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Electrochemical Distribution Characteristics Of Corrosion Behavior Of Carbon Steel Immersed In Seawater

Posted on:2011-11-10Degree:MasterType:Thesis
Country:ChinaCandidate:Y Y ZhengFull Text:PDF
GTID:2131330332464891Subject:Applied Chemistry
Abstract/Summary:PDF Full Text Request
Corrosion behavior of the metal covered with rust is a complex electrochemical system. The heterogeneous electrochemical characteristics of solution/metal interface can influence the corrosion behavior and corrosion rate which has drawn increasing attentions. Conventional electrochemical techniques, which have been successfully used in metal corrosion for many years can only obtain average data on metals surface. Therefore, it is very difficult to perform them in heterogeneous electrochemical research, but local electrochemical techniques can overcome these limitations. The current and potential distributions of metal surface are important parameters to study the micro-electrochemical characteristics. In order to study the current and potential distributions of the metal, scanning micro-electrode technique and wire beam electrode technique were used. The result showed that the current and potential had temporal and spatial distribution characteristics during the corrosion process though the metal surface was not uniform. When the samples were immersed into seawater horizontally, the cathodic region was chiefly distributed around the metal surface, while the middle area was the anode; when immersed vertically for only a few days, cathode was mainly located at the top of the electrode, with the immersion time increasing, potential on the surface shifted to positive direction and the bottom area with black products became the main cathodic region.The reasons for these distribution characteristics were explored. When immersed horizontally, dissolved oxygen could reach the edge of the electrode more easily than the middle area, so regions around the electrode had higher dissolved oxygen concentration, the dissolved oxygen reduced into OH-1 which made the edges became the cathode and the middle area was the main anode. When immersed vertically, the top of the strip electrode was closer to the surface of seawater and free oxygen from the atmosphere entered the water easily to form higher concentration of dissolved oxygen which resulted in the oxygen concentration cell. But then why the bottom of the electrode became the main cathode was not clear.A large number of experimental data have showed that the impetus for galvanic cell came from the potential difference between the cathode and anode. The potential difference on the surface was tested and the result indicated that the surface potential difference diminished with time, by which indicated that the corrosion rate was steadier than the early immersion.The composition and distribution characteristics of corrosion products which can influence the corrosion rate were tested by IR spectroscopy and ESEM. Studies indicated that after a period of time (about 8 weeks), rust layers were divided into two layers:the outer layer and the inner layer. The outer layer was loose and porous which was mainly composed ofγ-FeOOH and the rust composition changed little with time. The inner layer was black, more compact and closely attached to the metal surface and which components changed with time:theγ-FeOOH reduced or even disappeared while theβ-FeOOH and Fe3O4 gradually accumulated. With the increase of immersion time, the inner rust layer gradually thickened and the outer rust layer thinning. The result also indicated that there was a certain correlation between the corrosion products and potential distribution. Rust layer in the anode region was composed of (3-FeOOH and Fe3O4, in the meanwhile cathodic area was mainlyγ-FeOOH andα-FeOOH.All in all, the distribution of anode/cathode regions had certain regular by study the heterogeneous electrochemical characteristics on metal surface. These distribution features influenced the generation of the corrosion products as well as the composition of the rust. It was concluded that the potential distribution on metal surface was closely related to the corrosion products which have great significance for the further research on the corrosion electrochemical properties and mechanism of metal covered with rust.
Keywords/Search Tags:Carbon steel, Wire beam electrode, Current/potential distribution, Scanning micro-electrode technique, IR spectroscopy, Rust
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