Stains on the Arch

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Date: September 25, 2012
Contact: Tom Bradley, 314-655-1611
Contact: Frank Mares, 314-655-1633

After an extensive study of the interior and exterior of the Arch, as well as other stainless steel structures built before or at the same time as the Arch, the engineers concluded that the Arch is in excellent structural shape. The staining on the exterior is a cosmetic problem for which the engineers proposed several solutions.

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 Gateway Arch stands alone from other buildings and monuments researched in its extensive use of shop and field welds," the engineers stated. "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.

In addition to the natural corrosion emanating from the welds, another problem is posed by the unfortunate and growing habit of visitors who carve their names into the exterior base of the Arch legs. 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.

This most recent study is the second step in a four step process. Recommendations made by the engineers to eliminate the streaking on the exterior of the Arch include further close-up testing of the welds to determine the best method of cleaning the metal and stopping the streaking at its source. Samples will be taken from the exterior of the structure and studied to determine the exact type of corrosion present. This would be followed by the final step, a gentle cleaning of the stainless steel surfaces and dressing and polishing the original welds to reduce future staining. Lower welds will be easier to reach than those higher up, 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.

Superintendent Tom Bradley stated that "the report issued by Wiss, Janney, Elstner is good news for all of us. We now know that St. Louis's most iconographic structure is in good health. 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."

The effort to address the staining is part of the ongoing maintenance and long term stewardship responsibilities of the National Park Service and is separate from CityArchRiver 2015, a project currently underway to enhance the visitor experience and better connect the park to downtown St. Louis.

Last updated: April 10, 2015

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