Study Of Electrochemical Behavior & Life Prediction Of Two Kinds Of Epoxy Coatings During Failure Process

Organic coatings are one of the most effective,simple and economical way to prevent metals from corrosion.However,the coatings exposed in environmental conditions will suffer degradation,especially in aggressive solutions such as seawater.It is important to understand the failure process and because of the characteristics of longtime failure of organic coatings,it is also important to predict the service life of the coatings.In this thesis,two types of coatings including H900 epoxy anti-corrosive primer and ED 1000 epoxy primer were coated on 2024-T3 aluminum alloy.The thickness of H900 epoxy primer contains 35 μm,75 μm,130 μm and 245 μm and the thickness of ED1000 epoxy primer is 35 μm and 75 μm.The electrochemical impedance spectroscopy(EIS)technology was carried out to study the degradation behaviors of the Al alloy/epoxy coatings in 3.5 wt.%solution.ZSimpWin software was used to analyze the EIS data to obtain the electrochemical parameters of the coating samples.The chemical changes in the coatings were investigated by Fourier transform infrared spectroscopy(FTIR),the morphology of coatings and matrix was observed by scanning electron microscope(SEM)and elemental composition was analyzed by energy dispersion spectrum(EDS)method.The Corrosion Master Simulation software was used to predict the serve life of these two epoxy coatings in simulated sea water solution.The main conclusions are as follows:(1)Both the ED 1000 epoxy coating and H900 epoxy coating with 35 μm thickness have very good protective property to the 2024-T3 aluminum alloy substrate in 3.5 wt.%NaCl solution for a very long time(at least longer than 700 d).The coatings have low water absorption rate(φ)which remains basically less than 0.3%during the immersion time.With the thickness increasing,the protective property of the coating increases.For the studied ED 1000 coating,though the thickness increases twice(from 35 pm to 75 μm)the degree of the improvement of the coating barrier property is larger than twice.(2)The results of the EIS test,SEM and FRIT analysis indicate that the heat cycling test(8 h at 35℃+16 h RT)cans accelerate on the degradation of the epoxy coating,causing some blisters and micro pits in the coating.One important reason is probably due to the accelerating effect on the water adsorption of the coating and therefor the increase of the defects and porosity of the coating.(3)Pearson’s correlation coefficient(r)was used to study the correlation between the variations of phase angles at different frequencies and the variation of |Z]0.01Hz with immersion time.The results show that the phase angles in a middle frequency range show similar decreasing tendencies to that of |Z|0.01Hz.A strong positive correlation exists in the phase angles in this the middle frequency range and |Z|0.01Hz.Therefore,the phase angles can be suggested as a criterion to rapidly evaluate the degradation degree and protection performance of the coatings.(4)Corrosion Master software was used to simulate the failure process of the ED 1000 epoxy coating and H900 epoxy coating with different thicknesses in 3.5 wt.%NaCl solution.The service life of the coating was predicted.The results show that the simulation results are in good agreement with the experimental results under the same condition.The simulation technology can be used to study the failure behavior of metal/organic coating and the life prediction can be realized.