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.
In the 1960’s and 1970’s, weathering steel was commonly viewed as a “maintenance-free” cost-effective alternative for bridge superstructures, as the costs and environmental impacts associated with the maintenance/replacement of paint coatings would be theoretically eliminated. Weathering steel was first used for bridge applications in 1964, when weathering steel bridges were constructed in Michigan, Iowa and New Jersey. During the late 1970’s, Michigan evaluated many of their weathering steel bridges and found that the protective patina had not formed and the corrosion rates were not reduced. As a result,
Michigan instituted a partial moratorium on weathering steel bridges in 1979. It focused on avoiding
weathering steel in urban and industrial areas, as well as low-clearance, tunnel-like environments. In
1980, the moratorium was expanded to include all weathering steel bridges. However, by this time
Michigan had already constructed approximately 500 bridges using weathering steel.
After the Michigan moratorium, many states followed suit by limiting or eliminating weathering steel
bridges going forward, often stopping short of banning their use outright. The most common reasons for
the limitations were the problems reported by Michigan or other states, and/or similar conditions observed within their own state.
By the mid-1980’s, approximately 2,000 weathering steel bridge structures had been constructed in the
United States. A decade later, the number had only increased by approximately 300 structures, indicating
that many agencies had reservations regarding their use. The perceived problems with the use of
weathering steel for bridge structures resulted in the initiation of several research projects during the
1980’s and 1990’s.
In the early 1980’s, a task group was formed with the support of the American Iron and Steel Institute
(AISI) that included federal highway officials, state bridge engineers, and corrosion specialists. This task
group performed a field investigation of approximately 49 bridges in seven states (Michigan, Illinois,
Maryland, New York, North Carolina, Wisconsin, and New Jersey) in order to evaluate the performance
of weathering steel bridges in different environments. This study found that the performance of most
weathering steel bridges was either “good” or “good with moderate corrosion in some areas.”
Approximately 12% of the bridges inspected had “heavy” corrosion in some areas and deicing salts were
found to be a major contributor to the excessive corrosion.
In 1984, the National Cooperative Highway Research Program (NCHRP) set out to document the state of
the practice, evaluate the performance of weathering steel to date, and develop practical guidelines for
design, construction, maintenance, and rehabilitation. The findings of this study are presented in NCHRP
Report 272. Around the same time, a Michigan study identified several weathering steel bridges where a
fine-grained brownish-black patina had not formed; instead the surface consisted of large delaminating
flakes or thick exfoliating rust layers. Each of these studies identified examples of weathering steel bridge
structures that may not have been used in appropriate environments. Particularly, the studies noted that
weathering steel was not always “maintenance-free” and extended periods of wetness and heavy exposure
to deicing salts caused the weathering steel to be vulnerable to ongoing, advanced corrosion.
In 1988, the FHWA sponsored a forum on weathering steel to bring together owners, designers, suppliers,
fabricators, researchers, and maintenance personnel to examine the state of the art and develop guidelines
on the proper use and maintenance of weathering steel. Subsequently, FHWA issued a Technical
Advisory in 1989 providing guidelines for weathering steel use and maintenance. In addition, NCHRP
issued Report 314, which served as a comprehensive summary of weathering steel properties, guidelines
for use, and recommendations for inspection and maintenance. In general, the research revealed that
weathering steel performed satisfactorily in the right environments with proper detailing. This finding
resulted in renewed interest in the use of the material. Currently, it is reported that 40% to 45% of bridges
are being built with some form of weathering steel.
Assessment of Weathering Steel Bridge Structures in Iowa
The renewed interest in weathering steel bridge structures has also resulted in ongoing research on the
topics of patina performance, inspection, and maintenance. Recent research in Japan has reiterated that
chloride ions accelerate the growth of the rust layer and increase the rust particle size, which is not
beneficial to the formation of a protective patina. In addition, Japan evaluates the appropriateness of a
weathering steel brid