DEVELOPMENT OF NANO-PHASE IRON OXIDES FROM SHORT-TERM ATMOSPHERiC CORROSION OF CARBON STEEL

摘要

The performance of steels used for structural and industrial applications is affected by atmospheric corrosion. The study of development of corrosion products in atmospherically exposed steels is very important to understand the corrosion process. A systematic study has been made of the development of corrosion products formed on carbon steel atmospherically exposed for short times along the Gulf of Mexico. Carbon steel coupons were exposed between 1 and 12 months at a marine location in Campeche, Mexico, to study the corrosion as a function of time. The environmental parameters were monitored during the exposure period. The resulting corrosion products were analyzed by Mossbauer Spectroscopy, Micro-Raman Spectrometry and X-ray diffraction in order to completely identify the oxides and map their location in the corrosion coating. The most abundant oxides present in the corrosion products were lepidocrocite and goethite as evidenced by in-situ X-ray diffraction and transmission Mossbauer and micro-Raman analysis of the corroded coupons. Akaganeite was absent in the corrosion coatings even though the exposure site was in a marine environment. In a similar exposure of carbon steel coupons at one of the marine sites prior to this exposure, akaganeite was detected in the corrosion products. However during this present exposure the average chloride concentration was measured to be only 12 mg/m~2/day as opposed to an average chloride concentration of 45 mg/m2/day measured in the previous exposure. Mflssbauer analysis at 300 K and 77 K showed the presence of lepidocrocite and superparamagnetic goethite in amounts dependent on the exposure time. The average particle size of goethite in the corrosion coating was found to lie in the range of 7-10 nm. Mossbauer spectra of the corrosion products recorded at 4 K, showed the presence of an x-ray amorphous phase whose hyperfine parameters corresponded to ferrihydrite with particle size < 8 nm. The ratio of a-FeOOH/y-FeOOH, measured by Moesbauer spectroscopy was found to be nearly 3.5 in carbon steel for short-term exposures, compared to the reported ratio of 1.5 for weathering steel for similar exposure periods from X-ray diffraction measurements. Micro-Raman analysis showed that the corrosion coatings were multi-layered with lepidocrocite closer to the surface and goethite closer to the steel.