Assessing the Corrosion Resistance of Painted Steel by AC Impedance and Electrochemical Noise Techniques

Four organic coatings of commercial grade (polyurethane, epoxy-polyamide, medium-quality and low-cost alkyds) with an expected range of corrosion protective properties were applied to steel substrates and immersed for up to 4,300 h in 0.6 M NaCl solution. These painted electrodes were assessed by AC impedance in the frequency range 60 kHz to 0.1/0.01 Hz and also by a statistical analysis of the voltage and current noise transients generated under freely corroding conditions. Good corrosion-resistant coatings were associated with stable paint-film AC resistance (F_f) values and derived noise resistance (R_n) values of the order of 10⁸ to 10¹⁰ Ω · cm². Associated capacitances were also stable in the range 10^-10 F/cm². Relatively poor corrosion protection occurred when the R_f value decreased below ~ 10⁵ Ω · cm². Associated capacitance values in and above the μF/cm² range suggested increased exposed electrode areas under delaminating paint and/or formation of corrosion pits. For significantly degraded paints, the R_n values were somewhat higher than the equivalent R_f values because of charge-transfer and diffusion effects, which are not separated in the noise data. Immersion performance trends were consistent with results obtained from samples exposed for 12 months to an industrial atmosphere, suggesting that immersion testing may provide information relevant to atmospheric exposure conditions. In this respect, further work is necessary.