Atmospheric corrosion in the Gulf of Mexico

The corrosion products on steels exposed at two sites in Campeche, Mexico and one site at Kure Beach, USA, have been investigated to determine the extent to which different marine conditions and exposure times control the oxide formation. The corroded coupons were analyzed by Mossbauer, Raman and infrared spectroscopy as well as X-ray diffraction, in order to completely identify the oxides and map their location in the corrosion coating. The coating compositions were determined by Mossbauer spectroscopy using a new parameter, the relative recoilless fraction (F-value) which gives the atomic fraction of iron in each oxide phase from the Mossbauer sub-spectral areas. For shea exposure times, less than three months, an amorphous oxyhydroxide was detected after which a predominance of lepidocrocite (gamma-FeOOH), and akaganeite (beta-FeOOH), were observed in the corrosion coatings with the fraction of the later phase increasing at sites with higher atmospheric chloride concentrations. The analysis also showed that small clusters of magnetite (Fe3O4), and maghemite (gamma-Fe2O3), were seen in the micro-Raman spectra but were not always identified by Mossbauer spectroscopy. For longer exposure times, goethite (alpha-FeOOH), was also identified but little or no beta-FeOOH was observed. It was determined by the Raman analysis that the corrosion products generally consisted of inner and outer layers. The protective layer which acted as a barrier to slow further corrosion, consisted of the alpha-FeOOH and nano-sized gamma-Fe2O3 phases and corresponded to the inner layer close to the steel substrate. The outer layer was formed from high gamma-FeOOH and low alpha-FeOOH concentrations.