THE INHIBITIVE EFFECT OF ORGANIC CATIONS ON PASSIVE FILM BREAKDOWN OF IRON IN A CHLORIDE-CONTAINING BORATE BUFFER SOLUTION

The inhibitive effect of an organic cation inhibitor, alkyltrimethylammonium ions on the passive film breakdown of an iron electrode in the borate buffer containing Cl- was investigated by polarization measurement and X-ray photo-electron spectroscopy. The pitting potential of a passivated iron electrode shifted in the positive direction following the addition of the inhibitor, indicating inhibition of the breakdown. Since X-ray photo-electron spectra showed the absence of Cl- in the passive film near the iron substrate, it is concluded that the cation inhibitor suppresses migration of Cl- into the passive film via an electrostatic interaction between the adsorbed Cl- and the cation at the film surface. The inhibitive effect of alkyltrimethylammonium ion increased and then decreased with an increase in the carbon number of an alkyl chain. The increase of the effect is attributed to coverage of the passive film surface with the cation inhibitor to hinder adsorption of Cl- and the decrease is ascribed to lowering of the film thickness and the borate ion content in the film.