| Zinc is widely used as galvanized coating in power transmission system, particularly in transmission tower anti-corrosion field. With the fast development of electric network in China, the consumption of zinc increases with a high speed. The service condition of power transmission equipment is outdoor atmospheric environment, which means atmospheric corrosion is a common corrosion form of zinc and galvanized steel. Atmospheric corrosion is one kind of electrochemical corrosion, which is caused by the thin electrolyte layer(TEL) covering on the metal surface. The corrosion of transmission tower is not only influenced by atmospheric environment, but also by the interference voltage from high-voltage transmission line. In this paper, the atmospheric corrosion behaviour and mechanism of zinc and galvanized steel under the effect of direct current/alternating electric field have been investigated by cyclic wet/dry device and thin-layer electrochemical cell using weight gain method, electrochemical methods and Surface analysis methods. The main contents are listed as follows:1.The feasibility of using chronopotentiometry to determine diffusion coefficient of oxygen under thin electrolyte layer has been verified. Based on this result, the influence of direct current/alternating electric field to the transport process of oxygen under thin electrolyte layer has been investigated. The results showed that diffusion coefficient of oxygen under TEL increased with decreased TEL thickness and increased electric field strength. The thinner electrolyte layer and stronger electric field the easier oxygen could pass through the gas/liquid interface to the electrode surface, which means a promotion to cathodic process and results in a higher corrosion rate of the metal.2.The current distribution profile over copper surface with different diameter and the influence of copper wire diameter to the corrosion behaviour of copper under TEL have been investigated by polarization curve and EIS. The results indicated that when TEL thickness to copper wire diameter ratio was no less than 2, the phase shift θ went further than-45° on the plot of θ to frequency. In this condition the current distribution profile became uniformly and the accurate corrosion information of metal under TEL can be determined by EIS data.3.The influence of direct current/alternating electric field to the corrosion behaviours of zinc and carbon steel in a chloride-containing environment have been investigated by using multiple methods, such as weight gain method, electrochemical impedance spectroscopy(EIS) and weak polarization curve method. The results showed that the corrosion rate of zinc decreased first, then slowly increased to a stable level with the increased cyclic wet/dry time. And the corrosion behaviour of carbon steel contraries to zinc, with the increasing of cyclic wet/day time, the corrosion rate of carbon steel increased at first, and then decreased to a stable level. Direct current/alternating electric field accelerated corrosion of zinc and carbon steel, and the corrosion rate was proportional to the electric field strength.4.The influence of direct current/alternating electric field to the composition and structure of corrosion products of zinc and carbon steel has been studied by scanning electron microscopy/energy dispersive spectrometer(SEM/EDS) and X-ray diffraction(XRD).The results showed that(1)zincite, hydrozincite and simomkolleite are the main corrosion products of zinc but the structure of corrosion products of zinc samples under current/alternating electric field were very different from that blank samples. The corrosion products of blank sample spread uniformly and continuously over the surface, and its amount and size increased with the increasing of cyclic wet/dry time. Weight gain method, EIS and weak polarization curve showed that the corrosion products of blank sample had protective effect to the substrate; with the effect of direct current electric field, the corrosion products presented a morphology of porous and fibrous appearance. Aggressive chloride ion and oxygen, the main depolarizer, were easy to pass through the corrosion products layer to the surface of zinc and result in a corrosion of substrate due to these porous corrosion products.(2)The corrosion products of carbon steel under the effect of direct current electric field were loose and had a poor protectiveness. The main composition of the rust layer was lepidocrocite(γ-Fe OOH), which was an unstable corrosion product of carbon steel. The rust layer of blank sample of carbon steel was dense and showed good protectiveness to the substrate. |
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