Abstract The corrosion products formed on the interior weathering steel box-sections comprising one vertical support pier of the Luling Bridge in Louisiana, have been analyzed and compared with those formed on atmospherically exposed coupons for which the protective oxide layer is very adherent and stable. It was determined that very high fractions of large particle maghemite form in the hot and humid conditions inside the boxes. The high time-of-wetness also significantly reduces the fraction of nano-phase goethite which forms and this, along with the presence of maghemite, most likely prevents the formation of the adherent protective corrosion coating needed to reduce the long term corrosion of weathering steel. The spectroscopic analysis of the corrosion products was undertaken using Mössbauer spectroscopy, Raman spectrometry and X-ray diffraction, with morphology and elemental composition of the corrosion products being completed using scanning electron microscopy with energy dispersive spectroscopy facilities. The corrosion product composition varied with height of the box-sections in the pier, most likely due to the different local environmental conditions within each enclosed box. In general the corrosion products contained about 30% maghemite and 60% goethite which is very different to the compositions found on well protected weathering steels. The compositions were closer to those found on carbon steel structures exposed to a mild marine environment. The large amounts of exfoliation in the form of sheet-like rust within the box-sections, supports the argument that insufficient wet-dry cycling conditions are available for proper protective coating formation.