Theoretical And Experimental Study On Corrosion Behaviors Of The Coating/Metal Systems

In this paper,failure behavior of silicone-epoxy coatings in Na Cl solution investigated by the electrochemical theory and experiments.The main work is as follows:1.A silicone-epoxy hybrid coating cured with amino silane was developed to provide corrosion protection on 2024 Al-alloy using air spraying.Water uptake characteristics of the silicone-epoxy hybrid coatings were investigated by electrochemical impedance spectroscopy in a 5 wt.% Na Cl solution.The effect of mixture ratio of silicone-epoxy and amino-silane on the water uptake?solubility,diffusion coefficient and permeation?was studied by using a single frequency?10 k Hz?capacitance method.The glass transition temperature?T_g?was also investigated through differential scanning calorimeter?DSC?before and after immersion in the Na Cl solution.Consequently,the excess of silicone-epoxy resin or amino silane improved the solubility of water in the coatings.A low water permeation coefficient was obtained with the mixing ratio 8/2 of silicone-epoxy and amino-silane,in which the T_g value was found to be larger than other three mixing ratios before immersion.After immersion for 750 h,the impedance modulus of EFA 2 coating?mixing ratio 8/2?in the low frequency was still close to 108 Ohm·cm2 that accounts for the good protective performance.2.EIS of bare type A3 mild steel and silicone-epoxy hybrid coatings/mild steel system in 5 wt.% Na Cl solution has been investigated,respectively.The analyses of impedance models and water uptake behaviors of silicone-epoxy hybrid coatings have been studied during the aging process of the coatings.Taking into account the diffusion of oxygen,water and corrosion products as well as the formation of the passive film,six distinguishing impedance models were used to fit the EIS spectra of the coatings on type A3 mild steel.The water uptake was estimated from the Brasher-Kingsbury relation using CPE and two effective capacitances(C_effect^B andC_effect^H)which were also used to calculate the coating thicknesses.A comparison between the coating thicknesses calculated from the effective capacitance C_effect^B and thickness gauge measurements suggested a surface distribution of time constant on the surface of the coatings.The use of C_effect^B allows estimating the “true” values of water uptake in the silicone-epoxy hybrid coatings.3.Comparison of failure mechanisms of silicone-epoxy hybrid coatings on mild steel and Al-alloy immersed in 5 wt.% Na Cl solution was studied by electrochemical impedance spectroscopy,pull-off tests,differential scanning calorimetry and ultra-depth three-dimensional microscope.The results show that there were the effects on diffusion behavior in middle immersion and oxide film formed on metals in final immersion for mild steel and Al-alloy substrates.The effect of coating thickness on the cross-linking density of silicone-epoxy polymer was obvious.The delaminated area and the roughness of coating/metal interfaces were discussed from their respective break-point frequency and fractal dimensions deduced from impedance measurements.4.Water permeability and corrosion protection of silicone-epoxy coatings containing 3-glycidoxypropyltrimethoxy silane on 2024 Al-alloy were studied by electrochemical tests and glass transition temperature measurements.The results indicated that water permeation of silicone-epoxy coatings decreased after incorporated with GLYMO monomer.Glass transition temperature increased slightly after water because of the self-repairing effect and finally decreased lower than that of the coating without GLYMO.Electrochemical measurements confirmed that increased GLYMO content decreased the current density and increased the coatings pore resistance due to a decreased porosity,which resulted in an increase in corrosion protective performance of the coatings.Silicone-epoxy coatings that contained siloxane-modified polyacrylate were prepared.Further,the effect of the siloxane-modified polyacrylate on the hydrophobicity and water uptake behavior,and on corrosion resistance provided by the coatings were examined using contact angle measurements,glass transition temperatures and electrochemical impedance spectroscopy.The water uptake behavior of silicone-epoxy coatings with higher siloxane-modified polyacrylate content decreased to a larger extent for the coatings with higher hydrophobicity.The anticorrosion protection of 2024 Al-alloy with silicone-epoxy coatings containing siloxane-modified polyacrylate correlated well with water uptake and hydrophobicity.5.The electrochemical and water transport properties of silicone-epoxy coatings on Al-alloy pretreated with silane films are investigated in Na Cl solution.The results show the non-continuity of water concentration at the interface of silane film and silicone-epoxy coating.The silane film as pretreatment provides enhanced barrier properties and excellent corrosion protection.Considering water concentration jump on the silane film/silicone-epoxy boundary,diffusion coefficient of the coating obtained by validation model is similar to that obtained from the experimental method.In addition,relative to the other three coating systems,the best adhesion is maintained for the silicone-epoxy/silane film?p H=3.5?/Al-alloy system.