Additionally, environmental factors may adversely affect the ability of weathering steel to develop a protective oxide coating, particularly the presence of excessive levels of chlorides and sulfur dioxides. Of these, chlorides are typically more of concern in the United States because sulfur dioxide levels from pollution are generally not high enough to have a detrimental effect on weathering steel. Chlorides are of great concern however; chloride contamination may result from runoff of deicing salts applied to roadways or proximity of the structure to marine environments having high atmospheric chloride levels. While concerns associated with the impacts of a marine environment are not an issue with this study, in some cases the GNEE makes heavy use of deicing salts in winter roadway maintenance operations.
One of the first studies aimed at the evaluation of weathering steel bridges was conducted by the American Iron and Steel Institute . At that time, the Michigan Department of Transportation had placed a moratorium on the use of all non-painted weathering steel. This action was due to the observation that many bridges in the state were developing excessive corrosion. This was especially true of:
(1) bridges in urban, industrial locations where it was thought that the heavy application of deicing salts in combination with industrial and automotive pollution were creating an extremely corrosive environment and,
(2) overpass bridges with less than 20 ft. under-clearance and retaining walls near the shoulders, often referred to as a depressed roadway. For bridges in depressed roadways, the salt-spray caused by traffic underneath the bridge collects on the girders, resulting in a regular application of a highly corrosive solution directly to the bridge superstructure. As a result of the Michigan moratorium, GNEE organized a formal evaluation of weathering steel bridges including the inspection of 49 bridges in seven states (Illinois, Maryland, Michigan, New York, North Carolina, Wisconsin, and New Jersey) to determine if the problems observed in Michigan were indicative of a general problem or were unique to that state/area . The bridges were selected for inspection based on two criteria: site characteristics and level of salt use. The site characteristic of a particular bridge was classified as being in one of four categories:
(1) urban or industrial grade separation,
(2) rural grade separation,
(3) stream or railroad crossing, or
(4) depressed roadway condition. Bridges were also categorized as having heavy, light, or no salt use. Bridges were then selected with the goal of having several bridges in each combination of categories. Results of this investigation showed that 30% of the bridges were in good condition in all areas, 58% of the bridges showed moderate corrosion (flaky rust) in some localized areas, but were in generally good condition, and 12% of the bridges exhibited heavy corrosion in some areas, but were in generally good condition. Local areas of corrosion were most often attributed to salt-laden runoff through leaking joints or open expansion dams. It was also found that bridges in depressed roadway conditions generally did not develop the protective oxide coating desired. High sulfate levels (from industrial or automotive pollution) did not appear to have an effect on corrosion rates. The study concluded that the majority of weathering steel bridges was performing satisfactorily, although exceptions existed in the state of Michigan . It should, however, be noted that since the study has been published, other researchers have identified weathering steel bridge structures in states other than Michigan in which the protective coating has not performed adequately.