Study On The Preparation And Protection Mechanism Of Graphene-enhanced Green Dacromet Coating

Dacromet coating is an excellent anticorrosion coating,but it contains Cr⁶⁺which is harmful to human body.Therefore,abandoning chromium anhydride to develop chromium-free Dacromet technology has become the mainstream.Due to the lack of chromium anhydride,the anticorrosion performance of chromium-free Dacromet coating has declined.Graphene is a nano-material with two-dimensional flake structure.Its unique structure can effectively prevent the penetration of corrosive media.In order to improve the inadequate corrosion resistance of chromium-free dacromet,graphene was added to prepare composite coating which is expected to improve the corrosion resistance of the coating.1.The formulation of coating solution and the preparation technology were determined.Firstly,the main components of were screened.Secondly,the L₁₆?4⁵?orthogonal test was designed.The ratio of zinc to aluminium powder and the different content of graphene,dispersant,binder and corrosion inhibitor were taken as the main factors.The initial formulation of the coating solution was determined by evaluating the appearance,adhesion and corrosion resistance of the coating.On the basis of the initial formulation,the optimum formulation and preparation process were determined.The ideal formulation of the coating is:zinc powder 14%,aluminium powder 6%,graphene 0-0.36%,binder 8%,corrosion inhibitor 3%,dispersant 2.4%,protective agent 8%,thickener 0.5%,defoamer 3-4 drops and residual deionized water,in which graphene is graded by 0.12%to prepare four kinds of enhanced coatings with different graphene content;the preparation process is:90?drying for 10 minutes,280?sintering for 25 minutes,repeated baking and coating.2.The comprehensive properties of graphene reinforced coatings were analyzed.Firstly,the dispersion,stability and viscosity of the coating solution were tested.Secondly,the mechanical properties?adhesion,hardness?and physical properties?appearance,thickness?of the four coatings were tested and compared in detail.Then the corrosion resistance of the coatings was tested by ammonium nitrate rapid corrosion test,immersion test and salt spray test.The morphology and composition changes of the coatings before and after corrosion were observed and analyzed by SEM,EDS,XRD and Raman.The morphology and state of graphene in chromium-free Dacromet coatings were defined.Combining with electrochemical test,the simulation of four coatings was tested.The open-circuit potential,Tafel polarization curve and electrochemical impedance spectroscopy in seawater?3.5%NaCl solution?were compared,and the corrosion enhancement mechanism was analyzed with the corresponding equivalent circuit.The results show that the surface of the four coatings is smooth and the adhesion is good;graphene is embedded between zinc and aluminium powder in sheet structure,overlapping with sheet zinc and aluminium powder layer,and adding graphene nanosheet has no obvious effect on the micro-morphology of the coatings;the thickness of the coatings is about 20-40 um,the hardness of the coatings increases with the increase of graphene content,and the 0.36%graphene reinforced coatings is the hardest.which is 212.45 HV.The0.12%graphene reinforced coating has the best performance in ammonium nitrate rapid corrosion test,salt water immersion test and neutral salt spray test,and its corrosion resistance time is 5 h,1030 h and 800 h.The electrochemical test results show that the corrosion potential of all coatings with graphene nanosheets in 3.5%NaCl solution is lower than that of the matrix and higher than that of the chromium-free Dacromet coatings.Among them,the potential of 0.12%graphene reinforced chromium-free coatings is the lowest,which is-735 mV,and the current density is maintained in the order of 10^-7.The capacitive arc resistance radius and impedance value of the coatings are all 0.12%graphene addition.At the beginning of immersion,the Cc value of the coatings with graphene was smaller than that of the coatings with graphene.After 800 h salt spray test,the proportion of Zn and Al decreased significantly,and the proportion of O increased significantly.XRD spectra showed that Zn was consumed first in the corrosion process,and Al reacted with other substances to form dense corrosion products which were insoluble in water.In conclusion,the addition of graphene can improve the self-corrosion potential of the coating,reduce the self-corrosion current of the coating,delay the immersion of corrosion medium,and the optimum addition of graphene is 0.12wt.%.