Physics explains why gold doesn’t ‘rust’

Steel rusts over time. Copper turns green. But gold stays shiny, seemingly forever. Scientists have now discovered a property that explains why gold stays spotless.

Oxidation is a type of chemical reaction that can cause a metal to rust or tarnish when it meets oxygen. Atoms on gold’s surface, new data show, rearrange into a geometry that hinders that oxidation. If they didn’t, gold would begin to oxidize in seconds.

Santu Biswas and Matthew Montemore shared their new finding May 21 in Physical Review Letters. These researchers work at Tulane University in New Orleans, La.

In air, oxygen is bundled into molecules, each consisting of two oxygen atoms. For a metal to oxidize, it first has to split those oxygen pairings. Then the single oxygen atoms can form compounds that stick to the surface of a metal. The researchers calculated how well gold’s surfaces could split oxygen. They found that it depends on what happens on gold’s surface.

Imagine cutting a piece of gold in two. That would create a new surface that was once in the center of the gold hunk. Atoms on the newly exposed surface then shift from their original locations in the metal.

There are a few different possible arrangements those atoms can take. The researchers studied two common ones. In both, atoms that had been laid out in squares rearranged into hexagons.

The researchers did calculations to understand the effect of such shapeshifting. The hexagonal arrangement, they found, was much worse at splitting oxygen than the square one. To split oxygen, the hexagonal structure would first need to distort back into the original square shape. That’s a hurdle that stalls oxidation. (A third common surface structure of gold was already known to be bad at oxidizing. It’s hexagonal from the start.)

But there’s more to the story. Under certain conditions, gold can react with oxygen. When it does, it forms gold oxide. That’s unstable. So even if gold were able to split oxygen, the material would form only a thin layer of oxide, notes Montemore, a chemical engineer. His team’s new findings, he says, could help scientists understand how to design better catalysts. Those are materials that speed up chemical reactions.

Just how much slower the rearranged gold atoms oxidize was “definitely a surprise,” Montemore says. The tiny position shifts made a huge difference. “It’s something like a billion to a trillion times slower oxidation once you rearrange.”