Evaluation of the surface treatment effect on the anticorrosive performance of paint systems on steel

Electrochemical impedance spectroscopy studies were carried out for paint coated carbon steel sheets under free corrosion conditions when exposed to 3.5% NaCl solution for 405 days. Five anticorrosive primers [epoxy-polyamide/red lead/ferric oxide; alkyd/red lead; epoxy/zinc–aluminium; zinc-rich ethyl-silicate and epoxy/zinc phosphate] were used while changing the steel surface treatment condition. The EIS data was interpreted and discussed in terms of the time dependence of the electrical (paint coating) and electrochemical (steel substrate) parameters associated to interfacial processes describing the metal/paint system deterioration. Furthermore, the progress of the coating delamination process was estimated by the empirical expression relating the measured/specific double layer capacitance ratio. Experimental data withdrawn from salt spray cabinet tests performed for 261 days and adhesion measurements are also discussed. On the basis of the electrochemical, visual inspection and standardized tests results it was concluded that the best corrosion performance was provided by the epoxy-polyamide/red lead/ferric oxide anticorrosive paint without any marked effect of the surface treatment; followed by those anticorrosive systems including dry or wet sandblasting in presence of inhibitor. This result has been attributed to the fact that such surface treatments can improve the barrier properties and/or the inhibiting corrosion protection and the steel/paint adhesion properties or reduce the osmotic pressure effect.