Self-assembled monolayers of carboxylate ions on passivated iron for preventing passive film breakdown

Self-assembled monolayers (SAMs) of carboxylate ions C_n-1 H_2n-1CO₂^- (C_nA^-) with the carbon number, n = 12-18 and 16-hydroxyhexadecanoate ion HO(CH₂)₁₅CO^-₂ (HOC₁₆A^-) were prepared on an iron electrode previously passivated in a borate buffer at pH 8.49 by treatment in aqueous solutions of their sodium salts for many hours. Breakdown of the passive film on the electrode coated with the SAM was examined by anodic polarization measurement in the borate buffer containing 0.1 M of Cl^-. The pitting potentials of the passivated electrodes coated with the SAMs of C_nA^- and HOC₁₆A^- shifted toward a more positive potential than that of the uncoated electrode, indicating prevention of passive film breakdown by blocking diffusion of Cl^- through the SAM to defects of the passive film. No breakdown was observed over the potential range of the passive region by coverage of the passive film with the SAM in some cases. The SAMs on the passive film were characterized by contact angle measurements and X-ray photoelectron and Fourier transform infrared reflection spectroscopies.