Why does silver tarnish?

health : Why does silver tarnish?

Anyone who collects coins knows the difference: gold coins stay metallically pure even after a long time, silver coins discolour. In favorable cases, a patina forms on it, which emphasizes the age of the coins. Often, however, there are less beautiful changes that diminish their value.

"The tarnishing of silver is not an unusual property of metals," says Robert Schlögl, director at the Fritz Haber Institute in Berlin. There are hardly any metals that do not corrode. Steel is no exception. "Most of the time, the corrosion layer is so thin that you can't see it."

Precious metals such as gold or silver should not show any age spots. After all, they are called “noble” because they react little or no reaction with other substances, especially not with the oxygen in the air.

Oxygen has a special chemical structure. The element likes to grab foreign electrons and thus fills its outer electron shell. Oxygen therefore particularly likes to combine with substances that are ready to give up electrons because they have an insufficiently populated outer electron shell. Magnesium reacts violently with oxygen, iron rusts, especially in a damp environment, copper corrodes less, but silver and gold can hardly harm oxygen.

However, if a silver atom finds a reaction partner other than oxygen, things change. Hydrogen sulfide is such a substance. The compound is always present in traces in the air, but also in boiled eggs, which should not be eaten with a silver spoon if at all possible. In the presence of oxygen, hydrogen sulphide forms a compound with silver that is difficult to dissolve. Even when eating pea soup with a silver spoon, there is a risk that it will turn black and discolored. The subsequent silver cleaning is a laborious affair.

Even for chemists, it is not always easy to predict in which environment which substances will corrode particularly severely. The seemingly harmless milk can turn out to be extremely aggressive. It has already corroded many stainless steel pipes in dairies to the point of leaking, says Schlögl.

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