Research On Corrosion Behavior Of Nano-composite Polyurea Coatings Under The Wet-dry Cyclic Conditions

The marine environment is a very complex corrosive environments, and the metal structures is prone to corrosion under wet-dry environment. In order to solve this problem, polyurea elastomer coatings modified with nanoparticles are sprayed on metal surfaces to improve corrosion resistance of metal structures under marine environment.In this paper, nano-ZnO and nano-TiO2 are selected to modify the properties of polyurea based on the characteristics of polyurea elastomer and nanoparticles. According to the characteristics of marine environment in tidal zone, we design a wet-cycle experimental device. We spray the original and nano-modified polyurea on marine steel D36, and measure the coating thickness and adhesion of various polyurea before and after wet-dry cyclic experiment and immersion test with a thickness tester and coating adhesion tester. During the corrosion process of different types of polyurea coating, Electrochemical Impedance Spectroscopy(EIS), open circuit potential, coating resistance, coating capacitance and surface morphology are used to analyze effects of type and amount of nano particles to the corrosion protection performance under wet-dry cyclic conditions and full immersion conditions. The results show that:Under wet-dry cyclic conditions and full immersion conditions, different polyurea coating failure process can be divided into three stages: water permeating into coatings, corrosion initiation and corrosion extension underlying coatings. During the failure of various polyurea coating process, EIS parameters consist of open circuit potential, coating resistance and coating capacitance obtained by fitting method all have a high correlation with response characteristics of impedance spectroscopy.Adding nano-ZnO and TiO2 nanoparticles can increase the adhesion of the coating, and the adhesion is maximum when the dosage comes to 5%. The adhesion declined after experiment, and the modified polyurea coating with 5% nanoparticles has the minimum decline of adhesion force and has the maximum value; polyurea coating with 5% ZnO nanoparticles has the minimum change of thickness after experiment.Under wet-dry cyclic conditions and full immersion conditions, the polyurea with 5% nano-TiO2 and nano-ZnO particles can improve the corrosion protection performance. The polyurea coating protective performance with 10% nano particle is inferior to the original polyurea coatings,and the ZnO nano-modified polyurea coating works better than the TiO2 nano-modified polyurea coating. In addition, the failure rate of nano-modified polyurea coating under full immersion environment is greater than that under wet-dry cyclic conditions, but the failure rate of nano-modified polyurea coating in full immersion environment is higheer.At the beginning of dry-wet circulation, there is little difference of the corrosion protective performance between the 5% ZnO modified polyurea coating with surface scratch defect and the good one. But the defect coating has a short process of coating water penetration. Through the contrast of EIS parameters, thickness and adhesion of defect coatings and coatings in good condition, the coating defects will reduce the protective properties and service life.