Prevention of passive film breakdown on iron in a borate buffer solution containing chloride ion by coverage with a self-assembled monolayer of hexadecanoate ion

Breakdown of a passive film on iron in a borate buffer solution (pH 8.49) containing 0.1 M of Cl^- was suppressed by coverage of the passive film surface with a self-assembled monolayer (SAM) of hexadecanoate ion C₁₅H₃₁CO₂^- (C₁₆A^-). The pitting potential of an iron electrode previously passivated in the borate buffer at 0.50 V/SCE increased by treatment in an aqueous solution of sodium hexadecanoate for many hours, indicating protection of the passive film from breakdown caused by an attack on defects of the film with Cl^-. No breakdown occurred over the potential range of the passive region by coverage with the SAM of C₁₆A^- in some cases. Structures of the passive film and the monolayer were characterized by X-ray photoelectron and Fourier transform infrared reflection spectroscopies and contact angle measurement with a drop of water.