Preparation of ultrathin polymer coatings adsorbed on iron via a covalent bond by multistep modification of a hydroxymethylbenzene SAM with ethoxysilanes and alkanediol for preventing iron corrosion in 0.5Â M NaCl

The film thickness, d of a p-hydroxymethylbenzene - C6H4CH2OH (HOMB) self-assembled monolayer (SAM) adsorbed on iron via a covalent bond was increased by multistep modification with tetraethoxysilane (C2H5O)4Si (TES) and 1,8-octanediol HO(CH2)8OH (C8DO) and subsequently with 1,2-bis(triethoxysilyl)ethane (C2H5O)3Si(CH2)2Si(OC2H5)3 (BTESE) and akyltriethoxysilane CnH2n+1Si(OC2H5)3 (CnTES, n = 8 or 18). The protective ability of the film against iron corrosion was examined by polarization measurement of the iron electrode coated with the film in an oxygenated 0.5 M NaCl after immersion in the solution for many hours. The values of protective efficiency, P for the films, the HOMB SAM modified with TES and C8DO twice and subsequently with BTESE and CnTES were high, more than 81% at n = 8 and 85% at n = 18 in the range of the immersion time, t up to 240 h, respectively. The maximal P values of the respective films were 88.0% and 92.2%, of which approximate d values were 4.6 and 5.8 nm. The high protection of iron against corrosion was attributed to increases in the film thickness and interconnection between the adsorbed molecules with Si-O polymer linkages. The protective ability of the film was remarkably persistent during immersion in the solution for many hours. A slight enhancement of P was observed by additional modification of the modified HOMB SAM with C8TES due to increases in interconnection and close packed arrangement. The formation of strong adsorption bonds, σ-covalent bond and back-donating π-bond, between the carbon atom of HOMB moiety and iron atom was discussed.