The application of impedance spectroscopy to study the uptake of sodium chloride solution in painted metals

Impedance spectroscopy has been applied to study the permeation of sodium chloride solution through thin (25 μm) paint films and to determine whether the process is reversible. Painted Zn-55% Al alloy coated steel samples were immersed in 5% NaCl solutions at 50°C (referred to as the test solutions) and were periodically transferred to solutions of differing sodium chloride concentrations (referred to as measurement solutions). The impedance was measured in each measurement solution after only several minutes immersion, and the samples were transferred back to their respective test solution. At any given immersion time, the paint film resistance was characteristic of the concentration of the measurement solution and the extent of degradation of the paint film. The paint film resistance appears to be controlled by penetration of ions at direct or D-type areas. Paint film equilibrium is attained rapidly, is about 50% complete within several minutes, and the process of solution uptake is relatively reversible. Corrosion of the substrate occurs soon after immersion and is controlled by the resistance of the paint film. It is likely that the corroding area increases with time rather than the corrosion rate. Comparison of impedance results for free and attached films provides evidence that the high frequency Nyquist semicircle contains paint film information and not metal substrate information.