Salt that keeps eating bronze

In the picture

1. 1Green crust on the uncleaned half: corrosion layers that trap chloride salts from the sea
2. 2Crust flakes removed: the loose, salt-laden layer taken off by hand
3. 3Fresh-water bath: salts dissolve and slowly diffuse out of the metal into the water
4. 4Salt crystals in cracks: with damp air they form acid that digs new pits around them

Examples

• Cannons and anchors raised from shipwrecks are kept in water tanks until the salt is washed out, because letting them dry would make them crumble
• Cars driven on salted winter roads or parked by the sea rust faster, especially in seams where salt collects

Quiz: Salt that keeps eating bronze (Age 17)

1. 1

   A bronze statue raised from the sea is dried completely and then displayed in a humid room. What is most likely to happen to it?

   • Corrosion starts again, because the leftover chloride salts absorb moisture from the air and form acid
   • The dried salt hardens into a protective coating that shields the metal from the damp air
   • Nothing more, because once the seawater has dried, corrosion has nothing left to feed on
2. 2

   Conservators soak a salty metal object in fresh water and keep changing the bath. When do they know they can stop?

   • After one long soak, since a single bath takes up all the salt
   • When almost no new chloride shows up in the fresh bath water
   • When the green crust has fully dissolved into the water
3. 3

   Why is the corrosion crust on a metal object recovered from the sea a problem beyond its appearance?

   • Its layers trap chloride salts from the sea, which can keep attacking the metal
   • It is made of hardened sand that scratches the metal surface underneath
   • It blocks all air, so it is the only thing stopping the metal from corroding further

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.