You probably know that the Statue of Liberty underwent a massive restoration in the 1980s, but you may not know why. Well-meaning attempts to conserve the statue ended up turning it into a battery and ripping it apart.
To understand how we oopsed our way into nearly destroying one of the most iconic American monuments, let’s talk about the oldest form of battery, known as a “voltaic pile.” The voltaic pile is not really a pile. It’s more like a club sandwich in the sense that its components are placed on top of each other, in a pattern, in a specific order, and for a specific purpose. The pile consists of one kind of metal, a pad soaked in an electrolyte (usually an acid or salt water), and another kind of metal. Repeat.
Which two kinds of metal? People have made piles with all different kinds, but for the sake of our example, let’s say iron and copper. The acid in the electrolyte loosens up the electrons in the iron, allowing some of them to go free. Any iron atom that succumbs to this is turned into an ion, which has the freedom of movement to go through the electrolyte to the copper. The copper surrenders a couple of electrons to the iron, at which point the iron is fully stocked with electrons and immobile. Now because the iron ion left its electrons with the main body of iron before it went on the move, the iron layer of the pile grows more and more negative. The copper, donating its electrons, grows more positive. So between the copper end and the iron end, the pile maintains a voltage just waiting for a wire to connect the two ends so the current can flow.
The Statue of Liberty has that lovely green patina because it’s made of copper. Its inner frame is made of iron. It was never intended for the two to meet, but the statue was under the care of many different departments since it first went up, including the War Department during the First World War. Understandably, many of these departments had other things on their minds. The standard care given to the statue was layers and layers of varnish and paint on the interior. The paints were all different kinds of material, but most of them hadn’t been applied under perfectly dry conditions. Water, often corrupted with salt, was between the iron frame and the copper exterior.
How much voltage did this colossus produce? About a quarter of a volt. It wasn’t a strong battery, but it was running for over half a century, which led to the real problem. Remember that the iron ions travel to the copper, load up on electrons, and stop moving. That is what had happened to the iron frame holding up the statue. In some places, the iron beams were half as thick as they had originally been. This wasn’t the first time that people had made that mistake. A British ship, the HMS Alarm, had used iron rivets to attach copper plates to its hull. Soon the plates were popping off, and this kind of corrosion, known as galvanic corrosion, is why.
The statue had undergone worse damage than the ship. Where the iron beams had bent, more water had leaked in, corroding both the iron and the copper. The copper plating should have come off. In some places it wasn’t coming off only because the paint itself was holding the whole thing together. This is why the statue’s restoration took two years, and why people are now much more careful in their preservation attempts. It takes an understanding of materials science to keep the statue from electrifying itself to death.
[Source: Rust: The Longest War, by Jonathan Waldman, The Voltaic Pile]
Image: Dudva.