Diffusion behavior of water and chloride ion through acrylic polyurethane coating

Diffusion behavior of water and chloride ion through acrylic polyurethane coating was studied using water vapor, chloride ion transmission rate technique of organic coatings and electrochemical impedance spectroscopy (EIS) technique in this paper. The results obtained indicated that diffusion behavior of water was suited for Fick diffusion law in early stage, later water percolating capacity was constant, constant water percolating capacity was reduced with the increasing of film thickness. Chloride ion percolating capacity was constant in early stage, later increased linearly, diffusion coefficient of chloride ion through coatings could be obtained by slope coefficient; with the increasing of film thickness, time of chloride ion concentration keeping balance was prolonged and equilibrium concentration of chloride ion was reduced, diffusion coefficient of chloride ion through coatings was also reduced. According to the change of resistance and capacity of coatings and metal, and time of the second time constant appeared, diffusing behavior of electrolyte through coatings and corrosion of metals under the coating could be evaluated by EIS.