Studies of impedance models and water transport behaviors of polypropylene coated metals in NaCl solution

Abstract

Using electrochemical impedance spectroscopy (EIS) technique, the water uptake process and the evolution of impedance models of polypropylene coated mild steels and LY12 Al alloys immersed in 3.5% (weight fraction) NaCl solution were studied. The equivalent electrical circuit (EEC) fitting results suggested that there was one single semi-circle in the impedance spectra of immersed samples in the initial period of immersion, which was corresponded to one time constant. Along with the immersion time, the number of time constant in EEC for metal/coating electrode system increased. The diffusion process towards solution of corrosion products forming on the metal/coating interface might be restrained due to the influence of coating barrier, resulting in the initiation of diffusion component in EEC. This diffusion did not act as a Warburg behavior, leading to a deviation of dispersive number, n, from 0.5. The studies revealed that the water transport in polypropylene clear varnish followed the Fickian diffusion during the initial time of exposure to the solution. After a certain time of immersion, the water uptake reached a saturation value. According to the variation of coating capacitance with time, the water diffusion coefficient was deduced. The experimental results indicated that the water diffusion coefficients in polypropylene coatings were independent of substrate materials used and coating thickness. Relative dielectric constant of dry coatings ( ε d) and water volume fraction of the coatings at saturation ( φ ∞) were also calculated. The similar results were found in time dependence of coating capacitance calculated from the impedance imaginary component at a fixed high frequency and that obtained from the fitting result by EEC, which indicated that the coating capacitance from the impedance imaginary component was reliable to evaluate barrier performance of coatings against water.