Remediating areas of advanced corrosion on a weathering steel bridge superstructure where a protective patina has not formed is often costly and negates the anticipated cost savings for this type of superstructure. Therefore, proper inspection of a weathering steel superstructure is essential for determining the overall performance of the patina and identifying problem areas early so they can be addressed before deterioration progresses. Good patina performance is typically indicated by a fine grained, dark brownish-black, tightly adhered, stable rust layer on the surface of the weathering steel. The protective oxide layer depends on the formation of an initial amorphous layer of corrosion product. The introduction of salts and extended periods of wetness causes other types of corrosion product to form, such as less protective crystalline forms. While the difference between a good patina and a poor patina is determined at a microscopic level involving types of corrosion product and chloride content, early research correlated patina development and performance to various visual indicators. For example, the formation of less protective crystalline oxides resulted in the formation of loose rust scale on the patina surface. Furthermore, it was found that rust scale appearance is proportional to chloride content. Weathering steel surfaces with higher concentrations of chloride in the oxide layer were found to have developed larger, thicker rust flakes in the patina. Ultrasonic thickness gages have been used to measure the depth of corrosion penetration. If the depth of penetration is known, the approximate corrosion rate can be estimated from the age of the structure to assess if the corrosion rate has stabilized. The literature indicates that this testing technique has been used with varying degrees of success. Even with trained equipment operators, the potential exists for variations in the measured thicknesses and/or uncertainties in the results. Also, the initial thickness of the steel plate is typically not known due to fabrication tolerances. Based on these factors, the use of ultrasonic thickness gages for this project was not pursued. NCHRP Report 314 presents guidelines to evaluate the condition of the oxide layer on weathering steel structures. Particularly, the color and texture of the oxide layer can be used to evaluate if the patina is protective or not. However, it also suggests that visual observations alone can be misleading and recommends hammer tapping or wire brushing to determine if the layer is adherent or debonds in the form of granules, flakes, or laminar sheets. The inspector should be familiar with the appearance of weathering steel patinas that form in various environments and microclimates. For instance, the exterior face of a fascia girder may develop a dark-brown, tightly-adhered patina, but the interior face of the same girder may have a dark-brown color with non-adherent, coarse rust flakes.
Inspection of Weathering Steel Bridges
Carbon Steel, Corten Steel, Stainless steel
Rust, Bare, Powder Coated, Polished, Black Heat-resisting Paint, Iron Oxide
Diameter : 320mm--1200mm
Thickness : 1.2mm--3.0mm
Inside : Anti-wear foam paper
Outside : Plywood box
Free (without any condition)
OEM & ODM
Customized service is available.