Study On The Rapid Detection And Evaluation Of Coating Protective Performance And Failure Degree

Coated organic coatings are one of the most effective,simple and economical ways to prevent corrosion of metals.However,the coating under the action of the environment will occur aging,and then failure.At present,it depends on the appearance of the coating to judge the failure of the coating,the lack of rapid and accurate evaluation of the coating performance and failure degree of the method.In this thesis,two kinds of epoxy coatings were coated on the surface of 2024 aluminum alloy,which were H900 epoxy thick type anti-rust paint and ED 1000 epoxy primer respectively.Then,the coating/aluminum alloy samples with different thickness were soaked in 3.5wt%(FT-IR),scanning electron microscopy(SEM)and EDS were used to test the electrochemical parameters of the samples.The electrochemical parameters were analyzed by electrochemical impedance spectroscopy(EIS)Analysis of the degradation process and electrochemical parameters of the coating.The use of the characteristic frequency fb,the minimum phase angle ?min and its frequency fmin and the phase angle at the specific frequency and other rapid detection methods to evaluate the degree of deterioration of the coating and the protection of aluminum alloy,the main conclusions are as follows:1.The impedance at 0.01 Hz(|Z|0.01 Hz)is still high when the second time constant of the EGS spectrum of the H900 epoxy thin coating sample and the ED 1000 epoxy thin coating sample is 1×108?·cm2,while the infrared test results show that the coating does not occur aging,the coating still has a good protective properties of aluminum alloy matrix.2.The |Z|0.01 Hz of the two samples down to 1×106 ?·cm2,the coating water absorption is still low(less than 0.3%).The results of infrared test showed that the coating did not change chemically..The results of electron microscopy showed that the coating surface was intact and the corrosion surface of the aluminum alloy substrate was not.Previously proposed |Z|0.01Hz down to 1×106 ?·cm2,the coating failure standard is considered for the carbon steel substrate,for the aluminum alloy surface of the organic coating,when |Z|0.01 Hz down to 1×106 ?·cm2,the coating did not fail.3.In the two coating degradation process,the break-point frequency fb is increasing trend,and low frequency impedance |Z|0.01 Hz change trend is completely opposite.When the second time constant occurs in the EIS spectrum,fb is close to 10 Hz,where|Z|0.01 Hz is higher than 107 ?·cm2;when fb approaches 100 Hz,|Z|0.01 Hz is close to or less than 107 ?·cm2;When fb increases to about 500 Hz,|Z|0.01 Hz close to 106 ?·cm2,when the coating porosity of 10-3-10-2,infrared analysis coating did not occur hydrolysis,electron microscopy results show that corrosion products is not on the metal surface.4.The minimum phase angle ?min in the EIS phase angle and its corresponding frequency fmin appear,the impedance spectrum also presents a second time constant,so the occurrence of ?min and fmin may indicate that the solution penetrates into the coating/metal interface,the electrochemical reaction began to happen.fmin is approximately proportional to the peel rate of the coating,and the coating integrity can be determined by the size of fmin.5.The correlation between the change of phase angle with time and the change curve of low frequency impedance value |Z|0.01 Hz is analyzed by Pearson correlation analysis method.The results show that,the curve of the phase angle in the frequency range of 0.1 Hz?10 Hz with the time of the curve with the |Z|0.01 Hz curve has a high correlation,that is,in the coating failure process,the phase angle of 0.1 Hz?10 Hz frequency range of the phase angle can be used to quickly evaluate the coating.The highest phase correlation with the |Z|0.01 Hz curve is shifted to the high frequency end with the decrease of |Z|0.01 Hz.With the increase of the coating thickness,the highest correlation with the |Z|0.01 Hz curve which phase angle moves toward the low frequency side.6.The correlation between the phase angle(?0.85 Hz)at 0.85 Hz and the curve of |Z|0.01 Hz is the highest,and the corresponding relationship is:when?0.85Hz is higher than 20 °,|Z|0.01 Hz is higher than 1×108 ?·cm2;when ?0.85Hz is between 10°and 20°,|Z|0.01 Hz is between 1×107 ?·cm2 and 1×108 ?·cm2;when ?0.85Hz is at 5°-10 °,|Z|0.01 Hz is between 1×106 ?·cm2 and 1×107 ?·cm2.