Degradation of zinc-rich epoxy coating in 3.5% NaCl solution and evolution of its EIS parameters

The electrochemical characteristics in the degradation process of epoxy zinc-rich coating (80 wt% zinc particles) and epoxy varnish coating in 3.5% NaCl solution were studied comparatively by using open-circuit potential and electrochemical impedance spectroscopy tests. The coating morphology, the consumption and corrosion products of zinc particles and chemical changes in the coatings were observed and analyzed with the methods of scanning electron microscopy and X-ray diffraction. The results show that the zinc-rich coating provides effective cathodic protection to the carbon steel substrate at the initial stage and still provides certain cathodic protection in the period when the barrier protection by the formation of zinc corrosion products begins to dominate. Because the zinc particles are conductive and active, the values of the coating resistance and capacitance of zinc-rich coatings are lower than those of the varnish coating. When the value of |Z|0.01Hz decreases to 8 × 104 Ω cm2, the zinc-rich coatings still provide protection to the steel substrate, but when the value decreases to about 2 × 104 Ω cm2, rusted spots were observed on the coating surface. Based on the results, a reference criterion of failure for the zinc-rich epoxy coatings is suggested to be about 5 × 104 Ω cm2 (2 × 104 – 8 × 104 Ω cm2), which is different from the one (106 Ω cm2) reported in the literature for typical organic coatings without conductive particles.