Zinc And Carbon Steel Galvanized Layer In The Simulation Of Atmospheric Environment, The Failure Process Of Electrochemical Research

"Thin electrolyte layer (TEL)" experiment was applied to simulate the atmospheric corrosion in this thesis. Electrochemical techniques, including electrochemical impedance spectroscopy (EIS), cathodic polarization curves and etc., and materials characterization method like SEM-EDX were used to study the atmospheric corrosion of pure zinc and its alloy, Q235 steel and galvanized steel under indoor simulating acceleration environment.Corrosion rate of zinc alloy is faster than that of pure zinc due to the effect of the micro-galvanic couples between the primary phase and the eutectic phase. The results indicated corrosion rate of zinc alloy was greatly enhanced under TEL than that in bulk solution. The corrosion rate of pure zinc in bulk solution was higher than that under TEL 400μm and 300μm. Minimum corrosion resistance exhibited as TEL decreased to 198μm. There were two capacitive loops in high frequency (HF) and middle frequency (MF) respectively, with finite length diffusion in low frequency (LF) in EIS plots. For pure zinc under TEL below 300μm an additional inductive loop presented in MF-LF. Zinc and its alloy exhibited localized corrosion under TEL while it was more uniform in bulk solution. The component of the corrosion products is similar at different section of the specimen surface but different in content after the immersion tests.The corrosion resistance of Q235 steel under TEL was lower than that in bulk solution, as its corrosion rate was controlled by oxygen diffusion process. The rust layer comprised two layers but without any protection in Cl- containing environment. The outer layer of the rust was loose and porous, while the inner layer was mainly composed of oc-FeOOH and y-FeOOH. The inner layer became thicker with the thinner of TEL. However, there was only one layer of the rust on the steel surface after immersion in bulk solution which was very porous. With the effect of cathodic process as well as corrosion products, it behaved highest corrosion rate at TEL 200μm. The rust layer on Q235 steel provided little protection from corrosion that there were relatively high corrosion rate after long time immersion.After electrodeposition of zinc on Q235 steel surface, zinc corrosion was the main reaction during immersion. The corrosion rate of galvanized steel in 3.5wt.% NaCl bulk solution was lower than that under TEL. The corrosion process was controlled by cathodic process as there were limiting current plateau of oxygen reduction in cathodic polarization curves. A capacitive loop appeared in HF and MF respectively with finite length diffusion in low frequency (LF) in EIS. There was highest corrosion rate at TEL 99μm. Under relatively thicker TEL, the corrosion rate increased with the thinner of TEL at initial stage, but it closed to that in bulk solution in the later stage. The galvanized coatings behaved localized corrosion. Corrosion model based on EIS and SEM images was proposed to investigate the four-staged degradation of galvanized steel which comprised disruption of oxide coatings on coating surface, dissolution of intact galvanized coatings, galvanized corrosion after destroy of the galvanized coatings and corrosion of the steel substrate as galvanized coatings seriously destroyed.