Abstract
The corrosion products formed on weathering steels exposed in industrial environments in the United States and Japan for times between 11 and 29 years have been investigated using Mössbauer spectroscopy, Raman spectrometry and x-ray diffraction. Mössbauer spectroscopy was used to measure the fraction of each oxide in the corrosion coatings and Micro-Raman spectrometry was used to locate the oxides to 2 μm spatial resolution. Mössbauer spectroscopy identified the corrosion products as 80% goethite, 15% lepidocrocite and 5% maghemite. A very small amount of magnetite was also identified on some coupons. Raman analysis showed that the corrosion products generally formed as alternating layers containing different oxides. One type of layer, named the protective inner-layer closest to the steel substrate, consisted of nano-sized goethite ranging in size from 5-30 nm and having a mean particle size of about 12 nm. This layer contained small islands of nano-sized maghemite and/or magnetite. The other type of layer, the outer-layer close to the coating surface, consisted of lepidocrocite and goethite with the former oxide being most abundant. The fraction of goethite increased and that of lepidocrocite decreased with increased exposure time. Goethite is most likely the final stable oxide to form, and when present in large fractions and with nano-size particles, offers further corrosion protection to the steel substrate.
