THE KINETICS OF INHIBITOR ADSORPTION ON IRON

The kinetics of adsorption of Cl- and Br- ions and 2-naphthol-3,6-disulphonic acid (NDSA) on Armco iron electrodes cathodically polarized in 0.5 M H2SO4 was carefully analyzed. Contrary to the initial expectation that the adsorption of these species should be faster on such cathodically protected surfaces as compared to the rates of adsorption during anodic polarization when the metal surface changes permanently, the obtained adsorption rate constants were about 10(-2) mol(-1) dm3 s-1 both for halogen ions and NDSA with a small dependence on electrode potential for halogen ions. The kinetics were analyzed by using a computerized curve fitting procedure based on a Frumkin-type isotherm for the quasi-equilibrium state and Roginskii-Zeldovich-type equations for the adsorption and desorption processes. A computer analysis of the experimental theta vs. time data provided good estimations of k(a) and a, while the estimation of k(d) was less reliable, depending on the precision of the experimental data. For halide ion adsorption it was found that n = 1 and a < 0 (repulsive forces), while for NDSA n = 3 and a > 0 (attractive forces). The slow adsorption rates during cathodic polarizations are explained by the ''anomalous'' dissolution of iron during cathodic polarization by a mechanism similar to that for the anodic polarizations.