Nano-scale chemical analysis of rust on a 2% Si-bearing low alloy steel exposed in a coastal environment

Corrosion products of the rust layer formed on a 2% Si-bearing low alloy steel during atmospheric exposure at a coastal area in Japan for three years were characterized. The inner layer of the rust, i.e., near oxide/metal interface, has a layer structure and the average grain size of corrosion products is finer than that in the upper region that is few micro meter above the interface. A series of nano probe energy disperse X-ray spectroscopy (EDS) and selected area electron diffraction (SAED) analyses performed using a field emission gun (FEG) transmission electron microscope (TEM) revealed that the inner layer consists of an iron rich, coarse grained β-FeOOH matrix and the silicon enriched narrow band composed a mixture of α-FeOOH and nano-meter size silicon oxide. On the other hand, the α-FeOOH single phase area exists where the layer structure did not develop. The result of detailed microstructural analyses indicated that the silicon plays an important role in modifying the rust layer structure.