"Chemistry is in the News"
LADY LIBERTY: PLAN A & PLAN B
By GeoT, Geography Newsletter, August 22, 2001.
In 1886, the US received a gift from France in the form of a statue, which would later represent freedom of a nation. Frederic Auguste Bartholdi, the architect of the Statue, tried to conceive a structure that would avoid natural deterioration, but as years past it became obvious that this symbol of freedom couldn't avoid corrosion.
The Statue of Liberty is composed of several metals. The exterior layer of the statue is made of a metal alloy consisting of bronze and copper. Oxygen gas is a free diradical in the atmosphere, resulting in a highly reactive molecule. Copper is oxidized by the diradical causing the color of the statue to change from bright reddish-brown to malachite green. Also, on the inside of the statue, the skeletal iron frame is subject to rust by the same radical nature of oxygen gas causing rust to devour the iron. Once iron is oxidized it doesn't remain attached to the surface of the metal like copper does, but instead flakes off resulting in the weak internal structure of the Statue of Liberty.This corrosion led to a need for restoration of the Statue in the early 1980's and all work was completed by July 4, 1986. Despite these measures, copper and iron cannot avoid oxygen's "radical" nature and the Statue of Liberty will never be safe from corrosion.
Pertinent Text References
Wade: Chapters 4 & 11
The radical nature of oxygen gas causes copper to turn green over
time, but what other process accelerated the discoloration of the
Statue of Liberty?
Answer: Acid Rain. Acid rain helps in weakening structures. The Statue of Liberty will probably turn black due to the reaction between copper oxide on its surface and acid rain.
Write the chemical reactions for the oxidation of both copper and iron by
oxygen gas. Also include the oxidation numbers of the molecules across
Answer: 2 Cu + O2 ---> 2 Cu+2O-2.
Since iron in the presence of oxygen gas corrodes at a slow rate,
what molecules, when present, accelerate this process?
Answer: H2O. H2O allows the electrons to be transported form iron to oxygen.
Why was the copper/iron combination that was used in the construction of
the Statue a bad choice chemically?
Answer: Galvanic corrosion will occur when two dissimilar metals come into contact, and direct contact of Cu with Fe will cause corrosion. Iron is especially susceptible if in contact with Cu.