Weathering steel is commonly used as a cost-effective alternative for bridge superstructures, as the costs and environmental impacts associated with the maintenance/replacement of paint coatings are theoretically eliminated. The performance of weathering steel depends on the proper formation of a surface patina, which consists of a dense layer of corrosion product used to protect the steel from further atmospheric corrosion.
Under the right conditions, weathering steel will form a protective oxide coating that will eliminate the need for future painting. The oxide layer, or patina, is typically dense and well-adhered to the base metal, reducing the penetration of moisture, oxygen, and other corrosive contaminants. This protective patina thus reduces long-term corrosion rates. By eliminating the need for painting, the lower life-cycle costs of weathering steel structures provide an advantage over standard painted steel structures. Weathering steel evolved in the 1930’s, when United States Steel Corporation acquired various patents for high-strength low-alloy steel products. By alloying the steel with different elements, particularly copper, it was found that the corrosion resistance of the steel was improved, virtually eliminating the need for painting. In addition, the alloys resulted in an increased yield strength of the steel products. Marketed under the name “Cor-Ten,” United States Steel Corporation’s weathering steel products were first used in various applications, including coal-hopper cars, barges, transmission towers, and trolley cars. Recognizing the benefits of improved atmospheric corrosion resistance and higher strength, several other steel companies developed forms of weathering steel. Although “Cor-Ten” is the most recognizable, other weathering steel products included “Mayari R” by Bethlehem Steel Corporation and “Yolloy” by Youngstown Steel. Initially introduced in 1968, ASTM A588 Grade 50W (AASHTO M222) covered weathering steel products comprised of typical mild steel alloyed with 2% or less of various elements, including copper, phosphorus, chromium, nickel, and/or silicon. In 1974, ASTM A709 Grade 50W (AASHTO M270) was introduced to cover corrosion resistant steels for use in bridges. By alloying steels with copper or other selected elements, it was found that the corrosion resistance increased to approximately four times that of structural carbon steel without copper.
This project focused on the evaluation of weathering steel bridge structures, including possible methods to assess the quality of the weathering steel patina and to properly maintain the quality of the patina. The objectives of this project are summarized as follows:
Identify weathering steel bridge structures that would be most vulnerable to chloride contamination, based on location, exposure, environment, and other factors.
Identify locations on an individual weathering steel bridge structure that would be most susceptible to chloride contamination, such as below joints, splash/spray zones, and areas of ponding water or debris.
Identify possible testing methods and/or inspection techniques for inspectors to evaluate the quality of the weathering steel patina at locations discussed above.
Identify possible methods to measure and evaluate the level of chloride contamination at the locations discussed above.
Evaluate the effectiveness of water washing on removing chlorides from the weathering steel patina.
Develop a general prioritization for the washing of bridge structures based on the structure’s location, environment, inspection observations, patina evaluation findings, and chloride test results.