The EIS Characteristics Of Organic Coatings With Two Kinds Of Conductive Particles

Most metal corrosion can occur in service environments. Organic coating is an effective method to prevent metals from corrosion. EIS, is widely used to study the performance of coatings. The theory of EIS for organic coatings has been very mature, but, The EIS of the coatings with conductive particles need further study.Three kinds of organic coatings, including epoxy varnish coating,carbon black containing epoxy coating and aluminum powder containing epoxy coating, were coated on the carbon steel surface. All the coating thickness is about 150 μm. The electrochemical behaviors of the three types of epoxy coatings immersed in 3.5 wt% NaCl solution were researched by EIS method. The functional groups and degradation of coatings were analyzed by fourier transform infrared spectroscopy(FT-IR). The substrate surface was studied by SEM/EDS.In addition, several rapid evaluation methods, such as break point frequency, phase angle at intermediate frequency and phase angle at high frequency, were used to evaluate the protective performance of the three types of coatings. The obtained results are as follows.1. The EIS spectra of epoxy varnish coating shows only one time constant in the early immersion stage, While the EIS spectra of epoxy coating containing carbon black shows two time constants in the early immersion stage. This is probably due to a larger specific surface area of the carbon black which cause an obvious interface effects. with the amount of carbon black increasing, carbon black surface charge transfer resistance Rp decreases. For the coating with carbon black, the low-frequency impedance modulus|Z|0.01Hz and coating resistance Rc of epoxy coating decrease significantly. With the extension of immersion time, The|Z|0.01Hz and Rc of epoxy varnish coating decrease while the|Z|0.01Hz and Rc of carbon black containing epoxy coating increase slowly. FT-IR results of the epoxy coatings show no obvious degradation during the immersion. The SEM/EDS results showed that, after 200 d immersion in 3.5 wt% NaCl solution, the substrate coated by epoxy varnish coating corrode while the substrate coated by carbon black (1 wt% and 3 wt%) containing epoxy coating is intact. It might because that the reinforcing effect of carbon black can significantly improve the solvent resistance and anti-permeability of the coating.2. According to the equivalent circuit fitting results, the failure process of coatings with alumium powder was divided into three stages:the first stage, the corrosion reaction of aluminum powder particles and diffusion processes; the second stage, corrosive media and corrosion products in the coating/substrate interface lateral diffusion process; the third stage, the accumulation of corrosion products and the formation of corrosion product layer stage.Compared with epoxy varnish coating, epoxy coating containing aluminum particles shows a lower|Z|0.01Hz and Rc, and with extension of immersion time, the|Z|0.01Hz and Rc declines quickly. Coating capacitance (Qc) of epoxy coating slightly increase after adding aluminum powder and rapidly increase with the immersion time increasing while the Qc of epoxy varnish coating changes little. FT-IR results showed that aluminum powder epoxy coatings dosen’t an obviously degradation during immersion. SEM/EDS results showed that in the late immersion stage, the corrosion products on the substrate surface coated by epoxy varnish coating is mainly composed of Fe and O, While a mixed corrosion product layer containing Fe, Al, O and Cl is formed on the substrate surface coated by aluminum powder containing epoxy coating.3. Characteristic frequency method can be used to evaluate the epoxy coating which contains conductive particles. Phase angle at 0.1 Hz (θ0.01Hz) can more accurately evaluate the protection performance of carbon black containing coating, and phase angle at 10 Hz (θ10Hz) can accurately evaluate the protection performance of aluminum powder containing epoxy coating.