Research Of Aggressive Medium Uptake In Organic Coating

Currently, the organic coating technology is still the most effective and widely used method to prevent corrosion. The failure of the coating can cause metal corrosion, which result in resources and energy consumption, product reduction, environment pollution and threatening personal safety. Aggressive medium immerse into the substrate surface through organic coating, then chemical and electrochemical reaction take place, eventually led to the failure of the coating, thereby causing corrosion. Therefore, by immersed in corrosion solution, study on aggressive medium uptake in organic coating, so that establish organic coating life model, has the extremely vital guiding significance.In this research, by soaking organic epoxy coatings in simulated marine environment, studies the laws of weight, thickness, gloss change with different immersion time, the step of aggressive medium uptake in organic coating is obtained. Then, with the method of electrochemical impedance, study on the failure process organic coating. Through the equivalent capacitance and resistance variation of the organic coating, learn about the process of aggressive medium uptake in organic coating. At the same time, by scanning electron microscopy and energy spectrum analysis technology, the side face of organic coatings were obtained under different immersion time, to obtain the diffusion depth of different ions in the coating. Finally, creatively using nano-indentation method, through the variation of coating surface nano-mechanical properties at different immersion time, acquired the depth of aggressive medium uptake in organic coating under different immersion time. The electrochemical parameters and nano-mechanical parameters are related, to get the foundation life model of epoxy coating.The results show that:the weight and thickness variation of epoxy coating in different immersion time reflected a consistence rule. It supposed that in the early stage of immersion time, the initial swelling of the organic polymer epoxy coating and aggressive medium uptake in epoxy coating was significant. Then, with the organic polymer chain scission, small molecular chain was cut into pieces, and those hydrone diffused back into medium. Thus, it had become the key factor of coating weight and thickness change. Variation of gloss was not obvious, which could hardly reflect the change law of coating degradation process.The change of impedance AC reflected the process of aggressive medium uptake in organic coating, which caused electrical properties induce in the coating. The equivalent capacitance value of coatings increased rapidly in the initial stage, and then tended to be stable. The variation of electrical properties of the coating was actually caused by the penetration of aggressive medium. As the dielectric constant of aggressive medium was far greater than epoxy coating, equivalent capacitance of entire coating was increased at the early immersion stage. With the penetration of aggressive medium, coating swelling absorption gradually became saturated. The dielectric tended to be stable, and lead to equivalent capacitance stable.Nano-mechanical test results also reflected the process that aggressive medium diffusion in the coating. Aggressive medium diffused into organic coating through it defects, and coating swelling. In this process, coating surface nano hardness, nano elastic modulus decreased. Coating surface and nano-mechanical properties was decreased which confirmed that surface nano-mechanical was a sensitive property in the process of aggressive medium diffusion. Take advantage of the dimensionless quantity hardness modulus ratio(Hnano/Enano) variation, aggressive medium diffusion depth could be measured in different immersion time.SEM results showed the diffusion behavior of chloride ion(Cl-) and sodium ion(Na+) in the coating. Chloride ions in the coating diffusion amount were larger, and have obviously change regulation. On the other hand, sodium ion diffusion amount was small, and rule less. Compared chloride diffusion rate to water, it was found that chloride ion is faster.The comparative study of different experimental methods, indicate the limitation of common ways to describe aggressive medium diffusion. Comparison of nano-indentation test method and electrochemical results, indicate the feasibility of electrochemical method, and verified the superiority of the nano-indentation method. From here we can see that, the application of nano-indentation method in measuring aggressive medium diffusion process in the coating is promising.