The effect of trialkoxysilanes on the passivation of mild steel surface modified with a zinc-phosphonate solution

The effect of trialkoxysilanes, namely, [3-(2-aminoethylamino)propyl]trimethoxysilane, vinyltrimethoxysilane (VTMS) and octyltrimethoxysilane on the protective properties of thin passivating layers formed on the surface of low carbon steel was studied. The protective layers were obtained by two-stage treatment including preliminary modification of steel surface in an aqueous solution of aminotris(methylenephosphonic acid) zinc complex followed by passivation in solutions based on a carboxylate corrosion inhibitor. Among the studied trialkoxysilanes, VTMS is the most effective as an additive to a passivating solution. The mixture of sodium oleate and VTMS has shown the highest protective properties both on air-oxidized and modified steel surface, which is confirmed by the results of electrochemical measurements (potentiodynamic polarization, electrochemical impedance spectroscopy) and by accelerated corrosion tests (in a humid atmosphere and in a salt spray chamber). The criterion for assessing the protective properties of the films in polarization measurements was the difference in the pitting potentials of the passivated and air-oxidized electrodes, and in corrosion tests it was the time until the first corrosion lesions appeared. Preliminary modification of steel surface by phosphonate significantly enhanced the passivating effect of the mixture of sodium oleate and VTMS, and increased the resistance of the resulting films to the effects of humid atmosphere and chloride ions.