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Corrosion Behavior of Carbon Steel and Weathering Steel in Several Typical Conditions

Author: XiaZuo
Tutor: ZhangJianQing;CaoFaHe
School: Zhejiang University
Course: Physical and chemical
Keywords: Carbon steel Weathering steel TEL SECM EIS Polarization Curve Rust laver
CLC: TG142.1
Type: PhD thesis
Year: 2014
Downloads: 5
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Abstract

As the most important structural material, the steel is applied to almost the whole industrial domain for its excellent properties. Carbon steel is one of the foundation of steel, which is not only used widely in buildings, bridges, railways, vehicles, ships and all kinds of machine-making industrial, but also applied to petrochemical industry and ocean development abundantly. However, the corrosion resistance of carbon steel in highly aggressive medium, atmosphere, seawater and soil is so poor that he economic and social loss that the corrosion of steel brings about is particularly huge, and the cost for protection of steels is extremely high. Fortunately, the weathering steel which is low cost and highly protective can be obtained from the addition of trace alloy elements. At present, the corrosion resistance of weathering steel has reached to2-8times of ordinary steel. As important industrial materials, the investigations of the corrosion behavior and protection of carbon steel and weathering steel are increasingly paid attention to. In this dissertation, the corrosion behavior of weathering steel in TELs, the steels with rust and passive film were investigated via micro-and macro-electrochemical methods combining physical characterization exhaustively. The main contents and results are listed as follows:The corrosion behavior of weathering steel under thin electrolyte layer (TEL) at various relative humidities (RHs) was investigated by cathodic polarization curves, electrochemical impedance spectroscopy (EIS), OM, SEM/EDS, XRD and Raman spectroscopy. The results show that at the initial immersion stage (first2h), the100μm TEL has the largest corrosion rate, following by200μm and50μm, and then400μm. There is a point for the maximum value of corrosion rate between100and50μm, the corrosion under TEL is under the control of O2diffusion except50μm TEL which controlled by anodic process. For the50and100μm TEL, the corrosion rate is relatively large due to the sufficient supplement of oxygen in the initial stage and the rust accumulated, then the anodic reaction predominates the corrosion reaction, which results in the decrease of corrosion rate; In the case ofthe200μm TEL, the evolution of the corrosion resistance is similar to the first type at low RHs (75%and85%RH), while at97%RH, the corrosion rate keeps on increasing with the immersion time, and the relatively compact rust layer forms on the electrode. In the case of400and1000μm TEL, the porous and loose rust layers are formed on the electrode due to the insufficient oxygen in the beginning immersion, and the corrosion rate consistently increases due to the poor protectiveness of rust layer. In the later stage, the extent of the increasing of corrosion rate decrease due to the thick rust layers. For the thinner TELs less than400μm, the rust layers which form at75%RH are always porous and loose, while that of97%RH are compact, indicating that the RH and corrosion period has an significant influence on the density and then protectiveness of rust layer.SECM was used to investigate corrosion behavior of carbon steel and weathering steel under wet-dry cyclic conditions, combined with conventional electrochemical measurements such as polarization curves and EIS, and the physical characterization method of rust layers such as SEM and XRD. The results show that the rust layer formation in the initial stage reduces the Fe anodic dissolution rate, thereby improves the corrosion resistance of carbon steel and weathering steel, while the rust layer formation in the late period of the experimental results in higher corrosion rate due to the changes of its composition and structural characteristics. Weathering steel corrosion rate is higher than that of carbon steel, and the reduction rate of rust is also higher than that of carbon steel, which are conducive to the formation of rust layer, thus contributing to long-term protection of steel, but the rust layer of weathering steel is not very good protection in the short term. Rust layer is loose, porous and not dense enough, and the main composition of rust layer is crystalline γ-FeOOH, Fe3O4and γ-Fe2O3. The rust layer thickness of weathering steel is higher than that of carbon steel in the same wet-dry cycles condition.Passive films formed on carbon steel by different passivation potentials in borate buffer solution and their degradation behavior were studied by EIS, Mott-Schottky analysis and SECM, while the film composition and morphology were investigated by XPS and OM. The results show that the electrochemical reactivity on the surface of passive film is weakened as the film formation potential increases due to slower charge transfer rate. The heterogeneous electron transfer rate constant k calculated from approach curves decreases linearly with substrate potential increasing, resulting from different electrochemical reactivity on the surface of substrate. The electrochemical reaction resistance of the film increases with film formation potential increasing, while the point defect density decreases, indicating the film becomes less conductive. The ratio Fe3+/Fe2+and the percentage of oxyhydroxide species increase simultaneously as the film formation potential shifts from-0.1V to0.7V. Immersed in NaCl solution, the film formed at0.3V has the strongest corrosion resistance followed by that of0.7V which is porous and loose the self-repair ability in NaCl solution. The corrosion rate of-0.1V is fastest and accompanied by the formation of corrosion product with high protectiveness.The probe approach curves for different reaction rate constant of substrate were also simulated by COMSOL Multiphysics, and the heterogeneous rate constants at the interface of passive films formed at different potential and electrolyte are obtained. Comparing the fitting results by COMSOL and Origin, it is can be found that the variation tendency of ks by the potential of the substrate is similar to each other, but there is a difference in the real value of ks. The probable reason is the distinction of our experiment condition and the condition which in the expression of Origin. Since the Comsol simulation is based on the real corrosion spatial model, the fitting result is more accurate than that got from Origin fitting.

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