Relationship Between Adsorption of Anions on Metals or Metallic Oxide and the Hard and Soft Acids and Bases Principle

Abstract

Chemical adsorption of anions on metals or a metallic oxide and their effects on corrosion and passivity of metals were discussed on the basis of the hard and soft acids and bases principle (HSAB principle). The surface coverages of various anions on iron and nickel in 1N HClO₄ and borate buffer solutions and the corrosion and passive current densities of the metals in the solutions containing the anions were determined by polarization and impedance measurements. The coverages of anions on the metals were concluded as being closely related to the HSAB principle. Because a surface site around an adsorbed anion acts as a softer acid, a neutral molecule classified as a soft base is chemisorbed at the site without more difficulty. Since the passive current density of iron in the borate buffer with anions is closely related to the HSAB principle, the adsorption of anions on the oxidized iron surface is also associated with the principle. It was concluded that the pitting potential of an iron electrode in the borate buffer containing Cl^-, Br^-, I^-, or S^2-, being aggressive upon the passivity of iron, is also related with the HSAB principle. These anions were detected near the iron substrate in a passive film by XPS measurement. Other anions, F^-, ClO₄^-, CH₃CO₂^-, SO₄^2-, SO₃^2-, SCN^-, CN^-, and N₃^- scarcely induce the breakdown of passive film on the iron surface. Inhibition effects of a tetraalkylammonium ion in the presence of added anions on the iron corrosion and passivity are also shown.