THE REPASSIVATION KINETICS OF PURE NICKEL IN A NA2SO4 SOLUTION USING ABRADING ELECTRODE TECHNIQUE

The repassivating kinetics of Ni have been investigated by the abrading electrode technique. The early stage of repassivation kinetics of pure nickel have been studied as a function of applied potential and Cl- ion concentration in 0.75 M Na2SO4 solutions of pH 2.8. The applied potential and Cl- ion concentration ranged from -200 to 600 mV(sce) and from 0 to 0.2 M, respectively. Transition time t(T) at which current decay transients intersect for given applied potentials decreased with increasing applied potential. In the absence of the Cl- ion, the current decay rate increased with increasing applied potential from 0 to 600 mV(sce). As the Cl- ion concentration increased at a given applied potential, the current density increased below t(T), whereas it decreased above t(T). The repassivation kinetics with respect to the Cl- ion have been discussed in terms of competitive adsorption of Cl- and OH-/O2- ions. As a result it is suggested that below t(T), the increased current density by the addition of Cl- ion is caused by the preferential adsorption of Cl- ion and the inhibition of the adsorption of OH- ion on the nickel surface. Above t(T), the decreased current density by the addition of Cl- ion is based upon the fact that the adsorbed Cl- ion facilitates the place exchange reaction between metal and hydroxyl ions, and hence the film formation.