When looking at rusty objects I’ve noticed that some rust looks like its “dribbling” down from the edges of metal surfaces where two metal parts are directly touching, like where rivets/bolts are holding metal panels in place, almost like the rust is a liquid leaking out from beneath the metal. Why does it form like that?
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Rust is iron oxide. That is, it is when iron combines with oxygen in the air or from another source. Water, salt, and some other chemicals act as catalysts, speeding up the rusting process.
One factor with rust is that it is larger than the iron molecules it forms from, which results in the cracking and general damage caused by rusting.
The rate of rusting depends on how much iron is exposed, and the cracking from rusting ends up exposing more iron, which makes the rusting occur faster. As such, its very common to see rust spread out from areas it has already “infected”.
Similarly, rivets or bolts will often have some amount of friction, which will help break away tiny pieces and expose more. These types of surfaces will commonly accumulate water and other chemicals that can speed up the rusting process. Accumulated water will tend to drip down from these, increasing rusting along the path the water takes. When the water drips down, it will also take rust with it, some of which will settle onto the surface – much like how pebbles settle on the shore of a river, which may cause the appearance of rusting even if there is no actual rusting underneath, though there probably will be underlying rusting.
The process gets more complicated if different metals are used in the construction. If the main object is made of iron or steel, but the rivets are made of copper or brass, there is a process called [Galvanic corrosion](https://en.wikipedia.org/wiki/Galvanic_corrosion) that results in the iron rusting faster due to the presence of a more [“noble” metal](https://en.wikipedia.org/wiki/Noble_metal), that is, one that is more resistant to corroding. This same process can be used to protect iron from rusting, such as by plating iron in a thin layer of zinc (called [galvanization](https://en.wikipedia.org/wiki/Galvanization)), which is less noble than iron but also doesn’t damage when it corrodes. The same basic idea is how stainless steel works, though, its chromium, and normally nickel, actually getting mixed into the steel instead of being coated on top.
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