Improvement of healing treatment with NaNO₃ for a passivated iron electrode covered with an ultrathin polymer coating to prevent iron corrosion in some solutions containing aggressive anions

An ultrathin and ordered polymer coating was prepared on a passivated iron electrode by modification of a 16-hydroxyhexadecanoate ion HO(CH2)15CO2- self-assembled monolayer with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and octyltriethoxysilane C₈H₁₇Si(OC₂H₅)₃. Further, the passivated and polymer-coated electrode was healed by treatment in 1.0M NaNO₃ for 4h. Prevention of passive film breakdown and iron corrosion for the passivated, polymer-coated and healed electrode was examined by monitoring of the open-circuit potential and repeated polarization measurements in oxygenated 0.1M KClO₄, 0.1M Na₂SO₄ and 0.1M NaCl for many hours. The values of the time for passive film breakdown, t_bd were >240, 22.2 and 9.5h in these solutions, respectively. The protective efficiencies for the electrode were extremely high, more than 99.9% before t_bd, indicating complete protection of substrate iron against corrosion in these solutions, unless passive film breakdown occurred. The presence of NO3- on the passive surface by treatment in 1.0M NaNO₃ was detected by X-ray photoelectron and FTIR reflection spectroscopies. The self-healing activity of adsorbed NO3- to suppress passive film breakdown was discussed.