Structure and properties of the protective rust layers on systematically exposed weathering steels underneath 5 bridges in the main island of Japan for 17 years were investigated using various analytical methods. The weathering steels formed a protective rust layer containing alloying elements such as Cr and Cu. It was clarified that so obtained rust layers on the weathering steels were mainly composed of the goethite (α-FeOOH)-type structure. Protective properties of the rust layers were considered to be related with the suppression of ion transport due to its densely packed structure as indicated by nitrogen adsorption isotherm analysis. The influence of air-borne salt on the rust structure was also elucidated in such a way that content of β-FeOOH and rust particle size increase as the level of air-borne salt becomes high, which would cause less protective performance of the rust layer on weathering steels.
