AC Impedance Studies on Relationship between Adsorption Behavior of Polar Organic Compounds on Anodically Polarized Nickel and the HSAB Principle

It has been confirmed by impedance and polarization measurements that the adsorption behavior of polar organic compounds on anodically polarized nickel electrode in 3.0 M HClO 4 are closely related to the HSAB principle. The compounds used as corrosion inhibitors were those of which polar atoms were elements of the 5B and 6B groups in the periodic table. Double layer capacity of the electrode‐solution interface in the active region of the polarization curve showed that the compound classified as a soft base was more readily chemisorbed on the surface acting as a soft acid than that of a hard base. Hence, inhibition efficiency of a soft base inhibitor for nickel dissolution was higher than that of a hard base inhibitor. In the passive region, compounds classified as hard bases lowered the passive current density of the electrode, while some soft bases stimulated it. These results are discussed with respect to the HSAB principle by using the impedance data in the passive region.