single crystal Ni(111) in 0. 1N phosphate buffer solution as a function of potential (0. 1-0. 7 V//S//C//E) and pH from 7 to 12 is reported. At low frequencies, the interfacial impedance is dominated by a Warburg component, which is attributed to the transport of vacancies across the film. The product I//d//c sigma , where sigma is the Warburg coefficient and I//d//c is the steady-state passive current, is found to be independent of the applied voltage and pH. This finding is consistent with the electric field strength also being independent of these same parameters. The impedance data have been used to estimate the diffusivity of oxygen vacancies within the film, yielding a value of D approx. 2. 2 multiplied by 10** minus **2**0 cm**2/s.