Transmission Mössbauer Analysis of Nanophased Oxides Formed on High Strength Steels

Abstract

Nanophased oxides found in the corrosion coatings of atmospherically weathered steels have properties that are scientifically significant and industrially important. Mossbauer spectroscopy proves to be a very useful tool to accurately characterize the corrosion coatings. Samples of carbon steel were exposed in Campeche, along the Gulf of Mexico for up to one year and the development of corrosion products as a function of steel type and exposure time were studied using Mossbauer spectroscopy, micro-Raman spectrometry and X-ray diffraction. Both X-ray diffraction and transmission Mossbauer spectroscopic results indicated that lepidocrocite, maghemite and goethite were the dominant oxides. Transmission Mossbauer analysis at 77 K indicated that for up to three months of exposure, lepidocrocite and maghemite accounted for nearly 80% of the relative amount, with goethite contributing only 20% to the mixture. However, as the exposure time increased to 6 months, the relative contribution of goethite increased at the expense of decreasing amounts of maghemite. Monitoring the environment during the exposure time indicated that the average time of wetness decreased. The decrease in the relative contribution of maghemite to the total oxide concentration is related to the decreasing time of wetness, with increasing exposure time.