The patina of the HPS 70W steel ASTM A709 used in the delta frames of Centennial Bridge was analyzed using Fourier transform infrared spectroscopy, X-ray diffraction and Mössbauer spectroscopy. In general, the patina is formed by goethite (?-FeOOH), lepidocrocite (?-FeOOH), a spinel phase consisting of non-stoichiometric magnetite (Fe3-xO4) and/or maghemite (?-Fe2O3) and hematite (?-Fe2O3). Hematite, which is not a usual corrosion product in atmospheric corrosion, is a very stable oxide. FTIR also identified feroxyhyte (?'-FeOOH). A significant proportion, about 50%, is goethite with particle sizes below 15 nm. It is concluded that the patina, due to its composition and characteristics, is protective against corrosion. We recommend keeping a preventive inspection, particularly of moisture and standing water and rust.
