The corrosion of the St. Louis Gateway Arch—the nation’s tallest monument—is “a cosmetic problem” that does not affect the structure’s integrity, an in-depth engineering analysis has concluded.
A two-year study of the Arch’s creeping interior and exterior corrosion has determined that the structure “is in excellent structural shape,” according to the National Park Service.
Photos: National Park Service
The stainless steel exterior of the nation’s tallest monument was never coated. Experts in the 1950s considered stainless steel rustproof.
“The staining on the exterior is a cosmetic problem for which the engineers proposed several solutions,” the Park Service said.
The Park Service commissioned a structural study in September 2010 to determine the cause and extent of the decay on the Arch’s stainless-steel exterior and carbon-steel interior.
The monument’s exterior never received a protective coating, because planners of the 1950s considered stainless steel impervious to corrosion. They said cleaning every 50 years was all that was required for the structure to last 1,000 years. The Arch was completed in 1965 and opened in 1967.
A protective coating was applied to the interior, but the reinforced-concrete-and-steel structure has never been recoated or even thoroughly cleaned.
In 2006, a study commissioned by the Park Service noted corroding bolt heads and staircases; a rusting interior; and a degenerating exterior. But no action was taken until 2010, when the St. Louis Post-Dispatch reported and showed the spreading staining along the length of the 630-foot-tall monument.
Drilling and Sampling
The 2010 study was conducted by Wiss, Janney, and Elstner Associates of Northbrook, IL.
The $325,000 project included drilling concrete core samples; installing monitors above the 300-foot level to analyze steel composition and damage; and determining the source of the moisture infiltration—a chronic plague that channels water down the structure’s massive legs.
|A key area of corrosion is the base of the Gateway Arch legs. This is a September 2010 photo from the north leg.|
The work also involved comparing the Arch to other structures of similar vintage and composition.
Welds, Rogue Metals Cited
The conclusion: The Arch has fared more poorly in some ways than similar structures, the Park Service conceded.
“Research revealed that other stainless steel structures have not suffered the same exterior staining as the Arch, leading the engineers to believe that the welds that hold the Arch's sections together may be the culprit,” the Park Service reported in a release.
The agency said engineers involved in the analysis had concluded thus:
"The Gateway Arch stands alone from other buildings and monuments researched in its extensive use of shop and field welds. Many of the discolorations of concern are caused by atmospheric pollutants or inadequate cleaning and polishing of the Arch after erection.
“In addition, if a metal other than the exact type of stainless steel the Arch is made of (Type 304) got into the welds when they were made, the rogue metals might be the cause of the staining. The report notes that whatever the cause, the corrosion is natural and minor, and does not threaten the integrity of the welds or the structure.”
The monument's four million annual guests aren't helping the situation.
Aggravating the natural corrosion is the “unfortunate and growing habit of visitors who carve their names into the exterior base of the Arch legs,” the Park Service reported.
“The carved graffiti (which is a federal offense) shows streaking similar to the welds higher up, and will also be treated to try to eliminate it and to repair the damage. The cleaning of this graffiti might be followed up with a clear coating of some type to prevent future damage.”
The study is the second step in a four-part process, the Park Service said.
The engineers have recommended additional close-up testing of the welds “to determine the best method of cleaning the metal and stopping the [exterior] streaking at its source.”
Samples also will be taken from the exterior and studied to determine the exact type of corrosion present, the Park Service said.
The final step will be “a gentle cleaning of the stainless steel surfaces and dressing and polishing the original welds to reduce future staining.”
The Park Service press release did not mention any of the internal corrosion cited in earlier reports.
Testing the lower welds will be easier than reaching those higher up, the agency noted, “and some sort of rappelling solution—similar to what was recently done on the exterior of the earthquake-damaged Washington Monument—may be necessary to reach the upper welds.”
At 555 feet, the Washington Monument is 75 feet shorter than the Arch.
Arch Superintendent Tom Bradley called the new report’s conclusions “good news for all of us.”
“We now know that St. Louis's most iconographic structure is in good health,” said Bradley. “The Arch may be a little discolored in some places, but we are now certain that it is part of the aging process, and we will work to keep this one of a kind structure in the best shape possible for future generations.
“It may take some time to get up there to clean it, but we will get it done."