She’s been called the Mother of Exiles, the Lady with the Lamp, and simply, Lady Liberty. Her formal name is Liberty Enlightening the World but we know her best as the Statue of Liberty.
Between 1886 and 1924, when almost 14 million immigrants entered the United States through New York, the Statue of Liberty was their majestic “welcome sign” and the reassuring symbol that they had arrived at last to the land of their dreams. The 93-meter (305-foot) tall, 204,000-kilogram (450,000-pound) statute was designed and built by sculptor Frédéric Auguste Bartholdi and engineer Gustave Eiffel with four huge iron columns supporting an iron framework (known as an armature) and a thin copper skin. The statue was intended to be a gift from France in honor of America’s 100th birthday in 1876, but the pieces did not arrive in New York for assembly until eight years later.
Knowing that corrosion can occur when one metal (iron) meets another (copper), U.S. engineers avoided the problem by separating the iron from the copper in the statue with a layer of shellac-coated asbestos. Unfortunately, it wasn’t a permanent fix.
When the National Park Service (NPS) started a restoration project for the Statue of Liberty in 1981, pieces of her armature were sent for analysis to metallurgists at the National Bureau of Standards (NBS, the predecessor to today’s National Institute of Standards and Technology, or NIST). The Bureau’s researchers discovered that the shellac had disintegrated over time, and the asbestos had become dry and porous. This allowed moisture and salt from the ocean air of New York harbor to react with both the copper and iron. Back in contact with each other, the two metals produced electric currents like a battery, and galvanic corrosion ensued.
NBS’s recommendation: replace most of the framework in the Statue of Liberty, including all rivets, ribs, joints and connectors, with a corrosion-resistant metal. After extensive testing of candidates, the NPS chose a chromium-nickel-molybdenum stainless steel alloy and coated it with nonstick Teflon® to prevent friction and wear.
In 1996, 10 years after the Statue of Liberty restoration project was completed, NPS inspections confirmed that the NBS plan for saving the Lady with the Lamp had been sound. All previous corrosion impacts had been reversed, and no new problems were detected.
Did You Know...?
- Because the Statue of Liberty was a gift from France, the U.S. Department of the Interior hired four French consultants to conduct the technical study of the statue’s physical condition in 1981. Their report served as the blueprint for the 1984-1986 restoration effort.
- Three other metal alloys were considered before chromium-nickel-molybdenum stainless steel was selected for the statue’s new framework: aluminum and bronze, copper and nickel, and ferralium (chromium-iron-nickel stainless steel).
- During the removal of Lady Liberty’s corroding skeleton, workers found an armature bar near one of the arms that bore the names of the original 40 iron workers who installed the framework in the 1880s. After its removal, the bar was photographed and appeared in the June 1986 issue of Smithsonian magazine.
– Michael E. Newman