Physicovhemical And Eis Properties Evolution Of Two Typical Composite Coating Systems In Marine Atmosphere

In this paper,the failure process and mechanism of typical coating systems in simulated marine atmosphere were studied by a series of tests such as,electrochemical impedance spectroscopy(EIS),infrared spectroscopy(FTIR),glossiness,chromatism,adhesion and SEM-EDS.Degradation mechanism of epoxy zinc-rich/polyurethane coating system were studied and compared under salt spray test,UV/condensation test and cycle test.The relationship between the electrochemical properties of single layer coating and double layers coating was studied and the applicability of rapid evaluation by medium frequency phase angle under different acceleration tests was investigated.The main results are as follows:(1)The aging behavior and mechanism of two commonly used composite coating systems including epoxy zinc rich/polyurethane coating and zinc yellow epoxy/fluorocarbon coating were studied.The results show that,compared with the epoxy zinc rich/polyurethane system,the impedance of the zinc epoxy/fluorocarbon system decreases more slowly at the same aging time.There is almost no chemical bond formation and fracture in the coating,the adhesion and glossiness change are smaller,and the weatherability of the zinc yellow epoxy/fluorocarbon system is better.The further study of the epoxy zinc rich/polyurethane system shows that the aging phase of the coating can be divided into two parts,the reversible phase of impedance change and the irreversible phase of impedance change according to the changing law of the low frequency impedance of the coating.The FTIR results shows that the epoxy zinc/polyurethane coating has hydrolytic,photodegradation and thermo oxidative degradation.Photodegradation and thermal oxygen degradation can lead to the appearance of microcracks on the surface of topcoat surface and the roughness of the surface.The change of the coating impedance changes from reverse to irreversible when the surface is cracked.Adhesion test shows that the coating gradually decreases with the increase of aging time,and the spalling form changed compared with the unaged coating.(2)Coating degradation mechanism and the difference of the epoxy zinc rich/polyurethane coating system under the action of three kinds of experiments were studied by the method of salt spray test,ultraviolet test and cycle test.The results show that the electrolyte permeation speed is the fastest under the salt spray test,and it is slower under the cycle test.There is no electrolyte permeation during the UV test process.For the epoxy zinc rich/polyurethane coating system,the salt spray test has the most obvious acceleration effect.The surface of the coating is hydrolyzed,photodegradation and thermal oxygen degradation in the UV test and cycle test.The surface of the coating appears small holes and cracks,the glossiness decreases,and the adhesion strength of the topcoat decreases,and the coating is stripped mainly in the interface between the primer and the finish.Under the salt spray test,the topcoat only hydrolyzes and the surface morphology changes little.The peel off of the coating occurrs between the substrate and the primer.In the cycle test,the permeation process of electrolyte solution is delayed by UV/condensation process,but after a certain period of ultraviolet radiation,the aging degradation of polyurethane topcoat causes holes or cracks on the surface of the coating.After that,the impedance of the coating decreases rapidly.(3)Changes in the properties of the coating resistance,water absorption and porosity of acrylic polyurethane,epoxy zinc rich primer and epoxy zinc rich/polyurethane coating systems under the salt spray test,and the relationship between the phase angle and impedance of the polyurethane topcoat/epoxy zinc rich primer composite coating under different accelerated test conditions were studied.The results show that the primer improves the adhesion between the coating system and the substrate.The coverage of the top coat makes the reaction resistance of Zn in the epoxy zinc rich primer increase,prolongs the service life of the epoxy zinc rich primer,and after the application of the epoxy zinc rich primer to the acrylic polyurethane,the lifetime of coating system is postponed.The evaluation of the phase angle on the coating protection performance under 10Hz is also applicable to the epoxy zinc/polyurethane coating system under different accelerated conditions.