Protective Ability Index of Rust Layer Formed on Weathering Steel Bridge

Abstract

For a quantitative inspection on the performance of weathering steel bridges, we have investigated the relationship between the corrosion rate and the composition of the rust layers formed on weathering steel bridges located in various environments in Japan and applied a protective ability index (PAI) to the bridges. The corrosion rates were clearly classified by the PAI, α/γ* and sub index of (β+s)/γ*, where α, γ*, β and s are the mass ratio of crystalline α-FeOOH, the total of γ-FeOOH+ β-FeOOH + the spinel-type iron oxide (mainly Fe3O4), β-FeOOH and spinel-type iron oxide, analyzed by powder X-ray diffraction, respectively. In the case of α/γ*>1, the rust layer works protective enough to reduce the corrosion rate less than 0.01 mm/y. The sub index (β+s)/γ*<0.5 or >0.5 classifies the corrosion rate of the non-protective rust layers, therefore the former state of the rust layer terms inactive and the latter terms active. The quantitative inspection of a weathering steel bridge requires a performance-inspection (PI) and periodical deterioration- inspections (DI). The PI can be completed by checking of the PAI, α/γ*. The DI on the weathering steel bridges where deicing salt is sprinkled can be performed by checking the PAI, (β+s)/γ*.