Rust samples from unpainted weathering steel bridge spans from seven Louisiana bridges and from three other states were subjected to various types of analysis to determine the mechanism of rusting on the weathering steels. The rust is generally in the form of coarse flakes in the coastal area, sheets in areas in which water retention and chloride contents are high, and powdery and fine grained in the middle and northern regions of the state. Layered sheet samples exhibit γ·Fe2O3·H2O at the air contact surface and magnetite (Fe3O4) at the metal contact surface. In addition to these forms of rust, α-FeOOH and δ-FeOOH have been found in the intermediate layers and in flakes, identified primarily through infrared absorption analysis and scanning electron microscopy (SEM). It is theorized from general findings that the γ form of rust forms from an amorphous bulk and is converted to the α- and δ-FeOOH forms, which are electrochemically reduced to magnetite in chloride-containing solutions.
In addition to chloride, sulfur oxide, calcium, and silicon are picked up from the environment, with the latter two through dust particles. The fallout of the dust particles, especially those containing calcium, is beneficial in that they not only seal the pores in the rust layer, but also prevent the pH from becoming too low. The leached extracts from several samples exhibited pH values in the mildly acidic range, which would increase the corrosion rate and pitting.
