Study On Failure Process Of Several Organic Coating Under Wet/Dry Cycle Condition

The failure process of several single and composite organic coating under wet/dry and complete immersion of 3.5%NaCl were studied by Electrochemical Impedance Spectroscopy(EIS), the failure mechanism of the coating systems were characterized by Scanning Electron Microscope(SEM) and Fourier Transform Infrared Spectroscopy(FT-IR). Combined with special parameter, the protective performance of coating was quickly evaluated. The main test results are as follows:1. Electrochemical impedance technique, FT-IR and SEM were applied to study the failure process of fluorocarbon coating under wet-dry cyclic condition. The results show that, the failure rate of coating under wet-dry environment was faster than that under complete immersion. The wet-dry condition promoted formation of micro-pores in the coating and resulted in decreased coating resistance and increased water absorption. The mechanism for the coating failure was discussed. The coating volume experiences the variation of swelling and contraction during the wet-dry cycle, which results in increased micro-pores and cracks in the coating. The electrolyte enters the coating defects under the action of osmotic pressure, leading to blisters and reduced adhesion of the coating to the substrate.2. Electrochemical impedance technique has been applied to study the failure process of acrylic polyurethane coatings under wet-dry cyclic and complete immersion condition, the failure rate of coating under wet-dry environment was faster than that under complete immersion. The FT-IR and SEM results show that, For the Acrylic polyurethane coating, the hydrolysis of coating was one reason of the failure of coating. Physical effect of wet-dry cyclic might separate the filler from coating, accelerating the failure process of the coating.3. On the analysis of the porosity, water absorption rate and surface morphology of the coating before and after the failure, the failure process of top coating in composite coating was accelerated by wet/dry cycle, the micro-pores penetrated the top coating which resulted in the rapid failure of composite coating.4. The four coating systems under wet/dry and complete immersion condition were analyzed by phase angle in the specific frequency. The results show that the phase angle variation in specific frequency domain were same to variation of |Z|0.01Hz with test time, which could be applied to evaluated the protective performance of coating.5. The phase angle in 10Hz and 1Hz were used to evaluate the coating of naval ship on the pot, the results of the evaluation are in accord with the actual performance of the ship coating. The rapid evaluation of phase angle in specific frequency is practicable on site test, which may provide protection performance information for coating maintenance.