| Protection of aluminum substrates in aqueous environments against corrosion is important.Composition modifications can tune the properties of alloys but it can also alter their corrosion behavior and lower their service time.One of the methods of protection of metallic substrates is using organic coatings for isolation of the substrate from corrosive agents.These organic coatings are not entirely impermeable to electrolyte and cannot block the access of corrosive agents to the substrate indefinitely.They are also prone to physical damages that can expose the substrate to the electrolyte.It is important to have anticorrosion pigments imbedded inside the matrix of a coating to inhibit corrosion reactions in time.In order to achieve active corrosion protection for AA2024-T3 substrates,first,we selected an organic corrosion inhibitor,3-Amino-1,2,4-triazole-5-thiol(ATAT).After verifying its inhibition efficiency,the inhibitor was loaded in the mesoporous silica microparticles,wrapped with chitosan and poly(styrenesulfonate)polyelectrolytes.The loading of the inhibitor was confirmed with Thermogravimetric and infra-red analyses.The release of inhibitor in acidic and alkaline pH conditions was confirmed with UV-vis tests.The microcontainers were imbedded in the epoxy coating formula that was applied on AA2024-T3 substrates with the approximate thickness of 60 ?m.Using electrochemical impedance spectroscopy for 49 days in 0.1 M NaCl solution,the protection of the substrates was assessed.Using scanning vibrating electrode technique for ten days in 0.05 M NaCl solution,the changes in corrosion currents were measured on coated samples with artificial holes.The results showed that the microcontainers with ATAT protected AA2024-T3 substrates.Water at the interface determines the corrosion reactions,anticorrosion reactions,passive film formation and also the disbonding of coating.In order to be able to study water molecules under such a specific condition,atomistic simulation method was used.Three newly developed ReaxFF force fields were used to reproduce physical properties of the organic phase,inorganic phase,and inorganic/organic interactions.The structure and density of three different crosslinked epoxy molecules,DGEBA/IPDA,DGEBA/DETDA,DGEBA/T403,were reproduced and compared with the reported experimental values.The structure,density and diffusion constant of water were reproduced and compared with the reported experimental values.Hydration shells of Al(?)ion,and the gibbsite structure were also simulated and compared with the reported experimental values.The interaction energies of the epoxy with hydrated alumina substrates were also simulated and compared with the reported calculation results.Finally,the reaction of aluminum with water was simulated and compared with the reported experimental results.The results showed that,in all cases,one force field can be used for accurate simulations of all organic phase,inorganic phase,and the interactions of organic phase and inorganic phase.The selected accurate force field was used in simulation of water molecules at the interface of a hydroxylated alumina substrate and epoxy coating.Three configurations with 70,140,210 water molecules were simulated.Dynamic,structural,and energetic properties of water molecules were simulated and compared with reported bulk water properties.The results showed that by reduction of the amount of water at the interface,the dynamics and energetic properties of water were affected the most,but the structural properties of water molecules were remained qualitatively similar with the bulk water.It was shown that water molecules prefer to accumulate at the alumina interface at 3-5 A distance.Results also showed that water molecules lose their hydrogen bonds when they are confined at the interface.When a water molecule jumps from a larger confinement to a smaller one,the lost hydrogen bond can be compensated,which in return causes more water molecules to enter smaller confinements and cause disbonding of the coating.In addition to the interface of organic coating/alumina,water molecules can also accumulate at the interface of the added particles and pigments in a coating,Silica nanoparticles are commonly used and they have a large surface density that can become a favorable place for accumulation of water inside the matrix of a coating.The accumulated water at this interface can increase the water uptake of a coating and lower its barrier properties.The experimental section showed that addition of pigments in the coating reduces the barrier properties,but the mechanism was left unknown.We used atomistic simulations to study the mechanism of increased water uptake of coatings with untreated silica pigments.We used an accurate force field for simulations of untreated silica nanoparticles in the matrix of three different epoxy coatings,DGEBA/IPDA,DGEBA/DETDA,DGEBA/T403.Water molecules were added into the system using uniform distribution,and localized addition.Presence of water molecules at the interface of silica nanoparticle and the matrix of epoxy around the silica nanoparticle was studied.In order to observe the changes in the epoxy matrix,we used the concept of probe and by tracking the changes in the number of added probes,the structural changes of the epoxy matrix was revealed.The simulation results showed that untreated silica nanoparticles are not compatible with their surrounding epoxy matrix,therefore,are a favorable place for accumulation of water.But surface treatment of silica nanoparticles with 3-glycidoxypropyltrimethoxysilane,reduced the amount of interfacial water,and changed the structure of the epoxy matrices.Simulation results revealed that surface treatment of silica nanoparticles also alters the epoxy structure by creating more free volumes that can be filled with water.Therefore,water molecules will go in the created free volumes instead of the interface of silica nanoparticles,therefore reducing the overall uptake of the coating.Comparing experimental and computational results shows that the performance of anticorrosion coatings is determined by microstructure and interfacial characteristics of the coating/substrate and coating/pigments.Formation of easy path or water accumulation sites at the interface of pigments can affect the barrier properties of the coating.Coatings with low barrier properties cannot efficiently isolate the substrate from water and corrosive species.Consequently,water molecules reaching to the substrate will increases the chance of uninhibited corrosion reactions and disbonding process. |
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