| The adhesive marine fouling organisms on the surface of ships and offshore structures causes frictional resistance on the surface of structures,and accelerates metal corrosion and other hazards,which becomes an important obstacle to human maritime activities.In the view of the characteristics of fouling organisms attached to the surface of ships and offshore structures,Cu-Zn-Al/Zn-Al composite coatings were prepared on the surface of Q235 steel by aqueous chromium-free Dacromet technology.Micro-morphology,corrosion resistance,antifouling effect of the coatings were investigated.In addition,the corrosion resistance and antifouling mechanism of composite coatings were discussed.Based on the experimental results,it is possible for us to get a better understanding of the protection of Cu-Zn-Al/Zn-Al composite coatings for Q235 steel when the substrate is exposured in the harsh marine environment.In this paper,the coating formulation of the composite coating was designed by orthogonal test method,and the preparation process of the composite coating was determined.The optimum formulation of the solution which later transformed into coatings was as follows:Zn-Al alloy flakes?25%?,copper powder?7.5%?,AC-66?10%?,phosphorus sodium molybdate?1.5%?,Tween20?2%?and ethylene glycol?8%?.Small amounts of silane hydrolysis methanol,the antifoaming agent and thickener hydroxyethyl cellulose were also added to the solutions.After dipping in the paints,samples were pre-baked at 100°C and sintered at 280°C.The surface of the coating was uniform and metallic luster,and the thickness was 25-35?m.The coatings exhibited a good corrosive resistance in either salt spray for more than 1000h or 3.5%NaCl solution for approximately 150d.The microstructures and compositions of Cu-Zn-Al/Zn-Al composite coatings before and after corrosition tests were characterized by SEM,EDS and XRD.The formation mechanism of corrosion products was discussed.The results show that the composition of the composite coating is uniform and there is no obvious agglomeration.The corrosion morphology of the coating changes,and a dense corrosion product phase is formed on the surface of the coating with the increase of immersion time.By investigating the corrosion potential,|Z|0.01 and electrochemical impedance spectroscopy with time,the electrochemical corrosion behavior of the coating was studied.The reasonable equivalent circuit was used to fit and analyze the data,and the corrosion process was divided into three stages:permeation,the cathodic protection and the shield.Judging from the electrochemical results,the coatings provide the protection for substrates in two aspects:the cathodic protection of the sacrificial anode and the shield of the corrosion product.According to the antifouling test,the copper ion exudation rate is lower in the initial stage of soaking.As the soaking time increases,the exudation rate fluctuated at about 2030?g·cm-2·d-1.The killing effect of different concentrations of copper ions on N.crassifolia was investigated by the cultivation experiment of N.crassifolia with different exudates.The results show that the inhibition rate of the composite coating was over 95%,and the higher the concentration of copper ions in the exudate,the shorter the time to reach 95%inhibition rate.It indicates that when the concentration of copper ions in the exudate exceeds a certain value,the growth of N.crassifolia get inhibited,which can be mainly attributed to the destruction of the cell membrane and intracellular enzyme protein. |
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