Abstract
Impedance measurements, potentiostatic quasi steady state polarization data and scanning electron microscope studies were used to investigate the dissolution of nickel in 0·5–18·8M sulphuric acid. An active-passive transition was observed with a single-stage reaction to divalent nickel occurring in the active region. The activation energy associated with the critical passivating current density is 59 kJmol −1 and it is suggested that this is due to the rate-determining step being charge transfer through the double layer. The formation of the passivating film was shown to be associated with a solid state mechanism. The impedance data show that (i) The surface films become thinner as the acid concentration increases. (ii) There is no gross structural change in the film at 60°C as suggested by previous workers. Breaks at 60°C in Arrhenius plots of the current density in the passive region must be associated with changes in the mechanism./lt
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