Study On Failure Mechanism Of A Variety Of Typical Composite Coatings In The Simulation Of Marine Atmosphere

The marine atmospheric environment of the South China Sea has the characteristics of high temperature,high humidity,high salinity and strong solar radiation.In order to predict the maintenance cycle and service life scientifically,a variety of laboratory accelerated aging tests were carried out for the acrylic polyurethane composite coating system and fluorocarbon composite coating system used in aluminum alloy and carbon steel substrates.The differences and similarities of the failure mechanism were discussed and the optimum scheme was selected.At the same time,the natural aging test was carried out in Wanning,Hainan,the correlation between accelerated aging and natural aging was discussed,and the improvement of laboratory aging test was put forward.The conclusions are as follows:(1)The comparison of acrylic polyurethane composite coatings in three accelerated tests shows that the UV-condensation process has a strong destructive effect on the acrylic polyurethane topcoat,but periodic dry-wet alternations make the process of water permeating to the primer slower.Therefore,it has little effect on the shielding performance of the composite;coating,the experimental temperature of the UV-condensation process located in the transition temperature range of the glass transition of the surface lacquer and the destruction of carbamate group by ultraviolet light.Salt spray test shows that the loss of light and the value of color difference have little effect on the surface paint,but continuous salt spray penetration significantly reduces the shielding performance of the coating.The salt spray test also resulted in a significant decrease in adhesion between the coating and the substrate.The accelerated cycle test took into account factors such as strong solar radiation,high temperature,alternating dry and wet,sea salt particles,rainfall and temperature difference,etc.The results can more accurately reflect the failure process of composite coatings in the marine atmosphere.(2)The results of salt spray test on aluminum alloy and carbon steel substrate show that when the double layer coating is defective,the damage of the coating is mainly in the form of foaming and diffusing to the peripheral layers at the same distance.The corrosion of the aluminum alloy is limited to the initial defect,and the corrosion of the substrate is limited to the initial defect while the corrosion of the substrate is limited to the initial defect when there is a defect in the three-layer coating.The adhesion of double-layer coating is obviously decreased,while the adhesion of three-layer coating is not.The adhesion test by pull method can accurately and objectively evaluate the difference of adhesion between different coating systems,or before and after the test of the same coating system.Although the test of wet adhesion by grid method has high timeliness,the test results of coating system with high adhesion are inaccurate.With the addition of the epoxy cloud iron intermediate paint,the shielding performance of the three-layer coating system to salt fog is obvious due to the double layer system,and it has a good protective effect on the aluminum alloy and carbon steel metal substrates,indicating that the sea atmosphere environment of the South China Sea is high in salt content.In order to improve the shielding performance of the composite coating system,it is necessary to add intermediate coatings or additives which can effectively prevent the penetration of high salt solution under the environment of high humidity and sufficient precipitation.When the solution penetrates to the interface of primer/substrate,the corrosion of carbon steel substrate is obviously earlier than that of aluminum alloy substrate due to the difference of the corrosion resistance of the substrate itself,and the corrosion is easy to spread to the periphery.Therefore,in determining the service life or maintenance period of the substrate-coating system,the difference in the corrosion resistance of the substrate itself should also be fully taken into account.(3)The comparison of several fluorocarbon composite coatings in cyclic aging test shows that the light and color preservation performance of the topcoat is that manufacturer A is superior to manufacturer B better than manufacturer C,and the coating system of manufacturer A and B has excellent shielding performance,while factory C has obviously decreased.Adhesion test results show that manufacturer A is better than manufacturer C than manufacturer B,and the chemical stability of the three coatings is very high.Several fluorocarbon composite coatings were compared with each other in natural aging test.The results showed that A was better than B and C was better than C,and fluorocarbon topcoat from three factories had natural aging test in one year.Serious pulverization has occurred since then;All three coating systems exhibit excellent shielding performance.Comparing the cyclic aging test with the natural aging test,it is found that the color difference of topcoat is very similar between them,and the 100 days cycle test is equal to the natural test of one year;In the cycle test,the damage of the topcoat is lagging behind the natural aging test,while the shielding performance is relatively superior to the natural aging test.This may be related to the proportion of ultraviolet light exposure to only 3/14 of the cyclic test,and the sunlight exposure time in natural aging test is higher than this ratio,and there are other external forces such as wind,Rain Water scour,and so on.Therefore,we may consider increasing the number of days in each cycle of the cycle aging test during the UV-condensation process,or increasing the proportion of ultraviolet irradiation duration during the UV-condensation process,and changing the condensation to spray to simulate the scouring effect of Rain Water.In order to achieve better simulation effect,the variation of performance in the two tests is consistent.