Why does rust/corrosion appear to “dribble” from the edges of metal surfaces, or from between two pieces of metal?

Question

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?

In: 7

Answer 1

Rust can happen from oxidation due to oxygen in water or air.

If it happens because of water, the rust will dribble down with the water droplets.

If it happens in air, it can form a patina (a “good” rust) or “red” rust, which obviously won’t dribble down. “Red rust” being the bad rust which will weaken steel.