The Evolution Of Eis In The Degradation Process Of Several Coating Systems Under Different Accelerated Tests And Evaluation Of Coating Protection Performance

Organic coatings are effective ways to protect metals from corrosion in marine and atmospheric environments.Indoor accelerated corrosion tests are important ways to study the degradation of organic coatings.Compared with usual single factor accelerated tests,the cyclic accelerated test is suitable for simulating the synthetic action by various influencing factors in complex practical environment,which has better correlation.In this thesis,three kinds of indoor accelerated corrosion tests were used to study the corrosion failure process of different metal/coating systems.The UV-condensation/salt spray/low temperature exposure cyclic accelerated test was conducted to study the failure process of eight different metal/coating systems which are composed of six kinds of three-layer composite coating on three kinds of metal substrates.Based on these,the degradation process and EIS characteristics of five kinds of metal/single-layer coating systems which are composed of epoxy zinc-rich graphene primer and iron oxide primer coating on three kinds of metal substrates were investigated under salt spray and cyclic temperature conditions.Finally,the method based on EIS was discussed to rapidly evaluate the protective property of three kinds of organic coatings containing conductive particles on different substrates.The main conclusions obtained are as follows:(1)The eight composite coating systems show good anti-corrosion and anti-aging properties after six cycles in cyclic accelerated experiment(42 d).The topcoat,intermediate coat and primer play different roles in the coating systems.From the results of color difference and gloss loss tests,the fluorocarbon topcoat has better anti-aging performance than polyurethane topcoat.EIS fitting results show that the intermediate coatings hinder the diffusion of the electrolyte by its shielding effect.The flake graphene in the epoxy graphene primer could delay the diffusion of the electrolyte by extending the diffusion channels.The zinc powder in the epoxy zinc-rich graphene primer has the cathodic protection from the dissolution of zinc particles and shielding protection from graphene lamellar structure.The iron oxide particles in the epoxy iron oxide primer can reduce the diffusion channel of the electrolyte by filling the pores in the coating.(2)The epoxy zinc-rich graphene coating presents better behaviors than the epoxy iron oxide coating in both salt spray and cyclic temperature experiments.This is because the cathodic protection from the zinc particles dissolution and shielding action of the pores filling results from the zinc corrosion products,meanwhile the graphene lamellar structure enhance the barrier property of the coating and improve the cathodic protection efficiency by increasing the electric connection between zinc particles.Under the same condition,the results of gloss loss and color difference tests show that the epoxy zinc-rich graphene coating has better anti-aging property than that of iron oxide coating.(3)The results of salt spray and cyclic temperature tests show that,there are differences in the degradation degree and the electrochemical behavior of the same coating on different metals.The decreasing rate of the impedance at low frequency of the aluminum alloy/coating sample is smaller than that of the brass/coating sample,and then the 20#steel/coating sample.This might because the existence of the dense passive film on the aluminum alloy which has a protective effect.The salt film formed on the brass substrate is not stable enough to stop the corrosion reaction.While the corrosion products on the active 20#steel under the coating have not much protection protection.(4)The break-point frequency fb can effectively evaluate the degradation behavior of magnesium-rich epoxy coatings on magnesium alloy and aluminum alloy in 3.5%NaCl solution.When the value of fb is smaller than 10 Hz,manifesting good protection behavior,while the fb value bigger than 10 Hz means the coating basically has poor protection behavior.The variation of the phase angle has in 1-100 Hz frequency range is correlated very well with the variation of |Z|0.01hz,and the correlation coefficients are almost all higher than 0.9,so the phase angles in the intermediate frequency range which closely related to |Z|0.01Hz can be used to rapidly evaluate the coating performance of the studied magnesium-rich epoxy coating and zinc-rich epoxy coating.