| Atmospheric corrosion is one of the most widespread forms of corrosion. It is a gas/liquid/solid multiphase corrosion system. For atmospheric corrosion, the gas and solid phases are relatively stable while the liquid phase could change significantly with the fluctuations of atmospheric environment. Therefore, the liquid state is an important factor of influencing atmospheric corrosion. The main phenomena of liquid state on metal surface are the change of thin electrolyte layer (TEL) thickness, the change of ion concentration and the change of liquid dispersion degree. The change of liquid state has important influence on atmospheric corrosion rate. Another remarkable feature of atmospheric corrosion is the influence of wet-dry cycles. Because the amount of electrolyte is relatively little, the corrosion product could not migrate timely and it would deposit on metal surface quickly. The rust on carbon steel surface can influence a series of corrosion process and change the atmospheric corrosion rate a lot. Therefore, studying the influence of liquid state on atmospheric . corrosion and the corrosion process for rusted metal will have critical significance.In this paper, the wire beam electrode (WBE) method combined with electrochemical impedance spectroscopy (EIS) was firstly used to investigate the influence of TEL thickness and electrolyte concentration on corrosion rate of A3 carbon steel during the initial stage. The WBE method can measure the current value on metal surface under different condition. Therefore it is a simple and direct electrochemical method for atmospheric corrosion research. The WBE method can also achieve in situ monitoring for atmospheric corrosion. The results showed that TEL thickness is the main influence factor for atmospheric corrosion. For different concentration of electrolyte, the diffusion layer thickness was all about 500μm. When TEL is thicker than 500μm, the corrosion rate is independent of the TEL thickness and equal to full immersion in bulk solution. When the TEL thickness is less 500μm, the corrosion rate will increase quickly with the decrease of TEL thickness. But when the TEL on metal surface is very thin, the anodic reaction would become the rate determining step, and the corrosion rate would decrease quickly with the decrease of TEL thickness. Therefore a maximum corrosion rage was obsevered when corrosion rate controlling step changed. For different corrosion system, with the decline of electrolyte concentration, the maximum current decreased and the position of maximum current appeard under thicker TEL thickness.On the basis of the previous TEL experiment, the WBE method combined with the stereomicroscope observation method, was used to study the liquid dispersion degree on the corrosion rate of A3 carbon steel. For different size of droplets, the corrosion process and mechanism was same. The current under droplets presented as the cathode at the edge while the anode in the center. The OH- generated by the cathodic reaction migrated to the center and the Fe2+ generated by the anodic reaction migrated to the edge. Fe(OH)2 precipitated out and was quickly oxidized to Fe(OH)3. However, the liquid dispersion degree had importance influence on atmospheric corrosion. When the total amount of electrolyte on metal surface is certain, the more number of the droplets, the higher of the liquid dispersion degree. The dispersed droplets caused more serious corrosion. The WBE method can measure multiple droplets corrosion process on metal surface simultaneously, therefore, based on the dispersion droplets experiment, a corrosion model of atmospheric corrosion with liquid dispersion degree was established. The number of droplets and the radius of droplets followed the basic fractal law, the total circumference is proportional to the number of droplets, and the corrosion rate is in proportion to the degree of liquid dispersion.Wet-dry cycle is a comman atmospheric corrosion phenomena, the corrosion products on metal surface have importance influence on the corrosion behavior of A3 carbon steel during the wet-dry cycles. In this work, with the help of electrochemical research methods and physical characterization methods, the conversion and function mechanism of rust on carbon steel was studied. During the wetting stage, the active γ-FeOOH would participate in the corrosion process. Firstly, γ-FeOOH will directly act as oxidizer and consume the electron from metal dissolve. After that, the reduction product of γ-Fe·OH·OH is an electronic conductor which can act as cathodic reaction active area. The rust can enlarge the active cathodic reaction area, therefore the corrosion rate can be increased a lot. During the drying stage, the rust layer is oxidized and the γ-Fe·OH·OH becomes back to γ-FeOOH. γ-FeOOH will participate the corrosion process again for the next wet-dry cycle. The existed electrochemical active corrosion products caused the increase of atmospheric corrosion. In this paper, a corrosion model for rusted metal under TEL was established according to the experimental results and speculation. |
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