Protectiveness of Rust Formed on Weathering Steel in Atmosphere Rich in Air-Born Chloride Particles

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The influence of the chloride deposition on the protectiveness of rust formed on weathering steel in long term exposure was examined. Specimens of weathering steel containing Cu and P were exposed to a rural environment for 41 years. After the exposure, the corrosion behavior of the weathering steel with protective rust in the laboratory corrosion tests simulating coastal environments was observed. In the test, spots of orange rust appeared on the surface of the specimens, which increased in number and size with the increase of the deposited NaCl. Amount of dissolved ferrous ions also was enhanced when the deposited NaCl was over critical amount. Through EPMA inspection of the cross sections of the rust, it was confident that Cl ions penetrated into the rust to reach the rust/steel interface, which suggested that the dissolution of the iron occur at the same interface. From the result of X-ray diffraction analyses and polarized light microscopic observations, change in the structure of the rust was hardly seen. The protectiveness of the rust was found to have a certain limit regarding chloride deposition, although its basic structure was apparently maintained.
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