Abstract
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 hours in 0.6 M NaCl solution. These painted electrodes were assessed by AC impedance in the frequency range 60 kHz - 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 resistance coatings were associated with stable paint film AC resistance (Rf) values and derived noise resistance (Rn) values of the order of 108 -1010 Ω cm2. Associated capacitances were also stable in the range 10-10 F/cm2. Relatively poor corrosion protection occurred when the Rf value decreased below ~ 105 Ω cm2. Associated capacitance values in the μF/cm2 range and above suggested increased exposed electrode areas under delaminating paint and/or formation of corrosion pits. For significantly degraded paints, the Rn values were somewhat higher than the equivalent Rf values due to 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 a natural industrial atmosphere suggesting that immersion testing may provide information relevant to atmospheric exposure conditions. In this respect, further work is necessary.
