Many of the early studies performed on weathering steel bridges concentrated on the performance of existing bridges in order to correlate their performance to factors such as exposure and environment. This research is presented in reports prepared by AISI, the Michigan Department of Transportation (MDOT), NCHRP, as well as several others. Subsequent research confirmed many of the observations and findings of the early studies.
The performance of weathering steel depends on the proper formation of a dense layer of corrosion product, or patina, to protect the steel from further atmospheric corrosion. Initially, the corrosion rate of weathering steel is similar to that of plain carbon steel; however, wetting and drying cycles, or natural weathering, causes weathering steel to develop a dense oxide layer on the exposed surfaces. This dense, well-adhered patina helps to slow additional rust formation and the corrosion rate stabilizes.
The development of the weathering steel patina may be hindered by environmental factors such as humid environments, extended periods of wetness, sheltering, exposure to deicing chlorides, and design details that permit water to pond on steel surfaces. In the mid-1980’s, inspections performed by MDOT found bridges where the corrosion rate of the patina did not slow. At these bridges, the patina consisted of large flakes delaminating from the surface and/or exfoliating rust layers rather than a fine-grained, brownishblack oxide layer. The problems in Michigan appeared to correlate to heavy chloride exposure in urban or industrial areas, extended periods of wetness resulting from depressed or “tunnel-like” grade separations, and/or water contamination from deicing salts reaching the superstructure through leaking expansion joints, cracks in the concrete deck, or directly over the edge of the deck.
In most cases, early examples of poor weathering steel performance could be linked to two primary factors: heavy exposure to deicing salts and extended periods of wetness. Both of these factors are often present below leaking expansion joints or cracks in the concrete deck. In addition, traffic below bridge structures creates a “spray” or “fogging” that results in chlorides being deposited on the bridge structure from below. In this “splash zone” the chlorides often settle onto the top surface of the bottom flange along with dust, dirt, rust flakes, bird droppings, and other debris. Past research commonly referred to this contaminant layer as a “poultice” that retains moisture and keeps chlorides in close contact with these horizontal surfaces. Poultice corrosion was a common observation on horizontal surfaces above traffic or in areas where water and dirt could migrate and settle outside of the splash zone. It should be noted that mill scale on weathering steel does not have any significant effect on its overall performance. Mill scale does, however, create variation in the surface appearance of the protective patina. The recommendation of the AISI task group is to leave the mill scale unless aesthetics is a consideration.
Performance of Weathering Steel Bridges
Carbon Steel, Corten Steel, Stainless steel
Rust, Bare, Powder Coated, Polished, Black Heat-resisting Paint, Iron Oxide
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